VoIP and SS7 Chapter 7. Internet Telephony 2 Basic functional parts of the PSTN PSTN Switching in...

VoIP and SS7

Chapter 7

2Internet Telephony

Basic functional parts of the PSTN

PSTN

Switching in exchanges

Subscriber signalling (analog or ISDN=DSS1) Network-

internal signalling (SS7)

Transmission (PDH, SDH)

Databases in the network (HLR)

3Internet Telephony

PDH and SDH transmission bit rates

PDH (Plesiochronous Digital Hierarchy)

SONET (North Am.) SDH

STS-1 51.84 Mbit/sSTS-3 155.52 STM-1STS-12 622.08 STM-4STS-48 2.488 Gbit/s STM-16

Japan USA Europe

J1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s J2 6 T2 6 E2 8J3 32 T3 45 E3 34J4 98 T4 140 E4 140

4Internet Telephony

Structure of E1 frame (2.048 Mbit/s)

32 TDM time slots (with 8 bits each / frame)

Time slots 1-31 carry digital signals (usually PCM speech) with a bitrate of 64 kbit/s.

Time slot 0 is used for frame synchronization:

0 1 2 3116

... ...received bit stream ... where does a new frame begin?

Time slot 16 usually contains SS7 signalling information.

5Internet Telephony

Subscriber signalling

PSTN

Switching in exchanges





6Internet Telephony

Analog subscriber signalling

The calling party (user A) tells the local exchange to set up (disconnect) a call by generating a short (open) circuit in the terminal => off-hook (on-hook) operation.

The dialled called party (user B) number is sent to the local exchange in form of Dual Tone Multi-Frequency (DTMF) signal bursts.

Alerting (ringing) means that the local exchange sends a strong sinusoid to the terminal of user B.

In-channel information in form of audio signals (dial tone, ringback tone, busy tone) is sent from local exchange to user. User can send DTMF information to network.

1

2

3

4

7Internet Telephony

Analog subscriber signalling in action

LE AUser A User B

Ringing signal

Off-hook (user B

answers)

Off-hook SS7 signalling

(ISUP)Dial tone

B number

Ringback tone (or

busy tone)

LE B

Connection established

LE = local exchange

8Internet Telephony

ISDN subscriber signalling in action

LE AUser A User B

Ringing

Off-hook (user B

answers)

Off-hook SS7 signalling

(ISUP)B number

Tones generated in terminal

LE B

Setup

Call proc Setup

Alert

Conn

Alert

Conn

DSS1 signalling messages

Connection established

9Internet Telephony

PSTN vs. ISDN user access

300 … 3400 Hz analog transmission band

“Poor-performance” subscriber signaling

2 x 64 kbit/s digital channels (B channels)

16 kbit/s channel for signaling (D channel) => Digital Subscriber Signalling system nr. 1 (DSS1)

PSTN

Basic Rate

Access ISDN

Primary Rate

Access ISDN

30 x 64 kbit/s digital channels (B channels)

64 kbit/s channel for signaling (D channel)

Mainly used for connecting private branch exchanges (PBX) to the PSTN.

10Internet Telephony

End-to-end digital signalling

ISUPISUPQ.931Q.931

Q.921Q.921

I.430I.430

Q.931Q.931

Q.921Q.921

I.430I.430

MTP 3MTP 3

MTP 2MTP 2

MTP 1MTP 1

Q.931Q.931

Q.921Q.921

I.430I.430

Q.931Q.931

Q.921Q.921

I.430I.430

ISUPISUP

MTP 3MTP 3

MTP 2MTP 2

MTP 1MTP 1

contains the signalling messages for call control

User interface User interfacePSTN Network

DSS1

SS7

DSS1


Introduction

Channel Associated Signaling Still widely deployed today Considered as old telephony

Common Channel Signaling Separation of signaling and call

paths Signaling System 7 (SS7)

To enable a wide range of services to be provided to the end-user

Caller ID, toll-free calling, call screening, number portability, etc.

