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GSM PROTOCOL

skgochhayat

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Functions Of Protocols

Session establishment and termination between

users.

Orderly exchange of Data messages.

Coding of the information.

Routing and Sequencing

Flow control and Congestion control.

Error checking and recovery. Efficient network resource utilization

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Example Of Some Functions

Code conversion to facilitate understanding

of the meaning.

Routing of the messages through Network.

Error control to counteract effect of

disturbances.

Transmission of Electrical Signals.

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OSI Layers

The OSI model is built of seven ordered layers:

Layer-7: Application

Layer-6: Presentation

Layer-5: Session

Layer-4: Transport

Layer-3: Network

Layer-2: Data Link

Layer-1: Physical

ApplicationPresentation

Session

Transport

Network

Data Link

Physical

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OSI Layers

The seven layers can be thought of asbelonging to threesub groups

Network Support Layers (Layers 1-3)

Deal with the physical aspects of moving data from onedevice to another

User Support Layers (Layers 5-7)

Allow interoperability among unrelated software

systems Layer-4 ensures end to end reliable data

transmission

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OSI Layers

End System

Network NetworkNetwork Network

Data Link Data LinkData Link Data Link

Application

Presentation

Session

Transport

Physical PhysicalPhysical Physical

PSPDN PSTN Dedicated CSPDN

User Support

Layers Network Support Layers

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Hierarchical communication.

Within a single machine, each layer calls uponthe services of the layer just below it

The passing of data and network information

is made possible by an interface between eachpair of adjacent layers

The messages exchanged between the

adjacent layers, to obtain the requiredservices, are called Interface ControlInformation (ICI)

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Peer-to-Peer communication

Between machines, layer-n on one machinecommunicates with layer-n on another machine

This communication is governed by an agreed-uponseries of rules and conventions called protocols

The processes on each machine that communicates ata given layer are called peer-to-peer processes

At the physical layer, communication is direct

At higher layers, communication moves down throughthe layers on the transmitting machine and back upthrough the layers at receiving machine

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Hierarchical & Peer-to-Peer Communications

Peer-to-Peer

PCI

Interface Control Information (ICI)

Protocol Control Information (PCI)

Interface

N+1-Layer

N-Layer

Interface

N+1-Layer

N-Layer

Hierarchical

ICI

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(N+1)-Layer

(N)-Layer

Data Units in the OSI Model

(N+1)-SDU(N+1)-PCI

(N+1)-PDU

(N+1)-SDU(N+1)-PCI (N)-ICI

(N)-IDU

(N)-PCI

(N)-PDU

(N+1)-SDU(N+1)-PCI

(N)-PCI

(N)-ICI

(N+1)-SDU(N+1)-PCI(N)-SDU

(N)-ICI

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Data Units

Protocol Control Information (PCI) (N)-PCI is the protocol control information

exchanged between the (N)-entities to

coordinate their functions Service Data Units (SDU)

(N)-SDU is the data unit transferred betweenthe ends of a (N)-connection whose identityis preserved during the transfer

Protocol Data Unit (PDU) (N)-PDU is the combination of (N)-PCI and

(N)-SDU

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Data Units

Interface Control Information (ICI) (N)-ICI is the information exchanged between

(N+1)-entity and (N)-entity to coordinate their

functions Interface Data Unit (IDU)

(N)-IDU is the total data unit transferred across

the SAP between (N+1)-entity and (N)-entity

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OSI Layers

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

7-Application

6-Presentation

5-Session

4-Transport

3-Network

2-Data Link

1-Physical

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

7-Application

6-Presentation

5-Session

4-Transport

3-Network

2-Data Link

1-Physical

3-2 Interface

2-1 Interface

3-Network

2-Data Link

1-Physical

Intermediate

Node

Link Link

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Summary of OSI Layers Functions

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

Application

Presentation

Session

Transport

Network

Data Link

Physical

Allow access to network

resources

Translate, encrypt and

compress data

Establish, manage and

terminate sessions

Reliable end to end delivery

& error recovery

Movement of packets;

Provide internetworking

Transmit bits; Mechanical

and electrical specifications

Organise bits into streams;Node to node delivery

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Summary of OSI Layers Functions

6-5 Interface

5-4 Interface

4-3 Interface

3-2 Interface

2-1 Interface

7-6 Interface

Application

Presentation

Session

Transport

Network

Data Link

Physical

Allow access to network

resources

Translate, encrypt and

compress data

Establish, manage and

terminate sessions

Reliable end to end delivery

& error recovery

Movement of packets;

