atm switch

Graduation project

The AXD ATM Switch In

The National Telephone Network

Prepared by

Ahmed Abd-El Nasser Ramadan

Supervised By DR.Farid M. Badran

2008

Cairo UniversityFaculty of EngineeringCommunication & Electronic Department

Index

1

Index………………………………………………………………………………….1 INTRODUCTION ..............................................................................................3 CHAPTER 1 ...................................................................................................... 4 1.1 Introduction to PSTN ................................................................................... 4 1.2 The Beginning of the PSTN ........................................................................... 5 1.3 PSTN basics .............................................................................................. ... 8

1.3.1Analog and Digital Signaling .................................................................... 8 1.3.2 Pulse Code Modulation (PCM) .............................................................. .. 11

1.4 PSTN Architecture ...................................................................................... 13 1.4.1Class 1 (regional center) .............................................................................13 1.4.2 Class 2 (sectional center) .......................................................................... 14 1.4.3 Class 3 (primary center) ...................................................................... .... 14 1.4.4 Class 4 (toll center) ..................................................................... ............. 15 1.4.5 Class 5 (local exchange) ................................................................ ........... .. 15 1.5 PSTN signalling ........................................................................................ .. 17 1.5.1 User-to-Network Signalling ..................................................................... 18 1.5.2 Network-to-Network Signaling ................................................................ 20 1.6 Call Flow Throught The PSTN .................................................................... 21 1.7 PSTN Draw Backs ........................................................................................ 23 CHAPTER 2 .................................................................................................... .. 24 2.1 Introduction to AXE Digital Exchange .......................................................... 24 2.2 AXE Digital Exchange description ................................................................ 25 2.3 The reason behind dividing APT & APZ to SS ........................................... 26 2.4 The switching system ( APT ) ....................................................................... 28 2.5 The Control system ( APZ ) .......................................................................... 30 2.6 Subsriber Switching Subsystem ................................................................... 32

2.6.1 Line InterFace Card (LIC) .................................................................... 34 2.6.2 Key-set Reception Card (KRC) .............................................................. 35 2.6.3 Extention Terminal board (ETB) ................................................. ............ 35 2.6.4 Extension Module Regional processor (EMRP) ..................................... 35 2.6.5 Extension Module Time Switch (EMTS) ................................................ 36 2.6.6 Ringing Generator (RG) ....................................................................... 36 2.6.7 Sub. Line & Cct. Tester ....................................................................... 36 2.6.8 Remote Sub. Switch ............................................................................... 37

2.7 Group Switch Subsystem (GSS) ................................................................... 41 2.7.1 Time Switch Module .............................................................................. 42 2.7.2 Space Switch Module ............................................................................ 43 2.7.3 call establishment throught GS ............................................................. 45

2.8 Trunk & Signalling Subsystem .................................................................... 46 2.9 Common Channel Signalling (CCS) ............................................................ 47 2.10 Central Processor Subsystem .................................................................. 48 2.11 Regional Processor Subsystem (RPS) ....................................................... 51 2.12 Maintennance SubSystem (MAS) .............................................................. 53 2.13 Support Processor SubSystem (SPS) .......................................................... 54 2.14 Synchronization In Axe ..................................................... ....................... 55 2.15 Ericsson AXE10 ....................................................................................... 56 CHAPTER 3 ....................................................................... ............................. 58 3.1 Introduction to SDH ................................................................................... 58 3.2 SDH Basics .............................................................................................. ... 59

3.2.1 PDH (Plesiochronous Digital Hierarchy) ................................................ 59

Index

2

3.2.2 PDH limitation....................................................................................... 60 3.2.3 SDH advantage ...................................................................................... 60 3.2.4 Basic SDH Signal ................................................................................... 61 3.2.5 SDH Frame Structure ........................................................................... 62

3.3 SDH Multiplexing ....................................................................................... 72 3.4 SDH Multiplexer Structure ......................................................................... 74 CHAPTER 4 ......................................................................................................75 4.1 Introduction to SDH Network ...................................................................... 75 4.2 SDH Network Elements ...............................................................................76

4.2.1 Terminal multiplexer .............................................................................77 4.2.2 Regenerator ..........................................................................................77 4.2.3 Add/Drope Multiplexer ...........................................................................78 4.2.4 Digital crossconnects (DXC) ...................................................................78

4.3 SDH Network Configurations ...................................................................... 79 4.3.1 Point-to-Point ....................................................................................... 80 4.3.2 Point-to-Multipoint ............................................................................... 80 4..3.3 Mesh Architechture.............................................................................. 81 4.3.4 Ring Architecture................................................................................... 82

4.4 SDH Ring Protection Mechanisms ............................................................... 83 4.4.1 Unidirectional rings .............................................................................. 84 4.4.2 Bi-directional rings................................................................................. 85

CHAPTER 5 ..................................................................................................... 86 5.1 Introduction to ATM ................................................................................... 86 5.2 ATM Basic ..................................................................................................... 88 5.3 ATM Cell Formate ..................................................................................... 90 5.4 ATM Devices ............................................................................................. 91 5.5 ATM Network Interfaces............................................................................. 92 5.6 ATM Cell Header Formats ........................................................................... 93

5.6.1 UNI Cell Header Formats ...................................................................... 93 5.6.2 NNI Cell Header Formats ..................................................................... 95

5.7 ATM Reference Model ................................................................................ 96 5.7.1 Physical Layer ........................................................................................97 5.7.2 ATM Layer ............................................................................................98 5.7.3 ATM Adaptation Layer .......................................................................... 98 5.7.4 Sequence Of Layer Operation .......................................................... 100

5.8 ATM Switching .......................................................................................... 101 5.9 ATM Network Delay .................................................................................. 104

5.9.1 Paketisation Delay .............................................................................. 104 5.9.2 Transmission Delay............................................................................. 104 5.9.3 Fixed Switching Delay.......................................................................... 104 5.9.4 Queuing Delay................................................................................... 104 5.9.5 Depacketisation Delay........................................................................ 105 5.10 ATM Switch.......................................................................................... 106 5.10.1 Introduction to AXD........................................................................... 106 5.10.2 AXD 301 Structure............................................................................. 108 5.10.3 Switch layout..................................................................................... 109 5.10.3 AXD 301 20Gbps Structure................................................................ 111 5.10.4 Transition From Cicuit-Switched To Packet-Switched.................. 112

Introduction

3

The public switched telephone network has traditionally been a circuit switched network based on digital exchange and PCM transmission using SDH synchronous Digital Hierarchy multiplexers on fiber cables, however the present trend is to replace circuit switching by packet switching based on the Asynchronous Transfer Mode , ATM technology as introduce in PSTN in Egypt . This project gives an outline of the digital Exchange AXE and the SDH multiplexer system then explains the basic of ATM technology and its realization the Ericson AXD exchange system .the role of the AXD switch in the network is investigated and advantages outlined. The convergence of telephone , data and video service will be studied in this report. Chapter Contents 1- Public switched telephony network. 2- AXE digital exchange. 3- Synchronous digital hierarch multiplexer. 4- Synchronous digital hierarchy ring. 5- Asynchronous transfer mode.

Puplic Switched Telephone Network

Introduction 4

1.1 Introduction to PSTN:

PSTN (public switched telephone network) is the world's collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. It's also referred to as the Plain Old Telephone Service (POTS). The Public Switched Telephone Network (PSTN) is truly one of the marvels of the 20th century.

The PSTN provides voice and data communication over a circuit-switched network. It is the network which presently provides most international and national telephone service to end users.

P S T N

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PSTN Development 5

1.2 The Beginning of the PSTN:

The first voice transmission, sent by Alexander Graham Bell, was accomplished in 1876 through what is called a ring-down circuit. A ring-down circuit means that there was no dialing of numbers, Instead, a physical wire connected two devices. Basically, one person picked up the phone and another person was on the other end (no ringing was involved). Over time, this simple design evolved from a one-way voice transmission, by which only one user could speak, to a bi-directional voice transmission, whereby both users could speak. Moving the voices across the wire required a carbon microphone, a battery, an electromagnet, and an iron diaphragm. It also required a physical cable between each location that the user wanted to call. Theconcept of dialing a number to reach a destination, however, did not exist at this time.

To further illustrate the beginnings of the PSTN, see the basic four-telephone network shown in Figure (1.A).

Figure (1.A)


PSTN Development 6

As you can see, a physical cable exists between each location. Place a physical cable between every household requiring access to a telephone, however, and you’ll see that such a setup is neither cost-effective nor feasible (see Figure 1.B). To determine how many lines you need to your house, think about everyone you call as a value of N and use the following equation: N × (N–1)/2. As such, if you want to call 10 people, you need 45 pairs of lines running into your house.

Figure (1.B)

Due to the cost concerns and the impossibility of running a physical cable between everyone on Earth who wanted access to a telephone, another mechanism was developed that could map any phone to another phone. With this device, called a switch , the telephone users needed only one cable to the centralized switch office.


PSTN Development 7

At first, a telephone operator acted as the switch. This operator asked callers where they wanted to dial and then manually connected the two voice paths. Figure (1.C) shows how the four-phone network example would look today with a centralized operator to switch the calls.

Figure (1.C)

Now, skip ahead 100 years or so—the human switch is replaced by electronic switches as shown Figure (1.D).

Figure (1.D)

Before explaining the PSTN Architechture we need to explain some basics to understand the PSTN.


