is is called nonassociated mode of signaling. SS No. 7 supports only a special case of this routing, called quasiassociated mode, in which routing is static except for relatively infre- quent changes in response to events such as link failures or addition of new SPs. SS No. 7 does not include sufficient procedures to maintain in-sequence delivery of information if routing were to change completely on a packet-by-packet basis. The function of relaying messages at the network layer is called the signaling transfer point (STP) function.2 Although this practice results in some confusion, the logical and physical network nodes at which this function is performed are frequently called STPs, even though they may provide other functions as well. An important part in designing an SS No. 7 network is including sufficient equipment redundancy and physical-route diversity SO that the stringent availability objectives of the system are met. The design is largely a matter of locating signaling links and SPs with the STP function, SO that performance objectives can be met for the projected traffic loads at minimum cost. Figure 14.1 is an SS No. 7 network structure model. The STP function is concentrated in a relatively small number of nodes that are essentially dedicated to that function. The Signaling Link Signaling Point with STP Function Signaling Point without STP function (e.g., switch, database, ...) Figure 14.1 Signaling system No. 7 network structure model. 2Bellcore (Ref. 6) reports that "purists restrict this further to MTP relaying." (MTP stands for message trans- fer part.) 14.3 SS NO. 7 RELATIONSHIP TO OSI STPs are paired or mated, and pairs of STPs are interconnected with a quad configuration as shown in the figure. We could also say that the four STPs are connected in mesh. has proved to be an extremely reliable and survivable backbone network. Other nodes, such as switching centers and service control points (SCPs), are typically homed on one of the mated pairs of STPs, with one or more links to each of the mate