[nsp] Real IP troughput over ATM

[Noriega, Alejandro]

> Hello folks, 
> 	I have been reading all sorts of documents about real IP
> troughput over ATM link but I didn't find anything useful. 
> The last week my company bought 130Mbps IP over an OC3 link. 
> I was testing the link, only with downstream traffic, and the link
> never reaches more than 125Mbps (that was the info that the 'input
> rate' of "show int ATM2/0" showed me. I'm not sure if that rate is IP
> or ATM.) 
> 
> OC3 = 155.520Mbps 
> ATM AAL5 = 1 cell = 53bytes (48 payload + 5 header) 
> 
> I am not considering the bytes cosumed by the AAL5 cell trailer: 
> Should the real IP troughput over an ATM link be ~
> 155.520/(53*8)*(48*8)=140.850 ?. 
> Well, if my calculation is right my link SHOULD have ~143.542 of
> downstream to get 130Mbps IP, right? (143.54/(53*8)*(48*8)=130) 
> 
> My questions are: 
> lima10#sh int atm2/0
> ATM2/0 is up, line protocol is up 
>   Hardware is ENHANCED ATM PA
>   Description: Conexion: TeCo Fibra oscura - ID: 5601631
>   MTU 4470 bytes, sub MTU 4470, BW 155000 Kbit, DLY 80 usec, 
>      reliability 255/255, txload 88/255, rxload 187/255
>   Encapsulation ATM, loopback not set
>   Encapsulation(s): AAL5 
>   4095 maximum active VCs, 1 current VCCs
>   VC Auto Creation Disabled.
>   VC idle disconnect time: 300 seconds
>   0 carrier transitions
>   Last input 00:00:01, output 00:00:00, output hang never
>   Last clearing of "show interface" counters 02:19:19
>   Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops:
> 0
>   Queueing strategy: Per VC Queueing
>   5 minute input rate 113886000 bits/sec, 27138 packets/sec <--- This
> info talks about packets so I assume it's IP traffic, right? 
>   5 minute output rate 53787000 bits/sec, 17195 packets/sec
>      224971412 packets input, 2850386068 bytes, 0 no buffer
>      Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
>      0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
>      138570460 packets output, 2615814337 bytes, 0 underruns
>      0 output errors, 0 collisions, 0 interface resets
>      0 output buffer failures, 0 output buffers swapped out
> 
> If the input bits/sec referrs to IP traffic, in order to get my real
> throughput over the link I should sum the ATM overhead. 
> So if the 'show int' shows me a peak of 125Mbps, my "real" traffic
> SHOULD be 125/(48*8)*(53*8)=138. 
> Anyone has a PA-A3-OC3SMI handling more traffic? 
> 
> Thanks in advance. 
> 
> Regards, 
> Alejandro.