What is SS7 - ISUP?

All nodes in a network may have different functions but they all must have necessary "within node" features to communicate using SS7. A node with SS7 within node features is known as SP (Signaling Point). All SPs in the SS7 network must be connected dedicated signaling channels or links.

Possible SPs in a SS7 Network :

- Switching Offices

- OAM (Operational, Administrational and Maintenance) centers. Used in the management of SS7 network. Collecting statistics etc.

- SCP ( Service Control Point) is used to provide service to the network such as ANI, billing, 1-800 number translations etc. It has a associated SSP (Switching Service Point) which is a dedicated STP and it's role is to send queries to SCP and recive replies from SCP.

- STP (Signal Transfer Point) is used to concentrate signaling information from the switching office, switch messages in SS7 network, provide access to SCP.

The nodes in the SS7 networks are connected via SLs (Signaling Links). Sometimes they are connected with more than one link and this time it's called LS (Link Set). the types of links between the nodes of the SS7 network are below :

- A Link (Access Link) connect signaling end points such as offices, SCPs or STPs.

- B Link (Bridge Link) connects different mated STPs.

- C Link (Cross Link) connects members of a mated pair.

- D Link connects local STP pair to regional STP pair.

- E Link connects high traffic STP to an additional pair of STP.

- F Link enables load sharing between SS7 offices, give extra redundancy and reliability.

SS7 Node Architecture :

SS7 is developed in a layered architecture like OSI model. It provides flexibility because changes can be made to one level without affecting the others.

SS7 is a 4 Level Arthitecture :

3 of the 4 levels form MTP (Message Transfer Part) and the 4th level is the User Part.

MTP : To provide reliable transport and delivery for messages. Messages from the User and Application part of an SS7 node are passed to the MTP. MTP then packages and delivers them accross the network. When a message is received MTP passes it to the destination, when a failure occurs, it detects and responds them.

MTP corrosponds to the 3 OSI Layers . Physical, Data Link and Network layers

On the other hand, SCCP (Signaling Connection Control Part) is introduced to enhance the message carriying facilities of MTP. The combination of MTP and SCCP forms the SS7 Network Services Part. SCCP provides some network layer protocol functions.

The main users of SCCP are :

TCAP (Transcation Capabilities Application Part) to allow new applications to use SS7, communicating directly with SCCP services using nin-circuit related signalling.

ISUP (ISDN User Part) and TUP (Telephone User Part) . ISUP is the most widely used one.

MTP Layers :

MTP Layer 1 protocol is called signaling data link.

MTP Layer 2 is called signaling link and layer 2 packets are called signal units. The type of signal units are determined by it's LI (Length Indicator). There 3 tpes of signal units: MSU (Message Signal Unit), LSSU (Link Status Signal Unit), FISU (Fill in Signal Unit).

MTP also detects and corrects the errors in transmission. It uses 2 types of errr correction procedures. Basic Error Correction where the signal units without bit errors are acknowledged. The signal units with bit errors are rejected and bad SU and all subsequent SUs are retansmitted. Preventive Cyclic Retansmission, there are acknowledgement but no rejection. After the timer expires if there is no acknowledgement then signal unit is retansmitted.

Layer 2 also counts the errors and when count number reachs 64 then an indication is sent to upper layers for excessive error rates.

MTP Layer 3 defines the signaling network functions. It transports the messages between the signaling points of the network. It handles messages and does network management. Also handles maintenance of signaling services, restoration of signaling conditions.