Communication Networks

Transcription

1 Olum-fonoon Babol Computer networks course Communication Networks Chapter-3 Physical -Layer Lecture- 4,5 Dr. Eng. Samy Elmokadem Fall 2005 By: H. Veisi

2 Definitions OBJECTIVES :Physical Layer The Theoretical Basis for Data Communication - - Nyquist Theorem (for noiseless channels) - Shannon Channel Capacity ( for noisy channels) Guided transmission Media Unguided transmission Media( Wireless Transmission) The public Switched Telephone Networks. Trunks and Multiplexing ( FDM-WDM-TDM ) Switching (CS-MS-PS ) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 2

3 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical OSI REFERENCE MODEL 1. Physical Layer a) Convert the logical 1 s and 0 s coming from layer 2 into electrical signals. b) Transmission of the electrical signals over a communication channel. Main topics: Transmission mediums Encoding Modulation Repeaters Hubs (multi-port repeater) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 3

4 Physical Layer 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical 1. Signals Fourier analysis Maximum data rate of a channel 2. Transmission Media Guided and Unguided 3. Analog Transmission Modulation Modems RS-232, RS Digital Transmission Encoding schemes Repeaters and hubs 5. Transmission and Switching Multiplexing (FDM and TDM) Circuit switching vs. packet switching vs. packet switching Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 4

5 Definitions :Physical Layer The physical layer: defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systems. -Physical layer specifications: define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. - Physical layer implementations: can be categorized as either LAN or WAN specifications. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 5

6 Physical Layer definitions (Cont...) The physical layer : is the basis of all networks. Nature imposes two fundamental limits on all channels, and these determine their bandwidth ( H). * These limits are: 1.The Nyquist limit, which deals with noiseless channels 2.The Shannon limit, which deals with noisy channels Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 6

7 Physical Layer definitions (Cont...) The time required to transmit a character depends on both the encoding method and the signaling speed (i.e., the modulation rate - the number of times the signal changes its voltage per second. ) Baud (D) - the number of changes for signal level per second. Bandwidth (H) - the range of frequencies that is passed by a channel. The transmitted signal is constrained by the transmitter and the nature of the transmission medium in cycles/sec (hertz) Channel Capacity (C) the rate at which data can be transmitted over a given channel under given conditions {This is also referred to as Data rate (R).} Q. Explain the difference between data rate and baud rate? Data rate is the capacity of a channel in bits per second. Baud rate represents the number of times the line condition (i.e., frequency, amplitude or phase) changes each second. data rate = n bits x baud rate, where n is equal to the number of bits one signal represents. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 7

8 The Theoretical Basis for Data Communication Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 8

9 A Transmission System Transmitter Receiver Communication channel Transmitter Converts information into signal suitable for transmission Injects energy into communications medium or channel Telephone converts voice into electric current Modem converts bits into tones Receiver Receives energy from medium Converts received signal into form suitable for delivery to user Telephone converts current into voice Modem converts tones into bits Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 9

10 Transmission Impairments: Transmitter Transmitted Signal Communication channel Received Signal Receiver Communication Channel twisted pair wires Coaxial cable Optical fiber Radio Satellite Microwave Infrared Transmission Impairments Signal attenuation Signal distortion Spurious noise Interference from other signals Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 10

11 Transmission Impairments: Transmission Impairments: Attenuation - the loss of energy as the signal propagates. Delay Distortion - different frequencies travel at different speeds so the wave form spreads out. Noise - unwanted energy that combines with the signal - difficult to tell the signal from the noise. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 11

12 Analog transmission : Long-Distance Communications Transmission segment Source Repeater... Repeater Destination Each repeater attempts to restore analog signal to its original form Restoration is imperfect Distortion is not completely eliminated Noise, interference is only partially removed Signal quality decreases with no. of repeaters Communications is distance-limited Still used in analog cable TV systems Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 12

13 Digital transmission: Long-Distance Communications Transmission segment Source Regenerator... Regenerator Destination Regenerator recovers original data sequence and retransmits on next segment Can design so error probability is very small Then each regeneration is like the first time! Communications is possible over very long distances Digital systems vs. analog systems Less power, longer distances, lower system cost Monitoring, multiplexing, coding, encryption, protocols Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 13

14 Analog Transmission vs. Digital Transmission Analog transmission: all details must be reproduced accurately Sent signal Distortion Attenuation Received signal Digital transmission: only discrete levels need to be reproduced Sent signal Distortion Attenuation Received signal Simple Receiver: Was original pulse positive or negative? Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 14

15 Noise & Reliable Communications All physical systems have noise Electrons always vibrate at non-zero temperature Motion of electrons induces noise Presence of noise limits accuracy of measurement of received signal amplitude Errors occur if signal separation is comparable to noise level Bit Error Rate (BER) : increases with decreasing signal-to-noise ratio (SNR) Noise places a limit on how many amplitude levels can be used in pulse transmission Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 15

16 Signal-to-Noise Ratio High SNR Signal Noise Signal + noise t t t No errors Signal Noise Signal + noise Low SNR t t t SNR = Average signal power Average noise power error SNR (db) = 10 log 10 SNR Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 16

17 The Theoretical Basis for Data Communication - Information can be transmitted on wires by varying some physical property such as voltage or current. - By representing the value of this voltage or current as a single-valued function of time, f(t), we can model the behavior of the signal and analyze it mathematically. 1. Fourier Analysis: a) All signals can be represented mathematically. b) A periodic function can be constructed by adding a number of sine and cosine functions. - Fundamental frequency f : where f = 1/T - Harmonics integer multiples of the fundamental frequency - Baud number of signal level changes per second Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 17

