The Physical Layer Chapter 2

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1 The Physical Layer Chapter 2 Theoretical Basis for Data Communications Guided Transmission Media Wireless Transmission Communication Satellites Digital Modulation and Multiplexing Public Switched Telephone Network Mobile Telephone System Cable Television Revised: February 2018

2 The Physical Layer Foundation on which other layers build Properties of wires, fiber, wireless limit what the network can do Key problem is to send (digital) bits using only (analog) signals This is called modulation Application Transport Network Link Physical

3 Physical Layer Issues Media: wires, fiber, satellites, radio Signal propagation: bandwidth, attenuation, noise Modulation: how bits are represented as voltage signals Fundamental limits: Nyquist, Shannon

4 Abstract Model of a Link Sender Channel: bit rate, delay, error rate Receiver Bit rate: bits/sec depends on the channel s bandwidth Delay: how long does it take a bit to get to the end? Error rate: what is the probability of a bit flipping?

5 Bandwidth-Delay Product Bits have a physical size on the channel! Storage capacity of a channel is: bit rate x delay Example: 100 Mbps 5000-km fiber, delay = 50 msec In 50 msec we can pump out 5 million bits So the fiber can store 5 million bits in 5000 km 1 km holds 1000 bits so a bit is 1 meter long At 200 Mbps, a bit is 0.5 m long

6 Signal Propagation over a Wire The signal has a finite propagation speed (2/3 c) The signal is attenuated per km Frequencies above a cutoff are strongly reduced Noise is added to the signal

7 Theoretical Basis for Data Communication Communication rates have fundamental limits Fourier analysis Bandwidth-limited signals Maximum data rate of a channel

8 Sine wave g(t) = A sin (2 π f t + ϕ) A (Volts) Period = 1/f Time A is the amplitude = how strong the signal is f is the frequency (cycles/sec or Hz) = how fast it changes in time A A (Volts)

9 Fourier Analysis A time-varying periodic signal can be represented as a series of frequency components (harmonics): = Signal over time a, b weights of harmonics

10 Bandwidth At the signal level, bandwidth is cutoff frequency (HZ) For data transmission it is bits/sec

11 Bandwidth-Limited Signals Having less bandwidth (harmonics) degrades the signal Bandwidth 8 harmonics Lost! 4 harmonics Lost! 2 harmonics Lost!

12 Maximum Data Rate of a Channel - Nyquist Nyquist s theorem relates the data rate to the bandwidth (B) and number of signal levels (V) on a noiseless channel: Max. data rate = 2B log 2 V bits/sec Examples 3000 Hz channel (tel. line), binary signals = 6000 bps 3000 Hz channel (tel. line), 4-level signals = 12,000 bps 3000 Hz channel (tel. line), 16-level signals = 48,000 bps Nyquist is a property of mathematics that relate bandwidth to symbols/sec and bits/sec

13 Maximum Data Rate of a Channel - Shannon Signal to noise is signal power to noise power: - Expressed as log 10 signal power/noise power - S/N of 10 is written as 10 db - S/N of 100 is written as 20 db - S/N of 1000 is written as 30 db Shannon's theorem relates the data rate to the bandwidth (B) and signal strength (S) relative to the noise (N): Max. data rate = B log 2 (1 + S/N) bits/sec How fast signal can change How many levels can be seen

14 Example of Shannon s limit DSL line of 1 MHz Suppose S/N = 50 db (S = 100,000) Data rate = 10 6 log 2 (100,001) bit/sec Data rate = 16.6 Mbps To go higher, you have to cheat: - Fiber to the curb - Bonding: Use two or more pairs - Dynamic spectrum mgmt (basically, reduce noise)

15 Nyquist vs. Shannon Nyquist: - For noiseless channel - Depends on number signal levels per symbol Shannon - For noisy channel - Depends on S/N ratio, not bits/symbol

16 Guided Transmission (Wires & Fiber) Media have different properties, hence performance Reality check Physical transport of storage media Wires: Twisted pairs Coaxial cable Power lines Fiber cables

Ultrium 7 tape = 6 TB, 400 cm2 (costs 100) Typical van has capacity of 7 x 106 cm2 Van holds 17,500 tapes holding 105

17 Transporting Physical Media AST 1990: Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway. Ultrium 7 tape = 6 TB, 400 cm2 (costs 100) Typical van has capacity of 7 x 106 cm2 Van holds 17,500 tapes holding 105 x 1015 bytes One person can drive NYC to LA in 5 days = 4 x 105 s This is a bandwidth of 2 Tbps or 2000 Gbps Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

Enter Amazon s Snowmobile service It is for companies to put their data in the

18 Amazon s Snowmobile Service When I first wrote that, I meant it as a joke No longer. Enter Amazon s Snowmobile service It is for companies to put their data in the cloud The Truck holds 100 PB (100,000 terabytes) on HDs Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

