Fax over IP: Network Bandwidth and Interoperability Considerations

Transcription

1 Fax over IP: Network Bandwidth and Interoperability Considerations Voice over Internet Protocol (VoIP) has achieved wide acclaim and usage as a low-cost alternative for long-distance and international calling. Hence, VoIP networks are considered an attractive option for fax applications. Fax over Internet Protocol (FoIP) makes fax calls on the same Internet Protocol (IP) network that transports voice and data applications. Using VoIP for fax transmission offers a number of advantages, from cost savings and location independencies to the ability to store and retrieve fax later. The IP network bandwidth in some applications is conserved as the fax bit rates are lower than VoIP voice calls. While there are cost and service benefits, FoIP is impacted by a number of interoperability problems. These problems range from the type of fax machines used to the various IP network impediments. The interoperability concerns can be alleviated, by ensuring that the FoIP implementations are tolerant of the many anomalies that can occur. This document: Provides an overview of the FoIP Compares fax network bandwidth requirements with G.711 voice calls Examines the interoperability concerns that need to be addressed to ensure that the FoIP functions like a Public Switched Telephone Network (PSTN) based fax call.

2 Table of Contents About FoIP 3 Fax Pass Through 3 T.38 Based Fax Relay 3 PSTN and VoIP Fax Calls 3 Session Initiation Protocol (SIP) based FoIP Calls 4 Fax over IP Bandwidth Calculations 5 FoIP Interoperability 6 Interoperability with Fax Machines 7 Interoperability with VoIP Adapters and Gateways 7 Handling of Voice to T.38 Fax Call Switching 7 Deviations in Fax Call Tones 7 IP Network Impediments 7 Message Collisions Example with DIS and TCF 8 Packet Payload and Format Issues 8 Transmission Characteristics 8 End-to-end Gateway Clock Precisions 8 Interactions with Various Voice Modules 8 Achieving FoIP Interoperability 8 2

3 About FoIP The FoIP applications enable standard fax machines to work with the VoIP infrastructure. Voice interfaces on the VoIP Customer Premises Equipment (CPE), also called as the VoIP adapters or gateways will support fax calls much the same way the PSTN telephone interfaces support them. The popular real-time modes of sending FoIP are: Fax pass-through T.38 fax relay Fax Pass Through Fax pass through is similar to a G.711-based VoIP voice call, but additional care has been taken to modify the functionalities of voice modules. These functionalities include echo cancellation, Voice Activity Detection/Comfort Noise Generation (VAD/CNG), Packet Loss Concealment (PLC), Dual Tone Multi Frequency (DTMF) rejection, signal gain-loss settings and jitter buffers. T.38 Based Fax Relay The T.38-based fax relay is a true and a real-time FoIP call. The T.37 is a store and forward equivalent of T.38; it works in a similar way as the does. The T.38 or T.37 based fax uses the fax data pump modem (modem here refers to modulation and demodulation) that consists of ITU-T-V.21, V.27ter, V.29, V.17 and V.34 modules. Fax machines that support V.34 modules are known as Super Group-3 (SG3) and machines without V.34 modules are known as Group-3 (G3). These modules are used for extracting bits from modulations for delivering the payload bytes in VoIP packets. The management of a fax call is done through VoIP signaling, such as Session Initiation Protocol (SIP) and T.38. The T.30 ensures proper operation of end-to-end fax call in PSTN. A minimal part of T.30 is resident along with T.38 to handle the T.30 timeouts and simulate spoofing techniques. In the absence of T.38 support in the gateway or if interoperation of T.38 and data pump is not able to progress with the FoIP, a fax call is established in the G.711 pass-through mode. PSTN and VoIP Fax Calls Figure 1: Functional PSTN fax call as G.711 voice A VoIP fax call is shown in the Figure 2. In some of the VoIP deployments, a fax is sent in passthrough mode using G.711. When network conditions are perfect, fax pass-through will work exactly like a PSTN-based fax call. In most of the VoIP deployments packet impediments are unavoidable. Multiple redundant packets are sent as per RFC2198 (a guideline for redundant media packets), to counter packet impediments. In T.38 and T.37, modulations are extracted as 3

