Shikhar Bajaj bajaj at
Tue Mar 19 20:16:51 UTC 1996

> > >      > What kinds of routers can route at near wire speeds a bunch of ports at 
> > >      > 155Mb ATM?
> > >      
> > >      None right now. Gigarouter can handle it theoretically, but...
> > >      
> > >      I'm being picky, but it's a moot question since doing wirespeed OC3c using 
> > >      ATM is impossible in of itself. Hell, mapping of 
> > >      SONET framings to ATM  cells take off about 6 mbps right there.
> >        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
> By the way, I did not post the above..

Oops.  Replied to the right thread but wrong author.

> bajaj at (Shikhar Bajaj) wrote:
> > A couple of minor corrections here.  ATM cells are mapped into SONET frames and
> > not the reverse.  Also, I presume that the 6 Mbps that you are talking about is that
> > the ATM bit rate over an OC-3 is 149 Mbps as opposed to the OC-3 pipe
> > rate of 155 Mbps.  Those 6 Mbps are taken up by the SONET management overhead
> > (section, line, and path) in the frame.  This is independent of whatever goes in the
> > SONET payload envelope and has nothing to do with ATM.
> It seems to me that Sonet OC-3 is often taken as transport for 3 DS3s,
> which would reduce 155.530 Mbps to 134.208 Mbps (3 x 44.736 Mbps), less
> than the 149 Mbps you cite above. 

The 149 Mbps assumes that you have a native ATM/OC-3 interface where
the SONET transport is STS-3c ('c' for concatenated).  Here, a single connection 
can use the full bandwidth of the pipe. 

In your example, a DS-3 gets mapped into an STS-1 and then 3 STS-1s get muliplexed into an
STS-3.  The multiplexing is for trunking purposes and there is no relation between any
of the STS-1's (other than they are byte multiplexed).  Thus, the maximum bandwidth
available for any single connection is 51 Mbps (or DS-3 in this case).

Routers with ATM/OC-3 interfaces do STS-3c framing.  

> Do you have a good recommendation for Gordon Cook, TCP/IP direct over Sonet?

I've heard rumblings about such work going on but I am not personally familiar with
any of it.  The only thing I can think of is some work that the IETF did for
PPP over SONET (RFC 1619).  I *think* it became an Internet Standard.


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