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VoIP and SS7. Chapter 7. Basic functional parts of the PSTN. PSTN. Switching in exchanges. Transmission (PDH, SDH). Databases in the network (HLR). Subscriber signalling (analog or ISDN=DSS1). Network-internal signalling (SS7). PDH and SDH transmission bit rates.

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Voip and ss7

VoIP and SS7

Chapter 7


Basic functional parts of the PSTN

PSTN

Switching in exchanges

Transmission (PDH, SDH)

Databases in the network (HLR)

Subscriber signalling (analog or ISDN=DSS1)

Network-internal signalling (SS7)


PDH and SDH transmission bit rates

PDH (Plesiochronous Digital Hierarchy)

Japan

USA

Europe

J1 1.5 Mbit/s T1 1.5 Mbit/s E1 2 Mbit/s

J2 6 T2 6 E2 8

J3 32 T3 45 E3 34

J4 98 T4 140 E4 140

SONET (North Am.) SDH

STS-1 51.84 Mbit/s

STS-3 155.52 STM-1

STS-12 622.08 STM-4

STS-48 2.488 Gbit/s STM-16


Structure of E1 frame (2.048 Mbit/s)

0 1 2

16

31

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:

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

...

...

Time slot 16 usually contains SS7 signalling information.


Subscriber signalling

PSTN

Switching in exchanges

Transmission (PDH, SDH)

Databases in the network (HLR)

Subscriber signalling (analog or ISDN=DSS1)

Network-internal signalling (SS7)


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


Analog subscriber signalling in action

User A

LE A

LE B

User B

SS7 signalling (ISUP)

LE = local exchange

Off-hook

Dial tone

B number

Ringing signal

Ringback tone (or busy tone)

Off-hook (user B answers)

Connection established


ISDN subscriber signalling in action

User A

LE A

LE B

User B

SS7 signalling (ISUP)

Off-hook

DSS1 signalling messages

Setup

B number

Setup

Call proc

Ringing

Alert

Tones generated in terminal

Alert

Off-hook (user B answers)

Conn

Conn

Connection established


PSTN vs. ISDN user access

300 … 3400 Hz analog transmission band

“Poor-performance” subscriber signaling

PSTN

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

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

Basic 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.

Primary Rate Access ISDN


End-to-end digital signalling

User interface

PSTN Network

User interface

Q.931

Q.931

ISUP

ISUP

Q.931

Q.931

SS7

DSS1

MTP 3

MTP 3

DSS1

Q.921

Q.921

MTP 2

MTP 2

Q.921

Q.921

I.430

I.430

MTP 1

MTP 1

I.430

I.430

contains the signalling messages for call control


Introduction
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 is possible

Exchange

Exchange

Exchange

Signalling is not possible before previous circuit-switched link is established

Circuit switched connection

CAS has two serious draw-backs:

Setting up a circuit switched connection is very slow.

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


Common channel signalling (CCS)

In practice, CCS = SS7.

Signalling is possible anywhere anytime

Exchange

Exchange

Database

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

Signalling to/from databases is possible anytime.

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

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


The telephone network 1 2

SS7 Signaling

ISUP Messages

INAP/TCAP Messages

The Telephone Network [1/2]

Service

Control

Point

Service

Data

Point

+

Signal

Transfer

Point

Control Layer

Intelligent

Peripheral

Transport Layer

Class 4

Tandem Switch

Class 5

End Office Switch

Circuit Switched Network


The telephone network 2 2
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

SCP

SDP

TCAP messages

IP

STP

STP

SSP

SSP

ISUP messages

Voice


Signalling example

Tokyo

Oulu

Exch

User A (calling user)

User B (called user)

Exch

Exch

London

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


Ss7 protocol suite
SS7 Protocol Suite

OSI Layers

INAP

MAP

Application

Presentation

Session

ISUP

TCAP

SCCP

Transport

Network

MTP Level 3

MTP Level 2

Data Link

MTP Level 1

Physical


Mtp levels 1 2
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
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



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, 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
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
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
Signal Transfer Point (STP)

  • To transfer messages from one SPC to another


Service control point scp
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
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 msus1
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)



  • 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



F

CK

SIF

SIO

LI

Control

F

Same SPCs can be reused at different network levels

International

SPC = 277

SPC = 277

National

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

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)

Routing label (four octets)

CIC (two octets)

Must always be included in ISUP message

Message type (one octet)

Mandatory fixed part

E.g., IAM message

Mandatory variable part

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

Optional part


The isdn user part isup
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 labeldefines the signalling link which is used for transfer of the signalling information.

