Transcript cabling - University of Worcester

COMP1121
Computers and Computer
Networks
Richard Henson
University of Worcester
April 2008
Week 10 – The Physical
Layer: Network Hardware
• By the end of this session you should
be able to:
Identify and select network hardware
devices for a variety of purposes
Explain what a hardware driver is how
plug-and-play works
Explain how to install a network card and
ensure it provides network connectivity
Transmission Media
• “Connection” between computers on a
network is essential for sending/receiving
digital data
doesn’t need to be overtly physical
0’s and 1’s can be transmitted without an
apparent physical medium…
• Suggestions?
Hardware requirements:
• A networking “subsystem” on each computer,
which links in with the motherboard in some way
 at one time this always happened via a network card
connecting the motherboard via ISA or PCI connector
 nowadays, the networking hardware is often built onto
the motherboard
• A plug/socket arrangement with the physical
medium
 USUALLY… this is a “telephone-type” arrangement to
insulate twisted pair copper cable
• known as RJ45
 AT ONE TIME… co-axial cables and connectors like a
TV aerial system were used
• known as BNC
Network Card
software requirements
• Software interfaces effectively with level 3
protocol
 nowadays usually IP
 part of operating system networking component
 requires allocation of IP address
• Level 2 software converts packets into
•
“frames” of data
Level 1 software converts the binary frames
into electricity (high/low voltage) and sends
them out onto the physical medium
Network Card
software requirements
• Each card gets a unique identifier
during manufacture
known as the MAC address – where
“MAC” (media access control) is part of the
data link layer
• Provides software for physical
signal/binary number interconversion
OSI layer 1 signals - converts to layer 2
frames
OSI layer 2 frames converted to layer 1
signals
Binding Network Card
Software to an operating
system “protocol stack”
• Two way process:
 Down: Level 3 IP packets need to be converted to
frames
 Up: Level 2 frames need to be converted to IPcompatible packets
• As with other hardware devices…
 achieved through software called “drivers”
• Plug-and-play hardware can tell the operating
system what it is and provide the relevant
software on network card ROM
 otherwise, driver software will be needed…
Transmission Media
• From the network card/adaptor port, data can
be physically transmitted in a number of
ways:
 via insulated copper cable
 via optical fibre cable
 via “wireless” media e.g. radio waves,
microwaves, infra-red beams, etc.
• Transmission medium type greatly affects the
overall speed and resilience of the network
and the number of packets that get corrupted
Transmission Media - Cabling
• Two types of cables have historically been
used in LANs:
 thin Coaxial – up to 200 metres per cable run
 thick Coaxial – up to 500 metres per cable run
• Most cabled networks nowadays use either:
 Unshielded Twisted Pair (UTP) – up to 100 metres
 Fibre-Optic – up to 1 km per cable run
Transmission Media – Wireless
• “Wireless” means transmission using electromagnetic radiation
 means an electro-magnetic wave vibrating with a
specified frequency and moving forwards at the
speed of light…
 light itself is electromagnetic radiation
• We take wireless for granted now, but it was
first used to transmit and receive signals only
about 100 years ago
 theoretically shown to be possible by Maxwell, UK
 invention usually attributed to Marconi, Italian
Discovery of
Electro-magnetic Radiation
• Potential existence of radio waves were
predicted in 1864
 an amazing piece of maths…
• Started almost 30 years before Marconi with
Cambridge professor James Clerk Maxwell
 successfully predicted most of the physical laws
about propagation and speed of radio waves
• noted their resemblance to light waves
• showed how they could be reflected, absorbed and
focused like the beam from a torch
• and could change the very nature of the object on which
they were focused
Putting Theory into Practice
• Hardly anybody believed Maxwell in 1864!
 BUT his theory was quantified by Oliver Heaviside
into two equations
• Over the next 30 years they became a physical
reality
 in 1879, Prof. David Hughes walked up Portland
Place, London with a device that picked up
transmitted radio waves
 in 1887, German scientist Heinrich Hertz carried out
a famous set of experiments that proved
• that Maxwell had been right all along
• that some materials reflected radio waves back…
 in 1894, the British scientist Oliver Lodge transmitted
wireless signals over 150 yards
First Data Transmission by
Radio waves
• An Italian in London…
 ref: http://www.connectedearth.com/Galleries/Telecommunicationsage/Awirelessworld/Theoriginsofradio/index.htm
 Marconi arrived in 1895, 21 years old, with a new
system of 'telegraphy without wires'
• had already approached the Italian government - but it
showed no interest.
