Lecture 10

ISDN Contd.

ISDN Services

ISDN has three types of services :

– Bearer Services

– Teleservices, &

– Supplementary services

ISDN Services

• Text version of previous slide

Bearer ServicesBearer services provide means to transfer information

(voice, data & video) between users without network manipulating the content of information.

Network does not process the informationThese services belong to first three layers of the OSI

modelThese services can be provided using circuit switched,

packet switched, frame switched or cell switched networks

Teleservices

• Network may change or process the contents of the data

• These services correspond to layer 4-7 of OSI model

• Teleservices include telephony, teletex, telefax, videotex, telex & teleconferening

Supplementary Services

• Supplementary services are those services that provide additional functionality to the bearer services & teleservices

• Examples are reverse charging, call waiting, and message handling, all familiar from today’s telephone company

ISDN Services• ISDN Services

• The ISDN will provide a variety of services, supporting existing voice and data applications as well as providing for applications now being developed.

• Some of the most important applications are as follows.– Facsimile:

• Transmit the page of data at 64 kbps in 5 seconds.

• Service for the transmission and reproduction of graphics and handwritten and printed material.

Teletex• Transmission is at the rate of one page in 2 seconds at

9.6 kbps• Fast exchange of correspondences between terminals.• Communicating terminals are used to prepare, edit,

transmit and print message.

Videotext:• A page of data can be transmitted in 1 second at 9.6

kbps.• An interactive information retrieval service.

Most of the services can be provided with a transmission rate of 64 kbps or less. For higher rates B-ISDN will be used.

ISDN Architecture • Figure shown next is an architectural description of ISDN. The

ISDN will support completely physical connector for users, a digital subscriber line and a variety transmission services.

• Physical interface provides a standardized means of attaching to the network.

• The interface supports a basic service consisting of three time multiplexed channels, two at 64 kbps and one at 16 kbps.

• In addition, there is a primary service that provides multiple 64 kbps channels.

• For both basic and primary service, an interface is defined between the customer equipment (TE) and a device on the customer’s premises, known as a Network Termination (NT).

• The subscriber line is the physical path from subscriber’s NT to the ISDN central office. This line must support full duplex digital transmission for both basic and primary data rates.

• ISDN central office connects subscriber lines to the digital network, providing access to lower layer transmission facilities.

Circuit-switched capabilities: Same facility provided by other digital-switched telecommunications network. Data rate is 64 kbps.

Non-switched capabilities: A 64 kbps dedicated link, higher rates in B-ISDN by using ATM transmission.

Switched capabilities: This refers to high speed switched connections using ATM as part of B-ISDN.

Packet-switched capabilities: This facility resembles packet-switched service provided by other data networks.

Frame mode capabilities: A service that supports frame relay. Common-channel signaling capabilities: Used to control the

network and provide call management internal to network, SS7 is used.

Packet-switching

Capabilities

Frame mode

Capabilities

Common channel

signaling Capabilities

ISDN switch

ISDN switch

Subscriber site or service

provider

Network termination

(NT)

Terminal equipment

(TE)

Subscriber interface to ISDN

Digital subscriber loop to central office

User-network signaling

User-User signaling

Subscriber premises

ISDN Standards • The ISDN is governed by recommendations from ITU-T.

these recommendations are called as I-series of recommendations. They were first issued in 1984.

• These recommendations are as follows:– I.100 series: General concepts– I.200 series: Service capabilities– I.300 series: Network aspects– I.400 series: User-network interface– I.500 series: Internetwork interfaces– I.600 series: Maintenance principles– I.700 series: B-ISDN equipment aspects.

ISDN CHANNELS

Subscriber Access to The ISDNChannel Rates

Channel Data Rates

Bearer (B) 64 Kbps

Data (D) 16, 64 Kbps

Hybrid (H) 384 Kbps(H0), 1.536 Mbps(H11), 1.92 Mbps(H12)

B Channels

• A bearer channel (B channel) is defined at a rate of 64 kbps

• Its a basic user channel & carry any type of digital information(e.g. digital data, digital voice, digitized video) in full duplex mode as long as transmision rate doesnt cross 64 kbps data rate

• Several multiplxed signal can also travel on it but it cant demultiplex them in the way

• B channel carries end-to-end transmission

D Channels• A Data channel( D channel) can be either 16 or 64 kbps

depending on the need of the user

• Although the name says data, but the primary function of a D channel is to carry control signaling for B channel

