Modes and Media

1
Modes and Media

• How stuff is transmitted, and what we use to
  transmit it

2
Transmission modes

• simplex mode means that data can flow in one
  direction only, e.g. a public address system
• half-duplex means that data can flow in both
  directions but not at the same time
• full-duplex means that data can flow in both
  directions at the same time.
• (Note that there is some confusion over these
  terms. The above is American usage, which is
  what we will use.)

3
Types of data transmission
4
Parallel transmission

• in parallel transmission, a number of bits
  (usually a byte, i.e. 8 bits) are transmitted
  simultaneously
• this is fast
• convenient
• but expensive, because we need eight wires
• limited to very short distances (10m) because of
  the problem of skewing.

5
Serial transmission

• we transmit bits one by one and therefore only
  need one wire
• require parallel-to-serial and serial-to-parallel
  converters (because sender and receiver usually
  handle data in 8-bit chunks).

6
Asynchronous transmission

• timing of the signal is unimportant
• a byte is preceded by a start bit, which alerts
  the receiver to the fact that data is coming
• a byte is followed by a series of stop bits (or
  one stop bit and a gap)
• the stop bit and the start bit must be different.

7
Asynchronous transmission (cont)

• start bits, stop bits, and gaps slow down the
  communication
• cheap and effective if speed is not an issue
• used to be used widely for communication between
  terminals and computers.

8
Asynchronous transmission
9
Synchronous transmission

• data is transmitted in frames, that is units of
  (usually) many bytes
• bits within the frame are sent one after another
  without start/stop bits or gaps
• it is the receivers responsibility to separate
  the bit stream into bytes
• faster than asynchronous, it is the basis of
  modern comms.

10
Synchronous transmission
11
Factors affecting choice of medium

• cost
• channel capacity (measured in Mbps, megabits per
  second)
• robustness required
• security requirements.

12
Channel capacity (speed)

• The number of bits per second that can be
  transmitted over a communication channel. It
  depends on
• the nature of the transmission medium
• the transmission distance
• the characteristics of the terminal equipment.

13
Propagation delay

• The time required for a signal to get from one
  end of a channel to another.
• Electrical signals travel along wires at about
  two thirds the speed of light in a vacuum, i.e.
  about 2108 metres per second. If we have a 2km
  line, the propagation delay is thus 2103/ 2108
  10-5 secs, i.e. 10 microseconds.

i.e. 2000 metres
14
Types of transmission medium
15
Twisted pairs

• consists of a pair of insulated copper wires
  twisted round each other (the twists reduce the
  effect of noise)
• can be shielded (STP), that is, each pair of
  insulated wires is encased in metal foil or mesh,
  or unshielded (UTP)
• usually have a cable consisting of, say, five
  twisted pairs in one plastic sheath.

16
EIA (Electronic Industries Association) standards
for UTP

• Category 1 basic, old-fashioned, only suitable
  for voice telephony and very low speed data
  transmission.
• Category 3 must have at least three twists per
  foot suitable for speeds up to 10Mbps. Now
  standard for most telephone systems.
• Category 5 suitable for data transmission up to
  100Mbps.

17
Properties of twisted pairs

• UTP is cheap and easy to install
• up to 1Mbps (1.5Mbps for STP)
• suffers from high attenuation so can only be used
  over short distances
• easy to tap (http//www.edn.com/article/CA310392.h
  tml)
• STP is more expensive than UTP but offers better
  resistance to noise, in particular to crosstalk
  (when one wire picks up the signal being
  transmitted on another).

18
Coaxial cable (Coax)
19
Coax properties

• about the same cost as STP
• fast, 1 Mbps to 1Gps
• moderate susceptibility to attenuation and noise
  so can be used over 1 to 2 km
• easy to tap
• as for twisted pairs, there is a standard
  defining different grades of coax.

20
Fibre
Protective layer
Cladding
Core (glass or plastic)

• uses light to carry the signal rather than
  electricity
• much less susceptible to attenuation and
  interference
• very high channel capacities are possible, up to
  100Gbits over short distances
• difficult to tap
• but expensive, fragile and difficult to install.
• now forms the basis of most long distance voice
  telephony as well as much data transmission.
  Undersea fibre is used for inter-continental
  traffic.

21
Electromagnetic radiation

• All unguided transmission is based on the use of
  electromagnetic waves (radiation).
• The term electromagnetic waves includes radio
  waves, light, radiant heat, X-rays, and many
  other kinds of radiation.
• The properties of electromagnetic waves depend on
  their frequency.

