Alcatel Carrier Internetworking Solutions Switch/Router 사용자 설명서

Page 29-8

Use of Frame Relay lines tends to be “bursty,” with heavy use at times and light use at others. 
During heavy periods of congestion, data may be discarded. However, Frame Relay uses 
several software-configurable parameters and techniques to control congestion and to avoid 
data loss on the network during these heavy periods. These software parameters are set on a 
VC-by-VC basis. This section describes these parameters.

The parameters in this section describe how the Frame 
Relay network handles congestion. The 

WSX

 supports 

these parameters, but they must match those used by 
your Frame Relay service provider.

The 

, which is also referred to as “VC Throughput,” is the mini-

mum bandwidth a virtual circuit will provide under normal circumstances. Frames transmit-
ted within the CIR are not tagged by the Frame Relay network as being eligible for discard. 
Frames transmitted above the CIR are tagged for discard, but they will normally only be 
discarded if the virtual circuit or network becomes congested. For example, if the CIR is 16 
Kbps and you have a 56 Kbps line, then this virtual circuit will always get at least 16 of the 
available 56 Kbps. The extra 40 Kbps (56-16=40) is normally available to this virtual circuit as 
long as it is not being used by other virtual circuits and depending on how you have config-
ured the 

 and 

, which are described below.

The CIR is normally a rate given by your service provider. Your service provider may not 
allow a CIR, in which case your CIR would be 0 (no committed data rate for the virtual 
circuit).

The 

 is the amount of data that the network will guarantee to transfer 

under normal conditions. The data may or may not be contiguous and is expressed in kilo-
bits. This number is related to your CIR. In fact, the CIR is Bc divided by Tc where Tc is the 
time interval used to express the CIR. If Tc is equal to 1 second (a typical value for Tc) and 
your Bc is 16 kilobits, then your CIR is equal to 16 Kbps. So in many cases the Committed 
Burst rate will be the same number as the CIR expressed as a quantity of data (kilobits) rather 
than a data rate (kilobits per second).

The 

 is the amount of data over-and-above the Committed Burst Size (Bc) 

that the network will transmit as long as excess bandwidth is available on the virtual circuit. 
The number is also expressed in kilobits. Data at this level is not guaranteed transfer. Any 
data exceeding the Committed Burst Size may be part of the Excess Burst Size. If there is no 
bandwidth available on the virtual circuit or if the network is congested, the first data to be 
dropped is part of this Excess Burst data.

The Excess Burst Size is related to the Committed Burst Size and the access rate of the Frame 
Relay line. The Excess Burst Size plus the Committed Burst Size should be less than or equal 
to the access rate of the Frame Relay line. So, if you have a 56 Kbps line and the Committed 
Burst size is 16 kilobits, then the Excess Burst Size could range from 0 to 40 kilobits.

By default all of these congestion control parameters are set to zero (0), meaning that conges-
tion control is disabled and data flows at the access rate for learned virtual circuits. Conges-
tion control is not enabled until you set one or more of these parameters to a non-zero 
number.