Network Management

1
Network Management

• Physical Media

2
Transmission Modes

• Simplex - one direction only e.g. TV
• Half Duplex - either direction, but only one
  direction at a time e.g. Walkie-Talkie
• Full Duplex - both directions simultaneously
  (need not be symmetric)
• Most long distance applications use serial
  transmission, parallel is good for high speed
  local links.

3
Switching Techniques

• These control the way that information flows
  through the network over the physical media, in
  addition to simplex, half duplex and full duplex,
  we need to consider
• Message Switching
• Circuit Switching
• Packet Switching
• Hybrid models

4
Circuit Switching

• Circuit Switching - like phone system establish
  connections as needed and maintain during
  communication.
• A connection is essential for synchronous
  applications
• May waste bandwidth if the circuit is not
  available to other users.

5
Message Switching

• The entire message (which could be huge!) is
  forwarded from one node to another.
• Requires a lot of memory at the nodes.
• May waste bandwidth on a link basis.
• Does not support synchronous applications

6
Switching techniques

• Packet Switching
• Messages split into smaller units (packets)
• Each packet is handled separately
• Is fair because messages are multiplexed
  (interleaved) so making better use of bandwidth.
• Does not support synchronous applications

7
Circuit vs. Packet Switching

• In this circuit switching example B wants to send
  to D but can not do so because A is sending to C.
  cf. telephone system

8
Circuit vs. Packet Switching
A
C
B
D

• In this packet switching example, AC can
  communicate at the same time as BD because
  traffic is interleaved. (Multiplexed)

9
Circuit vs. Packet Switching

• Despite the need for sequencing and the
  asynchronous nature of packet switching it is
  widely used in networking.
• Sometimes fixed size packets are used, (called
  cells).
• We can superimpose packet switching on top of
  circuit switching to create virtual circuits.

10
Switching Techniques Summary

• WANs tend to be connection based and LANs tend to
  be connectionless - though switches are finding
  their way in LANs.
• Circuits are cumbersome for short interactions.
• The modern trend is to use fast circuit switching
  which combines the merits of packet and circuit
  switching- the call request packet carries data,
  this gives a connection for the duration of the
  communication and very low access time - this is
  one more example of technology convergence

11
Modulation Schemes

• Baseband - direct digital modulation, uses time
  division multiplexing TDM
• Broadband - several channels on the same physical
  medium, uses frequency division multiplexing. FDM
• The Above are not mutually exclusive, consider TV

12
Physical Media

• In most network architectures the lowest level
  is concerned with media and signalling schemes
• Media divide into two categories hard and soft
  wire
• Hardwire e.g. twisted pair, coaxial cable, optic
  fibres
• Softwire e.g. Radio, microwaves, free optics

13
Electrical Physical Media

• Twisted pair
• Ribbon Cable
• Coaxial Cable

14
Optical Physical Media

• Optic Fibres
• Free Space Optics (usually infrared)

15
Radio Physical Media

• VHF radio
• Satellite Microwave
• Terrestrial Microwave
• Also have hybrid technologies e.g. leaky wires

16
Electrical - Open Pair

• Lines separated by space
• Suffer from cross coupling and differential
  interference.
• Short distances (lt50m) and low speeds (lt20kbps)
• Often multi/flat ribbon type cables

17
Electrical - Open Pair

• Effects of interference may be felt in one cable
  more than the other.

18
Electrical Twisted Pair

• Overcome differential interference by wrapping
  lines around each other
• Get additional protection by using shielding and
  multiple pairs
• OK up to 100Mbps, lt100m

19
The Skin Effect
Current flows here
Radiation loss (minor compared to increased
resistance)
No Current

• As the frequency of the signal increases the
  current tend to flow around the outside surface
  of the conductor increasing the resistance and
  losing the signal via radiation. This is the
  limiting factor in copper technology.

