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New Member

atm cells

Hi all, can anyone tell me why using atm is better than ethernet for isp's, is it faster to transpot the 53 byte cells rather than ethernet 1500 byte cells ? whats the advantage ?

cheers

4 REPLIES

Re: atm cells

Hi Carl,

Having a fixed small size is one of the most advantages of ATM, please check this document:

http://www.cisco.com/univercd/cc/td/doc/product/atm/l2020/l2020r20/so/atmtech.htm

HTH,

Mohammed Mahmoud.

New Member

Re: atm cells

There are advantages to ATM, as Mohammed Mahmoud's link notes, but there's a some issues to be aware of when not using ATM end-to-end, such as running Ethernet or IP on top of it. These are most noticable when using ATM WAN links to connect to the provider's ATM cloud.

Most notable is cell tax. Although cells are 53 bytes, generally only 48 contain your payload. This means a T-1 ATM link will carry less of your data than a non-ATM link.

Also when running IP on top of ATM, you get packet fragmentation loss. A 49 byte packet will require two 53 bytes cells.

There also the issue of re-assembly of a packet when a cell has been loss. The whole packet often must be re-tranmitted even though all but one cell was received.

Don't misunderstand that ATM is bad, it's just different, and vendors tend to gloss over some of the differences.

New Member

Re: atm cells

can you tell me why isp's use it over ethernet ?

New Member

Re: atm cells

I think the two major reasons were ATM's additional capabilities for supporting different services, such as real-time traffic, and being able to switch cells very quickly. It is a superior technology especially attractive to TELCOs. For those of us working with business data networks, the advantages aren't as evident and the cost of moving your whole network to ATM is usually prohibitive.

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