18 The Theoretical Basis for Data Communication (Cont. ) Fourier Analysis In the early 19th century, the French mathematician Jean-Baptiste Fourier proved that any reasonably behaved periodic function, g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines: as in Eq. (2-1). where f = 1/T is the fundamental frequency, an and bn are the sine and cosine amplitudes of the nth harmonics (terms), and c is a constant. Such a decomposition is called a Fourier series. From the Fourier series, the function can be reconstructed; that is, if the period, T, is known and the amplitudes are given, the original function of time can be found by performing the sums of Eq. (2-1). Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 18

19 The Theoretical Basis for Data Communication (Cont. ) - A data signal that has a finite duration can be handled by just imagining that it repeats the entire pattern (i.e., the interval from T to 2T is the same as from 0 to T, etc.). - The an amplitudes can be computed for any given g(t) by multiplying both sides of Eq. (2-1) by sin (2pkft) and then integrating from 0 to T. Since Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 19

20 The Theoretical Basis for Data Communication (Cont. ) - only one term of the summation survives: an. The bn summation vanishes completely. Similarly, - By multiplying Eq. (2-1) by cos (2pkft) and integrating between 0 and T, we can derive bn. By just integrating both sides of the equation as it stands, we can find c. The results of performing these operations are as follows: Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 20

21 The Maximum Data Rate of a Channel Nyquist, Channel Capacity ( for noiseless channel) In 1924, Henry Nyquist, realized that even a perfect channel has a finite transmission capacity. He derived an equation expressing the maximum data rate for a finite bandwidth noiseless channel. 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 because the higher frequency components that such sampling could recover have already been filtered out. If the signal consists of V discrete levels, Nyquist's theorem states: Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 21

22 The Maximum Data Rate of a Channel 2 Nyquist's theorem : ( A noiseless ) Maximum data rate = 2 H log 2 V Where: Example: H = channel bandwidth (bits/sec) V = the number of discrete levels of the original signal A noiseless channel have 3kHz, cannot transmit binary signals (2 level) at a rate faster than 6000 bps, Calculate Maximum data rate. An : Maximum data rate = 2 H log 2 V (bits/sec) = 2 X (3k) log 2 2 = 6000 bps Q: Is the Nyquist theorem true for optical fiber or only for copper wire? Yes for both. It has nothing to do with technology or transmission media. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 22

23 The Maximum Data Rate of a Channel Shannon Channel Capacity ( for noisy 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 Shannon Channel Capacity: The maximum reliable transmission rate ( C) over an ideal channel with bandwidth H (Hz), with Gaussian distributed noise, and with SNR( S/N ) is : C = H log 2 ( 1 + PS/PN ) bits per second Reliable means error rate can be made arbitrarily small by proper coding Decibel To show that a signal has lost or gained strength, engineers use the unit of the decibel. The decibel (db) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 23

24 The Maximum Data Rate of a Channel Shannon Maximum data rate: c = H log 2 (1+ PS/PN) Where: H = line bandwidth PS = signal strength in watts PN = noise strength in watts SNR= PS/PN : signal to noise ratio (bits/sec) Example: A channel of 3000-Hz bandwidth with 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. An: - Shannon's result was derived from information-theory arguments and applies to any channel subject to thermal noise. - A 3kHz channel with a noise ratio of 30dB - Maximum data rate C = H log 2 (1+ PS/PN) bps Where: SNR = 10 log10 (PS/PN)=30 db, PS/PN = 1000 C =(3X1000) log 2 (1001) = 30,000 bps Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 24

25 The Maximum Data Rate of a Channel Shannon Channel Capacity ( for noisy channel) Q: Find the Shannon channel capacity for a telephone channel with H = 3400 Hz and SNR db = 40 An: C = H log 2 (1+SNR) SNR db =10 log 10 (SNR) bps SNR db = 40, 40 = 10 log 10 (SNR) 4 = log 10 (SNR), SNR =10,000 C = 3400 log 2 (10001) = 44.8 kbps Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 25

26 Example Decibel To show that a signal has lost or gained strength, engineers use the unit of the decibel. The decibel (db) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified. Note : 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 Q: If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio is 20 db, what is the maximum achievable data rate ( C)? Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 26

27 Transmission Media Two main categories: Guided wires, Coaxial cables, Fiber-optic cables Unguided wireless transmission, e.g. radio, microwave, infrared, sound, sonar We will concentrate on guided media here: Twisted-Pair cables: Unshielded Twisted-Pair (UTP) cables Shielded Twisted-Pair (STP) cables Coaxial cables Fiber-optic cables Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 27

28 Transmission Media Magnetic media Tapes, diskettes diskettes High bandwidth A 8 mm tape = 7 GB A (50*50*50 Cm^3 )box = 1000 tapes =7000 GB 7000 GB/24 Hrs= 648 Mbps 7000GB/1Hr=15 Gbps Sometimes it's cheaper and faster to load a box of tapes in your car! Problem: Delay! Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 28

29 Classes of Transmission Media Conducted or guided media use a conductor such as a wire or a fiber optic cable to move the signal from sender to receiver Wireless or unguided media use radio waves of different frequencies and do not need a wire or cable conductor to transmit signals Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 29

30 Electromagnetic Spectrum for Transmission Media Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 30

31 Guided Transmission Media Transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint Examples twisted pair wires coaxial cables optical fiber cables Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 31

32 Physical / Guided Transmission Media Twisted-pair wires: One or more twisted wires bundled together (why?) Made of copper Coaxial Cables: Consists of single copper wire surrounded by three layers of insulating and metal materials Typically used for cable TV Fiber-optics cables : Strands مسارات of glass or plastic used to transmit light Very high capacity, low noise, small size, less suitable to natural disturbances Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 32

33 Physical Transmission Media twisted-pair cable twisted-pair wire woven or braided metal copper wire plastic outer coating insulating material optical fiber core glass cladding protective coating Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 33

34 (1) Twisted Pair Wires Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference (EMI) between adjacent pairs Often used at customer facilities and also over distances to carry voice as well as data communications Low frequency transmission medium Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 34