Provide internetworking

Transmit bits; Mechanical

and electrical specifications

Organise bits into streams;Node to node delivery

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GSM System Architecture

BTS

BSC

BSS NSS

TRAU

HLR

VLR

GSTN

ISDN

MSC

SS7

NSS

R

A

DI

O

16Kb/s

64Kb/s

MS

BSS : Base Station Sub-system

BSC : Base Station Controller

BTS : Base Transceiver Station

TRAU : Transcoder / Rate Adapter Unit

NSS : Network and Switching Sub-system

MSC: Mobile service Switching Center

HLR : Home Location Register

VLR : Visitors Location Register

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GSM protocol layers for signaling

CM

MM

RR

MM

LAPDm

radio

LAPDm

radio

LAPD

PCM

RR BTSM

CM

LAPD

PCM

RR

BTSM

16/64 kbit/s

Um Abis A

SS7

PCM

SS7

PCM

64 kbit/s /

2.048 Mbit/s

MS BTS BSC MSC

BSSAP

BSSAP

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ISDN Protocol

Two types of ISDN Interfaces : Basic Rate Interface (BRI), and Primary rate

interface (PRI), provide multiple digital bearer channels over which

temporary connections can be made and data can be sent.

The result is digital dial access to multiple site concurrently.

Type of Interface Number of Bearer

Channels (B

channels)

Number of Signaling

Channels (D

Channels)

BRI 2 1 (16 Kbps)

PRI(T/1) 23 1 (64 Kbps)

PRI(E/1) 30 1 (64 Kbps)

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ISDN Channels

B Channels : Bearer channels (B channels) are used to

transport data. B Channels are called bearer channels

because they bear the burden of transporting the

data. B channels operate up to 64 Kbps, although the

speed might be lower depending on the service

provider.

D Channels are used for signaling. LAPD is used todeliver signaling message to the ISDN switch

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LAPD and PPP on D and B Channels

B0

B1D LAPD

BRI

B0

B1D LAPD

BRI

ISDN Network

PPP

B0

B1D

Call Setup Flows

LAPD

BRI

B0

B1D

Call Setup Flows

LAPD

BRI

Call Setup

Flows

ISDN Network

SS7

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LAPD and PPP on D and B Channels

The call is established through the service provider network; PPPis used as the data link protocol on the B channel from end toend. LAPD is used between the router and the ISDN switch ateach local central office (CO) and remains up so that newsignaling messages can be sent and received. Because thesignals are sent outside the channel used for data, this is calledout-of-band signaling.

The BRI encodes bits at 192 kbps, out of which 144 Kbps is used

by B and D Channels rest is used for framing.

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Use of ISDN as WAN Protocol

Dial on Demand RoutingLogic is configuredin the routers to trigger the dial when thattraffic needs to get to another site is sent by

user. Telecommuting Environment

Backup to leased linesWhen leased line

fails, an ISDN call is established between tworouters.

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ISDN as WAN Link

BRI BRI

Leased Line

ISDN Network

Dial on

Demand

Routing

Telecommuting

Leased Line Backup

ISDN Network

ISDN Network

Computer with

ISDN Inerface

NT1

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Layer 2 LAPDM Protocol

Establishment and release of signalling layer 2 connections.

Multiplexing and de multiplexing of several signalling layer 2connection on a dedicated control channel and discriminationbetween them by including different Service Access Point Identifiers(SAPI).

Mapping of signalling layer 2 service data units on protocol dataunit (in case of acknowledged operation service data units may besegmented and reassembled at destination).

Detection and recovery of errors due to loss, duplication, anddisorder.

Flow control.

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LAPDM Protocol

The establishment and release of layer 2 connection coincides withthe allocation release, and change of dedication radio channels.Signaling layer 2 connections are frequently established andreleased, and thus an average lifetime of a connection is short.Multiplexing and demultiplexing deals with arranging different user

(eight channels per frame) in a frame format. LAPDm uses on the two modes of operation for the transmission of

layer 3 message; unacknowledged operation of multiple frameoperation.

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LAPDM Protocol

On the DCCH both unacknowledged and acknowledged

operations are used,

DCCHs (SDCCH, SACCH and FACCH)

whereas on the CCCHs only unacknowledged operation

is applied. Thus, both modes are applicable for

transmission over on of the in contrast top information

transfer over CCCHs (BCCH, PCH and AGCH)

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LAPDM Protocol

For an unacknowledged information transfer,the use of layer 3 service implies that theinformation transfer is not acknowledged bythe data link layer, and thus error checkfacilities are not provided. The transmissionand reception of messages here use data link

service primitives, that is, DL-DATA-REQUESTand DL-DATA-INDICATION.

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LAPD and LAPDm

The main distinction between LAPD and LAPDm is theabsence of address and control fields. Thus, theprotocol is only used for the unacknowledged mode ofoperation, which applies to BCCHs and CCCHs only.