4 bits and bytes to be sent as packets. This conserves bandwidth compared to G.711 passthrough. A T.38-based fax call makes use of the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) based transport protocols to deliver packets on the IP network. On UDP, packets are sent using either Real-time Transport Protocol (RTP) or UDP Transport Layer (UDPTL). The T.38 fax over UDP with UDPTL for transport is the most popular and wellestablished method used in today's deployments. UDPTL makes use of redundancy and Forward Error Correction (FEC) techniques as per ITU-T T.38. Figure 2: Representation of VoIP Fax Call between Two Gateways Session Initiation Protocol (SIP) based FoIP Calls A SIP based FoIP call is shown in the Figure 3. The FoIP call initially establishes like a voice call. On detection of either answering tone (ANS, also referred to as caller tone-ced), or ANS amplitude and phase modulated (ANSam) and V.21 preamble indications, a SIP INVITE request is sent to the emitting gateway for T.38 fax call connection. At a high level, the following functional events occur with a FoIP call: The receiving or terminating gateway detects an ANS/ANSam/V.21 flag sequence and sends an INVITE with T.38 details in the Session Description Protocol (SDP) field to the emitting/originating gateway or to the SIP proxy server, depending on the network topology. The originating gateway receives the invitation to the call with INVITE message and sends back acceptance with a 200 OK message. SIP uses these capitalized message names to adhere to long-standing conventions in a number of protocols. The terminating/receiving gateway acknowledges the 200 OK messages and sends a confirmation with ACK message directly to the originating gateway. The fax call will be established in T.38 mode if the originating gateway supports the T.38 relay; otherwise the fax call will fall back to G.711 mode. In T.38 mode, the originating gateway starts sending Internet fax packets (IFP) over UDPTL, transport protocol data unit packet (TPKT) or RTP depending on the negotiations between the gateways. Most of the gateways use UDPTL to transmit fax packets At the end of the fax transmission, another INVITE message is sent to return the line to voice mode, or the originating machine disconnects the T.38 session after detecting disconnect (DCN) message. 4

5 OFF-Hook Calling Fax Machine Emitting VoIP Gateway Called Fax Machine Receiving VoIP Gateway Dial Tone Dialing SIP INVITE (Voice capabilities) in SDP 180 Ringing 200 OK ACK Voice Call Ringing OFF-Hook ON-Hook Send Calling Tone (CNG) 1100Hz 0.5 sec ON and 3 sec OFF ANS(CED)/ANSam Tone 2.4 to 4 sec Re INVITE(T.38 capabilities in SDP) 200 OK ACK Send Preamble; DIS(optional NSF and CSI) with V.21 FSK Send Preamble; DIS (optional NSS and TSI) with V.21 FSK High Speed Modem Training - TCF with V ms Send Preamble, Confirmation to Recieve (CFR) with V.21FSK Send Page-1 Data at 14.4 kbps (V.17) 75 ms Send Preamble, MPS (Multi Page Signal) with V.21 FSK Send Preamble, MCF (Message Confirmation) V.21 FSK Send Page-2 Data at 14.4 kbps (V.17) Preamble- Send End of Procedure (EOP) with V.21 FSK Send Preamble, MCF V.21 FSK Send Preamble, DCN with V.21 FSK BYE 200 OK ON-Hook Keywords of figure: DIS (Digital Identification Signal); CSI (Called Subscriber Identification); TSI (Transmitting Subscriber Identification); NSS (Non Standard Facilities); TCF (Training Check Flag); DCN (Disconnection Notice); FSK (Frequency Shift Keying) Figure 3: T.38 Basic Fax Call Fax over IP Bandwidth Calculations To cater to the network impediments, redundancy and FEC techniques are used in FoIP calls based on UDPTL/RTP. However, these techniques increase the bandwidth requirements for the FoIP call. Table 1 provides a summary and comparison of T.38 and G.711 bit rates. In the table, redundancies are marked with symbols "R0, R1 and R3." In this article, R0 means that there is no redundancy with the primary payload. R1 is redundancy-1, signifying that there is one extra previous payload; and R3 indicates three extra previous payloads. FoIP pass-through takes kbps on Ethernet interface. At R3, G.711 takes kbps. In practical systems, G.711 passthrough is used with 10 or 20 ms packets with R1. 5