PSTN Basics 8

1.3 PSTN basics:

1.3.1 Analog and Digital Signaling :

Everything you hear, including human speech, is in analog form. Until several decades ago, the telephony network was based on an analog infrastructure as well. Although analog communication is ideal for human interaction, it is neither robust nor efficient at recovering from line noise. ( Line noise is normally caused by the introduction of static into a voice network.) In the early telephony network, analog transmission was passed through amplifiers to boost the signal. But, this practice amplified not just the voice, but the line noise as well. This line noise resulted in an often unusable connection. Analog communication is a mix of time and amplitude. Figure 1-E , which takes a high-level view of an analog waveform, shows what your voice looks like through an oscilloscope.

Figure (1.E)


PSTN Basics 9

If you were far away from the end office switch (which provides the physical cable to your home), an amplifier might be required to boost the analog transmission (your voice). Analog signals that receive line noise can distort the analog waveform and cause garbled reception. This is more obvious to the listener if many amplifiers are located between your home and the end office switch. Figure 1.F shows that an amplifier does not clean the signal as it amplifies, but simply amplifies the distorted signal. This process of going through several amplifiers with one voice signal is called accumulated noise.

.

Figure (1.F)


PSTN Basics 10

In digital networks, line noise is less of an issue because repeaters not only amplify the signal, but clean it to its original condition. This is possible with digital communication because such communication is based on 1s and 0s. So, as shown in Figure 1-G , the repeater (a digital amplifier) only has to decide whether to regenerate a 1 or a 0.

Figure 1-G

Therefore, when signals are repeated, a clean sound is maintained. When the benefits of this digital representation became evident, the telephony network migrated to pulse code modulation (PCM).


PSTN Basics 11

1.3.2 Pulse Code Modulation (PCM):

PCM is the most common method of encoding an analog voice signal into a digital stream of 1s and 0s. All sampling techniques use the Nyquist theorem , which basically states that if you sample at twice the highest frequency on a voice line, you achieve good-quality voice transmission. The PCM process is as follows:

• Analog waveforms are put through a voice frequency filter to filter out anything greater than 4000 Hz. These frequencies are filtered to 4000 Hz to limit the amount of crosstalk in the voice network. Using the Nyquist theorem, you need to sample at 8000 samples per second to achieve good-quality voice transmission.

• The filtered analog signal is then sampled at a

rate of 8000 times per second.

• After the waveform is sampled, it is converted into a discrete digital form. This sample is represented by a code that indicates the amplitude of the waveform at the instant the sample was taken.

The telephony form of PCM uses eight bits for the code and a logarithm compression method that assigns more bits to lower-amplitude signals.


PSTN Basics 12

If you multiply the eight-bit words by 8000 times per second, you get 64,000 bits per second (bps). The basis for the telephone infrastructure is 64,000 bps (or 64 kbps). Two basic variations of 64 kbps PCM are commonly used: µ -law, the standard used in North America; and A-law, the standard used in Europe. The methods are similar in that both use logarithmic compression to achieve from 12 to 13 bits of linear PCM quality in only eight-bit words, but they differ in relatively minor details. The µ -law method has a slight advantage over the a-law method in terms of low-level signal-to-noise ratio performance, for instance. There is to units for the PCM :

1- T1 is a 1.544-Mbps digital transmission link normally used in North America and Japan (24 digital channel).

2- E1 is a 2.048-Mbps digital transmission link normally used in Europe (32 digital channel).

Now we will discuss the PSTN Architecture.


PSTN Architecture 13

1.4 PSTN Architecture:

1.4.1Class 1 (regional center):

The class 1 office was the Regional Center (RC). Regional centers served three purposes toll network:

(a) Their connections were the "last resort" for final setup of calls when routes between centers lower in the hierarchy were not available.

(b) They were initially staffed by engineers who had the authority to block portions of the network within the region in case of emergencies or network congestion - although these functions were transferred after 1962 to the Network Control/Operations Center and the distributed Network Management Centers.

(c) They provided collection points ( until the development of more advanced computer hardware and software for toll operators ) for circuits that would be passed along to one of the international overseas gateways. The regional centers updated each other on the status of every circuit in the network. These centers would then reroute traffic around the trouble spots and keep each informed at all times.



1.4.2 Class 2 (sectional center):

The class 2 office was the Sectional Center (SC).

The sectional center typically connected major toll centers within one or two states or provinces, or a significant portion of a large state or province, to provide interstate or interprovincial connections for long-distance calls. At various times, there were between 50 and 75 active class two offices in the network.

1.4.3 Class 3 (primary center):

The class 3 office was the Primary Center (PC). Calls being made beyond the limits of a small geographical area where circuits connected directly between class 4 toll offices would be passed from the toll center to the primary center. These locations use high usage trunks to complete connection between toll centers. The primary center never served dial tone to the user. The number of primary centers in the network fluctuated from time to time, ranging between 150 and 230.



1.4.4 Class 4 (toll center):

The class 4 office is the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP).

A call going between two end offices not directly connected together, or whose direct trunks are busy, is routed through the toll center. The toll center is also used to connect to the long-distance network for calls where added costs are incurred, such as operator handled services. This toll center may also be called the tandem office because calls have to pass through this location to get to another part of the network.

1.4.5 Class 5 (local exchange):

The class 5 office is the local exchange or end office. It delivers dial tone to the customer. The end office, also called a branch exchange, is the closest connection to the end customer.

In modern times only the terms Class 4 and Class 5 are much used, as any tandem office is referred to as a Class 4. This change was prompted in great part by changes in the power of switches and the relative cost of transmission, both of which tended to flatten the switch hierarchy.

Figure (1.H) shows the PSTN classes.



Figure (1.H)


PSTN Signalling 17

1.5 PSTN signalling:

Generally, two types of signaling methods run over various transmission media. The signaling methods are broken into the following groups:

• User-to-Network Signaling: This is how an end user communicates with the PSTN. • Network-to-Network Signaling: This is generally how the switches in the PSTN intercommunicate.


PSTN Signalling 18

1.5.1 User-to-Network Signalling:

Generally, when using twisted copper pair as the transport, a user connects to the PSTN through analog, Integrated Services Digital Network (ISDN), or through a E1/T1 carrier.

The most common signaling method for user-to network analog communication is Dual Tone Multi-Frequency (DTMF). DTMF is known as in-band signaling because the tones are carried through the voice path. Figure (1-I) shows how DTMF tones are derived.

Figure (1-I)


PSTN Signalling 19

When you pick up your telephone handset and press the digits (as shown in Figure 1-I), the tone that passes from your phone to the central office switch to which you are connected tells the switch what number you want to call. ISDN uses another method of signaling known as out-of-band. With this method, the signaling is transported on a channel separate from the voice. The channel on which the voice is carried is called a bearer (or B channel) and is 64 kbps. The channel on which the signal is carried is called a data channel (D channel) and is 16 kbps. Figure 1-J shows a Basic Rate Interface (BRI) that consists of two B channels and one D channel.

Figure (1.J)


PSTN Signalling 20

1.5.2 Network-to-Network Signaling:

Network-to-network signaling types includes

1- In-band signaling methods such as Multi-Frequency (MF) and Robbed Bit Signaling (RBS). MF is similar to DTMF, but it utilizes a different set of frequencies. As with DTMF, MF tones are sent in-band. But, instead of signaling from a home to an end office switch, MF signals from switch to switch.

2- Out-of-band signaling method known as Signaling System 7 (SS7) (or C7 in European countries). SS7 is beneficial because it is an out-of-band signaling method and it interconnects to the Intelligent Network (IN). Connection to the IN enables the PSTN to offer Custom Local Area Signaling Services (CLASS) services. SS7 is a method of sending messages between switches for basic call control and for CLASS. These CLASS services still rely on the end-office switches and the SS7 network. SS7 is also used to connect switches and databases for network-based services. (for example, 800-number services and Local Number Portability [LNP]).


Call Flow Throught The PSTN 21

1.6 Call Flow Throught The PSTN:

1- The calling user pick up the phone and send an off-hook indication to the end office switch.

2- The switch sends back a dial tone. 3- The calling User dial the digits to call the called

user (they are sent in-band through DTMF). 4- The switch interprets the digits and sends an

Initial Address Message (IAM, or setup message) to the SS7 network.

5- The SS7 network reads the incoming IAM and sends a new IAM to The called user’s switch.

6- The called user’s switch sends a setup message to the called user’s phone (it rings the phone).

7- An alerting message (alerting is the same as the phone ringing) is sent from The called user’s switch (not from the phone) back to the SS7 network through an Address Complete Message (ACM).

8- The SS7 network reads the incoming ACM and generates an ACM to the calling user`s switch.

9- The calling user can hear a ringing sound and know that the called user’s phone is ringing. (The ringing is not synchronized; the local switch normally generates the ringing when the ACM is received from the SS7 network.)

10- The called user picks up the phone, sending an off-hook indication to the switch.

11- The called User’s switch sends an ANswer Message (ANM) that is read by the SS7, and a new ANM is generated to the calling user switch.

12- A connect message is sent to the calling user phone (only if it’s an ISDN phone) and a connectacknowledgment is sent back (again, only if it’s an ISDN phone). (If it is not an ISDN phone, then on-hook or off-hook representations signal the end office switch.)

13- Now the two user can make a conversation tell the calling hang up (on-hoke indication).

