William Stallings Data and Computer Communications

1. William StallingsData and Computer Communications Chapter 4 Transmission Media

2. Overview • Guided - wire • Unguided - wireless • Characteristics and quality determined by medium and signal • For guided, the medium is more important • For unguided, the bandwidth produced by the antenna is more important • Key concerns are data rate and distance

3. Design Factors • Bandwidth • Higher bandwidth gives higher data rate • Transmission impairments • Attenuation • Interference • Number of receivers • Major factor in guided media • More receivers (multi-point) introduce more attenuation

4. Electromagnetic Spectrum

5. Guided Transmission Media • the transmission capacity depends on the distance and on whether the medium is point-to-point or multipoint • e.g., • Twisted Pair • Coaxial cable • Optical fiber

6. Twisted Pair • consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs • often used at customer facilities and also over distances to carry voice as well as data communications • low frequency transmission medium

7. Twisted Pair - Applications • Most common medium • Telephone network • Between house and local exchange (subscriber loop) • Within buildings • To private branch exchange (PBX) • For local area networks (LAN) • 10Mbps or 100Mbps

8. Twisted Pair - Pros and Cons • Cheap • Easy to work with • Low data rate • Short range

9. Twisted Pair - Transmission Characteristics • Analog • Amplifiers every 5km to 6km • Digital • Use either analog or digital signals • repeater every 2km or 3km • Limited distance • Limited bandwidth (1MHz) • Limited data rate (100MHz) using different modulation & signaling techniques • Susceptible to interference and noise

10. Unshielded and Shielded TP • Unshielded Twisted Pair (UTP) • Ordinary telephone wire • Cheapest • Easiest to install • Suffers from external electromagnetic interference (EM) • Shielded Twisted Pair (STP) • the pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference • More expensive • Harder to handle (thick, heavy)

11. UTP Categories • Cat 3 • up to 16MHz • Voice grade found in most offices • Twist length of 7.5 cm to 10 cm • Cat 4 (least common) • up to 20 MHz • Cat 5 • up to 100MHz • Commonly pre-installed in new office buildings • Twist length 0.6 cm to 0.85 cm

12. Twisted Pair Advantages • inexpensive and readily available • flexible and light weight • easy to work with and install

13. Twisted Pair Disadvantages • susceptibility to interference and noise • attenuation problem • For analog, repeaters needed every 5-6km • For digital, repeaters needed every 2-3km • relatively low bandwidth

14. Coaxial Cable

15. Coaxial Cable Applications • Most versatile medium • Television distribution • Aerial to TV • Cable TV • Long distance telephone transmission • Can carry 10,000 voice calls simultaneously • Being replaced by fiber optic • Short distance computer systems links • Local area networks

16. Coaxial Cable - Transmission Characteristics • Analog • Amplifiers every few km • Closer if higher frequency • Up to 500MHz • Digital • Repeater every 1km • Closer for higher data rates

17. Coax Advantages • higher bandwidth • 400 to 600Mhz • up to 10,800 voice conversations • can be tapped easily (pros and cons) • much less susceptible to interference than twisted pair

18. Coax Disadvantages • high attenuation rate makes it expensive over long distance • bulky

19. Optical Fiber

20. Optical Fiber - Benefits • Greater capacity • Data rates of hundreds of Gbps • Smaller size & weight • Lower attenuation • Electromagnetic isolation • Greater repeater spacing • 10s of km at least

21. Optical Fiber - Applications • Long-haul trunks • Metropolitan trunks • Rural exchange trunks • Subscriber loops • LANs

22. Optical Fiber - Transmission Characteristics • Act as wave guide for 1014 to 1015 Hz • Portions of infrared and visible spectrum • Light Emitting Diode (LED) • Cheaper • Wider operating temp range • Last longer • Injection Laser Diode (ILD) • More efficient • Greater data rate • Wavelength Division Multiplexing

23. Fiber Optic Types • multimode step-index fiber • the reflective walls of the fiber move the light pulses to the receiver • multimode graded-index fiber • acts to refract 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

24. Optical Fiber Transmission Modes

25. Fiber Optic Signals fiber optic multimode step-index fiber optic multimode graded-index fiber optic single mode

26. 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

27. Fiber Optic Disadvantages • expensive over short distance • requires highly skilled installers • adding additional nodes is difficult