19 Wires Twisted Pair Very common; used in LANs, telephone lines Twists reduce radiated signal (interference) UTP = Unshielded Twisted Pair Category 5 UTP cable with four twisted pairs

20 Kinds of Wire STP = Shielded Twisted Pair UTP = Unshielded Twisted Pair - Cat 3: Home telephone lines - Cat 5: Fast Ethernet (100 Mbps) - Cat 5e: Gigabit Ethernet (1 Gbps) - Cat 6: 10-Gigabit Ethernet (10 Gps) up to 100 m - Cat 6A: Better quality Cat 6 - Cat 7: Includes shielding (not in common use)

21 Connectors RJ11 4 wires RJ45 8 wires Modern buildings are wired for RJ45 but there are adaptors

22 Link Terminology Simplex link Only one fixed direction at all times; not common Half-duplex link Both directions, but not at the same time e.g., senders take turns on a wireless channel Full-duplex link Used for transmission in both directions at once e.g., use different twisted pairs for each direction

23 Wires Coaxial Cable ( Co-ax ) Also common. Better shielding and more bandwidth for longer distances and higher rates than twisted pair.

24 Wires Power Lines Household electrical wiring is another example of wires Convenient to use, but poor for sending data

25 Fiber Optics (1) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

26 Fiber Cables (2) Common for high rates and long distances 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

27 Fiber Cables (3) Fiber has enormous bandwidth (THz) and tiny signal loss hence high rates over long distances

28 Fiber Cables (3) 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

29 TAT-14 TransAtlantic Cable Fiber cable lies on the ocean floor (8000 m deep) Ring structure Two pairs of fibers used plus two pairs for backup Theoretical capacity is 3 Tbps Cables are not well protected and there is no backup

30 Wire vs. Fiber Comparison of the properties of wires and fiber: Property Wires Fiber Distance Short (100s of m) Long (tens of km) Bandwidth Moderate Very High Cost Inexpensive Less cheap Convenience Easy to use Less easy Security Easy to tap Hard to tap

31 Wireless Transmission Electromagnetic Spectrum Radio Transmission Microwave Transmission Light Transmission Wireless vs. Wires/Fiber

32 Electromagnetic Spectrum (1) Different bands have different uses: Radio: wide-area broadcast; Infrared/Light: line-of-sight Microwave: LANs and 3G/4G; Networking focus Microwave

33 Electromagnetic Spectrum (2) 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

34 Electromagnetic Spectrum (3) Fortunately, there are also unlicensed ( ISM ) bands: Free for use at low power; devices manage interference Widely used for networking; WiFi, Bluetooth, etc b/g/n a/g/n/ac

35 Radio Waves Radio waves have a frequency, f, in Hz They have a wavelength, λ in meters λf = c in vacuum Speed of radio/light = 1 foot/nsec For microwaves, megahertz x meters = MHz waves are 1 meter long 1 GHz waves are 30 cm long 2.4 GHz waves are 12.5 cm long

36 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.

37 Microwave Transmission 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.

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

39 Wireless vs. Wires/Fiber Wireless: + Easy and inexpensive to deploy + Naturally supports mobility + Naturally supports broadcast Transmissions interfere and must be managed Signal strengths hence data rates vary greatly Wires/Fiber: + Easy to engineer a fixed data rate over point-to-point links Can be expensive to deploy, esp. over distances Doesn t readily support mobility or broadcast

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

41 Kinds of Satellites Satellites and their properties vary by altitude: Geostationary (GEO), Medium-Earth Orbit (MEO), and Low-Earth Orbit (LEO) Sats needed for global coverage

42 Geostationary Satellites (1) GEO satellites orbit 36,000 km above a fixed location VSAT (computers) can communicate with the help of a hub Up and down time is about 250 msec Big problem for voice GEO satellite VSAT

43 Geostationary Satellites (2) The principal satellite bands Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

44 Low-Earth Orbit Satellites Systems such as Iridium use many low-latency satellites for coverage and route communications via them The 66 Iridium satellites form six necklaces around the earth.

