Cell Relay Archive

[fgoldstein@bbn.com (Fred R. Goldstein)]

In article <JACKSON.95Mar13101838@vision.icrl.mew.mei.co.jp> jackson@icrl.mew.mei.co.jp (Spencer Jackson) writes:

me> The classical example is T1, which carries 1.544 Mbps (or E1 at 2.048)
>>> using the AMI bit transitions to provide exact clock to the receiver.
>>> Other fixed-speed channels are then derived against that clock.

>I think you are mixing up 'plesiochronous' and 'isochronous'.
>Plesiochronous refers to self-clocked lines, like T1, as opposed to
>synchronous lines, like SONET.  Isochronous refers to data that has
>timing limitations, like voice or video.

Uh, not exactly.  Plesiochronous is nowadays used to mean "self-clocked
without reference to a central timing source", or "nearly synchronous" (which 
is its etymology).  Thus T1, E1, and SDH are all isochronous, but T1 and E1
are normally allowed to be plesiochronous while SDH normally is not.

Plesiochronous comes about from the old digital islands in an analog sea.
Each T1 in a T2, or T2 in a T3, etc., could have its own clock, thus requiring
"justification" bits and a complex demux.  SDH/SONET assumes that everybody
has a connection to the network's master clock, so it's all synchronized. 
Thus "synchronous" and "plesiochronous" are both capable of accompanying
"isochronous".

Since isochronous carries a clock mixed in with the data, it is capable of
easily supporting timing-sensitive payloads; however, the term does not refer 
to the payload but to the nature of the medium. 

___
Fred R. Goldstein   k1io    fgoldstein@bbn.com
Bolt Beranek & Newman Inc., Cambridge MA  USA   +1 617 873 3850
Opinions are mine alone; sharing requires permission.