VoIP Network

What is the maximum MTU size I can transmit and receive to SIP traffic if I’m using UDP?

There are two transport protocols that we use to carry VoIP traffic between your network and ours. UDP and TCP/IP. UDP is the most common standard for VoIP but is limited in capability.

The maximum MTU (Maximum Transmission Unit) size of UDP we can receive un-fragmented is 1460 bytes. Most broadband routers are set to an MTU default of 1454 bytes, so its uncommon you can transmit more than our limits. You can send us larger sizes, but that would mean our receipt is fragmented and we won't receive all the data we may need to make your call work.

The data you send us with each call contains information about how our platform can communicate with you, who you are calling, how you are presenting your call, your settings such as the codecs you support and a heap of other information we need to make a call function.

If your MTU size is larger than 1460 bytes, it can be reduced by removing some of the optional needless information you may transmit to us. We commonly see users limiting the number of Codecs in their device's settings to the four main codecs of OPUS, G.722, G.711A (Also referred to as PCMA) and G.711U(Also referred to as PCMU) in that order.

Whilst we do fully appreciate network engineers will often prefer to use UDP for VoIP, MTU size is a limited and repetitive issue we see causing communication problems. TCP/IP has no material limit of concern, so its worth considering switching your devices from UDP to TCP/IP as their transport.

If you need any further help today, please don't hesitate to contact our friendly support team on 0330 122 6000 or by email!

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