Message Routing and Management Summary

In order to support SS7 signalling between two nodes, a signalling link must exist. This is usually (but not always) timeslot 16 of a G.704 2-Mbps link in Europe, often referred to as an E1. There are certain attributes that a signalling link must have in order for it to function, one of these being that it must be a member of a linkset, which is simply a set of signalling links. Signalling links normally connect exchanges and nodes, and are usually constructed in pairs for both diversity and security. While different manufacturers set their own limitations, there is a maximum permissible value of 16 links per linkset. Generally, most real world setups incorporate no more than 4 links per linkset.
In order to get a message from one node in the network to another, it needs to traverse a signalling link. The originator does not have to know which link in a linkset the signalling messages are using, as long as it goes via a link within a particular linkset. Therefore the route a message takes to a specific destination is determined by the linkset over which it is transmitted. This is referred to as the Signalling Route. Signalling messages can sometimes reach the destination by leaving on two or more linksets, which means multiple signalling routes. This is referred to as a routeset, and as long as this routeset remains in service then messages can be sent. The general rule is that there can be up to 16 linksets within a routeset.
The MTP (Message Transfer Part) layer transfers signalling messages on a point code basis. The signal unit for origin of a message will contain a DPC (Destination Point Code) and OPC (Originating Point Code). An STP (Signalling Transfer Point) in the network needs to look at the DPC to decide if the message is for itself, as the STP is a routing node it needs to make a routing decision. If the receiving node is a SCP (Signalling Control Point) or SSP (Service Switching Point) and the DPC does not tally then the signal unit is merely discarded and a log report will be generated.
A TFP (Transfer Prohibit) is sent to an adjacent node when a message cannot reach its final destination, which is normally due to the loss of a route. The TFP will contain the DPC for an affected node, and this information will help an originating node to search for alternative routing. Messages will ultimately be sent via the most direct route but this is not always possible. Following a route being reinstated, a Transfer Allowed Message is sent to reverse the process from the Transfer Prohibit.
A TFR (Transfer Restricted) message is generated by an STP to all adjacent nodes when a failure occurs between the access point and the STP itself. In the event of a long term failure, the STP will receive a changeover from the access point ensuring all traffic is diverted over an alternative route. If the signalling point had a TFR and is now available then a TRF is sent until the links in the route are working to full capacity. If traffic is to be routed via the restricted path then it is operator dependent as to how this is administered.