TA2 CH5 Long Distance Data Transmission PDF

Long Distance Data Transmission CHAPTER - 5 LONG DISTANCE DATA TRANSMISSION 5.0 Introduction In this chapter, topics on Modems, their working and diagnostics features, which are applicable to Railway owned long distance dat...

Long Distance Data Transmission CHAPTER - 5 LONG DISTANCE DATA TRANSMISSION 5.0 Introduction In this chapter, topics on Modems, their working and diagnostics features, which are applicable to Railway owned long distance data networks like PRS, FOIS, UTS etc. discussed. The topics on Digital Subscriber Loop (DSL) and extending Networks using LAN extenders and Media Converters for Internet / Railnet connectivity are elaborately covered. 5.1 Modem A modem is like a telephone set for a computer. Modems let digital devices like computers talk to each other over the ordinary analog telephone system. 5.1.1 Modem working The conventional Plain Old Telephone System (POTS) is more than 100-years old now. Another common term for POTS is the PSTN (Public Switched Telephone Network). The PSTN was not designed for transmission of digital electronic computer signals, as electronic computers did not exist with the original design of the PSTN. The PSTN was originally designed only to send analog Voice frequency signals; therefore it can only pass low frequency analog signals in the range of 300-to-4000 cycles per second. Modem Modem DIGITAL DIGITAL ANALOG Fig. 5.1 Application of modem for Data communication over Analog media Therefore, modems were invented to use the PSTN to send digital data as shown in fig 5.1 A modem changes, or modulates, digital data into electronic analog signals that the telephone network can carry. At the other end of the connection, another modem demodulates, or interprets the telephone signals and converts them back to digital computer data signals. The word "modem" is an acronym that comes from combining the words "Modulate" and "Demodulate." i. Data sent over ordinary telephone lines: Fundamentally, modems send analog data bits sequentially over telephone lines similar to how the telegraph systems sent signals in the form of dots and dashes (Morse Code) to represent information. Morse code is a binary coding system that represents all the letters of the alphabet, punctuation and numerals. Like Morse code used in telegraph systems, modems also use a binary coding system called "ASCII" (American Standard Computer Information Interchange) code. ASCII code is used to represent the alphabet, punctuation and numerals with a unique series of seven ones or zeros in all combinations, which gives the possibility of sending 128 different characters. High-speed modems can send data over the PSTN at rates ranging from 300-to-56,000 bits per second (bps). IRISET 79 TA2 – Data Communication & Networking Long Distance Data Transmission ii. Modem speed: Signaling standards determine the modem's speed. Modem speed is measured in bits-per-second (bps). Bps refers to how many bits of data per second a modem can send and receive over the telephone line. 56K for example is 56,000 bits per second. 56K is defined under the ITU V.90 standard. The ITU V.34 standard defines an upper limit data speed of 33,600 bps or 33.6K bps. There are also several other lower speed standards defined by the ITU. 5.2 CLASSIFICATION OF MODEMS i. Classifying Modems according to Range a. Short Haul: Short haul modems are cheap solutions to systems of short ranges (up to 15 km), which use private lines and are not part of a public system. Short haul modems can also be used, even if the end-to-end length of the direct connection is longer than 15 km, when both ends of the line are served by the same central office in the telephone system. These lines are called "local loops". Short haul modems are distance-sensitive, because signal attenuation occurs as the signal travels through the line. The transmission rate must be lowered to ensure consistent and error-free transmission on longer distances. Short haul modems tend to be cheaper than other modems for two reasons: 1. No circuitry is included in them to correct for differences between the carrier frequency of the demodulator and the frequency of the modulator. 