The SS7 protocol stack

The SS7 protocol differs somewhat from the OSI model.While the OSI moden consists of seven different layers, the SS7 standar uses only four level.The functions carried out by these four levels correspond with the OSI model's seven layer.Some of the functions called for in the OSI model have no purpose in the SS7 network and are therefore undifined

It should also be noted that the functions in the SS7 protocol have been refined over the years and tailored for the specific requirments of the SS7 network.For this reason,there are many discrepancies betwen the two protocol and their corresponding functions

Regardless of the differences, the SS7 protocol has proven to be a highly reliable packet switching protocol,providing all of the services and function required bt the telephone service providers.THis protocol continues to evolve as the network grows and the services provided by the telephone companies change

1.Level one -Physical level

it is virtually the same as that of the OSI model.THe OSI model does not specify any specific interface to be used, as that will always differ from network to network.In SS7,we can specify which interface will be used, since the Bellcore standard and the ANSI standard all call for one or two types or interface - the DS0A or V.35
THe DS0A interface is the most forward for the application with the V.35 acceptable as the second choice

2.Level two - data link level

it provides the SS7 network with error detection/correction and sequenced delivery of all SS7 message packets.A with OSI model,this level is concerned only with the transmission of data from one node to the next node in the network.It does not concerned it self with the final destination of the message.As the message travels from one node to node, each node examine the dialied digits (contained in level four) and use that information to determine the next route of the message
Level two is provided the inforamation by level three which determines message routing

3.Level three-network level

it rpovides three functions;routing,message discrimination and distributin.All three functions depend on the servicee of the level two.When a mesaage is received, it is passed by level two to level three for message discrimination
Message discrimination determines who the message is addressed to.If the mesage contains the local addess (of the receiving node), then the message is passed to message distribution.If the message isnot addressed to the local node,then it is passed to the message routing function.The message routing function reads the called and dialling party address int the message to determine which physical address to route to.The called and the calling party addresses can be considered logical addresses and the physical address the node address

4.Level four-user parts

it consists of several different protocols, all called user parts and applicaiton parts.For basic telephone call connection and disconnect, the Telephone User part (TUP) or ISDN user part protocol (ISUP) are used.TUP is used in Europe and other countries following ITU-TS standards, while ISUP is used primarily in Nort America