CHAPTER 2 LITERATURE REVIEW

Transcription

1 CHAPTER 2 LITERATURE REVIEW 2.1 FILE FORMATS Facsimile file format defines the typical way in which the information will be encoded Group 3 terminal file format The characteristics of G3 facsimile devices such as the terminal supported dimensions, coding scheme, modulation, demodulation and transmission time per total coded scan line which is the sum of data bits, fill bits and end-of-line bits are given in T.4 recommendation (ITU-T 2003). These devices can send black and white documents and also color documents, provided the device satisfies the color mode recommendation. Coding scheme The following is the coding scheme recommended for G3 terminals by T.4: One dimensional coding scheme o Data: Data has a chain of variable length code words and each code may represent either all white or all black. Black runs and white runs 21

2 alternate. One scan line (horizontal) of 215 mm length can have 1728 picture elements. The maximum length of one scan line of white and black run lengths are defined by code words. There are two types of code words such as terminating code word and make-up code word. Each run length in the range of 0 to 63 pels (picture elements) can have a terminating word. Make-up code word is used for run lengths which are a multiple of 64 up to a maximum run length of 1728 pels. The run length can be denoted by either one make-up code word followed by a terminating code word or one terminating code word. If the run length is in the range of 0 to 63, they are encoded with their terminating code and if the run length is in the range to 1728, first they will be encoded by the make-up code word representing the run length which is equal to or shorter than that required. This will be followed by terminating code word which represents the difference between the required run length and the run length of the make-up code. All the data lines should begin with white run length code word in order to make sure that the receiver maintains color synchronization. A white run length of zero will be sent if the actual scan line begins with a black run. o End-of-line (EOL): Every line of data will be followed by this code word. EOL signal happens before the first data line of a page. This is 22

3 a unique code word. It cannot be found in a valid line of data. Hence, resynchronization is possible after an error burst. o Fill: The Fill can be placed between EOL and the line of data. This cannot be placed within a line of data. Fill is used for sending a pause in the message flow. The Fill can be added in such a way the Fill, EOL and the transmission time of data should be greater than the minimum transmission time of the total coded scan line given in the pre-message control procedure. The maximum time for sending the Fill bits is less than five seconds. o Return To Control (RTC): Six consecutive EOL signals are sent to specify the end of the transmission of a document. The transmitting terminal will send a post message command after the RTC signal. These messages will be in the framed format Internet fax file format Fax over Internet uses the Tag Image File Format (TIFF) to represent the data. McIntyre et al. (1998) presents the TIFF fields and field values that are compatible with ITU-T recommendations for G3 facsimile. Fax data can be compressed into a TIFF file and TIFF fields will encode the parameters that describe the image data (Parsons & Rafferty 1998). The main aim of 23

4 TIFF is to give an atmosphere within which applications can exchange image data. TIFF gives a means for describing, storing and interchanging raster image data. TIFF file structure: TIFF is basically planned for scanned images and so it is best suited for facsimile. Facsimile image has a number of scanlines which has the same number of pixels. Typical TIFF file has an eight byte image file header. The byte order within the file is indicated by the first two bytes. The next two bytes are used for indicating the number 42. The number 42 recognizes the file as TIFF file. In addition to this, ordering is done in accordance with the value given in the first two bytes of the header. Finally, the last four bytes indicate the offset. This offset points to the first Image File Directory (IFD). IFD has a series of tagged fields which are in ascending order by tag value. TIFF file structure for facsimile: File structure of the TIFF is very flexible. There is no specific order for field values, IFDs and image data in a file. Ordering can be done by the individual applications. There some principles for the ordering of TIFF files such as: 1) A multipage fax document should have an IFD for each page. 2) The order of the IFD should be the same in the file as that of the page in the 24

5 document. 3) The IFD can precede the image data to which it have offsets. 4) The value data, IFD and the image data which has offsets should precede the next image IFD. 2.2 FAX MODEM STANDARDS Fax modems along with fax software are required to send and receive facsimile using a computer by connecting it to the telephone network. Fax modems can be connected externally or internally. The fax modems that can be connected internally are called fax boards. Table 2.1: Fax and modem standards Standard Speed Modulation V bit/s Trellis coded modulation V baud Audio frequency shift keying V.22bis V bit/s bit/s Quadrature amplitude modulation Phase shift keying V Phase shift keying & quadrature amplitude bit/s modulation 25

