Table of Contents:

Definition and Overview

1. What is Signaling?

2. What is Out-of-Band Signaling?

3. Signaling Network Architecture

4. The North American Signaling Architecture

5. Basic Signaling Architecture

6. SS7 Link Types

7. Basic Call Setup Example

8. Database Query Example

9. Layers of the SS7 Protocol

10. What Goes Over the Signaling Link

11. Addressing in the SS7 Network

12. Signal Unit Structure

13. What are the Functions of the Different Signaling Units?

14. Message Signal Unit Structure

Self-Test

Glossary

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Three types of SUs are defined in the SS7 protocol.

1. message signal units (MSUs)
2. link status signal units (LSSUs)
3. fill-in signal units (FISUs)

SUs are transmitted continuously in both directions on any link that is in service. A signaling point that does not have MSUs or LSSUs to send will send FISUs over the link. The FISUs perform the function suggested by their name; they fill up the signaling link until there is a need to send purposeful signaling. They also facilitate link transmission monitoring and the acknowledgment of other SUs.

All transmission on the signaling link is broken up into 8-bit bytes, referred to as octets. SUs on a link are delimited by a unique 8-bit pattern known as a flag. The flag is defined as the 8-bit pattern "01111110". Because of the possibility that data within an SU would contain this pattern, bit manipulation techniques are used to ensure that the pattern does not occur within the message as it is transmitted over the link. (The SU is reconstructed once it has been taken off the link, and any bit manipulation is reversed.) Thus, any occurrence of the flag on the link indicates the end of one SU and the beginning of another. While in theory two flags could be placed between SUs (one to mark the end of the current message and one to mark the start of the next message), in practice a single flag is used for both purposes.