Both FCCH and SCH under BCCH do not requireunacknowledged. Similarity, no acknowledgement isneeded for PCH and AGCH.

The LAPD frame is used internal to BSS, namely,between BTS and BSC.

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MTP3, SCCP, and TCAP

Protocols.

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Class 5

End Office Switch

The Telephone Network [1/2]

Circuit Switched Network

Intelligent

Peripheral

Signal

Transfer

Point

Service

Control

Point

Class 4

Tandem Switch

Service

Data

Point

+

Transport Layer

Control Layer

SS7 Signaling

ISUP Messages

INAP/TCAP Messages

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The Telephone Network [2/2]

5 Basic Components in Intelligent Networks

SSP/Service Switching Point

switching, service invocation

STP/Service Transfer Point

signal routing

SCP/Service Control Point

service logic execution

SDP/Service Data Point

subscriber data storage, access

IP/Intelligent Peripheral

resources such as customized voiceannouncement, voice recognition,

DTMF digit collection

SSP

SCP SDP

STPIP

SSP

STP

TCAP messages

ISUP messages

Voice

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Signalling example

ExchExchUser A

(callinguser)

Database

A typical scenario:

User A calls mobile user B. The call is routed to a specific

gateway exchange (GMSC) that must contact a database(HLR) to find out under which exchange (MSC) the mobileuser is located. The call is then routed to this exchange.

CUTTACKBBSR

KOLKOTA

User B(calleduser)

Exch

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SS7 Protocol Suite

ISUP

TCAP

SCCP

MAP

MTP Level 3MTP Level 2MTP Level 1

OSI LayersApplication

Presentation

Session

TransportNetworkData LinkPhysical

INAP

Signaling

Connection

Control Part

Transaction

Capabilities

Applications

PartISDN User

Part

IN Application

Part

Mobile

Application

Part

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MTP Levels 1 & 2

Message Transfer Part

Level 1

Handling the issues related to the signals on the physicallinks between one signaling node and another

Closely to layer 1 of the OSI stack

Level 2

Dealing with the transfer of messages on a given link fromone node to another

Providing error detection/correction and sequenced deliveryof the SS7 messages

signalling network supervision and maintenance functions

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MTP Level 3

Signaling message handling

Providing message routing between signaling points inthe SS7 network

May pass a number of intermediate nodes (STP, SignalTransfer Point)

MTP level 3 users are ISUP and SCCP

Signaling network management

Rerouting traffic to other SS7 signaling links in thecase of link failure, congestion or node failure

Load-sharing

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Services

Provides a number of services to the protocol

layer above it

The transfer of messages

Indicating availability of resources MTP-Transfer request, MTP-Transfer indication,

MTP_Pause indication, MTP-Resume indication, and

MTP-Status indication

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ISUP

ISDN User Part

Used as the protocol for setting up and tearing down phone

calls between switches

Initial Address Message (IAM) To initiate a call between two switches

Answer Message (ANM)

To indicate that a call has been accepted by the called party

Release Message (REL)

To initiate call disconnection

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Connection-Oriented Protocol

A connection-oriented protocol

Related to the establishment of connections between

users

The path of messages and the path of the bearer mightbe different

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SCCP

Signaling Connection Control Part

Used as the transport layer for TCAP-basedservices

Free phone (800/888), calling card, wireless roaming

Both connection-oriented and connectionless

Mostly connectionless signaling

Global title translation (GTT) capabilities The destination signaling point and subsystem

number is determined from the global title

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TCAP, MAP and INAP

TCAP (Transaction Capabilities Applications Part)

Supporting the exchange of non-circuit related

information between signaling points

Queries and responses sent between SSPs and SCPs

are carried in TCAP messages

Provides services to

INAP (IN Application Part)

MAP (Mobile Application Part)

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SS7 Network Architecture

Figure 7-4 depicts atypical SS7 networkarrangement.

This configuration servesseveral purposes.

No direct signaling links

A fully meshed signalingnetwork is not required.

The quad arrangementensures great robustness.

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Signaling Point (SP)

Each node in an SS7 network is an SP.

The signaling address of the SP is known as

a signaling point code (SPC). Linkset

Group of signaling links directly connecting two

SPCs

For capability and security reasons

Service Switching Point (SSP)

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Signal Transfer Point (STP)

To transfer messages from one SPC to another

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Service Control Point (SCP)

A network entity that contains additionallogic and that can be used to offer

advanced services The switch sends a message to the SCP

asking for instructions.

The SCP, based upon data and service logic that

is available, will tell the switch which actionsneed to be taken.

