Computer Networks

1
Computer Networks

• Chapter 2
• Physical Layer
• Prof. Jerry Breecher
• CSCI 280
• Spring 2002

2
The Weeks Ahead
Date
Lecture
Lab/Project
Project 0 Due
Jan 21 - 23
The Physical Layer
Start Project 1
Project 1 Checkin
Jan 28 - 30
Medium Access Layer
Medium Access Layer
Feb 4 - 6
Project 1 Checkin
Feb 11
Exam I
Project 1 Due
3
Chapter Overview

• 2.1 Theoretical Basis For Data Communication
• What every sophomore EE knows !!! How much data
  can be put on a wire? What are the limits
  imposed by a medium?
• 2.2 Transmission Media
• Wires and fibers.
• 2.3 Wireless Transmission
• Radio, microwave, infrared, unguided by a medium.
• 2.4 The Telephone System
• The system invented 100 years ago to carry
  voice.
• 2.5 Narrowband ISDN
• Mechanisms that can carry voice and data.

4
DATA COMMUNICATION THEORY
Overview
2.1 Theoretical Basis For Data
Communication 2.2 Transmission Media 2.3
Wireless Transmission 2.4 The Telephone
System 2.5 Narrowband ISDN
This is Physics or Electrical Engineering
stuff. Its how we understand what is actually
happening on a wire.
5
DATA COMMUNICATION THEORY
Some Basic Terms

• Analog and Digital Signaling

Amplitude Modulation
Frequency Modulation
6
DATA COMMUNICATION THEORY
Some Basic Terms

• Baseband and Broadband

Phase Modulation
Modems
7
DATA COMMUNICATION THEORY
Some Basic Terms

• Synchronous theres a clock embedded in the
  wire that guarantees all users of that wire are
  based on that clock.

Asynchronous theres no clock. Users of the
wire send signals when they feel like it.
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DATA COMMUNICATION THEORY
FOURIER ANALYSIS

• Harmonics - Signals of any flavor are made up of
  harmonics. One could in theory have a pure sine
  wave, but that's pretty dull - it carries no real
  information other than its frequency (musically
  it's pretty dull too.)
• In practice, information-carrying-signals are
  made up of a number of frequencies. These
  typically are the fundamental frequency, plus
  other frequencies that are multiples of the
  fundamental. These higher frequency components
  are called harmonics.
• Fourier Series - Any wave, no matter what its
  shape, can be formed by the addition of a
  (possibly infinite) number of sinusoidal waves.
  So if the wave g is a function of time t, then

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DATA COMMUNICATION THEORY
FOURIER ANALYSIS

• g(t) c/2 Sn1 to inf An sin( 2 p n f
  t)
• Sn1 to inf Bn cos( 2 p n f t)
• The coefficients An and Bn are determined for
  each component - they represent the amplitude of
  the individual waves.
• The way to solve this equation is to take the
  function you're trying to analyze (say for a
  square wave)
• g(t) 1 ( 0 lt t lt 1, 2 lt t lt 3,
  .... )
• 0 ( 1 lt t lt 2, 3 lt t lt 4,
  .... )
• then solve for
• An 2/T ? g(t)sin(2 p n f t) dt
• Bn 2/T ? g(t)cos(2 p n f t) dt

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DATA COMMUNICATION THEORY
FOURIER ANALYSIS

• Let's look at this case for a couple of elements
  in the series.

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DATA COMMUNICATION THEORY
BANDWIDTH-LIMITED SIGNALS

• Unfortunately, life isn't perfect. All
  frequencies aren't possible in a transmission
  medium. We'll do a quick detour in order to
  discuss capacitance here - this is what damps out
  higher frequencies. The Figure shows the affect
  of this distortion. Later on we'll see that some
  media are opaque to various frequencies - a
  second reason why signals can't get through.
• Signals can also be intentionally distorted or
  constrained. Perhaps only a certain range of
  frequencies is allowed for each channel. This
  allows multiple channels per medium.