6 A modem modulates analog carrier signals to digital signals and demodulates the signals back to the transmitted information. Modems are often categorized by the amount of data they can send which is usually expressed in bits/bytes per second (Schulzrinne 2006). Modems are also categorized by their symbol rate which is expressed in baud (symbols per second). One of the differences between facsimile device and modem transmission is that in a facsimile device the payload initiates as a paper image which has been converted as digital data but in modem the payload initiates as digital data. Some standards that are common to facsimile devices and modems are given in Table FACSIMILE PROTOCOLS T.38 T.38 is the only protocol standardized for implementing real time fax over IP. T.38 protocol can be used for communication between facsimile devices that are directly connected to an IP network. This can also be used for communication between PSTN facsimile devices through Internet by using fax gateways. The gateway can be an Analog Telephone Adapter (ATA) box, which can connect the traditional fax to the Internet. The facsimile will be sent as image data and this will reduce the bandwidth required for transmission. The traditional T.38 specification uses a V.17 26

7 modem with operation up to bps (ITU-T 1991). The new version uses V.34 modem with operation up to bps. T.38 can work using TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) or RTP (Real-time Transport Protocol) as the transport layer protocol (Stevens 1999, Comer 2007, Comer & Stevens 2003). TCP is best for communication when there is no real-time constraint between the communicating terminals. TCP provides proper error control but the gateways will have only little control over the fax data timing. Hence, UDP is preferred and more popular as transport layer protocol for real-time facsimile communication than TCP. Figure 2.1: Elements of a T.38 facsimile terminal 27

8 The reporting statistics provided by UDPTL (UDP Transport Layer) was not as good as RTP (Jones & Tamura 2006). In addition to this, it did protect the integrity and did not provide encryption. RTP provides header compression and it is interoperable with a wider range of devices such as Network Address Translation (NAT), firewalls etc. TPKT which is ISO (International Standards Organization) Transport Service on top of the TCP offers a technique to carry data over TCP/IP networks (Pouffary & Young 1997). The messages that pass between T.38 terminals called the Internet Facsimile Protocol (IFP) messages are dependent on the transport layer protocol. In contradictory, the second stage packetiser is dependent on the transport layer protocol used. In order to improve the reliability of T.38 protocol when UDPTL and RTP are used, Forward Error Correction (FEC) information is added to the packets. TPKT encapsulation describes the starting and ending of IFP packets in a TCP stream. Figure 2.1 shows the elements of a T.38 facsimile terminal. HDLC (High-level Data Link Control) decoder outputs HDLC frames octet by octet and thus prevents unacceptable latency. HDLC encoder takes the incoming message and adjusts the length if the data is falling behind and does the flow control. 28

9 Figure 2.2: Elements of a T.38 gateway 29

10 During T.30 message analysis and manipulation, the content of the message are tracked and passes the frames with minimum possible delay. Figure 2.2 shows the elements of a T.38 gateway. TCF (Training Confirmation) has a stream of zero bits. It is not required to exchange the TCF data between the T.38 entities and it can be checked locally. This can be done when TCP/TPKT is used as the transport layer protocol. In this case, a TCF checker and a TCF generator are required T.37 The store and forward method of faxing uses the T.37 protocol and this method has the capability of storing the faxes between the sending and receiving stages if required. This method is simple and reliable. Figure 2.3 shows the facsimile transmission using T.37 protocol. Store and forward faxing has the capability of delivering the faxes as messages. In addition to this standard , messages can be delivered, as facsimile messages to the G3 facsimile equipment. The sending facsimile will be connected to the IP network using on-ramp gateway. This gateway will convert the facsimile into messages which can be sent over IP network. Accordingly, the on-ramp gateway converts the facsimile from the PSTN based fax equipment to TIFF (Tagged Image File Format) file attachment. This gateway assigns the TIFF file to by producing MIME 30

