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SIGTRAN (SS7 over IP)

Common Channel Signaling System #7 ( SS7) transports SS7 messages over an SS7network (normally a DS0A connection) and then there is SIGTRAN. SIGTRAN is a set of

protocols defined to transport SS7 messages over IP networks. SIGTRAN allows IP networks

to inter-work with the Public Switched Telephone Network (PSTN) and vice versa.

Notes: There are two other IETF standards which define transport of PSTN signaling (non-

SS7) over IP networks: ISDN User Adaption (IUA) which provides adaption at the the Q.921

layer to allow transparent transport of Q.931 messages to, say, a VoIP gateway. IUA is

defined by RFC4233. V5.2 User Adaption (V5UA) which provides for the transport of ETSI

standard V5.2 messages in an IP network by adaption at the LAPV5 level. V5UA is defined

by RFC3807.

Contents

SIGTRAN Stack

SCTP Overview

M2PA Adaption Layer

M2UA Adaption Layer

M2UA vs M2PA Adaption

M3UA Adaption Layer

SUA Adaption Layer

SS7 & SIGTRAN Glossary

The SIGTRAN Protocol Stack, RFC 2719

The SIGTRAN protocol stack consists of 3 components:

A standard IP layer.

A common signaling transport protocol, Stream Control Transmission Protocol

(SCTP): SCTP provides connection oriented reliable transfer of user messagesbetween peer SCTP users (Adaptation layer protocols). Note: The SCTP layer

replaces a normal TCP/UDP layer.

An Adaptation layer: Protocols defined for this layer are M2PA, M2UA, M3UA, and

SUA.


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Figure 1: SIGTRAN Protocol Stack Model

SCTP (Stream Control Transmission Protocol)

Defined by RFC 3309 and RFC 4166. SCTP is designed to transport SS7 signaling messages

over IP networks. It operates directly on top of IP at the same level as TCP. SCTP's basic

service is connection oriented reliable transfer of messages between peer SCTP users. SCTP

offers the following services (not exhaustive) that may be used by SIGTRAN:

Multi-homing: is the ability of an association (that is, a connection) to support

multiple IP addresses or interfaces at a given end point. In case of network failures,

use of more than one address can allow re-routing of packets, and also provide an

alternate path for retransmissions. End-points exchange lists of addresses during

initiation of the connection. One address is designated as the primary address to

receive data. A single IP port number is used across the entire address list at an

endpoint for a specific session.

Multi-streaming (Chunking): Allows for multiple virtual connections on the same

physical line. Each user application might be assigned its own stream (virtual

connection). SCTP's multi-streaming allows data to be delivered in multiple,

independent streams, so that if there is data loss in one stream, delivery will not be

affected for the other streams. The SCTP user can specify at startup time the numberof streams to be supported by the association.

Sequenced Delivery: Allows for messages to be delivered in the order in which there

were sent.

Block-Level: Unlike TCP in which transmission is viewed as a continuous stream of

data, SCTP transmits blocks.


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Self-Fragmentation: SCTP uses MTU path discovery to ensure that messages are not

fragmented by intermediate nodes.

Heart-Beat: SCTP uses periodic heart-beat messages to confirm the status of each

end point.

Adaptation Layer / SCTP Boundary

Primitives for this interface are defined in the RFC 2960, Section 10.

M2PA (MTP2 Peer-to-Peer Adaptation Layer)

Defined by RFC 4165. MP2A is a protocol which supports the transport of SS7 MTP3

signaling messages over IP using the services of the SCTP. M2PA operates similarly to MTP2

so as to provide peer-to-peer communication between SS7 endpoints. M2PA extends the

SS7 network into the IP network in a manner transparent to the SS7 network. Because the

MTP3 layer (which routes on point codes) is present in all Signaling Points each IPSP

requires a point code.

Figure 1 - M2PA Adaption Layer

SEP = Signaling End Point ( for example, an SSP)

IPSP = IP Signaling Point

SG = Signaling Gateway


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Notes:

1. MTP3, which is preserved in every IPSP with M2PA, uses Point Codes as its address

mechanism and thus all IPSP require a Point Code.

2. The term UL (Upper Layer) has been used to indicate any layer which runs over

TCAP, for example, MAP.

3. With M2PA the SG may act as an STP simply routing messages (based on Point

Codes) from the SS7 network to the IPSP (IP Signaling Point).

4. In essense M2PA adaption allows SS7 functionality to be located within an IP

network.

5. The Signaling Gateway (SG) will be a fully function SS7 SP and therefore can have

any SS7 functionality such as SCP, STP or SSP.

6. If ISUP is supported by any IPSP then it would be acting as a SSP and have a VoIP

stack, such as SIP, which is not shown to keep the diagram simple.

7. Other STPs may be present in the path between the SEP (SSP) and the SG.

8. M2PA allows for one or more IPSP in the network. Each IPSP must have its own point

code.

9. M2PA can transport any MTP3 user messages, for example, SCCP and ISUP).

10.M2PA supports all primitives between MTP3 and MTP2 and is essentially transparent

to MTP3.

11.M2PA provides MTP2 functionality that is not provided by SCTP, so that together

M2PA and SCTP provide functionality similar to that of MTP2.

Definition of the M2PA / MTP3 boundary

The primitives are described in Q.703, Q.704, T1.111, and Q.2140. These are the same

primitives that define the MTP2/MTP3 boundary.

M2PA receives the primitives sent from MTP3 to its lower layer. M2PA processes these

primitives or maps them to appropriate primitives at the M2PA/SCTP interface. Likewise,

M2PA sends primitives to MTP3 like those used in the MTP3/MTP2 interface.

M2UA (MTP2 User Adaptation Layer)

Defined by RFC 3331. M2UA is a protocol for the backhauling of SS7 MTP3 messages over IP

using the services of SCTP. This protocol is used between a Signaling Gateway (SG) and a

Media Gateway Controller (MGC) such as a VoIP gateway but it could equally well be SCP.


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Figure 2 - M2UA Adaption Layer

SEP = Signaling End Point (An SSP)

SG = Signaling Gateway

NIF = Nodal Inter-working Function

IPSP = IP Signaling Point

MGC = Media Gateway Controller: A device that converts from one protocol to another.

Notes:

1. With M2UA the SG functions as a transparent relay that converts SS7 messages to IP

format.

2. All SPs in the SS7 domain have Point codes. Only the MGC (IPSP) in the IP domain

has a Point Code (it has an MTP3 layer). The SG does not have a Point Code (since it

has no MTP3 Layer).

3. Other STPs may be present in the SS7 path between the SEP and the SG.

4. The SG cannot have upper ISUP/SCCP layers because it has no MTP3 layer

5. The Nodal Inter-working Function (NIF) simply accepts messages from the MTP2

layer and writes them to the M2Ua layer. It is a 'glue' layer and its functionality is

not defined by the standards.6. With M2UA the IPSP's MPT3 uses the SG's MPT2 as its lower SS7 layer. Both MTP2

on the SG and MTP3 on IP signaling point remain unaware of the fact that their

upper and lower layer respectively is not running locally.

7. The whole IP network is represented by a single point code that addresses the MGC

at the network edge. Therefore all messages from the SS7 side going to the IP side

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