GSM PROTL with full gsm architecture

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GSM PROTOCOL

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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



Example Of Some Functions Code conversion to facilitate

understanding of the meaning.

Routing of the messages throughNetwork.

Error control to counteract effect of

disturbances. Transmission of Electrical Signals.



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



OSI Layers

The seven layers can be thought of as

belonging to three sub groups

• Network Support Layers (Layers 1-3)• Deal with the physical aspects of moving data from

one device to another

• User Support Layers (Layers 5-7)

• Allow interoperability among unrelated softwaresystems

• Layer-4 ensures end to end reliable data

transmission



OSI Layers

End System

Network Network Network Network

Data Link Data Link Data Link Data Link

Application

Presentation

Session

Transport

Physical PhysicalPhysical Physical

PSPDN PSTN Dedicated CSPDN

User Support

Layers Network Support Layers



Hierarchical communication.

Within a single machine, each layer calls

upon the services of the layer just below

it The passing of data and network

information is made possible by an

interface between each pair of adjacentlayers

The messages exchanged between the

adjacent layers, to obtain the required

services, are called Interface Control



Peer-to-Peer communication

Between machines, layer-n on one machine

communicates with layer-n on another

machine

This communication is governed by an agreed-

upon series of rules and conventions called

protocols

The processes on each machine thatcommunicates at a given layer are called peer-

to-peer processes

At the physical layer, communication is direct

At higher layers, communication moves down



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



(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



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 identity

is preserved during the transfer Protocol Data Unit (PDU)

• (N)-PDU is the combination of (N)-PCI and(N)-SDU



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



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



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



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



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





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





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)



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 usedto deliver signaling message to the ISDN switch



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



LAPD and PPP on D and B

Channels

The call is established through the service provider network; PPP is used as the data link protocol on the Bchannel from end to end. LAPD is used between the router

and the ISDN switch at each local central office (CO) andremains up so that new signaling messages can be sentand received. Because the signals are sent outside thechannel used for data, this is called out-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.



Use of ISDN as WAN Protocol

Dial on Demand Routing – Logic is

configured in the routers to trigger the

dial when that traffic needs to get toanother site is sent by user.

Telecommuting Environment

Backup to leased lines – When leasedline fails, an ISDN call is established

between two routers.



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



Layer 2 LAPDM Protocol

Establishment and release of signalling layer 2connections.

Multiplexing and de multiplexing of several signallinglayer 2 connection on a dedicated control channel and

discrimination between them by including differentService Access Point Identifiers (SAPI).

Mapping of signalling layer 2 service data units onprotocol data unit (in case of acknowledged operationservice data units may be segmented and reassembled atdestination).

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

Flow control.



LAPDM Protocol

The establishment and release of layer 2 connectioncoincides with the allocation release, and change of dedication radio channels. Signaling layer 2 connectionsare frequently established and released, and thus an

average lifetime of a connection is short. Multiplexing anddemultiplexing deals with arranging different user (eightchannels per frame) in a frame format.

LAPDm uses on the two modes of operation for thetransmission of layer 3 message; unacknowledged

operation of multiple frame operation.



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)



LAPDM Protocol

For an unacknowledged informationtransfer, the use of layer 3 service impliesthat the information transfer is notacknowledged by the data link layer, andthus error check facilities are not provided.The transmission and reception of

messages here use data link serviceprimitives, that is, DL-DATA-REQUESTand DL-DATA-INDICATION.



LAPD and LAPDm

The main distinction between LAPD andLAPDm is the absence of address and controlfields. Thus, the protocol is only used for theunacknowledged mode of operation, whichapplies to BCCHs and CCCHs only. BothFCCH and SCH under BCCH do not requireunacknowledged. Similarity, noacknowledgement is needed for PCH and

AGCH. The LAPD frame is used internal to BSS,

namely, between BTS and BSC.



MTP3, SCCP, and TCAPProtocols.



