# Chapter 2: The Physical Layer

## Presentation on theme: "Chapter 2: The Physical Layer"— Presentation transcript:

Chapter 2: The Physical Layer

Recap: Physical Layer Physical layer is the lowest layer in the hierarchy of the hybrid reference model The purpose of physical layer is to transport a raw bit stream from one machine to another. CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

The Theoretical Basis for Data Comm.
Information can be transmitted on wires by varying some physical properties such as voltage or current. We represent the value of voltage or current as a function of time (f(t)) to model the behavior of signal. Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate of a Channel CEN 4500, S. Masoud Sadjadi

Fourier Analysis Any reasonably behaved periodic function can be constructed as the sum of a (possibly infinite) number of sines and cosines. You can see how it works by trying it at: g(t)=1/2 c + n=1-  an sin(2nft) + n=1-  bn cos(2nft) A data signal that has finite duration (which all of them do) can be handled by just imagining that it repeats the entire pattern over and over forever. Let’s see how we can use Fourier analysis to transmit the ASCII character “b” encoded in an 8-bit byte. (“b” is 98 or ) CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals
(a) A binary signal and its root-mean-square Fourier amplitudes. (b) The signal resulted from a channel that allows only the first harmonic to pass. (c) The resulted signal from a channel passing the first two harmonics. CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals (2)
(d) The resulted signal from a channel passing the first four harmonics. (e) The resulted signal from a channel passing the first eight harmonics. CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals (3)
Attenuation: No transmission facility can transmit signals without loosing some power in the process. Distortion: Unfortunately, all transmission facilities diminish different frequencies by different amount, thus introducing distortion. Bandwidth: The range of frequencies transmitted without being strongly attenuated. The cutoff in practice is often from 0 to the frequency at which half the power gets through. CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals (4)
is a physical property of a transmission medium depends on construction, thickness, and length of the medium. Also, a filter might be introduced to limit the amount of bandwidth available to a customer A telephone wire may have a bandwidth of 1MHz for short distances, but telephone companies add a filter restricting each customer to about 3100 Hz CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals (5)
Data rate and harmonics (terms) The time required to transmit “b”, 8 bits, on a b bits/sec line is 8/b sec. So, the frequency of the first harmonic (term in Fourier transform) is b/8 Hz. Ordinary telephone line Often called a voice-grade line Has an artificially-introduced cutoff frequency just about 3000 Hz The number of the highest harmonic for 8-bit data that can pass is 3000/(b/8) or 24000/b. CEN 4500, S. Masoud Sadjadi

Bandwidth-Limited Signals (6)
Making accurate reception of original bit stream is tricky, when going over 4800 bps Limiting the bandwidth limits the data rate. We need sophisticated coding schemes. Relation between data rate and harmonics. CEN 4500, S. Masoud Sadjadi

Maximum Data Rate of a Channel
Even perfect channel has a finite transmission capacity. An arbitrary signal that has been run through a low-pass filter of bandwidth H can be completely reconstructed by making only 2H (exact) samples per second. Sampling faster than 2H times per second is pointless, because the higher frequency components that such samples can recover have already been filtered out. CEN 4500, S. Masoud Sadjadi

Maximum Data Rate of a Channel (2)
Nyquist’s Theorem States (Noiseless Channel): maximum data rate = 2H log2V bits/sec For example, a noiseless 3-kHz channel cannot transmit binary (i.e., two level) signals at a rate exceeding 6kbps. Shannon’s Results (Channel with thermal noise): maximum data rate = H log2(1 + S/N) bits/sec For example, a channel of 3-kHz bandwidth with a signal to thermal noise ratio of 30 dB (typical in analog part of the telephone system) can never transmit much more than 30kpbs, no matter how many or how few signal levels are used and how often the samples are taken. The minimum of the above two should be considered. CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

Guided Transmission Data
Magnetic Media Magnetic tape or Removable media (DVD) Excellent bandwidth, but poor delay! Twisted Pair One of the oldest and still most common Can be used for both analog and digital signal Coaxial Cable Better shielding, so it can span longer distances at higher speed. Fiber Optics Computer industry, a gain of 20 per decade! Communication industry, a gain of 125 per decade! CEN 4500, S. Masoud Sadjadi

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

Coaxial Cable Better shielding than twisted pair.
Thick (75-ohm) and thin (50-ohm) Modern cables have up to 1GHz bandwidth Largely has been replaced by fiber optics on long-haul routes A coaxial cable. CEN 4500, S. Masoud Sadjadi

Fiber Optics Each ray is said to have a different mode
Current single-mode fibers can transmit data at 50 Gbps for 100 km without amplification. (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. CEN 4500, S. Masoud Sadjadi

Transmission of Light through Fiber
Attenuation of light through fiber in the infrared region. f = c /2 CEN 4500, S. Masoud Sadjadi

Fiber Cables (a) Side view of a single fiber.
(b) End view of a sheath with three fibers. CEN 4500, S. Masoud Sadjadi