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

    Signalling link

    STP

    Exchange

    Exchange

    Circuit


    Isup call establishment and release
    ISUP Call Establishment and Release

    • A given circuit between two switches is identified by OPC, DPC and CIC.


    Signalling using IAM message

    STP

    STP

    SL 4

    SL 7

    Exchange

    SPC = 82

    Exchange

    SPC = 22

    Exchange

    SPC = 60

    Circuit 20

    Circuit 14

    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).

    Outgoing message:

    OPC = 82 CIC = 14

    DPC = 22 SLS = 4


    Setup of a call using ISUP

    User A

    LE A

    Transit exchange

    LE B

    User B

    Setup

    IAM

    IAM

    Setup

    DSS1 signalling assumed

    Number analysis

    Alert

    ACM

    ACM

    Alert

    Connect

    ANM

    ANM

    Connect

    Charging of call starts now



    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):

    STP with GTT capability

    Exchange

    STP

    Database

    1.

    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.

    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

    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.

    SCCP/GTT functionality

    STP

    SCCP

    SPC = 32

    SCCP

    MSC located in Espoo

    HLR located in Oslo

    SPC = 99

    SPC = 82

    Outgoing message:

    OPC = 82 DPC = 32

    SCCP: IMSI global title

    Processing in STP:

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


    To sum it up with an example…

    Part B, Section 3.3 in ”Understanding Telecommunications 2”

    PSTN

    Typical operation of a local exchange

    Transmission (PDH, SDH)

    Databases in the network (HLR)

    Subscriber signalling (analog or ISDN=DSS1)

    Network-internal signalling (SS7)


    Basic local exchange (LE) architecture

    Modern trend: Switching and control functions are separated into different network elements (separation of user and control plane).

    Switching system

    Subscriber stage

    TDM links to other network elements

    LIC

    Time switch

    ETC

    Group switch

    Tone

    Rx

    ETC

    LIC

    Tone generator

    Sign.

    Exchange terminal circuit

    Line interface circuit

    • Switch control

    • E.164 number analysis

    SS7 Signalling equipment

    • Charging

    • User databases

    • O&M functions

    Control system


    Setup of a call (1)

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

    Local exchange of user A

    Switching system

    4. Tone Rx is connected

    1. Off hook

    LIC

    Time switch

    ETC

    Group switch

    Tone

    Rx

    ETC

    LIC

    5. Dial tone is sent (indicating “network is alive”)

    Tone generator

    Sign.

    2. Check user database. For instance, is

    user A barred for outgoing calls?

    3. Reserve memory for user B number

    Control system


    Setup of a call (2)

    Phase 2. Exchange receives and analyzes user B number.

    Local exchange of user A

    Switching system

    LIC

    Time switch

    ETC

    Group switch

    Tone

    Rx

    ETC

    LIC

    2. Number (DTMF

    signal) received

    1. User A dials user B number

    Sign.

    3. Number analysis

    4. IN triggering actions? Should an external

    database (e.g. SCP, HLR) be contacted?

    Control system


    Setup of a call (3)

    Phase 3. Outgoing circuit is reserved. ISUP Initial address message (IAM) is sent to next exchange.

    Local exchange of user A

    Switching system

    E.g.,

    CIC = 24

    LIC

    Time switch

    ETC

    Group switch

    Tone

    Rx

    ETC

    LIC

    IAM (contains information CIC = 24)

    1. Tone receiver

    is disconnected

    Sign.

    2. Outgoing circuit is reserved

    3. Outgoing signalling message (ISUP IAM)

    contains user B number

    Control system


    Setup of a call (4)

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

    Local exchange of user A

    Switching system

    LIC

    Time switch

    ETC

    Group switch

    ETC

    LIC

    ACM, ANM

    2. Ringback or busy tone is locally generated

    4. Call continues…

    Tone generator

    Sign.

