[c-nsp] Help Understanding Traffic Rates for different interfaces

[Brian McMahon]

On May 3, 2006, at 17:12, Thanh_Mai at 3com.com wrote:
> Multiple Part Question:
> 1) I know an Ethernet frame has a preamble of 8 bytes and a minimum  
> of 12
> bytes for the inter frame gap. With an FE line rate of 100Mbps and  
> 64 byte
> packets only, the maximum rate that is possible is
> 100*(64/(64+8+12))=76.19Mbps.
> Where can I find similar (maximum rate per packet size)  
> calculations for
> T1/T3/E1/E3/POS/ATM, etc? If you have this information, I'd really
> appreciate it if you pass it along. I can't find it anywhere.

The calculation is much simpler for synchronous point-to-point links  
(like T carriers).  Preamble and IFG are specific to Ethernet (and  
potentially other multiaccess networks like it).

> 2) If a T1 is 24 channels and 1.544Mbps and a T3 is 28 T1s, How is  
> the rate
> for a T3 44.736Mbps? It's obvious that you can't just simply  
> multiply 28 by
> 1.544. So what am I missing?

Framing.  Just like if you multiply the 24 DS-0 of a DS-1 (T-1) line,  
you don't get the familiar (well, on this side of the pond) one-point- 
five-four-four.  24 * 64kbps = 1.536 Mbps of data, and there's 8kpbs  
of framing and signalling overhead.

-- 
Brian McMahon <brian dot mcmahon at cabrillo dot edu>
Computer Networking and System Administration Instructor
Cabrillo College, Aptos, California