Tech2301: Data
Communications
Mohammed A. Saleh
http://ifm.ac.tz/staff/msaleh/TECH2301.html
1
Transmission Media
Transmission rates
 One of the more important media considerations is the
supported data transmission rate or speed.
 The different network media vary greatly in the
transmission speeds they support.
 Application-intensive networks require more than the
10Mbps. In some cases, even 100Mbps, which is found
in many modern LANs.
 Many organizations deploy 1Gbps networks, and some
now even go for 10Gbps implementations.
2
Transmission Media


This is the physical path between a transmitter and a
receiver in a data communication system.
They may be classified into two type:


Guided: waves are guided along a solid medium, such as copper
twisted pair, copper coaxial cable or fiber optic.
Unguided: provides a means for transmitting electromagnetic
signals through air but do not guide them

This is also known as wireless transmission/ communication
3
Cont …
Transmission
Media
Guided
Media
Twisted-pair Cable
Unguided Media
Coaxial Cable
Fiber- optic Cable
Air
4
Media Selection Criteria

Cost
 For
actual media and connecting devices
such as NICs hubs etc

Installation
 Difficulty
to work with media
 Special tools, training
Cont …

Capacity
 The
amount of information that can be
transmitted in a giving period of time
 Measured as
Bits per second bps (preferred)
 Baud (discrete signals per second)
 Bandwidth (range of frequencies)

Cont …

Node Capacity
 Number
of network devices that can be
connected to the media

Attenuation
 Weakening
of the signal over distance
Cont …

Electromagnetic Interference (EMI)
 Distortion
of signal caused by outside
electromagnetic fields
 Caused by large motors, proximity to
power sources

Other noise sources
 Crosstalk
 Echo
Twisted Pair Cable




The popularity can be attributed to the fact that it is
lighter, more flexible, and easier to install than coaxial or
fiber optic cable
It is also cheaper and can achieve greater speeds than
its coaxial competition.
Ideal solution for most network environments.
Two main types of twisted-pair cabling are:

Unshielded Twisted Pair (UTP)


more commonplace than STP and is used for most networks
Shielded Twisted Pair (STP)


used in environments in which greater resistance to EMI and attenuation is
required.
the greater resistance comes at a price.
9
Cont …





This extra protection increases the distances
that data signals can travel over STP but also increases
the cost of the cabling
UTP: one or more pairs of twisted copper wires
insulated and contained in a plastic sheath
Uses RJ-45 telephone connector
STP: Same as UTP but with a aluminium/
polyester shield.
Connectors are more awkward to work with
10
Cont …



Twisted nature to reduce crosstalk.
Any interference from a physically adjacent channel that
corrupts the signal and causes trans- mission errors is
what is known as crosstalk.
UTP Categories:

Categories 1 and 2 (CAT 1 and CAT 2)



voice grade
low data rates up to 4 Mbps
Category 3 (CAT 3)


suitable for most LANs
up to 16 Mbps
11
Cont …

Category 4


Category 5





up to 1000 Mbps
Category 6


Supports Fast Ethernet
100Mbps
more twists per foot
more stringent standards on connectors
Category 5e


up to 20 Mbps
up to 1000 Mbps +
Data grade UTP cable usually consists of either 4 or 8
wires, two or four pair
12
Unshielded Twisted Pair (UTP)
Shielded Twisted Pair (STP)
Coaxial Cable




Commonly referred to as coax
Coax found success in both TV signal transmission as
well as in network implementations.
Constructed with a copper core at the centre that carries
the signal, plastic insulation, braided metal shielding, and
an outer plastic covering
Constructed this way to avoid:

Attenuation


Crosstalk


the loss of signal strength as it travels over distance
the degradation of a signal caused by signals from other cables running
close to it
EMI

Electromagnetic Interference
15
Cont …



Networks can use two types of coaxial cabling: thin
coaxial and thick coaxial
Thin coax is only .25 inches in diameter, making it fairly
easy to install
Disadvantages of all thin coax types are that they are
prone to cable breaks
16
Cont …
Cont …

Size of Coax

RG-8, RG-11




50 ohm Thick Ethernet
RG-58
 50 ohm Thin Ethernet
RG-59
 75 ohm Cable T.V.
RG-62
 93 ohm ARCnet
Fiber Optic



Addresses the shortcomings associated with copperbased media
Use light transmissions instead of electronic pulses
Advantages:




