Frame Relay Virtual Circuit Parameters

Time to turn the tables and invest in a VC for a change. However, we're referring to Virtual Circuit parameters for Frame Relay... From Cisco Press' Network Consultants Handbook.

 By Cisco Press
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Network Consultants Handbook - Frame Relay
by Matthew Castelli

Network Consultants Handbook -- Click here to go to publisher's site

Frame Relay Virtual Circuit (VC) Parameters
Frame Relay VCs, both permanent (PVC) and switched (SVC), have three configurable parameters that must be agreed upon between each end node (origination and termination) and the Frame Relay network service provider.

These parameters are as follows:

  • CIR
  • DE
  • VC identifiers
    • DLCIs for PVCs
    • X.121/E.164 addresses for SVCs

Frame Relay CIR
The CIR is the amount of bandwidth that will be delivered as "best-effort" across the Frame Relay backbone network. Network providers typically have provisions in their tariffs guaranteeing delivery of CIR traffic at some percentage. For example, a tariff might state a guarantee such as guaranteeing delivery of 99.9% CIR marked traffic.

CIR is measured in bytes over a periodic interval of time, expressed as TC. BC is the committed burst rate across the VC for that period of time. Bc can be represented by the formula Bc = CIR 4 Tc.

Bc is the negotiated maximum amount of bits that a Frame Relay internetwork is committed to accept and transmit at the CIR. Excess of CIR is measured as Be. BE is the number of bits that a Frame Relay internetwork will attempt to transfer after Bc is accommodated and is marked as DE.

TC is the periodic interval of time over which BC and BE are measured. The TC interval counter starts when data begins to enter into the Frame Relay network, and ends when data is no longer entering the network. When a new data stream enters the network, the TC counter starts over.

Frame Relay PVCs are simplex connections. Each VC is configured with its own CIR, meaning an A -> B PVC could be configured for 64 kbps CIR, and a B -> A PVC could be configured with a 32 kbps CIR. It is up to the network designer/engineer to determine the proper amount of CIR required, generally based on user and application traffic.

Frame Relay Discard Eligibility (DE)
Frame Relay uses a bit in the frame header to indicate whether that frame can be discarded if congestion is encountered during transmission. The DE bit is part of the Frame Relay frame header's address field.

The DE bit can be set by the transmitting Frame Relay networking device to prioritize the frame because it has a lower priority than other outgoing frames. If the network becomes congested, frames with the DE bit marked will be discarded prior to frames that do not have the DE bit marked to relieve this congestion.

This article was originally published on Jan 22, 2002
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