22
The electromagnetic spectrum

• up to 1GHz radio waves
• 1 GHz to 300 GHz microwaves
• 300 GHz to 1014Hz infra-red (radiant heat)
• 1014Hz to 1015Hz visible light
• 1015Hz to 1017Hz ultra-violet
• 1017Hz to 1019Hz X-rays
• 1019Hz upwards ?-rays

23
Use of frequencies

• low frequencies, up to 300KHz, are used for long
  range radio navigation, submarine communication
  and other specialised purposes. LW radio starts
  around 150 KHz
• frequencies between 300KHz and 300MHz are used
  for radio, VHF TV and aircraft communication

24
Use of frequencies (cont)

• frequencies between 300MHz and 3GHz are used for
  mobile telephones, UHF TV, LANs, pagers, etc.
  Bluetooth uses 2.4GHz
• frequencies between 3GHz and 30GHz are used for
  microwave links, both terrestrial and satellite
• higher frequencies used for wireless comms.

25
Radio waves, microwaves, infra-red and visible
light

• Infra-red is used for some local area networks
  (IEEE 802.11) and for certain special purposes
  (e.g. remote controls).
• Infra-red and visible light can be focused by
  lasers and used in systems based on free space
  optics.
• Radio waves and infra-red are normally broadcast.
• For communications purposes, microwaves need to
  be focused.
• The use of radio frequencies, including
  microwaves, is governed by international
  agreement and regulated by national governments.

26
Terrestrial microwaves

• microwaves travel in straight lines so that
  communication is restricted to line of sight
• repeaters are used for greater distances
• a single microwave channel can only operate in
  one direction
• susceptible to interference and attenuation
  (depending on atmospheric conditions)

27
Terrestrial microwaves (cont)

• high capital cost but not as high as laying
  fibre
• possible to tap them but you need a lot of money
  and technology
• microwave links are used extensively for data
  communications (the high-speed data link between
  the University and the outside world is a
  microwave link as far as Swansea)
• they are still used for voice telephony but have
  largely been replaced by fibre.

28
Satellite links

• use microwaves but distances mean they are not
  tightly focused so possible to tap
• used for satellite phones, for (cut-price) voice
  telephony and for data communications, but most
  of all for TV broadcasting
• need geostationary satellites (see next slide)
• satellites are expensive but they provide
  enormous capacity so using a bit of it is quite
  cheap.

29
Communications satellites
30
Satellite links (cont)

• Geostationary satellites must orbit the earth
  above the equator at a height of 35,863 km.
• Propagation delays are therefore significant,
  which affects speech quality.
• Now largely replaced by undersea fibre for voice
  but heavily used for data communications.

31
Satellite frequencies

• Satellite transmission works best when
  frequencies in the range 1GHz to 10GHz are used.
• Most existing satellites use a range of 5.925 to
  6.425 GHz for uplink (earth to satellite)
  transmission and 3.7 to 4.2 GHz for the downlink
  (known as the 4/6 GHz band).
• At these frequencies, an angular separation of
  at least 4 is needed, which limits the number of
  communication satellites.
• The 4/6 band is getting saturated.

32
Wireless (radio) transmission

• broadcast, so inherently insecure (but can be
  made secure)
• very subject to attenuation, distortion,
  dispersion and interference
• more or less line of sight at the very high
  frequencies (depends on atmospheric conditions)
  lower frequencies are also used for radio/TV and
  other things
• reflection leads to the problem of multiple paths.

33
Wireless communications

• wireless LANs
• cellular communication systems (mobile telephony,
  mobile computing)
• satellite phones.

34
Wireless LANs

• cheap to install
• convenient (no wires)
• reasonably fast (up to 100 Mbps)
• most are inherently insecure and are easier to
  tap than any other medium
• but there are perfectly good ways of making them
  secure.

35
Infra-red vs. radio

• Infra-red
• advantages
• simple and cheap
• no licences needed for use of spectrum
• shielding simple
• no interference with or from electrical devices
• reasonably secure
• disadvantages
• cannot penetrate walls
• line of sight connection needed for good quality
• low bandwidth.

36
Infra-red v. radio (cont)

• Radio
• advantages
• covers larger areas and can penetrate obstacles
• line of sight generally unnecessary
• transmission rates up to 54 Mbits/sec
• disadvantages
• shielding is difficult
• generates and is subject to interference
• easy to tap
• very limited range of frequencies available, with
  licences needed outside this range.

37
Free space optics

• uses infra-red, focused by a laser
• line of sight required but can pass through
  windows
• gives a channel capacity of up to 2.5 Gbps over
  distances up to about 3 km
• used for the last mile and for linking sections
  of local area networks
• no licensing problems.