 1896:
• called upon the Engineer-in-Chief of the Post Office to
demonstrate his system
• allowed him to set up his transmitter on the roof of the
Central Telegraph Office, and a receiver on the roof of a
building 300 yards away.
• On July 27 succeeded in sending morse code signals
between the two locations - world's first recorded wireless
message. By 1901, signals had crossed the Atlantic!
What vibrates, in electromagnetic waves?
• Put simply:
 electricity through a coil produces a magnetic effect
 magnetism through a coil generates electricity
• If the electricity is varied, or “pulsed”, the
magnetic field will also pulse at the same rate
 magnetism travels even through a vacuum
 can be used to carry a signal
• Maxwell’s brainwave was suggesting that this
effect could be used to transmit signals
without wires
 need a energy source, & transmitter/receiver coils
Transmission Media – Wireless
• Data carried most efficiently nowadays on
extremely high frequency radio waves:
patented as “radar”; now called microwaves
used in ww2 - bounced off enemy planes

http://www.fi.edu/weather/radar/video/ /historyrad.mov
• Less bandwidth & lower reliability than
optical fibre or cable, but becoming very
popular… e.g.
 Cellular Mobile Phone networks
• Connecting mobile phones to each other & the Internet
 Satellite microwave
• Data to/from satellite in geocentric orbit (22300 miles up!)
Transmission Media - Wireless
• Point-point microwave
 data transmitted either across roofs of adjacent
campus buildings, or “line of sight” point-point
across open land (up to 30 miles away)
• Radio wave
 Either “spread-spectrum” or “narrow-band”
 Useful for connecting mobile laptops to a LAN
Mechanism of data transfer
• Coaxial or twisted pair:
 data is transmitted by electrical conduction
 cabling system consist of two (or groups of two)
conducting wires
• Fibre optic
 Data transmitted by light internally reflected
through a thin fibre-glass tube
 Data can be safely transmitted separately in both
directions
Cabling and Crosstalk
• Two parallel wires, as used in domestic
•
electrical cabling, cannot be used for data
Reason - “crosstalk”:
 electrical interference between signals in the two
wires
 signals jumping from one wire to the other
• The longer the cable, the greater the chance
of crosstalk
 Therefore there will always be a limit on cable run
between data storage devices
Crosstalk and Coaxial Cabling
• Magnetic fields produced by electricity in the
•
•
two wires tends to cancel out
This greatly reduces, but does not eliminate
cross talk
There is a recommended maximum length for
Ethernet cables for this reason:
 thin Ethernet - 185 metres
 thick Ethernet - 500 metres
Thin Coaxial Cable
• Also known as:
 Thin Ethernet
 Base band
• Cable consists of:
 single copper central wire covered with a layer of
insulation
 itself covered by wire braiding (a patchwork made
of very thin copper wire)
 Whole arrangement wrapped in a (usually black)
plastic tube
Thin Coaxial Cable
• Also known as IEEE 10base2
 IEEE - Institute of Electrical and Electronic
Engineers (more on this erudite body later)
• Not very flexible because of the nature of its
•
•
•
construction
Available in a range of different qualities
Generally used for networks using a bus
topology
Recommended maximum data transmission
rate - 10Mbits/sec
Connecting Thin Coaxial Cable
• Computers connected to a thin coaxial bus
•
•
•
network need a network card with a BNC
socket
Such network cards are becoming very rare…
The coaxial bus connects to the network card
through a metal BNC “T connector”
The coaxial cable itself must be “terminated” at
each end with a BNC “terminator” that
completes the electrical circuit
Thick Coaxial Cable
• Also known as Thick Ethernet or broadband
cable
 very expensive and cumbersome to use…
• Includes two shielding layers between the
•
wires to allow for “harsh” environments (lots of
electrical “noise” caused by nearby electric
motors, etc.)
Superior to thin Ethernet is two ways:
 Higher data transmission rates (100Mbits/sec recd
maximum)
 Larger cable lengths (500 metres recd. maximum)
Twisted Pair Cable
• The current standard in most LANs
• Compared to coaxial:
 cheaper
 much more flexible
 easy to use
 doesn’t need BNC T connectors or terminators
• Twisted Pair construction tends to cancel out
magnetic fields - greatly reducing cross talk
(but not as effectively as coaxial)
Twisted Pair v Thin Coaxial
Disadvantages
-
• Use of the “twisted pairs” - wrapping the
•
individual wires around one another - does
not reduce cross talk as effectively as coaxial
cable
More susceptible to “ harsh” environments
(especially rapidly changing magnetic fields)
 Extra insulation of twisted pair cable
recommended in such circumstances
 Cable therefore becomes more expensive
Topology - Twisted Pair Cable
• Normal use - Star topology
• Connections could go directly from network
card sockets to hub ports
 using RJ45 plastic end connectors
 similar to RJ11 telephone line connectors (but not
the same!)