Fiber Cables (2) A comparison of semiconductor diodes and LEDs as light sources. CEN 4500, S. Masoud Sadjadi

A fiber optic ring with active repeaters.
Fiber Optic Networks A fiber optic ring with active repeaters. CEN 4500, S. Masoud Sadjadi

Fiber Optic Networks (2)
A passive star connection in a fiber optics network. CEN 4500, S. Masoud Sadjadi

Wireless Transmission
The Electromagnetic Spectrum Radio Transmission AM and FM Microwave Transmission Microwave Oven Infrared and Millimeter Waves Remote control Lightwave Transmission Laser beam CEN 4500, S. Masoud Sadjadi

The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for communication. f = c  300,000 km/sec CEN 4500, S. Masoud Sadjadi

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. CEN 4500, S. Masoud Sadjadi

Politics of the Electromagnetic Spectrum
Microwave: Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. The ISM bands in the United States. Industrial, Scientific, Medical for unlicensed usage CEN 4500, S. Masoud Sadjadi

Lightwave Transmission
Laser beams cannot penetrate rain or thick fog, but they normally work well on sunny days. Convection currents can interfere with laser communication systems. A bidirectional system with two lasers CEN 4500, S. Masoud Sadjadi

Communication Satellites
Geostationary Satellites GEO Medium-Earth Orbit Satellites MEO Low-Earth Orbit Satellites LEO Satellites versus Fiber CEN 4500, S. Masoud Sadjadi

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. CEN 4500, S. Masoud Sadjadi

Communication Satellites (2)
Orbit slots are not the only bone of contention. Frequencies are too! The downlink transmission interferes with existing microwave users The principal satellite bands. CEN 4500, S. Masoud Sadjadi

Communication Satellites (3)
Very Small Aperture Terminals (VSATs) using a hub. CEN 4500, S. Masoud Sadjadi

Low-Earth Orbit Satellites Iridium
Iridium is element 77, and originally there were supposed to be 77 satellites, but it was reduced to 66 (which is Dysprosium). (a) The Iridium satellites from six necklaces around the earth. (b) Each satellite have a maximum of 48 cells, so 1628 moving cells cover the earth. CEN 4500, S. Masoud Sadjadi

Globalstar Based on 48 LEO satellites, but different switching scheme than that of Iridium (a) Relaying in space. (b) Relaying on the ground. CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

Public Switched Telephone System
Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching CEN 4500, S. Masoud Sadjadi

Structure of the Telephone System
(a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. CEN 4500, S. Masoud Sadjadi

Structure of the Telephone System
A typical circuit route for a medium-distance call. CEN 4500, S. Masoud Sadjadi

Major Components of the Tel. Sys.
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 CEN 4500, S. Masoud Sadjadi

The Politics of Telephones
The relationship of LATAs, LECs, and IXCs. All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it. CEN 4500, S. Masoud Sadjadi

The Local Loop Computer to computer call using both analog and digital transmissions. Modem: A device for transmitting usually digital data over telephone wires by modulating the data into an audio signal to send it and demodulating an audio signal into data to receive it. Codec: device that converts analog signals to digital form for transmission and converts signals traveling in the opposite direction from digital to analog form. Derived from coder-decoder. CEN 4500, S. Masoud Sadjadi

Modems (a) A binary signal (c) Frequency modulation
(b) Amplitude modulation (c) Frequency modulation (d) Phase modulation CEN 4500, S. Masoud Sadjadi

Modems (2) (a) QPSK: Quadrature Phase Shift Keying.

Modems (3) (a) (b) (a) V.32 for 9600 bps. (b) V.32 bis for 14,400 bps.
Note: baud is the number of samples per second and might be different from number of bits per second (bps). CEN 4500, S. Masoud Sadjadi

Digital Subscriber Lines
Bandwidth versus distance over category 3 UTP for DSL. CEN 4500, S. Masoud Sadjadi

Digital Subscriber Lines (2)

Digital Subscriber Lines (3)
Splitter: An analog filter that separates the Hz band used by POTS from the data. Network Interface Device (NID): marks the end of the telephone companies property. DSLAM: DSL Access Multiplexer. A typical ADSL equipment configuration. CEN 4500, S. Masoud Sadjadi

Wireless Local Loops Architecture of an LMDS system.

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network Trunks and Multiplexing The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

Frequency Division Multiplexing
The frequency spectrum is divided into frequency bands, with each user having exclusive possession of some bands. (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel. CEN 4500, S. Masoud Sadjadi

Wavelength Division Multiplexing
A variation of FDM used for fiber optic channels. Basically FDM at high frequency. CEN 4500, S. Masoud Sadjadi

Time Division Multiplexing
Transmits multiple signals simultaneously over a single transmission path by time slicing lower-speed signals into one high-speed transmission channel. FDM (analog circuitry); TDM (entirely by digital electronics) The T1 carrier (1.544 Mbps). CEN 4500, S. Masoud Sadjadi

Time Division Multiplexing (2)
Differential pulse code modulation Jumps of 16 or more are unlikely on a scale of 128 Delta modulation CEN 4500, S. Masoud Sadjadi

