The Telephone Network [1/2]

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VoIP and SS7

Chapter 7



„ 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


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Class 5

End Office Switch

The Telephone Network [1/2]

Circuit Switched Network

Intelligent Peripheral

Signal Transfer Point

Service Control Point

Class 4

Tandem Switch

Service Data Point +

Transport Layer Control Layer SS7 Signaling

ISUP Messages

INAP/TCAP Messages


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





TCAP messages

ISUP messages Voice


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SS7 Protocol Suite



MTP Level 3 MTP Level 2 MTP Level 1

OSI Layers Application Presentation

Session Transport


Data Link Physical



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


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


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MTP Level 3


Signaling message handling

„ Providing message routing between signaling points in the SS7 network


Signaling network management

„ Rerouting traffic to other SS7 signaling links in the case of link failure, congestion or node failure

„ Load-sharing




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)


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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 (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


INAP (IN Application Part)


MAP (Mobile Application Part)


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SS7 Network Architecture

„ Figure 7-4 depicts a typical SS7 network arrangement.

„ This configuration serves several purposes.

„ 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).



„ Group of signaling links directly connecting two SPCs

„ For capability and security reasons


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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



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


ISUP Call Establishment and Release

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


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.


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Softswitch Architecture

Signaling (SS7) Gateway

Trunking Gateway

Call Agent SCP

Residential Gateway


SS7 Network


Trunking GatewayTrunking

Gateway CO


Residential GatewayResidential

Gateway MGCP/







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 ???




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


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SIGTRAN Architecture

„ Signaling over standard IP uses a common transport protocol that ensures reliable signaling delivery.

„ Error-free and in-sequence

„ Stream Control Transmission Protocol (SCTP)

„ An adaptation layer is used to support 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 inetworking between the SS7 and IP networks


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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)


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Adaptation Layer [2/3]

„ M2PA (MTP-2 Peer-to-Peer Adaptation Layer)

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

„ It is effectively an IP-based STP.

„ SG can processing higher-layer signaling functions, such as SCCP GTT.


Adaptation Layer [3/3]

„ 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)


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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



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,


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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



„ 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.)



„ A protocol relationship between SCTP endpoints.

Two SCTP endpoints MUST NOT have more than


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Multi-Homed Host

Host A


Host B

One IP address

One SCTP association with multi-homed redundant



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.


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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.



„ Data or control

Source Port Number Destination Port Number Verification Tag


Chunk Type Chunk Flags Chunk Length

Chunk Value

Common Header

Chunk 1

0 . . . 15 16 . . . 31


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Chunk Type

„ ID Value Chunk Type

„ --- ---

„ 0 - Payload Data (DATA)

„ 1 - Initiation (INIT)

„ 2 - Initiation Acknowledgement (INIT ACK)

„ 3 - Selective Acknowledgement (SACK)

„ 4 - Heartbeat Request (HEARTBEAT)

„ 5 - Heartbeat Acknowledgement (HEARTBEAT ACK)

„ 6 - Abort (ABORT)

„ 7 - Shutdown (SHUTDOWN)

„ 8 - Shutdown Acknowledgement (SHUTDOWN ACK)

„ 9 - Operation Error (ERROR)

„ 10 - State Cookie (COOKIE ECHO)

„ 11 - Cookie Acknowledgement (COOKIE ACK)

„ 12 - Reserved for Explicit Congestion Notification Echo (ECNE)

„ 13 - Reserved for Congestion Window Reduced (CWR)

„ 14 - Shutdown Complete (SHUTDOWN COMPLETE)

„ - Reserved for IETF


INIT Chunk

Advertised Receiver Window Credit (a_rwnd) Number of Outbound Streams

Type = 1 Chunk Flags Chunk Length

Initial TSN (Transmission Sequence Number)

. . .

0 . . . 15 16 . . . 31

Initial Flag

Number of Inbound Streams

Optional / Variable-Length Parameter


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Association Establishment


INIT [I-Tag=Tag_A]

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

COOKIE [Cookie_Z]


allocating resources


User Data Transfer

SCTP user

SCTP Control Chunks



User Messages

SCTP packets


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DATA Chunk

Stream ID = S

Type = 0 Reserved Chunk Length

Payload Protocol ID

. . .

0 . . . 15 16 . . . 31


Stream Sequence Number = n

User Data (Sequence n of Stream S) U B E

U : unordered B : begin E : end


SACK Chunk

Advertised Receiver Window Credit (a_rwnd) Number of Gap Ack Blocks = n

Type = 3 Chunk Flags Chunk Length

0 . . . 15 16 . . . 31

Cumulative TSN Ack

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

. . . . . .

Duplicate TSN #1


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SCTP Robustness

„ Robustness is a key characteristic of any carrier- grade network.

„ To handle a certain amount of failure in the network without a significant reduction in quality

„ INIT and INIT ACK chunks may optionally 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




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