The Physical Layer - Chapter 2


  1. Theoretical Basis for Data Communications Information can be transmitted by varying some physical property such as voltage or current. * See pages 77-80.
  2. The Maximum Data Rate of a Channel As early as 1924, H. Nyquist realized the existence of the fundamental limit and derived an equation expressing the maximum data rate for a finite bandwidth noiseless channel. In 1948, Claude Shannon carried Nyquist's work further and extended it to the case of a channel subject to random (that is, thermodynamic) noise.
  3. Nyquist's Theorem Nyquist proved that if an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless. If the signal consists of V discrete levels, Nyquist's theorem states: Maximum data rate = 2H log2 V bits /sec (log base 2 of V) For example, a noiseless 3-KHz channel cannot transmit binary (i.e., two-levels) signals at a rate exceeding 6000 bps.
  4. Shannon's Law So far we have considered only noiseless channels. If random noise is present, the situation deteriorates rapidly. The amount of thermal noise present is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio. If we denote the signal power by S and the noise power by N, the signal- to-noise ratio is S/N. Usually, the ratio itself is not quoted; instead, the quantity 10 log10 S/N is given. These units are called decibels (dB). An S/N ratio of 10 is 10 dB, a ratio of 100 is 20 dB, a ratio of 1000 is 30 dB and so on. Shannon's major result is that the maximum data rate of a noisy channel whose bandwidth is H Hz, and whose signal-to-noise ratio is S/N, is given by Maximum number of bits /sec = H log2 (1 + S/N) For example, a channel of 3000-Hz bandwidth, and a signal to thermal noise ratio of 30 dB (typical parameters of the analog part of the telephone system) can never transmit much more than 30,000 bps, no matter how many or few signal levels are used and no matter how often or how infrequent samples are taken. It should be noted, however, that this is only an upper bound and real systems rarely achieve it. Example, Assume that a communication line has a bandwidth of 3000 Hz and a typical signal-to-noise ratio of 20 dB and hence the maximum theoretical information rate that can be obtained is derived as follows: 20 = 10 log10 (S/N) Therefore, S/N = 100 and C = 3000 log2 (1+100) that is C = 19963 bps. Nyquist's result is rather open ended. We can increase the data rate indefinitely by increasing the encoding (number of voltage levels). Shannon's result is independent of the number of voltage levels and therefore gives us an absolute upper bound.
  5. Transmission Media The purpose of the physical layer is to transport a raw bit stream from one machine to another. Various physical media can be used for the actual transmission. Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and maintenance. Media are grouped into guided media, such as copper wire and fiber optics, and unguided media, such as radio and lasers through the air.
  6. Magnetic Media One of the most common ways to transport data from one computer to another is to write them onto magnetic tape or floppy disks, physically transport the tape or disks to the destination machine, and read them back again. While this method is not as sophisticated as using a geosynchronous communication satellite, it is often much more cost effective, especially for applications in which high bandwidth or cost per bit is the key factor. Example, Assume an industry standard 8-mm tape can hold 7 gigabytes. A box 50*50*50 cm can hold about 1000 of these tapes, for a capacity of 7000 gigabytes. A box of tapes can be delivered anywhere in the United States in 24 hours by Federal Express. The effective bandwidth of this transmission is 56 gigabits/86400 sec or 648 Mbps, which is slightly better than the high-speed version of ATM (622 Mbps). If the destination is only an hour away by road, the bandwidth is increased to over 15 Gbps.
  7. Twisted Pair (see page 83) a. Open Wire: Attached to ceramic or glass insulators. They are still in use in parts of the US and in other parts of the world, particularly when traffic is slow. Picture: b. Twisted Wire Pair Cables: Consist of copper wires insulated by plastic or other meterials and twisted into pairs. There are many considerations in the construction of such cables; it is important, for instance, to minimize cross talk (the undesirable detection of a signal from one pair on another) Picture: . The most common application of twisted pair is the telephone system . Twisted pairs can run several km without amplification; but for longer distance, repeaters are needed. . Twisted pairs acn be used for either analog or digital transmission. . The bandwidth depends on the tickness of the wire and the distance traveled. . Several megabits/sec can be achieved for a few km.
