Key data communication concepts

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Key data communication concepts

• Week 7 Lecture 1

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Introduction to key concepts

• Physical media
• Analogue Digital transmission
• Multiplexing
• Circuit switching Packet switching
• Bandwidth Latency

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Wide Area Network
The last kilometre
ISP
Routers
Home
ISP or Telco
Local Area Network
Business
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Physical Media

• Connecting home to ISP via Public Switched
  Telephone Network (PSTN)
• Connecting a user to the organisations LAN, and
  the LAN to the WAN
• Connecting the mobile user not so physical
  via the ether
• The WAN copper, optical fibre, Radio wave

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Twisted pair

• Colour coded pair of insulated copper wires
  twisted around each other
• One carries the signal, the other is grounded and
  absorbs interference
• Electrical noise is a big issue the tighter the
  twist the more resistant the cable is to noise
• Lots of variations
• STP shielded twisted pair
• UTP unshielded twisted pair
• Quality of copper, number of twists, length of
  segment, devices on the line

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Fibre optic cable

• Consists of
• One or more glass fibre at the core
• Clad by a glass layer that acts as a mirror
• Layer of plastic
• Braiding of Kevlar
• Plastic jacket
• Data is transmitted by a pulsating light
  generated by a laser or LED

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Fibre characteristics

• Currently used as a LAN backbone or in the WAN
  for high capacity links
• It is more expensive may replace copper to the
  desktop one day
• Basically unlimited bandwidth up to 1gbps at
  present
• Cannot be easily tapped
• Does not need repeaters or amplifiers
• Does not transmit in both directions 2 strands
• NIC and Hubs more expensive
• More difficult to splice

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Wireless

• Big range of complex products being introduced
• Mobile telephones
• Packet Switched Data Networks
• Point to point links
• Local loop
• Wireless LANs
• Wireless personal area networks - Bluetooth
• Satellites

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Mobile Network Generations

• 1G Analogue phased out in Oz
• 2G Digital
• GSM in Europe Asia other technologies in the
  US
• Data to 9.6kbps,
• SMS messages to 160 characters
• 2.5G - Digital
• Higher data rates than 2G but lower initial cost
  than 3G,
• GPRS (General Packet Radio Service) can go up to
  115kbps,
• Oz intro first GPRS network at 24kbps to go to
  48kbps
• Uses multiple slots to give higher data rates

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Mobile Network Generations (cont.)

• 3G
• Now being introduced
• Expected to offer data rates up to 2mbps
• Frequencies sold recently by auction in Europe
  Oz
• Three competing protocols (CDMA) each backwards
  compatible with the 3 main 2G technologies
• Proving to be difficult to introduce
• 4G
• Some small implementations have been made
• Expected to go up to 10mbps

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Analogue Transmission

• Until recently telephone networks were analogue
• Sound is carried along the wire in sine wave form
• Put simply, there are two attributes, the height
  and length of the wave, known as amplitude and
  frequency.
• Loudness varies the amplitude and pitch varies
  the frequency
• Data can be carried by varying Amplitude,
  Frequency or Phase

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Amplitude Shift Keying ASK
Frequency Shift Keying FSK
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Digital Transmission

• Digital signals do not use the sign wave
• It turns the electrical signal on and off. On
  representing 1 and Off Zero
• Usually represented as a square wave form
• Not as clear as On or Off because of noise and
  voltage variation

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• Problem of synchronisation if a long row of
  zeros or ones sent
• This is one of a number of solutions Return to
  Zero

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Multiplexing

• Transmission capacity of the media is often much
  greater than the needs of any one user
• This capacity can be shared by allowing
  simultaneous transmission of multiple signals on
  a single data link
• This technique is known as multiplexing
• There is one device to combine the signals a
  mux and one to separate them again a demux at
  each end of the link

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Mux
Demux
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An example is Telstras ADSL

• Data downstream up to 1.5mbps
• Data upstream up to 256kbps
• Telephone on the same line
• Always on Internet connection

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Circuit switching

• The resources required for a connection are
  reserved for the duration of the connection.
• Good for voice, with low bandwidth and relatively
  constant usage
• Set up time to establish a circuit
• While the circuit is reserved this does not imply
  physical resources totally reserved
  (multiplexing)
• When used by data, transfer rate is constant

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Packet switching

• Data is bursty, often high data rates for short
  periods, and low usage for other parts of the
  connection. Inefficient use of circuits
• Packet switching breaks the message into small
  packets, wraps an electronic envelope with
  address and sends it through the network with
  other packets from other users

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Circuit switching
Packet switching
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Packet switching
Packets can be routed through the network
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Packet switching like public road usage

• Driveway
• Many vehicles share the road
• Junctions allow cars to change direction
• Map determines which roads to take to get to
  destination

• Telephone connects to PSTN
• Data packets share the link with data from other
  connects
• Switches allow data also to change direction
• Packet address instructs switch as to which links
  should be taken

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Multiplexing is not the same as Packet switching

• Multiplexing allows a physical link to carry
  multiple circuits
• Any one circuit can carry packets for a number of
  user connections
• Think of how a road can be divided into multiple
  lanes
• One lane can carry a large number of cars going
  to different destinations
• Another lane may be only available to buses going
  from A to B.

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Store Forward

• Packets are routed through a number of switches
  in their journey
• Each switch uses a store forward mechanism
• The packet must be totally received and checked
  before being sent on the outward link
• Sometimes the outward link is busy, thus the
  packet must be buffered
• Sometimes buffers are full and the packet is
  dropped, and must be sent again
• This delay is called latency, and the uncertain
  nature of this latency is one of the problems
  with packet switching

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Packet V Message switching

• Why break the message into packets?
• Messages can be quite long eg on a file
  transfer
• With packets you get a parallel processing
  through the various switches
• In some cases many messages fit into a packet
• But packets have an overhead of data and
  processing

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Quick quiz You have a 10 second (1k at 100bps)
message to transmit through two switches. Will
sending it as one whole message, or as 10x1
second packets take longer, end to end?
10 second file transfer
C
B
D
A
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10 second file transfer
C
B
D
A
Message switch A to B 10 B to C 10 C to
D 10 Total transmit time is 30
Packet switch (10 x 1)
A-B
B-C C-D
P1
1 sec
1 sec
1 sec
A-B
B-C C-D
P2
1 sec
1 sec
1 sec
A-B
B-C C-D
P3
1 sec
1 sec
1 sec
Total transmit time is 102 12 seconds
(ignoring latency the overhead of the envelope)
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Latency Bandwidth

• Bandwidth is the speed at which data is passed
  down a specific link. Usually expressed in bits
  per second bps
• Latency is the collective delay from all of the
  switches in the trip to the final destination
• As bandwidth increases, then latency becomes more
  of an issue