6 In T.38, redundancy up to R3 is used. Low speed (V.27ter-2400 bps) fax with T.38 takes only 8.48 kbps on Ethernet interface. High-speed fax of 9600 or bps is popularly used worldwide. Fax at bps with R3 takes kbps, which is comparable to basic PSTN 64 kbps and lower than G.711 VoIP voice call. In general, T.38 offers a bandwidth utilization advantage of two to 20 times compared to a G.711 fax pass-through call. In this article, bandwidth calculation examples are considered with G.711 at 10 ms intervals and T.38 payloads at 40 and 100 ms intervals. Smaller packetization reduces end-to-end delay and the packet payload size in case of V.34 higher bit rates. V.34 at 40 ms packetization is noted as the highest bandwidth used in T.38 FoIP call. In some countries, users receive about 50 Mbps to 100 Mbps of Internet bandwidth. As a result, bandwidth from G.711 pass-through, even with R3, is negligible compared to the available bandwidth. In general, G.711-based FoIP can be used when the available guaranteed network bandwidth is at least 256 kbps per channel and there are no impediments in end-to-end packet delivery. Table 1: Summary on G.711 and T.38 Fax Bandwidth on Ethernet Interface Fax module Rate in bps V V.29/V V V V ms packets Redundancy T ms bandwidth (kbps) G ms bandwidth (kbps) Remarks R T.38 of 8.48 kbps is 15 times lower than G.711 R R T.38 is 19 times lower R R R T.38 is 7 times lower. This is most popular in deployment. R R R T.38 is 5 times lower R T.38 is at least 2 times better R R R T.38 is 2 times lower than G.711 even at 40 ms packets FoIP Interoperability The primary challenge in FoIP implementation is end-to-end interoperability. By improving interoperability, FoIP calls can be comparable to PSTN-based fax calls. In most situations, PSTN fax calls are completed successfully. Nevertheless, PSTN fax calls can occasionally fail because of line conditions from user fax machine to the PSTN digital loop carrier or central office. Fax machine anomalies, mismatched message and capability exchanges contribute to these failures. Even in the PSTN, intermediate long distance routing of fax calls may happen as FoIP or G.711 VoIP voice calls. FoIP interoperability can be impacted by the type of fax machines used, VoIP gateway features, fax call switching and deviations in fax call tones. IP network impediments - such as delays and timing issues, packet formats, redundancy, error correction mode (ECM), end-to-end transmission characteristics, clock drifts, various configurations and interactions between voice and fax modules also affect interoperability. However, interoperability concerns can be alleviated by ensuring that FoIP implementations are tolerant of the many anomalies that may occur. The FoIP interoperability issues include the following: 6

7 Interoperability with Fax Machines There are several deviations among the available fax machine timings in delivering messages and responses. T.38 FoIP adds delays that may exceed fax-timing limits, resulting in failed fax calls or forcing the fax call to G.711 pass-through mode. The new SG3 with V.34 support is creating interoperability with other low-speed machines and computers. Personal computers, in combination with a telephone interface, also are used for faxing document files. In many situations, users may not have upgraded the bug patches or use of the computer as fax machine is not fully evaluated with hardware and operating system combinations. The goal here is to make the VoIP gateway interoperate with several fax machine anomalies. Interoperability with VoIP Adapters and Gateways Many of the VoIP gateways primarily will be used for voice services and may not support T.38. There are several revisions in fax standards with new features and optional messages given in RFC4161 (as "Guidelines for Optional Services for Internet Fax Gateways") that may be handled in a fax call. These changes introduce gateway interoperability issues. To ensure proper function, VoIP gateways must continually be upgraded with the latest fax revisions. Handling of Voice to T.38 Fax Call Switching Gateways initially establish VoIP voice calls using compression codecs, such as G.729A, G and G.711, then switch to the fax call after analyzing for CNG (calling tone), CED/ANS family of fax/modem tones and V.21 preambles. Some FoIP Gateways also wait for the V.21 preamble from originating fax machine to switch from Voice-to-T.38 Fax call. In in-band operation, tones and V.21 preamble tone and DIS signals may be distorted with compression codecs. Out-of-band packet creation will introduce huge delays in detecting and regenerating tones and messages. In general, voice-to-fax call switching is established with CNG, ANS, and V.21 preamble in-band and out-of-band combinations. In all these situations, T.38 relay should be generic so it can handle several combinations of fax call switching modes. Deviations in Fax Call Tones CED/ANS tones have several deviations. The ANS family of tones consists of ANS, ANS/, ANSam and ANSam/. Here suffix '/' in keywords denotes phase modulation. Some fax machines also send modem tones in place of fax tones. This requires extra validation for fax and modem, which is not supported in all the gateways. Validating ANS or ANS/ with additional CNG or V.21 preamble detections is a useful option. Some machines omit the ANS and just begin with the V.21 preamble first handshake message. A G3 fax machine sends an ANS tone, while a highspeed SG3 fax machine sends the ANSam/ tone during call set-up phase. These deviations create interoperability challenges. IP Network Impediments The IP network creates impediments of delay, jitter, packet drop, fragmentation and errors. With packet impediments, message collisions occur as a result of increased delays and unacknowledged commands. In T.30 protocol, procedures are defined with timeout mechanisms. If a response to the message is not received within specified time of three seconds, then the fax machine re-transmits the message or disconnects the call after repeating the message for three times. With more end-to-end impediments, users will experience more time outs and disconnects. Fax spoofing techniques and logic for collisions are used to improve call success with IP impediments. In the absence of required data from network, spoofing logic sends the known pattern to fax machine that makes fax connection alive. 7

