User`s guide

124 XSR User’s Guide
Controlling Congestion in Frame Relay Networks Chapter 7
Configuring Frame Relay
CIR is the minimum rate of service that a public Frame Relay provider
guarantees for a given PVC under normal conditions. Frame Relay provides
the ability to burst beyond the CIR if bandwidth is available.
You can transmit traffic at a rate exceeding the CIR using Excess Information
Rate (EIR), but excess traffic might be discarded in the event of congestion.
Traffic shaping prevents traffic from being sent in excess of a value such as
CIR, which considerably reduces the likelihood of network congestion.
Without this feature, one VC could use all the access-link bandwidth before
Frame Relay congestion techniques even begin.
Several other parameters work hand-in-hand with CIR in controlling traffic
flow. Committed burst (Bc) is the maximum number of bits that the network
attempts to deliver during a given period.
Bc differs from CIR - it is a number, not a rate. CIR is equal to the committed
burst divided by time interval Tc, expressed in the formula: CIR = Bc/Tc. The
frame-relay bc command sets outgoing committed burst size.
Excess burst (Be) is the maximum number of bits that you may send in excess
of Bc. Sent on a best-effort basis, these bits will likely be discarded during
congestion. The
frame-relay be command sets outgoing excess burst size.
Another method of traffic shaping is the use of queues to limit surges that can
congest a network. Data is buffered and then sent to the network in regulated
amounts to ensure that traffic will fit within the promised traffic envelope for
the particular connection. Traffic shaping is also known as metering, shaping,
and smoothing.
Forward Explicit Congestion Notification (FECN)
Forward Explicit Congestion Notification (FECN) sets a bit to inform the DTE
device receiving the frame that congestion was experienced in the path from
source to destination. A DTE device receiving frames with the FECN bit set
can request that higher-level protocols take flow-control action as
appropriate.
Receiving a frame with the FECN bit set indicates that the received frame
experienced congestion en route, and that a method to slow down the peer
shall be used. The XSR does not act upon receiving a frame with the FECN bit
set.