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DATA COMMUNICATION THEORY
BANDWIDTH-LIMITED SIGNALS

• Wave Shape - A pure sine wave (the fundamental
  only) doesn't cut it. The receiver needs the
  signal to electrically stay high for some amount
  of time so it can distinguish the voltage - some
  degree of "squareness" is necessary requiring
  some harmonics in the signal.
• Baud - The number of changes in the signal per
  second. A b baud line does not necessarily
  transmit b bits/second - each signal may convey
  several bits - for example if 8 voltages are
  possible per signal, then 3 bits are sent on
  every signal. If the signal is BINARY (only two
  voltage levels), then the bit rate is equal to
  the baud rate.

13
DATA COMMUNICATION THEORY
BANDWIDTH-LIMITED SIGNALS

• Voice-grade Line - is an ordinary telephone line.
  Its cutoff frequency is near 3,000 Hz.
• If we assume
• 1) 1 bit per baud,
• 2) that there are, in the best case, 8 harmonics,

then 1) the bit rate is b bits/sec, 2) the
frequency of the fundamental (also called the
first harmonic) is b/8 Hz. 3) the highest
harmonic passed through a voice grade line
3000/(b/8) 24,000/b. 4) The Table shows how
this equation works in practice.
14
DATA COMMUNICATION THEORY
MAXIMUM DATA RATE OF A CHANNEL

• The Nyquist equation tries to talk about the
  realistic amount of data that can be pushed
  through a channel with a given bandwidth, H. If
  the signal consists of V levels (for example
  binary 2), then assuming no noise (i.e.,
  perfect signals)
• maximum data rate 2 H log2 V ( in bits/sec )
• Signal to noise ratio - random (thermal) or
  interference causes a degradation of the signal.
  This is measured in terms of the ratio of signal
  power to noise power. Usually this is measured
  in decibels, in terms of 10 log10 S/N. So an
  S/N of 100 20 dB.
• Shannon's equation is another way of expressing
  maximum data rate. It's given as
• maximum data rate H log2 ( 1 S/N )
• When do you use which of these equations??

15
Transmission Media
Overview
2.1 Theoretical Basis For Data
Communication 2.2 Transmission Media 2.3
Wireless Transmission 2.4 The Telephone
System 2.5 Narrowband ISDN
This section discusses the various types of
wires/fibers/etc that can be used to carry data.
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Transmission Media
Hardware Stuff

• MAGNETIC MEDIA
• Sometimes it's cheaper and faster to load a box
  of tapes in your car !!!
• TWISTED PAIR
• Simply two wires twisted together - the twisting
  cuts down on electrical interference.
• Heavily used in the phone system - the typical
  office has four pairs for phones, etc.
• Category 3 and 5 - with 5 having more twists and
  better insulation.
• BASEBAND COAXIAL CABLE
• Used for digital transmissions (called baseband.)
  
• Good noise immunity.
• Data rates as high as 1 Gbps for short distances.
  
• Now being replaced by fiber.

17
Transmission Media
Hardware Stuff

• BROADBAND COAXIAL CABLE
• Used for analog transmissions (called broadband.)
  
• Can run 300 MHz for long distances.
• Analog signaling has better S/N than digital
  signaling.
• Interfaces must convert digital signals to analog
  and vice versa.
• Designed for long distances - can use amplifiers.
• FIBER OPTICS
• Transmission of light through fiber - properties
  include total internal reflection and attenuation
  of particular frequencies.
• Fiber Optic Networks - can be used for LANs and
  long-haul.

18
Transmission Media
Hardware Stuff
19
Transmission Media
Connectors
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Transmission Media

• Signal Regeneration
• Clean up
• Amplify
• Distance Extension
• Repeater functionality, plus...
• Concentration Point
• Signal Distribution Device
• Management Functions

