With IP telephony, each function from a PBX is replicated in some fashion on the IP network, through a combination of hardware, firmware, and software.

In a traditional PBX environment, as shown in Figure 2-1. a look inside the chassis/main cabinet of a PBX reveals that the PBX is a TDM switching environment. Each voice session is assigned a timeslot, or a piece of bandwidth, for the duration of the session. The telephones connect (through a cable plan) to a line card on one of the PBX shelves. This line card can either be a digital card or an analog card, depending upon the type of telephone. The line card provides the hardware connectivity between the telephone and the PBX switching environment.

Additionally, external trunks (links) to the public switched telephone network (PSTN) allow external connectivity for incoming and outgoing calls (i.e., calls not initiated and terminated on an internal extension). Just as with the line cards, the trunk cards can either be analog or digital. Digital cards usually are T-1, E-1, or ISDN links.

Figure 2-1 shows a time-division multiplexing (TDM) bus connecting the shelves within a cabinet. This TDM bus is the highway that allows information to travel within the PBX.

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