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



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

voice announcement, voice

recognition, DTMF digit collection

SSP

SCP SDP

STPIP

SSP

STP

TCAP messages

ISUP messages

Voice



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



SS7 Protocol Suite

ISUP

TCAP

SCCP

MAP

MTP Level 3 MTP Level 2 MTP Level 1

OSI Layers Application

Presentation

Session

Transport Network Data Link Physical

INAP

Signaling

Connection

Control Part

Transaction

Capabilities

Applications

PartISDN User

Part

IN Application

Part

Mobile

Application

Part



MTP Levels 1 & 2

Message Transfer Part

Level 1

• Handling the issues related to the signals on thephysical links 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 linkfrom one node to another

• Providing error detection/correction and sequenceddelivery of the SS7 messages

• signalling network supervision and maintenancefunctions



MTP Level 3

Signaling message handling

• Providing message routing between signaling points

in the SS7 network

• May pass a number of intermediate nodes (STP,Signal Transfer Point)

• MTP level 3 ”users” are ISUP and SCCP

Signaling network management• Rerouting traffic to other SS7 signaling links in the

case of link failure, congestion or node failure

• Load-sharing



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



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



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 might be different



SCCP

Signaling Connection Control Part

Used as the transport layer for TCAP-based

services

• 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



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)



SS7 Network Architecture

Figure 7-4 depicts atypical SS7 networkarrangement.

This configurationserves severalpurposes.• No direct signaling links

• A fully meshed signalingnetwork is not required.

• The quad arrangementensures great robustness.



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)



Signal Transfer Point (STP)

To transfer messages from one SPC to another



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 SCPasking for instructions.• The SCP, based upon data and service logic

that is available, will tell the switch whichactions need to be taken.

An good example – toll-free 800 number



• 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

turn uses the services of SCCP



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

•Backward Sequence Number

•

BSN Indicator Bit•Forward Sequence Number

•Length Indicator



Message Signal Units (MSUs)

The messages sent in the SS7 network

The format of an MSU

• SIO – Service Information Octet• Indicate the upper-level protocol (e.g., SCCP or

ISUP)

• A sub-service field indicating the signaling

numbering plan• SIF – Signaling Information Field

• The actual user information

• The ANSI version and the ITU-T version

• The routing label•



• Signaling Link Selection (SLS)

• The particular signaling link to be used



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



International Signaling

Same SPCs can be reused at different



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 international or national signalling point codes.



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



The ISDN User Part (ISUP)

ISUP• The most-used SS7 application

• The establishment and release of telephone

calls

• IAM• Called number, calling number, transmission

requirement, 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



• 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



Difference between SLS and CIC

The four-bit signalling link selection (SLS) field in the routinglabel defines the signalling link which is used for transfer of thesignalling information.

The 16-bit circuit identification code (CIC) contained in theISUP message defines the TDM time slot or circuit with which

the ISUP message is associated.

Exchange

STP

Exchange

Circuit

Signalling link

ISUP Call Establishment and



Release

• A given circuit between two

switches is identified by OPC,DPC and CIC.



Signalling using IAM message

Exchange ExchangeExchange

SPC = 82

Circuit14

SPC = 22 SPC = 60 Circuit20

STP

SL 4

SL 7

STP

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

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



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



ISUP message format



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 of the 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.



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.



To sum it up with an example…

PSTN

Typical operation of a 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”



Basic local exchange (LE) architecture

Timeswitch

TDM linksto othernetworkelements

• Switch control

Switching system

• E.164 number analysis• Charging

• User databases

LIC

LIC

Tone

Rx

Groupswitch

Sign.

ETC

ETC

Exchangeterminalcircuit

Lineinterfacecircuit

SS7 Signallingequipment

Control system• O&M functions

Subscriber stage

Modern trend: Switching and control functions are separated into

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

Tone generator



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(indicating “network is

alive”)

Tone generator



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)




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


E.g.,CIC = 24

IAM(containsinformationCIC = 24)



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)


ACM,ANMTone generator2. Ringback

or busy toneis locallygenerated

4. Callcontinues…



Performance Requirements for

SS7

Bellcore spec. GR-246-Core

• MTP

• A given route set should not be out of service for

more than 10 minutes per year

• < 1*10-7 messages should be lost

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

sequence

• ISUP

• Numerous timing requirements

A VoIP network that uses SS7

• Must meet the stringent requirements



Performance Requirements for

SS7 Long-distance VoIP network A given route set should not be out

of service for more than 10 minutes

per year.

No more than 1x10-7 messages

should be lost. No more than 1x10-10 messages

should be delivered out of

sequence.

In ISUP, numerous timing

requirements must be met. How to make sure that VoIP

networks can emulate the signaling

performance of SS7.

SIGTRAN (Signaling Transport)

group of IETF

S ft it h A hit t



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



Signaling Transport (SIGTRAN) Addressing the issues regarding the

transport of signaling within IP networks

• The issues related to signaling performance

within 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 IP

address ???





