An MTSI client in terminal supporting FoIP is typically either a facsimile gateway or a facsimile end-point and does not need to support both cases. An MTSI client in terminal may support FoIP where the MTSI client in terminal is used as a facsimile gateway between an external Group 3 facsimile equipment and the IMS network. In this case, the MTSI client in terminal acts as an Internet Facsimile Protocol (IFP) peer, either as an emitting gateway or as a receiving gateway depending on whether the MTSI client in terminal initiates the Internet Facsimile Transfer (IFT) or whether it accepts the IFT, [93]. An MTSI client in terminal may support FoIP where the MTSI client in terminal is the end-point for the facsimile transmission. In this case, the facsimile transmission originates or terminates in the MTSI client in terminal. An MTSI client in terminal supporting FoIP and used as a facsimile gateway shall support:

• input/output to Group 3 facsimile devices, [91]

An MTSI client in terminal supporting FoIP and used either as a facsimile gateway or as a facsimile end-point should support the recommended configuration defined in clause L.2.3, Table L.1.

• encapsulating and decapsulating T.30 to/from Internet Facsimile Protocol (IFP) packets, [93];

An MTSI MGW may support FoIP where the MGW is used as a facsimile gateway between the IMS network and a Circuit Switched (CS) network, e.g. PSTN or CS GERAN, in order to connect to another Group 3 facsimile device. An MTSI MGW supporting FoIP should support the recommended configuration defined in clause L.2.3, Table L.1.

The recommended configuration for T.38 UDPTL-based FoIP is defined in Table L.1.

It is recommended that the MTSI client supports sending and receiving facsimile with 200% redundancy when UDP redundancy is used, even if the SDP attributes and parameters ('T38FaxUdpECDepth' with 'minred' and 'maxred') are not supported. This allows for transmitting each IFP message three times, once as a primary message and twice as redundancy messages.

An MTSI client supporting facsimile transmission shall support facsimile transmission in stand-alone sessions without any other media types. An MTSI client supportings facsimile versions (T38FaxVersion) higher than 0 shall be capable of downgrading the session to any lower facsimile version, if indicated by a received SDP message. An MTSI client sending an SDP for a facsimile session shall include the following in the SDP (offer or answer):

• MIME media type and subtype names as defined in RFC 3362;
• bandwidth information, both on media level and session level;
• T38FaxVersion attribute, if the offered version is higher than 0;
• T38FaxRateManagement attribute, with the value according to the offered method.

Absence of the T38FaxVersion attribute indicates that only version 0 is supported. SDP examples for facsimile calls can be found in clause L.7.

When the MTSI client supports facsimile transmission according to the recommended profile in Annex L.2.3 and initiates a session for UDPTL-based facsimile transmission then:

• the following SDP lines shall be used in the SDP offer:
  • b=AS with the bandwidth set to at least 46 kbps for IPv4 or 48 kbps for IPv6;
  • T38FaxVersion attribute indicating at least version 2;
  • T38FaxRateManagement attribute with value 'transferredTCF';
• the following SDP attributes should be included in the SDP offer:
  • T38MaxBitRate, the value should be set to 14400;
  • T38FaxMaxBuffer with value 1800;
  • T38FaxMaxDatagram with value 150;
  • T38FaxUdpEC with value 't38UDPRedundancy'.

Other SDP attributes defined in ITU-T T.38 Annex D may be included, if supported. When the MTSI client supports facsimile transmission according to the recommended profile in Annex L.2.3 and accepts an offer for a session initiation for facsimile transmission then:

• the following SDP lines shall be included in the SDP answer:
  • T38FaxVersion attribute indicating at least version 2;
  • T38FaxRateManagement, the value shall be the same as in the SDP offer;
  • T38FaxUdpEC, the value to include depends both on what error correction schemes the MTSI client supports and what error correction schemes that are declared in the SDP offer;
  • b=AS, the value indicates the bandwidth needed for facsimile transmission and should be aligned with T38MaxBitRate (if included);
• and the following SDP attributes should be included:
  • T38MaxBitRate, the value should be set to 14400;
  • T38FaxMaxBuffer, the value indicates the receiver buffer size.

This example shows the media scope of the SDP offer and SDP answer for a facsimile-only session when the recommended configuration in Annex L.2.3 is offered by both end-points.

m=image 49150 udptl t38
b=AS:46
a=T38FaxVersion:2
a=T38FaxRateManagement:transferredTCF
a=T38MaxBitRate:14400
a=T38FaxUdpEC:t38UDPRedundancy
a=T38FaxMaxBuffer:1800
a=T38FaxMaxDatagram:150

m=image 49154 udptl t38
b=AS:46
a=T38FaxVersion:2
a=T38FaxRateManagement:transferredTCF
a=T38MaxBitRate:14400
a=T38FaxUdpEC:t38UDPRedundancy
a=T38FaxMaxBuffer:1800
a=T38FaxMaxDatagram:150

Comments: The session bandwidth is set to 46 kbps based on the following calculation:

• the maximum fax bitrate is 14.4 kbps;
• it is recommended that the MTSI client supports 200% redundancy;
• 14.4 kbps and 150 bytes in each IP/UDP packet gives 12 packets per second;
• IPv4 and UDP gives 28 bytes overhead for each IP/UDP packet (IPv6 and UDP gives 48 bytes overhead).

This gives 3*14400 +12*28*8 = 45888 bps, which is rounded up to 46 kbps for IPv4 (48 kbps for IPv6). ITU-T Recommendation T.38 states that only 'T38FaxRateManagement' is mandatory to include in the SDP. There are however other reasons to include the other SDP lines:

• b=AS is included both because this is useful information both for resource allocation in network nodes and the remote end-point, and because it is required by the current specification, see clause 6.2.5.1;
• the 'T38FaxVersion' attribute needs to be included to declare that version 2 is supported, since the other end-point would otherwise assume that only version 0 is supported;
• the 'T38MaxBitRate', 'T38FaxMaxBuffer' and 'T38FaxMaxDatagram' attributes are very useful to ensure that the remote end-point sends fax media within the limitations of the local end-point;
• The 'T38FaxUdpEc' attribute is very useful information for the remote end-point in case of bad channel conditions.

The SDP attributes 'T38FaxFillBitRemoval', 'T38FaxTranscodingMMR' and 'T38FaxTranscodingJBIG' are not included since the MTSI client is not required to support these options. The SDP attributes 'T38FaxMaxIFP', 'T38FaxUdpECDepth' and 'T38FaxUdpFECMaxSpan' are not included since these attributes are not defined for T.38 fax version 2.