[j-nsp] Usable Bandwidth on STM-1 with Cisco HDLC or PPP Encapsulation

sthaug at nethelp.no sthaug at nethelp.no
Mon Feb 6 07:24:03 EST 2006

> > can anybody help me with the following issue :
> > 
> > What is the real usable bandwidth on a STM-1 link for IP traffic in
> > consideration of the encapsulation and transmission overhead ?
> With PPP you can pretty much get away with 130-135 Mbit/s
> but that will give you short bursts that get into queueing
> already. Mind you, 1 kbit = 1000 and not 1024 here.

Minor addition: In principle you should be able to go somewhat higher:

- An STM-1 is 149.76 Mbps after SDH overhead is removed
- PPP is 8 or 10 bytes overhead depending on 2 or 4 byte CRC

Assuming 1500 byte packets (including IP header) and 4 byte CRC, the
bandwidth available for IP is:

149.76 * 1500 / 1510 = 148.768 Mbps

In real life, of course, traffic is a mix of various packet sizes, and
the 130-135 Mbps mentioned above is pretty realistic.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no

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