• Control information such as call establishment, ringing, call interrupt or synchronization is carried by this channel

• D channel may be used for packet switching or low speed telemetry at times when no signalling information is waiting

H Channels

• Hybrid channels (H channels) are provided for high data transmission rate

• Hybrid channels (H channels) are available with data rates of 384 kbps(H0), 1536Kbps(H11) or 1920 Kbps(H12)

• These rates suit H channels for high data rate applications such as video, teleconferencing & so on

ISDN User Interface

• Digital subscriber loops are of two typesBasic Rate Interface (BRI)Primary Rate Interface (PRI)

• Each type of services suits to different types of users

• Both include one D channel & some number of B or H channels

Basic Rate Interface (BRI)• Basic rate interface (BRI) specifies a digital pipe

consisting of 2 B channels & one D channel

• The BRI requires a digital pipe of 192 kbps: B1(64)+ B2(64) + 16(D)+ 48 (operating overhead) =

192 kbps

• It meets the requirement of residential & small office customers

• Same earlier used twisted pair local loop can be used in BRI, no need to replace local loop

BRI

Digital pipe can be assumed to contain 3 pipes inside: 2 pipes for B channels & 1 pipe for D channel, remaining portion can be assumed to carry overhead bits

Primary Rate Interface(PRI)

• A usual PRI specifies one D & 23 B channels

• 23 B channels of 64k + 1 D channel of 64 K = 1.536 Mbps

• Overhead = 8 kbps

• Therefore, PRI requires a Digital pipe of 1.536+ 8 = 1.544 Mbps

PRI

• Transmission speed of PRI exactly matches DS-1of American telephone services

• E-1 standard of Europe transmission is met by 30B + 2D channels = 2.048 Mbps

• Some other standards are:• 3H0+D, • 4H0+D & • H12+D

Functional GroupingIn the ISDN standard, the devices that enable users to access the services of the BRI or PRI are described by their functional duties and collected in functional groupings. Subscribers choose the specific devices best suited to their needs from these groupings. Each functional grouping is a model that can be implemented using devices or equipment chosen by the subscriber. Functional groupings used at the subscriber’s premises include network terminations (types 1 and 2), terminal equipment (types 1 and 2), and terminal adapters.

Functional Grouping

Network Termination 1 (NT1)• A network termination 1 (NT1) device controls

the physical and electrical termination of the ISDN at the user’s premises and connects the user’s internal system to the digital subscriber loop. These functions are comparable to those defined for the OSI physical layer.

Network Termination 2 (NT2)• A network termination 2 (NT2) device performs functions

at the physical, data link, and network layers of the OSI model (layers 1, 2, and 3). NT2s provide multiplexing (layer 1), flow control (layer 2), and packetizing (layer 3). An NT2 provides intermediate signal processing between the data-generating devices and an NT1.

• NT2s can be implemented by a variety of equipment types. For example, a private branch exchange (digital PBX) can be an NT2; it coordinates transmissions from a number of incoming links (user phone lines) and multiplexes them to make them transmittable by an NT1. A LAN also can function as an NT2.

Terminal Equipment 1 (TE1)• The term terminal equipment is used by the

ISDN standard to mean the same thing as DTE in other protocols. It refers to digital subscriber equipment. Terminal equipment 1 (TE1) is any device that supports the ISDN standards. Examples of TE1s are digital telephones, integrated voice/data terminals, and digital facsimiles.

Terminal Equipment 2 (TE2)• To provide backward compatibility with a customer’s

existing equipment, the ISDN standard defines a second level of terminal equipment called terminal equipment 2 (TE2). TE2 equipment is any non-ISDN device, such as a terminal, workstation, host computer, or regular telephone. TE2 devices are not immediately compatible with an ISDN network but can be used with the help of another device called a terminal adapter (TA).

Terminal Adapter (TA)• A terminal adapter (TA) converts information received in

non-ISDN format from a TE2 into a format capable of being carried by the ISDN.

ISDN Layers • ITU-T has devised to expand an expanded model

for the ISDN layers.

• Instead of a single seven layers architecture like OSI, the ISDN is defined in three separate planes1. User Plane

2. Control Plane

3. Management Plane

• All these three planes are divided into seven layers that corresponds to the OSI model.

ISDN Layers

Simplified Layers of ISDN

Physical Layer is identical for B & D channel both.

2B/1Q Encoding