Figure (1.k) showes that


Call Flow Throught The PSTN 22

Figure (1.k)


PSTN Draw Backs 23

1.7 PSTN Draw Backs:

Although the PSTN is effective and does a good job at what it was built to do (that is, switch voice calls), many business drivers are striving to change it to a new network, whereby voice is an application on top of a data network. This is happening for several reasons:

• Data has overtaken voice as the primary traffic on many networks built for voice. Data is now running on top of networks that were built to carry voice efficiently. Data has different characteristics, however, such as a variable use of bandwidth and a need for higher bandwidth. Soon, voice networks will run on top of networks built with a data-centric approach. Traffic will then be differentiated based upon application instead of physical circuits. New technologies (such as Fast Ethernet, Gigabit Ethernet, and Optical Networking) will be used to deploy the high-speed networks that needed to carry all this additional data.

• The PSTN cannot create and deploy features quickly enough. With increased competition due to deregulation in many telecommunications markets, LECs are looking for ways to keep their existing clientele. The primary method of keeping customers is by enticing them through new services and applications. The PSTN is built on an infrastructure whereby only the vendors of the equipment develop the applications for that equipment. This means you have one-stop shopping for all your needs. It is very difficult for one company to meet all the needs of a customer. A more open infrastructure, by which many vendors can provide applications, enables more creative solutions and


PSTN Draw Backs 24

applications to be developed. It is also not possible with the current architecture to enable many vendors to write new applications for the PSTN. Imagine where the world would be today if vendors, such as Microsoft, did not want other vendors to write applications for its software.

• Data/Voice/Video (D/V/V) cannot converge on the PSTN as currently built. With only an analog line to most homes, you cannot have data access (Internet access), phone access, and video access across one 56-kbps modem. High-speed broadband access, such as digital subscriber line (DSL), cable, or wireless, is needed to enable this convergence. After the last bandwidth issues are resolved, the convergence can happen to the home. In the backbone of the PSTN, the convergence has already started.

• The architecture built for voice is not flexible enough to carry data. Because the bearer channels (B channels and T1 circuits), call-control (SS7 and Q.931), and service logic (applications) are tightly bound in one closed platform, it is not possible to make minor changes that might improve audio quality. It is also important to note that circuit-switched calls require a permanent 64-kbps dedicated circuit between the two telephones. Whether the caller or the person called is talking, the 64-kbps connection cannot be used by any other party. This means that the telephone company cannot use this bandwidth for any other purpose and must bill the parties for consuming its resources. Data networking, on the other hand, has the capability to use bandwidth only when it is required. This difference, although seemingly small, is a major benefit of packet-based voice networking.

AXE Digital Exchange

Introduction 25

2.1 Introduction to AXE Digital Exchange :

AXE is the most widely deployed switching system in the world . It is used in public telephony-oriented applications of every type, including traditional fixed network applications in local, transit, international and combined networks. AXE is also deployed for all major mobile standards – analogue as well as digital. AXE is compatible with other systems world wide and with PSTN. AXE is the foundation of modern day telephony. When people lift the receiver or pick up their mobile phones all over the world, it is likely that AXE is involved. For more than twenty years AXE has been the world’s favorite means of personal and business communication.

AXE Digital exchange


AXE description 26

2.2 AXE Digital Exchange description :

The AXE Architecture is logically divided into two main parts, the switching system (APT) and the control system (APZ). The switching system (APT) performs traffic and operation / maintennance functions. It comprises four main hardware subsystems:

• Subscriber Switching SubSystem. (SSS) • Group Switching SubSystem. (GSS) • Trunk and Signalling SubSystem. (TSS) • Common-Channel Signalling SubSystem. (CCS)

And the following software subsystems :

• Traffic Control SubSystems. (TCS) • Charging SubSystem. (CHS) • Operation and Maintennance SubSystem. (OMS) • Subscriber Services SubSystem. (SUS) • Network Management SubSystem. (NMS)

The control system (APZ) is made up of centralised and distributed logic achieved through the following subsystems:

• Central Processor SubSystem. (CPS) • Maintennance SubSystem. (MAS) • Regional Processor SubSystem. (RPS) • Support Processor SubSystem. (SPS)


AXE description 27

2.3 The reason behind dividing APT & APZ to SS :

The division of the systems (APT and APZ) into subsystems is determined by conditions and requirements that arise from features, traffic handling and operations and maintennance functions. Each subsystem is built from a number of function blocks which in themselves comprise hardware, central software, regional sofware and data components, or just central software and data components. Each function block is designed to execute a specific set of functions or sub functions. A library of several hundred functional blocks exists to satisfy all applications and individual exchange requirements. Standardised interface signalling is extensively used between blocks not only to achieve the neccesary flexibility to satisfy customer's requirements, but to also enable new technology to be introduced within the system in an efficient manner. The next Figure (2.a) descripes the over all AXE system.


AXE Digital Exchange Block Diagram 28

Figure (2.A)


The switching system APT 29

2.4 The switching system ( APT ) :

As we mention before the switching system ( APT ) consist of number of subsystems . Each subsystem has a function to do & now we will discusse each subsystem function :

1- Sub. Switch Subsystem (SSS) :

The subscriber switching subsystem contains the digital subscriber switch. And is built up of 16 line switch modules (LSMs). An LSM serves 128 analogue subscribers or can support four 30-Channel systems for ISDN Customers. Its principal function is to supervise the state of connected subscriber lines, and to set up release connections by sending and receiving signals to and from subscribers.

2- Group Switch Subsystem (GSS) :

GSS is the heart of the switching system and is responsible for connecting and supervising speech paths. The group switching subsystem houses a time-space-time digital switch built up of duplicated time switch modules (TSMs) and duplicated space switch modules (SPMs).


The switching system APT 30

3- Trunk & Signalling Subsystem (TSS) :

The trunk and signalling subsystem includes the equipment for connecting trunks to the group switch.

TSS handles the connections to other exchanges. Its main job is to match the AXE exchange to various junction signalling systems.

4- Common Channel Subsystem (CCS) :

This subsystem handles the common channel signalling messages between the AXE and other exchanges. The Common-channel signalling subsystem implements the message transfer part (MTP).


The Control System APZ 31

2.5 The Control system ( APZ ) :

As we mention before the switching system ( APZ ) consist of number of subsystems .

Each subsystem has a function to do & now we will discusse each subsystem function :

1- Centeral Processor Subsystem (CPS) :

CPS contains two processors (CPs) which carry out all the complex processing needed to control the AXE10 exchange. The duplication of central processors is necessary for system security.

2- Regional Processor Subsystem (RPS) :

RPS consists of a number of Regional Processors (RPs). These processors perform simple, routine, high capacity tasks, such as scanning of subscribers lines and the operation of switches. The regional processors are usually mounted next to the equipment they are serving and so are spread around the exhange equipment.


The Control System APZ 32

3- Maintennance SubSystem. (MAS) :

The maintennance subsystem consists of both hardware and software. The major role is to supervise the operation of the APZ control system and takes theappropriate action should a malfunction occur.

4- Support Processor SubSystem. (SPS) :

The support processor subsystem consists of one or more independent processors which drive input/output equipment such as personal computers, visual display units and disc drives which are connected to the subsystem.

Now we will discuss each subsystem of the two systems (APT & APZ) in detail ...


Subscriber Switching Subsystem 33

2.6 Subsriber Switching Subsystem :

SSS consist of a number of LSM [ Line Subscriber Module ] (up to 16 LSM) each LSM connected to 128 subscribers . The LSM consiste of number of boards called Extension Module: 1- 16 Line Interface Card (LIC). 2- Extension Module Time Switch (EMTS). 3- Key-set Reception Circuit (KRC) 8 KRC / board. 4- Extension Module Time Switch (EMTS). 5- Subscriber Line Circuit Tester (SLCT). 6- Ring Generator (RG).

The Following is a schematic & a block diagram of LSM.

Figure (2.B)


Line Subscriber Module block diagram 34

Figure (2.C)


Subscriber Switch Subsystem 35

2.6.1 Line InterFace Card (LIC) :

The LSM contains 16 Line Interface Card board. each LIC board is connected to 8 Sub. Line to have 8 × 16 = 128 Sub. per LSM. The LIC is divided to two main parts ( SLIC & SLAC ) and we will discuss the function of each part separetlly . SLIC is Sub. Line Interface Cct. which provides to the Sub. Line :

1- current feed. 2- Polarety revausal. 3- Reception of dialling pulses. & it also has arelay

for connects ringing current signal , & relay for connecting test equipment.

SLAC contains two parts :

1- Analoge to Digital Converter (ADC) : To convert the analog speech signal to digital.

2- Hybird cct : Which is a 4 port network. that prevent the signal to spread in the same direction.

Each LIC board has aDevice Processor (DP) with related software for scanning the sub line state & report the EMRP by its states.



2.6.2 Key-set Reception Card (KRC) :

KRC recevies digits (as tones) from the Subscriber Key-set telephone . There is 8 KRC per board & 1 DP with its related software for scanning operation , & reporting the EMRP.

2.6.3 Extention Terminal board (ETB) :

ETB works as an interface between the LSM & GSS via PCM channel . The ETB card has a DP with its related software for scanning operation & reporting the EMTP.

2.6.4 Extension Module Regional processor (EMRP) :

The LSM contains one EMRP board with its regional software. Its main role to scannes the DPs of each board & reporting the CP.



2.6.5 Extension Module Time Switch (EMTS) :

It connects the LICs with KRC & ETB it also has a DP with its related software for scanning operation & reporting the EMTP.

2.6.6 Ringing Generator (RG) :

The RG board in the LSM generates ringing current , which is connected to sub-line by a relay in LIC.

