RE: stds-80220-requirements: 802.1q/p
I am assuming PPPOE
5) Analysis of PPPoE overhead
From the PPPoE Informational RFC document, we see that:
The Maximum-Receive-Unit (MRU) option MUST NOT be negotiated to a
larger size than 1492. Since Ethernet has a maximum payload size
of 1500 octets, the PPPoE header is 6 octets and the PPP Protocol
ID
is octets, the PPP MTU MUST NOT be greater than 1492.
This means that PPPoE imposes an overhead of 8 bytes for every 1500
bytes of data for as long as the negotiated MTU is 1492 bytes. 8 over
1500 is thus .53% over overhead on packets that fill up the complete
ethernet payload capacity. But if the PPP MTU is negotiated to be
smaller, in order to improve the performance of VoIP or real-time
gaming applications for example, the overhead becomes drastically
more
apparent on smaller packets. For example, the MTU on dial-up modems
is usually as small as 256 bytes. With a 256 bytes MTU, PPPoE would
thus present an overhead of 3.125%. This number may not be small, but
this means that a voice packet, would incur a total rountrip delay
that is over 6.25% higher than it would be without the overhead of
PPPoE.
David S. McGinniss
Sprint Broadband Wireless Group
Principal Engineer II
(630) 926-3184
david.s.mcginniss@mail.sprint.com
-----Original Message-----
From: Marc Goldburg [mailto:marcg@arraycomm.com]
Sent: Thursday, November 20, 2003 21:18
To: Mcginniss, Dave S [GMG]
Cc: Branislav Meandzija; stds-80220-requirements@ieee.org
Subject: RE: stds-80220-requirements: 802.1q/p
On Thu, Nov 20, Mcginniss, Dave S [GMG] writes ...
Dave> PPP also increases overhead.
Dave,
Relative to ethernet, I don't believe that this is correct. PPP adds 2
bytes of overhead to an IP packet. Standard ethernet adds 14 bytes
(802.3
adds 17), and 802.1Q adds an additional 4 bytes.
Regards,
Marc