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Physical Layer Propagation. Chapter 3 Updated January 2009 Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book. 3-1: Signal and Propagation.

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physical layer propagation

Physical Layer Propagation

Chapter 3Updated January 2009

Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book

3 1 signal and propagation
3-1: Signal and Propagation

A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver

If propagation effects are too large, the receiver will not be able to read the received signal

binary encoded data
Binary-Encoded Data
  • Computers store and process data in binary representations
    • Binary means “two”
    • There are only ones and zeros
    • Called bits

Non-Binary Data Must Be Encoded into Binary




encoding alternative
Encoding Alternative

An N-bit field can represent 2N alternatives

Each additional bit doubles the number of possibilities

Start with one you know and double or halve until you have what you need

E.g., if you know 8 is 256, 10 must be 4 times as large or 1,024

3 3 binary encoding for a number of alternatives
3-3: Binary Encoding for a Number of Alternatives

1There are 2N alternatives with N bits

3 3 binary encoding for a number of alternatives1
3-3: Binary Encoding for a Number of Alternatives
  • Examples:

1. You have N bits. How many alternatives can you represent?

2. You have 4 bits. How many alternatives can your represent?

3. You need to represent 8 things. How many bits must you use?

4. You need to represent 6 things. How many bits must you use?

3 4 ascii
3-4: ASCII
  • Purpose
    • To represent text (A, a, 3, $, etc.) as binary data for transmission
    • Traditional code to represent text data in binary
    • Seven bits per character
    • 27 (128) characters possible
    • Sufficient for all keyboard characters (including shifted values)
3 4 ascii1
3-4: ASCII

Data Byte










for Character


Value does

not matter

  • Each ASCII Character is Sent in a Byte
    • 8th Bit in Data Bytes Normally Is Not Used
3 4 ascii2
3-4: ASCII

To send “Hello world!” (without the quotes), how many bytes will you have to transmit?

3 6 data encoding and signals
3-6: Data Encoding and Signals

We have just seen this

We will now

see this

Before transmission, two things must happen

First, data must be converted into a bit streamWe have already seen this

Second, the 1s and 0s need to be converted intosignals—disturbances that travel down the medium

3 11 multistate digital signaling
3-11: Multistate Digital Signaling


  • Concepts
    • Bit rate: Number of bits sent per second
    • Baud rate: Number of clock cycles per second
      • If 1,000 clock cycles per second, 1 kbaud
      • If each clock cycle is 1/1,000 second = 1,000 clock cycles/second = 1 kbaud
3 11 multistate digital signaling1
3-11: Multistate Digital Signaling


  • Computing the Bit Rate

Bit rate = Baud rate X Bits sent per clock cycle


    • If baud rate is 10,000 baud
    • If two bits per clock cycle
    • Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps
3 11 multistate digital signaling2
3-11: Multistate Digital Signaling


  • Computing the Bit Rate
    • Know the baud rate and the number of states
    • Compute the number of bits from the number of states
        • States = 2Bits per clock cycle

Bit rate = Baud rate X Bits sent per clock cycle


    • If baud rate is 10,000 baud (not bauds)
    • If four states, can send 2 bits per clock cycle
    • Then bit rate is 2 x 10,000 or 20,000 bps = 20 kbps
3 11 multistate digital signaling3
3-11: Multistate Digital Signaling


  • Computing the Required Number of States
    • Know the required bit rate and baud rate
    • Bits sent per clock cycle =Bit rate / Baud rate
    • Compute the required number of states
  • EX:
    • Required bit rate is 4 Mbps
    • Baud rate is 1 Mbaud
    • Bit rate / baud rate = 4 bits per clock cycle
    • 4 bits per clock cycle are required
bit rate versus baud rate
Bit Rate versus Baud Rate

Number of

Possible States

Bits per Clock


If a Baud Rate is 1,200 Baud,

Bit Rate is

2 (Binary)


1,200 bps



2,400 bps



3,600 bps



4,800 bps

Each Doubling of States Gives One More Bit per Clock Cycle


There are eight states.

Each clock cycle is 1/8000 of a second.

What is the baud rate?

What is the bit rate?

utp propagation

UTP Propagation

Unshielded Twisted Pair wiring


Transmission Media

  • Two main categories:
    • wires, cables
    • wireless transmission, e.g. radio, microwave, infrared, …
  • Wired
    • Twisted-Pair cables:
    • Coaxial cables
    • Fiber-optic cables
3 12 unshielded twisted pair utp wiring
3-12: Unshielded Twisted Pair (UTP) Wiring
  • UTP Characteristics
    • Inexpensive and to purchase and install
    • Dominates media for access links between computers and the nearest switch
3 12 unshielded twisted pair utp wiring1
3-12: Unshielded Twisted Pair (UTP) Wiring
  • Cord Organization
    • A length of UTP wiring is a cord
    • Each cord has eight copper wires
    • The wires are organized as four pairs
      • Each pair’s two wires are twisted around each other several times per inch
    • There is an outer plastic jacket that encloses the four pairs
3 12 unshielded twisted pair utp wiring2
3-12: Unshielded Twisted Pair (UTP) Wiring





