i am wondering if this is true
Bandwidth=384=>CIR=384*0.95=364.8=100% available bandwidth
on the LLQ we can configure 75% of the 364.8 and the 25% is for the default queue
priority percent 50
bandwidth percent 10
bandwidth percent 15
map-class frame-relay FRTS
frame-relay cir 364800
frame-relay mincir 182400
frame-relay bc 3648
frame-relay be 0
Is this right above
1) Why we only use 95% of the existing bandwidth
2)what drives the FRTS config, i mean the min CIR is half the CIR and the bc is 1% of the CIR: this is a very commun configuration !!!!
Ok - back to basics here about frame relay.
If you owned the full bandwidth of the link you could transmit at the actual line (access) rate. This would be a leased line.
Since bandwidth is (was) expensive, providers offered a Committed Information Rate (CIR) where they guarantee to transmit a certain number of bits every second. The CIR is always less than the actual access rate.
Frame relay traffic shaping (FRTS) allows you to use this CIR better. To ensure you don't send over the CIR, FRTS allows you to shape the traffic rate to the cponfigured CIR - this ensures you don't exceed the rate at which the provider will transport your data without droppoing it.
As an example, if you have a 256K access rate and a 128K CIR, you can reliably transmit traffic for exactly half a second over any one second period. This is what the minCIR value sets.
If you want to run the risk of sending above what you purchased off the provider, you can set a higher rate by configuring the CIR to be above your minCIR. The router will send at this rate until it gets some kind of notification (by BECN or FECN) , it will then drop back down to minCIR.
The Bc sets the shaping interval - in the example above we COULD send 256Kbits over a 1 second period, but the minCIR ensures we only send 128Kbits every second.
The question is should we send this 128K in the first 0.5 seconds, or should we break it into smaller intervals and shape the traffic over the full 1 second period. We still send 128K over the 1 second period, but we send it as smaller groups with small periods in between.
These smaller groups of traffic are set by the Bc. For voice , the best setting is a Bc of 1/100th of the minCIR. This sets the shaping interval Tc based on this formula -
so in this case Tc=1280/128000 = 0.01 seconds , or 10msec.
So we have broken the actual minCIR down to 100 seperate groupings of 1280 bytes, which will be sent in any 10msec period.
We still send 128Kbits of traffic over a 1 seconds period, but it gets sent as 100 seperate groups of 1280 bits.
Having 100 seperate periods to send the voice is perfect - a normal voice call sends 50 packets per second, so you can fit 50 voice packets and 50 data intervals - the traffic flow is very smooth, and overall jitter is minimised. Since you do not exceed the CIR the provider offers you, there is no risk of packet loss. The traffic shaping minimises overall jitter of the voice traffic and prevents packet loss affecting the voice quality.
Hope this explains things :-)