2.6.7 Sub. Line & Cct. Tester :

The SLCT board in LSM performs routine testing of Sub-Line , as well interface ccts. The SLCT is connected to Sub. line by a relay in the Line Interface Cct..



2.6.8 Remote Sub. Switch :

It`s a group of 4 LSM connected together via a bus to form a group of 512 subscriber that can be installed away from AXE. It`s also called Concentrator. The Remote Subscriber Switch scheme resultes into Great saving in the cost of Sub. line. We achieve this by connecting the EMTS of the 4 LSM via duplicated Time switch Bus (TSB) (for reliability). Just 3 of the 4 LSM has a direct PCM link to the parent AXE. The 1st LSM has no direct PCM link to the AXE so by the TSB it can use the PCM link of any other free LSM. The Concentrator have 3 PCM E1 link connect the 4 LSM to the parent AXE the 1st channel of each E! Used for synchronization & the channel 16 of the 1st two E1 link is used for signalling to have a 91 simultaneous voice channel . We have a larger concentrator by connecting up to 16 LSM to serve up to 2048 subscribers. The next figure shows the connection of the 4 LSM in one switch group.


Remote Sub. Switch Block Diagram 39

Figure (2.D)



The signalling in Remote Subscriber Switch :

In AXE the CP sends the signalling information to the STC which reformate the signalling & send it to ETC.

In the Remote Sub. Concentrator the signalling information is extracted in the Etension Terminal Block (ETB) & reformates it by the Signalling Terminal Regional (STR) then sends it to the EMRP concerned on the EMRP bus. & the next is aFigure to show this(2.E).

Figure (2.D)



The Remote Group Switch has so meny advantages:

1- The Remote Sub. Switch resultes into Great saving in

the cost of Sub. line.

2- In the Remote Sub. Switch the traffic Requirments manage the number of PCM links so every LSM does not need to have a separate link to the parent link.

3- If any PCM link of acertain LSM has a free channels Any other LSM with no free channels can use these free channels.

4- If the connections between the concentrator & the parent AXE are failed the 4 LSM will keep connecting together so we can serve the calles between the Subscribers connected to the same concentrator(the block which responsible for this function is Autonomous Traffic at Link failure (ATL)).


Group Switch Subsystem 42

2.7 Group Switch Subsystem (GSS) :

The Group Switch main role is to establish the connection (PCM channel) between the calling Sub. & the called Sub. & the connection (PCM channel) between the called Sub. & the calling Sub. The Axe , GSS is mainly consists of two parts

1- Time Switch Module (TSM). TSM consist of number of memories called Stores.

2- Space Switch Module (SPM). SPM consist of cross points matrix.

Each SPM can handle up to 32 TSM. The GSS structure is called ( Time – Space – Time ) switch.

Now we will discuss the two previous modules in detail.



2.7.1 Time Switch Module :

Each TSM has a 16 PCM. Each of 32 channel to have 32 × 16 = 512 digital telephone channel. The TSM has two types of stores : 1- Speech Stores (SS):

To save the speech signal .

2- Control Stores (CS): Which controls the read-out from the speech stores(ss).

The telephony is a two way communication so the TSM has 2 Speech Stores (SSA & SSB) & 2 Control Speech (CSA & CSB) & 1 Control Store (CSC) for the control of the SPM as chown in Figure (2.F).

Figure (2.F)



2.7.2 Space Switch Module :

It consists of a matrix with crosspoints in the form of logic gates. The logic gates in one raw of the matrix are controlled by an csc, connected to it. Each cross point (Gate) is enabled or disabled by a control store CSC of the TSM connected to that crosspoint. The capacity of one SPM = 32 TSM × 16 PCM link × 32 channel =16384 called 16 K GS as shown in figure (2.G) There`s another configuration whit ahigher capacity by connecting 16 SPM (as 4 × 4 matrix) to have a capacity of 4 × 16 K = 64 K as shown in figure (2.H).



Figure 2.G

Figure 2.H


Call Setup 46

2.7.3 call establishment throught GS :

In the establishment of a call the CP uses its software to assign a PCM channel for the calling Sub. & another PCM channel for the called channel. Using the SPM to select a crosspoint for each direction. Where information of the two cross points are stored in the 2 (CSC) stores as shown in figure (2.I).

Figure (2.I)


Trunk & Signalling Subsystem 47

2.8 Trunk & Signalling Subsystem:

TSS handles the connections to other exchanges. Its main job is to match the AXE exchange to various junction signalling systems. It has a board called Extension Terminal Circuit (ETC) whitch work as an interface to E1 line to other digital exchange.


Common Channel Signalling 48

2.9 Common Channel Signalling (CCS):

This subsystem handles the common channel signalling messages between the AXE and other exchanges. The Common-channel signalling subsystem implements the message transfer part (MTP). It has a board called Signalling Terminal.

Now we will discripe the APZ system.


Central Processor 49

2.10 Central Processor Subsystem:

The Central Processor subsystem is realised in both hardware and software. The CPS executes the complex software tasks which are stored in the various APT blocks.

The main hardware parts of the central processor, which is duplicated and runs in the synchronous mode, are the central processing unit (CPU). Each central processor is divided to two main processor & each has a function to do:

1- Instruction Processor Unit (IPU): for executing programes.

2- Signal Processing Unit: For job administration, prepares next job.

The CP SubSystem also has memory stores. These stores is divided to two types:

1- Programe Stores (PS) or main stores (MS).

2- Data Referance Stores (DRS). which is divided to two types: - Data Store (DS). - Reference Store(RS).

Each CP has also a Regional Processor Handler (RPH) which handels the signalling with Regional Processor.



The purpose of the CPS is to execute the following functions:

• Program Control including supervision of functions and for measuring processor load.

• Loading and storage of tasks. • Output and updating reloading information. • Controlling fault tracing programs resident in

the MAS or RPs. • Processor Maintaenance Statistics collects

information on CP status. And events in the telephony system (APZ).

• Programe Test. • Initial loading. • System back-up.

The next is a Figure to show the CPs A&B connection Figure (2.J)



Figure (2.J)

UMBI : Updatin & Matching Bus for IPU. UMBS : Updatin & Matching Bus for SPU.


Regional Processor 52

2.11 Regional Processor Subsystem (RPS):

It`s a special purpose small , fast processor , which with there programs are associated with the APT modules. The RP are controlled witk one CP. All the RPS are duplicated for reability

The regional processor subsystem consists of both hardware and software blocks. The purpose of the RPS is to run the simple, routine and very frequent tasks to drive the RP part of the APT software and hardware. The number of functions performed by an RP pair depends upon the complexity of these functions. The number of RP pairs required for a given exchange depends upon its size and the complexity of its signalling systems. One RP can handle up to 16 extension Moduls .


Regional Processor 53

The RPs mainly consist of 5 boards: 1- Board for power supply. 2- Two boards for communication with CP-A & CP-B. 3- PRO board , is processor unit consisting of ALU,

microprograms, and a special circuits for address calculation.

4- EMU board, is memory, with circuits for communication with EM bus.

Figure (2.k) shows the connection between RP & both of CPs & EMs

Figure (2.k)


Maintennance Subsystem 54

2.12 Maintennance SubSystem (MAS):

The maintennance subsystem consists of both hardware and software. The major role is to supervise the operation of the APZ control system and takes the appropriate action should a malfunction occur.


Support Processor Subsystem 55

2.13 Support Processor SubSystem (SPS):

The support processor subsystem consists of one or more independent processors which drive input/output equipment such as personal computers, visual display units and disc drives which are connected to the subsystem.


Synchronization In AXE 56

2.14 Synchronization In Axe:

TO have an accurate rate of reading & writting into the speech store. These units provide the synchronization in GS:

1- The CLock Module (CLM) : Which provides clock pulses.

2- Two highly accurate Refrence Clock Module (RCM) : Works as areference to (CLM).

3- Two Incoming Clock Reference Boards. (ICR): To connecte additional clock reference from the other Exchange.

4- Regional Processor (RP): To connect the Synchronization Equipment.

5- Cesium Clock Module (CCM): We found this equipment just in the international AXE exchange.


Ericsson AXE10 57

2.15 Ericsson AXE10 :

As a pratical example for a digital AXE we will study the Ericsson AXE10.

The Ericsson AXE system is the most widely deployed switching system in the world. It is used in public telephony-oriented applications of every type, including traditional fixed network applications in local, transit, international and combined networks. AXE is also deployed for all major mobile standards – analogue as well as digital. An Ericsson AXE10 exchange can be split into three main parts; these are known as APT, APZ, and IOG (called APG in later generations). APT which handles the call switching, customer access and junction access, contains the following subsystems: Subscriber Switching Subsystem (SSS) to provide customer access.

• Trunk and Signalling Subsystem (TSS) which deals with junction access.

• Group Switching Subsystem (GSS) which handles switching.


Ericsson AXE10 58

• Common Channel Subsystem (CCS) which handles CCITT No 7 (C7) signalling

APZ which is responsible for the control in the exchange contains the following subsystems:

• Regional Processor Subsystem (RPS) • Central Processor Subsystem (CPS)

IOG which handles the Input and output connections to terminals, printers, alarms, storage devices, and data links contains the following subsystems:

• Support Processor Subsystem (SPS) to supervise the operation of all IOF functions.

• Man-machine Communication Subsystems (MCS) to handle communications between input/output devices and the rest if the AXE10 exchange.

• Data Communications Subsystem (DCS) to handle communications over digital links.

• File Management Subsystem..

