Cabletron Systems CSX1000, CSX1200 manual Allowed

USER’S GUIDE

--the rate at which data frames may be sent into the network without incurring congestion. This is generally accepted as the end-to-end available bandwidth at which frame relay service devices may enjoy sustained frame transmission. By definition this must be less than the throughput that the actual physical access link can support. However, for short periods of time, service devices may exceed this rate by defined values. This excess is known as the excess information rate and is defined as the bandwidth available above and beyond the committed rate. The reason this is possible is because statistically, not each PVC within the access will make use of its complete bandwidth allocation. Busy PVCs may essentially borrow bandwidth from underutilized PVCs. The Frame Relay software has the capability to transmit data above the committed information rate up to the excess information rate. Note that the sum of the committed and the excess information rates must not exceed the rate defined by the physical link. These rates are user-configurable options.

CONGESTION CONTROL OVERVIEW

Congestion occurs when traffic arriving at a resource, whether network or user equipment, exceeds that node’s capacity. Congestion notification in the device plane is used to inform the equipment (at the ingress point to the network) of the congestion, and allows the user equipment to initiate congestion avoidance procedures. The intent is to reduce the negative effects on both network and user equipment: the user equipment should take corrective action to reduce the congestion, or to notify the source that throughput has been exceeded. Congestion control is very important in providing reliable frame relay services. Congestion can be detected in two ways, implicitly and explicitly. Implicit indications are provided by lost frames whereas explicit congestion indications are provided for within the frame relay protocol.

CURRENT RESTRICTIONS

Currently, only PVC (Permanent Virtual Circuit) frame relay connections are implemented. Through configuration, PVC connections establish a permanent association between two DTEs.

The only types of facilities to be supported for frame relay access are channeled interfaces such as T1 and BRI. However, currently only 1 port per channeled interface is supported until SVC standards are available.

Frame relay supports only a single Permanent Virtual Circuit connecting any two given systems. To illustrate this point, the following diagram shows a frame relay network configuration that would be allowed:

DLCI 1 -> NE2

CSX1200

		Frame Relay
	DLCI 2 ->NE 3
"SITE1"	CSX1200	CSX1200

ALLOWED

"SITE2""SITE3"

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