2. Generally no circuitry is included to reduce/correct for noise rejection, which is less of a problem over short distances than over long distances. There are two main types of short haul modems: Analog modems Using a simple modulation method, without sophisticated devices for error control or equalizers. These modems usually operate at a maximum rate of 9600 bps, but there are some which supports higher rates (up to 64,000 bps). Line drivers: Increase the digital signal, which transmit to the communication channel and do not transmit the carrier signal, as conventional modems. Line drivers are very cheap and tiny and connect to the RS232 connector of the terminal (since they lack a power supply, they use the signal voltage of the DTE-DCE interface for DC power supply). b. Voice Grade (VG): Voice-grade modems are used for unlimited destination, using a moderate to high data rate. These modems are expensive and their maintenance and tuning are sophisticated. Communication channels are leased lines and dial-up. Voice-band telephone network is used for data transmission. A user-to-user connection may be either dedicated or dialed. The links in the connection are the same in the two cases, and the only difference for the user is that for some impairment (particularly attenuation and delay distortion), a dedicated (private or leased) line is guaranteed to meet certain specifications, whereas a dialed connection can only be described statistically. c. Wideband: Wideband modems are used in large-volume telephone-line multiplexing, dedicated computer-to-computer links. These modems exceed high data rates. IRISET 80 TA2 – Data Communication & Networking Long Distance Data Transmission ii. Classifying Modems according to: Line Type a. Dial up: Dial-up modems can establish point-to-point connections on the PSTN by any combination of manual or automatic dialing or answering. The quality of the circuit is not guaranteed, but all phone companies establish objectives. The links established are almost always 2-wire because 4-wire dialing is tedious and expensive. b. Leased: Leased lines (usually 4-wire) are for the exclusive use of "leased-line" modems - either pair (in a simple point-to-point connection) or several (on a multidrop network for polling or a contention system). If the medium is the telephone network, their transmission characteristics are usually guaranteed to meet certain specifications, but if the link includes any radio transmission, the quality of it may be as variable as that of a switched (i.e. non dedicated) line. iii. Classifying Modems according to: Operation Mode a. Half Duplex: Half duplex means that signals can be passed in either direction, but not in both simultaneously b. Full Duplex: Full duplex means that signals can be passed in either direction, simultaneously. Full duplex operation on a two-wire line requires the ability to separate a receive signal from the reflection of the transmitted signal. This is accomplished by either FDM (frequency division multiplexing) in which the signals in the two directions occupy different frequency bands and are separated by filtering, or by Echo Canceling (EC). The implication of the term full-duplex is usually that the modem can transmit and receive simultaneously at full speed. c. Simplex: Simplex means that signals can be passed in one direction only. A remote modem for a telemeter system might be simplex and a 2-wire line with a common unidirectional amplifier is Simplex. iv. Classifying Modems according to: Synchronization a. Asynchronous Modems: Most of the modems that operate in slow and moderate rates, up to 1800 bps, are asynchronous (using asynchronous data). Asynchronous modems operate in FSK modulation and use two frequencies for transmission and another two for receiving. In a 2-wire line, full duplex operation can be achieved by splitting the channel into two-sub channels as shown in fig. 5.2. Asynchronous data is not accompanied by any clock, and the transmitting and receiving modems know only the nominal data rate. To prevent slipping of the data relative to the modems' clocks, this data is always grouped in very short blocks (characters) with framing bits (start and stop bits). The most common code used for this is the seven-bit ASCII code with even parity. IRISET 81 TA2 – Data Communication & Networking Long Distance Data Transmission Fig. 5.2 Operating Asynchronous modem in a 2 Wire line b. Synchronous Modems: Synchronous modems operate in the audio domain, at rates up to 28800 bps in audio lines, used in telephones systems (using synchronous data). The usual modulation methods are the phase modulation and integrated phase and amplitude (at higher rates than 4800 bps). In synchronous modems, equalizers are used, in order to offset the misfit of the telephone lines. These equalizers are inserted in addition to the equalizers that sometimes already exist in the telephone lines. These equalizers can be classified into three main groups: Fixed/statistical equalizer - these equalizers offset the signal according to the average of the known attenuation in each frequency. Tuning the equalizer is sometimes done in the factory and stays fixed; usually they are used to operate at low rates in a dial up line. Manually adjusted equalizer - these equalizers can be tuned to optimal performance to a given line. These equalizers should be re-tuned when the line is replaced and periodically. Specially, it should be tuned frequently when the line is of a low quality and it's parameters are changed frequently. Tuning is done using a button inside the modem (or on the external board). Automatic equalizer - these