Repeaters
Hubs
21
Wireless Transmission
Overview
2.1 Theoretical Basis For Data
Communication 2.2 Transmission Media 2.3
Wireless Transmission 2.4 The Telephone
System 2.5 Narrowband ISDN
So how do those signals magically get through the
air to give us wireless computing.
22
Wireless Transmission
Hardware Stuff
Though we will say little about this topic in the
present course, it's clearly an extremely
important topic. It's especially relevant since
wireless transmission bypasses a great amount of
infrastructure. It means that developing
countries can leap ahead in technology without
running miles of physical media. Cell
Phones Wireless Computing
23
The Telephone System
Overview
2.1 Theoretical Basis For Data
Communication 2.2 Transmission Media 2.3
Wireless Transmission 2.4 The Telephone
System 2.5 Narrowband ISDN
How is the phone system put together? And how is
voice and data transmitted on that system?
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The Telephone System
STRUCTURE OF THE PHONE SYSTEM
The use of analog and digital signals has pros
and cons
25
The Telephone System
The Local Loop

• This is the connection from the local switching
  station to your house. This is ultimately what
  controls the transmission speed to your house.
• Transmission Impairments
• Attenuation - the loss of energy as the signal
  propagates.
• Delay Distortion - different frequencies travel
  at different speeds so the wave form spreads out.
• Noise - unwanted energy that combines with the
  signal - difficult to tell the signal from the
  noise.

26
The Telephone System
Modems
A device that converts digital data to and from
an analog signal for transmission over phone
lines. Because attenuation is frequency
dependent, modems use a sine wave carrier of a
particular frequency, and then modulate that
frequency. Various modulations include

• Amplitude modulation Two different amplitudes of
  sine wave are used to represent 1's and 0's.
• Frequency modulation Two (or more) different
  frequencies, close to the carrier frequency, are
  used.
• Phase modulation The phase of the sine wave is
  changed by some fixed amount.