20
Coaxial Cable
Braided outer conductor
Insulator
Outer Cladding
Solid inner Conductor

• Overcomes the skin effect by shielding one
  conductor inside the other.
• Good for 10-100Mbps over 100s of metres
• May have several inner cables

21
Microwaves

• Modulate data onto a collimated beam
• Different frequencies for up down link
• Size of footprint could vary widely
• Usually geosynchronous orbit - long lag times but
  LEO MEO are also used

22
Terrestrial Microwave
Strictly line of sight communication

• Used where cabling is impractical
• Suffers from atmospheric conditions, moisture
  etc.
• Buildings, planes etc.can reflect the signal
• Basis of mobile phone systems

23
Hybrid Radio-Electrical

• Systems which use leaky cables have been around
  for years. A cable is laid under the floor and
  this carries the signal. Provided a transceiver
  is within a few metres of the cable communication
  is possible.
• This is used in universities (e.g. Columbia) and
  allows frequencies to be reused since adjacent
  rooms can not interfere with each others
  signals.
• Since no physical connection is needed
  maintenance and management costs will be reduced
  - also damage to connectors is eliminated.

24
Fibre Optics
Total Internal Reflection

• Much greater bandwidth than any other media.
• Immune to electrical interference but can be
  costly due to precise engineering needed.
• Tend to be used wherever high speed and security
  are essential, or for safety reasons.

25
Fibre Optics

• Three main types of fibre
• Step index (older - uses LEDs)
• Graded index or multimode (very common - uses
  LEDs)
• Monomode (used for long distance very high speed
  links - very expensive needing a laser for
  signalling)
• Although plastic fibres exist, they should be
  avoided - they are little more than a marketing
  novelty
• Since fibres are immune from electrical impulses,
  they are ideal for long horizontal cable runs -
  even when high speed is not required
• Dense Wave Division Multiplexing increases fibre
  carrying capacity (sort of FDM for fibre)
• Fibres can give 10Gbps and more.

26
Free Space Optics

• Can be infrared like TV remote operating as a
  common medium, broadcast system.
• Sometimes can be point to point using lasers.
  Terabeam provides high speed final mile access
  using lasers on high buildings and in offices.
  This saves investment in cable and cuts
  relocations costs. Although this is strictly line
  of sight operation, relatively few base stations
  are needed (17 for Chicago CBD). Terabeam has
  been very useful in New York in the recovery from
  S11.

27
Problems with signals

• Attenuation, amplitude of signal decreases with
  distance
• This limits the length of cables that can be used
• Need to use amplifiers and repeaters to boost the
  signal at intervals

28
Problems with signals

• Attenuation is frequency dependent, higher
  frequencies are attenuated more than lower ones.
  This leads to distortion of analog signals which
  use more than one frequency.
• We use selective amplifiers or equalisers to
  overcome this problem.
• Obviously digital signalling is simpler, cheaper
  and more reliable than analog.

29
Noise

• Impulse noise comes from external electrical
  activity
• Thermal noise is caused by agitation of the
  electrons in the device or line, the higher the
  temperature the greater the noise level.
• Thermal noise is often called white noise because
  it occurs in all parts of the spectrum.
• Keep cables cool and away from electrical
  activity where possible.

30
Types of Error

• Errors can be
• single bit errors - very rare
• Burst errors - quite common
• A burst may contain correct bits e.g. the string
• 0XX1010XXXXX101XXXXXXXXXXX1X
• is burst of length 27 and contains 8 bits that
  are not in error
• A 10 error rate almost halves the carrying
  capacity of a channel 1 is considered a high
  rate

31
Design And Cost Issues

• The main cost of cabling is labour - make sure
  that you install enough data points. This is
  often called structured cabling - one per 2
  square metres of floor is common. It is cheaper
  to overbuild than come back and upgrade - leave
  extra cables to accommodate growth
• Also check on the local cabling codes - assume
  nothing! You may not have the right to carry out
  cabling.
• Older cabling may be slow but it has scrap value!
  Reuse it if you can/must.

32
Softwire Systems

• These tend to be more expensive to install but
  allow greater flexibility later on - most LANs
  require reconfiguration on a yearly basis as the
  result of staff movements.
• These may be the only option in heritage listed
  buildings.
• Progress on standards has been slow here but
  16Mbps is common now.
• Softwire systems are more expensive to install,
  tend to be slower but are cheaper to manage -
  management costs for one year for a LAN will
  exceed total purchase costs

33
Summary

• You need to be aware of the different types of
  media, their relative merits and criteria for
  their use.
• There are many developments taking place so keep
  up your awareness
• In all probablity you will bring in people to
  carry out the cabling but you still need to
  understand what they are doing.
• Cabling is the most important aspect of LAN
  design - get this right and most of the rest will
  fall into place - get it wrong and problems will
  follow.