equalizers are tuned automatically when the connection is established. Depending on the line quality in a specific moment, in a process of about 15ms to 25ms, after the first tuning, the equalizer samples the line continually and adjusts itself to the changed conditions, so the modem operates at each moment under optimal conditions. The fitness process operates, in some modems, at rates of 2400 times in a second. Synchronous modems operate in the same manner asynchronous modems. However, synchronous modems operate at higher rates and since the requirements to transmit at these rates are increasing, most of the innovations are implemented for synchronous modems. In synchronous modems the channel can be split for several consumers at various speeds. Modems who have this ability are called SSM - Split System Modem. These modems can use a simple split or a split using multipoint connection. Synchronous data is accompanied by a clock signal. Synchronous data is almost always grouped in blocks, and it is the responsibility of the data source to assemble those blocks with framing codes and any extra bits needed for error detecting and/or correcting according to one of many different protocols (BISYNC, SDLC, HDLC, etc.). The data source and destination expect the modem to be transparent to this type of data; conversely, the modem can ignore the blocking of the data. IRISET 82 TA2 – Data Communication & Networking Long Distance Data Transmission v. Classifying Modems according to MODULATION: Communication channels like telephone lines are usually analog media. Analog media is a bandwidth-limited channel. In the case of telephone lines the usable bandwidth frequencies is in the range of 300 Hz to 3300 Hz. Data communication means moving digital information from one place to another through communication channels. These digital information signals have the shape of square waves and the meaning of "0" and "1" If such digital signals were transmitted on analog media the square waves of the digital signals would be distorted by the analog media. The receiver, which receives these distorted signals, will be unable to interpret accurately the incoming signals. These digital signals must be converted into analog signals so that the communication channels can carry the information from one place to another. The technique, which enables this conversion, is called modulation like QAM, QPSK etc. 5.3 Modem Diagnostics: Base band modems are provided with inbuilt loop diagnostics (V.54 protocol ITU-T standard) to check the integrity of the leased line connectivity. V.54 is an ITU standard for various loop back tests that can be incorporated into modems for testing the telephone circuit and isolating transmission problems. Operating modes includes local & remote loop backs with Digital as well as Analog tests. Local ANALOG loop: (defined in the V.54 protocol as Loop 3) tests the integrity of the Serial connector chord, the cable connecting the chord to the modem, and the local modem. Remote DIGITAL loop: (defined in the V.54 protocol as Loop 2) tests the integrity of the Serial connector chord, the cable connecting the chord to the modem, the local modem, the carrier connection, and the remote modem. Built in BERT, if activated (the modem starts generating and checking standard 511- bit pseudo random pattern) in remote digital loop test mode for quick fault isolation on communication link. Local DIGITAL loop: To extend digital loop from local modem for fault isolation on communication link on remote side. Bit-error-rate testing and loopbacks are used by carriers and ISPs to help resolve problems as well as test the quality of T1/E1 links. By early detection of poor quality links and quick problem isolation to improve network's quality of service. Local analog loop back Analog Digital line line RS232 V.54 modem V.54 modem RS232 Remote digital loop FIG.5.3 Modem Diagnostics IRISET 83 TA2 – Data Communication & Networking Long Distance Data Transmission 5.4 Digital subscriber line & XDSL Modems Telephone Company developed another technology called as DSL, to provide high speed access to Internet. DSL technology supports High-speed digital communication Over the existing local loops. DSL technology is a set of technologies, each differing in first letter (ADSL, VDSL, HDSL and SDSL). The set is often referred to as XDSL, where X can be replaced by A, V, H, or S. 5.4.1 ADSL The first technology in the set is asymmetric DSL (ADSL). ADSL, like a 56K modem, provides higher speed (bit rate) in the downstream direction (from the Internet to the resident) than in the upstream direction (from the resident to the Internet). That is the reason it is called asymmetric. Unlike the asymmetry in 56K modems, the designers of ADSL specifically divided the available bandwidth of the local loop unevenly for the residential customer. The service is not suitable for business customers who need a large