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Frame Relay "Committed Information Rates"

Access to a frame relay network is generally available at "port speeds" of 64 kbps, up to 1.984 Mbps -- obviously related to ISDN line speeds. The port speed is the rate, in bps, of the point-to-point physical link between a user's premises and the frame relay service.
 
Internally, the frame relay network is engineered on the basis that not all nodes will continuously attempt to transmit at their full port speed all of the time. In fact, each port is only guaranteed reliable service at an agreed "Committed Information Rate" (CIR). This is typically less than half of the actual port speed in bps.

The charge for a frame relay PVC is mainly based on the agreed CIR, and to a lesser extent on the actual port speed. To minimise the cost, some providers even allow a CIR of 0 bps.
 
It is possible for a frame relay user to transmit up to the port speed -- in other words, continuously. However, the network is engineered so that reliable frame delivery becomes less and less probable as the average offered data rate rises above the CIR: ultimately, the network is permitted to drop frames.
 

Useful links

Telstra Bigpond Direct
NSFNET history
Ameritech's Chicago NAP
Google Links to Internet History pages
Another bit of NSFNET history
The aus.net.access newsgroup sometimes has interesting discussions about Internet access in Australia.
 

The tutorial for this lecture is Tutorial #14.
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