SS7 is the foundation for Intelligent Network (IN) services.


Channel-associated signalling (CAS)

CAS means in-band signalling over the same physical channels as the circuit-switched user traffic (e.g. voice).

Signalling to/from databases is not feasible in practice (setting up a circuit switched connection to the database and then releasing it would be extremely inconvenient).

ExchangeExchange ExchangeExchange

ExchangeExchange

Circuit switched connection

Signalling is possible

Signalling is not possible before previous circuit-

switched link is established

CAS has two serious draw-backs:

Setting up a circuit switched connection is very slow.•

•


Common channel signalling (CCS)

In practice, CCS = SS7.

ExchangeExchange ExchangeExchange

Signalling is possible anywhere anytime

DatabaseDatabase

End-to-end signalling is possible before call setup and also during the conversation phase of a call.

The packet-switched signalling network is totally separated from the circuit-switched connections. Consequently:

Signalling to/from databases is possible anytime.•

•

There is one drawback: It is difficult to check if the circuit-switched connections are really working (= continuity check).


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

SSPSSP

SCPSCP SDPSDP

STPSTPIPIP

SSPSSP

STPSTP

TCAP messages

ISUP messages

Voice


Signalling example

ExchExchExchExchUser A

(calling user)

User A (calling user)

DatabaseDatabase

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 mobile user is located. The call is then routed to this exchange.

OuluTokyo

London

User B (called user)

User B (called user)

ExchExch


SS7 Protocol Suite

ISUPTCAP

SCCP

MAP

MTP Level 3

MTP Level 2

MTP Level 1

OSI Layers

Application

Presentation

Session

Transport

Network

Data Link

Physical

INAP


MTP Levels 1 & 2

Message Transfer Part Level 1

Handling the issues related to the signals on the physical 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 link from one node to another

Providing error detection/correction and sequenced delivery of the SS7 messages

signalling network supervision and maintenance functions


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


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


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 freephone (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 a typical SS7 network arrangement.

This configuration serves several purposes.

No direct signaling links A fully meshed signaling network is not

required. The quad arrangement ensures 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 additional logic 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 actions 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

The Destination Point Code (DPC) The Originating Point Code (OPC)


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 network levels

SPC = 277SPC = 277

SPC = 277SPC = 277

International

National

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

FF CKCK SIF SIF SIOSIO LILI ControlControl FF

The Service Information Octet (SIO) indicates whether the DPC and 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 be used 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)SIO (one octet)

Routing label (four octets)Routing label (four octets)

CIC (two octets)CIC (two octets)

Message type (one octet) Mandatory fixed part

Message type (one octet) Mandatory fixed part

Mandatory variable partMandatory variable part

Optional partOptional part

Must always be included in ISUP message

E.g., IAM message

E.g., contains called (user B) number in IAM 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 routing label defines the signalling link which is used for transfer of the signalling information.

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

ExchangeExchange

STPSTP

ExchangeExchange

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

ExchangeExchange ExchangeExchangeExchangeExchange

SPC = 82

Circuit 14

SPC = 22 SPC = 60Circuit 20

STPSTPSL 4

SL 7

STPSTP

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 of ISUP) and selects new DPC (60) and CIC (20).


Setup of a call using ISUP

LE A LE BTransit exchange User 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 between exchanges and databases in the network.

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

STPSTP DatabaseDatabaseExchangeExchange

STP with GTT capability

Exchange knows the global title (e.g. 0800 number or IMSI number 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 or IMSI number => DPC) and the SCCP message can now be routed to the database.

2.


Example: SCCP usage in mobile call

SCCPSCCP

MSC located in EspooMSC located in Espoo HLR located in OsloHLR located in Oslo

STPSTP

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 location register (HLR) of a mobile user identified by his/her International Mobile Subscriber Identity (IMSI) number.