Synchronous Digital Hierarchy Multiplexer

Introduction

59

3.1 Introduction to SDH:

SYNCHRONOUS : One master clock & all elements synchonise with it. DIGITAL: Information in binary. HIERARCHY: Set of bit rates in a hierarchical order. SDH is an ITU-T standard for a high capacity telecom network. SDH is a synchronous digital transport system,Aim to provide a simple,economical and flexible telecom infrastructure. SDH was first introduced into the telecommunications network in 1992 and has been deployed at rapid rates since then. It’s deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It’s based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fibre-optic cable. SDH is also defined for use on radio relay links, satellite links, and at electrical interfaces between equipment.

SDH Multiplexer


SDH Basics

60

3.2 SDH Basics:

Before talking a bout SDH Multiplexer we need to ilustrate some basics.

3.2.1 PDH (Plesiochronous Digital Hierarchy):

Traditionally, digital transmission systems and hierarchies have been based on multiplexing signals which are plesiochronous (running at almost the same speed). Also, various parts of the world use different hierarchies which lead to problems of international interworking; for example, between those countries using 1.544 Mbit/s systems (U.S.A. and Japan) and those using the 2.048 Mbit/s system. To recover a 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH requires “steps” (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel (see Figure 3.A). This is due to the bit-stuffing used at each level.

Figure 3.A


SDH Basics

61

3.2.2 PDH limitation:

The main limitations of PDH are:

• Inability to identify individual channels in a higher-order bit stream.

• Insufficient capacity for network management; • Most PDH network management is proprietary. • There’s no standardised definition of PDH bit

rates greater than 140 Mbit/s. • There are different hierarchies in use around the

world. Specialized interface equipment is required to interwork the two hierarchies.

3.2.3 SDH advantage:

The primary reason for the creation of SDH was to provide a long-term solution for an optical mid-span meet between operators; that is, to allow equipment from different vendors to communicate with each other. This ability is referred to as multi-vendor interworking and allows one SDH-compatible network element to communicate with another, and to replace several network elements, which may have previously existed solely for interface purposes. The second major advantage of SDH is the fact that it’s synchronous. Currently, most fibre and multiplex systems are plesiochronous. This means that the timing may vary from equipment to equipment because they are synchronised from different network clocks. In order to multiplex this type of signal, a process known as bit-stuffing is used. Bit-stuffing adds extra bits to bring all input signals up to some common bit-rate , thereby


SDH Basics

62

requiring multi-stage multiplexing and demultiplexing. Because SDH is synchronous, it allows single-stage multiplexing and demultiplexing. This single-stage multiplexing eliminates hardware complexity, thus decreasing the cost of equipment while improving signal quality. In plesiochronous networks, an entire signal had to be demultiplexed in order to access a particular channel; then the non-accessed channels had to be re-multiplexed back together in order to be sent further along the network to their proper destination. In SDH format, only those channels that are required at a particular point are demultiplexed, thereby eliminating the need for back-to-back multiplexing. In other words, SDH makes individual channels “visible” and they can easily be added and dropped.

3.2.4 Basic SDH Signal

The basic format of an SDH signal allows it to carry many different services in its Virtual Container (VC) because it is bandwidth-flexible. This capability allows for such things as the transmission of high-speed packet-switched services, ATM, contribution video, and distribution video. However, SDH still permits transport and networking at the 2 Mbit/s, 34 Mbit/s, and 140 Mbit/s levels, accommodating the existing digital hierarchy signals. In addition, SDH supports the transport of signals based on the 1.5 Mbit/s hierarchy.


SDH Basics

63

3.2.5 SDH Frame Structure:

The STM-1 (Synchronous Transport Module ) frame is the basic transmission format for SDH. The frame lasts for 125 microseconds, therefore, there are 8000 frames per second. The STM-1 frame consists of overhead plus a virtual container capacity (see Figure 3.B). The first nine columns of each frame make up the Section Overhead, and the last 261 columns make up the Virtual Container (VC) capacity. The VC plus the pointers (H1, H2, H3 bytes) is called the AU (Administrative Unit). Carried within the VC capacity, which has its own frame structure of nine rows and 261 columns, is the Path Overhead and the Container (see Figure 3.C). The first column is for Path Overhead; it’s followed by the payload container, which can itself carry other containers. Virtual Containers can have any phase alignment within the Administrative Unit, and this alignment is indicated by the Pointer in row four, as described later in the Pointers section. Within the Section Overhead, the first three rows are used for the Regenerator Section Overhead, and the last five rows are used for the Multiplex Section Overhead. The STM frame is transmitted in a byte-serial fashion, row-by-row, and is scrambled immediately prior to transmission to ensure adequate clock timing content for downstream regenerators.

Figure 3.D shows the steps to creat a STM-1


SDH Basics

64

Figure 3.B


SDH Basics

65

Figure 3.C


SDH Basics

66

Figure 3.D

SDH Overhead: As we mention before the STM-1 frame consist of a payload & an overhead the section over head is divided to 3 parts :

• Regenerator Section Overhead. • Multiplex Overhead. • Path Overhead.

• Regenerator Section Overhead:

The Regenerator Section Overhead contains only the information required for the elements located at both ends of a section. This might be two regenerators, a piece of line terminating equipment and a regenerator, or two pieces of line terminating equipment. The Regenerator Section Overhead is found in the first three rows of Columns 1 through 9 of the STM-1 frame.


SDH Basics

67

• Multiplex Section Overhead:

The Multiplex Section Overhead contains the information requiredbetween the multiplex section termination equipment at each end of theMultiplex section (that is, between consecutive network elements excluding the regenerators). The Multiplex Section Overhead is found in Rows 5 to 9 of Columns 1 through 9 of the STM-1 frame . Byte by byte, the Multiplex Section Overhead.

This shown in Figure (3.E)

Figure (3.E)

The following is a table to ilustrate the function of each byte.


SDH Basics

68

• Path overhead:

The path overhead (POH) plus a container forms a virtual container. The POH has the task of monitoring quality and indicating the type of container. The format and size of the POH depends on the container type. A distinction is made between two different POH types:


SDH Basics

69

SDH Pointers:

SDH provides payload pointers to permit differences in the phase and frequency of the Virtual Containers (VC-N) with respect to the STM-N frame. Lower-order pointers are also provided to permit phase differences between VC-1/VC-2 and the higher-order VC-3/VC-4. On a frame-by-frame basis, the payload pointer indicates the offset between the VC payload and the STM-N frame by identifying the location of the first byte of the VC in the payload. In other words, the VC is allowed to “float” within the STM-1 frame capacity. To make this possible, within each STM-N frame, there’s a pointer, known as the VC Payload Pointer, that indicates where the actual payload container starts. For a VC-4 payload, this pointer is located in columns 1 and 4 of the fourth row of the Section Overhead. The bytes H1 and H2 (two 8-bit bytes) of the Overhead can be viewed as one value (see Figure 3.F). The pointer value indicates the offset in bytes from the pointer to the first byte of the VC, which is the J1 byte. Because the Section Overhead bytes are not counted, and starting points are at 3-byte increments for a VC-4 payload, the possible range is: Total STM-1 bytes – Section Overhead bytes = Pointer value range. For example: (2430 – 81)/3 = 783 valid pointer positions.

Figure (3.F)


SDH Basics

70

That is, the value of the pointer has a range of 0 to 782. For example, if the VC-4 Payload Pointer has a value of 0, then the VC-4 begins in the byte adjacent to the H3 byte of the Overhead; if the Payload Pointer has a value of 87, then the VC-4 begins in the byte adjacent to the K2 byte of the Overhead in the next row. The pointer value, which is a binary number, is carried in bits 7 through 16 of the H1-H2 pointer word. The first four bits of the VC-4 payload pointer make provision for indicating a change in the VC, and thus an arbitrary change in the value of the pointer. These four bits, the N-bits, are known as the New Data Flag. The VC pointer value that accompanies the New Data Flag will indicate the new offset.

Payload Pointers

When there’s a difference in phase or frequency, the pointer value is adjusted. To accomplish this, a process known as byte stuffing is used. In other words, the VC payload pointer indicates where in the container capacity a VC starts, and the byte stuffing process allows dynamic alignment of the VC in case it slips in time.

Positive Pointer Justification:

When the data rate of the VC is too slow in relation to the rate of the STM-1 frame, bits 7, 9, 11, 13, and 15 of the pointer word are inverted in one frame, thus allowing 5-bit majority voting at the receiver (these bits are known as the I-bits or Increment bits). Periodically, when the VC is about one byte off, these bits are inverted, indicating that positive stuffing must occur.


SDH Basics

71

An additional byte is stuffed in, allowing the alignment of the container to slip back in time. This is known as positive stuffing, and the stuff byte is made up of non-information bits. The actual positive stuff byte immediately follows the H3 byte (that is, the stuff byte is within the VC portion). The pointer is incremented by one in the next frame, and the subsequent pointers contain the new value.

Simply put, if the VC is running more slowly than the STM-1 frame, every now and then “stuffing” an extra byte in the flow gives the VC a one-byte delay (see Figure (3.G).

Negative Pointer Justification:

Conversely, when the data rate of the VC is too fast in relation to the rate of the STM-1 frame, bits 8, 10, 12, 14, and 16 of the pointer word are inverted, thus allowing 5-bit majority voting at the receiver (these bits are known as the D-bits, or Decrement bits). Periodically, when the VC is about one byte off, these bits are inverted, indicating that negative stuffing must occur. Because the alignment of the container advances in time, the payload capacity must be moved forward. Thus, actual data is written in the H3 byte, the negative stuff opportunity within the Overhead; this is known as negative stuffing. The pointer is decremented by one in the next frame, and the subsequent pointers contain the new value. Simply put, if the VC is running mor quickly than the STM-1 frame, every now and then pulling an extra byte from the flow and stuffing it into the Overhead capacity (the H3 byte) gives the VC a one-byte advance (see Figure 3.H). In both positive or negative cases, there must be at least three frames in which the pointer remains constant before another stuffing operation (and, therefore a pointer value change) can occur.


