Lecture 3: Transmission Media

Transcription

1 Lecture 3: Transmission Media Dr. Mohd Nazri Bin Mohd Warip High Performance Broadband Networks Research Group Embedded, Networks and Advanced Computing Research Cluster School of Computer and Communication Engineering Universiti Malaysia Perlis Session 2013/2014

2 Lecture Outline Transmission Medium Transmission Media Transmission System Transmission Delay 2

3 Bits, numbers, information Bit: number with value 0 and 1 n bits: digital representation for 0, 1,..., 2 n Byte or Octet, n = 8 Computer word, n=16, 32 or 64 n bits allows enumeration of 2 n possibilities n-bit field in a header n-bit representation of a voice sample Message consisting of n bits The number of bits required to represent a message is measure of its information content More bits More content 3

4 Block vs. Stream Information Block Information that occurs in a single block Text message Data file JPEG image MPEG file Size = Bits / block or bytes/block 1 kbyte = 2 10 bytes 1 Mbyte= 2 20 bytes 1 Gbyte= 2 30 bytes Stream Information that is produced & transmitted continuously Real-time voice Streaming video Bit rate = bits / second 1 kbps = 10 3 bps 1 Mbps = 10 6 bps 1 Gbps=10 9 bps 4

5 Transmission Medium Transmission medium is the physical path between transmitter and receiver guided media (wired/cables) guided along a solid medium unguided media (wireless) atmosphere, space, water characteristics and quality determined by medium and signal guided media - medium is more important unguided media - bandwidth produced by the antenna is more important key concerns are data rate and distance 5

6 Data rate and Distance Factors Parameters Bandwidth Transmission Impairments Interference Number of Receivers Effects higher bandwidth gives higher data rate impairments, such as attenuation, limit the distance overlapping frequency bands can distort or wipe out a signal more receivers introduces more attenuation 6

7 Terminology Unipolar: all signal elements have the same sign Polar: one logic state represented by positive voltage and the other by negative voltage Data rate: rate of data ( R ) transmission in bits per second Duration or length of a bit: time taken for transmitter to emit the bit (1/R) Modulation rate: rate at which the signal level changes, measured in baud = signal elements per second. mark and space binary 1 and binary 0 7

8 Key Data Transmission Terms Term Units Definition Data element Bits A single binary one Data Rate Bits per second (b/s) The rate at which data elements are transmitted. Signal element Digital: a voltage That part of a signal pulse of constant that occupies the amplitude. shortest interval of a Analogue: a pulse of signalling code constant frequency, phase and amplitude. Signalling rate or modulation rate Signal elements per second (baud) The rate at which signal elements are transmitted. 8

9 Layer 1: The Physical Layer Fundamental to next layers Properties of wires, fibre, wireless limit what the network can do Key problem is to send (digital) bits using only (analogue) signals This is called modulation Application Transport Network Link Physical 9

10 Guided Transmission Guided Transmission viz. Wires (cables), Fibre Optics Properties of Media: Wires (Cables): Twisted Pairs, coaxial cables, Power Lines Fibre optic: Multimode, Single Mode Reality Check: Storage Media 10

11 Unguided Transmission Unguided Transmission viz. Wireless, Microwave, WiFi, WiMax, 4G, LTE Properties of Media: Electromagnetic Spectrum Radio Transmission Microwave Transmission Light Transmission (No Fiber Optic) 11

12 Cables/Wires Twisted Pair Uses in LAN Twists reduce radiated signal (interference) Category 5 UTP cable with four twisted pairs 12

13 Twisted Pair UTP Cat 5, 6, 7 13

14 Link Terminology Full-Duplex link: Used for transmission in both directions at once e.g., use different twisted pairs for each direction Half-Duplex link: Both directions, but not at same time e.g., use different twisted pairs for each direction Simplex link: Both Only one fixed direction at all times; not common 14

15 Wired Coaxial Cable ( Co-ax ) Old LAN cabling. Better shielding and more bandwidth for longer distances and higher rates than twisted pair. 15

16 Wired Power Lines Also known as Homeplug Power Line. Powerline use household electrical wiring: Convenient to use, but horrible for sending data 16

17 Wired Fibre Optic Cable Uses for high rates and long distances at Core Networks. Long distance ISP links, Fiber-to-the-Home Light carried in very long, thin strand of glass Light source (LED, laser) Light trapped by total internal reflection Photodetector 17

18 Wired Fibre Optic Cable Fibre has enormous bandwidth (THz) and tiny signal loss hence high rates over long distances 18

19 Wired Fibre Optic Cable Single-Mode Core so narrow (10um) light can t even bounce around Used with lasers for long distances, e.g., 100km Multi-Mode Other main type of fiber Light can bounce (50um core) Used with LEDs for cheaper, shorter distance links Fibers in a cable 19

20 Wired vs. Fibre Comparison Property Wires (Cables) Fibre Optic Distance Short ( < 100m) Long (> 500m - 70km) Bandwidth Moderate Very High Cost Cheap Expensive Convenience Easy to use Requires skill Security Easy to tap Hard to tap 20

