From: Michelle T (mtruman@xxxxxxxxxxxxxxx)
Date: Wed Jan 10 2001 - 15:50:09 GMT-3
All, I went to a colleague for more clarification on this topic, as he is a
frame-relay wizard in
the truest sense. I asked about Tc and Tc as it relates to Bc and FRTS. I
thought I would share his
response with you, because it further clarified a few things about the
Committed Burst rate that I
did not understand.
Tc is a Frame Relay standards term and refers to the time constant for
Committed Bursts (Bc). Generally your committed information rate (CIR) is
described in terms of a rate in BPS and a maximum committed burst size in
bits (Bc). (Hence the relationship of CIR = Bc / T). These terms have been
adopted by Cisco and Nortel for the configuration of Frame Relay traffic
shaping.
For Frame Relay Traffic Shaping (in a customer's edge router)...
When configuring traffic shaping for a PVC, threre are two relevant
parameters.
- The rate for the shaper
For a given frame relay connection, the value would normally be set to the
smaller of the two ports on the connection. In cisco, the parameter that
sets this rate is refered to as 'cir'. (This parameter has no relationship
to the actual cir of the pvc.)
- The time constant for the shaper (Tc)
This dictates how often the shaper does its job.
For example, if CIR is 64 kbps and Tc is 1 second, then the shaper will run
once a second and allow 64,000 bits to be transmitted
If Tc is 1/8 second, then the shaper will run 8 times and second and allow
8,000 bits each time.
You configure Tc in a cisco indirectly with the Bc (committed burst)
setting. This configures the number of bytes the shaper will transmit each
time it runs.
So (for the example above), to get Tc of 1 second, set Bc to 8000 (bytes).
To get Tc of 1/8 second, set Bc to 1000 (bytes).
----------
Typically, you are using a protocol prioritization scheme in conjunction
with shaping. In these configs, each time the shaper 'runs', it chooses
data from the highest priority source. If high priority traffic arrive an
instant after the shaper runs, it must wait till the next instance before it
can be transmitted. For this reason, you generally want to configure Tc to
the smallest possible setting in order to minimize this waiting time for
priority traffic.
Some vendors (vendor delted here) counsel setting Tc to match your service
provider. This is not a good idea. Generally, you want to have a really
small Tc for the reasons mentioned above; but you want your service provider
to have a big Tc to minimize the likelihood of discards.
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