  8. Coaxial Cables Picture: a. Single Wire Conductor b. Cylindrical Conductor c. Insulator d. protective jacket Two kinds of Coaxial cables are used: a. 50-ohm cable is used for digital transmission b. 75-ohm cable is used for analog transmission . Current tends to flow on the inside of the outer shell. This gives more surface area . Since current flows on the inside of the outer shell, signal is shielded from noise & crosstalk . Can transmit at higher frequencies. . The construction and shielding of the coaxial cable give it a good combination of high bandwidth and excellent immunity. The bandwidth possible depends on the cable length. For 1-km cables, a data rate of 1 to 2 Gbps is feasible. Longer cables can also be used, but only at lower rates or with periodic amplifiers. Coaxial cables used to be used within the telephone system but have now largely been replaced by fiber optics on long-haul routes. . There are two ways to connect computers to coaxial cables: a. T junction b. Vampire Tap . There are three coding techniques (see pages 279-280): a. Straight Binary b. Manchester encoding A bit period is divided into two equal intervals. A binary 1 bit is sent by having the voltage set high during the fist interval and low in the second one. A binary 0 is just the reverse: first low and then high. This scheme scheme ensures that every bit period has a transition in the middle, making it easy for the receiver to synchronize with the sender. A disadvantage of manchester encoding is that it requires twice as much bandwidth as straight binary encoding, because the pulses are half the width. c. Differential Manchester encoding, is a variation of basic Manchester encoding. In it, 1 bit is indicated by the absence of a transition at the start of the interval. In both cases, there is a transition in the middle as well. The differential scheme requires more complex equipment but offers better noise immunity. . Advantages a. Moderate cost b. Low bit error rate c. High bandwidth d. Multidrop configuration e. Security
  9. Baseband Coaxial Cable (see page 84) . Single square wave digital signal placed directly on the cable. . Signals propagate in both directions . Can support data rates of 50 Mbps over distances of a few miles. . Error rates: 1 bit in 10^7 and 1 bit in 10^8 (in practice appears error free)
  10. Broadband Coaxial Cable (see page 85) . Directional broadcast transmission using FDM to devide cable into a number of independent channels . Supports multiple channels of 12 Mbps with error rate like baseband . Broadband systems are divided up into multiple channels, frequently the 6-Mhz channels used for television broadcasting. Each channel can be used for analog television, CD-quality audio (1.4 Mbps), or a digital bit stream at, say, 3 Mbps, independent of the others. . Broadband can reach up to 450 Mhz and can run over 100 km. . To transmit digital data over a Broadband network, we convert binary to analog. . Typically, a 300 MHz cable will support a total data rate of 150 Mbps.
  11. Satellites . Use Geo-Stationary (Rotating at the same rate as earth) satellites. . Orbit at about 23,300 miles (36,000 Km). Generally in plane of the equator . Each satellite incorporates a number of transponders which receive earth-origin signals in one frequency range and broadcast back to earth in a second frequency range. . Frequencies a. C-band (most) 3.7 - 4.2 Ghz down 5.925 - 6.425 GHz up (4/6 GHz) b. Ku-band 10-15 Ghz (12/14 GHz) c. High frequencies: 20/30 GHz . Most satellites have between 10 and 25 transponders, each having a bandwidth of 30-50 MHz. . Propagation delay of 0.24 - 0.27 second (Most ground links have delays in the micro to msec range) . Error rates 1 bit in 10^6 to 10^7 bits 95% of the time 1 in 10^4 during poor conditions (e.g., heavy rain) . Limitations . Satellites transmitting at same frequencies can not be closer than 4 degrees (90 satellites) or 8 degrees for TV satellites (due to higher power). High band (12/14 GHz) can be spaced as close as 1 degree. . Longer delay than ground links: Satellites: 250-300 msec Microwave links have a delay of about 3 micro sec /km coaxial cables have a delay of about 5 micro sec / km . Typically use TDM . Communication Satellites (page 163) . Low-Orbit Satellites (page 167) . Satellites versus Fibers (page 168)
  12. Fiber Optics (see page 87-91) . Transmitting data by pulses of light. . A light pulse can be used to signal a 1, the absence of pulse signal a 0 bit. . Bandwidth of 10^8 Mhz is possible . An optical transmission system has three components: 1. The transmission medium: cylinder of glass surrounded by a substance of low refractive index . Components: thin fiber of glass or fused silica Glass of lower refractive index absorpative jacket 2. The light source. Converts electrical signal to light pulses . LED (Light Emitting Diode) . Laser diode 3. The detector. Is a photodiode, which generates an electrical pulse when light falls on it. . Types of fibers 1. Large core fiber 2. Graded Index fiber . refractive index is variable accross the cross section of glass, causing rays to be continually refocused as they travel down the fiber. . Rays speed up as they pass from higher to lower refractive index . All rays arrive at end at approximately the same time. . It is called mulltimode fiber. . Problem of dispersion . Limits transmission capacity . Sources of the problem 1. Rays can travel different paths. Rays traveling a straight line will reach end faster than one experiencing many reflections. 2. Different frequencies have different velocities in glass. . A Comparison of Laser and LEDs as light sources ITEM LED Semiconductor Laser -------------------------------------------------------------- Data Rate Low High Mode Multimode Multimode or Single mode Distance Short Long Lifetime Long life Short life Temperature sensitivity Minor Substantial Cost Low cost Expensive --------------------------------------------------------------- . Techniques to lesson dispersion 1. Thinner Fibers 2. Graded Index Fiber 3. Small core fiber . Core is made small enough to allow propagation in only 1 electronic mode (traveling as axial rays only). It is called single mode fiber . Single mode fiber requires (expensive) laser diodes to derive them, rather than (inexpensive) LEDs, but they are more efficient and can be run for longer distances. . Currently available fiber systems can transmit data at about 1000 Mbps and with powerful lasers a transmission over 100 km long without repeaters. . Characteristics of Optical Fibers 1. Small size 2. Light weight 3. Very wide bandwidth (excess of 1 Gbps) 4. Low cost potential 5. Interference and noise immunity 6. Complete electrical isolation 7. Greater repeater spacing 8. Low crosstalk 9. Security 10. Error rates on the order of 1 in 10 ^9 bits 11. Usually used in point-to-point and rings
  13. Fiber Optic Networks (see page 91) Fiber optics can be used for LANs as well as for long-haul transmission, although tapping onto it is more complex than connecting to an Ethernet. One way around the problem is to realize that a ring network is really just a collection of point-to-point links.
