Fax and Modem Services over IP Overview

The call originator prepares a fax and dials a destination number. The destination fax device picks up the call. The originator and the destination are now connected in a voice call, but to transition to fax transmission one party must signal that it is a fax device. Either device can send its signal first, using one of the following methods:

• The calling device sends a Calling Tone (CNG) to the destination device. The CNG identifies the calling device as a fax machine. The CNG is a repeating 1100-Hz tone that is on for 0.5 seconds and then off for 3 seconds.
• The called device sends a Called Station Identifier (CED) tone, which identifies the called device as a fax machine. CED is a 2100-Hz tone that is on for 2.6 to 4 seconds.

Once these messages have been exchanged, the transaction can move to phase B.

The following sequence of events identifies facilities and capabilities for fax transmission:

1. The called device sends a Digital Information Signal (DIS), which describes the called fax machine's reception facilities, such as maximum page length, scan line time, image resolution, and error correction mode. Many standard facilities are contained in the DIS message, and they are defined in the T.30 specification.
2. The calling device examines the DIS message and in response sends a Digital Command Signal (DCS) that tells the called device which facilities to select for the reception of the fax transmission.
3. The called device may also choose to send the following optional messages:
4. Called Subscriber Identification (CSI) provides some detail as to the identity of the called device.
5. Non-Standard Facilities (NSF) informs the calling device that the called device may have some extra features that can be utilized during the fax transmission.
6. The calling device can then choose to send a Transmitting Subscriber Identification (TSI) message. Also, in response to an NSF message, the calling device can send a Non-Standard facilities Setup (NSS) message to select extra reception parameters on the called device.
7. The calling device now sends the Training Check (TCF) message, which includes a stream of 0s for about 1.5 seconds through the HS modulation that was agreed upon during the DIS-DCS handshake. The called device then responds with a Failure To Train (FTT) if the modulation speed is not acceptable or with a Confirmation to Receive (CFR) if the modulation speed is acceptable. Training is a process that verifies the communication path.
8. Once the training has been completed and the modulation speed is agreed upon, the fax devices move to phase C and start the transmission of T.4 page data using HS modulation.

Phase C is referred to as the In-message Procedure. During this phase, high-speed T.4 page data is sent one line at a time. Each burst of line data is followed by an End Of Line (EOL) message. Because the EOL information is sent as T.4 data, it would not necessarily be seen in a T.30 trace. When the sending device has finished sending pages or wishes to return back to control mode, it sends 6 EOLs in a series that constitutes a Return To Control (RTC) message. The RTC message indicates the end of phase C, and the call progresses to phase D.

Note

If the fax machines decide during phase B to use Error Correction Mode (ECM), the format of the data sent during phase C may be different. With ECM, the T.4 page data is grouped into high-level data link control (HDLC) frames rather than being sent in a raw stream. This means that if the HDLC blocks of T.4 page data are not received error-free, a Partial Page Request (PPR) message can be sent, listing the frames that were not received and asking for them to be resent. The details of the transmission differences during phase C with ECM enabled are explained in Annex A of the T.30 specification.

After the T.4 transmission and the subsequent return to control mode, the sending device must send one of the following signals:

• Partial Page Signal (PPS)--Devices that send faxes with ECM can send a PPS, which must be acknowledged by a Message Confirmation (MCF) signal from the receiving device.
• End Of Procedure (EOP)--This signal indicates that transmission of pages is complete and that there are no more pages to send. The EOP must be acknowledged with an MCF from the receiving device, after which the devices can move to phase E.

Following the fax transmission and the postmessage transactions, either the calling device or the called device can send a Disconnect (DCN) message, at which point the devices tear down the call, and the telephony call control layer releases the circuit. DCN messages do not require a response from the opposite device.

A Cisco voice gateway provides an interface between the IP network and the public switched telephone network (PSTN) or telephony (fax) device. When a call comes into the IP network over a gateway, that gateway is called an originating gateway (OGW). Similarly, a gateway over which a call passes out of the IP network is called a terminating gateway (TGW).

