Introduction to Computer Networks Yan Yunyi, Ph.D Associate Prof. Xidian University yyyan@xidian.edu.cn

Chapter 2 The Physical Layer

Functions of Physical Layer Physical layer defines the mechanical, electrical, and timing interfaces to the network. Three examples of communication systems used in practice for wide area computer networks: the (fixed) telephone system, the mobile phone system, and the cable television system. Three kinds of transmission media: guided (copper wire and fiber optics), wireless (terrestrial radio), and satellite

Ch2.1 The Theoretical and Physic Basis for Data Communication Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate of a Channel Typical Lined Mediums

Fourier Analysis A function g(t) with period T can be constructed as the sum of a (possibly infinite) number of sines and cosines: Fourier Series Where: f = 1/T is the fundamental frequency, an and bn are the sine and cosine amplitudes of the nth harmonics, and c is a constant. And

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

Bandwidth-Limited Signals (2) (d) – (e) Successive approximations to the original signal.

Bandwidth-Limited Signals (3) Cutoff frequency: the frequency at which half the power gets through. Half power point P.S. (postscript) In practice, the cutoff is not really sharp, so often the quoted bandwidth is from 0 to the cutoff frequency. Bandwidth: the range of frequencies transmitted without being strongly attenuated. The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium.

Bandwidth-Limited Signals (4) Relation between data rate and harmonics. (to send a BYTE (8 bits) periodically in 3K Hz bandwidth) limiting the bandwidth limits the data rate, even for perfect channels

Noiseless Channel (Nyquist theorem ) The Maximum Data Rate Noiseless Channel (Nyquist theorem ) Where: H: the bandwidth (in Hz). V: discrete levels of which signal consists . For example: a noiseless 3-kHz channel cannot transmit binary (i.e., two-level) signals at a rate exceeding 6000 bps . Extention: why it is 2H? If an arbitrary signal has been run through a low-pass filter of bandwidth H, the filtered signal can be completely reconstructed by making only 2H (exact) samples per second. Sampling the line faster than 2H times per second is pointless because the higher frequency components that such sampling could recover have already been filtered out.

What is the value of V? (a) QPSK. (b) QAM-16. (c) QAM-64. QPSK: Quadrature Phase Shift Keying (a) QPSK. (b) QAM-16. (c) QAM-64. QAM: Quadrature Amplitude Modulation.

The Maximum Data Rate (3) Noisy Channel (Shannon theorem): Often quoted as 10log10(S/N) in dB units. p.s. power to amplitude with factor 2. Where: H: the bandwidth (in Hz). S/N: signal-to-noise ratio (SNR). For example: A channel of 3000-Hz bandwidth with a signal to thermal noise ratio of 30 dB (typical parameters of the analog part of the telephone system) can never transmit much more than 30,000 (exact,29902) bps , no matter how many or how few signal levels are used and no matter how often or how infrequently samples are taken.

Typical Lined Mediums Twisted Pair Coaxial Cable Fiber Optics

Twisted Pair (a) Category 3 UTP (~10MHz). (b) Category 5 UTP (~100MHz). UTP (Unshielded Twisted Pair)

Coaxial Cable A coaxial cable(~1GHz).

Fiber Optics An optical transmission system has three key components: the light source, the transmission medium, and the detector. Conventionally, a pulse of light indicates a 1 bit and the absence of light indicates a 0 bit. The transmission medium is an ultra-thin fiber of glass. The detector generates an electrical pulse when light falls on it.

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.

Fiber Optics By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it by light pulses, and then reconverts the output to an electrical signal at the receiving end.

Transmission of Light through Fiber Attenuation of light through fiber in the infrared region. 25,000 ~ 30,000 GHz

Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

A comparison of semiconductor diodes and LEDs as light sources. Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources.

Homewoks Qestion1: what data rate can we get in the following cases? Suppose the systems have 2400 bauds (symbols or samples) per second and channel is free of noise. Qestion2: Why date rate is 56Kbps for most laptop when it uses the traditional telephone line?

Wireless Transmission The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission

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.

