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Careful planning and engineering of SS7 networks is crucial for efficient and effective operation of modern telecommunictions networks.

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SS7 Links

SS7 messages are carried by a physical medium referred to as a link. While there is some variation throughout the world, the traditional facility type utilized for an SS7 link is a 64 kbps circuit outside the US and a DS-0 circuit, with a load carrying capacity of 56 kbps within the US.

SS7 Nodes

A Service Switching Point (SSP) is a telecommunications switch that contains the control logic (software) necessary to send/receive SS7 messages to other nodes in the network. SSPs are able to send/receive IN messages (ANSI-41/WIN, INAP, and CAMEL) over the TCAP portion of the SS7 protocol.

Signal Transfer Points (STP) are provided in "mated pairs" and operate in what is called "load sharing mode", meaning that, at any given time, each STP should be processing 40% of the total signal-processing load. In the event of a STP and/or link(s) failure, the network is designed to changeover to the remaining STP so that it can process at 80% load.

Like STPs, Service Control Points (SCP) are (almost always) deployed in a mated pair configuration. This designed redundancy makes allowance for a back-up SCP should the other go out of service for some reason.

Planning and Engineering

SS7 networks are designed for extremely high reliability and to survive any sort of disaster. To accomplish this design goal, SS7 networks have built-in fault prevention and isolation technologies. In addition, SS7 network nodes and links are deployed in a geographically diverse manner for survivability in the event of a disaster or equipment/facility isolation event.

SS7 networks have various capacity-limiting elements including processing capacity, disc space, table size for translations, input/output capability, and other resources. All of these components must be considered in engineering a node. The most limiting capacity element at any given time must be used in network capacity planning.

Traffic engineering of SS7 networks involves defining calls/data use cases, defining SS7 message attributes, gathering data and/or estimating demand, computing capacity requirements, determining the amount of calls/sessions that may be handled based on capacity demands and assumptions, monitoring growth of traffic demands, and planning for capacity exhaust.

SS7 network nodes and links are engineered to allow for adequate capacity so that a failure in any one component will not prevent the remaining components from handling the entire remaining load. Therefore, STPs, SCPs, and links are often engineered at 40% capacity, allowing for the remaining node/links to carry 80% of the load (the other 20% is for traffic "peakedness") in the event of a node or link failure.

Ongoing planning and management of SS7 networks requires network monitoring, capture of usage information, and projection of SS7 signaling requirements based on anticipated introduction and usage of various features.

Planning of large SS7 networks also requires consideration of network topology and the size and routing of messages. For example, it does not always make the most sense to directly connect a SSP to the closest STP pair. The SSP may have most of its SS7 traffic (and/or data size of messages) with a SCP that is located at a distant side of the overall SS7 network. Therefore, it may be more economically sound to connect the SSP to a STP closer to the SCP pair in question, even if that connection would incur additional facilities costs, those costs may be off-set by reducing the capacity demand on the SS7 backbone.

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