1 CSCD 433 Network Programming Fall 2011 Lecture 4 Physical Layer Transmission

Physical Layer Topics Motivation for studying this topic Definitions of terms Analog vs Digital Line encoding Characteristics of physical media 2

Motivation Why study the physical layer? Need to know basic data transmission concepts Didn't really cover them in CSCD330 Should understand physical layer to better understand how various media influence network performance and efficiency What transmission speed is possible with various media? Where and how are errors introduced? Need to know current implementations of physical layer and future trends

Physical Layer - Purpose To transmit bits, by encoding them onto signals To receive the signals, interpreting them as bits Signal 1. Mechanism used to carry information over time or distance 2. Sign or gesture giving information 3. Sequence of electrical or optical impulses or waves

Signals Examples Physical gesture, wave, hand signal Flashes of light (eg, Morse code)‏ Sound: vary tone, loudness or duration Flags Smoke Electical voltages

Transmission 1. Action of conveying electrical or optical signals from 1 point to 1 or more other points in space 2. Process of sending information from 1 point to another What do you need for a Transmission System ? Medium for signal transfer Transform signal to appropriate form Way to transmit the signal Way to remove, receive or detect the signal

Digital vs. Analog Signals Digital Signal 1. Limited to finite number of values 2. Has meaning only at discrete points in time Examples: Text, bits, integers

Digital vs. Analog Signals Analog Signal 1. Signal that is an analog of the quantity being represented 2. Continuous range of values 3. Also continuous in time, always valued Examples: Sound, vision, music

Analog vs. Digital

Analog Signals An analog signal is continuous has infinite number of values in a range Primary shortcomings of analog signals is difficulty to separate noise from original waveform An example is a sine wave which can be specified by three characteristics:  t  sin (2  f t + p)‏ A: amplitudef : frequencyp  phase

Sine Wave Examples ~emusic/acoustics/phase.htm

Modems, Codecs Modem (Modulator-Demodulator)‏ What does a modem do? Translates digital signal (bit) into an analog signal, for transmission as an analog signal Receives corresponding analog signal, and translates back into digital (bit)‏ Purpose: Use analog medium for digital data/signals Example: PC modem, phone lines, TV cable modems

Modems, Codecs, Baud Rates Codec (codec/decoder)‏ Converts analog data into digital form (bits), and the reverse. Main technique: PCM PCM (pulse code modulation)‏ Absolute values, based on sampling theory (nearly) total information

Pulse Code Modulation Analog signal amplitude is sampled (measured) at regular time intervals. Sampling rate, number of samples per second, Several times maximum frequency of the analog waveform in cycles per second or hertz Amplitude of analog signal at each sampling is rounded off to nearest of several specific, predetermined levels Process is called quantization

A Transmission System Transmitter Converts information into signal suitable for transmission Injects energy into communications medium or channel Telephone converts voice into electric current Modem converts bits into tones Receiver Receives energy from medium Converts received signal into form suitable for delivery to user Telephone converts current into voice Modem converts tones into bits Receiver Communication channel Transmitter

Analog Long-Distance Communications Each repeater attempts to restore analog signal to its original form Restoration is imperfect Distortion not completely eliminated Noise & interference only partially removed Signal quality decreases with increased repeaters Communications is distance-limited Still used in analog cable TV systems Analogy: Copy a song using a cassette recorder Source Destination Repeater Transmission segment Repeater...

Analog vs. Digital Transmission Analog transmission: all details must be reproduced accurately Sent Received Distortion Attenuation Digital transmission: only discrete levels need to be reproduced Distortion Attenuation Simple Receiver: Was original pulse positive or negative?

Digital Long-Distance Communications Regenerator recovers original data sequence and retransmits on next segment Can design so error probability is very small Each regeneration is like the first time! Analogy: Copy an MP3 file Communications possible over very long distances Digital systems vs. analog systems Less power, longer distances, lower system cost Source Destination Regenerator Transmission segment Regenerator...

Spectra & Bandwidth Spectrum of a Signal magnitude of amplitudes as a function of frequency x 1 (t) varies faster in time & has more high frequency content than x 2 (t) Bandwidth W s is defined as range of frequencies where a signal has non-negligible power, e.g. range of band that contains 99% of total signal power Spectrum of x 1 (t)‏ Spectrum of x 2 (t)‏

Sampling Theorem Nyquist–Shannon sampling theorem Theorem shows that an analog signal that has been sampled Can be perfectly reconstructed from an infinite sequence of samples if the sampling rate exceeds 2W samples/Sec, where W is the highest frequency of the original signal

Ws = 4KHz, so Nyquist sampling theorem  2W = 8000 samples/second Suppose 8 bits/sample, m PCM (“Pulse Code Modulation”) Telephone Speech: Bit rate= 8000 x 8 bits/sec= 64 kbps Example: Telephone Speech

