SS7 Basics, Second Edition
This guidebook provides an overview of SS7 technologies, how they are structured, and why they are important.
TABLE OF CONTENTS SS7 Basics, Second Edition
Chapter 3: Message Transfer Part (MTP) Level 2
The Message Transfer Part (MTP) Level 2, together with MTP Level 1, provides a signaling link for reliable transfer of signaling messages between two directly connected signaling points. Figure 3.1 shows the relationship between MTP Level 2 maps as compared to the OSI 7-Layer Model.
Figure 3.1: MTP Level 2 of the SS7 Layers
Message Formats
There are three types of signal unit (SU) message formats. They are differentiated by the value contained in the length indicator (LI) field. The signal unit field types include:
F (flag): The beginning and end of a signal unit are indicated by a unique 8-bit pattern, called the flag, which does not appear elsewhere in the signal unit. Measures are taken to ensure that the flag pattern is not imitated elsewhere in the SU. The flag pattern is 01111110.
CK (cyclic redundancy check): The CK is a 16-bit checksum transmitted with each signal unit. If the checksum does not match at the receiving signaling point, the SU is considered to have errors and is discarded.
SIF (signaling information field): This field contains the routing and signaling information of the message.
SIO (service information octet): This octet is made up of the service indicator and the sub-service field. The service indicator is used to associate the signaling message with a particular MTP user at a signaling point, for instance, the layers above the MTP level. The sub-service field contains the network indicator that is used to differentiate between national and international calls, or between...
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