(ss7) signaling system 7
the signaling system #7 (ss7) is an international standard network signaling protocol that allows common channel (independent) signaling for calls establishment, billing, routing, and information-exchange between nodes in the public switched telephone network (pstn). ss7 system protocols are optimized for telephone system control connections and they are only directly accessible to telephone network operators.
common channel signaling (ccs) is a separate signaling system that separates content of telephone calls from the information used to set up the call (signaling information). when call-processing information is separated from the communication channel, it is called "out-of-band" signaling. this signaling method uses one of the channels on a multi-channel network for the control, accounting, and management of traffic on all of the channels of the network.
an ss7 network is composed of service switching points (ssps), signaling transfer points (stps), and service control points (scps). the ssp gathers the analog signaling information from the local line in the network (end point) and converts the information into an ss7 message. these messages are transferred into the ss7 network to stps that transfer the packet closer to its destination. when special processing of the message is required (such as rerouting a call to a call forwarding number), the stp routes the message to a scp. the scp is a data that can use the incoming message to determine other numbers and features that are associated with this particular call.
in the ss7 protocol, an address, such as customer-dialed digits, does not contain explicit information to enable routing in a signaling network. it then will require the signaling connection control part (sccp) translation function. this is a process in the ss7 system that uses a routing tables to convert an address (usually a telephone number) into the actual destination address (forwarding telephone number) or into the address of a service control point (data) that contains the customer data needed to process a call.
intelligence in the network can be distributed to datas and information processing points throughout the network because the network uses common channel signaling a set of service development tools has been developed to allow companies to offer advanced intelligent network (ain) services
below picture shows the basic structure of the ss7 control signaling system. shows that a customer's telephone is connected to a local switch end office (eo). the service switching point (ssp) is part of the eo and it converts dialed digits and other signaling indicators (e.g. off-hook answer) to ss7 signaling messages. the ss7 network routes the control packet to its destination using its own signal transfer point (stp) data packet switches using separate interconnection lines. in some cases, when additional services are provided, service control point (scp) datas are used to process requests for advanced telephone services. this picture also shows that the connections used for signaling are different than the voice connections. this diagram shows that there are multiple redundant links between switches, switching points, and network datas to help ensure the reliability of the telephone network. the links between points in the ss7 system have different functions and message structures. access links (a-links) are used for access control between eos and scps. bridge links (b-links), cross links (c-links), and diagonal links (d-links) interconnect stps. extended links (e-links) are optionally used to provide backup connections from an eo to the ss7 network. fully associated links (f-links) share (associate with) the connection between eos.
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