45 Low-Earth Orbit Satellites (2) Relaying in space. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

46 Low-Earth Orbit Satellites (3) Relaying on the ground Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

47 Satellite vs. Fiber Satellite: + Can rapidly set up anywhere/anytime communications (after satellites have been launched) Fiber: + Can broadcast to large regions Limited bandwidth and interference to manage + Enormous bandwidth over long distances Installation can be more expensive/difficult Doesn t work at sea or in remote areas

48 Digital Modulation and Multiplexing Modulation schemes send bits as signals Multiplexing schemes share a channel among users. Baseband Transmission Passband Transmission Frequency Division Multiplexing Time Division Multiplexing Code Division Multiple Access

49 Baseband Transmission Line codes send symbols that represent one or more bits NRZ is the simplest, literal line code (+1V= 1, -1V= 0 ) Other codes tradeoff bandwidth and signal transitions Four different line codes

50 Clock Recovery To decode the symbols, signals need sufficient transitions Otherwise long runs of 0s (or 1s) are confusing, e.g.: um, 0? er, 0? Strategies: Manchester coding, mixes clock signal in every symbol 4B/5B maps 4 data bits to 5 coded bits with 1s and 0s: Data Code Data Code Data Code Data Code Scrambler XORs tx/rx data with pseudorandom bits

51 Passband Transmission (1) Modulating the amplitude, frequency/phase of a carrier signal sends bits in a (non-zero) frequency range NRZ signal of bits Amplitude shift keying Frequency shift keying Phase shift keying

52 Passband Transmission (2) Constellation diagrams are a shorthand to capture the amplitude and phase modulations of symbols: BPSK 2 symbols 1 bit/symbol QPSK 4 symbols 2 bits/symbol QAM symbols 4 bits/symbol QAM symbols 6 bits/symbol BPSK/QPSK varies only phase QAM varies amplitude and phase

53 Frequency Division Multiplexing FDM (Frequency Division Multiplexing) shares the channel by placing users on different frequencies: Overall FDM channel

54 Frequency Hopping Spread Spectrum WiFi and Bluetooth change frequencies many times/sec Called frequency hopping Invented by sex-goddess Hedy Lamarr She patented it, but Navy wasn t interested

55 Time Division Multiplexing (TDM) Time division multiplexing shares a channel over time: Users take turns on a fixed schedule; this is not packet switching or STDM (Statistical TDM) Widely used in telephone / cellular systems

56 Code Division Multiple Access (1) (a) Chip sequences for four stations. (b) Signals the sequences represent Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

57 Code Division Multiple Access (2) (c) Six examples of transmissions. (d) Recovery of station C s Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

58 The Public Switched Telephone Network Structure of the telephone system Politics of telephones Local loop: modems, ADSL, and FttH Trunks and multiplexing Switching

59 Structure of the Telephone System (1) (a) Fully interconnected network. (b) Centralized switch. (c) Two-level hierarchy. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

60 Structure of the Telephone System (2) A hierarchical system for carrying voice calls made of: Local loops, mostly analog twisted pairs to houses Trunks, digital fiber optic links that carry many calls Switching offices, that move calls among trunks

61 Structure of the Telephone System (3) Major Components Local loops analog twisted pairs to houses, businesses). Trunks (digital fiber optic links between switching offices). Switching offices (calls are moved from one trunk to another) Core of phone system is optical & digital in Europe, US Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

62 The Politics of Telephones There is a distinction for competition between serving a local area (LECs) and connecting to a local area (at a POP) to switch calls across areas (IXCs) Customers of a LEC can dial via any IXC they choose

63 Physics of Cat 3 Wiring Bandwidth versus distance over Category 3 UTP for DSL. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

64 Local loop (2): modems Telephone modems send digital data over an 3.3 KHz analog voice channel interface to the POTS Rates <56 kbps; early way to connect to the Internet

65 Local loop (1): Acoustic Couplers Until 1968 in U.S. and ca in Europe, modems were not allowed People could use acoustic couplers to connect terminals

66 Local loop (3): Digital Subscriber Lines DSL broadband sends data over the local loop to the local office using frequencies that are not used for POTS Telephone/computers attach to the same old phone line Rates vary with line ADSL2 up to 24 Mbps VDSL2 to 100 Mbps OFDM used to 1.1 MHz Most bandwidth down

67 Local loop (4): Fiber To The Home FttH broadband relies on deployment of fiber optic cables to provide high data rates customers One wavelength can be shared among many houses Fiber is passive (no amplifiers, etc.)

68 Pulse Code Modulation (1) Calls are carried digitally on PSTN trunks using TDM A call is an 8-bit PCM sample each 125 µs (64 kbps) Traditional T1 carrier has 24 call channels each 125 µs (1.544 Mbps) Europe uses 8 bits for data: E1 at Mbps

69 Pulse Code Modulation (2) Multiplexing T1 streams into higher carriers Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

70 SONET/SDH (1) Different carriers had to interconnect For international calls, T3 and E3 had to be harmonized Need for standards above T4 and E4 Better network management was needed

71 SONET/SDH (2) SONET (Synchronous Optical NETwork) is the worldwide standard for carrying digital signals on optical trunks Keeps 125 µs frame; base frame is 810 bytes (52Mbps) Payload floats within framing for flexibility