  14. Comparison of Fiber Optics and Copper Wire(see page 92)
  15. The Electromagnetic Spectrum (pages 94-97)
  16. Radio Transmission (Page 97)
  17. Microwave Transmission (page 98)
  18. Infrared and Millimeter Waves (page 100)
  19. The Telephone System (Page 102) The telephone system is organized as a highly redundant, multileveled hierarchy. Each telephone has two copper wires coming out of it that go directly to the telephone company's nearest END OFFICE (also called a local central office). The distance is typically 1 to 10 km. In the United States alone there are about 19,000 end offices. The concatenation of the area code and the first three digits of the telephone number uniquely specify an end office. The two-wire connecting between each subscriber's telephone and the end office are known as the local loop. If the world's local loops were stretched out end to end, they would extend to the moon and back 1000 times. .Types of nodes (offices) Class#5 Central Office (End Office) There are about 19,000 Class#4 Toll Center Class#3 Primary Center Class#2 Sectional Center Class#1 Regional Center AT&T has 7 regional centers. In addition to tandem center in large cities. It is unfeasible to connect all Central Offices together. . In summary, the telephone system consists of three major components: 1. Local loops (twisted pairs, analog signaling) 2. Trunks (fiber optics or microwave, mostly digital). 3. Switching offices
  20. Transmission Impairments Analog signaling consists of varying a voltage with time to represent an information stream. If transmission media were perfect, the receiver would receive exactly the same signal that the transmitter sent. Unfortunately, media are not perfect, so the received signal is not the same as the transmitted signal. For digital data, this defference can lead to errors. Transmission lines suffer from three major problems: 1. Attenuation: is the loss of energy as the signal propagates outward (decibels) 2. Delay distortion: It is caused by the fact that different Fourier components travel at different speeds. 3. Noise: which is unwanted energy from sources other than the transmitter.
  21. Transmission Techniques (Page 110) 1. Baseband Signaling: . Simply a train of pulses . Problem: Square waveform is lost due to capacitance and inductance effects . Good for slow speeds and short distances. 2. Amplitude Modulation (AM) . Continous tone in 1000-2000 hz range (sine wave carrier) . Amplitude of this tone is modulated to introduce information 3. Frequency Modulation (FM) 4. Phase Modulation (PM) 5. Combination of AM, FM, and PM . Example, 4 phases and 2 amplitudes give 8 levels. 3 bits encoded per signal
  22. Modulation .The process used to "imprint" the binary (square-edged) values on an analog signal is called MODULATION. .Essentially, modulation is the modufication of some characteristics of an otherwise continuous carrier signal and is the whistle that is often heard on a simple acoustical modem.
  23. Signaling Speed Number of times per second a signal changes its value- measured in units called BAUD. If each state represents a 0 or 1, then baud is the same as bits/second. If there are four possible states, then one line condition change of state represents a dibit in which case n bauds is equal to 2*n bits/sec.
  24. Signaling Techniques An electronic signal is used to move information from one computer system to another or from a computer to a terminal device. Different states, often two, can be represented by the modification of a particular signal; these modified signals are then interpreted as a zero or one.
  25. Analog Signal . Continuously varying signal . A sine wave is an analog signal . The frequency of a signal, is the number of cycles per second. Example, a frequency of 1100 hertz has 1100 such cycles occuring in a period of one second.
  26. Digital Signal . Propagation of information via discrete signal levels. . Square wave is a digital signal . Repeaters can restore the original digital signal . Typically have higher data rates, and provide more multiplexing potential.