SDH Basics

72

Figure (3.G) Figure (3.H)


SDH Multiplexing

73

3.3 SDH Multiplexing:

The multiplexing principles of SDH follow, using these terms and definitions: 1- Mapping: A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information. 2- Aligning: This process takes place when a pointer is included in a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located. 3- Multiplexing: This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.

4- Stuffing: As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all the various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this space capacity is filled with “fixed stuffing” bits that carry no information, but are required to fill up the particular frame.

Figure (3.I) showes the multiplexing steps.


SDH Multiplexing

74

Figure (3.I)


SDH Multiplexer Structure

75

3.4 SDH Multiplexer Structure:

SDH synchronous multiplexer consists functionally of the following modules:

1. Cross-Connect or Switching Network : Which is a switching matrix with cross-point logic gates. 2. Two line port units: East side and west side for connection with fiber. 3. Triburatury units: For adding and dropping of tributary signals e.g. E1, E3 or E4 from the fiber line STM-1 or STM-4 higher order signals.

The line port unit provide the SDH physical interface to the regenerator section termination (RSOH) and the multiplexer section termination (MSOH). Line port units for STM-1 155Mbps STM-4 622Mbps and STM-16 2.5Mbps are available.

Line Port

Unit

Line Port

Unit

Cross Connect Fiber Line Fiber Line

Triburatury Unit

Synchronous Digital Hierarchy Network

Introduction

76

4.1 Introduction to SDH Network:

A transport network using SDH provides much more powerful networking capabilities than existing asynchronous systems.

Current SDH networks are basically made up from four different types of network element.

Network has four types of topology. The topology is governed by the requirements of the network provider. The Network elements is constant for different types of toplogy. In this chapter we will discuse in detail the SDH-Network in Ring architechture.

SDH Network


SDH Network Element

77

4.2 SDH Network Elements:

The Network elements is constant for different types of toplogy. Current SDH networks are basically made up from four different types of network element. 1- Terminal Multiplexer. 2- Regenerator. 3- Add/Drope Multiplexer. (ADM) 4- Digital Crossconnects. (DXC)

Now we will discuse each element in detail.


SDH Network Element

78

4.2.1 Terminal multiplexer:

Terminal multiplexers are used to combine plesiochronous and synchronous input signals into higher bit rate STM-N signals. (See Figure 4.A)

Figure 4.A

4.2.2 Regenerator:

Regenerators, as the name implies, have the job of regenerating the clock and amplitude relationships of the incoming data signals that have been attenuated and distorted by dispersion. They derive their clock signals from the incoming data stream. Messages are received by extracting various 64 kbit/s channels (e.g. service channels E1, F1) in the RSOH (regenerator section overhead). Messages can also be output using these channels. (see Figure 4.B)

Figure 4.B

Regenerator


SDH Network Element

79

4.2.3 Add/Drope Multiplexer:

Plesiochronous and lower bit rate synchronous signals can be extracted from or inserted into high speed SDH bit streams by means of ADMs. This feature makes it possible to set up ring structures, which have the advantage that automatic back-up path switching is possible using elements in the ring in the event of a fault. (see Figure 4.C)

Figure 4.C

4.2.4 Digital crossconnects (DXC):

This network element has the widest range of functions. It allows mapping of PDH tributary signals into virtual containers as well as switching of various containers up to and including VC-4. (see Figure 4.D)

Figure 4.D


SDH Network configuration

80

4.3 SDH Network Configurations:

we have 4 types of configurations: 1- Point-To-Point. 2- Point-To-Multipoint. 3- Mesh Architecture. 4- Ring Aarchitecture.

Now we will discuse each one in detail.



81

4.3.1 Point-to-Point: The simplest network configuration involves two terminal multiplexers linked by fibre with or without a regenerator in the link (see Figure 4.E). In this configuration, the SDH path and the Service path (for example, E1 or E3 links end-to-end) are identical and this synchronous island can exist within an asynchronous network world. In the future, point-to-point service path connections will span across the whole network and will always originate and terminate in a multiplexer.

Figure 4.E

4.3.2 Point-to-Multipoint:

A point-to-multipoint (linear add/drop) architecture includes adding and dropping circuits along the way (see Figure 4.F). The SDH ADM (add/drop multiplexer) is a unique network element specifically designed for this task. It avoids the current cumbersome network architecture of demultiplexing, cross-connecting, adding and dropping channels, and then re-multiplexing. The ADM typically is placed in an SDH link to facilitate adding and dropping tributary channels at intermediate points in the network.

Figure 4.F



82

4..3.3 Mesh Architechture:

The meshed network architecture accommodates unexpected growth and change more easily than simple point-to-point networks. A crossconnect function concentrates traffic at a central site and allows easy re-provisioning of the circuits (see Figure 4.G). There are two possible implementations of this type of network function: 1. Cross-connection at higher-order path levels, for example, using AU-4 granularity in the switching matrix. 2. Cross-connection at lower-order path levels, for example, using TU-12 granlarity in the switching matrix.

Figure 4.G



83

4.3.4 Ring Architecture:

The SDH building block for a ring architecture is the ADM (see Figure 4.H). Multiple ADMs can be put into a ring configuration for either Bi-directional or Uni-directional traffic. The main advantage of the ring topology is its survivability; if a fibre cable is cut, for example, the multiplexers have the local intelligence to send the services affected via an alternate path through the ring without a lengthy interruption. The demand for survivable services, diverse routing of fibre facilities, flexibility to rearrange services to alternate serving nodes, as well as automatic restoration within seconds, have made rings a popular SDH topology.

Figure 4.H


SDH Ring Protection

84

4.4 SDH Ring Protection Mechanisms:

Ring protection The greater the communications bandwidth carried by optical fibers, the greater the cost advantages of ring structures as compared with linear structures. A ring is the simplest and most cost-effective way of linking a number of network elements. Various protection mechanisms are available for this type of network architecture, only some of which have been standardized in ITU-T Recommendation G.841. A basic distinction must be made between ring structures with unidirectional and bi-directional connections.


SDH Ring Protection

85

4.4.1 Unidirectional rings:

Figure 4.I shows the basic principle of APS for unidirectional rings. Let us assume that there is an interruption in the circuit between the network elements A and B. Direction y is unaffected by this fault. An alternative path must, however, be found for direction x. The connection is therefore switched to the alternative path in network elements A and B. The other network elements (C and D) switch through the back-up path. This switching process is referred to as line switched. A simpler method is to use the so-called path switched ring (see Figure 4.I). Traffic is transmitted simultaneously over both the working line and the protection line. If there is an interruption, the receiver (in this case A) switches to the protection line and immediately takes up the connection.

Figure 4.I


SDH Ring Protection

86

4.4.2 Bi-directional rings

In this network structure, connections between network elements are bi-directional. This is indicated in figure 4.J by the absence of arrows when compared with figure 4.I. The overall capacity of the network can be split up for several paths each with one bi-directional working line, while for unidirectional rings, an entire virtual ring is required for each path. If a fault occurs between neighboring elements A and B, network element B triggers protection switching and controls network element A by means of the K1 and K2 bytes in the SOH.

Even greater protection is provided by bi-directional rings with 4 fibers.Each pair of fibers transports working and protection channels. This results in 1:1 protection, i.e. 100 % redundancy. This improved protection is coupled with relatively high costs.

Asynchronous Transfer Mode

Introduction 87

5.1 Introduction to ATM:

The standards for ATM were first developed in the mid-1980s. For those too young to remember, at this time there were predominately two types of networks: telephone networks, which were (and still are) primarily used to carry real-time voice, and data networks, which were primarily used to transfer text files, support remote login, and provide e-mail. There were also dedicated private networks available for video conferencing. The Internet existed at this time, but few people were thinking about using it to transport phone calls, and the World Wide Web was as yet unheard of. It was therefore natural to design a networking technology that would be appropriate for transporting real-time audio and video as well as text, e-mail, and image files. Asynchronous transfer mode (ATM) achieved this goal. Two standards committees, the ATM Forum [ATM 2002] and the International Telecommunications Union [ITU 2002] developed standards for broadband digital services networks.

ATM


Introduction 88

The ATM standards call for packet switching with virtual circuits (called virtual channels in ATM jargon). The standards define how applications directly interface with ATM, so that ATM provides a complete networking solution for distributed applications. Paralleling the development of the ATM standards, major companies throughout the world made significant investments in ATM research and development. These investments have led to a myriad of high-performing ATM technologies, including ATM switches that can switch terabits per second. In recent years, ATM technology has been deployed very aggressively within both telephone networks and the Internet backbones. Although ATM has been deployed within networks, it has been less successful in extending itself all the way to desktop PCs and workstations. And it is now questionable whether ATM will ever have a significant presence at the desktop. Indeed, while ATM was brewing in the standards committees and research labs in the late 1980s and early 1990s, the Internet and its TCP/IP protocols were already operational and making significant headway. In this chapter we will discuss the ATM structure & Atm switches in detal.


