# [nsp] Real IP troughput over ATM

Jay Young 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
significant.

Jay

Noriega, Alejandro wrote:
>>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,
>>  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
>>     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
>>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?
>>
>>
>>Regards,
>>Alejandro.
>
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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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