Basic SS7

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Why SS7?

Public Telephone Networks(PTNs) typically are composed of a signaling and transport network. The signaling network controls the setup and message transfer of cells, as well as advanced features such as calling card and toll-free service. The signaling network controls the voice and data circuts in transport network. The primary signaling protocol in use today is signaling systems 7(SS7), which interconnects networks such as SSP, SCP

The SS7 architecture comprises three signaling points: services switching points(SSPs), signal transfer Points (STPs) and service control points (SCPs). Each signaling point is identified a unique point code, similar to how each host on an IP network has a unique IP address, Signaling points are connected via signaling links, typically 56 or 65 Kbps DS0 links. SSPs are typically central office switches with SS7 capabilities. PBX trunks, ISDN and POTS lines connectes to SSPs. SSP generate the appropriate SS7 messages to provide end-office functionality, such as call setup & database query initiation. All SSPs are connected to at least one matted pair of STPs.

STPs are SS7 network routers, directing signaling units to the destination signaling points. They are large, specialized devices that offen incorporate several racks. Messages to outside destinations must travel through dedicated gateways STPs.

SCPs provides access needed for advanced services.They directly connects to STPs. SCPs responds to queries with information nessasary to process calls, accessing database that maintain subscriber and call information, including all management services database, Number portability.

SS7 Protocol

The SS7 Protocol stak consist in several individual protocols, including Message Transfer Part(MTP) level1,2 and 3, Integrated services Digital Network User Part, Signaling Connection Control Part (SCCP), and the Transmission Capabilities Application Part (TCAP) working together, these protocols define the format for all SS7 messages, as well as the procedures by which messages are generated and processed.

Message Transfer Part – 1

This defines the physical and electrical characteristics of the signaling links of the SS7 networks. Signaling links utilize DS-0 channels and carry raw signaling data at a rate of 56Kbps or 64Kbps.

Message Transfer Part – Level 2

The level2 portion of the message transfer part provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling message. It incorporates such capabilities as error checking, flow control, and sequence checking.

Message Transfer Part – Level 3

The level 3 portion of the message transfer part extends the functionality provided by MTP level 2 provide network layer functionality. It ensures that messages can be delivered between signaling points across the SS7 network regardless of whether they are directly connected. It includes such capabilities as node addressing, routing, alternate routing, and congestion control.

Collectively, MTP levels 2 & 3 are referred to as the message transfer part(MTP).

Signaling Connection Control Part

The network level functions provided by the MTP are relatively minimal. The Signaling Connection Control Part (SCCP) augments the services of MTP to provide user parts with enriched connectionless or connection-oriented services. It is specified by CCITT recommendations Q.711-Q.714. The combination of MTP and SCCP is called the network service part.

SCCP provides services (similar to those of the OSI network layer) to the SS7 user parts. It serves as a means of using higher layer OSI protocols to communicate with SS7. Five classes of SCCP service are identified:

0. Basic connectionless class.
1. Sequenced connectionless class.
2. Basic connection-oriented class.
3. Flow control connection-oriented class.
4. Error recovery and flow control connection-oriented class.

Class 1 denotes the MTP service described earlier. The connection-oriented service is defined in terms of the same set of service primitives as presented for the OSI network layer (see Chapter 4) and will not be further discussed here. SCCP messages appear as the SIF field in the signal unit.

Routing Label  : The routing label in SIF as explained earlier.
Message Type : Denotes the type of SCCP message.
Mandatory Fixed Part : Denotes the mandatory message parameters.
Mandatory Variable Part:  Denotes additional parameters for variable messages.
Optional Part : Denotes optional parameters.

ISDN User Part (ISUP)

The ISDN User Part (ISUP) defines the SS7 signaling functions and messages
for supporting ISDN applications. ISUP provides a variety of services: control of
circuit-switched connections between subscribers, calling line identification, malicious
call identification, call waiting, call redirection, as well as others. ISUP either uses the
SCCP for end-to-end signaling transport between local exchanges or uses its own
internal service for this purpose. The structure of ISUP messages is very similar to
SCCP messages. The ISUP is defined by CCITT recommendations Q.761-Q.766.

Telephone User Part (TUP)

The Telephone User Part (TUP) defines the SS7 telephone signaling messages
and functions for national and international calls. It directly uses the services of the
MTP. The structure of a TUP message is dependent on the message type and,
because of its complexity, is beyond the scope of our discussion. The TUP is defines
by CCITT recommendations Q.721-Q.725.