  27. Types of signals and transmission systems 1. Analog signal over analog system 2. Analog signal over digital system 3. Digital signal over digital system 4. Digital signal over analog system
  28. Analog to Digital (Digitizing) 1. Delta Modulation (page 123) Output 1 if current signal >= previous signal Output 0 otherwise. . Disadvantages . Small levels of speech output result in sampling error . Slope limiting rapid changes of waveform can not be kept up with . Advantages . Simple and cheap. 2. Pulse-Code Modulator (PCM) . CODEC (Coder-Decoder). digitizes the analog signal . CODEC makes 8000 samples/sec (it samples every 125 micro seconds) . It uses 128 levels (7 bits are used) . 7 * 8000 = 56000 bps is required to transmit the digital signal. . T1 carrier consists of 24 channels multiplexed together. Each of the 24 channels, in turn, gets to insert 8 bits into the output stream. 7 bits are data, and one is for control, yielding 7*800 = 56000bps of data, and 1 * 8000 = 8000 bps of signaling information per channel. . A frame consists of 24 * 8 = 192 bits, plus one extra bit for framing, yielding 193 bits every 125 micro seonds. This gives a gross data rate of 1.544 Mbps.
  29. Multiplexing Techniques 1. Frequency Division Multiplexing (FDM) . Partitions a limited bandwidth channel into a group of independent lower speed channels, each using a permanently assigned portion of the total frequency spectrum. . Guard bands are required between adjacent channels to prevent electrical overlapping of signals. That means not all the available bandwidth is used. . Example, a wide spread standard is 12 4000hz channels (3000 for user and two guard bands of 500 hz each) multiplexed into the band 60-108 khz. This is called a group. . Advantages: . Low cost . Readily cascadable (one can drop and insert at intermediate points along a multiplexed channel. 2. Time Division Multiplexing . Creates a permanently dedicated time slot . Incoming data from input ports is cyclically scanned, characters or bits are peeled off, and interleaved into frames on a single high speed data stream . Makes more efficient use of bandwidth than FDM . Demultiplexing assumes an implicit relationship between the output line and the relative position of the time slot in the arriving frame. . Stations need buffers to assemble data. . Time Division Multiplexing allows multiple T1 carriers to be multiplexed into higher-order carriers. 4 T1 - T2 (4*1.544 = 6.312 Mbps) 6 T2 - T3 (44.736 Mbps) 7 T3 - T4 (274.176) 3. Statistical Time Division Multiplexing (STATDM) . Unlike TDM, a dedicated time slot is not provided for each port in the sharing group . Time slots are dynamically allocated in a frame to the currently active users . Need addressing info . Need buffers . Do better when a few users are active.
  30. Switching Two main switching techniques are used inside the telephone system: Circuit switching and packet switching
  31. Circuit Switching When you or your computer places a telephone call, the switching equipment within the telephone system seeks out a physical path all the way from your computer to the receiver's telephone. This technique is called circuit switching. In the early days of the telephone, the connection was made by having the operator plug a jumper cable into the input and output sockets. An important property of circuit switching is the need to set up an end-to-end path before any data can be sent.
  32. Message Switching An alternative switching strategy is message switching. No physical path is established in advance between sender and receiver. Instead, when the sender has a block of data to be sent, it is stored in the first switching office (i.e., router) and then forwarded later, one hop at a time. Each block is received in its entirety, inspected for errors, and then retransmitted. A network using this technique is called a store-and-forward network. With message switching, there is no limit on block size, which means that routers must have disks to buffer long blocks. It also means that a single block may tie up a router-router line for minutes, rendering message switching useless for interactive traffic.
  33. Packet Switching Packet-switching networks place a tight upper limit on block size, allowing packets to be buffered in router main memory instead of on disk. Packet switching networks are well suited to handling interactive traffic. A further advantage of packet switching over message swicthing is that the first packet can be forwarded before the second one has fully arrived, reducing delay and improving throughput. A comparision of Circuit-switched and packet-switched networks Item Circuit_switched Packet-swicthed ---------------------------------------------------------------- Dedicated path Yes No Bandwidth available Fixed Dynamic Potentially wasted Yes No bandwidth Store-and-forward No Yes Each Packet follows the same route Yes No Call setup Required Not needed When can congestion At setup time On every packet occur Charging Per minute Per packet ------------------------------------------------------------------
  34. ISDN (Integrated Services Digital Network) (Page 139-144)
  35. ATM (Asynchronous Transfer Mode).(Page 144-148) .Packet switching .Original rate is 155.52 Mbps (capable to tranmit HDTV) .Additional rate is 622.08 Mbps.
  36. Cellular Radio (page 155)
  37. Paging Systems (page 155)
  38. Cordless Telephones (page 157)
  39. Analog Cellular Telephones
  40. Advanced Mobile Phone System (page 158)
  41. Problems (Pages 170-172) .Numbers 2, 3, 4, 5, 14, 23, and 24)

Last update April 9, 1998.