An good exampletoll-free 800 number

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An example

A subscriber dials a toll-free 800 number

The SSP knows that it needs to query the SCP

The SCP contains the translation information The SCP responds to the SSP with a routable number

The SSP routes the call

Connectionless signaling

The application use the services of TCAP, which in turnuses the services of SCCP

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Message Signal Units (MSUs) The messages sent in the SS7 network

Backward Sequence Number

BSN Indicator BitForward Sequence Number

Length Indicator

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Message Signal Units (MSUs)

The messages sent in the SS7 network

The format of an MSU

SIOService Information Octet

Indicate the upper-level protocol (e.g., SCCP or ISUP)

A sub-service field indicating the signaling numbering plan

SIFSignaling Information Field

The actual user information

The ANSI version and the ITU-T version

The routing label

The Destination Point Code (DPC)

The Originating Point Code (OPC)

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Signaling Link Selection (SLS)

The particular signaling link to be used

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SS7 addressing

The ANSI version, 24 bits

Member, cluster, network codes

An operator has a network code The ITU-T version, 14 bits

International Signaling Gateway

Use sub-service field

National, Nation Spare, International, International Spare

An international gateway has one national point code and one

international code

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International Signaling

Same SPCs can be reused at different

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Same SPCs can be reused at differentnetwork levels

SPC = 277

SPC = 277

International

National

SPC = 277 means different signalling points (network elements)at different network levels.

F CK SIF SIO LI Control F

The Service Information Octet (SIO)indicates whether the DPCand OPC are internationalor nationalsignalling point codes.

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ISDN User Part (ISUP)

ISUP is a signalling application protocol that is used for establishing

and releasing circuit-switched connections (calls).

Only for signalling between exchanges (ISUP can never beused between an exchange and a stand-alone database)

Not only for ISDN (=> ISUP is generally used in the PSTN)

Structure of ISUP message:

SIO (one octet)

Routing label (four octets)

CIC (two octets)

Message type (one octet)Mandatory fixed part

Mandatory variable part

Optional part

Must always be included in ISUP message

E.g., IAM message

E.g., contains called (user B) number inIAM message

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The ISDN User Part (ISUP) ISUP

The most-used SS7 application

The establishment and release of telephone calls

IAM

Called number, calling number, transmissionrequirement, type of caller,

ACM

The call is through-connected to the destination

A one-way-audio path is opened for ring-back tone

Optional

If not returned, no ring-back tone at all

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CPG, Call Progress Optional; provide information to the calling switch

ANM, Answer Message

Open the transmission path in both directions

Instigate charging for the call

REL, Release

RLC, Release Complete

CIC, circuit identification Code

Indicates the specific trunk between two switches

OPC, DPC, and CIC

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Difference between SLS and CIC

The four-bitsignalling link selection (SLS) field in the routinglabeldefines the signalling linkwhich is used for transfer of thesignalling information.

The 16-bitcircuit identification code (CIC) contained in theISUP messagedefines the TDM time slot or circuitwith which

the ISUP message is associated.

Exchange

STP

Exchange

Circuit

Signalling link

ISUP C ll E t bli h t d

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ISUP Call Establishment and

ReleaseA given circuit between two

switches is identified by OPC,DPC and CIC.

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Signalling using IAM message

Exchange ExchangeExchange

SPC = 82

Circuit14

SPC = 22 SPC = 60Circuit20

STP

SL 4

SL 7

STP

Outgoing message:OPC = 82 CIC = 14DPC = 22 SLS = 4

Processing in (transit) exchange(s):Received IAM message contains B-number.Exchange performs number analysis(not part ofISUP) and selects new DPC (60) and CIC (20).

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Setup of a call using ISUP

LE A LE BTransit exchangeUser A User B

Setup IAMIAM

Setup

Alert

Connect

ACM

ANM

ACM

ANM

Alert

Connect

Charging of call starts now

Number analysisDSS1

signalling

assumed

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ISUP message format

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Signalling Connection Control Part (SCCP)

SCCP is required when signalling information is carried betweenexchanges and databases in the network.

An important task of SCCP is global title translation (GTT):

STP DatabaseExchangeSTP with GTT capability

Exchange knows the global title (e.g. 0800 number or IMSInumber in a mobile network) but does not know the DPC ofthe database related to this global title.

1.

SCCP performs global title translation in the STP (0800 orIMSI number => DPC) and the SCCP message can now berouted to the database.

2.

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Example: SCCP usage in mobile call

SCCPSCCP

MSC located in Espoo HLR located in Oslo

STP

SPC = 82 SPC = 99

SPC = 32

SCCP/GTT functionality

Outgoing message:OPC = 82 DPC = 32SCCP:IMSI global title

Processing in STP:Received message is given to SCCP for GTT.SCCP finds the DPC of the HLR: DPC = 99

Mobile switching center (MSC) needs to contact the home locationregister (HLR) of a mobile user identified by his/her InternationalMobile Subscriber Identity (IMSI) number.

