[nsp] Real IP troughput over ATM
jay at net.ohio-state.edu
Fri Nov 7 10:01:12 EST 2003
Also in addition to the ATM overhead, there is a potential for loss do
to packet size. A 64 byte IP packet doesn't fit well into a 48 byte ATM
cell payload. This leaves a 32 byte PAD every two cells under AAL5.
This would be:
53 + 53 = 106 (Total bytes for two cells)
64 bytes (Total bytes in one packets)
64 / 106 = 60.3% of actual throughput
155 * .60 = 93.4 mbps
Of course this is with 100% 64 bytes packets. Unrealistic, however I
have averaged 30 - 40% 64 byte packets on an ATM link which is still
Noriega, Alejandro wrote:
>> 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
>>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 ~
>>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:
>> 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.
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Jay Young |Office of Information Technology
Network Engineer |The Ohio State University
Phone: (614) 292-7350 |320 West 8th
Fax: (614) 292-9525 |Columbus, OH 43201
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