35 (1) Twisted Pair Wires If the pair of wires are not twisted, electromagnetic noises from, e.g., motors, will affect the closer wire more than the further one, thereby causing errors 3 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 35

36 Types of Twisted Pair UTP (unshielded twisted pair) each wire is insulated with plastic wrap, but the pair is encased مغلف in an outer covering STP (shielded twisted pair) رقائق the pair of wires are wrapped with metallic foil or braid ضفيرة to insulate the pair from electromagnetic interference Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 36

37 (1)Twisted Pair Wires Twisted pair (1) Simply two wires twisted together thickness=1mm The twisting cuts down on electrical interference. Heavily used in the phone system Until some Kilometers/ Some Mbps Used For Analog and Digital Twisted pair (2) Bandwidth depends on thickness and distance Need repeater for long distances UTP Category 3 and 5 - Category 5 UTP. having more twists and better insulation. Popular by UTP (Unshielded Twisted Pair) (a) Category 3 UTP Cat3 Cat 5 (b) Category 5 UTP. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 37

38 Unshielded Twisted-Pair (UTP) Typically wrapped inside a plastic cover (for mechanical protection) A sample UTP cable with 5 unshielded twisted pairs of wires Insulator Metal 3 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 38

39 Categories of UTP Cables EIA classifies UTP cables according to the quality: Category 1 the lowest quality, only good for voice, mainly found in very old buildings, not recommended now Category 2 good for voice and low data rates (up to 4Mbps for low-speed token ring networks) Category 3 at least 3 twists per foot, for up to 10 Mbps (common in phone networks in residential buildings) Category 4 up to 16 Mbps (mainly for token rings) Category 5 (or 5e) up to 100 Mbps (common for networks targeted for high-speed data communications) Category 6 more twists than Cat 5, up to 1 Gbps 3 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 39

40 Shielded Twisted-Pair (STP) STP cables are similar to UTP cables, except there is a metal foil or braided-metal-mesh cover that encases each pair of insulated wires 4 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 40

41 Ratings تصنيفات of Twisted Pair wires Category 3 UTP (unshielded twisted pair) data rates of up to 16 Mbps are achievable Category 5 UTP (unshielded twisted pair) data rates of up to 100 Mbps are achievable more tightly twisted than Category 3 cables more expensive, but better performance STP (shielded twisted pair) More expensive, harder to work with Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 41

42 Twisted Pair Advantages Inexpensive and readily available Flexible and light weight Easy to work with and install Twisted Pair Disadvantages Susceptibility to interference and noise Attenuation problem For analog, repeaters needed every 5-6 km For digital, repeaters needed every 2-3 km Relatively low bandwidth (3000Hz) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 42

43 (2) Transmission Media (Coaxial cable) Used for cable television, LANs, telephony مضفرة شبكة Has an inner conductor surrounded by a braided mesh Both conductors share a common center axial, hence the term co-axial Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 43

44 (2) Coaxial Cables In general, coaxial cables, or coax, carry signals of higher freq (100KHz 500MHz) than UTP cables Outer metallic wrapping serves both as a shield against noise and as the second conductor that completes the circuit 4 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 44

45 (2) Transmission Media (Coaxial cable) outer jacket (polyethylene) shield (braided wire) insulating material copper or aluminum conductor Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 45

46 (2) Transmission Media (Coaxial cable) Baseband Coaxial cable Used for digital transmissions (called baseband.) Good noise immunity. Data rates as high as 2 Gbps for 1 Km distance. Now being replaced by fiber. Broadband Coaxial cable Used for analog transmissions (called broadband.) Can run 300 MHz for long distances. Analog signaling has better S/N than digital signaling. Interfaces must convert digital signals to analog and vice versa. Designed for long distances - can use amplifiers. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 46

47 Coaxial Cable Advantages Higher bandwidth 400 to 600 Mhz up to 10,800 voice conversations Can be tapped easily Much less susceptible to interference than twisted pair Coaxial Cable Disadvantages High attenuation rate makes it expensive over long distance Bulky Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 47

48 (3) Transmission Media (Fiber Optic Cables ) Many people in the computer industry take enormous pride in how fast computer technology is improving. -The original (1981) IBM PC ran at a clock speed of 4.77 MHz. Twenty years later, PCs could run at 2 GHz - In the same period, wide area data communication went from 56 kbps (the ARPANET) to 1 Gbps (modern optical communication, while at the same time the error rate went to almost zero. -Furthermore, single CPUs are beginning to approach physical limits, such as speed of light and heat dissipation problems. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 48

49 (3) Transmission Media (Fiber Optic Cable) Relatively new transmission medium used by telephone companies in place of long-distance trunk lines Also used by private companies in implementing local data communications networks Require a light source with injection laser diode (ILD) or light-emitting diodes (LED) Fiber Optic (1) Transmission of light through fiber Bandwidth more than 50,000 Gbps! But now restricted to 1 Gbps! Reason: Electrical and optical signal conversion Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 49

50 (3) Transmission Media (Fiber Optic Cables ) - In contrast, with current fiber technology, the achievable bandwidth is certainly in excess of 50,000 Gbps (50 Tbps) and many people are looking very hard for better technologies and materials. -The current practical signaling limit of about 10 Gbps is due to our inability to convert between electrical and optical signals any faster, although in the laboratory, 100 Gbps has been achieved on a single fiber. An optical transmission system has three key components: 1- light source 2- transmission medium, 3- detector. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 50

51 (3) Transmission Media (Fiber Optic Cable) Electrical signal Modulator Optical fiber Receiver Electrical signal Optical source Light sources (lasers, LEDs) generate pulses of light that are transmitted on optical fiber Very long distances (>1000 km) Very high speeds (>40 Gbps) Nearly error-free (BER of ) Profound influence on network architecture Dominates long distance transmission Distance less of a cost factor in communications Plentiful bandwidth for new services Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 51