Threats such as EMI, crosstalk, and attenuation become a
nonissue
Well suited for the transfer of data, video, and voice
transmissions
It is the most secure of all cable media
Disadvantages:



difficult installation and maintenance procedures of fiber
often require skilled technicians with specialized tools
the cost of a fiber-based solution limits the number of
organizations that can afford to implement it
Cont …




incompatible with most electronic network equipment; you have to purchase
fiber-compatible network hardware.
Composed of a core glass fiber surrounded by cladding
An insulated covering then surrounds both of these
within an outer protective sheath
Two types of fiber-optic cable are available: single and
multimode fiber




multimode fiber, many beams of light travel through the cable
bouncing off of the cable walls
weakens the signal, reducing the length and speed the data
signal can travel
Single-mode fiber uses a single direct beam of light
allows for greater distances and increased transfer speeds
Cont …
Cont …

Common types of fiber-optic cable include the following:
62.5 micron core/125 micron cladding multimode
 50 micron core/125 micron cladding multimode
 8.3 micron core/125 micron cladding single mode
The main advantage of optical fiber is the great bandwidth it can
carry.
There are three main bands of wavelength used.



Characteristics of Cable Media
Media connectors
attach to the transmission media and allow the physical
connection into the computing device
BNC – Connector
 Associated with coaxial media
 Common BNC connectors include a barrel connector, Tconnector, and terminators

BNC - connectors
RJ - 11



RJ (Registered Jack) -11 connectors are small plastic
connectors used on telephone cables
have capacity for six small pins
Not all the pins are used




a standard telephone connection only uses two pins
a cable used for a DSL modem connection uses four
Similar to RJ-45 connectors
Have small plastic flange on top of the connector to
ensure a secure connection
Cont …
RJ – 11
RJ – 45
 used with twisted-pair cabling
 resemble the aforementioned RJ-11 phone jacks
 support up to eight wires instead of the six
RJ - 45
Wire Propagation Effects
Propagation


Effects
Signal changes as it travels
Receiver may not be able to recognise it
Original
Signal
Final
Signal
Distance
Propagation Effects: Attenuation

Attenuation: signal gets weaker as it
propagates
 Attenuation
becomes greater with distance
 May become too weak to recognise
Signal
Strength
Distance
Propagation Effects: Distortion

Distortion: signal changes shape as it
propagates
 Adjacent
bits may overlap
 May make recognition impossible for receiver
Distance
Propagation Effects: Noise

Noise: thermal energy in wire adds to signal
 Noise
floor is average noise energy
 Random signal, so spikes sometimes occur
Signal
Strength
Signal
Noise
Spike
Noise Floor
Time
Propagation Effects: SNR

Want a high Signal-to-Noise Ratio (SNR)
 Signal
strength divided by average noise
strength
 As SNR falls, errors increase
Signal
Strength
Signal
SNR
Noise Floor
Distance
Propagation Effects: Interference

Interference: energy from outside the wire
 Adds
to signal, like noise
 Often intermittent, so hard to diagnose
Signal
Strength
Signal
Interference
Time
Propagation Effects: Termination

Interference can occur at cable terminator (connector,
plug)

Often, multiple wires in a bundle
 Each radiates some of its signal
 Causes interference in nearby wires
 Especially bad at termination, where wires are
unwound and parallel
Termination
Channel Types



A channel is any conduit for sending information between
devices.
There are three basic types of channel: simplex, halfduplex and full-duplex.
A simplex channel is unidirectional, which means data
can only be sent in one direction. For example, a TV
channel only carries data from the transmitter to your TV
set. Your TV set cannot send information back.
Channel Types


A half-duplex channel allows information to flow in either
direction (but not simultaneously).
Devices at either end of the channel must take it in turns
to transmit information whilst the other listens. For
example, a walky-talky either transmits or receives but
not both at the same time.
Channel Types



A full-duplex channel allows data to be sent in both
directions simultaneously.
A full-duplex channel can be formed from two simplex
channels carrying data in opposite directions. This may
make it more expensive than a half-duplex channel.
There is no waiting for turns or for the devices swap
roles, as is the case with a half-duplex channel. This
means full-duplex can be faster and more efficient.
Unguided Media



Provides a means for transmitting electro-magnetic
signals through air but do not guide them.
Also referred to as wireless transmission
Why is wireless communication so important?




