Thursday, March 17, 2011

How did we get a 53 byte packet size in ATM?

I'll be honest, I don't know squat about ATM, but I was having lunch with this fellow, and he told me the story of the 53 byte ATM packet. 

You can find more details on Wikipedia, but here’s the synopsis:

(Disclaimer: I’m not an expert in ATM; nor am I trying to teach you technical details about ATM networks; so I’ll hand wave and trade off accuracy for simplicity. For example, ATM does have variable sized packets which it divides into cells, and it is the cells which are 53 bytes long. However, since the closest thing to a cell in common networks is an Ethernet packet, I’ll simply refer to cells as packets.)

ATM is designed to be shared between data network applications, and voice network applications(+).

In data networks we want large packets because this gives maximum efficiency.  This is because each packet has a fixed size header and thus the more data you can transmit per packet , the higher your ‘real’ throughput.

For voice networks we want to reduce latency.  Latency is the fancy word for delay.

If voice packets are delayed, you’ll hear them at the receiver as an echo; this is un-acceptable and thus to mitigate delays you’d need to install an echo canceller (++).
In the USA, the country was big enough (+++) that no matter how small the packet size was chosen to be, you’d always need echo cancellers – therefore America proposed an optimal packet size of 64 bytes.

In France, which was much smaller then America, if the packet size was chosen to be 32 bytes the packet could get from one end of the country to the other without enough delay to require echo cancellers. Thus France pushed for a 32 byte packet size.

To achieve consensus, a compromise was required and a 48 byte payload size was chosen.  This did not meet France’s need, nor was it optimal for the US; however, this is what happens when you require consensus.
But wait you ask, where do the last 5 bytes come from? Ah, the committee was very concerned about the overhead of the packet header, so the committee decided the packet header could not exceed 10% of the packet size, thus the committee chose a 5 byte header.  48 + 5 == the 53 byte ATM packet size; which is the optimal solution for no one.

(+) At the time ATM was designed, people made long distance calls over voice networks.
(++) If you’re old enough to remember the early days of skype, you needed to use headphones otherwise the echo was unbearable. Today computers are powerful enough to do echo cancellation without blinking.
(+++) Compute big enough by calculating time required for light to traverse the USA.

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