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SS7 Signaling Review

Old Signaling (i.e. R2, C5 etc.)

New Signaling = SS7


In response to more features and integrated data services, CCITT developed the Signaling System 7 specification.

Signaling System 7 is a common channel signaling system. This means one channel (either analog and almost always digital) is used only for sending the signaling information whether the system has one bearer channel or multiple bearer channels.

In order to support this architecture, a new protocol was developed and is a variation of data packet switching. This means the signaling channel uses framing words, checksums , packets for address and information packets. The order of these packets are well defined and flexible for the user requirements.

Signaling System 7 on T1 circuits are typically split into what is called the A link and the B link. This provides link redundancy in case of equipment failure and link failure whereby the entire network does not fail. Normally, systems do not even include voice channels on the signaling links. (They normally put them on separate T1 circuits which are called "bearer" channels and they may be fixed mapped or connected through a digital crossconnect system (DACCS).

One timeslot on the signaling T1 link is used for transmission of signaling system 7. The applications have the flexibility to define any of the 24 timeslots as a signaling channel so the converter equipment must also have this flexibility.

There appears to be two modes of operation for the signaling channel. They are redundant load share and redundant standby.

Redundant load share splits traffic between the A and B signaling links.

Redundant standby links use a method where the signaling receivers receive the same information off the active link and update the memory of the standby link but the standby link is prohibited from transmitting on the link until the active link fails. (A protocol function in the SS7 determines link failure).

SS7 Structure

There are many "buzz" words thrown out about SS7, (i.e. TUP,ISUP,TCAP, etc.)

These are a simple description of the type of protocol used for the user functions but they omit the complete nature of the protocol which is as follows:

Physical Layer (1)

Message Transfer Part (MTP) Layer (2) (Protocol Framing etc.)

Message Transfer Part (MTP) Layer (3) (Message Transfer Protocol functions)

TUP = Telephone User Part

ISUP = ISDN User Part

More Resources on SS7

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2004 Goodman's