Binary Signal
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The Telephone System
Modems
The 3 kHz phone line can only be sampled at 6
kHz. So it doesn't do any good to sample more -
instead try to get in more bits per sample. For
example, the Figure shows a combination of phase
and amplitude modulation leading to multiple
bits/baud.
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The Telephone System
Modems
In addition, modems use compression and error
correction to increase the effective bits per
second. Full Duplex - Able to transmit in both
directions on a wire at the same
time. Electrical Standards - take a look at the
connector between an external modem and a
computer. Those pins all have meanings and
definitions.
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The Telephone System
TRUNKS AND MULTIPLEXING
The cost of a wire is pretty much constant,
independent of the bandwidth of that wire - costs
come from installation and maintenance of the
physical space (digging, etc.), not from the
media or the electrical support structure. So,
how can we stuff more through that medium?
Frequency Division Multiplexing The frequency
spectrum is divided up among the logical channels
- each user hangs on to a particular frequency.
The radio spectrum (and a radio) are examples of
the media and the mechanism for extracting
information from the medium. So how does a
filter work? Note that this is analog stuff.
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The Telephone System
TRUNKS AND MULTIPLEXING
Wavelength Division Multiplexing The same as
FDM, but applied to fibers. There's great
potential for fibers since the bandwidth is so
huge (25,000 GHz). Time Division Multiplexing
In TDM, the users take turns, each one having
exclusive use of the medium in a round robin
fashion. TDM can be all digital.
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The Telephone System
TRUNKS AND MULTIPLEXING
4 KHZ Analog/Voice ? 8,000 samples/sec (
sample every 125 usecond ). T1 is the
combination of 24 of these voice channels.
See Figure on previous slide. 24 X 8 1 Framing
Bit 193 bits/125 usec --gt 1.544 Mbps. When T1
is being used for digital data, the 24th channel
is converted for use as synchronization. T2
combines 4 X T1 T3 combines 6 X T2 T4
combines 7 X T3. Differential Code Pulse
Modulation Assumes that a particular sample
doesn't vary much from the previous one on that
channel. Then we don't need 8 bits to
represent the level (0 - 255), but simply 5 bits
in order to indicate that the sample is (-16 -
15) as compared to the last sample.
32
The Telephone System
SONET
(Synchronous Optical NETwork). Most long
distance traffic in the US uses SONET. Design
goals include 1. Common among different carriers
- requires frequency, timing standards. 2. Common
among different countries - needed to supersede
previous national standards. 3. Multiplexed
multiple digital channels together in a standard
fashion. Sonet is TDM - uses a highly accurate
master clock. Data is transmitted
SYNCHRONOUSLY. A SONET frame of 810 bytes is
transmitted every 125 usec. Because it's
Synchronous, the frame is sent whether there's
data to be carried or not. Data rate is 51.84
Mbps. This basic channel is called STS-1.
Multiple channels can be multiplexed to get
higher bandwidth.
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The Telephone System
SWITCHING
This is what happens inside the phone company -
the various wires or fibers interconnect the
switching centers. Methods of switching
include Circuit Switching A connection
(electrical, optical, radio) is established from
the caller phone to the callee phone. This
happens BEFORE any data is sent. Message
Switching The connection is determined only when
there is actual data (a message) ready to be
sent. The whole message is re-collected at each
switch and then forwarded on to the next switch.
This method is called store-and-forward. This
method may tie up routers for long periods of
time - not good for interactive traffic. Packet
Switching Divides the message up into blocks
(packets). Therefore packets use the
transmission lines for only a short time period -
allows for interactive traffic.
34
The Telephone System
COMPARISON OF CIRCUIT SWITCHED AND PACKET
SWITCHED NETWORKS
What are the relative characteristics of these
two technologies?
35
ISDN
Overview
2.1 Theoretical Basis For Data
Communication 2.2 Transmission Media 2.3
Wireless Transmission 2.4 The Telephone
System 2.5 Narrowband ISDN
This is a method of combining Voice and Data over
a single wire. Used heavily by the phone system
in a number of applications.
36
ISDN
NARROWBAND - WHAT IS IT?
Integrated Services Digital Network A
completely digit, circuit-switched phone system.
Integrates voice and non-voice services. ISDN
allows integration of computers and voice. It
means that caller ID can be used to look up your
account on the computer so that by the time a
human answers the phone, a screen has your
information already available.
37
ISDN
WHAT IS IT?
ISDN SYSTEM ARCHITECTURE ISDN uses TDM to
handle multiple channels. See Figure on previous
page. For home use, the NT1 (Network
Terminator) connects the twisted pair going to
the phone company with the house wiring. Various
ISDN devices can be connected to this
NT1. Businesses may have more channels active
than the home configuration internal bus can
handle. So a PBX ( Private Branch eXchange ) is
used to provide the internal bus containing more
switching capacity. This in turn is connected to
NT1.
38
ISDN
WHAT IS IT?
THE ISDN INTERFACE Typically a number of
channels are combined together. In the USA,
Primary Rate ISDN contains 23 channels (each 64
kbps carrying voice or data) 1 channel for
signaling and control (16 kbps digital channel.)
In Europe, instead of 23 channels, 30 are used.
The primary Rate is designed to connect to a
business with a PBX. As it turns out, most
companies now need far more capacity than 64 kbps
for the many uses beyond voice. So this is less
than adequate. N-ISDN may have a life as a
connection to homes for people wanting to
download images etc. But it's not useful for
serious business applications.
39
ISDN
BROADBAND - WHAT IS IT?

• This is a digital virtual circuit capable of 155
  Mbps. Characteristics include
• ATM Packet Switched Technology.
• The obsolescence of a vast amount of telephone
  technology which is based on circuit switching.

40
ISDN
Comparing Virtual Circuits and Circuit Switching
The service offered is connection oriented (from
the customer's point of view) but is implemented
with packet switching. Services offered include

• Permanent virtual circuits that remain in place
  for long periods of time.
• Switched virtual circuits that are set up and
  torn down with each request.
• The method for establishing these circuits is
  shown in the Figure. The circuit is really
  entries in a series of switches, each mapping a
  circuit number onto a forwarding line.

41
SUMMARY

• 2.1 Theoretical Basis For Data Communication
• What every sophomore EE knows !!! How much data
  can be put on a wire? What are the limits
  imposed by a medium?
• 2.2 Transmission Media
• Wires and fibers.
• 2.3 Wireless Transmission
• Radio, microwave, infrared, unguided by a medium.
• 2.4 The Telephone System
• The system invented 100 years ago to carry
  voice.
• 2.5 Narrowband ISDN
• Mechanisms that can carry voice and data.