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To sum it up with an example

PSTN

Typical operation ofa local exchange

Subscriber signalling(analog or ISDN=DSS1)

Network-

internalsignalling(SS7)

Transmission(PDH, SDH)

Databases inthe network(HLR)

Part B, Section 3.3 in UnderstandingTelecommunications 2

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Basic local exchange (LE) architecture

Timeswitch

TDM linksto othernetworkelements

Switch control

Switching system

E.164 number analysisCharging

User databases

LIC

LIC

Tone

Rx

Groupswitch

Sign.

ETC

ETC

Exchangeterminalcircuit

Lineinterfacecircuit

SS7 Signallingequipment

Control systemO&M functions

Subscriber stage

Modern trend: Switching and control functions are separated into

different network elements (separation of user and control plane).

Tone generator

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Setup of a call (1)

Timeswitch

2. Check user database. For instance, is

user A barred for outgoing calls?

Switching system

3. Reserve memory for user B number

LIC

LIC

Tone

Rx

Groupswitch

Sign.

ETC

ETC

Control system

Phase 1. User A lifts handset and receives dial tone.

1. Off hook

Local exchange of user A

4. Tone Rx is connected

5. Dial toneis sent(indicatingnetwork is

alive)

Tone generator

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Timeswitch

3. Number analysis

Switching system

4. IN triggering actions? Should an externaldatabase (e.g. SCP, HLR) be contacted?

LIC

LIC

Tone

Rx

Groupswitch

Sign.

ETC

ETC

Control system

Phase 2. Exchange receives and analyzes user B number.

2. Number (DTMFsignal) received

1. User Adials user Bnumber

Setup of a call (2)

Local exchange of user A

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Timeswitch

2. Outgoing circuit is reserved

Switching system

LIC

LIC

Tone

Rx

Groupswitch

Sign.

ETC

ETC

Control system

3. Outgoing signalling message (ISUP IAM)contains user B number

Phase 3. Outgoing circuit is reserved. ISUP Initial address message

(IAM) is sent to next exchange.

Setup of a call (3)

1. Tone receiveris disconnected

Local exchange of user A

E.g.,CIC = 24

IAM(containsinformationCIC = 24)

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Timeswitch

1. ISUP ACM message indicates free or busy

user B

Switching system

LIC

LICGroupswitch

Sign.

ETC

ETC

Control system

3. Charging starts when ISUP ANM messageis received

Phase 4. ACM received => ringback or busy tone generated. ANM

received => charging starts.

Setup of a call (4)

Local exchange of user A

ACM,ANMTone generator2. Ringback

or busy toneis locallygenerated

4. Callcontinues

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Performance Requirements for SS7

Bellcore spec. GR-246-Core MTP

A given route set should not be out of service for more than10 minutes per year

< 1*10-7messages should be lost

< 1*10-10messages should be delivered out of sequence

ISUP Numerous timing requirements

A VoIP network that uses SS7 Must meet the stringent requirements

Signaling Transport (Sigtran) group of the IETF

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Performance Requirements for SS7

Long-distance VoIP network A given route set should not be out of

service for more than 10 minutes peryear.

No more than 1x10-7messages shouldbe lost.

No more than 1x10-10messagesshould be delivered out of sequence.

In ISUP, numerous timingrequirements must be met.

How to make sure that VoIP networks

can emulate the signalingperformance of SS7.

SIGTRAN (Signaling Transport) groupof IETF

S ft it h A hit t

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Softswitch Architecture

Signaling(SS7)

Gateway

Trunking

Gateway

Call

Agent

SCP

Residential

Gateway

InternetSS7 Network

STP

TrunkingGatewayTrunking

Gateway

CO

Switch

ResidentialGatewayResidential

Gateway

MGCP/

MEGACO

MGCP/

MEGACO

RTP

SIGTRAN

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Signaling Transport (SIGTRAN)

Addressing the issues regarding thetransport of signaling within IP networks

The issues related to signaling performancewithin IP networks and the interworking with

PSTN SIP/MEGACO/ISUP Interworking

Translating the MTP-based SS7 message (e.g.,IAM) to IP-based message (e.g., IP IAM)

Just a simple translation from point code to IPaddress ???