ATM basic 89

5.2 ATM Basic: ATM is a aspecial form of fast packet switching, desidend to handle traffic inputs with different band widths, as basis for broadband ISDN. It uses short , fixed-length packetss , called cells. The cells, which consist of 5 header bytes and 48 data or payload bytes , are short in order to minimize delay (since short delay,is essential for telephony) . the system operates at a fixed digit rate (appriximately 155 Mbit/s). And different services are handeled by changing the intervals between cells, for example , a 33Mbit/s video codec will require many more cells in agiven time , than a 64 Kbit/s speech codec . Atm is the switching technique that provides the subscriper with a multiservice terminal for narrowband voice (telephony) as well as broadband for video , data, text, graphics and images. This can be called Bandwidth on Demand< where the subscriber pays only for the bandwidth actually used. The ITU-T describes the Broadband Integrated Services Digital Network [B-ISDN] as a network, public or private, bulit on the concepts of the ISDN modle, which is implemented with ATM and SDH technologies. Actually, ATM and SDH are complementing each other. In its simplest form, SDH system provide the long distance physical caeeier transport systems for the user payload. The user payload is carried in ATM cells. In other words, the SDH network acts as the long distance service provider for the ATM traffic.


ATM basic 90

Beside telephny , subscriber now need video broad band, and high data Pc rates, requiring greater throughput, and lower delay. ATM and SDH are designed to provide these applications. Optical fiber systems are providing the foundation for the transmission media support for the SDH and ATM system, with very low noise, while hight-speed processor are providing the speed necessary to process the traffic in the ATM switches. ATM is defined as a high-speed tranport (i.e. trans-mission) and switching method, in which information does not occur periodically with some time reference such as a frame , hence it is called Asynchronous. With ATM, data arrives and is processed across the network randomly. There is no timing associated with ATM traffic, so the cells are generated as data needs to be transmitted. When no traffic exists, idle cells may be present on the network, or cells carrying other payloads will be present. Each ATM Switch queses ATM cells for transmission in a logical schedule that allows control over such parameter as delay , delay variation and cell loss , based on type of service : voice, video, data,etc, as indicated when the virtual connection is established For example, a data file transfer is much more sesitive to cell loss than to delay, while a voice connection is more sensitive to delay, and video is sensitive to delay variation. These parameters are called Quality Of Service (QOS).


ATM Cell Formate 91

5.3 ATM Cell Formate:

ATM transfers information in fixed-size units called cells. Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell-header information, and the remaining 48 contain the payload (user information). Small, fixed-length cells are well suited to transferring voice and video traffic because such traffic is intolerant of delays that result from having to wait for a large data packet to download, among other things. Figure 5.A illustrates the basic format of an ATM cell.

Figure 5.A


ATM Devices 92

5.4 ATM Devices:

An ATM network is made up of an ATM switch and ATM endpoints. An ATM switch is responsible for cell transit through an ATM network. The job of an ATM switch is well defined: It accepts the incoming cell from an ATM endpoint or another ATM switch. It then reads and updates the cell header information and quickly switches the cell to an output interface toward its destination. An ATM endpoint (or end system) contains an ATM network interface adapter. Examples of ATM endpoints are workstations, routers, digital service units (DSUs), LAN switches, and video coder-decoders (CODECs). Figure 5.B illustrates an ATM network made up of ATM switches and ATM endpoints.

Figure 5.B


ATM Devices 93

5.5 ATM Network Interfaces:

An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or interfaces. ATM switches support two primary types of interfaces: UNI and NNI. The UNI connects ATM end systems (such as hosts and routers) to an ATM switch. The NNI connects two ATM switches. Depending on whether the switch is owned and located at the customer’s premises or is publicly owned and operated by the telephone company, UNI and NNI can be further subdivided into public and private UNIs and NNIs. A private UNI connects an ATM endpoint and a private ATM switch. Its public counterpart connects an ATM endpoint or private switch to a public switch. A private NNI connects two ATM switches within the same private organization. A public one connects two ATM switches within the same public organization. An additional specification,the broadband intercarrier interface (B-ICI), connects two public switches from different service providers. Figure 5.C illustrates the ATM interface specifications for private and public networks.

Figure 5.C


ATM Cell Header Formate 94

5.6 ATM Cell Header Formats:

The ATM standards groups have defined two header formats. The UNI header format is defined by the UNI specification, and the Network-Node Interface (NNI) header format is defined by the NNI specification.

5.6.1 UNI Cell Header Formats:

The UNI specification defines communications between ATM endstations (such as workstations and routers) and ATM switches in private ATM networks. The format of the UNI cell header is shown in Figure 5.D.

Figure 5.D.

The UNI header consists of the following fields:

• GFC—4 bits of generic flow control that can be used

to provide local functions, such as identifying multiple stations that share a single ATM interface. The GFC field is typically not used and is set to a default value.



• VPI—8 bits of virtual path identifier, which is used, in conjunction with the VCI, to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination.

• VCI—16 bits of virtual channel identifier, which is used, in conjunction with the VPI, to identify the next destination of a cell as it passes through a series of ATM switches on its way to its destination.

• PT—3 bits of payload type. The first bit indicates whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.

• CLP—1 bit of congestion loss priority, which indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network.

• HEC—8 bits of header error control, which is a checksum calculated only on the header itself.



5.6.2 NNI Cell Header Formats: The NNI specification defines communications between ATM switches. The format of the NNI header is shown in Figure 5.E.

Figure 5.E

The GFC field is not present in the format of the NNI header. Instead, the VPI field occupies the first 12 bits, which allows ATM switches to assign larger VPI values. With that exception, the format of the NNI header is identical to the format of the UNI header.


ATM Reference Model 97

5.7 ATM Reference Model:

Figure 5.F is a reference model that illustrates the organization of ATM functionality and the interrelationships between the layers of functionality.

Figure 5.F

In the ATM reference model, the ATM layer and the ATM adaptation layers are roughly analogous parts of the data link layer of the Open System Interconnection (OSI) reference model, and the ATM physical layer is analogous to the physical layer of the OSI reference model. The control plane is responsible for generating and managing signaling requests. The user plane is responsible for managing the transfer of data. Above the ATM adaptation layer are higher-layer protocols representing traditional transports and applications.



5.7.1 Physical Layer:

The ATM physical layer controls transmission and receipt of bits on the physical medium. It also keeps track of ATM cell boundaries and packages cells into the appropriate type of frame for the physical medium being used. The ATM physical layer is divided into two parts: 1- the physical medium sublayer. 2- the transmission convergence sublayer. The physical medium sublayer is responsible for sending and receiving a continuous flow of bits with associated timing information to synchronize transmission and reception. Because it includes only physical-medium-dependent functions, its specification depends on the physical medium used.

The transmission convergence sublayer is responsible for the following: 1- Cell delineation—Maintains ATM cell boundaries. 2- Header error control sequence generation and

verification—Generates and checks the header error control code to ensure valid data.

3- Cell rate decoupling—Inserts or suppresses idle (unassigned) ATM cells to adapt the rate of valid ATM cells to the payload capacity of the transmission system.

4- Transmission frame adaptation—Packages ATM cells into frames acceptable to the particular physical-layer implementation.

5- Transmission frame generation and recovery Generates and maintains the appropriate physical-layer frame structure.



5.7.2 ATM Layer:

The ATM layer is responsible for establishing connections and passing cells through the ATM network. To do this, it uses the information contained in the header of each ATM cell.

5.7.3 ATM Adaptation Layer: AAL1:

AAL1, a connection-oriented service, is suitable for handling constant bit rate sources (CBR), such as voice and videoconferencing. ATM transports CBR traffic using circuit-emulation services. Circuit-emulation service also accommodates the attachment of equipment currently using leased lines to an ATM backbone network. AAL1 requires timing synchronization between the source and the destination. For this reason, AAL1 depends on a medium, such as SDH, that supports clocking. The AAL1 process prepares a cell for transmission in three steps. First, synchronous samples (for example, 1 byte of data at a sampling rate of 125 microseconds) are inserted into the Payload field. Second, Sequence Number (SN) and Sequence Number Protection (SNP) fields are added to provide information that the receiving AAL1 uses to verify that it has received cells in the correct order. Third, the remainder of the Payload field is filled with enough single bytes to equal 48 bytes. Figure 5.G illustrates how AAL1 prepares a cell for transmission.



Figure 5.G

AAL2:

Aal-2 defines the trasport of variable Bit Rate (VBR) real time traffic, such as compressed audio and video. Actually, AAL-2 is still under development. It will be useful for the Universal Mobile Telecom [UMTS] real time traffic.

AAL3/4

For supporting Non-real Time services, with aal-3 is connection oriented(e.g. X25 & Frame Relay). AAL-4 is connectionless (e.g. TCP/IP on the internet).



AAL-5 AAL-5 is less complex than AAL-3/4. Also has significantly lower overhead than AAL-3/4 . Therefore, AAL-5 is very widely used in the tranport of both connection oriented, and connectionless services. Signaling ATM Adaptation Layer[SAAL]: SAAL for dividing the ATM signalling message (e.g. UNI , PNNI) into 48-byte packages. It consists of AAL-5 plus additional functions for signalling links.

5.7.4 Sequence Of Layer Operation:

1- User Traffic : Bit Stream at I/P of ATM Switch:

• Constant Bit Rate (CBR). • Data Bursts (DB). • Variable Bit Rate (VBR).

2- AAL Layer : • Sgmentation in 48-bytes. • Specify class of Service.

3- ATM Layer : Add header to 48bytes to form ATM cell in the send direction. Remove the header from cell in the receive direction. The header enables the cell to be royted from ATM nodes (Exchange) to the next one.

