ATM doubt

[ankurbhasin]

Hi Champs,

Got a little confusion.Please make me clear.

As ATM is a asynchronous transfer mode tech and in asynchronous mode the recurrence of cells containing information is not periodic.

But TC sub-layer performs a function "Cell Rate Decoupling" in this function fake/idle cells are transmitted whenever no actual cell is there to transfer to maintain the link rate.

My confusion is that when we are using Cell Rate Decoupling we are providing synchronous transmission then how can we say ATM is asynchronous.

Thanks in advance.

Regards,

Ankur

[pkhatri]

Hi Ankur,

ATM came about as a technology that provided a combination of the best parts of circuit-switched networks and packet-switched networks.

SDH/SONET is synchronous because it associates a fixed relationship between timeslots and the data it carries. For instance, in a channelised E1 with timeslots 1-20 assigned to circuit1 and timeslots 21-31 assigned to circuit2, there will always be a fixed relationship between timeslots and circuits - timeslots 1-20 will always carry circuit 1 traffic and timeslots 21-31 will always carry circuit 2 traffic.

The asynchronous in ATM does not refer to physical layer synchronisation. It refers to the fact that there is no relationship in timing between ATM cells and the data it carries. For a simplistic case, consider a link with a cell rate of 3 cells/sec. In one second, cell1 can carry data for PVC1, cell2 can carry data for PVC2 and cell3 can carry data for PVC3. In the next second cells 1 to 3 can all carry data for PVC2. That's what makes it asynchronous, that there is no correspondence between cell number and the PVC.

Hope that helps - pls rate the post if it does.

Paresh

[ankurbhasin]

Hi Paresh,

Thanks for a quick one.

So "Cell Rate Decoupling" is to maintain the physical medium synchronisation?

Fake/idle cells are transmitted whenever no actual cell is there to transfer to maintain the link rate. Is this statement correct to maintain the sync transmission on the physica media?

Regards,

Ankur

[pkhatri · Accepted solution]

No probs, Ankur.

Yes, that is correct. Remember that ATM does not use a de-limiting mechanism such as the frame-relay flag so the mechanism used to maintain sync transmission is the use of idle cells.

Hope that helps - pls rate posts that help.

Regards,

Paresh