Communications Channels A physical medium is an inherent part of a communications system Copper wires, radio medium, or optical fiber Communications system include electronic or optical devices that are part of the path followed by a signal Equalizers, amplifiers, signal conditioners By communication channel we refer to the combined end-to-end physical medium and attached devices Sometimes we use the term filter to refer to a channel especially in the context of a specific mathematical model for the channel

Digital Binary Signal For a given communications medium How do we increase transmission speed? How do we achieve reliable communications? Are there limits to speed and reliability? +A+A -A-A 0 T 2T2T 3T3T 4T4T5T5T 6T6T Bit rate = 1 bit / T seconds

Pulse Transmission Rate Objective: Maximize pulse rate through a channel, that is, make T as small as possible Channel t t Question: How frequently can these pulses be transmitted without interfering with each other? Answer: 2 x W c pulses/second where W c is the bandwidth of the channel T

Multilevel Signaling Nyquist pulses achieve the maximum signaling rate with zero Inter Symbol Interference (ISI)‏ 2W c pulses per second or 2W c pulses / W c Hz = 2 pulses / Hz With two signal levels, each pulse carries one bit of information Bit rate = 2W c bits/second With M = 2 m signal levels, each pulse carries m bits Bit rate = 2W c pulses/sec. * m bits/pulse = 2W c m bps Bit rate can be increased by increasing number of levels r(t) includes additive noise, that limits number of levels that can be used reliably.

signal noise signal + noise signal noise signal + noise High SNR Low SNR SNR = Average Signal Power Average Noise Power SNR (dB) = 10 log 10 SNR virtually error-free error-prone Channel Noise affects Reliability

If transmitted power is limited, then as M increases spacing between levels decreases Presence of noise at receiver causes more frequent errors to occur as M is increased Shannon Channel Capacity: The maximum reliable transmission rate over an ideal channel with bandwidth W Hz, with Gaussian distributed noise, and with SNR S/N is C = W log 2 ( 1 + S/N ) bits per second Reliable means error rate can be made arbitrarily small by proper coding Shannon Channel Capacity

What is Line Coding? Mapping of binary information sequence into the digital signal that enters the channel Ex. “1” maps to +A square pulse; “0” to –A pulse Line code selected to meet system requirements: Transmitted power: Power consumption = $ Bit timing: Transitions in signal help timing recovery Bandwidth efficiency: Excessive transitions wastes bw Low frequency content: Some channels block low frequencies Long periods of +A or of –A causes signal to “droop” Waveform should not have low-frequency content Error detection: Ability to detect errors helps Complexity/cost: Is code implementable in chip at high speed?

Desirable Properties Line Code Clock Signal Synchronization between transmitter and receiver is of critical importance in digital communications systems Ideally, spectrum of line code should contain a frequency component at the clock frequency to permit clock extraction This avoids having to transmit a separate clock signal between the transmitter and receiver

Desirable Properties Line Code Signal Interference and Noise Immunity Ideally, line code should be rugged in terms of exhibiting an immunity to interference and noise In more technical terms, line code should have a low probability of error for a given level of transmitted power Certain line codes are more rugged than others, e.g. polar codes have a better error performance compared to unipolar codes.

Line coding examples NRZ-inverted (differential encoding)‏ Unipolar NRZ Bipolar encoding Manchester encoding Differential Manchester encoding Polar NRZ

NRZ vs RZ In telecommunication, a non-return-to-zero (NRZ) line code is a binary code in which 1's are represented by one significant condition (usually a positive voltage)‏ 0's are represented by some other significant condition (usually a negative voltage), with no other neutral or rest condition Pulses have more energy than a RZ code Unlike RZ, NRZ does not have a rest state. RZ NRZ

Unipolar & Polar Non-Return-to-Zero (NRZ)‏ Unipolar NRZ “1” maps to +A pulse “0” maps to no pulse Long strings of A or 0 Poor timing Low-frequency content Simple Polar NRZ “1” maps to +A/2 pulse “0” maps to –A/2 pulse Long strings of +A/2 or –A/2 Poor timing Low-frequency content Simple Unipolar NRZ Polar NRZ

Bipolar Code Three signal levels: {-A, 0, +A} “1” maps to +A or –A in alternation “0” maps to no pulse Every +pulse matched by –pulse so little content at low frequencies String of 1s produces a square wave Long string of 0s receiver loses synchronization Bipolar Encoding

Manchester code & mBnB codes “1” maps into A/2 first T/2, -A/2 last T/2 “0” maps into -A/2 first T/2, A/2 last T/2 Every interval has transition in middle Timing recovery easy Uses double the minimum bandwidth Simple to implement Used in 10-Mbps Ethernet & other LAN standards mBnB line code Maps block of m bits into n bits Manchester code is 1B2B code 4B5B code used in FDDI LAN 8B10b code used in Gigabit Ethernet 64B66B code used in 10G Ethernet Manchester Encoding

Summary Looked at Physical layer Analog vs. Digital Line encoding Next, we will map this knowledge to Ethernet Choice of physical media in relation to performance and/or efficiency

37 New Assignment up Some problems from the Book