Components of SS7

Signal Switching Points (SSP’s). SSP’s are telephone switches (end offices or tandems) equipped with SS7 capable software and terminating signalling links. An SSP sends signalling message to other SSP’s to setup, manage, and release voice circuits required to complete a call. An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call. Sometimes SSP also referred  to as service switching point.

Signal Transfer Point (STPS). STPS are the packet switches of the SS7 network. An STP routes each incoming message to an outgoing signalling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signalling points.  An STP may perform global title translation, a procedure by which the destination signalling point is determined from digit present in the signalling message. An STP can also act as a ‘‘fire wall’’ to screen SS7 messages exchanged with other networks.

Signal Control Points (SCP’s). SCP’s are databases that provide information necessary for advanced call processing capabilities. An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number. An alternate routing number may be used by the SSP if the prime number is busy or the call is unanswered within a specified time.

SCP’s and STP’s are generally deployed in mated pair configurations in separate physical locations to ensure network wide service in the event of isolated failure.

SS7 signalling link types. Signalling links are logically organized by link type (A through F)
according to their use in the SS7 signalling network. SS7 signalling links are characterized
according to their use in the signalling network. Virtually all links are identical in that they are 56 kbps or 64 kbps bidirectional data links that support the same lower layers of the protocol

A links. ‘A’ links interconnect an STP and either an SSP or SCP which are collectively referred to as signalling end points. ’A’ stands for access, because only messages originating from or destined to the signalling end point are transmitted on an ‘A’ link. Examples of ‘A’ links are 1–9, 3–10, 3–4, 3–8, 4–7, 5–9, 7–8 and 7–10 in Fig. 7.13.
Signalling that SSP or SCP switches to send to any other node is sent on either of it’s a links to its home STP, which in turn, processes or routes the messages. Examples are 4–7–10 (or) 8–7–10 in Fig. 7.13. Similarly, message intended for an SSP or SCP will be routed to one of its home STP which will forward them to the addressed node over its ‘A’ links.