| The traffic shaping over Frame Relay
feature applies to Frame Relay PVCs and SVCs.
You should be familiar with some
terminology related to Frame Relay traffic flow. Common Frame Relay
terms related to traffic flow follow:
- Local 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, regardless of other settings.
- CIR - The rate, in bits per
second, that the Frame Relay switch agrees to transfer data. The
rate is usually averaged over a period of time, referred to as
the committed rate measurement interval (Tc). In general, the
duration of Tc is proportional to the "burstiness" of
the traffic.
- Oversubscription - Oversubscription is when the sum of the
CIRs on all the VCs coming in to a device exceeds the access
line speed. Oversubscription can occur when the access line can
support the sum of CIRs purchased, but not of the CIRs plus the
bursting capacities of the VCs. If oversubscription occurs,
packets are dropped.
- Committed Burst (Bc) - The
maximum number of bits that the switch agrees to transfer during
any committed rate measurement interval (Tc). The higher the Bc-to-CIR
ratio, the longer the switch can handle a sustained burst. For
example, if the Tc is 2 seconds and the CIR is 32 Kbps, the Bc
is 64 Kbps. The Tc calculation is Tc = Bc/CIR.
Note: Tc is not a recurrent
time interval. It is used strictly to measure inbound data, during
which time it acts like a sliding window. Inbound data triggers
the Tc interval.
- Excess Burst (Be) - The
maximum number of uncommitted bits that the Frame Relay switch
attempts to transfer beyond the CIR. Be is dependent on the
service offerings available from your vendor, but it is
typically limited to the port speed of the local access loop.
- FECN - When a Frame Relay
switch recognizes congestion in the network, it sends a FECN
packet to the destination device, indicating that congestion has
occurred, as shown in Figure
.
- BECN - 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, as shown in Figure
.
With Cisco IOS Release 11.2 or later, Cisco routers can respond
to BECN notifications. This topic is discussed later in this
chapter.
- DE indicator - When the
router detects network congestion, it will mark the packet
"Discard Eligible." The DE bit is set on the
oversubscribed traffic-that is, the traffic that was received
after the CIR was met. These packets are normally delivered, but
in periods of congestion, the Frame Relay switch will drop
packets with the DE bit set first.
Note: These are generic
Frame Relay terms. They may be the same or slightly different from
the terms your Frame Relay service provider uses.
The CIR, by itself, does not provide
much flexibility when dealing with varying traffic rates. In
practice, the Frame Relay switch measures traffic over a time
interval that is specific to each logical connection, as you can see
in Figure .
Bc and Be are amounts of data that a
Frame Relay network agrees to transfer over a time interval, T. Be
is the maximum amount in excess of the Bc that the network attempts
to transfer under normal conditions. However, the traffic beyond the
Bc will be marked DE.
Notice in the Figure that the actual
frame transfer rate parallels the access rate; when a frame is being
transmitted on a channel, that channel is dedicated to that
transmission. The horizontal lines represent no frames being
transmitted. In general, the sum of CIRs for all logical connections
must be less than or equal to the physical line rate.
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