• In practice, for flexibility, a combination of
•
CAT5 cables, connectors, patch leads/sockets
used to connect network cards to hubs
Most modern network cards are known as
“combo” - this means they have sockets for
both BNC (coaxial) and RJ45 (twisted pair)
EIA/TIA Cabling Standards for
Twisted Pair cable
• EIA - Electronics Industries Association
• TIA - Telecommunications Industries Association
• EIA/TIA is a Cabling standards body
 joint venture between the EIA & TIA
• The EIA/TIA 568 standard covers five types of
•
unshielded twisted pair cable known as (in
increasing quality) CAT1 through to CAT5
CAT5 standard has evolved to CAT5f
Existing EIA/TIA 568 standards
•
•
•
•
•
•
CAT1 is OK for voice communications, but not
suitable for digital data
CAT2 can only support digital data transfer rates of up
to 4 Mbits/sec
CAT3 can only support digital data transfer rates of up
to 10 Mbits/sec (this is the lowest standard for IEEE
802.3 10BaseT Ethernet networks - next week’s
session)
CAT4 can only support digital data transfer rates of up
to 16 Mbits/sec
CAT5 can officially handle up to 100 Mbits/sec,
although it is being used on faster (e.g. 155Mbit/sec)
FDDI networks
CAT6 can handle faster rates, theory up to 1 Gigabit
Features of Twisted Pair Cable
• Most popular type currently known as
Category 5 UTP (unshielded twisted pair)
 5e still widely used
 5f, 6 current preferred standards
 CAT5e upwards can carry data at high transmission
rates (up to 200 Mbits/sec)
 CAT5f, CAT6: even higher (1 Gigabit/sec)
• Because of the greater susceptibility of twisted
pair to cross talk, the maximum recommended
cable length for CAT5 is 100 metres
Optical Fibre Cable construction
• The cable itself consists of:
 a glass or plastic central light conductor
 surrounded by a further layer of glass or plastic
cladding
 and a protective outer casing
• The cable must be connected directly to:
 light emitter (one end)
 light detector (other end)
Optical Fibre Cable
• First of all, the electrical signal needs to be
converted into light pulses. This done by:
 either an LED (light emitting diode)
 or a Laser
• The light pulses are then directed into the
•
central tube
The light is repeatedly totally internally
reflected as it passes along the inside of the
tube - as if it were on the inside of a mirror
Optical Fibre Cable (continued)
• Thanks to total internal reflection, a cable can
carry light considerable distances, including
round bends, without significant energy loss
 1 km cable run quite possible…
 cable must be bent carefully, otherwise internal
structure could be damaged…
• On emerging from the cable, the pulse is
•
converted back into an electrical signal by a
photodiode
More detail:
 http://www.arcelect.com/fibercable.htm
Optical Fibre Cable advantages
• Speed of transmission (up to Gbits/sec)
• Ability to support voice and digital data along
•
•
the same cable
Security (very difficult to tap) and reliability of
transmission (almost immune to electrical
interference)
A pair of optical fibres can simultaneously
carry light/data in each direction (full duplex)
with no danger of signal attenuation
 With copper cables, signals in adjacent cables
could interfere with each other
Optical Fibre Cable disadvantages
• Expensive
• Expensive to install
• Not as flexible to use in tight areas of twisted
•
pair
Needs expensive hardware to reliably convert
light into electricity and vice versa
Health, Safety &
Transmission Media
• Some people object to cable being visible
 apart from aesthetic reasons, also a potential safety
hazard
• Different types of twisted pair cable are
•
•
available for different environments (e.g. under
carpets or in the plenum space above a
“lowered” ceiling)
This usually increases the likelihood of
crosstalk, and effectively reduces the
recommended minimum cable length
Standards laid down by IBM in the 1980s…
IBM Cabling Categories
Category
Type 1
Type 6
Contents
Shielded Twisted Pair x2
with casing
as above, with further 4
pairs added outside the
shield for voice
4 twisted pairs
unshielded (as above)
2 x 62.5/125 micron
Optical Fibre
as 1, with foil-braid shield
Type 8
as 1 - flattened
Type 9
as 1 - fire-resistant case
(e.g. PVC)
Type 2
Type 3
Type 5
Purposes
Data
Data & Voice
Voice
Data
Data (2/3 distance limits of
category 1)
Data (under carpets - 1/2
distance limits of category 1)
Data (in plenum space above
ceiling)