Due to availability of mobile devices, people demand for
communication from anywhere and to anyplace.
Mobility and flexibility are some of the reasons for having
wireless communication
Wireless communications uses specific frequency bands
which separates the ranges.
Main types, radio waves, microwaves, Bluetooth and
Infrared.
39
Cont …

Transmission and reception are achieved by means of
antennas




1.
2.
For transmission, an antenna radiates and electromagnetic
radiation in the air
For reception, the antenna picks up electromagnetic waves from
the surrounding medium
The antenna plays a key role; the characteristics of the antenna
and the frequency that it receives
Basic types of antenna configuration
Point-to-point communications
Broadcast communications
40
Cont …
Point-to-point communication
 The
transmitting antenna puts out a focused
electromagnetic beams
 The transmitter and receiver must be carefully aligned
Broadcast communication
 Transmitted signals spreads out in all directions
 Transmissions can be received by many antennas
41
42
Cont …
The bands are the official ITU names and are based on
the wavelengths
The terms LF, MF, and HF refer to low, medium, and high
frequency
Politics of EM spectrum
 There exist national and international agreements about
who gets to use which frequencies
 Allocate spectrum for AM and FM radio, television, and
mobile phones
 Ways of allocating frequencies
 Beauty contest - proposal serves the public interest best
 Lottery – interested company's
 Auctioning off the bandwidth to the highest bidder

43
Cont …



A different approach is to use ISM (Industrial, Scientific
Medical) bands for unlicensed usage
Garage door openers, cordless phones, radio-controlled
toys, wireless mice use ISM bands
Spread spectrum techniques are used to minimize
interferences
Fig. 3.0: ISM Bands in the US
ITU-T, “Whenever we exchange voice, data or video messages,
communications cannot take place without standards linking
the sender and the receiver”
44
Cont …
-
-
-
900-MHz band works best, but it is crowded and not
available worldwide.
2.4-GHz band is available in most countries; subject to
interference from microwave ovens and radar
installations (Bluetooth and some of the 802.11 wireless
LANs operate in this band)
5.7-GHz band is new and relatively undeveloped;
equipment relatively expensive
45
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.
Radio Wave





Most radio frequencies are regulated
Must obtain a license from a regulatory board (CRTC,
FCC)
A range of radio frequencies are unregulated
Low power single frequency
 uses one frequency
 limited range 20 to 30 meters
 usually limited to short open environments
High power single frequency
 long distance may use repeaters to increase distance
 line of sight or bounced of the earth's atmosphere
 uses a single frequency
Cont …
The spread spectrum maintains security of the radio
transmission by:
Spreading the carrier signal frequency
Modulating the carrier frequency:
It employs two different schemes for using
frequencies:

1.
2.



Frequency Hopping Spread Spectrum (FHSS)
Direct Sequence Spread Spectrum (DSSS
Cont …
Frequency Hoping Spread Spectrum

The frequency spectrum is divided into channels.

Data packets are split up and transmitted on these
channels in a random pattern known only to the
transmitter and receiver

Multiple networks can operate in close proximity
without interfering.

If interference is present on one channel, data
transmission is blocked.

The transmitter and receiver ‘hop’ to the next channel
in the hop table and the transmitter resends the data
packet.
Cont …

Frequency hopping technology works best for small
data packets in high interference environments
Cont …
Direct Sequence Spread Spectrum

The DSSS encoder spreads the data across a broad
range of frequencies using a mathematical key.

The receiver uses the same key to decode the data.

Sends redundant copies of the encoded data to ensure
reception.

Narrowband and DSSS transmissions use the same
total power to send data
Cont …

DSSS uses a lower power density (power/frequency),
making it harder to detect

Narrowband interference appears to the receiver as
another narrowband transmission
When broadband interference is present, however, the
resulting decoded broadband interference can give a
much higher noise floor, almost as high as the
decoded signal.

Cont …


DSSS works best for large data packets in a low to
medium interference environment
As a general rule, FHSS can resist interference from
spurious RF signals ten times better than DSSS
Radio Wave
Microwave



Terrestrial
 line of sight
 use relay towers
 uses license frequencies
General characteristics
Radio Wave (RW) and Micro Wave (MW)
 Licensing needed
 Global span
 Not secure
 Cheap
 RW Good for broadcast
 Attenuation, loss
Communication Satellites

Used the moon as permanent weather balloon to bounce
off signals from the sky




Thought of a big microwave repeater in the sky
It contains several transponders which:





Until an artificial one was built, the moon was no longer used
Key difference between an artificial and a real satellite is that the
artificial one can amplify the signals before sending them back
listens to some portion of the spectrum
amplifies the incoming signal
rebroadcasts it at another frequency to avoid interference with
the incoming signal
Downward beams can be broad, to cover a substantial
area or narrow to cover small areas
Mode of operation is known as a bent pipe.
56
Cont …


1.
2.

