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IP over ATM overhead


Anybody know why when i using below traffic descriptor in DSLAM the cpe (ADSL) can use just 484 byte/sec.

Service = CBR

PCR = 302 (302cells)

is this true that we must have 302 * 48 byte or 302 * 53 byte rate per secend from cpe?

how much overhead on ip over atm?

i would be grateful if you advice me.


M.Arshad Rad


Re: IP over ATM overhead

My understanding is that you calculate everything based on 53 byte cells, since that is the size of the cell as it would be switched.

I'm also not entirely sure about this, but if you declare traffic as CBR, the the other values (sustained, peak, and burst cell rates) don't really come into play - you are asking that a CBR connection automatically get that much bandwidth carved out of the overall available bandwidth.

Overhead should be 5 bytes regardless of the protocol. The additional protocol overhead for IP (or IPX or whatever) is already in the packet and would be constant from end-to-end.

Generally speaking, try to limit routers in the path as much as possible. Each time a packet passes through a router, the cells have to be re-assembled back into a packet, the packet has to be evaluated, routing decisions must be made, and the packet has to re-segmented and put out. The double SAR (segmentation and re-assembly) can be a pretty fair performance hit in latency and throughput ... depending on the routers and processors involved.

It's been a while since I messed with ATM, so some of the info could old or off some, but this is my recollection.

Good Luck


Community Member

Re: IP over ATM overhead

Hi. Just one thing: All that Scott wrote is true, but.... :-)

When you write the PCR on a cisco router, or switch running IOS you have to specify the PCR in Kbps!! Strange I know, but that is the case. The 302 Kbps would correspond to a cell rate of: (302000/8)/53=713 cps.

You say that it is a DSLAM; I don't know if they are different, but it seems that the PCR in the ATM networks may be wrong.

If you try to push more cells into the ATM network than you have configured, the ATM-switches will discard the cells, even if it is a CBR service. A CBR service type, just means that you have low latency trough the switches, as CBR cells will be switched first, to put it very simple :-)

You should be able to see discards on your ATM-switch.

Discards of cells will give a very low troughput, as the routers in both ends has to retransmit all the time.

If you are using this connection for LAN traffic, you shuold consider to run UBR, UBR+ or ABR. These are designed for LAN traffic, as this can be very bursty. The CBR does not handle bursty traffic very good.

For me it sounds like you have a problem somewere, where you maybe have misconfigured the PCR.

BTW: Some vendors use 48Bytes to calculate the Kbps, but Cisco has desided to use 53bytes. My opinion is that this is a more correct way of calculationg PCR, but it can be a bit confusing to calculate the Kbps.

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