The Physical Layer

Chapter 2
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Outline of Physical Layer
It defines the electrical, timing and their interfaces by which bits are
sent as signals over the channel.
Foundation on which the network is built.

The properties of different kinds of physical channel determine the
performance throughput, latency, and error rate.

Data Transmission rate is limited
Three types of transmission media:
Guided (copper wire and fiber optics)
Wireless (terrestrial radio), and
Satellite

Outline of Physical Layer
Digital Modulation: How analog signals are converted into digital
bits and back again.
Multiplexing Schemes: Exploring how multiple conversations can
be put on the same transmission medium at the same time without
interfering with one another.

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.


Theoretical Basis for Data
Communication
Fourier analysis
Bandwidth-limited signals
Maximum data rate of a channel

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fourier Analysis
We model the behavior of variation of voltage or current
with mathematical functions
Fourier series is used to expand any periodic function
with period T



f=1/T fundamental frequency.
a
n
, b
n
are the sine and cosine amplitudes of the nth
harmonic.
c is a constant.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fourier Analysis
Function reconstructed with
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (1)
A binary signal and its root-mean-square
Fourier amplitudes.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (2)
(b)-(e) Successive approximations
to the original signal.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (3)
(b)-(e) Successive approximations
to the original signal.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (4)
(b)-(e) Successive approximations
to the original signal.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (5)
(b)-(e) Successive approximations
to the original signal.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Bandwidth-Limited Signals (6)
Relation between data rate and
harmonics for our example.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Maximum Data Rate of a Channel
Nyquists theorem


Shannons formula for capacity of a noisy channel
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Guided Transmission Media
Magnetic media
Twisted pairs
Coaxial cable
Power lines
Fiber optics

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Magnetic Media
Write data onto magnetic media
Disks
Tapes
Data transmission speed
Never underestimate the bandwidth of
a station wagon full of tapes hurtling
down the highway.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Twisted Pairs
Category 5 UTP cable with four twisted pairs
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Coaxial Cable
A coaxial cable
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Power Lines
A network that uses household electrical wiring.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fiber Optics (1)
Three examples of a light ray from inside a
silica fiber impinging on the air/silica boundary
at different angles.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fiber Optics (2)
Light trapped by total internal reflection.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Transmission of Light Through Fiber
Attenuation of light through fiber
in the infrared region
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fiber Cables (1)
Views of a fiber cable
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fiber Cables (2)
A comparison of semiconductor diodes
and LEDs as light sources
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Wireless Transmission
The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared Transmission
Light Transmission

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Electromagnetic Spectrum (1)
The electromagnetic spectrum and
its uses for communication
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Electromagnetic Spectrum (2)
Spread spectrum and ultra-wideband
(UWB) communication
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Radio Transmission (1)
In the VLF, LF, and MF bands, radio waves follow the curvature
of the earth
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Radio Transmission (2)
In the HF band, they bounce off the ionosphere.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Politics of the Electromagnetic
Spectrum
ISM and U-NII bands used in the
United States by wireless devices
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Light Transmission
Convection currents can interfere with laser communication
systems. A bidirectional system with two lasers is pictured here.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Communication Satellites
Geostationary Satellites
Medium-Earth Orbit Satellites
Low-Earth Orbit Satellites
Satellites Versus Fiber

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Communication Satellites
Communication satellites, some properties, including: altitude
above earth, round-trip delay time, number of satellites for
global coverage.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Geostationary Satellites (1)
The principal satellite bands
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Geostationary Satellites (2)
VSATs using a hub.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Low-Earth Orbit Satellites (1)
The Iridium satellites form six necklaces
around the earth.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Low-Earth Orbit Satellites (2)
Relaying in space.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Low-Earth Orbit Satellites (3)
Relaying on the ground
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Modulation and Multiplexing
Baseband Transmission
Passband Transmission
Frequency Division Multiplexing
Time Division Multiplexing
Code Division Multiplexing

