Unformatted text preview:

PHY LayerENEE 426 | Communication Networks | Spring 2008 Lecture 4PHY• Specified by a standard• Major areas of specification– How to modulate bits onto a physical medium• Optical, Electrical, RF– Error detection / correction• Checksum to identify bit errors• Codes to correct bit errorsENEE 426 | Communication Networks | Spring 2008 Lecture 4Electrical PHYs• Two major types of cablesTwisted Pair CoaxialENEE 426 | Communication Networks | Spring 2008 Lecture 4Cable Performance• Different qualities of cables have different performance– Loss = f(length, frequency)• Signal loss per footfrequencyLoss per footLMR-400 specsENEE 426 | Communication Networks | Spring 2008 Lecture 4Twisted Pair• Pairs of copper conductors wound together• Each pair transmits independent voltages• Measured voltage is difference between two• Minimizes crosstalk and interference– Copper cables act as antennas– Cable can induce a charge on neighboring cable (crosstalk)– External devices can cause interference– Induced charges equal between two cables; cancel outENEE 426 | Communication Networks | Spring 2008 Lecture 4Classes of Twisted PairClassBandwidthUsesCat 120 kHzTelephone, doorbell wiringCat 26 MHzToken ring 802.5Cat 316 MHz10 Mbps EthernetCat 420 MHzHigh-speed Token ringCat 5100 MHz100 Mbps EthernetCat 5e100 MHz1 Gbps EthernetCat 6250 MHzLonger Ethernet runsCat 6a500 MHz10 Gbps Ethernet (future)ENEE 426 | Communication Networks | Spring 2008 Lecture 4Coaxial Cable• Higher bandwidths thantwisted pair cabling• More shielding• Less leakage• Standards– RG-6: CATV– RG-8: 10base5– RG-58: 10base2, common RF cable– RG-59: CCTV– RG-213: common RF cableENEE 426 | Communication Networks | Spring 2008 Lecture 4Electrical Encodings• Nonreturn to Zero (NRZ)– Bit 0 = low voltage, Bit 1 = high voltage– Problem• string of 0s or 1s results in sustained voltage (or no voltage)• receiver can lose synchronization• NRZ Inverted (NRZI)– Differential encoding– Bit 0 = same voltage, Bit 1 = switch voltage– Solves consecutive 1s but not 0s• MLT-3– Three voltage levels –V, 0, +V, – Bit 0 = same voltage, Bit 1 = cycle voltage– Cycle: -V, 0, +V, 0, -V, …– Uses less bandwidth• Longer distances on cheaper cable• Manchester Coding– XOR clock pulse with bit stream– No more runs of 1 or 0ENEE 426 | Communication Networks | Spring 2008 Lecture 4Electrical Encodings• Manchester doubles the bandwidth– Bit 0 = 01; Bit 1 = 10– Half bit per baud, very inefficient• 4B/5B Encoding– Every 4-bit pattern has 5-bit set of voltages– Design to never have more than • 3 consecutive 0s• 8 consecutive 1s– Unused patterns have special meaning• Line idle• Line dead• Control symbols4B5B000011110000101001001010100001110101010001010010101011011001110011101111100010010100110011101010110101110111110011010110111011111011100111111101ENEE 426 | Communication Networks | Spring 2008 Lecture 4Ethernet (IEEE 802.3)• Packet-based protocol• Early versions used coax, now either fiber or twisted pair• 10 Mbps mode used multiple access– Multiple users share same electrical medium– Half duplex: can only transmit or receive but not both– Manchester coded signal• 100M/1G versions are switched– Direct connection to switch; no contention with other users– Full duplex: transmit and receive packets simultaneously– 100 Mbps: 4B5B MLT-3 signal– 1 Gbps: PAM-5 signalENEE 426 | Communication Networks | Spring 2008 Lecture 4Error Detection• Receive a pattern of bits– How do we know if the pattern is correct?– Single bit error can completely alter processing of a message• Early solution: parity bit– (8, 7) code– 7 bits of data, add 8thbit to make sum even or odd (even vs odd parity)– Only detects odd number of bit errors• Two-dimensional parity– Perform parity check across N bytes– Byte N+1 is parity check across first N– Detect 1-, 2-, 3-, most 4-bit errors– Additional overheadTransverseLongitudinalENEE 426 | Communication Networks | Spring 2008 Lecture 4Error Detection• Checksum– Add up all the transmitted words (16 bit blocks) using ones-complement arithmetic– Transmit the sum at the end of the message– Receiver verifies the sum is correct– Very low overhead– Not very robust, but very easy to implement• Cyclic Redundancy Check (CRC)– Mathematically much more complex– Treat packet as polynomial of high degree– Divide polynomial by standardized divisor– Attach remainder to the messageENEE 426 | Communication Networks | Spring 2008 Lecture 4Error Correction• Error detection: know there’s an error, just not which bits• Error correction: fix error• 2D parity: can correct single bit error• Early scheme: repetition code– (N, 1) code: send every bit N times (N odd)– Vote on received bits: detect N-1 bit errors, correct (N-1)/2– Very inefficient, but simple to implement• Hamming codes– Revolutionized field, applied mathematics to the problem (like CRC)– (7, 4) code common: maps 4 bits into 7– Each parity bit covers different bits– Detect 2, correct 1• Many more modern codes– Reed-Solomon Codes– Convolutional Codes– Turbo CodesEncoded data bitsp1p2d1p3d2d3d4Paritybitcoveragep1XXXXp2XXXXp3XXXXENEE 426 | Communication Networks | Spring 2008 Lecture 4Optical Fiber• Primarily used in core networks• Two types of transmitters– Light Emitting Diodes (LEDs)• Inexpensive• Incoherent light source -> short distances– Lasers• Expensive• Coherent light source -> long distancesENEE 426 | Communication Networks | Spring 2008 Lecture 4Optical Fiber• Glass or plastic core, covered by cladding• Cladding has low refractive index– Total internal reflection of light• Two types of fiber:– Multimode• Larger diameter, suitable for LEDs• Easier to connect, less precision required• Short distances– Single Mode• Carries single “ray” of light, no dispersion• Small diameter, suitable for laser• Longer distancesENEE 426 | Communication Networks | Spring 2008 Lecture 4Optical Fiber• Receivers– Photodiode converts light into electricity– Amplifier magnifies signal• Wavelength Division Multiplexing– Parallel communications channels, different wavelengths of light– Spectrometer (like a prism) separates wavelengths– Photodiodes convert each wavelength


View Full Document

UMD ENEE 426 - PHY Layer

Download PHY Layer
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view PHY Layer and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view PHY Layer 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?