ability. Signaling takes place in two key parts of the network: in the access network, where it's called loop signaling, and in the core, where it's called interoffice signaling (see Figure 4.10). With analog loop signaling, two types of starts exist: Ground start-Ground start means that when you seize a particular line, it is immediately grounded SO that no other call can potentially conflict with it. Ground start is used with a contentious system, perhaps a PBX at a cor- porate enterprise, to avoid collisions. For example, say you seize a trunk and place a call, and now you're in the ringing state. There are short periods of silence between ringing tones. The local exchange could mistake one of these periods of silence to mean that the trunk is available and try to send in a call over that same trunk over which you're trying to place a call out; Switch Switch Trunk Loop Signaling Interoffice Signaling Figure 4.10 Customer loop and interoffice signaling Signaling Systems this would cause a collision (referred to as glare). Consequently, when you're dealing with systems and contention for the resource, grounding the trunk up front is the most efficient procedure. Loop start-Pay telephones and residential phones use loop start, which means that the circuit is grounded when the connection is completed. There are various start standards for digital subscriber signaling, and they are defined in accordance with the service being provided. Interoffice signaling has evolved through several generations of signaling approaches. In the first generation, called per-trunk signaling, the complete path- all the way to the destination point-is set up in order to just carry the signaling information in the first place (see Figure 4.11). This method uses trunks very inef- ficiently; trunks may be put into place to carry 20 or 30 ringing tones, but if nobody is on the other end to take that call, the network trunk is being used but not generating any revenue. Also, when a call is initiated and begins to progress, you can no longer