Univ. of TehranIntroduction to Computer Network1 An Introduction to Computer Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Physical Layer Lecture 5: Physical Layer

Univ. of TehranIntroduction to Computer Network2 Concepts: Data signal Links Link functions Modulation Shannon’s Theorem Transmission media Outline

Univ. of TehranIntroduction to Computer Network3 Data Discrete data: an instance is binary. Computer works with discrete data. Discrete is encoded in 0s and 1s. Continuous data: change with time or space. It is converted to discrete data by sampling Data is delivered by signals in the links

Univ. of TehranIntroduction to Computer Network4 Link Functions Functions 1. Construct Frame with Error Detection Code 2. Encode bit sequence into analog signal 3. Transmit bit sequence on a physical medium (Modulation) 4. Receive analog signal 5. Convert Analog Signal to Bit Sequence 6. Recover errors through error correction and/or ARQ Adaptor Signal Adaptor: convert bits into physical signal and physical signal back into bits

Univ. of TehranIntroduction to Computer Network5 Link Components NRZI

Univ. of TehranIntroduction to Computer Network6 Link Properties Function Duplex/Half Duplex One stream, multiple streams Characteristics Bit Error Rate Data Rate (this sometimes mistakenly called bandwidth!) Degradation with distance

Univ. of TehranIntroduction to Computer Network7 Example: Optical Links

Univ. of TehranIntroduction to Computer Network8 Electromagnetic waves propagating in the light speed. Frequency Wavelength A (periodic) signal can be viewed as a sum of sine waves of different frequencies and strengths. Every signal has an equivalent representation in the frequency domain. » What frequencies are present and what is their strength (energy)Signals

Univ. of TehranIntroduction to Computer Network9 Signals (cont)

Univ. of TehranIntroduction to Computer Network10 Sender changes the nature of the signal in a way that the receiver can recognize. » Similar to radio: AM or FM Digital transmission: encodes the values 0 or 1 in the signal. » It is also possible to encode multi-valued symbols Amplitude modulation: change the strength of the signal, typically between on and off. » Sender and receiver agree on a “rate” » On means 1, Off means 0 Similar: frequency or phase modulation. Can also combine method modulation types.Modulation

Amplitude Modulation

Frequency Modulation

l Baseband modulation: send the “bare” signal. l Carrier modulation: use the signal to modulate a higher frequency signal (carrier). »Can be viewed as the product of the two signals »Corresponds to a shift in the frequency domain Baseband vs. Carrier Modulation

Amplitude Signal Carrier Frequency Amplitude Modulated Carrier Amplitude Carrier Modulation

Univ. of TehranIntroduction to Computer Network15 Noise: “random” energy is added to the signal. Attenuation: some of the energy in the signal leaks away. We need repeaters. Dispersion: attenuation and propagation speed are frequency dependent. » Changes the shape of the signal Limits in sending signals

Univ. of TehranIntroduction to Computer Network16 Noise A signal s(t) sent over a link is generally Distorted by the physical nature of the medium This distortion may be known and reversible at the receiver Affected by random physical effects Shot noise Fading Multipath Effects Also interference from other links Wireless Crosstalk Dealing with noise is what communications engineers do

Univ. of TehranIntroduction to Computer Network17 Link rate and Distance Links become slower with distance because of attenuation of the signal Amplifiers and repeaters can help

Univ. of TehranIntroduction to Computer Network18 Every transmission medium supports transmission in a certain frequency range. This is called channel capacity. » The channel bandwidth is determined by the transmission medium and the nature of the transmitter and receivers A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H. » E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second Assumes binary amplitude encoding Shannon extended this result by accounting for the effects of noise. More aggressive encoding can increase the channel bandwidth. » Example: modems Channel capacity

Nyquist’s Theorum How is the data rate constrained by bandwidth? Maximum data rate(bits/second) = 2 * bandwidth (hz) Nyquist’s Theorum considers only the limit imposed by the bandwidth not noise, encoding, or other factors.

Nyquist’s Theorum Why Double The Bandwidth? In addition to looking at a signal in the time domain, we can view it in the frequency domain. In other words, instead of asking the question, “What is the amplitude at time X?”, we can ask the question, “How much energy is present every X units of time?” Energ y Frequency

Nyquist’s Theorum Why Double The Bandwidth? As an analog signal is transmitted through some media, it is filtered by that media. Not only is noise introduced, but energy at certain frequencies is lost – and nearly completely so above and below some threshold frequencies. As a result, the signal has no harmonics above a certain frequency or below another.

A fundamental theoretical finding is that to reproduce an analog signal accurately at a certain frequency, we must sample it twice as frequently. Otherwise, we could lose information. If we sample less often, we might miss an event – we sample just before it happens. If we sample more often, we just sample the same thing twice – we can’t get more information than is there – and the data has already been limited to a certain bandwidth of information. Nyquist’s Theorum Why Double The Bandwidth?