Politics of the Electromagnetic Spectrum The ISM bands in the United States. For example, garage door openers, cordless phones, radio-controlled toys, wireless mice, and numerous other wireless household devices use the ISM bands without licenses or registration.

Lightwave Transmission Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

Communication Satellites Geostationary-Earth Orbit Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber

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 (2) The principal satellite bands.

Communication Satellites (3) Very Small Aperture Terminals VSATs using a hub. These tiny terminals have 1-meter or smaller antennas (versus 10 m for a standard GEO antenna) with 1 watt power, sometimes they need a hub (ground station) to relay the traffic.

Low-Earth Orbit Satellites Iridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth. The 24 GPS (Global Positioning System) satellites orbiting at about 18,000 km are examples of MEO satellites (not LEO).

Globalstar (a) Relaying in space (Iridium plan). (b) Relaying on the ground.

Public Switched Telephone Network Structure of the Telephone System The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching

Structure of the Telephone System (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

Structure of the Telephone System (2) A typical circuit route for a medium-distance call.

Major Components of the Telephone System Local loops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another

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.

Modem: A device that accepts a serial stream of bits as input and produces a carrier modulated by some methods (or vice versa) is called a modem (for modulator-demodulator). The modem is inserted between the (digital) computer and the (analog) telephone system. Modem can translate digital data into analog signal, and vice versa.

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

Modems (2) (a) QPSK. (b) QAM-16. (c) QAM-64. QPSK: Quadrature Phase Shift Keying (a) QPSK. (b) QAM-16. (c) QAM-64. QAM: Quadrature Amplitude Modulation.

2400 bauds (symbols or samples) per second. Modems (3) 2400 bauds (symbols or samples) per second. (a) (b) (a) V.32 for 9600 bps (QAM-32, 4 data bits and 1 parity bit per symbol ). (b) V32 bis for 14,400 bps (QAM-128, 4 data bits and 1 parity bit per symbol ).

Why 56 kbps modem? The telephone channel is about 4000 Hz wide (including the guard bands). Due to the Nyquist theorem, the maximum number of independent samples per second is thus 8000. The number of bits per sample in the U.S. is 8, one of which is used for control purposes, allowing 56,000 bit/sec of user data. In Europe, all 8 bits are available to users, so 64,000-bit/sec modems could have been used, but to get international agreement on a standard, 56,000 was chosen. This modem standard is now in wide use, called V.90. It provides for a 33.6-kbps upstream channel (user to ISP), but a 56 kbps downstream channel (ISP to user) because there is usually more data transport from the ISP to the user than the other way (e.g., requesting a Web page takes only a few bytes, but the actual page could be megabytes).

All modern modems allow traffic in both directions at the same time (by using different frequencies for different directions). A connection that allows traffic in both directions simultaneously is called full duplex. A two-lane road is full duplex. A connection that allows traffic either way, but only one way at a time is called half duplex. A single railroad track is half duplex. A connection that allows traffic only one way is called simplex. A one-way street is simplex. Another example of a simplex connection is an optical fiber with a laser on one end and a light detector on the other end.

Digital Subscriber Lines The lower the chosen speed, the larger the radius and the more customers covered. But the lower the speed, the less attractive the service. Bandwidth versus distanced over category 3 UTP for DSL.

Digital Subscriber Lines (2) Divide the available 1.1 MHz spectrum on the local loop into 256 independent channels of 4312.5 Hz each. Channel 0 is used for voice or POTS POTS (Plain Old Telephone Service). Channels 1–5 are not used, to keep the voice signal and data signals from interfering with each other. The remaining 250 channels are used for upstream and downstream. Operation of ADSL using discrete multitone modulation.

