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Re: ATM vs. DS3
From: "Avitosh Pal" <apal () nortel ca>
Date: 10 Jul 1997 13:59 EDT

In message "ATM vs. DS3", you write:

From: Josh Beck <jbeck () connectnet com>
Now, the question is, what is the overhead of the serial (ppp?) protocol
running over the T3? Since it is point to point, and not addressed, it
should be less, or is this not the case?

The framing overhead for PPP over DS3 is about 7 bytes per packet.  You
can make it as few as 4 (PPP header compression) or as long as 10
(32-bit checksum).

I don't know anyone shipping PPP data compression yet at those speeds,
although I've heard of a project using multiple Stac LZS chips.

Anyway, you will get a lot better performance out of a PPP link than an
ATM link; certainly in the short term, and probably in the long term.

WSimpson () UMich edu
    Key fingerprint =  17 40 5E 67 15 6F 31 26  DD 0D B9 9B 6A 15 2C 32
BSimpson () MorningStar com
    Key fingerprint =  2E 07 23 03 C5 62 70 D3  59 B1 4F 5E 1D C2 C1 A2

I think what the question is  how much is lost in overhead for 
Frame Relay over ATM vs straight PPP on a DS3 link. (I'm assuming
that since he says CIR he means FrameRelay and does not really mean

My understanding is that you really arent loosing anything to cellification since the 
4 Mb CIR is what the provider guarantees to you. i.e. the overhead has
already been accounted for in getting to 4 Mb. The raw bandwidth  
required to achieve to get 4 Mb is; 

4 Mb * 53/48 = 4.42 Mb

To see what the PPP overhead consult RFC 1661. 

Avitosh Pal
Nortel Magellan Networks


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