Following are some terms that are used
in this chapter to discuss Frame Relay:
- Access rate -- The clock speed
(port speed) of the connection (local loop) to the Frame Relay
cloud. It is the rate at which data travels into or out of the
network.
- Data-link connection identifier (DLCI) -- As
shown in Figure
a DLCI is a number that identifies the end point in a Frame Relay
network. This number has significance only to the local network.
The Frame Relay switch maps the DLCIs between a pair of routers to
create a permanent virtual circuit.
- Local management interface (LMI) -- A
signaling standard between the customer premises equipment (CPE)
device and the Frame Relay switch that is responsible for managing
the connection and maintaining status between the devices. LMIs
can include support for a keepalive mechanism, which verifies that
data is flowing; a multicast mechanism, which can provide the
network server with its local DLCI; multicast addressing,
providing a few DLCIs to be used as multicast (multiple
destination) addresses and the ability to give DLCIs global (whole
Frame Relay network) significance, rather than just local
significance (DLCIs used only to the local switch); and a status
mechanism, which provides an ongoing status on the DLCIs known to
the switch. There are several LMI types, and routers need to be
told which LMI type is being used. Three types of LMIs are
supported: cisco, ansi, and q933a.
- Committed information rate (CIR) -- The
CIR is the guaranteed rate, in bits per second, that the service
provider commits to providing.
- Committed burst -- The maximum number
of bits that the switch agrees to transfer during a time interval.
(It is noted Bc)
- Excess burst -- The maximum number of
uncommitted bits that the Frame Relay switch attempts to transfer
beyond the CIR. Excess burst is dependent on the service offerings
available by the vendor, but is typically limited to the port
speed of the local access loop.
- Forward explicit congestion
notification (FECN) -- A bit set in a frame that notifies a DTE
that congestion avoidance procedures should be initiated by the
receiving device. When a Frame Relay switch recognizes
congestion in the network, it sends a FECN packet to the
destination device, indicating that congestion has occurred.
- Backward explicit congestion
notification (BECN) -- A bit set in a frame that notifies a DTE
that congestion avoidance procedures should be initiated by the
receiving device. As shown in Figure
when a Frame Relay switch recognizes
congestion in the network, it sends a BECN packet to the source
router, instructing the router to reduce the rate at which it is
sending packets. If the router receives any BECNs during the
current time interval, it decreases the transmit rate by 25%.
- Discard eligibility (DE)
indicator -- A set bit that indicates the frame may be discarded
in preference to other frames if congestion occurs. When the router detects network congestion, the Frame
Relay switch will drop packets with the DE bit set first. The DE
bit is set on the oversubscribed traffic (that is, the traffic
that was received after the CIR was met).
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