Absolutely not. The MTU is the Maximum Transmission Unit, and sip
packets are about 214 bytes in size (including all pkt headers). Way
smaller then the MTU.
If the only thing on my network are these Cisco Phones, would lowering the
MTU encourage more efficient transfer of data as per here:
No. The reason is that "if" the phones are the only thing on this, the
size of the sip packets will never be greater then 214 bytes. The mtu
value would need to be set to something smaller then 214 bytes in order
for the parameter to have any impact whatsoever. If you selected 150
bytes (for example only), the sip/rtp packets would be fragmented into
smaller pieces forcing the destination device to reassemble those
fragments. (I'm not sure, but I'd have to guess the Cisco's don't even
support udp fragmentation.) Fragmentation is generally considered a
high-cost overhead and should be avoided when possible.
Given your table below, there "are" other devices on your network and 6%
of those are sending packets of in the 512 to 1023 byte range. If you
set an mtu of 512, then _those_ devices would be forced to fragment
larger packets, giving the sip/rtp packets a greater chance of getting
onto the external network. If your sdsl circuit is running 256kb/s, an
mtu value of 512 would generate "at best" a 16 millisecond benefit
roughly 6% of the time. At all other times it would generate no
improvement whatsoever. If the sdsl circuit is operating at a speed
greater then 256k, the best case improvement would be smaller (eg, 8 ms
for a 512k circuit).
http://www.voiptroubleshooter.com/problems/mtu.html
http://www.opalsoft.net/qos/VoIP.htm
Here is the breakdown of packets on the port that has the SDSL modem:
SDSL Router
Amount % of Whole
64 BytePkts 0 0%
65-127 BytePkts 20257 1%
128-255 BytePkts 3623518 91%
256-511 BytePkts 237087 6%
512-1023 BytePkts 112900 3%
1024-1522 BytePkts 48 0%
Total 3993810 100%
The larger packets I'm sure are the bootloader stuff and config file
downloads, etc.
That's very possible.
I do feel like I am reaching for straws here!
You're reaching for _air_ by even thinking about mtu as an issue. In 20+
years of professional network management and performance assessments,
the only time mtu is ever brought up, its generally by some first-year
technical type that doesn't have a clue (that's not directed to you
either). (FWIW, mtu adjustments use to be fairly popular for those
attempting to balance response time vs throughput on 28kb dialup circuits.)
If you don't believe any of the above, go ahead and change the mtu and
see for yourself. Bet the phones don't even work at all.
Just as an update, the users used to be on two 2mb down/512 up ADSL lines
(PPPoE) (4 users on each) and they never reported a problem. Now that they
are on one SDSL (PPPoA) line (2mb) is when they report the issues.
There is something else going on besides adsl vs sdsl, and mtu is not
even close to being the root cause.
Have you tried the previous suggestion relative to two simultaneous ftp
sessions?
What city/state are you located in?
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