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SIGTRAN

Issues discussed in SIGTRAN Address translation

How can we deploy an SS7 application (e.g., ISUP)that expects certain services from lower layerssuch as MTP when lower layers do not exist in the

IP network? For transport layer, the ISUP message must be

carried in the IP network with the same speed andreliability as in the SS7. UDP x

TCP x

RFC 2719, Framework Architecture forSignaling Transport

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SIGTRAN Architecture Signaling over standard IP uses a common transport

protocol that ensures reliable signaling delivery.

Error-free and in-sequence

Stream Control Transmission Protocol (SCTP)

An adaptation layer is used to support specificprimitives as required by a particular signaling

application.

The standard SS7 applications (e.g., ISUP) do not realize that

the underlying transport is IP.

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ISUP Transport to MGC

NIF (Nodal Interworking Function) is responsible forinterworking between the SS7 and IP networks

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SIGTRAN Protocol Stack

SCTP: fast delivery of messages (error-free, in sequencedelivery), network-level fault tolerance

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Adaptation Layer [1/3] M2UA (MTP-2 User Adaptation Layer)

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Adaptation Layer [2/3] M2PA (MTP-2 Peer-to-Peer Adaptation Layer)

An SG that utilizes M2PA is a signaling node for the MGC.

It is effectively an IP-based STP.

SG can processing higher-layer signaling functions, such

as SCCP GTT.

Adaptation Layer [3/3]

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p y [ / ]

M3UA (MTP3-User Adaptation Layer)

SUA (SCCP-User Adaptation Layer) Applications such as TCAP use the services of SUA.

IUA (ISDN Q.921-User Adaptation Layer)

V5UA (V5.2-User Adaptation Layer)

SCTP

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SCTP

To offer the fast transmission and reliabilityrequired for signaling carrying.

SCTP provides a number of functions that

are critical for telephony signaling transport. It can potentially benefit other applications

needing transport with additional performance

and reliability.

SCTP must meet the Functional

Requirements of SIGTRAN.

Wh TCP?

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Why not use TCP?

TCP provides both reliable data transfer andstrict order-of-transmission, but SS7 may notneed ordering.

TCP will cause delay for supporting order-of-

transmission. The limited scope of TCP sockets complicates

the task of data transmission using multi-homed hosts.

TCP is relatively vulnerable to DoS attack,such as SYN attacks.

Wh S d B U i SCTP?

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What Supported By Using SCTP?

To ensure reliable, error-free, in-sequencedelivery of user messages (optional).

To support fast delivery of messages and

avoid head-of-line blocking. To support network-level fault tolerance

that is critical for carrier-grade network

performance by using multi-home hosts.

To provide protection against DoS attack by

using 4-way handshake and cookie.

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SCTP Endpoint & Association

Endpoint

The logical sender/receiver of SCTP packets.

Transport address = IP address + SCTP port number

An endpoint may have multiple transport addresses (formulti-homed host, all transport addresses must use the

same port number.)

Association

A protocol relationship between SCTP endpoints. Two SCTP endpoints MUST NOT have more than one SCTP

association.

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Multi-Homed Host

Host A

SCTP User

Host B

One IP address

One SCTP association with

multi-homed redundant

SCTP

SCTP User

SCTP

One IP address One IP address

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SCTP Streams

A stream is a one-way logical channel

between SCTP endpoints.

The number of streams supported in an

association is specified during the

establishment of the association.

To avoid head-of-line blocking and to

ensure in-sequence delivery In-sequence delivery is ensured within a single

stream.

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SCTP Functional View

SCTP UserApplication

Acknowledgementand

Congestion Avoidance

Chunk Bundling

Packet Validation

Path Management

Associationstartup

and

takedown

Sequenced deliverywithin streams

User Data Fragmentation

SCTP P k t & Ch k

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SCTP Packets & Chunks

A SCTP packet can comprise several chunks.

Chunk

Data or control

Source Port Number Destination Port Number

Verification Tag

Checksum

Chunk Type Chunk Flags Chunk Length

Chunk Value

. . .

Common

Header

Chunk 1

Chunk N

0 16 3115. . . . . .

Ch k T

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Chunk Type ID Value Chunk Type

-------- ---------------

0 - Payload Data (DATA)

1 - Initiation (INIT)

2 - Initiation Acknowledgement (INIT ACK)

3 - Selective Acknowledgement (SACK)

4 - Heartbeat Request (HEARTBEAT)

5 - Heartbeat Acknowledgement (HEARTBEAT ACK)

6 - Abort (ABORT)

7 - Shutdown (SHUTDOWN)

8 - Shutdown Acknowledgement (SHUTDOWN ACK)

9 - Operation Error (ERROR)

10 - State Cookie (COOKIE ECHO)

11 - Cookie Acknowledgement (COOKIE ACK)

12 - Reserved for Explicit Congestion Notification Echo (ECNE)

