Overview

When implementing a Fax over IP (FoIP) solution, customers can choose between T.38 and G.711 as their two main transport codec options. The purpose of this article is to explain some of the differences between T.38 and G.711 to prepare agents to discuss it with customers, as it continues to come up.

Solution

T.38 Description

T.38 fax relay is an ITU-T recommendation that allows for fax data to be carried over IP networks. When you send a fax over an IP network, the data is directly transmitted in T.38 without any conversion to an audio stream.

Advantages

• Without data conversion to an audio stream, there is a significant reduction in bandwidth needed, which can help lower operating costs.
• T.38 also supports data and controls redundancy to mitigate the effects of packet loss. Note: Packet loss results in significant gaps in fax data and is prevented by switching to a lower transmission speed.

Disadvantages

• Support for some fax parameters, such as V.34 transmission speeds or Error Correction Mode, is not universal.
• Since the PSTN network is full of packet and circuit-switched connections, T.38 often has a transcoding overhead, which can add cost and latency to faxing services.

G.711 Description

G.711 is an ITU-T recommendation for Pulse Code Modulation (PCM) of voice frequencies. It uses an uncompressed format and requires high bandwidth, typically about 64 kbps. The digital fax data is converted to a PCM audio stream and then sent as G.711 Real-time transport packets (RTP).

Advantages

• G.711 is a more straightforward and often more cost-effective approach than T.38 because of its ability to leverage voice data infrastructure.
• It is less likely to suffer interoperability issues with different vendors' products.

Disadvantages

• It is not optimized for fax transport over IP networks.
• G.711 does not typically support packet redundancy.
• It can result in significant packet loss when used to transmit modem data.
• Increased transmission errors are likely.