52 (3) Transmission Media (Fiber Optic Cables ) - Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The transmission medium is an ultra-thin fiber of ألياف رقيقة جدا من الزجاج glass. The detector كاشف : generates an electrical pulse when light falls on it. By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 52

53 (3) Transmission Media (Fiber Optic Cable) Fiber Optic Cable :Including 3 components: 1. Light source: Pulse of light=1, absence of light=0 2. Transition medium: an ultra-thin fiber of glass 3. detector: generate an electrical pulse when light falls on it (صفيره Similar to coax (without braid Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 53

54 Fiber Optic Layers consists of three concentric sections plastic jacket glass or plastic cladding fiber core Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 54

55 Fiber-Optic Cables Light travels at ms -1 in free space and is the fastest possible speed in the Universe Light slows down in denser media أكثر كثافة,مادة e.g. glass Refraction اآلنكسار occurs at interface, with light bending away from the normal when it enters a less dense medium مادة أقل كثافة Beyond the critical angle total internal reflection Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 55

56 (3) Transmission Media (Fiber Optic Cables ) 7 Application 6 Presentation 5 Session Fiber Optic Cables Two general types are multimode and single mode. In multimode, light is reflected internally. Light source is an LED. 4 Transport 3 Network 2 Data Link In single mode, the light propagates in a straight line. Light source come from expensive laser diodes. Faster and longer distances as compared to multimode. 1 Physical * Fiber optic cables are difficult to tap (higher security) and are normally used for backbone cabling. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 56

57 Fiber Optic Types Multimode step-index fiber The reflective إنعكاس walls of the fiber move the light pulses to the receiver Multimode graded-index fiber Acts to إنكسارrefrac the light toward the center of the fiber by variations in the density Single mode fiber The light is guided down the center of an extremely narrow core Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 57

58 Fiber Optic Signals fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 58

59 Multimode & Single-mode Fiber Multimode fiber: multiple rays follow different paths Reflected path Direct path Single-mode fiber: only direct path propagates in fiber Multimode: Thicker core, shorter reach Rays on different paths interfere causing dispersion & limiting bit rate Single mode: Very thin core supports only one mode (path) More expensive lasers, but achieves very high speeds Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 60

60 An optical fiber consists of a core (denser material) and a cladding كسوة (less dense material) Simplest one is a multimode step-index optical fiber مادة أكثر كثافة Multimode = multiple paths, whereas step-index = refractive index follows a step-function profile (i.e. an abrupt change of refractive index between the core and the cladding) Light bounces back and forth along the core Common light sources: LEDs and lasers 6 Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 61

61 (3) Transmission Media (Fiber Optic Cable) Fiber Optic (2) Thickness of core: 8~10 microns or 50 microns Two typically light sources: 1. LED (Light Emitting Diode) response time=1ns data rate = 1Gbps 2. Semiconductor laser ** A comparison of semiconductor diodes and LEDs as light sources. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 62

62 (3) Transmission Media (Fiber Optic Cable) Fiber Optic (3) Properties include total internal reflection and attenuation of particular frequencies. Fiber Optic Networks - can be used for LANs and long-haul. A fiber-optic LAN (A fiber optic ring with active repeaters ) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 63

63 Fiber Optic Advantages greater capacity (bandwidth of up to 2 Gbps) smaller size and lighter weight lower attenuation immunity to environmental interference highly secure due to tap difficulty and lack of signal radiation Fiber Optic Disadvantages expensive over short distance requires highly skilled installers adding additional nodes is difficult Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 64

64 Transmission Media Comparison Fiber optic Copper wire Bandwidth Higher Lower Distance between repeaters 30 KM 5 Km Interference Low High Physical Smaller / Lighter - Flow of data Uni-directional Bi-directional Why does fiber-optic cable have greater capacity than copper-based media? Fiber is immune to EMI. Fiber supports higher frequencies with out signal reflections or noise. Fiber frequencies can be multiplexed to increase bandwidth considerably. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 65

65 Bit Rates of Digital Transmission Systems System Bit Rate Observations Telephone twisted pair kbps 4 khz telephone channel Ethernet twisted pair 10 Mbps, 100 Mbps 100 meters of unshielded twisted copper wire pair Cable modem 500 kbps-4 Mbps Shared CATV return channel ADSL twisted pair kbps in, Mbps out Coexists with analog telephone signal 2.4 GHz radio 2-11 Mbps IEEE wireless LAN 28 GHz radio Mbps 5 km multipoint radio Optical fiber Gbps 1 wave length Optical fiber >1600 Gbps Many wave lengths Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 66

66 Examples of Channels Channel Bandwidth Bit Rates Telephone voice channel 3 khz 33 kbps Copper pair 1 MHz 1-6 Mbps Coaxial cable 5 GHz radio (IEEE ) 500 MHz (6 MHz channels) 300 MHz (11 channels) 30 Mbps / channel 54 Mbps / channel Optical fiber Many TeraHertz 40 Gbps / wavelength Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 67

67 Lecture- 5 Unguided transmission media (Wireless transmission ) 1. Microwave Transmission 2. Satellites microwave 3. Radio transmission 4. Infrared Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 68

68 Unguided transmission media Wireless Examples (Wireless transmission ) 1. Microwave Transmission 2. Satellites microwave 3. Radio transmission 4. Infrared The fundamental relation between f,, and c (in vacuum) is Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 69

69 Wireless Media Wireless media - natural parts of the Earth s environment that can be used as paths to carry electrical signals; no physical transmission media Microwave/satellite transmission High frequency radio Bluetooth Pagers Cellular Wi-Fi & Wi-Max Infrared light Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 70

70 Unguided transmission media (Wireless transmission ) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 71

71 Wireless (Unguided Media) Transmission Transmission and reception are achieved by means of an antenna Directional transmitting antenna puts out focused beam transmitter and receiver must be aligned Omnidirectional signal spreads out in all directions can be received by many antennas Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 72