11 (Multipurpose Internet Mail Extension) message (Freed & Klensin 2005). This type of is called the fax mail. Figure 2.3: T.37 facsimile transmissions The on ramp gateway will forward the fax mail and the attachment to the concerned SMTP (Simple Mail Transfer Protocol) server of the messaging infrastructure. The message will be routed and transported to the 31

12 destination by the messaging infrastructure. The gateway at the terminating end called the off-ramp gateway will convert these messages back to facsimile messages. 2.4 PERFORMANCE EVALUATION OF FACSIMILE IN AN IP NETWORK The facsimile sent using the IP network should work efficiently and must be as reliable as that in PSTN. The performance of facsimile will be the same in IP and PSTN networks during ideal network conditions. Achieving this ideal condition during transmission is challenging. The research work by Aljaz, Imperl & Mrak (2007) evaluated the performance of FoIP under various characteristics of IP networks such as jitter, packet loss and delay using G.711 codec and T.38 protocol. In an IP network, which is also known as packet based network, the data moves in small blocks called packets. Each packet may follow different routes to reach the destination. The packets are reassembled at the destination on arrival. Delay is the time required for the packet to travel from the source to the destination. Jitter is the variation in delay and it is caused mostly due to network congestion. Packet loss happens when some packets fail to reach the destination. The G.711 codec is mainly used in telephony and it is the 32

13 first ITU-T recommendation for speech coding. It is also known as Pulse Code Modulation (PCM). G.711 can be used for sending PSTN signals, modem signals and facsimile documents through the IP network. It provides end to end connectivity and all the signals and modem tones and negotiations are passed transparently through the IP network. Test environment Figure 2.4: Test environment The performance analysis by Aljaz, Imperl & Mrak (2007) was conducted in a test environment which created conditions similar to a public 33

14 packet based network as shown in Figure 2.4. G3 facsimile equipment was connected to TDM exchange directly. Two FoIP gateways were used to connect PSTN network to the IP network. A packet based network emulator device was used to emulate various network parameters such as delay, jitter and packet loss. Test methodology Test methodology was derived considering that the image quality evaluation is a matter of subjective judgment and recurrent assessment of the same image may produce different results. The transmission speed and quality of the facsimile document can depend on the quality of the sending and receiving facsimile equipment. The network characteristics such as delay, jitter and packet loss cannot be exactly controlled. So the two facsimile documents sent between same equipment using the same protocol under same network characteristics may yield various results. The test image used for the experiment was standardized, digitized image set with high density and patterns that offer enough observation points for the analysis of image quality. The quality of a transmitted image (FoIP IQ ) evaluation was done by evaluating ten observation points. Ten observation points were defined. These 10 observation points were assigned 2 points if all 34

15 the details of the observation point were clear, 1 point if some were clear and some were blurred, and 0 if most of the observation points were not clear. Average transmission time (FoIP TT ) is the difference between the start time of the facsimile transmission (t start ) and the end time of the successful transmission at the receiving facsimile device (t end ): The Normalized Transmission Time (FoIP NTT ) is the ratio of FoIP TT and FoIP TTO parameters: where FoIP TTO was defined as the average measured transmission time in the case of ideal network condition, when there is no traffic congestion. The FoIP SR parameter was defined as the ratio of number of successful transmission (n ok ) to the total number of facsimile documents sent (n fax ): Facsimile Satisfactory Rate parameter (FSR) is given by: 35

16 One hundred subjects, who are familiar with the facsimile communication, were interviewed and they agreed with a minimum value points for FoIP IQ, 0.8 for FoIP SR and FoIP NTT can be below 1.1. Considering the above values for each parameter, the acceptable value of FSR derived is In order to study the influence of delay on the facsimile transmission, the emulation device simulated a delay of 0 to 500 ms. To study the influence of packet loss on the facsimile transmission, the emulation device simulated 0% to 9 % packet loss. The jitter was set according to the Gaussian distribution as ( ) ( ) where ( ) indicates the probability that a packet arrives in time x with an average delay µ and standard deviation δ. For each facsimile, three average one way delay values µ=98, 132 and 166 ms, were used. The values of standard deviation δ used were 5 to 45 ms to study the influence of packet delay variation. The experimental result showed that there was no degradation in the image quality when a constant delay was introduced during the facsimile transmission using the G.711 codec and T.38 protocol. The most important 36

