SS7 PROTOCOL STACK
At each signaling point is some type of computer element that has a network card connecting the point to the network. These network cards are designed to operate in accordance with the SS7 protocol, which defines standards for communication between signaling points.
The SS7 protocol at signaling points is divided into several parts spread across seven "layers." These layers are collectively known as the SS7 protocol stack. The SS7 protocol stack was modeled after the seven-layer Open Systems Interconnection (OSI) model. It can be loosely mapped to the OSI model.
Only streams of digital bits (i.e., 1s and 0s) can be processed by the SS7 protocol. Therefore, it is often referred to as a "bit-oriented" protocol.
SS7 Layers
Dividing the protocol stack into layers and assigning specific responsibilities to specific parts is a main advantage in that changes can be quickly implemented. If a change in technology occurs, only one part of the stack will likely be affected. In this way, it is easier to capitalize on new technology.
MTP Level 1
Message Transfer Part Level 1 (MTP L1) is responsible for the physical, electrical, and functional characteristics of the digital signaling links that go out to the network. This encompasses network cards and cables used to interface with the SS7 network.
Standards used by MTP L1 for a signaling point's electrical interface may be one of the following
MTP Level 2
Message Transfer Part Level 2 (MTP L2) is responsible for guaranteeing the delivery of signaling messages between two signaling end points (i.e., SSPs or SCPs). It performs message error detection and correction functions.
It is advantageous to check messages when they first arrive to determine if they have been received without error; resources should never be wasted on corrupt messages. For this reason, MTP L2, which is located "low" in the stack, is given such a responsibility; problems can be detected and corrected early.
MTP Level 3
Message Transfer Part Level 3 (MTP L3) is a message identifier and distributor. MTP L3 is responsible for identifying a signaling message's type, and then distributing it up the stack to the correct user part (i.e., SCCP or ISUP).
Another responsibility of MTP L3 at an STP is to route an inbound message back out on the proper link to the proper point on the network.
MTP L3 is also the network manager. It performs functions that provide rules and procedures for opening and maintaining links, and also for placing them back into service after a link or point failure.
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