It’s useful to understand from which CSI a network transport service comes. The CSI to which a transport service belongs affects the way in which that transport service is implemented. sider the case of VoIP. An integral part of VoIP is the concept of transmitting voice packets over the transport. But each CSI, each transport, and the VoIP transport service provided therein support voice packets differently. This is because each CSI operates with different underlying protocols.
A transport line on the PSTN CSI, for example, is a limited bandwidth, circuit- switched medium that can support telephone services. A T1 transport is a much higher bandwidth line out of the DS CSI that can support many trans- port service applications that use packet-switching protocols. Although VoIP works under both the PSTN and DS CSIs, the faster T1 transport on the DS CSI affects the efficiency with which VoIP operates. If you’re thinking about using VoIP or have concerns about the operation of your existing VoIP, evaluate your network transports and transport services, paying particular attention to which CSIs are involved. The following sections examine, in some detail, the various CSIs available, along with how VoIP works in relation to those CSIs.
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