The higher the satellite, the longer the period
Altitude
Period
35,800 km
24
hours
384, 000 km 1 month
Where
would you place a satellite?
Altitude – to know the period it can last
The presence of the Van Allen belts - layers of highly
charged particles
These factors lead to three regions in which satellites
can be placed safely.
57
Cont …
Communication satellites and their properties
58
Cont …
GEO (Geostationary)

35800 km from Earth

Stationary position relative to the Earth

up to 180 in orbit at a time

High propagation delay

Major existing telecoms and broadcasting
satellites. E.g. Thuraya, Inmarsat
MEO (Medium Earth Orbit)

10,000 – 20,000 km from Earth

10 to cover earth surface

GPS and mobile telephony services. E.g. ICO
59
Cont …
LEO (Low Earth Orbit)

700km-1,500km, 50 to cover Earth surface.

Telephony, navigation, messaging

Lower propagation delay.

Little LEO: Small satellites providing mainly mobile
data services. E.g. Orbcomm, Teledesic.

Big LEO: Larger satellite, providing mainly mobile
telephony services. E.g. Iridium, Globalstar.
60
Cont …

The principal satellite bands

The C band is designated for commercial satellite traffic



Assigned two frequency ranges
Overcrowded
L and S bands used for international agreement

Narrow and crowded
Cont …

Ku is a higher band, used for commercial telecomm
carriers






Not yet congested
A new development is the invention of VSATs (Very
Small Aperture Terminals)
Antenna size is 1m compared to 10m of GEO satellites
The UL is 19.2kbps but the DL is more, around 512kbps
Biggest problem is that the micro stations do not have
enough to directly communicate with one another
High-gain antenna known as a hub is needed to relay
traffic between VSATs
62
VSATs
VSATs using a hub.
Cont …



The trade-off is a longer delay in return for having
cheaper end-user stations
VSATs have great potential in rural areas
SAT dishes powered by solar cells is often feasible
64
One disadvantage of
satellite transmission is the
delay that occurs because
the signal has to travel out
into space and back to
Earth (propagation delay).
One problem associated
with some types of satellite
transmission is raindrop
attenuation (some waves
at the high end of the
spectrum are so short they
can be absorbed by
raindrops).
Cont …
Properties
 Radically different from terrestrial point-to-point links.
- substantial delay for GEO satellites
- inherently broadcast media
- Security and privacy is a complete disaster
- cost of transmitting a message is independent of the
distance traversed
66
Cont …
Satellites Vs Fiber
 Bandwidth – fiber exceeds satellite
 Mobile communication
 Broadcasting
 Poorly developed terrestrial infrastructure
 Satellites cover areas where obtaining the right of way
for laying fiber is difficult or unduly expensive.
 Rapid deployment is critical
67
Infrared






Uses same technology as remotes for T.V.
signals can not penetrate objects
Can be point to point or broadcast
Point to point requires precise alignment of devices
Point less immune to eavesdropping
General characteristics:
 1-4 Mbps, one room
 Easily blocked (LoS)
 Secure
 Short range
Synchronous Vs Asynchronous
There are two general strategies for communicating
over a physical circuit: Asynchronous and
Synchronous. Each has it's advantages and
disadvantages.
Asynchronous
 Asynchronous communication utilizes a transmitter, a
receiver and a wire without coordination about the
timing of individual bits
 There is no coordination between the two end points
 The transmitting device simply transmit
 Asynchronous systems do not send separate
information for encoding or clocking

Cont …





The receiver must decide the clocking of the signal.
The process performed by the receiver to derive how
a signal is organized without consulting the
transmitting device is known as asynchronous
communication
There is no synchronization between the two ends
before communicating.
More efficient when there is low loss and los error
rates over a transmission medium.
Some called “best effort”:


One side simply transmits and the other side does its best to
receive it
Think of asynchronous as a faster means of
connecting, but less reliable.
Cont …
Synchronous
 Negotiate the communication parameters before
communication begins
 Synchronize clocks before transmission
 Advanced systems may negotiate things like error
correction and compression
 Once a connection is established the transmitter
sends out a signal, and the receiver sends back data
regarding that transmission, and what it received.
 Synchronous transmissions are much more reliable.
Questions