Merit Network
Can't find what you're looking for? Search the Mail Archives.
  About Merit   Services   Network   Resources & Support   Network Research   News   Events   Home

Discussion Communities: Merit Network Email List Archives

North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: value of co-location

  • From: Shikhar Bajaj
  • Date: Fri Jan 19 20:25:51 1996

> > What's the average packet size now?  I think the overhead on DS-3 ATM/SMDS
> > is worse than on OC-3, if I recall correctly, because the DS-3 PLCP is so
> > wasteful as well.  Someone should redo these calculations more accurately.
> 
> With PLCP, maximum bit rate available for ATM cells =
> 
> 53 cells * 12 rows * 8bits / 125us = 40.704 mpbs.
> 
> So, 40.704 * 48/53 = 36.864 mpbs = data rate. 
> 
> Now if you use AAL5, then it's another 2 bytes for SAR, and 2 bytes for 
> CS, so, 40.704 * (48-4)/53 = 33.792 mpbs.

You are correct up til you get to the efficiency at the AAL level.  AAL-5 does
not have any SAR overhead but it does have 8 bytes of CS overhead.  Now, this
overhead is on a per PDU basis and NOT a per cell basis.  Thus, you maximum
efficiency will be 

36.864 * [(x-8) / x] where x = PDU size in bytes.

Thus, if you just send 64-byte packets, your min throughput will 
be 36.8 * (56 / 64) = 32.3 Mbps.

Of course, larger PDUs are more efficient.



Shikhar
 




Discussion Communities


About Merit | Services | Network | Resources & Support | Network Research
News | Events | Contact | Site Map | Merit Network Home


Merit Network, Inc.