28. Wireless Transmission • Unguided media • Transmission and reception via antenna • Two techniques are used: • Directional • Focused beam • Careful alignment required • Omnidirectional • Signal spreads in all directions • Can be received by many antennas

29. Frequencies • 2GHz to 40GHz • Microwave • Highly directional • Point to point • Satellite • 30MHz to 1GHz • Omnidirectional • Broadcast radio • 3 x 1011 to 2 x 1014 • Infrared • Local

30. Wireless Examples • terrestrial microwave transmission • satellite transmission • broadcast radio • infrared

31. Terrestrial Microwave • uses the radio frequency spectrum, commonly from 2 to 40 Ghz • transmitter is a parabolic dish, mounted as high as possible • used by common carriers as well as by private networks • requires unobstructed line of sight between source and receiver • curvature of the earth requires stations (called repeaters) to be ~30 miles apart

32. Microwave Transmission Applications • long-haul telecommunications service for both voice and television transmission • short point-to-point links between buildings for closed-circuit TV or a data link between LANs

33. Microwave Transmission Advantages • no cabling needed between sites • wide bandwidth • multichannel transmissions

34. Microwave Transmission Disadvantages • line of sight requirement • expensive towers and repeaters • subject to interference such as passing airplanes and rain

35. Satellite Microwave • a microwave relay station in space • Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency • 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

36. 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

37. Satellite Transmission Process satellite transponder dish dish 22,300 miles uplink station downlink station

38. Satellite Transmission Applications • television distribution • a network provides programming from a central location using direct broadcast satellites (DBS) • long-distance telephone transmission • high-usage international trunks • private business networks

39. Principal Satellite Transmission Bands • C band: 4(downlink) - 6(uplink) GHz • the first to be designated • Ku band: 12(downlink) -14(uplink) GHz • rain interference is the major problem • Ka band: 19(downlink) - 29(uplink) GHz • equipment needed to use the band is still very expensive

40. Satellite Advantages • can reach a large geographical area • high bandwidth • cheaper over long distances

41. Satellite Disadvantages • high initial cost • susceptible to noise and interference • propagation delay

42. Broadcast Radio • Omnidirectional • FM radio • UHF and VHF television • Requires line of sight • Suffers from multipath interference • Reflections

43. Infrared • Achieved using tranceivers that modulate noncoherent infrared light • Requires line of sight (or reflection) • Blocked by walls • e.g. TV remote control, Infrared port

44. Common Carriers • a government-regulated private company • involved in the sale of infrastructure services in transportation and communications • required to serve all clients indiscriminately • services and prices from common carriers are described in tariffs

45. Leased (or Dedicated) Lines • permanently or semi-permanently connect between two points • economical in high volume calls between two points • no delay associated with switching times • can assure consistently high-quality connections

46. Leased (or Dedicated) Lines • voice grade channels • normal telephone lines • in the range of 300 Hertz to 3300 Hertz • conditioning or equalizing • reduces the amount of noise on the line, providing lower error rates and increased speed for data communications

47. T-1 Carrier • also referred to as DS-1 signaling • provides digital full-duplex transmission rates of 1.544Mbps • usually created by multiplexing 24 64-Kbps voice or 56-Kbps data lines • higher speeds are available with T-3 (45Mbps) [sometimes referred to a DS-3 lines; can be multiplexed into 28 T-1 signals; T-3 consists of 672 individual channels, each of which supports 64-Kbps] and T-4 services (274Mbps) • in Europe, E-1 (2.048Mbps) is used instead of T-1

48. Integrated Services Digital Network (ISDN) • all-digital transmission facility that is designed to replace the analog PSTN • basic ISDN (basic rate access) • two 64Kbps bearer channels + 16Kbps data channel (2B+D) = 144 Kbps • broadband ISDN (primary rate access) • twenty-three 64Kbps bearer channels + 64 data channels (23B+D) = 1.536 Mbps

49. Past Criticism of ISDN • “Innovations Subscribers Don’t Need” • “It Still Doesn’t Network” • “It Still Does Nothing” • Why so much criticism? • overhyping of services before delivery • high price of equipment • delay in implementing infrastructure • incompatibility between providers' equipment.

50. ISDN Channel Definitions • B (bearer) channels • 64 kbps channels that may be used to carry voice, data, facsimile, or image • D (demand) channels • mainly intended for carrying signaling, billing and management information to control ISDN services (out-of-band control messages) • may be either 16 or 64 kbps