A traditional voice call over the PSTN uses a dedicated 64-kbps circuit end to end. In contrast, a voice call over the packet network contains several discrete segments or call legs. A call leg is a logical connection between two routers or between a router and a telephony device. A voice call comprises four call legs, inbound into and outbound from both the OGW and the TGW.

Dial peers are software constructs that sort calls, route calls, and define characteristics applied to each call leg in the call connection, based on call source and destination endpoints. Dial peers are used for both inbound and outbound call legs. It is important to remember that these terms are defined from the perspective of the router. An inbound call leg is created by any call that comes in to a router, regardless of whether the router is an OGW or a TGW. An outbound call leg is created by any call that leaves a router, regardless of whether the router is an OGW or a TGW, as shown in the figure below.

Different types of dial peers handle different kinds of call legs. The following types of dial peers are used for fax over Cisco IP networks:

• Plain old telephone service (POTS) dial peers handle call legs between a voice gateway and the PSTN or a telephony device.
• Voice over IP (VoIP) dial peers handle call legs between a voice gateway and the IP network.
• Multimedia Mail over IP (MMoIP) dial peers handle call legs between a voice gateway and a Simple Mail Transfer Protocol (SMTP) server or Extended SMTP (ESMTP) server.

Note	For more information on voice gateways and dial peers, see Dial Peer Configuration on Voice Gateway Routers.

Figure 1

Call Legs and Dial Peers on Cisco IP Networks

Tool Command Language (TCL) is used for scripts that direct interactive voice response (IVR) applications, which are used in Cisco voice networks for various purposes. IVR applications typically involve the real-time gathering of data from callers by means of digit collection and voice prompts. For example, you might have a debit card application that asks a user to enter a personal identification number (PIN) and then collects and verifies the digits that the user enters.

A gateway can have several IVR applications to accommodate different gateway services, and you can customize IVR applications to present different interfaces to various callers. IVR applications are used to implement the following fax services:

• T.37 Store-and-Forward Fax
• Fax Detection IVR Application
• Fax Rollover IVR Application

TCL scripts are provided on the Cisco Software Center website. You download them to a location that is accessible to the voice gateway that is running the fax application and then configure the gateway with the name and location of the script.

Note	For more information on TCL IVR, see the Cisco IOS TCL and VoiceXML Application Guide .

Quality of service (QoS) refers to the ability of a network--whether the network is a complex network, small corporate network, Internet service provider (ISP), or enterprise network--to provide better service to selected network traffic over various technologies, including Frame Relay, ATM, Ethernet and 802.1 networks, and SONET, as well as IP-routed networks that may use any or all of these underlying technologies.

The primary goals of QoS are to provide better and more predictable network service by providing dedicated bandwidth, controlled jitter and latency, and improved loss characteristics. QoS achieves these goals by providing tools for managing network congestion, shaping network traffic, using expensive wide-area links more efficiently, and setting traffic policies across the network.

QoS for fax transmissions means assuring that echo cancellation (EC) and voice activity detection (VAD), which are normally enabled for voice calls, are turned off as soon as a call is identified as a fax call. If EC and VAD are enabled, they can interfere with the successful reception of fax traffic.

The advantages of carrying fax over packet networks are reduced cost and saved bandwidth and are associated with QoS issues that are unique to packet networks. A major issue in the implementation of fax over IP networks is the problem of inaccurate timing of messages caused by delay through the network.

The delay of fax packets through a packet network causes the precise timing that is required for many portions of the fax protocol to be skewed and can result in the loss of the call. The fax-over-packet protocol in the interworking function must compensate for the loss of a fixed timing of messages over the packet network so that the T.30 protocol operates without error. Error Correction Mode (ECM) is enabled in the T.30 protocol.