Why Asymmetry DSL or ADSL? In principle, the 250 channels can be used for downstream or upstream. But It is up to the provider (the ISP) to determine how many channels are used for upstream and how many for downstream. A 50%-to-50% mix of upstream and downstream is technically possible, but most providers allocate something like 80%–90% of the bandwidth to the downstream channel since most users download more data than they upload. So the upstream is not equal to the upstream, and the bandwidth allocation is not symmetry, i.e. Asymmetry. A common split is 32 channels for upstream and the rest downstream. The ADSL standard (ANSI T1.413 and ITU G.992.1) allows speeds of as much as 8 Mbps downstream and 1 Mbps upstream. However, few providers offer this speed. Typically, providers offer 512 kbps downstream and 64 kbps upstream (standard service) and 1 Mbps downstream and 256 kbps upstream (premium service). ISP always lift the speed progressively, and customers will pay more at the same time.

Digital Subscriber Lines (3) NID (Network Interface Device) DSLAM (Digital Subscriber Line Access Multiplexer) A typical ADSL equipment configuration.

Frequency Division Multiplexing ADSL and radio broadcast are typical FDM systems. FDM: Analog circuitry required; Not suitable for computer to do this. (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel.

Wavelength Division Multiplexing A Variation of FDM Wavelength division multiplexing. Dense WDM: large number of channels (200 or more), wavelengths closer (0.1nm)

Time Division Multiplexing T1 Just used in USA & Japan, others E1 8K Hz Sample for 4K Hz voice channel. TDM can be processed by digital circuits and suitable for Computer to do. E1:(30+2)x8x8k=2.048Mbps [(7+1)bits x (23+1)Channels+1bit]/125us=1.544 Mbps The T1 carrier (1.544 Mbps).

Time Division Multiplexing (2) DPCM: Differential Pulse Code Modulation --not the value but the difference A Variation of DPCM Distortion, or information lost Predictive Encode is an improvement. Delta modulation. Digitized Signal can be compressed by using statistical techniques.

Time Division Multiplexing (3) Multiplexing T1 streams into higher carriers.

Switching Circuit Switching Message Switching Packet Switching

Circuit Switching & Packet Switching (a) Circuit switching. (b) Packet switching.

Message Switching (a) Circuit switching (b) Message switching (c) Packet switching

and a single block can tie up a router-router line for minutes. Packet Switching Message Switching: No limit on block size (needs disk to restore large blocks) and a single block can tie up a router-router line for minutes. A comparison of circuit switched and packet-switched networks.

The Mobile Telephone System First-Generation Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data

Advanced Mobile Phone System(1G) (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used.

GSM(2G) Global System for Mobile Communications GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

A portion of the GSM framing structure. Each user’s data are transmitted once every 4.615 ms, and the voice data during the 4.615ms are compressed to 114 bits. A portion of the GSM framing structure.

CDMA (2G) Code Division Multiple Access How To Do? (1) Bipolar notation by binary 0 being -1 and binary 1 being +1. For example, a 1 bit for station A (0 0 0 1 1 0 1 1) now becomes (-1 -1 -1 +1 +1 -1 +1 +1). (2) All chip sequences are pairwise orthogonal, i.e. the normalized inner product of any two distinct chip sequences, S and T, is 0. In fact, we have For complement or negation,

CDMA (2G) Code Division Multiple Access orthogonal To recover the bit stream of an individual station, the receiver must know that station's chip sequence in advance. If the receiver is trying to listen to the station C, it just computes the normalized inner product, S•C. (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal

Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace e-mail, fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.) WCDMA-Ericsson China Unicom CDMA2000-Qualcomm China Telecom TD-SCDMA- DaTang China Mobile GPRS (General Packet Radio Service)-2.5G

Cable Television Community Antenna Television Internet over Cable Spectrum Allocation Cable Modems

Community Antenna Television An early cable television system.

Internet over Cable Cable television

Internet over Cable (2) The fixed telephone system.

Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access

Cable Modems Typical details of the upstream and downstream channels in North America.

Summary Nyquist and Shannon principles Transmission medias. Guided: twisted pair, coaxial cable, and fiber optics Unguided: radio, microwave, infrared, laser C) Telephone System. Local Loop: Analog signals, twisted pair, modem, ADSL Trunks: Multiplexing-FDM TDM WDM Switch: Circuit Switching , Packet Switching. D) Mobile Telephone System GSM, CDMA