72 1- Terrestrial Microwave used for long-distance telephone service uses radio frequency spectrum, from 2 to 40 Ghz parabolic dish transmitter, mounted high used by common carriers as well as private networks requires unobstructed line of sight between source and receiver curvature of the earth requires stations (repeaters) ~30 miles apart Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 73

73 Microwave A high frequency radio signal that is sent through the air using either terrestrial (earth-based) or satellite systems Terrestrial Microwave - A line-of-site technology (unobstructed) دون عائق used to cross inaccessible terrain or to connect buildings where cable installation would be expensive. Attenuation is low over short distance but higher over longer distances. High winds, heavy rain, EMI and eavesdropping problems. areالتنصت also Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 74

74 Microwave Transmission Disadvantages line of sight requirement expensive towers and repeaters subject to interference such as passing airplanes and rain Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 75

75 2- Communication Satellites A communication satellite can be thought of as a big microwave repeater in the sky. It contains several transponders, each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal. The downward beams can be broad, covering a substantial fraction of the earth's surface, or narrow, covering an area only hundreds of kilometers in diameter. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 76

76 Communication Satellites The first artificial communication satellite, Telstar, was launched in July Since then, communication satellites have become a multibillion dollar business and the only aspect of outer space that has become highly profitable.مربح These high-flying satellites are often called : GEO (Geostationary ثابت Earth Orbit) satellites. Satellite : a microwave relay station in space can relay signals over long distances Geostationary satellites Remain above the equator at a height of 22,300 miles (geosynchronous orbit) Travel around the earth in exactly the time the earth takes to rotate Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 77

77 Communication Satellites MEO (Medium-Earth Orbit) satellites. As viewed from the earth, taking something like 6 hours to circle the earth.. Because they are lower than the GEOs, they have a smaller footprint on the ground and require less powerful transmitters to reach them. Currently they are not used for telecomminication, The 24 GPS (Global Positioning System) satellites orbiting at about 18,000 km are examples of MEO satellites. Low-Earth Orbit Satellites (LEO) Moving down in altitude, we come to the LEO (Low-Earth Orbit) satellites. Due to their rapid motion, large numbers of them are needed for a complete system. - because the satellites are so close to the earth, the ground stations do not need much power, and the round-trip delay is only a few milliseconds. its used in voice communication and Internet service. Compare and contrast GEO and LEO satellite communications systems. GEO satellites orbit at altitude of 22,000 miles. Speed of satellite is same as of earth. Total global coverage with 8 satellites. LEO satellites orbit at altitude of 300 to 1200 miles. LEO satellite Computer speed is much networking, greater than Olum-Fonoon GEO. Total global Babol coverage H. Veisi takes 48 Fall satellites Page 78

78 Satellite Microwave Transmission Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 79

79 Examples for : Low-Earth Orbit Satellites (LEO) Iridium satellites are positioned at an altitude of 750 km, in circular polar orbits. They are arranged in north-south necklaces, with one satellite every 32 degrees of latitude. With six satellite necklaces, the entire earth is covered. Iridium is targeted at telephone users. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 80

80 USING SATELLITE TECHNOLOGIES IN BUSINESS An interesting property of Iridium is that communication between distant customers takes place in space, with one satellite relaying data to the next one,, Here we see a caller at the North Pole contacting a satellite directly overhead. The call is relayed via other satellites and finally sent down to the callee at the South Pole Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 81

81 Satellite Transmission Links Earth stations communicate by sending signals to the satellite on an uplink The satellite then repeats those signals on a downlink The broadcast nature of the downlink makes it attractive for services such as the distribution of television programming Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 82

82 Satellite Transmission Process satellite transponder dish 22,300 miles dish uplink station downlink station Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 83

83 Satellite Transmission Applications Satellite Microwave - A line-of-site technology that uses relay stations to transfer signals between antennae located on earth and a satellite orbiting the earth. It can be used to access very remote locations and like terrestrial microwave, attenuation, EMI and eavesdropping are also problems Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 84

84 Satellite Transmission Applications Television distribution a network provides programming from a central location direct broadcast satellite (DBS) long-distance telephone transmission high-usage international trunks private business networks Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 85

85 Satellite Microwave Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 86

86 USING SATELLITE TECHNOLOGIES IN BUSINESS Global positioning system (GPS) a "constellation" of 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpoint their geographic location by determining current latitude, longitude, speed, and direction of movement Location accuracy is anywhere from 10 to 100 meters Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 87

87 Global Positioning System (GPS) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 88

88 USING SATELLITE TECHNOLOGIES IN BUSINESS Geographic Information System (GIS) - is designed to work with information that can be shown on a map Most location based applications use a GIS combined with database and GPS technology Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 89

89 USING SATELLITE TECHNOLOGIES IN BUSINESS Location-based services (LBS) - are wireless mobile content services which provide location-specific information to mobile users moving from location to location Common Location Based Services based on GIS/GPS technology: Finding what is nearby Routing information Information alerts Mapping densities Mapping quantities Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 90

90 Principal Satellite Transmission Bands Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 91

91 Fiber vs Satellite Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 92

92 3- Radio transmission Radio : is a general term often used to encompass frequencies in the range 3 khz to 300 GHz. Mobile telephony occupies several frequency bands just under 1 GHz. Radio : is omni-directional microwave is : directional Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 93

93 Radio transmission Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. -Radio waves also are om-nidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically. -Sometimes omni-directional radio is good, but sometimes it is bad. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 94

94 Radio transmission Broadcast Radio Distribute signals through the air over long distance Uses an antenna Typically for stationary locations Can be short range Cellular Radio A form of broadcast radio used for mobile communication High frequency radio waves to transmit voice or data Utilizes frequency-reuse Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 95

95 Radio transmission Microwaves Radio waves providing high speed transmission They are point-to-point (can t be obstructed) Used for satellite communication Infrared (IR) Wireless transmission media that sends signals using infrared light- waves - Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 96