8 Message Collisions Example with DIS and TCF Fax works as half-duplex. Fax message collisions with digital identification signal (DIS), digital command signal (DCS), and training check (TCF) contribute to the fax call-switching failures. To avoid DIS and TCF collision, the DCS spoof timers are used. Sometimes it is useful to deny the first DIS message and keep using subsequent DIS messages. This will allow enough time to check for collisions and prevent further collisions and re-transmissions. Packet Payload and Format Issues FoIP has several options in IP packet creation. T.38 IFP fax data is sent as TCP using TPKT and UDP using UDPTL or RTP. The problems grow with combinations of fax machines, redundancy, FEC and ECM modes. T.38 relay should be capable of handling several combinations of packets. The deviations include: T.38 implementation sends one T.30 signal frame as one or multiple packets. High Level Data Link Control (HDLC) frames and packets have different frame boundaries when inserting the data into packets. Redundancy and duplicate packets are used without distinction. T.38 relay implementations follow different packetization intervals and redundancies for low-speed data and indicator packets. There are some T.38 implementations that follow duplicate indicator packets with same sequence number for multiple times. In ECM, fax image data is divided into blocks and frames. Frames are sent with HDLC and cyclic redundancy check (CRC). Transmitting and receiving fax machines work in coordination to get complete error free blocks through re-transmission. Retransmission of lost packets can cause long delays making fax call to time out. Transmission Characteristics VoIP gateways should comply with local PSTN transmission guidelines. Ensuring these transmission characteristics through proper selection of telephone interface components and proper tuning to comply with local PSTN standards helps reduce transmission errors. End-to-end Gateway Clock Precisions The clocks used in VoIP gateways create several issues if they drift. VoIP boxes use ±50 parts per million (PPM) clock as a reference for generating voice and fax packets. This clock PPM rating can result in fax transmission errors when several pages of fax are sent. By incorporating clock matching very close to the Stratum-3 precision of ±4.6 PPM, these fax transmission errors can be reduced. Interactions with Various Voice Modules As shown in Figure 3, a fax call is established as a VoIP voice call, and then switched to FoIP call. In the transition of voice to fax call, configurations are modified for various voice modules like echo canceller, VAD/CNG, PLC and jitter buffer. Any mismatch in configurations will create fax page errors. Achieving FoIP Interoperability FoIP transmission offers many of the same benefits as VoIP voice calls. However, there are significant differences, too. T.38-based FoIP calls take relatively lower bandwidth than G.711 VoIP call. Voice calls are interactive and listeners can adapt to voice impediments. In fax calls, 8

9 tones, messages, and page data have to operate in an automated way without human interaction. Several deviations and options must be taken into consideration when implementing FoIP solutions. In spite of these issues with T.38 fax interoperability, it is possible to achieve quality comparable to PSTN fax calls by making the system to be tolerant of anomalies and standards violations, while giving the solution intelligence to handle several unexpected events. FoIP seamlessly integrates with VoIP and provides the advantages expected from an IP network. FoIP working as G.711 pass through, T.38 or T.37 - in combination with perfecting the solutions to provide robust high quality fax services - will serve to support the migration of PSTN-based fax calls to VoIP. 9

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