To sum it up with an example…

PSTN

Typical operation of a local exchange





Part B, Section 3.3 in ”Understanding Telecommunications 2”


Basic local exchange (LE) architecture

Time switch

TDM links to other network elements

• Switch control

Switching system

• E.164 number analysis• Charging• User databases

LIC

LIC

ToneRx

Group switch

Sign.

ETC

ETC

Exchange terminal circuit

Line interface circuit

SS7 Signalling equipment

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)

Time switch

2. Check user database. For instance, is user A barred for outgoing calls?

Switching system

3. Reserve memory for user B number

LIC

LIC

ToneRx

Group switch

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 tone is sent (indicating “network is alive”)

Tone generator


Time switch

3. Number analysis

Switching system

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

LIC

LIC

ToneRx

Group switch

Sign.

ETC

ETC

Control system

Phase 2. Exchange receives and analyzes user B number.

2. Number (DTMF signal) received

1. User A dials user B number

Setup of a call (2)



Time switch

2. Outgoing circuit is reserved

Switching system

LIC

LIC

ToneRx

Group switch

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 receiver is disconnected


E.g., CIC = 24

IAM (contains information CIC = 24)


Time switch

1. ISUP ACM message indicates free or busy user B

Switching system

LIC

LIC Group switch

Sign.

ETC

ETC

Control system

3. Charging starts when ISUP ANM message is 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 tone is locally generated

4. Call continues…


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 Signaling Transport (Sigtran) group of the IETF


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


Softswitch Architecture

Signaling(SS7)

Gateway

TrunkingGateway

CallAgent

SCP

ResidentialGateway

InternetSS7 Network

STP

TrunkingGatewayTrunking

GatewayCO

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 not exist in the IP network?

For transport layer, the ISUP message must be carried in the IP network with the same speed 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 not realize 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 sequence delivery), 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] 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 and reliability 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 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.


SCTP Functional View

SCTP User Application

Acknowledgement and Congestion Avoidance

Chunk Bundling

Packet Validation

Path Management

Association startup and takedown

Sequenced delivery within streams

User Data Fragmentation


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

Only the gaps need to be resent


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 …


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 Can be multiplexed; must be the first chunk


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

allocating resources


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 TSN ++ for each new 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 = x

Gap 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 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


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 primary address + several secondary addresses).

Multi-homes hosts SCTP ensures that endpoint is aware of the

reachability of another endpoint through the following 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 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


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 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


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

becoming unavailable


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 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 Management MSGs

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 The M3UA of the ASP

Create MTP-Pause indication


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 Mapped to the MTP-Status indication


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 One or more SS7 destinations are restricted

from The M3UA may use a different SG


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 INACTIVE ACK – ASP Inactive 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 To communicate the occurrence of certain

events


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


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


STATE REQUEST Sent from a CA to the SG to cause the SG to

perform some action on a signaling link Link alignment, or flushing transmit buffers

STATE CONFIRM STATE INDICATION

The SG can autonomously send During link changeover

The CA must retrieve certain information from the SG

DATA RETRIEVAL REQUEST DATA RETRIEVAL CONFIRM DATA RETRIEVAL INDICATION


M2PA Operation

IP-based SS7 links No FISUs sent; only LSSUs and MSUs Establish SCTP associations between

M2PA peers Two streams One for MSU The other for LSSU


Interworking SS7 with VoIP Arch

Interworking softswitch and SS7 At least two SGs Use SCTP as MEGACO Transport

An Internet draft Reliable and quick transport

Use SCTP to transport SIP message? Might not be easy No semi-permanent relationship


ISUP encapsulation in SIP When the softswitch network provides a

transit function Interworking often leads to a lowest

common denominator result Retry-after header ISUP -> SIP -> ISUP

Are the ISUP messages the same? The reliable delivery of provisional response


Interworking H.323 and SS7

The H.323 gateway Terminate SS7 links and voice trunks A close relationship exits between ISUP and

Q.931 IAM and Setup ANM and Connect

VoIP and SS7 Chapter 7. Internet Telephony 2 Basic functional parts of the PSTN PSTN Switching in...

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