Voice Over IP

THE SS7 LAYERS

Figure B-4 provides a more detailed description of the SS7 layers, and will serve as an introduction to subsequent material on these levels. MTP level 1 performs the functions of a traditional OSI physical layer. It generates and receives the signals on the physical channel.

MTP level 2 relates closely to the OSI layer 2. It is a conventional data link level and is responsible for the delivery of traffic on each link between SS7 nodes. The traffic in the upper layers of SS7 are encapsulated into MTP 2 "signal units" (this term is used instead of the conventional HDLC "frame"), and sent onto the channel to the receiving node. This node checks for errors that may have occurred during transmission and takes remedial action (discussed ...