13 - Reserved for Congestion Window Reduced (CWR)

14 - Shutdown Complete (SHUTDOWN COMPLETE)

- Reserved for IETF

SCTP t l h k

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SCTP control chunks

INIT chunk Initiate an SCTP association between two endpoints

Cannot share an SCTP packet with any other chunk

INIT ACK Acknowledge the initiation

Must not share a packet with any other chunk

SACK

Acknowledge the receipt of Data chunks Inform the sender of any gaps

Only the gaps need to be resent

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HEARTBEAT When no chunks need to be sent

Send periodic HEARTBEAT messages

Contain sender-specific information

HEARTBEAT ACK Containing heartbeat information copied form

HEARTBEAT

ABORT End an association abruptly

Cause information

Can be multiplexed with other SCTP control chunks Should be the last chunk, or

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SHUTDOWN

A graceful termination of an association

Stop sending any new data

Wait until all data sent has been acknowledged

Send a SHUTDOWN to the far end Indicate the chunk received

Upon receipt of a SHUTDOWN

Retransmit data that are not acknowledged

Send a SHUTDOWN ACK SHUTDOWN ACK

SHUTDOWN COMPLETE

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ERROR Some error condition detected

E.g., a chunk for a non-existent stream

COOKIE ECHO Used only during the initiation of an association

An INIT ACK includes a cookie parameter

Information specific to the endpoint, a timestamp, a cookie lifetime

Upon receipt of an INIT ACK

Return the cookie information in COOKIE ECHO

Can be multiplexed; must be the first chunk COOKIE ACK

Can be multiplexed; must be the first chunk

INIT Chunk

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INIT Chunk

Advertised Receiver Window Credit (a_rwnd)

Number of Outbound Streams

Type = 1 Chunk Flags Chunk Length

Initial TSN (Transmission Sequence Number)

. . .

0 16 3115. . . . . .

Initial Tag

Number of Inbound Streams

Optional / Variable-Length Parameter

Association Establishment

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Association Establishment

A Z

INIT [I-Tag=Tag_A]

INIT ACK [V-Tag=Tag_A, I-Tag=Tag_Z, Cookie_Z]

COOKIE [Cookie_Z]

COOKIE ACK

allocatingresources

User Data Transfer

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User Data Transfer

SCTP user

SCTP Control Chunks

SCTP

SCTP DATA Chunks

User Messages

SCTP packets

Connectionless Packet Transfer Service (e.g. IP)

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DATA Chunk

Stream ID = S

Type = 0 Reserved Chunk Length

Payload Protocol ID

. . .

0 16 3115. . . . . .

TSN

Stream Sequence Number = n

User Data (Sequence n of Stream S)

U B E

U : unorderedB : beginE : end

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Payload data chunk Carry information to and from the ULP

U: unordered bit The information should be passed to the ULP without regard

to sequencing

B and E: beginning and end bits Segment a given user message

TSN: Transmission Sequence Number (32-bit) Independent of any streams

Assigned by SCTP An INIT has the same TSN as the first DATA chunk

TSN ++ for each new DATA chunk

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S: Stream Identifier (16-bit)

n: stream sequence number (16-bit)

Begins at zero

Increments for each new message Payload protocol identifier

For the users to pass further information about the

chunk but is not examined by the SCTP

SACK Chunk

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SACK Chunk

Advertised Receiver Window Credit (a_rwnd)

Number of Gap Ack Blocks = n

Type = 3 Chunk Flags Chunk Length

0 16 3115. . . . . .

Cumulative TSN Ack

Number of Duplicate TSNs = xGap Ack Block #1 Start Gap Ack Block #1 End

. . .

. . .

Duplicate TSN #1

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Transferring data Reliable transfer

SACK chunk

Cumulative TSN The highest TSN value received without any gaps

4

The number of Gap Ack Blocks

The number of fragments received after the unbroken sequence

2

The number of duplicate TSNs

2

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Gap Ack Block number 1 start

The offset of the first segment from the unbroken sequence

3 (7-4)

Gap Ack Block number 1 end

The offset of the first segment from the unbroken sequence

8 (8-4)

a_rwnd

The updated buffer space of the sender

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SCTP Robustness Robustness is a key characteristic of any carrier-

grade network. To handle a certain amount of failure in the network

without a significant reduction in quality

INIT and INIT ACK chunks may optionally includeone or more IP addresses (a primary address +several secondary addresses). Multi-homes hosts

SCTP ensures that endpoint is aware of thereachability of another endpoint through the

following mechanisms. SACK chunks if DATA chunk have been sent

HEARTBEAT chunks if an association is idle

M3UA Operation

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M3UA Operation

M3UA over SCTP Application Server

A logical entity handling signaling for a scope

A CA handles ISUP signaling for a SS7 DPC/OPC/CIC-

range An AS contains a set of Application Server Processes

(ASPs)

