The Physical Layer

The Theoretical Basis for Data Communication Fourier analysis Niquist chriterium for bandwidth-limited channel Shannon maximum data rate of a noisy channel

Fourier Transform Periodic signals with period T=2π/w Non-periodic signals

Bandwidth-Limited Signals A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

Bandwidth-Limited Signals (d) – (e) Successive approximations to the original signal.

Bandwidth-Limited Signals Relation between data rate and harmonics.

Band-Limited Channel Fourier transform of a typical signal 1/Ts 2/Ts -1/Ts w

Power Spectrum Density Autocorrelation function of signal or noise Power spectrum density

Filtering Channel behaves as a filter When the noise is white (uncorrelated) Gaussian optimum filter has transfer function H(w)=X * (w).

Niquist Theorem If the signal bandwidth has width of W, then it can be reconstructed by taking 2W samples per second. Maximum data rate is where V is the number of different symbols

Niquist Chriterium T s sampling interval, ∆ sampling pulse width

Niquist Chriterium s(t) x(t) t t t

Niquist Chriterium S(f) X(f) f f 1/T s -2/T s -1/T s 2/T s W -W

Shannon Theorem If the channel bandwidth has width of W, and S/N is the signal-to-noise ratio, then the maximum data rate is

Modulation (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation

Modulation Signal is located around carrier frequency w 0, and its amplitude and phase depend on the data symbol in each time slot

Modulation Schemes (a) QPSK. (b) QAM-16. (c) QAM-64.

Guided Transmission Twisted Pair Coaxial Cable Fiber Optics

Twisted Pair (a) Category 3 UTP 16 MHz. (b) Category 5 UTP 100MHz.

Coaxial Cable A coaxial cable 1GHz.

Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection.

Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. Bands 25-30THz, and last two bands have attenuation less than 5%/km

Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers, diameter 8-10μm.

Transmission Devices Light emitting diode (LED) Semiconductor lasers Mach-Zehnder external modulator EDFA Photodiode

Optical Transmitters A comparison of semiconductor diodes and LEDs as light sources.

Wireless Transmission Relationship between wavelength and frequency: 100MHz waves are about 3m long, 1000MHz waves are 0.3m long. An object distracts those waves, whose length is smaller or equal to the object dimension.

The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

Radio Transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.

Issues in Wireless Transmission Radio signals are omnidirectional, and penetrate through objects. Throughput is low. HF radio and microwave signals are directed. Suffer from multipath fading, and are reflected against the buildings. Above 4GHz, signals are absorbed by the rain.

Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here. Fog and rain are disruptive too.

Communication Satellites Geostationary Satellites Several kWs. 40 transponders with 80MHz. TDMA. Medium-Earth Orbit Satellites 24 GPS satellites. Low-Earth Orbit Satellites Iridium project started by Motorola

Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

Communication Satellites The principal satellite bands.

Frequency Division Multiplexing (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.

Wavelength Division Multiplexing Wavelength division multiplexing.

Time Division Multiplexing The T1 carrier (1.544 Mbps).

Time Division Multiplexing Multiplexing T1 streams into higher carriers.

TDM US and Japan T Mbps 24 channels one sync. bit 23 data channels, 7 data bits + 1 signalling bit Multiplexing degrees 4,7,6 Others E Mbps 32 channels 30 data channels, 8 data bits, 1 bit signalling in every sixth frame Multiplexing degree 4, bit rates: 2.048Mbps, 8.848Mbps,

SONET and SDH Bellcore and CCITT: Synchronous Optical Networks (SONET), Synchronous Digital Hierarchy (SDH) Define frames for bit-rates 50Mbps and up

SONET Two back-to-back SONET STS-1 frames comprising 810 bytes

Time Division Multiplexing SONET and SDH multiplex rates.

CDMA – Code Division Multiple Access IS-95 X g 1 (t) r 1 (t) X g 2 (t) r 2 (t) X g K (t) r K (t) r(t)

Walsh-Hadamard Sequences

CDMA – Code Division Multiple Access (e.g. IS-95) (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal

The Local Loop: Modems, ADSL, and Wireless The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

Modems (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. (a) (b)

Higher Bit-rate Modems 2400 samples (bauds) per second V32 to 14.4Kbps, V34 to 33.6Kbps V90 35Kbps upstream, 56Kbps downstream V92 48kbps upstream, 56Kbps downstream

Digital Subscriber Lines Bandwidth versus distanced over category 3 UTP for DSL.