bandwidth in both directions. i. Using Existing Local Loops: One interesting point is that ADSL uses the existing local loops. But how does ADSL reach a data rate that was never achieved with traditional modems? The answer is that the twisted-pair local loop is actually capable of handling bandwidths up to 1.1MHz, but the filter installed at the end office of the telephone company where each local loop terminates limits the bandwidth to 4 KHz (sufficient for voice communication). If the filter is removed, however, the entire 1.1MHz is available for data and voice communications. ii. Adaptive Technology: Unfortunately, 1.1 MHz is just the theoretical bandwidth of the local loop. Factors such as the distance between the residence and the switching office, the size of the cable, the signaling used, and so on affect the bandwidth. The designers of ADSL technology were aware of this problem and used an adaptive technology that tests the condition and bandwidth availability of the line before settling on a data rate. The data rate of ADSL is not fixed; it changes based on the condition and type of the local loop cable. iii. Discrete Multi tone Technique: The modulation technique that has become standard for ADSL is called the discrete multi tone technique (DMT), which combines QAM and FDM. There is no set way that the bandwidth of a system is divided. Each system can decide on its bandwidth division. Typically, an available bandwidth of 1.104 MHz is divided into 256 channels. Each channel uses a bandwidth of 4.312 KHz, figure 5.3 shows how the bandwidth can be divided into the following: Voice. Channel 0 is reserved for voice communication Idle. Channels 1 to 5 are not used and provide a gap between voice and data communication. iv. Band width in ADSL Upstream data and control. Channels 6 to 30 (25 channels) are used for upstream data transfer and control. One channel is for control and 24 channels are for data transfer. If there are 24 channels, each using 4 KHz (out of 4.312 KHz available) with QAM modulation, we have 24 x 4000 x 15, or a 1.44 Mbps bandwidth, in the upstream direction. However, the data rate is normally below 500 kbps because some of the carriers are deleted at frequencies where the noise level is large. In other words, some of channels may be unused. IRISET 84 TA2 – Data Communication & Networking Long Distance Data Transmission Fig. 5.4 Discrete Multi tone Technique Downstream data and control. Channels 31 to 255 (255 channels) are used for downstream data transfer and control. One channel is for control and 224 channels are for data. If there are 224 channels, we can achieve up to 224 x 4000 x 15, for 13.4 Mbps. However, the data rate is normally below 8 Mbps because some of the carriers are deleted at frequencies where the noise level is large. In other words, some of channels may be unused. Refer Fig. 5.5 Fig. 5.5 Bandwidth division in ADSL v. Customer Site: ADSL Modem: Figure 5.6 shows an ADSL modem installed at a customer’s site. The local loop connects to a splitter, which separates voice and data communications. The ADSL modem modulates and demodulates the data, using DMT (Discrete Multitone Technique), and creates downstream and upstream channels. Note that the splitter needs to be installed at the customer’s premises, normally by a technician from the telephone company. The voice line can use the existing telephone wiring in the house, but the data line needs to the installed by a professional. All this makes the ADSL line expensive. We have an alternative technology - Universal ADSL or (ADSL Lite). Fig. 5.6 Customer Site: ADSL modem IRISET 85 TA2 – Data Communication & Networking Long Distance Data Transmission vi. Telephone Company Site: DSLAM: At the telephone company site, the situation is different. Instead of an ADSL modem, a device called a digital subscriber line access multiplexer (DSLAM) is installed that functions similarly. In addition, it packetizes the data to be sent to the Internet (ISP server). Figure 5.7 shows the configuration. Fig. 5.7 Telephone Company Site: DSLAM 5.4.2 HDSL The high-bit-rate digital subscriber line (HDSL) was designed as an alternative to the T-1 line (1.544 Mbps). The T-1 line uses alternate mark inversion (AMI) encoding which is very susceptible to attenuation at high frequencies. This limits the length of a T-1 line to 3200 ft (1 km). For longer distances, a repeater is necessary, which means increased costs. HDSL uses 2B1Q encoding (see chapter4), which is less susceptible to attenuation. A data rate of 1.544 Mbps (sometimes up to 2 Mbps) can be achieved without repeaters upto a distance of 12,000 ft (3.86km). HDSL uses two twisted pairs (one pair for each direction) to achieve full- duplex transmission. 