Bearers and Signaling

ISUP allows the call control signaling to be separated from the circuit that carries the voice stream over interoffice trunks. The circuit that carries the voice portion of the call is known within the telephone industry by many different terms. Voice channel, voice circuit, trunk member, and bearer all refer to the digital time slot that transports the voice (fax, modem, or other voiceband data) part of a call. The term "voice circuit" can be somewhat ambiguous in this context because sometimes it is used to refer to the trunk span that is divided into time slots, or to an individual time slot on a span.

The signaling component of the call is, of course, transported over SS7 signaling links. This creates two independent paths for call information between nodes: the voice path and the signaling path. The signaling mode describes the signaling relation between the two paths. Following is a brief review of the associated and quasi-associated signaling modes as they relate to ISUP, which we discussed in earlier chapters.

If the signaling travels on a single linkset that originates and terminates at the same nodes as the bearer circuit, the signaling mode is associated. If the signaling travels over two or more linksets and at least one intermediate node, the signaling mode is quasi-associated. In Figure 8-1, part A shows quasi-associated signaling between SSP A and SSP B and between SSP B and SSP C. In part B of Figure 8-1, the same SSP nodes are shown using associated signaling. Notice that the signaling links in part B terminate at the same point as the trunks. Also, the signaling link is shown as a separate entity in part B to illustrate the signaling mode; however, it is typically just another time slot that is dedicated for signaling on a trunk span.

The signaling mode used for ISUP depends greatly on what SS7 network architecture is used. For example, North America uses hierarchical STPs for aggregation of signaling traffic. Therefore, most ISUP trunks are signaled using quasi-associated signaling. Using this mode, the signaling is routed through the STP before reaching the destination SSP. In contrast, while the U.K. uses quasi-associated signaling for some SSPs, they also heavily use associated signaling with directly connected signaling links between many SSPs.