SIGTRAN

Issues discussed in SIGTRAN• Address translation

• How can we deploy an SS7 application (e.g.,ISUP) that expects certain services from lower

layers such as MTP when lower layers do notexist in the IP network?

• For transport layer, the ISUP message mustbe carried in the IP network with the samespeed and reliability as in the SS7.

• UDP x

• TCP x

RFC 2719, “Framework Architecture for

Signaling Transport”



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 specific

primitives as required by a particular signaling

application.

• The standard SS7 applications (e.g., ISUP) do notrealize that the underlying transport is IP.



ISUP Transport to MGC NIF (Nodal Interworking Function) is

responsible for interworking between the SS7

and IP networks



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



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

Layer)



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]



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

To offer the fast transmission andreliability required 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.



Why not use TCP?

TCP provides both reliable data transfer and strict order-of-transmission, but SS7

may not need 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.



What Supported By Using SCTP?

To ensure reliable, error-free, in-sequence delivery 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.



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 (for multi-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.



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



SCTP Streams

A stream is a one-way logical channel

between SCTP endpoints.

• The number of streams supported in an

association is specified during theestablishment of the association.

To avoid head-of-line blocking and to

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

stream.



SCTP Functional View

SCTP User Application

Acknowledgementand

Congestion Avoidance

Chunk Bundling

Packet Validation

Path Management

Associationstartup

and

takedown

Sequenced deliverywithin streams

User Data Fragmentation

SCTP P k t & Ch k



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. . . . . .



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 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



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 controlchunks

•



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



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



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

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

SCTP user

SCTP Control Chunks

SCTP

SCTP DATA Chunks

User Messages

SCTP packets

Connectionless Packet Transfer Service (e.g. IP)



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



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



• 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

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



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



• Gap Ack Block number 1 start

• The offset of the first segment from the unbrokensequence

• 3 (7-4)

• Gap Ack Block number 1 end

• The offset of the first segment from the unbrokensequence

• 8 (8-4)

• a_rwnd

• The updated buffer space of the sender



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

include one or more IP addresses (a primaryaddress + several secondary addresses).• Multi-homes hosts

SCTP ensures that endpoint is aware of the

reachability of another endpoint through thefollowing mechanisms.• SACK chunks if DATA chunk have been sent

• HEARTBEAT chunks if an association is idle



M3UA Operation

M3UA over SCTP

Application Server • A logical entity handling signaling for a scope

• A CA handles ISUP signaling for a SS7DPC/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



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

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 thesame PC

• A single SS7 signaling endpoint

• All ASPs share a PC != that of the SG

• ASPs: a si nalin end oint SG: an STP



Robust Signaling Architecture



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



Transferring application message

• A CA sends an ISUP message

• MTP-Transfer request

• A SCTP DATA chunk

• Transmitted to a SG

• M3UA – MTP3

• To the SS7 network



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 SS7signaling-network management messages

• The SG pass M3UA messages to the CA

• The ISUP application at the CA is made aware

• MTP-Pause indication

gna ng e wor anagemenMSGs



S7ISO – SS7 Network Isolation

• When all links to the SS7 network have been

lost

DUNA – Destination 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 MTP3 network management

message



DAVA - Destination Available• Sent from SG to all concerned ASPs

• Mapped to the MTP-Resume indication

DAUD – Destination 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

SCON – SS7 Network Congestion• Sent from the SG to ASPs

• The route to an SS7 destination is congested



DUPU – Destination 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

DRST – Destination Restricted

• Sent from the SG to ASPs



ASP management

ASPUP – ASP Up

• Used between M3UA peers

• The adaptation layer is ready to receive traffic

or maintenance messages

ASPDN – ASP Down

• An ASP is not ready

UP ACK – ASP Up Ack

DOWN ACK – ASP Down Ack



ASPAC – ASP 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

ASPIA – ASP Inactive

ACTIVE ACK – ASP Active Ack



BEAT – Heartbeat• 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

ERR – Error message• A received message with invalid contents

NFTY – Notify• Between M3UA peers

•

Routing Key Management



Routing Key Management

Messages

Registration Request (REG REQ)

• An ASP = a DPC/OPC/CIC range

Registration Response Deregistration Request

Deregistration Response



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 by M3UA

• ASPUP, ASPDN, ASPAC, ASPIA and ERR

•



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 linkout of service

• RELEASE CONFIRM

Documents

GSM PROTL with full gsm architecture