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i'm studing for a multimedia networks exams and i don't understand why ATM is asynchronous. i know that it is asyncrhonous if used with SONET but this makes no sense..

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This gives a decent explanation as to why ATM was designed as an asynchronous protocol.

Though TDM is very efficient carrying delay sensitive voice traffic, it wastes bandwidth because individual time slots in the synchronous frame cannot be easily reallocated in real time between variable and constant bit rate traffic sources. By the late sixties, researchers at Bell Labs had the idea of introducing a label into each cell to identify traffic sources. Thus, a given call would be transported by an asynchronous series of fixed length cells identified by a virtual circuit identifier in the header, instead of being assigned to a fixed time slot.

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  • It might be worth noting that the reference article uses terms and acronyms with the original meaning, many of which have changed over time. For instance, TDM used to reference the protocol by which phone calls were encoded on T1 lines; today it generically means a type of encoding, and the protocol is known as PDH.
    – Chris S
    Jan 19 '11 at 17:07
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The really short version:

In sychronous communication the two devices have a common clock that sychronizes the communication. Both send and receive on some schedule, based on the clock. This usually requires another wire in the system just for the clock.

With asynchronous, the data is sent at a particular speed, and the reciever has to figure out the speed so it can recover the clock rate and the data. This doesn't require a clock wire, but requires slightly more complex receiving equipment.

More information available from the Wikipedia: ATM SONet

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