Compaq W25 manual T.38 and T30 Protocol, Access Network

2.4	T.38 and T30 Protocol
	There are two protocols used to transfer a fax over IP; T.30 [2] and T.38 [3].
	T.30 is the protocol that describes the communication process between two
	fax machines on a circuit-switched network.
	T.38 is the protocol that describes the process for sending and receiving
	fax in real-time over a packet network. The protocol makes adjustments for
	delay, jitter, and dropped packets, which comes with the characteristics of
	IP networks and radio networks. Since Fax devices are sensitive to timing it
	would not be reliable to send and receive faxes over an IP network using a
	normal voice codec e.g. G.711.
	The gateway (MGW/MGC) receives T.30 data from the fax (PSTN-side),
	converts the data to IP-packets, wraps it in T.38 packets, and sends it to
	the destination, over the IP network.
	The MGW/MGC works the other way round as well, receives the T.38 data
	from the IP-network, un-wraps it to T.30-data, and converts and forwards it
	to the PSTN-side.
	Since T.30 data is encapsulated in T.38 packets, the same protocol used
	for faxing over traditional circuit switched networks, remains for every IP fax
	connection. Thus it’s important to have a high level of T.30 interoperability
	to be able to communicate with legacy fax devices.
	The normal bandwidth for the T.38 protocol is approx 30 kbps, but
	depending on configuration it can be up to 80 kbps.

2.5 Access Network

The access network used for transmitting fax pages to and from the Ericsson W25 is the WCDMA/HSDPA Radio access network and belonging core network.

IP packets are sent in packet switch data channels over the radio network. To increase the success rate of fax calls it’s important to have a connection with as little as possible latency and jitter since, despite T.38, Fax devices are sensitive to timing. WCDMA is a minimum requirement, but it’s recommended to have a HSPA connection since this reduce the latency and jitter. Future radio network improvements, mainly regarding QoS, the fax transmission success rate will further improve.

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