4- Physical Layer : Muliplexed ATM cells (i.e. mixed

Voicecells, Video cells)are mapped (i.e. loaded in an SDH frame on Fiber Cable).


ATM Switching 102

5.8 ATM Switching:

ATM switches use the VPI and VCI fields of the cell header to identify the next network segment that a cell needs to transit on its way to its final destination. A virtual channel is equivalent to a virtual circuit—that is, both terms describe a logical connection between the two ends of a communications connection. A virtual path is a logical grouping of virtual circuits that allows an ATM switch to perform operations on groups of virtual circuits. The main function of an ATM switch is to receive cells on a port and switch those cells to the proper output port based on the VPI and VCI values of the cell. This switching is dictated by a switching table that maps input ports to output ports based on the values of the VPI and VCI fields, as shown in Figure 5.H.

Figure 5.H


ATM Switching 103

Say, for example, that two cells arrive on port 1 of the ATM switch in Figure 5.H. First, the switch examines the VPI and VCI fields of cell 1 and finds that the fields have a value of 6 and 4, respectively. The switch examines the switch table to determine on which port it should send the cell. It finds that when it receives a VPI of 6 and a VCI of 4 on port 1, it should send the cell on port 3 with a VPI of 2 and a VCI of 9. So, for cell 1, the switch changes the VPI to 2 and the VCI to 9 and sends the cell out on port 3. Next, the switch examines cell 2, which has a VPI of 1 and a VCI of 8. The table directs the switch to send out on port 2 cells received on port 1 that have a VPI of 1 and a VCI of 8, respectively, and to change the VPI and VCI to 4 and 5, respectively. Conversely, when a cell with a VPI and VCI of 2 and 9, respectively, comes in on port 3, the table directs the switch to send the cell out on port 1 with a VPI and VCI of 6 and 4, respectively. When a cell with a VPI and VCI of 4 and 5, respectively, comes in on port 2, the table directs the switch to send the cell out on port 1 with a VPI and VCI of 1 and 8, respectively. Note that VPI and VCI values are significant only to the local interface. Figure 5.I shows how the VPI field is used to group virtual channels (identified by their VCI values) into logical groups. By reducing the number of fields that have to be changed as each cell passes through the switch, the performance of the switch increases.


ATM Switching 104

Figure 5.I

In Figure 5.I , cells that enter the ATM switch on port 1 and have a VPI value of 4 are processed through the “VP switch,” which changes the VPI value of each cell to 5, but leaves the VCI value intact, and sends the cell out on port 3. Cells that have a VPI value of 1 are processed through the “VC switch.” For cells that have a VCI value of 1, the VC switch changes the VPI to 4 and the VCI to 4 and sends the cell out on port 2. For cells that have a VCI value of 2, the VC switch changes the VPI to 3 and the VCI to 3 and sends the cell out on port 3.


ATM Network Delay 105

5.9 ATM Network Delay:

There is so meny types of network delays . we will discuss each one of them in detail.

5.9.1 Paketisation Delay:

This introduced at the sending terminal when sample of a real-time signal (such as speech) are assembled to form packets. For cells having 48 octets, sent at 150 Mbit/s, the delay resulting for Kbit/s PCM is 6 milisec. For 2 Mbit/s Video coder, the delay is 212µsec.

5.9.2 Transmission Delay:

Depends on Transmission medium and distance. For modem transmission systems, it is typically 4-5 µsec/Km.

5.9.3 Fixed Switching Delay: This is the time for cell to pass through the ATM switch. For cells of 53 bytes, it is or the order of 30µsec per ATM Exchange.

5.9.4 Queuing Delay:

Queues are necessary in ATM switch to avoid loss of cells. Delay increases with traffic load on the switch. For a cell length of 53 bytes, and a digit rate of 150 Mbit/s, the Queueing delay is of the order of 664µsec.


ATM Network Delay 106

5.9.5 Depacketisation Delay:

Cells arrive at the receiving terminal with intervals between them. How ever ,for a real-time services (e.g. telephony). Output samples must be generated at a constant rate. Abuffer is provided into whitch cells are written as they arrive, and from which samples are read out at constant rate, this buffer must accomodate delay variations in queuing delay during a cell. However, the max delay has already been included 1n (3,4) the 664µsec delay.


ATM Switch 107

5.10 ATM Switch: 5.10.1 Introduction to AXD:

The AXD from LM Ericsson (Sweden) is a high technology, multi service ATM Sxchange, for cell switching of cell-packet of Vouce (telephony), Data & Video, in standard format. The AXD consist of repeated modules, which makes the extension of the system from 10 Gbps capacity on site, whithout affecting the Exchange operation. The AXD has a simple, easily overviewed structure. It consists of the following units, (see Figure 5.j)

1- A Switch Core (SC): based on space switching, which is duplicated.

2- Exchange Termilnals (ET) or Line Modules: These act as external interfaces, and ATM layer functions.

3- Control Processors (CP) : for call control, signalling and Operation and Maintenance.


ATM Switch 108

Figure 5.j


ATM Switch 109

5.10.2 AXD 301 Structure:

The AXD 301 is a new asynchronous transfer mode (ATM) switching system from Ericsson. Combining features associated with data communication, such as compactness and high functionality, with features from telecommunications, such as robustness and scalability, the AXD 301 is a very flexible system that can be used in several positions in a network [blau]. The AXD 301 supports standardized ATM service categories, including constant bit rate (CBR), variable bit rate (VBR) and unspecified bit rate (UBR). The system supports ITU-T and ATM Forum signaling specifications. The AXD301 Switch core, Exchange Terminal, and Control Processors consist of boards, housed in Subracks. The subracks are mounted in Cabinet, with 2 subracks in one cabinet. An AXD301 system of 10Gbps capacity consist of the following hardware, as an ATM switch, in one subrack.

1- two 10 Gbps Switch core modules, running in parallel, performing space switching, of virtual Pathes and virtual channels, carrying ATM cells.

2- Up to 16 Exchanges Terminals Modules (ET), performing ATM packet layer functions(cell buffering, policing and shaping), and different types of external interfaces.


ATM Switch 110

3- Two co-operating Control Processor (CP) • One CP handling call control. • One CP performing Operation & Maintenance.

Each CP runs as hot stand-by to the other, (i.e. when one CP fails, the other CP cancarry out call control as well asoperation & Maintenance). The Front & Rear View of a fully equipped 10 Gbps AXD301 is shown in Figure 5.K, as an ETSI standard Subrack.

5.10.3 Switch layout:

An AXD301 Subrack is divided into a front Part, & Rear part, with a midplane in between. All equipment modules consist of a Front Board, and a corresponding Rear Board, interconnected by the Midplane.

Front Board Rear Board Switch Core (SC). Clock & Network Syn. Central Processor (CP).

Input/Output (CP-IO) Board • Hardware for AAL-5. • External Alarm Connectors. • LAN-Interface for PC-system

Management. • Rs-232 Consol Interface.

External Terminal (ET). ATM Termination Board (ATB).

Line Interface Board (LI)

The Front & Rear View of a fully equipped 10 Gbps AXD301 is shown in Figure 5.K, as an ETSI standard Subrack.


ATM Switch 111

Figure 5.K


ATM Switch 112

• Each ET has Mbps Switch Port, which is connected to

both Switch Cores. ATM cells pass through both switch cores.

• Each CP is connected to the Switch Core for communication the Switch Core support 16 Switch Ports, per Subrack.

5.10.3 AXD 301 20Gbps Structure:

Two 10 Gbps Switches can be used to build a 20 Gbps AXD Switch (showes in Figure 5.L) installed in one Cabinet.

Figure 5.L

The Switch Core used in each 10 Gbps Subrack will be one of two sharing segments of the 20 Gbps Switch Core. By connecting through cabling the midplanes of Two 10 Gbps subracks, the 20 Gbps AXD301 Switch can built. No additional equipment will be needed for this purpoe, beside the two Gbps Switch Subracks.


ATM Switch 113

Finally The AXD 301 can be equipped with Two types of boards : series A & series B.

1- series A is based on the first release of ATM IC chip set [QRT/QSE].

2- Series B is based on the second generation of ATM

IC chip set with the following advantages:

• Improved ATM operation, with Upgrading to 160G.

• Echo cancellation. • Tone sending. • Performance Monitoring.

5.10.4 Transition From Cicuit-Switched To Packet-Switched:

In present Telecom Services, seperat network for telephony, data, etc based on circuit switching are used, with increased costs for expansion, as well as for operation and maintenance. The introduction of packet telephony based on ATM leads to a great reduction in these costs. The transition from circuit switching to ATM can be implemented by a common connectivity network in the form of an ATM packet Backbone Network, and connecting the existing circuit switched nodes (e.g. AXE exchange) to it, for this purpose, AXD301 the ATM switch is combined with the existing AXE exchange, to form together the Telephony Server (TeS). Such transsion solution by Ericsson company is called ENGINE, the Multi Service Network, Beside the TeS, the ENGINE also uses a Media GateWay (MGW), which connects different types of Accsses to the Backbone in the Multi Service Network. The MGW is also based on th ATM switch AXD301.


ATM Switch 114

ENGINE can be greatly reduce the Number of switching system compared to traditional number of switching system compared to traditional network, since few Telephony Severs can replace many Local Exchanges. Transit Switches can replaced as well as international switches by fewer TES and MGW, This is due to direct optical fiber connection,& combining data & telephony on one Backbone>

atm switch

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