SS7 Signaling Review

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

• Transmission links require a protocol to convey information about a telephone call in order to place the call successfully from the originator to the destination.
• Earlier systems used many variations involving rotary dialling (i.e. dial pulses) or tone pulses to represent the called number.
• Over the years these systems were expanded to generate and detect tone pulses representing the status of the called number (i.e. subscriber free, busy, etc.) or the calling number for automatic number identification (ANI) for billing.
• The number of combinations and signal types are limited by the number of available tones in the signaling system. The call setup time is extended each time additional features are required. This results in loss of revenue per circuit.

New Signaling = SS7

Introduction

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)

• This is typically the requirement of the link (i.e. T1 or E1). The specification of the link applies to this layer.
• T1 uses any timeslot of 24 channel link.
• E1 typically uses timeslot 16 for the SS7 access.
• T1 and E1 systems use 64KB/s data transfer on the timeslot for SS7.

 

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

• The 64KB/s data from a timeslot in Layer 1 is processed here.
• This layer forms the protocol framing, alarming, error correction and detection and provides a smooth data path to the next higher layer

 

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

• The location of where to send messages and control of the link is performed here.
• Management of the user part (Layer 4) messages is performed.

User Part (Layer 4) (TUP, ISUP, etc.)

• This layer actually contains the information about the call
• Typical information consists of called number, calling number, class of call, routing specifics, types of lines, etc.

TUP = Telephone User Part

• TUP was an earlier implementation of SS7 and does not allow for data type applications (except in special applications).

ISUP = ISDN User Part

• ISUP is turning out to be requested more of often then other protocols.
• ISUP means Integrated Services User Part.
• ISUP supports both voice and data applications.
• ISUP provides a direct link to ISDN primary rate applications.
• Other user parts - There are other user parts for mobile cellular applications, data only and specific functions which are not addressed at this time.

More Resources on SS7

• SS7 Technology and Applications Overview
• SS7 Overview

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