96 Radio transmission Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 97

97 4-Infrared Uses transmitters / receivers (transceivers) that modulate noncoherent infrared light. Transceivers must be within line of sight of each other (directly or via reflection ). Unlike microwaves, infrared does not penetrate walls. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 98

98 Infrared - Unguided infrared and millimeter waves are widely used for short-range communication. -The remote controls used on televisions, and stereos all use infrared communication. They are relatively directional, cheap, and easy to build but have a major drawback: - Drawback: they do not pass through solid objects (try standing between your remote control and your television and see if it still works). In general, as we go from long-wave radio toward visible light, the waves behave more and more like light and less and less like radio. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 99

99 Infrared -The fact that infrared waves do not pass through solid walls well is also a plus. It means that an infrared system in one room of a building will not interfere with a similar system in adjacent rooms or buildings: - you cannot control your neighbor's television with your remote control. -Furthermore, security of infrared systems against eavesdropping is better than that of radio systems precisely for this reason. -Therefore, no government license is needed to operate an infrared system, in contrast to radio systems, which must be licensed outside the ISM bands. Infrared communication has a limited use on the desktop, for example, connecting notebook computers and printers, but it is not a major player in the communication game Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 100

100 The public Switched Telephone Networks (PSTN) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 101

101 The public Switched Telephone Networks (PSTN) When two computers owned by the same company or organization and located close to each other need to communicate, -it is often easiest just to run a cable between them. LANs work this way. However, when the distances are large or there are many computers or the cables have to pass through a public road or other public right of way, - the costs of running private cables are usually prohibitive. Furthermore, in just about every country in the world, stringing private transmission lines across (or underneath) public property is also illegal. - Consequently, the network designers must rely on the existing telecommunication facilities. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 102

102 The public Switched Telephone Networks (PSTN) -These facilities, especially the PSTN (Public Switched Telephone Network), were usually designed many years ago, with a completely different goal in mind: transmitting the human voice in a more-or-less recognizable form. Their suitability for use in computer-computer communication is often marginal at best, but the situation is rapidly changing with the introduction of fiber optics and digital technology. - In any event, the telephone system is so tightly intertwined with (wide area) computer networks, that it is worth devoting some time to studying it. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 103

103 Public Switched Telephone System (PSTN) For connecting computers in near distances (in a company) run a cable between them=lan For long distances and more computers use existing telecommunication facilities = PSTN A cable running faster than 10 9 bps A dial-up line has max. 56 Kbps Difference factor = 20,000!! Transmission of voice and data on system Computer system designer try to figure out how to use PSTN efficiently Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 104

104 Structure of the Telephone System Telephone structures Fully-interconnected network Centralized switch Two-level hierarchy Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 105

105 Structure of the Telephone System Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 106

106 Structure of the Telephone System A key element in most wide area networks is the telephone system. Its main components are : the local loops, trunks, and switches. Local loops : are analog, twisted pair circuits, which require modems for transmitting digital data. ADSL offers speeds up to 50 Mbps by dividing the local loop into many virtual channels and modulating each one separately. Wireless local loops are another new development to watch.. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 107

107 The public Switched Telephone Networks (PSTN) The telephone system consists of three major components: 1-Local loops: (analog twisted pair wires going into houses and business). 2- Trunks: (digital fiber optics cables connecting the switching offices). 3-Switching offices: (where calls are moved from one trunk to another). Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 108

108 The public Switched Telephone Networks (PSTN) Can be viewed to have two types of components: External (communication medium last mile, long haul trunks etc) and, Internal (Switching Offices) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 109

109 PSTN (3) Structure of the Telephone System(PSTN) A typical circuit route for a medium-distance call Major Components of the Telephone System Local loops: Analog twisted pairs going to houses and businesses ( the two-wire local loop coming from a telephone company end office into houses and small businesses ) Trunks: Digital fiber optics connecting the switching offices Switching offices: Where calls are moved from one trunk to another Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 110

110 Structure PSTN (3) of the Telephone System - variety of transmission media are used for telecommunication. Local loops consist of category 3 twisted pairs nowadays,- Between switching offices, coaxial cables, microwaves, and especially fiber optics are widely used - In the past, transmission throughout the telephone system was analog, with the actual voice signal being transmitted as an electrical voltage from source to destination. - With the advent of fiber optics, digital electronics, and computers, all the trunks and switches are now digital, Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 111

111 PSTN (3) Structure of the Telephone System leaving the local loop as the last piece of analog technology in the system. Digital transmission is preferred because it is not necessary to accurately reproduce an analog waveform after it has passed through many amplifiers on a long call. Being able to correctly distinguish a 0 from a 1 is enough. This property makes digital transmission more reliable than analog. It is also cheaper and easier to maintain. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 112

112 PSTN ( contd ) The use of analog and digital signals has pros and cons Analog Digital Signals Originally Increasingly Attenuation/Noise Low High Amplification/Regeneration Hard Easy Information loss Some Little Maintain - Easier/cheaper Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 113

113 A coder accepts an arbitrary analog signal and generates a digital signal form it. A demodulator accepts a modulated sine wave only and generate a PSTN (The Local Loop: Modems, ADSL, and Wireless ) What is the difference, if any, between the demodulator part of a modem and the coder part of a codec? (After all, both convert analog signals to digital ones.) A coder accepts an arbitrary analog signal and generates a digital signal form it. A demodulator Computer networking, accepts a modulated Olum-Fonoon sine Babol wave only H. and Veisi generate Fall 2005 a digital Page signal. 114

114 PSTN ( contd ) The main parts of the system are illustrated in Fig we see The local loops, The trunks, and The toll offices and End offices, both of which contain switching equipment that switches calls. An End office has up to 10,000 local loops (in the U.S. and other large countries) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 115

