After reading basic concept of SS7, we can see the seven layer of SS7. The first three Layer SS7 has function to assure reliability which is known as Message Transfer Part. Here we have
- Signalling Data Link : Physical, Electrical and Procedural Characteristics.
- Signalling Link : Error Detection/Correction,Sequencing, Alignment, Link Synchronization and Flow control
- Signalling Network : Messages and Procedures for Network Management. Responsible for end-to-end routing
The other layer part is 4th layer for user application which has :
SCCP (Signaling Connection Control Part) , TCAP (Transaction Capabilities Application Part ), ISUP (ISDN User Part), TUP (Telephone User Part ). Now I concern to discuss about first three layer element : Signalling link, Signalling Linkset and Signalling Route which is very important when you setup new network connection.
The signaling link is a physical link connecting various Signalling Points ( SP ) and transmitting signaling messages among them. A number of parallel signaling links that directly interconnect two signaling points constitute a signaling Link Set. We can see in this picture about the relation :
Signalling-Linkset
We can conclude like this :
1) There is one and only one signaling linkset between two adjacent DSPs.
2) The basic unit composing the signaling linkset is the signaling link.
3) One signaling linkset can contain one or more signaling links.
A group of links within a linkset that have the same characteristics (data rate, terrestrial/satellite, and so on) are called a link group. Normally the links in a linkset have the same characteristics, so the term link group can be synonymous with linkset.
Signaling Route
The signaling route is a logical concept. It is the path the signaling message passes from the Originating Point Code (OPC) to the Destination Signalling Point (DSP).
To describe a signaling route, at least three basic elements are needed:
- Destination Point Code DPC : indicates the destination that the signaling route reaches finally;
- Signalling linkset : related to the adjacent DSP and indicates the start direction of this signaling route;
- Signaling link : indicate the specific path to the adjacent DSP.
Signalling-Route
There are two options for message transmission from the OPC D1 to the DSP D2:
D1 –> STP1 ->D2
D1 –> STP2 ->D2
All the preprovisioned routes to a particular SP destination are called the routeset
CIC : Circuit Identification Code
CIC is used for circuit identification between two SPs.
The CIC field only exists in the messages of circuit switching services such as Telephone User Part (TUP) and Integrated Services Digital Network User Part (ISUP). Its length is defined as 12bit. Therefore, there can be a maximum of 4096 circuits between two SPs. There is no CIC field in the network management message.
ANSI uses a larger CIC value of 14 bits, allowing for up to 16,384 circuits.
SLS: signaling Link Selection
ITU Network SLS is a value of one 4 bit (00~0F) used for routing of the SS7 message.
For the TUP and ISUP messages, its value is the lower 4 bits of the CIC value of the speech channel circuit.
ANSI networks use an 8-bit SLS code. The SLS code was originally 5 bits, but was later increased to 8 bits to provide better distribution across links
For the MTP message, its value is the signaling link code of the corresponding link.
SLC: signaling link code
Each link in one linkset must be numbered uniformly, it is called SLC.
the SLC values of the same link at both sides should be consistent
Signalling LinkSet-Route-SLC
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