17 network characteristic that affected the performance was the packet loss. In this scenario, T.38 protocol performed better than G.711 codec. For T.38 protocol, the facsimile transmission was successful for a packet loss upto 4% and 0.3 % to 0.8% for G.711 codec. As in the case of packet loss, the T.38 protocol performed better in the case of packet delay variation (jitter). 2.5 ADAPTIVE INTERNET FAX PROTOCOL Yeo et al. (2000) delivers a framework for facsimile transmission through Internet using Adaptive Internet Fax Protocol (AIFP). AIFP provides delivery confirmation for real time facsimile transmission. It is designed to be used by all venders and interoperates with other Internet facsimile protocols. AIFP consists of three protocols such as control secondary protocol, feedback secondary protocol and data transfer secondary protocol. Control secondary protocol is used for exchanging capabilities by the facsimile equipment before starting the data transmission. In addition, this protocol is also used for synchronization by sender and receiver when they change from TCP socket to UDP socket or vice versa for the data transfer. Feedback secondary protocol provides dynamic QoS using network initiated control. The core protocol of AIFP is the data transfer secondary protocol. This protocol is responsible for the data transmission using TCP or UDP. 37

18 Integrating these three secondary protocols, AIFP can handle issues such as capability exchange, change of data socket, data transfer and network load feedback. AIFP is fast in delivery of the facsimile document. It is inexpensive in its operating system and hardware requirements. 2.6 SIP CONTACT HEADER IN FoIP SIP (Session Initiation Protocol) can be used for the real time facsimile communication (Choudhary, Perl & Sidhu 2001). The research work by Sarvakar et al. (2009) suggests that the utilization of SIP in combination with SDP (Session Description Protocol) can reduce the load on proxy servers for the FoIP applications. SIP/SDP can be used for negotiating fax parameters. SDP attribute fields can be used to specify the parameters of the T.38 facsimile. SIP along with SDP provides efficient fax session establishment, management etc. during real time facsimile transmission. 2.7 MULTICASTING IN REAL TIME FoIP Multicasting is the transmission of a message to a group of destinations simultaneously from a source. In computer networking, it is referred as one-tomany communication. Irshad, Nawaz & Mohsin (2006) have proposed architecture and demonstrate several practices for implementing multicasting of real time facsimile transmission. In this architecture, when the sending 38

19 terminal initiates the call, the facsimile transmission can take place with the same traditional five phases and the multicasting work will be accomplished by the IP multicast and IGMP (Internet Group Management Protocol). IGMP is the most common protocol used for multicasting in an IP network. This architecture requires the sending and the receiving devices to share similar capabilities such as exchange data and flags at the same rates. In this case, a local database is maintained on gateways with IP addresses of the destination gateways with respective fax numbers. This aids in the address calculation of the receiving facsimile devices. 2.8 JUNK FAX Junk faxes are often unwanted advertisements sent through facsimile transmissions. They are in fact a form of telemarketing. They are same as junk or spam mail. Generally, a firewall is a software or hardware that is used to protect the resources in a network. Junk fax can be prevented by installing fax firewall (Home & office phone depot 2012) on the fax line which has caller ID service. Hence, only the invited senders facsimile will be received. The senders can be invited using number, name or wildcard. The firewall facilitates blocking of anonymous numbers. The blocking can be done based on numbers, area codes or prefixes. The faxes can also be directed to additional fax machines. 39

20 2.9 IP FAX SOFTWARE FaxBack (2006) examines the aspects of IP fax software such as NET SatisFAXtion. It evaluates how this software can be used in a VoIP environment and how to avoid buying expensive fax hardware. It details the advantages of using IP fax software and how to make advantage of built-in fax capabilities of VoIP systems. 40