Enterasys csx400 Manuel De Montage
Chapter 2: About the CSX400
2-8
CyberSWITCH CSX400 and CSX400-DC Installation Guide
Frame Relay Protocol
Frame Relay can be defined as a “packet mode” service, organizing data into individually
addressed units known as “frames”. Frame Relay eliminates all Layer 3 processing. Only a few
Layer 2 functions are used, such as checking for a valid, error free frame, but not requesting
retransmission 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.
addressed units known as “frames”. Frame Relay eliminates all Layer 3 processing. Only a few
Layer 2 functions are used, such as checking for a valid, error free frame, but not requesting
retransmission 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 is
not a single physical connection between one endpoint and another. Frame Relay protocol is based
on the concept of Virtual Circuits (Vcs). Vcs are two-way, software defined data paths between two
ports that take the place of private lines in the network. There are two types of Frame Relay
connections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).
not a single physical connection between one endpoint and another. Frame Relay protocol is based
on the concept of Virtual Circuits (Vcs). Vcs are two-way, software defined data paths between two
ports that take the place of private lines in the network. There are two types of Frame Relay
connections; Switched Virtual Circuits (SVCs), and Permanent Virtual Circuits (PVCs).
Permanent Virtual Circuits, or PVCs, are set up via a network management system, and initially
defined as a connection between two sites, or endpoints. PVCs may be added as the demand arises
for more bandwidth, alternate routing, or more sites. PVCs are fixed paths, not available on
demand, or on a call-by-call basis. Although the actual path through the network may change from
time to time, such as when automatic rerouting takes place, the beginning and end of the circuit
will not change.
defined as a connection between two sites, or endpoints. PVCs may be added as the demand arises
for more bandwidth, alternate routing, or more sites. PVCs are fixed paths, not available on
demand, or on a call-by-call basis. Although the actual path through the network may change from
time to time, such as when automatic rerouting takes place, the beginning and end of the circuit
will not change.
Switched Virtual Circuits, or SVCs, are available on a call-by-call basis using the SVC signaling
protocol (Q.933). The network must quickly establish the connection, and allocate bandwidth
based on the user’s request.
protocol (Q.933). The network must quickly establish the connection, and allocate bandwidth
based on the user’s request.
In a Frame Relay frame, user data packets are not changed in any way. A two byte header is
appended to the frame. Contained in this header is a 10-bit number called the Data Link
Connection Identifier (DLCI). The DLCI is the “virtual circuit” number which corresponds to a
particular destination. The DLCI allows data coming into a Frame Relay switch to be sent across
the network using a three-step process: Check the integrity of the frame and discard it if it is in
error, look up the DLCI in a table and if not intended for this link, discard the frame. If the frame
passes the previous tests, relay the frame toward its destination out the specific port specified in the
table.
appended to the frame. Contained in this header is a 10-bit number called the Data Link
Connection Identifier (DLCI). The DLCI is the “virtual circuit” number which corresponds to a
particular destination. The DLCI allows data coming into a Frame Relay switch to be sent across
the network using a three-step process: Check the integrity of the frame and discard it if it is in
error, look up the DLCI in a table and if not intended for this link, discard the frame. If the frame
passes the previous tests, relay the frame toward its destination out the specific port specified in the
table.
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) and
BECN (Backward ECN) bits to notify end user devices about network congestion. Although the
Frame Relay protocol does not respond to congestion, some higher layer protocols for end user
devices may respond to ECNs by recognizing that delays have increased, or that frames have been
dropped.
called Explicit Congestion Notification (ECN) bits. Frame Relay uses FECN (Forward ECN) and
BECN (Backward ECN) bits to notify end user devices about network congestion. Although the
Frame Relay protocol does not respond to congestion, some higher layer protocols for end user
devices may respond to ECNs by recognizing that delays have increased, or that frames have been
dropped.