Cabletron Systems CyberSWITCH CSX400 Installation Manual

Also see for CyberSWITCH CSX400: Setup guide Configuration guide User guide User guide User guide

Contents

Chapter 2: About the CSX4002-8CyberSWITCH CSX400 and CSX400-DC Installation GuideFrame Relay ProtocolFrame Relay can be defined as a “packet mode” service, organizing data into individuallyaddressed units known as “frames”. Frame Relay eliminates all Layer 3 processing. Only a fewLayer 2 functions are used, such as checking for a valid, error free frame, but not requestingretransmission if an error is found. Frame Relay uses a variable length framing structure, which,depending on user data, can range from a few to more than a thousand characters.A Frame Relay Network will often be depicted as a cloud, because the Frame Relay Network isnot a single physical connection between one endpoint and another. Frame Relay protocol is basedon the concept of Virtual Circuits (Vcs). Vcs are two-way, software defined data paths between twoports that take the place of private lines in the network. There are two types of Frame Relayconnections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).Permanent Virtual Circuits, or PVCs, are set up via a network management system, and initiallydefined as a connection between two sites, or endpoints. PVCs may be added as the demand arisesfor more bandwidth, alternate routing, or more sites. PVCs are fixed paths, not available ondemand, or on a call-by-call basis. Although the actual path through the network may change fromtime to time, such as when automatic rerouting takes place, the beginning and end of the circuitwill not change.Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signalingprotocol (Q.933). The network must quickly establish the connection, and allocate bandwidthbased on the user’s request.In a Frame Relay frame, user data packets are not changed in any way. A two byte header isappended to the frame. Contained in this header is a 10-bit number called the Data LinkConnection Identifier (DLCI). The DLCI is the “virtual circuit” number which corresponds to aparticular destination. The DLCI allows data coming into a Frame Relay switch to be sent acrossthe network using a three-step process: Check the integrity of the frame and discard it if it is inerror, look up the DLCI in a table and if not intended for this link, discard the frame. If the framepasses the previous tests, relay the frame toward its destination out the specific port specified in thetable.The ANSI standard defines a mechanism for the network to signal the existence of congestion,called Explicit Congestion Notification (ECN) bits. Frame Relay uses FECN (Forward ECN) andBECN (Backward ECN) bits to notify end user devices about network congestion. Although theFrame Relay protocol does not respond to congestion, some higher layer protocols for end userdevices may respond to ECNs by recognizing that delays have increased, or that frames have beendropped.