We need to have two points within the same period to know exactly which sine function we have. More points provide no additional information.

Better Than Nyquist’s Limit If clocks are synchronized sender and receiver, we only need one point per period. This is because the synchronized starting point counts as one of the two points.

Noisy Channel Consider ratio of signal power to noise power. Consider noise to be super-imposed signal Decibel (dB) = 10 Log (S/N) S/N of 10 = 10 dB S/N of 100 = 20 dB S/N of 1000 = 30 dB

Shannon’s Theorem Maximum data rate (bits/second) = bandwidth (Hz) Log 2 (1 + S/N) As before, this only gives us the limit on the data rate imposed by the noise, itself. It does not consider the encoding or bandwidth limitations. The bandwidth parameter can be confusing. It is there because it governs the effect that the noise has. More bandwidth either dilutes the noise, or gives the data more places to hide, or both.

Increased bandwidth decreases the effects of noise. The signal has either more frequency space to call its own, or the noise gets diluted across the frequency space, or some combination of the two. noise signal Shannon’s Theorum

Univ. of TehranIntroduction to Computer Network28 Signaling bits on a link Multi-level Signaling 1 Levels Levels bits per symbol 3-bits per symbol Ultimately, what limits the number of bits I can send per symbol?

Univ. of TehranIntroduction to Computer Network29 Maximum Capacity/Data Rate Shannon Capacity: Bandwidth of linkSignal-to-Noise ratio For example:  Bandwidth of telephone link from telephone to a typical home is approx 3300Hz – 300Hz = 3kHz  Signal-to-noise ratio is approx 30dB = 10log 10 (S/N)  Therefore, C = 3000*log 2 (1001) ~= 30kb/s Optical fiber has a higher capacity because the bandwidth, B, of a fiber is much greater than for wire; and it is less susceptible to noise, N.

Univ. of TehranIntroduction to Computer Network30 Sampling Result (Nyquist) Suppose a signal s(t) has a bandwidth B. Sampling Result: Suppose we sample it (accurately) every T seconds. If T≤ 1/2B then it is possible to reconstruct the s(t) correctly Only one signal with bandwidth B has these sample points There are multiple signals with these sample points for signals with bandwidth greater than B Increasing the bandwidth results in a richer signal space No noise allowed in the sampling result

Univ. of TehranIntroduction to Computer Network31 Unshielded Twisted Pair (UTP) » Two copper wires twisted - avoid antenna effect » Grouped into cables: multiple pairs with common sheath » Cat 3 (voice grade) versus Cat 5 for data » 100 Mbps up to 100 m, 1 Mbps up to a few km » Cost: ~ 10cents/foot Coax cables. » One connector is placed inside the other connector » Holds the signal in place and keeps out noise » Gigabit up to a km Copper Wire

Univ. of TehranIntroduction to Computer Network32 Copper Wire

Univ. of TehranIntroduction to Computer Network33  Thin thread of glass or plastic  Lightweight.  Fibers act as wave-guides for light which is usually produced by lasers.  Visible light has frequency around 5*10 15 Hz, which ensures an extremely high bandwidth.  The raw materials are cheap.  Immune to electrical interference.  Difficult to join and tap.  Security advantages Fiber Optic

lower index of refraction core cladding (note: minimum bend radius of a few cm) Ray Propagation, Fiber

l Multimode fiber. »62.5 or 50 micron core carries multiple “modes” »used at 1.3 microns, usually LED source »subject to mode dispersion: different propagation modes travel at different speeds, depending on where source reflects bounces within cable – different paths are different lengths »Mode dispersion can be combated with a graded refraction index. Cable has variable refraction index to squeeze things back together. »typical limit: 1 Gbps at 100m l Single mode »Narrow cable so that it holds only “one beam” of light »8 micron core carries a single mode »used at 1.3 or 1.55 microns, usually laser diode source »typical limit: 1 Gbps at 10 km or more »still subject to chromatic dispersion Fiber Types

Univ. of TehranIntroduction to Computer Network36 Leased Lines Dedicated link from Telephone Companies DS1 24 digital voice of 64Kbps DS3 28 DS1 STS stands for Synch. Transfer signal LineBandwidth DS1 (T1)1.544 Mbps DS3 (T3) Mbps STS-1 (OC1) Mbps STS-3 (OC3) Mbps STS-12 (OC12) Mbps STS-48 (OC48)2.48 Gbps STS-12 (OC12) Mbps STS-192 (OC192)9.95 MGbps

Univ. of TehranIntroduction to Computer Network37 Last-Mile Links Connect from home to network service providers xDSl (Digital Subscriber line), runs on local loop on telephone line. ADSL, VDSL CATV- Cabel TV, BW of 6 MHZ, is asymmetric. LineBandwidth POTS (modem) Kbps ISDN Kbps XDSL16Kbps-55.2 Mbps CATV20-40 Mpbs