Cabletron Systems CyberSWITCH CSX150 User Manual

Also see for CyberSWITCH CSX150: User guide Quick start guide

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Small Office Remote Access Switch 197CONFIGURING A LTERNATE ACCESSESFrame Relay Accesses-- the rate at which data frames may be sent into the network without incurring congestion. This isgenerally accepted as the end-to-end available bandwidth at which frame relay service devices mayenjoy sustained frame transmission. By definition this must be less than the throughput that theactual physical access link can support. However, for short periods of time, service devices mayexceed this rate by defined values. This excess is known as the excess information rate and isdefined as the bandwidth available above and beyond the committed rate. The reason this ispossible is because statistically, not each PVC within the access will make use of its completebandwidth allocation. Busy PVCs may essentially borrow bandwidth from underutilized PVCs.The Frame Relay software has the capability to transmit data above the committed information rateup to the excess information rate. Note that the sum of the committed and the excess informationrates must not exceed the rate defined by the physical link. These rates are user-configurableoptions.C ONGESTION C ONTROL O VERVIEWCongestion occurs when traffic arriving at a resource, whether network or user equipment, exceedsthat 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 initiatecongestion avoidance procedures. The intent is to reduce the negative effects on both network anduser equipment: the user equipment should take corrective action to reduce the congestion, or tonotify the source that throughput has been exceeded. Congestion control is very important inproviding reliable frame relay services. Congestion can be detected in two ways, implicitly andexplicitly. Implicit indications are provided by lost frames whereas explicit congestion indicationsare provided for within the frame relay protocol.C URRENT R ESTRICTIONSCurrently, 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 asT1/PRI and BRI. However, currently only 1 port per channeled interface is supported until SVCstandards 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 thatwould be allowed:"SITE1""SITE2" "SITE3"DLCI 1 -> NE2DLCI 2 ->NE 3ALLOWEDFrame RelayCSX150CSX150 CSX150