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Baseband Transmission
Line codes: (a) Bits, (b) NRZ, (c) NRZI,
(d) Manchester, (e) Bipolar or AMI.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Clock Recovery
4B/5B mapping.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Passband Transmission (1)
(a) A binary signal. (b) Amplitude shift keying.
(c) Frequency shift keying. (d) Phase shift keying.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Passband Transmission (2)
(a) QPSK. (b) QAM-16. (c) QAM-64.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Frequency Division Multiplexing (1)
Gray-coded QAM-16.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Frequency Division Multiplexing (2)
Frequency division multiplexing. (a) The original bandwidths.
(b) The bandwidths raised in frequency.
(c) The multiplexed channel.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Frequency Division Multiplexing (3)
Orthogonal frequency division
multiplexing (OFDM).
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Time Division Multiplexing
Time Division Multiplexing (TDM).
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Code Division Multiplexing (1)
(a) Chip sequences for four stations.
(b) Signals the sequences represent
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Code Division Multiplexing (2)
(a) Six examples of transmissions.
(b) Recovery of station Cs
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Public Switched Telephone Network
Structure of the telephone system
Politics of telephones
Local loop: modems, ADSL, and fiber
Trunks and multiplexing
Switching
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Structure of the Telephone System (1)
(a) Fully interconnected network.
(b) Centralized switch.
(c) Two-level hierarchy.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Structure of the Telephone System (2)
A typical circuit route for a long-distance call.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Structure of the Telephone System (3)
Major Components
1. Local loops analog twisted pairs to houses, businesses).
2. Trunks (digital fiber optic links between switching offices).
3. Switching offices (calls are moved from one trunk to
another).
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
The Politics of Telephones
The relationship of LATAs, LECs, and IXCs. Circles are LEC
switching offices. Hexagons belong to
IXC whose number is in it.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Telephone Modems
Use of both analog and digital transmission for computer -to-
computer call. Conversion done by modems and codecs.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Subscriber Lines (1)
Bandwidth versus distance over Category 3
UTP for DSL.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Subscriber Lines (2)
Operation of ADSL using discrete
multitone modulation.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Subscriber Lines (3)
A typical ADSL equipment configuration.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Fiber To The Home
Passive optical network for Fiber To The Home.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Time Division Multiplexing (1)
The T1 carrier (1.544 Mbps).
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Time Division Multiplexing (2)
Multiplexing T1 streams into higher carriers
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
SONET/SDH (1)
Two back-to-back SONET frames.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
SONET/SDH (2)
SONET and SDH multiplex rates.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Wavelength Division Multiplexing
Wavelength division multiplexing
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Circuit Switching/Packet Switching (1)
(a) Circuit switching. (b) Packet switching.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Circuit Switching/Packet Switching (2)
Timing of events in (a) circuit switching,
(b) packet switching
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Circuit Switching/Packet Switching (3)
A comparison of circuit-switched and packet-switched networks.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Mobile Telephone System
First-Generation (1G) Mobile Phones Analog Voice
Second-Generation (2G) Mobile Phones Digital Voice
Third-Generation (3G) Mobile Phones Digital Voice + Data

Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Advanced Mobile Phone System
(a) Frequencies are not reused in adjacent cells.
(b) To add more users, smaller cells can be used.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
GSMThe Global System for Mobile
Communications (1)
GSM mobile network architecture.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
GSMThe Global System for Mobile
Communications (2)
GSM uses 124 frequency channels, each of which uses an
eight-slot TDM system.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
GSMThe Global System for Mobile
Communications (3)
A portion of the GSM framing structure.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Voice and Data (1)
Basic services intend by IMT-2000 network
High-quality voice transmission.
Messaging (replacing email, fax, SMS, chat).
Multimedia (music, videos, films, television).
Internet access (Web surfing, incl. audio, video).
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Digital Voice and Data (2)
Soft handoff (a) before, (b) during, and (c) after.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Cable Television
Community antenna television
Internet over cable
Spectrum allocation
Cable modems
ADSL versus cable


Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Community Antenna Television
An early cable television system
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Internet over Cable (1)
Cable television
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Internet over Cable (2)
The fixed telephone system.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Spectrum Allocation
Frequency allocation in a typical
cable TV system used for Internet access.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
Cable Modems
Typical details of the upstream and downstream
channels in North America.
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011
End
Chapter 2
Computer Networks, Fifth Edition by Andrew Tanenbaum and David Wetherall, Pearson Education-Prentice Hall, 2011