It passes NSS, NSF and NSC frames, but modifies their contents. A receiving FAX machine must be prevented from acting upon these messages in a manufacturer specific manner. The T.38 protocol is incapable of handling such manufacturer specific things. However, simply dropping these packets would upset the timing of the T.30 protocol. Instead, most T.38 implementations modify the country and manufacturer codes nesr the start of these messages, so the messages will be ignored by any receiver. They are typically set to 0x00, 0x55, 0xAA or 0xFF. Some implementations set them to the country and manufacturer code of the T.38 implementor. As long as they are set to something the receiver will not recognise, problems are avoided. To avoid removing useful information, the original country and manufacturer code bytes might be reinserted in a different part of the modified NSF message, provided space permits.

It tracks the contents of the DCS messages. During flow control of non-ECM data, the minimum row length must be preserved, so the current minimum must be found from the DCS messages. Similarly, the current encoding must be known to apply non-ECM flow control correctly. ECM data must be flow controlled in a completely different way than non-ECM data, so ECM mode information must be found from the DCS messages.

It may optionally modify the minimum row length in the DIS messages. Because flow control will be applied to non-ECM data, it can make sense to tell the sending FAX machine it has no need to impose a minimum row length, and let the emitting T.38 entity impose the minimum as part of its flow control operation.

It passes on the stripped and modified HDLC frames, with the minimum possible delay. This implies that is must analyse and modify the messages octet by octet.