Digital Subscriber Lines Operation of ADSL using discrete multitone modulation. Up to 8Mbps downstream, and up to 1Mbps upstream. Modulation similar to V34, 15 bits per sample, 4000 bauds per sec

Digital Subscriber Lines A typical ADSL equipment configuration.

Internet over Cable Cable television

Internet over Cable The fixed telephone system.

Community Antenna Television (CATV) antenna HEADEND o HOME RF Spectrum: 55 MHz 350 MHz AM-VSB signals Long chains of RF amplifiers: limited bandwidth, poor reliability. Sheryl Woodward, AT&T Labs-Research

Linear Lightwave Revolution 55 (E 85)MHz 350 MHz 550 (E 606)MHz RF Spectrum: 80 AM-VSB channels HEADEND o HOME Fiber Node o Hybrid-Fiber-Coax Architecture: Improved reliability and performance, BUT to transmit 80 channels of AM-VSB, an optical link must operate near fundamental limits. Sheryl Woodward, AT&T Labs-Research

Compressed Digital Video a)MPEG-3 compresses a video channel to <5 Mbps. b)Quadrature Amplitude Modulation (QAM) can be used to transmit multiple television channels in a single 6 (E 8)MHz RF channel. Around 38Mbps can be transmitted through this channel. c)A much lower Carrier-to-Noise Ratio (CNR) is required to transmit these QAM signals than is required by AM-VSB. d)A set top box is required to receive these channels. 55 (E 85)MHz 350 MHz 550 (E 603)MHz 750 (E 862)MHz RF Spectrum: 80 AM-VSB channels30 QAM channels (~150 video channels) Sheryl Woodward, AT&T Labs-Research

Upstream Transmission 5-40 (E 65)MHz 350 MHz 550 (E 603)MHz 750 (E 862)MHz RF Spectrum: 80 AM-VSB channels30 QAM channels (~150 video channels) HEADEND o HOME Fiber Node o Can now offer interactive services Sheryl Woodward, AT&T Labs-Research

Upstream Transmission a)RF band is 5-42 (E 65)MHz, this band can carry multiple RF channels. Modulation schemes are QPSK or 16QAM b)5-15 MHz is plagued with ingress noise. c)All frequencies suffer from the funnel effect. d)Up to 10 Mbps transmission per RF channel is provided in the standard, but a peak rate of ~3 Mbps is more realistic. e)Bandwidth is shared. f)Services can be segregated by RF frequency. g)For data the standard is DOCSIS (Data Over Cable Service Interface Specification). h)Telephony can be carried over DOCSIS 1.1. i)A cable modem or set top box resides in the home, a CMTS, which coordinates traffic, resides in the headend. Sheryl Woodward, AT&T Labs-Research

The Mobile Telephone System Improved mobile telephone system (IMTS): 23 channels, MHz; Advanced mobile telephone system (AMTS) is cellular system: 832x30kHz channels, MHz Digital: D-AMPS, 30KHz channels, MHz; Global System for Mobile Communications (GSM) 124x200kHz channels MHz; Code Division Multiple Access (CDMA)

GSM Global System for Mobile Communications GSM uses kHz frequency channels, each of which uses an eight-slot TDM system

GSM A portion of the GSM framing structure.

Control Channels Broadcast control channel: Base station broadcasts control signal Dedicated control channel: Registration, location and connection status of users Common control channel Paging subchannel: announces calls Random access channel: connection requests Access grant channel: connection grants

3G W-CDMA or universal mobile telecommunication system (UMTS) compatible with GSM, uses 5MHz. CDMA2000 extension of IS-95 uses 5MHz Enhanced data rates for GSM evolution (EDGE) uses more bits per baud General radio packet service (GPRS) is overlay packet network over D-AMPS or GSM WLAN at 2.5 or 5.7GHz 10-50Mbps; WMAN at GHz up to 200Mbps; (As oppose to around 50Mbps total for D-AMPS and GSM, and 8Kbps and 13Kbps per user.