An end-to-end fax over IP call is susceptible to the following sources of delay:

• Network delay--Network delay is caused by the physical medium and protocols that are used to send fax data and by buffers that are used to remove packet jitter on the receiving end. This delay is a function of the capacity of the links in the network and the processing that occurs as the packets transit the network. The jitter buffers add delay when they remove the packet delay variation of each packet as it transits the packet network. This delay can be a significant part of the overall delay because packet delay variations can be as high as 70 to 100 milliseconds in some Frame Relay networks, and even higher in IP networks.
• Processing delay--Processing delay is caused by the process of demodulating and collecting digital fax information into a packet for transmission over the packet network. Encoding delay, which is one type of processing delay, is a function of both the processor execution time and the amount of data collected before a packet is sent to the network.

Delay issues are compounded by the need to remove jitter, which is the variable interpacket arrival time that is caused by conditions in the network that a packet traverses. An approach to removing the jitter is to collect packets and hold them long enough so that even the slowest packets arrive in time to be played in the correct sequence. This approach, however, causes additional delay. In most fax over IP methods, a time stamp is incorporated in the packet to ensure that packet data is played out at the proper instant.

The T.30 standard provides for ECM that allows a fax page to be broken into HDLC-like frames that allow transmission errors to be detected. ECM works by sending a fax page in a series of blocks. After receiving the complete page data, the receiving fax identifies any frames with errors. The sending fax then retransmits those frames. This process is repeated until all frames have been received without errors.

If a receiving fax machine is not able to receive an error-free page, the fax transmission may fail, and one of the fax machines may disconnect. If a network has packet-loss levels greater than 3 to 5 percent, fax transmissions consistently fail when ECM is enabled. Fax relay packet loss concealment disables ECM so that fax calls with up to 9 percent packet loss succeed and calls with packet loss of 5 to 7 percent succeed with acceptable quality.

For more information, see the following documents:

• QoS: Quality of Service for Voice.
• Fax relay packet loss concealment: Chapter 1, "Configuring Cisco Fax Relay" or Chapter 1, "Configuring T.38 Fax Relay."

When a DSP is put into voice mode at the beginning of a VoIP call, the DSP is informed by the call control stack whether fax relay is supported and if it is supported, whether it is Cisco fax relay or T.38 fax relay. If Cisco fax relay is supported, the following events occur:

• Initially a VoIP call is established as if it were a normal speech call. Call control procedures are followed and the DSP is put into voice mode, after which human speech is expected to be received and processed.
• At any time during the life of the call, if a fax answer or calling tone (ANSam or CED) is heard, the DSP does not interfere with the speech processing. The ANSam or CED tone causes a switch to modem passthrough, if enabled, to allow the tone to pass cleanly to the remote fax.
• A normal fax machine, after generating a CED or hearing a CNG, sends a DIS message with the capabilities of the fax machine. The DSP in the Cisco IOS gateway attached to the fax machine that generated the DIS message (normally the TGW) detects the HDLC flag sequence at the start of the DIS message and initiates fax relay switchover. The DSP also triggers an internal event to notify the call control stack that fax switchover is required. The call control stack then instructs the DSP to change the RTP payload type to 96 and to send this payload type to the OGW.
• When the DSP on the OGW receives an RTP packet with payload type set to 96, it triggers an event to inform its own call control stack that a fax changeover has been requested by the remote gateway. The OGW then sends an RTP packet to the TGW with payload type 97 to indicate that the OGW has started the fax changeover. When the TGW receives the payload type 97 packet, the packet serves as an acknowledgement. The TGW starts the fax codec download and is ready for fax relay.
• Once the OGW has completed the codec download, it sends RTP packets with payload type 96 to the TGW. The TGW responds with an RTP packet with payload type 97, and fax relay can begin between the two gateways. As part of the fax codec download, other parameters such as VAD, jitter buffers, and echo cancellation are changed to suit the different characteristics of a fax call.

Cisco fax relay fax setup is shown in the figure below.

