[Asterisk-Users] MTU and Voice Delay (latency??)

[Rich Adamson]

> Our users are experiencing some unacceptable delay when trying to have a
> conversation. The delay is so noticeable that they keep stepping on each
> others words and resort to calling the customers via cell phone.
> 
> Here is the setup
> 
> SDSL Connection (PPPoA)
> Speedtouch 610 SDSL Modem
> 3Com 2224PWR Plus Switch (phones on separate VLAN)
> 8 Cisco 796 Phones
> 
> All connecting to a remote Asterisk Server.
> 
> We found that the MTU for the SDSL modem was set to 1500 and I have since
> changed it to 1458 which is the ISP's recommended setting.
> 
> Can this MTU difference cause the delay my users are experiencing? All the
> voice packets would become fragmented so it sounds logical.

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.

> And simply changing the MTU on the modem, will that fix it, I can't find a
> way to change it at the Cisco phone level.

There will be a delay associated with any sip-to-sip call, but it should
not be all that noticable unless both the talker and listener are in the
same room. 

Are you sure this is a delay problem, or might it be a half-duplex problem?

If any of the hardware mentioned is operating in half-duplex-only mode,
it is entirely possible to create what might be construed as a delay.