72 SONET/SDH (3) Hierarchy at 3:1 per level is used for higher rates Each level also adds a small amount of framing Rates from 52 Mbps (STS-1) to 40 Gbps (STS-768) SONET/SDH rate hierarchy

73 Wavelength Division Multiplexing WDM (Wavelength Division Multiplexing), another name for FDM, is used to carry many signals on one fiber:

74 Circuit Switching/Packet Switching (1) (a) Circuit switching. (b) Packet switching. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

75 Circuit Switching/Packet Switching (2) Timing of events in (a) circuit switching, (b) packet switching Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

76 Circuit Switching/Packet Switching (3) A comparison of circuit-switched and packet-switched networks. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

77 QUEUEING DELAY Packet Switch Queueing delay for M/M/1 system: Q = 1/(1-ρ) * T Where ρ is the line utilization Examples: - ρ = 0.01 means Q = 1.01T - ρ = 0.5 means Q = 2T - ρ = 0.8 means Q = 5T

78 Mobile Telephone System Generations of mobile telephone systems Cellular mobile telephone systems GSM, a 2G system UMTS, a 3G system 4G LTE 4G

79 Generations of mobile telephone systems 1G, analog voice AMPS (Advanced Mobile Phone System) is example, deployed from 1980s. Modulation based on FM (as in radio). 2G, analog voice and digital data GSM (Global System for Mobile communications) is example, deployed from 1990s. Modulation based on QPSK. 3G, digital voice and data UMTS (Universal Mobile Telecommunications System) is example, deployed from 2000s. Modulation based on CDMA LTE, digital data including voice LTE (Long Term Evolution) is example, deployed from 2010s. Modulation based on OFDM 4G based on CDMA and m (WiMax)

80 Cellular mobile phone systems All based on notion of spatial regions called cells Each mobile uses a frequency in a cell; moves cause handoff Frequencies are reused across non-adjacent cells To support more mobiles, smaller cells can be used

81 2G GSM Global System for Mobile Communications (1) Mobile is divided into handset and SIM card (Subscriber Identity Module) with credentials Mobiles tell their HLR (Home Location Register) their current whereabouts for incoming calls Cells keep track of visiting mobiles (in the Visitor LR)

82 2G GSM Global System for Mobile Communications (2) Air interface is based on FDM channels of 200 KHz divided in an eight-slot TDM frame every ms Mobile is assigned up- and down-stream slots to use Each slot is 148 bits long, gives rate of 27.4 kbps

83 2G GSM The Global System for Mobile Communications (3) A portion of the GSM framing structure. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

84 Goals for UMTS (3G) Basic services desired High-quality voice transmission. Messaging (replacing , fax, SMS, chat). Multimedia (music, videos, films, television). Internet access (Web surfing, incl. audio, video). Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

85 3G UMTS Universal Mobile Telecommunications System (1) Architecture is an evolution of GSM; terminology differs Not compatible with 2G GSM Internet

86 3G UMTS Universal Mobile Telecommunications System (2) Air interface based on CDMA over 5 MHz channels Rates over users <14.4 Mbps (HSPDA) per 5 MHz CDMA permits soft handoff (connected to both cells) Soft handoff

87 4G ITU defined spec in 2008, before the technology existed ITU can t enforce what carriers do or call their services Pure IPv6 packet switching, no circuit switching No voice (except as VoIP) 1 Gbps for stationary user, 100 Mbps for moving user Uses carrier aggregation (multiple bands together) Uses OFDMA (Orthogonal Freq. Div. Mux Access)

88 OFDMA Channel 1 Channel 2 Channel 3 Channel Each channel is broadcast in parallel on different frequency bands

89 Cable Television Internet over cable Spectrum allocation Cable modems ADSL vs. cable

90 Community Antenna Television An early cable television system Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

91 Internet over Cable Internet over cable reuses the cable television plant Data is sent on the shared cable tree from the headend, not on a dedicated line per subscriber (DSL) ISP (Internet)

92 Internet over Telephone System The fixed telephone system. Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011

93 Spectrum Allocation Upstream and downstream data are allocated to frequency channels not used for TV channels:

94 Cable Modems Cable modems at customer premises implement the physical layer of the DOCSIS standard QPSK/QAM is used in timeslots on frequencies that are assigned for upstream/downstream data

95 Comparison of Cable and Telephone Item Cable Internet Telephone Internet Type of wiring Shared Dedicated Interference from neighbors Possible Impossible Wiring Coax CAT 3 twisted pair Age of system Newer Very old Max speed 400 Mbps 100 Mbps (copper) Fiber possible? No Yes Security Poor Good

96 End Chapter 2

The Physical Layer Outline

The Physical Layer Outline Theoretical Basis for Data Communications Digital Modulation and Multiplexing Guided Transmission Media (copper and fiber) Public Switched Telephone Network and DSLbased Broadband