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refereeoppositeNetworking and Communications

Oct 30, 2013 (3 years and 9 months ago)

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Networks
3

Network
Hardware

Dr John Cowell


phones off

(please)

1

Overview


Network hardware


workstations


multiplexors, hubs, bridges


gateways and routers


Interface devices


NIC’s


modems


Interconnection media


direct


indirect

2

3

Modems and Multiplexers


A
modem

is a device that converts data bits into sound so
that data can be transmitted over public telephone lines


it is an acronym for
mo
dulator /
dem
odulator


A
multiplexer

is a device which allows two or more signals
to be sent over the same line. They can be separated
because they occupy different frequencies.


A
demultiplexer

separates the signals.



4

Multiplexers
-

MUX


Several low
-
speed terminals share a high
-
speed data line


Signals from terminals combined into a single data
channel


separated again at the other end

5

Hubs


Allow simple
interconnectivity of
workstations


peer
-
to
-
peer network


little/no processing in hub


bus architecture


level 1

6

Switches


Allow faster
interconnectivity of
workstations


peer
-
to
-
peer network


routing in switch


any two machines can talk
simultaneously


level 2

7

Bridges


Interconnect LANS of
same/similar types


i.e. protocol


Convert

data type


Level 3

8

Gateways and Routers


Interconnect networks


convert protocols


Gateways


interface to Internet


level 7


Routers


operate within Internet


decide pathway for signals


usually level 3


NB the term
gateway
is also used by network engineers for
any device that interconnects networks, or subnetworks

9

10

Interface Devices


Some device must physically connect a computer to a
network


Three principal devices are


network interface cards


a card that plugs into a PC expansion slot to provide a direct
(permanent) connection to a network


modems


a device that converts bits into sound to be transmitted through the
public telephone network


terminal adaptors


a device that converts bits into format required for a digital
telephone service such as ISDN

11

Network Interface Card


Network interface card
-

NIC


an expansion card plugged into a PC expansion slot so that the
computer can connect to a network


often now on the motherboard


it is a direct (permanent) physical link to a network


Each NIC (usually)


plugs into one sort of expansion slot


e.g. ISA, PCI, PCA


supports one type of network protocol


e.g. Ethernet, Token Ring


connects to one type of media


e.g. twisted
-
pair (cat 5), wireless

12

Ethernet Cards


An ethernet NIC contains
protocol control firmware and
Ethernet controller needed to
support the media access
control (MAC) data link
protocol used by Ethernet


each card has its own unique
MAC address


eg 00
-
01
-
02
-
B1
-
64
-
1D


assigned by manufacturer


card is configured in Control
Panel/Networks

13

Modulation and Demodulation


Standard telephones work by
analogue

signals


sounds are converted into electrical signals of varying
amplitude
(volume) and
frequency
(pitch)


Computers work by
digital

signals


encoded somehow: e.g. 0 low voltage; 1 high voltage


Modulation

is the process of encoding digital signals in
some way into analogue signals


Demodulation

is the opposite process of converting the
analogue signals back to digital

14

DSL


A
D
igital
S
ubscriber
L
ine (DSL) modem connects a single
computer to a DSL.


If more than one computer is connected a
DSL router

is
needed.


A DSL modulate data into high frequency tones which
are transmitted to a
D
igital
S
ubscriber
L
ine
A
ccess
M
ultiplexer (DSLAM).


The DSLAM separates the voice and data components.


A DSL modem uses frequencies for data between 25KHz
and about 1Mhz.


Voice is below 4KHz.



15

ADSL


A
symmetric
D
igital
S
ubscriber
L
ine (ADSL)


Distinguished from DSL because data transfer rates from the
Internet to the computer are much faster than those in the
other direction.


Why? The DSLAM at the ISP site is the noisiest part of the
circuit. This is where the signal from the customer is weakest
and that to be downloaded at its strongest.


ADSL is full
-
duplex.


ADSL filters are needed at each phone socket


a low pass
filter for voice and a high pass filter for data.


More than 5 filters cause a degradation of the signal.




16

ADSL


Range about 4km.


Data rates 1.5 to 9 Mbps when receiving data


downstream rate


Typically 16 to 640 Kbps when sending data


upstream rate


17

18

Interconnection Media


Directly connected networks


unshielded twisted pair


fibre optic


co
-
axial cable


now rarely used


Indirectly connected networks


microwave


radio


infra
-
red


laser

19

Unshielded Twisted Pair


UTP
-

strands of wire twisted in pairs


‘cat 5’


Two insulated copper wires are twisted around each other


this reduces crosstalk and interference (noise)


Different cable lengths can cause signals to be received out
of phase


Each twisted pair can carry a single signal


Usually four pairs in outer sheathing


Cheapest form of cabling

20

Fibre Optic


Optical fibre


strand of glass fibre


not prone to electromagnetic interference


Coherent beam of mono
-
frequency light


produced by light
-
emitting
-
diode (LED) or laser


Coated to prevent loss of signal by refraction


Multiple strands are bundled together


thousands of signals can be carried by each strand


mostly used for long distance cabling


more expensive than UTP cable


signal repeaters required after several miles

21

Broadcast Networks


An early broadcast radio network was Aloha


used a packet radio system


established connection between Hawaiian Islands


Originally a user could transmit at any time


had to retransmit if a collision occurred


this was fine when network traffic was very light


Later, ‘slotted Aloha’ reduced the chances of collision by
requiring users to only start transmitting at the beginning
of fixed time slots

22

Satellite Networks


Arthur C. Clarke discussed use of geo
-
stationery satellites
for telecommunications in 1945

23

Footprints


Earth’s surface could be totally
covered by just 3 satellites


if the surface was smooth!

24


Iridium system used 66 in
Low Earth Orbit


250 miles


now being left to decay their
orbits and burn


US GPS system


24 satellite military system


now widely used for
navigation


Galileo


European 30 (eventually)
satellite system


purely for civilian users

Galileo System


Four different navigation service:


The
Open Service

(
OS
)


Free with an accuracy of <4 m horizontally and <8 m vertically.


The encrypted
Commercial Service (CS)


Not free.
Has

an accuracy of better than 1 m. Can be complemented by
to give an accuracy of < 0.1m.


The encrypted
Public Regulated Service

(
PRS
) and
Safety of
Life Service

(
SoL
)



Protection against jamming and the reliable detection of problems
within 10 seconds.


Similar accuracy to the OS


Aimed at the police, military and safety
-
critical transport applications
(air
-
traffic control, automated aircraft landing, etc.), respectively.


Global Maritime Distress Service.


Able to detect and report signals from the
Corpas

Sarsat

search and
rescue beacons


25

Microwave


Microwaves


line of sight


parabolic dish reflector


electromagnetic radiation


Other electromagnetic
systems


in general the longer the
wavelength, the further the
signal will travel


laser (visible light)


infra
-
red


radio

26

BT Tower, Birmingham

Summary


Network hardware


workstations


multiplexors, hubs, bridges


gateways and routers


Interface devices


NIC’s


modems


Interconnection media


direct


indirect

27