Figure 3

Cisco Fax Relay Fax Setup Call Flow

During fax relay operation, the T.30 analog fax signals are received from the PSTN or from a directly attached fax machine. The T.30 fax signals are demodulated by a DSP on the gateway and then packetized and sent across the VoIP network as data. The TGW decodes the data stream and remodulates the T.30 analog fax signals to be sent to the PSTN or to a destination fax machine.

The messages that are demodulated and remodulated are predominantly the phase B, phase D, and phase E messages of a T.30 transaction. Most of the messages are passed across without any interference, but certain messages are modified according to the constraints of the VoIP network.

During phase B, fax machines interrogate each other's capabilities. They expect to communicate with each other across a 64-kbps PSTN circuit, and they attempt to make best use of the available bandwidth and circuit quality of a 64-kbps voice path. However, in a VoIP network, the fax machines do not have a 64-kbps PSTN circuit available. The bandwidth per call is probably less than 64 kbps, and the circuit is not considered a clear circuit.

Because transmission paths in VoIP networks are more limited than in the PSTN, Cisco IOS CLI is used to adjust fax settings on the VoIP dial peer. The adjusted fax settings restrict the facilities that are available to fax machines across the VoIP call leg and are also used to modify values in DIS and NSF messages that are received from fax machines.

The call flow of the Cisco fax relay data transfer phase is shown in the figure below.

Figure 4

Cisco Fax Relay Data Transfer Call Flow

The T.38 Annex B standard defines the mechanism that is used to switch over from voice mode to T.38 fax mode during a call. The ability to support T.38 must be indicated during the initial VoIP call setup. If the DSP on the gateway is capable of supporting T.38 mode, this information is indicated during the H.245 negotiation procedures as part of the regular H.323 VoIP call setup.

Once the VoIP call setup is completed, the DSP continues to listen for a fax tone. When a fax tone is heard, the DSP signals the receipt of fax tone to the call control layer, which then initiates fax changeover as specified in the T.38 Annex B procedures. The H.245 message flow shown in the figure below contains the following events:

1. The detecting TGW sends a ModeRequest message to the OGW, and the OGW responds with a ModeRequestAck.
2. The OGW sends a closeLogicalChannel message to close its VoIP UDP port, and the TGW responds with a closeLogicalChannelAck while it closes the VoIP port.
3. The OGW sends an openLogicalChannel message that indicates to which port to send the T.38 UDP information on the OGW, and the TGW responds with an openLogicalChannelAck.
4. The TGW sends a closeLogicalChannel message to close its VoIP UDP port, and the OGW responds with a closeLogicalChannelAck.
5. Finally the TGW sends an openLogicalChannel message that indicates to which port to send the T.38 UDP stream, and the OGW responds with an openLogicalChannelAck.
6. T.38-encoded UDP packets flow back and forth. At the end of the fax transmission, either gateway can initiate another ModeRequest message to return to VoIP mode.

Figure 5

H.323 T.38 Fax Relay Call Flow

When the call control protocol is SIP, T.38 Annex D procedures are used for the changeover from VoIP to fax mode during a call. Initially, a normal VoIP call is established using SIP INVITEs. The DSP needs to be informed that it can support T.38 mode while it is put into voice mode. Then, during the call, when the DSP detects fax HDLC flags, it signals the detection of the flags to the call control layer, and the call control layer initiates a SIP INVITE mid-call to signal the desire to change the media stream.

Note	For initial INVITE in SIP calls, Cisco UBE and voice gateways do not support having both fax (T.38) and audio capabilities together.

The SIP T.38 fax relay call flow shown in the figure below contains the following events:

1. The TGW detects a fax V.21 flag sequence and sends an INVITE with T.38 details in the SDP field to the OGW or to the SIP proxy server, depending on the network topology.
2. The OGW receives the INVITE message and sends back a 200 OK message.
3. The TGW acknowledges the 200 OK message and sends an ACK message direct to the OGW.
4. The OGW starts sending T.38 UDP packets instead of VoIP UDP packets across the same ports.
5. At the end of the fax transmission, another INVITE message can be sent to return to VoIP mode.