    1. ISUP ACM message indicates free or busy

    user B

    3. Charging starts when ISUP ANM message

    is received

    Control system


    Performance requirements for ss7
    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 ss71
    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
    Softswitch Architecture

    SS7 Network

    Internet

    Signaling

    (SS7)

    Gateway

    SCP

    MGCP/MEGACO

    Call

    Agent

    SIGTRAN

    STP

    MGCP/MEGACO

    Residential

    Gateway

    Trunking

    Gateway

    Residential

    Gateway

    Trunking

    Gateway

    CO

    Switch

    Residential

    Gateway

    Trunking

    Gateway

    RTP


    Signaling transport 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
    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
    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
    ISUP Transport to MGC

    • NIF (Nodal Interworking Function) is responsible for interworking between the SS7 and IP networks


    Sigtran protocol stack
    SIGTRAN Protocol Stack

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


    Adaptation layer 1 3
    Adaptation Layer [1/3]

    • M2UA (MTP-2 User Adaptation Layer)


    Adaptation layer 2 3
    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
    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
    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
    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
    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
    Multi-Homed Host

    Host A

    Host B

    SCTP User

    SCTP User

    SCTP

    SCTP

    One IP address

    One IP address

    One IP address

    One SCTP association with multi-homed redundant


    Sctp streams
    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 Functional View

    SCTP User Application

    Sequenced delivery

    within streams

    Association

    startup

    and

    takedown

    User Data Fragmentation

    Acknowledgement

    and

    Congestion Avoidance

    Chunk Bundling

    Packet Validation

    Path Management


    Sctp packets chunks

    0

    . . .

    15

    16

    . . .

    31

    Source Port Number

    Destination Port Number

    Common

    Header

    Verification Tag

    Checksum

    Chunk Type

    Chunk Flags

    Chunk Length

    Chunk 1

    Chunk Value

    . . .

    Chunk N

    SCTP Packets & Chunks

    • A SCTP packet can comprise several chunks.

    • Chunk

      • Data or control


    Chunk type
    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
    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

    0

    . . .

    15

    16

    . . .

    31

    Type = 1

    Chunk Flags

    Chunk Length

    Initial Tag

    Advertised Receiver Window Credit (a_rwnd)

    Number of Outbound Streams

    Number of Inbound Streams

    Initial TSN (Transmission Sequence Number)

    Optional / Variable-Length Parameter

    . . .

    INIT Chunk


    Association establishment
    Association Establishment

    INIT [I-Tag=Tag_A]

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

    Z

    A

    COOKIE [Cookie_Z]

    COOKIE ACK

    allocating resources


    User data transfer
    User Data Transfer

    User Messages

    SCTP user

    SCTP DATA Chunks

    SCTP Control Chunks

    SCTP packets

    SCTP

    Connectionless Packet Transfer Service (e.g. IP)


    Data chunk

    0

    . . .

    15

    16

    . . .

    31

    Type = 0

    Reserved

    U

    B

    E

    Chunk Length

    TSN

    Stream ID = S

    Stream Sequence Number = n

    Payload Protocol ID

    User Data (Sequence n of Stream S)

    . . .

    U : unordered

    B : begin

    E : end

    DATA Chunk


    • 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

    0

    . . .

    15

    16

    . . .

    31

    Type = 3

    Chunk Flags

    Chunk Length

    Cumulative TSN Ack

    Advertised Receiver Window Credit (a_rwnd)

    Number of Gap Ack Blocks = n

    Number of Duplicate TSNs = x

    Gap Ack Block #1 Start

    Gap Ack Block #1 End

    . . .

    Duplicate TSN #1

    . . .

    SACK Chunk


    • 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
    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 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
    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



    Services provided by m3ua
    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



    • 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
    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
    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
    Routing Key Management Messages

    • Registration Request (REG REQ)

      • An ASP = a DPC/OPC/CIC range

    • Registration Response

    • Deregistration Request

    • Deregistration Response


    M2ua operation
    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
    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 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
    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


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