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WAN
Figure 79: Data-link connection identifiers over an interexchange carrier Frame Relay
network
This hypothetical implementation shows the Dallas corporate office connected to three branch
offices in a common star topology (or hub and spoke). Each office connects to a LEC CO over a
fractional T1 circuit, which terminates onto a Frame Relay port at the CO, and onto a Frame
Relay capable router at the customer premise. The port rates and the access circuit rates
match. PVCs are provisioned within the Frame Relay network between Dallas and each branch
office. The CIR of each PVC is sized so that it is half the respective port rate, which is a
common implementation. Each branch office is guaranteed its respective CIR, but it is also
allowed to burst up to the port rate without any guarantees.
The port rate at Dallas is not quite double the aggregate CIR, but it does not need to be,
because the expectation is that not all three branch offices will burst up to the maximum at the
same time. In an implementation like this, the service is probably negotiated through a single
vendor. But it is likely that Dallas and Houston are serviced by the same LEC, and that the
Frame Relay is intra-lata, even if it was negotiated through an IXC, such as AT&T, WorldCom,
or Sprint. The service between Dallas and the other two branch offices, however, is most likely
inter-lata.
A frame relay issue and alternatives
The obstacle in running IP Telephony over Frame Relay involves the treatment of traffic within
the CIR and outside of CIR, commonly termed the "burst range."
Figure 80: Committed information rate (burst range)
Issue 3.4.1 June 2005
305
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