Time Division Multiplexing (3)
PCM: Pulse Code Modulation. 8000 samples per second, 8bits per sample (64kbps) for digitizing 4-kHz channels. T1 consists of 24 voice channels (1.536mps). Multiplexing T1 streams into higher carriers. CEN 4500, S. Masoud Sadjadi

Time Division Multiplexing (4)
SONET: Synchronous Optical NETwork It sends 810-byte frames, without gaps, even when there is no data. 8000 frames/sec, which exactly matches the PCM sampling rate. SPE: Synchronous Payload Envelope. Two back-to-back SONET frames. CEN 4500, S. Masoud Sadjadi

Time Division Multiplexing (5)

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network Trunks and Multiplexing Switching The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

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

Message Switching Problems:
There is no limit at all on block size. Buffers are limited No good for interactive applications (a) Circuit switching (b) Message switching (c) Packet switching CEN 4500, S. Masoud Sadjadi

Packet Switching A comparison of circuit switched and packet-switched networks. CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

The Mobile Telephone System
First-Generation Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Digital Voice and Data CEN 4500, S. Masoud Sadjadi

AMPS: Bell Labs, 1982, geographical regions are divided into cells, hence the name cell phones. The key idea is to reuse the same transmission frequencies in the nearby (but not adjacent) cells. (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. CEN 4500, S. Masoud Sadjadi

Channel Categories The 832 full-duplex channels
832 simplex send and receive ( & MHz) Each simplex channel is 30 kHz wide They are divided into four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data Since the same frequencies cannot be reused in adjacent cells, the actual number of voice channels available per cell is about 45. CEN 4500, S. Masoud Sadjadi

D-AMPS Digital Advanced Mobile Phone System
Digital and analog channels can be mixed. Higher frequencies and shorter antennas. (a) A D-AMPS channel with three users (compress to 8kbps). (b) A D-AMPS channel with six users (compress to 4kbps). CEN 4500, S. Masoud Sadjadi

GSM Global System for Mobile Communications
GSM uses 124 frequency channels, each of which uses an eight-slot TDM system CEN 4500, S. Masoud Sadjadi

GSM vs D-AMPS Similarities Differences Both are cellular systems
Both use FDM Each cell phone transmitting on one frequency and receiving on a higher frequency (80 MHz higher for D-AMPS and 55 MHz for GSM). Single frequency pair is split by TDM Differences GSM channels are much wider 200 kHz vs 30 kHz Hold relative few additional users (8 vs 3) CEN 4500, S. Masoud Sadjadi

GSM (2) A portion of the GSM framing structure.

CDMA: Code Division Multiple Access
Does not use FDM and TDM Each bit time is subdivided into m short intervals called chips Typically there are 64 to 128 chips per bit Each station is assigned a unique m-bit code called a chip sequence 1 is the chip sequence and 0 is the one’s complement of this sequence For each bit, we need to sent m bit, so the bandwidth should be m times more Example If we have 1 MHz bandwidth, with FDM, 100 stations can each use 10 kHz, effectively 10 kbps (1 bit per Hz) With CDMA, all stations will use the 1 MHz bandwidth, effectively 1 Mega chip per second With fewer than 100 chips per bit, the effective bandwidth per station is higher for CDMA than FDM And the channel allocation problem is also solved. CEN 4500, S. Masoud Sadjadi

CDMA: Example Four stations and 8 chips/bit for simplicity
S is the m-chip vector for station S All chip sequences are pair-wise orthogonal S ● T = 0, S ● S = 1, (a) Binary chip sequences for four stations (b) Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C’s signal CEN 4500, S. Masoud Sadjadi

Third-Generation Mobile Phones
Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace , fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.) CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

Cable Television Community Antenna Television Internet over Cable

Community Antenna Television

Internet over Cable (2) The fixed telephone system.

Spectrum Allocation Frequency allocation in a typical cable TV system used for Internet access CEN 4500, S. Masoud Sadjadi

Cable Modems Typical details of the upstream and downstream channels in North America. CEN 4500, S. Masoud Sadjadi

Agenda Theoretical Basis Transmission Media
Guided Transmission Media Wireless Transmission Communication Satellites Examples of Communication Systems The Public Switched Telephone Network The Mobile Telephone System The Cable Television System Summary CEN 4500, S. Masoud Sadjadi

Summary The physical layer is the basis of all networks
It transports raw bits from one machine to another one. Natural limitations on all physical channels Nyquist limit deals with noiseless channels Shannon limit deals with noisy channels Transmission media Guided: twisted pair, coaxial cable, fiber optics. Unguided: radio, microwaves, infrared, lasers, satellites. Examples Telephone system Local loops, trunks, switches; ADSL, Wireless local loops (LMDS); Trunks can be multiplexed: FDM, TDM, WDM. Mobile Telephone System AMPS, D-AMPS, GSM, and CDMA. Cable Television System Community antenna to hybrid fiber coax. CEN 4500, S. Masoud Sadjadi