8-pin RJ-45 connectors

  • Connector
    • RJ-45 connector is the standard connector
    • Plugs into an RJ-45 jack in a NIC, switch, or wall jack
3 14 attenuation and noise
3-14: Attenuation and Noise





Noise Spike


Noise Floor

(Average Noise level)






Ratio (SNR)




  • The signal attenuates (falls in power) as it propagates
  • There is noise (random energy) in the wire that adds to the signal
  • The average noise level is called the noise floor
  • Noise is random. Occasionally, there will be large noise spikes
  • Noise spikes as large as the signal cause errors
  • You want to keep the signal-to-noise ratio high
limiting utp cord length
Limiting UTP Cord Length
  • Limit UTP cord length to 100 meters
    • This keeps the signal-to-noise ration (SNR) high
    • This makes attenuation and noise problems negligible
    • Note that limiting cord lengths limits BOTH noise and attenuation problems

100 Meters Maximum

Cord Length

utp wiring
UTP Wiring
  • Electromagnetic Interference (EMI)
    • Electromagnetic interference is electromagnetic energy from outside sources that adds to the signal
      • From fluorescent lights, electrical motors, microwave ovens, etc.
3 16 electromagnetic interference emi and twisting
3-16: Electromagnetic Interference (EMI) and Twisting

UTP is twisted

to reduce EMI


Interference (EMI)



Interference on the Two Halves of a Twist Cancels Out

3 16 crosstalk interference and terminal crosstalk interference
3-16: Crosstalk Interference and Terminal Crosstalk Interference


at Ends


Crosstalk Interference

Terminal crosstalk interference

normally is the biggest EMI problem for UTP

Terminal Crosstalk


utp limitations
UTP Limitations


  • Limit cords to 100 meters
    • Limits BOTH noise AND attenuation problems to an acceptable level
  • Do not untwist wires more than 1.25 cm (a half inch) when placing them in RJ-45 connectors
    • Limits terminal crosstalk interference to an acceptable level
  • Neither completely eliminates the problems but they usually reduce the problems to negligible levels
optical fiber transmission

Optical Fiber Transmission

Light through Glass

Spans Longer Distances than UTP

3 20 optical fiber transceiver and strand
3-20: Optical Fiber Transceiver and Strand

An optical fiber strand has a thin glass core

This core is 8.3, 50, or 62.5 microns in diameter

This glass core is surrounded by a tubular glass cladding

The outer diameter of the cladding is 125 microns,

regardless of the core’s diameter

The transceiver injects laser light into the core

3 20 optical fiber transceiver and strand1
3-20: Optical Fiber Transceiver and Strand

When a light wave ray hits the core/cladding boundary,

there is perfect internal reflection. There is no signal loss

two strand full duplex optical fiber cord with sc and st connectors
Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors


A fiber cord has two-fiber strands for full-duplex (two-way) transmission



SC Connectors

ST Connectors

radio propagation1
Radio Propagation

Radio signals also propagate as waves.

Radio waves are measured in hertz (Hz),

which is a measure of frequency.

Radio usually operates in the MHz and GHz range.

Hertz (Hz) is the term for cycles per second

3 28 wireless propagation problems
3-28: Wireless Propagation Problems

UTP and optical fiber propagation are fairly predictable.

However, radio suffers from many propagation effects.

This makes radio transmission difficult to manage.

We will look at these problems one at a time.

3 28 wireless propagation problems1
3-28: Wireless Propagation Problems

The first propagation problem is electromagnetic

interference (EMI) from nearby radio sources

This includes other wireless devices

It can include microwave ovens an other devices

3 28 wireless propagation problems2
3-28: Wireless Propagation Problems

Another problem is inverse square law attenuation.

As a signal propagates, its energy spreads out over the

Surface of an ever-expanding sphere.

3 28 wireless propagation problems3
3-28: Wireless Propagation Problems




Comm. Tower



3 28 wireless propagation problems4
3-28: Wireless Propagation Problems




Comm. Tower

Signals Arriving by Different Paths

May Cancel Out



Network topology is the physical

arrangement of a network’s computers,

switches, routers, and transmission lines

It is a physical layer concept

3 29 major topologies
3-29: Major Topologies

The simplest topology is the point-to-point topology

3 29 major topologies1
3-29: Major Topologies

Ethernet uses a star topology

Note that the switch does not have to be in the middle of the star

3 29 major topologies2

Mesh (Routers, Frame Relay, ATM)

3-29: Major Topologies









In a mesh topology, there are many connections

between switches or routers

Consequently, there are many alternative routes between hosts

3 29 major topologies3
3-29: Major Topologies

In the ring topology, messages travel around a loop

3 29 major topologies4
3-29: Major Topologies

The bus topology uses broadcasting.

The message receives each host at almost the same time.

All wireless transmission uses a bus topology.