115 The Local Loop Modems ADSL Wireless Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 116

116 Modem ( Modulator-demodulator ) Modem: A device that converts digital data to analog signal ( for transmission over phone lines) and from an analog signal to digital data. Modulator : converts the binary data into band-pass analog signal. Demodulator : recovers the binary data from the modulated signal to the digital data To convert binary data into analog signals, A sine wave is used and one of the characteristics (amplitude, phase or frequency) is modulated to carry the binary information. The sine wave is called the carrier wave. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 117

117 Modem : Kinds of modulation Amplitude modulation: Two different amplitudes of sine wave are used to represent 1's and 0's. Binary Signal Frequency modulation: Two (or more) different frequencies, close to the carrier frequency, are used. Phase modulation: The phase of the sine wave is changed by some fixed amount. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 118

118 Modulation / demodulation Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 119

119 Modem : Conversions Computer (binary data) to analog signals done by modems scheme is TCM: modulation schemes like QPSK, QAMs Local loop :. computer/modem to codec Analog to Digital Codecs scheme is PCM done through sampling - codec to telephone net to codec Digital to Analog Codecs Inverse PCM Codec to modem Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 120

120 Figure 5.18 Telephone line bandwidth 300 Hz 3300 Hz For voice the entire range is used because some distortion and noise can be tolerated But for data, for integrity of data, edges of this range are not used. The range used for data is 600 Hz 3000 Hz = 2400 Hz Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 121

121 The Electromagnetic Spectrum -When electrons move, they create electromagnetic waves that can propagate through space (even in a vacuum). -These waves were predicted by the British physicist James Clerk Maxwell in 1865 and first observed by the German physicist Heinrich Hertz in Frequency ( F) : The number of oscillations of a wave per second and is measured in ( Hz.) -The wavelength ( λ) : The distance between two consecutive maxima (or minima) is measured in ( Meter ) - All wireless communication is based on this principle. In vacuum, all electromagnetic waves travel at the same speed, This speed, usually called the speed of light, c, is approximately 300,000 km/sec Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 122

122 The Electromagnetic Spectrum Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 123

123 ADSL: Asymmetric Digital Subscriber Line ADSL uses a frequency spectrum of 1.1 MHz. Divides it into 256 channels each of size roughly Hz. Channel 0 : POTS Channels 1-5 ; guard band between voice and data Two for control channels, one for downstream and one for upstream Remaining are partitioned between upstream and downstream : depends on the service provider; usually it is asymmetric giving 80-90% for download and remaining for upstream hence the word Asymmetric Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 124

124 ADSL: Asymmetric Digital Subscriber Line Operation of ADSL using discrete multitone modulation. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 125

125 Installation requirement of ADSL A typical ADSL equipmnid (Network Interface Device) ent configuration NID (Network Interface Device) At the other end of the wire, on the end office side, a corresponding splitter is installed. Here the voice portion of the signal is filtered out and sent to the normal voice switch. The signal above 26 khz is routed to a new kind of device called a DSLAM (Digital Subscriber Line Access Multiplexer), which contains the same kind of digital signal processor as the ADSL modem. Once the digital signal Computer has been recovered networking, into a bit Olum-Fonoon stream, packets Babol are formed H. Veisi and sent Fall off 2005 to the ISP Page 126

126 Cable broadband Vs DSL Cable Broadband is a public network and is shared by several users, hence Bandwidth reduces as more users log in, and Less secure ADSL is a private network..works on leased lines from old PSTN, hence Dedicated bandwidth, and More secure Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 127

127 Cable broadband Vs DSL : (Speeds ) Can t distinguish on the basis of speeds Different companies offer different packages Cable modem speeds vary widely. While cable modem technology can theoretically support up to about 30 Mbps, most providers offer service with between 1 Mbps and 6 Mbps bandwidth for downloads, and bandwidth between 128 Kbps and 768 Kbps for uploads. Both take flat monthly or yearly rents Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 128

128 Wireless Local Loops Local Multipoint Distribution Service ( LMDS): uses Millimeter waves (because of low BW of MMDS) band in US : GHz and band in Europe: 40GHz (both MM wave bands) were not allocated because it was difficult to build silicon integrated circuits that operate so fast. With the invention of Gallium arsenide ICs the speed became achievable and hence people started thinking of using MM waves for communication. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 129

129 Wireless Local Loops Architecture of an LMDS system. The FCC responded to the demand by allocating 1.3 GHz to a new wireless local loop service called LMDS (Local Multipoint Distribution Service).. The operation of LMDS Here a tower is shown with multiple antennas on it, each pointing in a different direction. Since millimeter waves are highly directional, each antenna defines a sector, independent of the other ones. At this frequency, the range is 2 5 km, which means that many towers are needed Computer to cover networking, a city Olum-Fonoon Babol H. Veisi Fall 2005 Page 130

130 Long-Haul Trunks The next thing now is to combine the signals received in the end office (switching offices of the telephone co.s) from various local loops into one signal that is transmitted on the long-haul trunk. This is done with the help of various multiplexing schemes : - FDM - WDM - TDM Trunks are digital, and can be multiplexed in several ways, including FDM, TDM, and WDM. Both circuit switching and packet switching are important. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 131

131 Trunks and Multiplexing Telephone companies have developed elaborate schemes for multiplexing many conversations over a single physical trunk. These multiplexing schemes can be divided into two basic categories: FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing). In FDM, the frequency spectrum is divided into frequency bands, with each user having exclusive possession of some band. In TDM, the users take turns (in a round-robin fashion), each one periodically getting the entire bandwidth for a little burst of time. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 132

132 Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (C) The multiplexed channel. shows how three voice-grade telephone channels are multiplexed using FDM. Filters limit the usable bandwidth to about 3100 Hz per voice-grade channel. When many channels are multiplexed together, 4000 Hz is allocated to each channel to keep them well separated. First the voice channels are raised in frequency, each by a different amount. Then they can be combined because no two channels now occupy the same portion of the spectrum. Notice that even though there are gaps (guard bands) between the channels, there is some overlap between adjacent channels because the filters do not have sharp edges. This overlap means that Computer a strong spike networking, at the edge of Olum-Fonoon one channel will Babol be felt H. in the Veisi adjacent Fall one 2005 as nonthermal Page 133