ASP

A process instance of an AS Can be spread across multiple IP addresses

Active ASPs and standby ASPs

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Routing Key A set of SS7 parameters that identifies the

signaling for a given AS

OPC/DPC/CIC-range

Network Appearance

A mechanism for separating signaling traffic

between an SG and an ASP E.g., international signaling gateway

Signaling Network Architecture

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Signaling Network Architecture

No single point of failure SGs should be set up at least in pairs

ASPs A redundant or load-sharing configuration

Spread over different hosts Point code

All ASPs and the connected SG share the same PC A single SS7 signaling endpoint

All ASPs share a PC != that of the SG ASPs: a signaling endpoint; SG: an STP

A group of ASPs share a PC

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Robust Signaling Architecture

Services Provided by M3UA

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Services Provided by M3UA

Offer the same primitives as offered by MTP3 MTP-Transfer request

MTP-Transfer indication

MTP-Pause indication

Signaling to a particular destination should be suspended

MTP-Resume indication

Signaling to a particular destination can resume

MTP-Status indication

Some change in the SS7 network

E.g., network congestion or a destination user part becomingunavailable

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Transferring application message A CA sends an ISUP message

MTP-Transfer request

A SCTP DATA chunk

Transmitted to a SG

M3UAMTP3

To the SS7 network

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M3UA Messages Messages between peer M3UA entities

A header + the M3UA message content

The entities can communicate information regarding

the SS7 network If a remote destination becomes unavailable

The SG becomes aware of this through SS7 signaling-network management messages

The SG pass M3UA messages to the CA The ISUP application at the CA is made aware

MTP-Pause indication

Signaling Network ManagementMSGs

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MSGs

S7ISOSS7 Network Isolation When all links to the SS7 network have been lost

DUNADestination Unavailable

Sent from the SG to all connected ASPs

Destination(s) within the SS7 network is not available

Allocate 24 bits for each DPC

DUNA is generated at the SG

It determines from MTP3network management message

The M3UA of the ASP

Create MTP-Pause indication

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DAVA - Destination Available Sent from SG to all concerned ASPs Mapped to the MTP-Resume indication

DAUDDestination State Audit

Sent from an ASP to an SG To query the status of one or more destination The SG responds with DAVA, DUNA, or SCON

SCONSS7 Network Congestion Sent from the SG to ASPs The route to an SS7 destination is congested Mapped to the MTP-Status indication

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DUPUDestination User Part Unavailable Sent from the SG to ASPs

A given user part at a destination is not available

The DPC and the user part in question

Mapped to MTP-Status indication

Cause codes

DRSTDestination Restricted

Sent from the SG to ASPs

One or more SS7 destinations are restricted from

The M3UA may use a different SG

ASP management

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ASP management

ASPUPASP Up Used between M3UA peers

The adaptation layer is ready to receive traffic or

maintenance messages ASPDNASP Down

An ASP is not ready

UP ACKASP Up Ack DOWN ACKASP Down Ack

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ASPACASP Active Sent by an ASP

Indicate that it is ready to be used

To receive all messages or in a load-sharing mode

Routing context

Indicate the scope is applicable to the ASP

DPC/OPC/CIC-range

ASPIAASP Inactive

ACTIVE ACKASP Active Ack

INACTIVE ACKASP Inactive Ack

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BEATHeartbeat Between M3UA peers

Still available to each other

When M3UA use the services of SCTP

The BEAT message is not required at the M3UA level SCTP includes functions for reachability information

ERRError message A received message with invalid contents

NFTYNotify Between M3UA peers

To communicate the occurrence of certain events

Routing Key Management Messages

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Routing Key Management Messages

Registration Request (REG REQ) An ASP = a DPC/OPC/CIC range

Registration Response

Deregistration Request

Deregistration Response

M2UA Operation

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M2UA Operation

MTP3/M2UA/SCTP

The CA has more visibility of the SS7 network More tightly coupled to the SG

MTP3 Routing and distribution capabilities

M2UA uses similar concepts to those used byM3UA ASPUP, ASPDN, ASPAC, ASPIA and ERR

Exactly the same functions In M2UA, the ASP is an instance of MTP3

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M2UA-specific messages DATA

Carry an MTP2-user Protocol Data Unit

ESTABLISH REQUEST

To establish a link to the SG ESTABLISH CONFIRMATION

RELEASE REQUEST Request the SG to take a particular signaling link out of

service

RELEASE CONFIRM RELEASE INDICATION

The SG autonomously take a link out of service