Transcript Lesson_3B.pps

Transmission Media
• Wired
– Coaxial
– Twisted Pair (UTP / STP)
– Optical Fibres
• Wireless
– IR
– Microwave signals
– Radio Frequency
• Wired LAN (Coaxial cable)
– high capacity communication cable
– using BNC Connector
– 2 concentric conductor
• one inner (inner core)
• one outer (metal mesh)
– nowadays mostly for cable TV transmission
– disadv :
• much thicker
• not flexible
• more expensive (c.f. UTP / STP)
• Wired LAN (Twisted pair)
– Using ordinary copper wire
– the copper wires are twisted together
• to reduce crosstalk / electromagnetic interference
– by electrical appliances (motors, transformers)
– radio frequency interference
– if the twisted pair is
• enclosed inside a shield
– STP
• not enclosed inside a shield (unshield)
– UTP
– nowadays mostly for networking cabling
• Wired LAN (Twisted pair) … cont’d
– Using RJ45 connector
– Adv :
• more flexible
• less expensive (c.f. coaxial cable)
– Categories
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Cat. 1 --- for analogue voice (old telephone system)
Cat. 2
Cat. 3 --- 10baseT Ethernet (16Mbps)
Cat. 4
Cat. 5 --- 100baseT Ethernet (100Mbps)
– Ref : http://www.siemon.com/us/standards/1310_twisted-pair_balanced_cabling.asp
• Wired LAN (Twisted pair) … cont’d
– Types of cables
• Straight through cable
– connecting PC to network devices (hub / switch)
• Crossover cable
– connection between PC & PC
– connection between network device with network
device
• Rollover cable
– with connector
– RJ45 to DB-9 converter then to COM 1 or COM 2 of
PC
• Wired LAN (Optical fibres)
– made up of a bundle of thin tubes of glass
• thinner than a human hair
– do not conduct or transmit electrical signals
• not interfere by electromagnetic wave
– Transmission of info via light pulse
– Carry more info than copper wire at the
same period of time
– signal can travel a very long distance
• because no / little attenuation
– But fragile … need more protection
• installation .. More labour intensive
• Interference
– Original Signal
– Noise
– When signal and noise combined together
– Resulting Signal detected
• Wireless LAN
– use electromagnetic waves to transmit and
receive data from one device to another
– suitable to env. where cabling is difficult / not
practical / not economical
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construction site
warehouse
school campus
hospitals
– usually as an extension or an alternative for
a wired LAN
– transmitter : Access Point (AP)
• Wireless LAN … cont’d
– Using IR / radio signals / microwaves
• Some of them require direct line of sight (e.g. : IR)
– Signal travels in straight line
• Stations using antenna & satellite
– To avoid the
» curvature of the Earth
» Mountains
» Tall buildings
» Other signal obstacles
• Advanced wireless technology
– Eliminate the limitation of direct line-of-sight
– More flexible
• Wireless LAN … cont’d
– Problems faced by wireless transmission
• Signals can be weakened by bad weather and
solar activity
• A satellite can be used for around 7-10 years
• Anyone can detect the signal (Security problem)
– Signal / data must be encrypted before sending
• Wireless LAN … cont’d
–Advantages
–Easy installation
–Flexibility in usage
–Economical
–Expandability
– http://iclassroom.hkedcity.net/directory/index
.phtml?iroom_id=1387&tool_id=853&folder
_id=1520