Figure 6

SIP T.38 Fax Relay Call Flow

The MGCP T.38 fax relay feature conforms to ITU-T T.38, Procedures for Real-Time Group 3 Facsimile Communication over IP Networks, which determines procedures for real-time facsimile communication in various gateway control protocol (XGCP) applications.

MGCP T.38 fax relay provides two modes of implementation:

• Gateway-controlled mode--Gateways negotiate fax relay transmission by exchanging capability information in Session Description Protocol (SDP) messages. Transmission of SDP messages is transparent to the call agent. Gateway-controlled mode allows use of MGCP-based T.38 fax without the necessity of upgrading the call agent software to support the feature.
• Call-agent-controlled mode--Call agents use MGCP messaging to instruct gateways to process fax traffic. For MGCP T.38 fax relay, call agents can also instruct gateways to revert to gateway-controlled mode if the call agent is unable to handle the fax control messaging traffic; for example, in overloaded or congested networks.

MGCP-based T.38 fax relay enables interworking between the T.38 application that already exists on Cisco gateways and the MGCP applications on call agents.

MGCP-based T.38 fax relay has the following call flow:

1. A call is initially established as a voice call.
2. The gateways advertise capabilities in an SDP exchange during connection establishment.
3. If both gateways do not support T.38 fax relay, fax pass-through is used for fax transmission. If both gateways support T.38, they attempt to switch to T.38 upon fax tone detection. The existing audio channel is used for T.38 fax relay, and the existing connection port is reused to minimize delay. If failure occurs at some point during the switch to T.38, the call reverts to the original settings it had as a voice call. If this failure occurs, a fallback to fax pass-through is not supported.
4. Upon completion of the fax image transfer, the connection remains established and reverts to a voice call using the previously designated codec, unless the call agent instructs the gateways to do otherwise.

A fax relay MGCP event allows the gateway to notify the call agent of the status (start, stop, or failure) of T.38 processing for the connection. This event is sent in both call-agent-controlled and gateway-controlled mode.

Fax detection supports the use of a single E.164 number for both voice mail and fax mail by providing the capability to detect through an interactive voice response interface whether an incoming call is voice or fax. Fax detection can be configured to use either the distinctive fax calling tones (CNG) or a manually dialed digit or both to distinguish fax calls from voice calls. Fax detection supports the following modes of operation:

• connect-first--The gateway connects incoming calls immediately to a voice-mail server, which plays a greeting, or audio prompt, based upon the number called. The gateway also listens for CNG throughout the duration of the call and connects the call to the configured fax application if CNG is detected.
• listen-first--The gateway listens for CNG for 9 seconds; an audio prompt can be played during this time. If CNG is detected, the call is connected to a fax application or server. If CNG is not detected, the call is connected to a voice application or server.
• default-voice--The gateway is configured to recognize a particular dual tone multifrequency (DTMF) tone to indicate voice calls and a different DTMF tone to indicate fax calls. If no DTMF tone is heard and no CNG tone is heard for 9 seconds, the call is treated as a voice call.
• default-fax--The gateway is configured to recognize a particular DTMF tone to indicate voice calls and a different DTMF tone to indicate fax calls. If no DTMF tone is heard and no CNG tone is heard for 9 seconds, the call is treated as a fax call.

For configuration information, see Chapter 1, "Configuring Fax Detection."

The fax rollover IVR application provides a configured fallback to T.37 store-and-forward fax if a call attempts to use fax relay and fails. An OGW must be configured with fax relay, store-and-forward fax, and also with the fax rollover application. Then, if a fax relay attempt fails, the call is forwarded to an SMTP server by a mail transfer agent (MTA) using T.37-standard protocols for store-and-forward fax.

For configuration information, see Chapter 1, "Configuring Fax Rollover."