133 Wavelength Division Multiplexing (WDM): Wavelength Division Multiplexing (WDM): The same as FDM, but applied to fibers. There's great potential for fibers since the bandwidth is so huge (25,000 GHz). In optical fibers, the scheme used is WDM instead of FDM. As more and more wavelengths are being discovered in a single fiber WDM is getting denser and now the name DWDM (dense WDM) is being used when the number of channels is vary large in a single fiber. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 134

134 Wavelength WDM (Wavelength Division Division Multiplexing Multiplexing). (WDM): Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 135

135 Time Division Multiplexing (TDM): TDM WDM : applicable only on optical fiber and not on copper, but a lot of copper is there on the last mile, also analog. WDM is common for multiplexing optical signals because it allows the multiplexing of signals with a very high frequency FDM : used on copper and microwave but requires analog circuitry and cannot be done by a computer, Solution : TDM : unfortunately can be used only for digital data. So, Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 136

136 Time Division Multiplexing (TDM): TDM Time Division Multiplexing Time divided into slots each user has time slot Users take turns in round robin fashion In TDM, the users take turns, each one having exclusive use of the medium in a round robin fashion. TDM can be all digital. The method used in North America and Japan is the T1 carrier Ex. T1=24 channels, each 8 bits =192 bits Mbps Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 137

137 Time Division Multiplexing (TDM): Trunks and Multiplexing Time Division Multiplexing (TDM): Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 138

138 Digital Trunks What we need is to convert the analog signals received in the end office (switching offices of the telephone co.s) from various local loops into digital signals and combine them into one signal that is transmitted on the digital trunk. This is done with the help of TDM. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 139

139 CODEC : PCM (Pulse Code Modulation) The codec makes 8000 samples per sec or one sample per 125 microsec. This is because Nyquist theorem says that this is sufficient to capture all the information from the 4 KHz * bit rate = No. of samples x log L => sample rate = 2B from Nyquist theorem). This technique is called : PCM. All the time intervals (a pulse) within the telephone system are multiples of 125 microsecond. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 140

140 Time Division Multiplexing : T1 Carrier T1 carrier is used on long-haul trunks. Supports Codec with 24 Local Loops i.e. 24 channels Codec picks signals from these 24 channels on a Round Robin basis to insert 8 bits (7 data + 1 error) for each sample( i.e. for each channel) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 141

141 T1 Carrier The T1 carrier (1.544 Mbps). 193 X 8000 = Mbps 193 rd bit is used for frame synchronization : a pattern of is looked for --- analog nodes cannot generate this pattern, digital users can but the chances are less. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 142

142 PSTN ( contd ) Can be viewed to have two types of components: External (communication medium last mile, long haul trunks etc) and, Internal (Switching Offices) Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 143

143 Circuit Switching Circuit switching refers to a communication mechanism that establishes a path between a sender and receiver with guaranteed isolation from paths used by other pairs of senders and receivers Circuit switching is usually associated with telephone technology because a telephone system provides a dedicated connection between two telephones Figure 13.1 illustrates the concept Circuit switching networks use electronic devices to establish circuits Instead of having each circuit correspond to a physical path multiple circuits are multiplexed over shared media and the result is known as a virtual circuit Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page

144 Circuit Switching Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page

145 Circuit Switching Three general properties define a circuit switched : Point-to-point communication means that a circuit is formed between exactly two endpoints Separate steps for circuit creation, use, and termination distinguishes circuits that are switched (i.e., established when needed) from circuits that are permanent Performance equivalent to an isolated physical path communication between two parties is not affected in any way by communication among other parties circuit switching must provide the illusion of an isolated path for each pair of communicating entities Switched circuits use a three-step process analogous to placing a phone call a circuit is established between two parties the two parties use the circuit to communicate the two parties terminate use Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 147

146 Packet Switching A packet switching system requires a sender to divide each message into blocks of data that are known as packets the size of a packet varies each packet switching technology defines a maximum packet size Three general properties define a packet switching Arbitrary, asynchronous communication allows a sender to communicate with one recipient or multiple recipients a given recipient can receive messages from one sender or multiple senders communication can occur at any time and a sender can delay arbitrarily long between successive communications No set-up required before communication begins system remains ready to deliver a packet to any destination at any time a sender does not need to perform initialization before communicating system does not need to notify the underlying system when communication terminates Performance varies due to statistical multiplexing among packets multiplexing occurs among packets rather than among bits or bytes Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page

147 Packet Switching A packet switching system uses statistical multiplexing multiple sources compete for the use of shared media Figure (below) illustrates the concept Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page

148 Switching Circuit Switching: A physical connection (electrical, optical, radio) is established from the caller phone to the callee phone. This happens before any data is sent. Message Switching: The connection is determined only when there is actual data (a message) ready to be sent. The whole message is recollected at each switch and then forwarded on to the next switch. This method is called store-and-forward. - This method may tie up routers for long periods of time - not good for interactive traffic. Packet Switching: Divides the message up into blocks (packets). Therefore packets use the transmission lines for only a short time period - allows for interactive traffic. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 150

149 Switching Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 151

150 Switching Figure Timing of events in (a) circuit switching, (b) message switching, (c) packet switching. Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 152

151 Packet Switching VS. circuit-switching One of the chief advantages of packet switching is the lower cost that arises from sharing To provide communication among N computers With a circuit-switched network must have a connection for each computer plus at least N/2 independent paths With packet switching, a network must have a connection for each computer, but only requires one path that is shared Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page

152 Switching Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 154

153 Switching Q : What is the essential difference between message switching and packet switching? Computer networking, Olum-Fonoon Babol H. Veisi Fall 2005 Page 155