21 Transmission Mode - Fibre Optical 21

22 Wavelength Division Multiplexer Wavelength Division Multiplexing (WDM) is a technique to transmit data onto a common optical fibre simultaneously on various wavelength channels. In optical networks, each channel is known as a wavelength and the term lambda (λ) is used to designate the carrier. Transmitter/Receiver + MUX/DeMUX Point-to-Point application Dr. Mohd Nazri Mohd Warip August 2013 / EKT 544 Lecture 1 22

23 Wavelength Division Multiplexing (WDM) Lower speed, lower chromatic dispersion. Capacity can be increased by adding wavelengths WDM channels can be designed to be transparent (in data rate and format, nice, but; loss of monitoring functions). One transceiver per channel needed. Effectively bridging the gap between optical transmission speed and electronic processing speed (known as the electro-optical bottleneck). 23

24 ITU WDM Channel Spacing (G.692) Frequency (THz) Wavelength in Vacuum (nm) 50 GHz 100 GHz 200 GHz X X X X X X X X X X X ,12 X X X X X X X X X X X X 24

25 Transmission Characteristics: Guided Media Characteristics Twisted Pair (with loading) Twisted Pair (multi-pair cables) Frequency Range Typical Attenuation 0 to 3.5kHz khz 0 to 1 MHz khz Coaxial cable 0 to 500 MHz 7 10 MHz Fibre Optic 186 to 370 MHz 0.2 to 0.5 db/km Typical Delay Repeater Spacing 50 µs/km 2 km 5 µs/km 2 km 4 µs/km 1 to 9 km 5 µs/km 40 km 25

26 Wireless: Electromagnetic Spectrum Different bands have different uses. Long Radio: wide-area broadcast; Infrared/Light: line-of-sight Microwave: LANs and 3G/4G; Networking focus Microwave 26

27 Wireless: Electromagnetic Spectrum To manage interference, spectrum is carefully divided, and its use regulated and licensed, e.g., sold at auction. 300 MHz 3 GHz WiFi(ISM bands) 3 GHz Source: NTIA Office of Spectrum Management, GHz Part of the US frequency allocations 27

28 Wireless: Electromagnetic Spectrum Unlicensed Spectrum Band - ISM Radio Band. ISM (Industrial, Scientific and Medical) reserved RF Free for use at low power; devices manage interference Widely used for networking; WiFi, Bluetooth, Zigbee, etc b/g/n a/g/n 28

29 Wireless: Radio Transmission Radio signals penetrate buildings well and propagate for long distances with path loss In the VLF, LF, and MF bands, radio waves follow the curvature of the earth In the HF band, radio waves bounce off the ionosphere. 29

30 Wireless: Microwave Transmission Unlicensed Microwaves () have much bandwidth and are widely used indoors (WiFi) and outdoors (3G, satellites). Signal is attenuated/reflected by everyday objects Strength varies with mobility due multipath fading, etc. 30

31 Wireless: Microwave Transmission 31

32 Wireless: Light Transmission Line-of-sight light (no fibre optic) can be used for links Light is highly directional, has much bandwidth Use of LEDs/cameras and lasers/photodetectors 32

33 Wireless vs. Wired(Cables)/Fibre Medium Advantages Disadvantages Wireless Wired / Fibre Easy and inexpensive to deploy Naturally supports mobility Naturally supports broadcast Easy to engineer a fixed data rate over point-topoint links Transmissions interfere and must be managed Signal strengths hence data rates vary greatly Can be expensive to deploy, esp. over distances Doesn t readily support mobility or broadcast 33

34 Communication Satellites Satellites are effective for broadcast distribution and anywhere/anytime communications. Kinds of Satellites Geostationary (GEO) Satellites use Low-Earth Orbit (LEO) Satellites Satellites vs. Fibre : 34

35 Types of Satellites Satellites and their properties vary by altitude: Geostationary (GEO), Medium-Earth Orbit (MEO), and Low- Earth Orbit (LEO) Satsneeded for global coverage 35

36 Geostationary Satellites GEO satellites orbit 35,000 km above a fixed location VSAT (Very Small Aperture Terminal) can communicate with the help of a hub. Two-way Satellite Ground Station. Antenna <3m Different bands (L, S, C, Ku, Ka) in the GHz are in use but may be crowded or susceptible to rain. VSAT 36

37 Low-Earth Orbit Satellites Systems such as Iridium use many low-latency satellites for coverage and route communications via them Providing voice and data coverage to satellite phone, GPS The Iridium satellites form six necklaces around the earth. 37

38 Satellite vs. Fibre Optic Medium Pros Cons Satellite Can rapidly set up anywhere/anytime communications (after satellites have been launched) Can broadcast to large regions Limited bandwidth and interference to manage Fibre Enormous bandwidth over long distances Installation can be more expensive/difficult 38

39 References Kurose, J. F. and Ross, K. W., Computer Networking A Top-Down Approach, 6th. Edition, Pearson, ISBN: Alberto Leon-Garcia and Indra Widjaja., Communication Networks: Fundamental Concepts and Key Architectures, 2nd. Edition, McGraw Hill, Tanenbaum, A. S., Computer Network, 5th Edition. Prentice-Hall, William Stallings, Data & Computer Communications, 8th. Edition, Prentice Hall,