5.4.3 SDSL The symmetric digital subscriber line (SDSL) is a one twisted- pair version of HDSL. It provides full-duplex symmetric communication supporting up to 768 kbps in each direction. SDSL which provides symmetric communication can be considered an alternative to ADSL. ADSL provides asymmetric communication, with a downstream bit rate that is much higher than the upstream bit rate. Although this feature meets the needs of most residential subscribers, ti is not suitable for business that sends and receives data in large volumes in both directions. 5.4.4 VDSL The very high – bit-rate digital subscriber line (VDSL), and alternative approach that is similar to ADSL, uses coaxial, fiber-optic, or twisted-pair cable for short distance. The modulating technique is DMT. It provides a range of bit rates (25 to 55 Mbps) for upstream communication at distances of 3000 to 10,000 ft. The downstream rate is normally 3.2 Mbps. Configurations XDSL provides for both symmetric and asymmetric configurations as shown in table 5.1 Asymmetric Symmetric Bandwidth is higher in one direction Bandwidth same in both directions Suitable for Web Browsing Suitable for video-conferencing Table 5.1Asymmetric and symmetric configuration of XDSL MODEMs IRISET 86 TA2 – Data Communication & Networking Long Distance Data Transmission 5.4.5 Variations In XDSL there are currently six (6) variations available p as shown in table 5.2 XDSL Meaning Rate Technology 2 x 64Kbps circuit switched DSL Digital Subscriber Line 1 x 16Kbps packet switched (similar to ISDN-BRI) 2.048Mbps over two pairs at HDSL High-bit-rate DSL a distance up to 4.2Km Single-pair or Symmetric S-HDSL/SDSL 768Kbps over a single pair High-bit-rate DSL ADSL Asymmetric DSL up to 6Mbps in one direction An extension of ADSL which supports RADSL Rate Adaptive DSL a variety of data rates depending upon the quality of the local loop Very High-bit-rate Up to 52Mbps in one direction and VDSL asymmetric DSL 2Mbps in the other direction. Table 5.2 XDSL variants 5.5 LAN extender: A LAN extender (also network extender or Ethernet extender) is a device used to extend an Ethernet or network segment beyond its inherent distance limitation which is approximately 100 meters (330 ft) for most common forms of twisted pair Ethernet. The extender forwards traffic between LANs transparent to higher network-layer protocols over distances that far exceed the limitations of standard Ethernet. Extenders that use copper wire include 2 and 4 wire variants using unconditioned copper wiring (without load coils), to extend a LANs. Network extenders use various methods (line encodings), such as TC-PAM, 2B1Q or DMT, to transmit information. The LAN extender (Ethernet-Extender) is used in pairs. Different Types of LAN extenders: 2BASE-TL — Full-duplex long reach Point-to-Point link over voice-grade copper wiring. 2BASE-TL PHY can deliver a minimum of 2 Mbit/s and a maximum of 5.69 Mbit/s over distances of up to 2700 m (9,000 ft), using ITU-T G.991.2 (G.SHDSL.bis) technology over a single copper pair. 10PASS-TS — Full-duplex short reach Point-to-Point link over voice-grade copper wiring. 10PASS-TS PHY can deliver a minimum of 10 Mbit/s over distances of up to 750 m (2460 ft), using ITU-T G.993.1 (VDSL) technology over a single copper pair IRISET 87 TA2 – Data Communication & Networking Long Distance Data Transmission Fig. 5.8 LAN Extender Unit 5.6 Media Converters: Media Converters (also Ethernet-Fiber Converters) enable connections of UTP copper-based Ethernet equipment over a fiber optic link to take advantage of the benefits of fiber which include; ✓ Extending links over greater distances using fiber optic cable ✓ Protecting data from noise and interference ✓ Future proofing your network with additional bandwidth capacity Copper-based Ethernet connections are limited to a data transmission distance of only 100 meters when using unshielded twisted pair (UTP) cable. By using an Ethernet to fiber conversion solution, fiber optic cabling can now be used to extend this link over a greater distance. An Ethernet to Fiber Media Converter can also be used where there is high level of electromagnetic interference or EMI which is a common phenomenon found in industrial plants. This interference can cause corruption of data over copper-based Ethernet links. Data transmitted over fiber optic cable however is completely immune to this type of noise. An Ethernet to Fiber Optic Converter therefore enables you to inter-connect your copper-Ethernet devices over fiber ensuring optimal data transmission across the plant floor. The copper transceiver used in an Ethernet-Fiber Converter transforms the signal from a UTP / RJ45 Ethernet link to one that can be used by a fiber optic transceiver. Media converters can connect to various optical fiber cable such as multimode, single mode or single strand fiber cable. Options exist for many distances to suit the needs of a particular Ethernet to fiber application. And, fiber interface connectors can be dual ST, dual SC, dual LC or single SC type. Fig. 5.9 , Media Converter IRISET 88 TA2 – Data Communication & Networking