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Telecom Signaling System 7 [SS7]
The standards [protocols] that control the routing of the call content are referred to as Telephony Signaling.
Signaling System Number 7 [SS7]SS7 or Signaling System Number 7 is a layered set of protocols describing communication between telephone switches in public telephone networks, mainly set by the International Telecommunications Union or ITU-T. It is used for ISDN and also outside ISDN. SS7 is used for interoffice signalling. The primary function of SS7 is to provide:
SS7 exchanges control messages between SS7 telephone exchanges [signalling points or SPs] and SS7 signalling transfer points [STPs]. The switching offices [SPs] handles both the SS7 control network and the user circuit-switched network, SS7 telling the switching office which paths to establish over the circuit-switched network. The STPs route SS7 control packets across the signalling network. NOTE: A switching office may or may not be an STP. 
In-BandIn-band signalling used the same physical path for both call-control signalling and the actual connected call. In-band signalling techniques are inefficient and rapidly being replaced by out-of-band or common-channel signalling techniques. It helps to understand SS7 by knowing a bit about how In-Bank was used in the Public Switched Telephone Network [PSTN]. A network utilizing common-channel signalling is two networks in one:
SS7 LayersPhysical Layer [MTP-1]Defines the physical and electrical characteristics of the signaling links of the SS7 network, using DS–0 channels. Raw signaling data is carried at a rate of 56 kbps or 64 kbps. Message Transfer Part—Level 2 [MTP-2]Level 2 portion of the message transfer part [MTP Level 2] provides link-layer functionality to ensure both end points of a signaling link can reliably exchange signaling messages. It includes capabilities such as error checking, flow control, and sequence checking. Message Transfer Part—Level 3 [MTP-3]Level 3 portion of the message transfer part [MTP Level 3] adds
to the functionality provided by MTP-2 by providing network layer
functionality to ensure that messages can be delivered between signaling
points across the SS7 network regardless of whether they are directly
connected. This includes node addressing, routing, alternate routing,
and congestion control. 
Signaling Connection Control Part [SCCP]The signaling connection control part [SCCP] provides two major functions lacking in the MTP: Capability to explicitly address applications within a signaling point - MTP can only receive and deliver messages from a node, not deal with software applications within a node. Separate applications [subsystems] within a node include:
ISDN User Part [ISUP]Messages and protocol used to establish and tear down of voice and data calls over the public switched network [PSN], and to manage the trunk network. ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes. Transaction Capabilities Application Part [TCAP]TCAP defines the messages and protocol used to communicate between applications [subsystems] in nodes. TCAP messages must be delivered to individual applications within the nodes they address, using the SCCP for transport. Operations, Maintenance, and Administration Part [OMAP]OMAP defines messages and protocol designed to support administration of the SS7 network. This includes procedures for validating network routing tables and for diagnosing link troubles. OMAP includes messages that use both the MTP and SCCP for routing.
Other SS7 Resourceswww.pt.com SS7 Tutorial [html] [pdf] Cisco SS7 Fundamentals[html] [pdf] Next: Telco Switches |
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