Network Management

1
Network Management

• Local Area Networking

2
Intoduction

• Local area networking accounts for the majority
  of the work in network management
• In most cases we do not build WANs - we buy time
  on other peoples networks or just buy in the
  communication lines we need
• We usually build and own LANs

3
LANs

• The IEEE has defined layered LAN standards under
  the 802 project. These have been adopted by ISO,
  essentially they operate at level two in the
  model.
• Because there are so many different types of
  media, the IEEE subdivided the layer into a Media
  Access Control (MAC) part and a Logical Link
  Control (LLC) part.

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LANs
LLC Layer
Logical Link Control 802.2
802.5 Token Ring
802.6 Metro LAN
802.3 CSMA/CD
802.4 Token Bus
Layer 2
MAC Layer
Baseband Ring
Baseband Bus
Broadband Bus
802.6 Physical
Physical (most common)

• LLC gives a consistent interface to higher layers
  irrespective of physical medium used.

5
802.3 CSMA/CD
1
5
3
2
4
6

• Physically a common medium or bus system joined
  by Repeaters or Bridges (Carrier Sense
  Multiple Access / Collision Detection)

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Carrier Sensing

• Simply listening to the medium
• Could be persistent or non persistent
• Varying the probability of sensing introduces
  fairness by enforcing turn taking
• CSMA/CD is a 1 persistent protocol since it
  seizes the medium as soon as it is free with a
  probability of 1

7
CSMA/CD Operation

• When the medium is quiet a station may transmit
• Non addressees ignore the frame
• Sender listens during transmission, if a
  collision is detected it sends a jamming signal
  and tries again after a pseudo random wait.
• Jamming signal must be greater than twice the
  end-end delay time

8
802.3 CSMA/CD

• Bridges allow larger networks to be built because
  they filter out unnecessary traffic. So 1 2 can
  communicate at the same time as 5 6.

1
5
3
2
4
6
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CSMA/CD MAC Layer
Preamble SOF Destination Source Length Data
Pad CRC
Bytes
7 1 6 6
2 0-1500 0-46 4

• Defines operation and frame format
• Will consider the operation
• Pad field needed because frames must have a
  minimum size to avoid undetected collisions

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CSMA/CD Operation

• Addresses are 6 byte, the high order bit can be
  set to 1 for broadcasting - can address
  70,368,744,177,663 devices!
• Frames have to be a minimum length to prevent
  undetected collisions hence the PAD field.
• No acknowledgements in the standard

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CSMA/CD Problems

• Collisions are inevitable, so we have
  nondeterministic message delay times.
• The standard has no priority scheme so it can not
  be used for control systems.
• It is widely used in office environments and can
  use a wide variety of media e.g coaxial cable, 10
  BaseT etc.
• Ethernet is generally used to denote CSMA/CD
  networks - after Bob Metcalfes work in the 1970s

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802.4 Token Bus

• General Motors wanted Bus topology with Ring
  simplicity - produced token bus.
• Possession of the token confers exclusive rights
  to transmit.
• This is really a logical ring superimposed on a
  physical bus.
• Control passes from station to station in an
  ordered sequence based on address not physical
  position. Very complicated MAC layer.

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Token Bus

• Medium usage divides into two categories token
  passing and data transfer. Control is distributed
  and stations can join or leave the ring.

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Token Bus

• A station needs to keep information about its
  successor in the token passing scheme. When it
  wishes to leave the ring it sends a message to
  its predecessor saying who to pass the token to.
  Stations are invited to join the ring
  periodically.
• Physically much more robust than 802.3, it has
  priorities but the two are incompatible.

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Token Bus

• Performance is much better than 802.3 under heavy
  loading.
• Performance under light load poor due to
  complexity of token passing scheme.
• Because token ownership gives exclusive rights to
  transmit there are no collisions.
• Designed for a manufacturing environment.

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802.5 Token Ring

• Ring networks tend to be digital and use the
  propagation delay of the ring to store data.
  There is high degree of multiplexing at ring
  level so they are fair. The length of cable
  that a single bit occupies is a design factor.
• e.g. at 10Mbps at a propagation speed of
  200,000,000 metres per sec. a single bit is 20
  metres long.

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Token Ring
1 bit delay
Station (Listening)
Station (Transmitting)

• The interfaces/repeaters have a 1 bit delay which
  allows bits to be modified/copied. Stations
  switch from listening to sending in 1 bit time.

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Token Ring

• Possession of the token gives exclusive use of
  the medium. The ring must be long enough to allow
  the token to circulate, the cabling and the 1 bit
  delay give this length. Interfaces have to be
  powered separately from the stations to ensure
  that the ring remains long enough.

19
Token Ring

• Interfaces need to buffer frames to avoid wasting
  time.
• The physical medium is usually twisted pairs, and
  we use a robust form of signalling that combines
  clock and data.
• What happens if the ring gets broken?

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Token Ring
Station working normally Relay is open
Wire Centre
Station not working Relay is closed

• In the event of cable breakage or station failure
  the relay closes connecting input directly to
  output.

21
Token Ring

• The use of wire centres not only gives failsoft
  potential, it also allows cabling and cabling
  costs to be reduced.
• Wire centres simplify the process of network
  design.

22
Token Ring MAC layer

• Frame formats similar to 802.3
• Acknowledgments are simple, the receiver just
  modifies a single bit.
• One station is the monitor, it maintains the
  ring, removes garbage and frames that have
  already passed the monitor.
• Any station can become the monitor in the event
  of failure

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Token Ring

• 802.5 was designed for office environments and is
  very widely used.
• Very good throughput.
• Has deterministic message delay times.
• However it is seen as proprietary (IBM) and its
  market share is declining - estimated lt20
• Has a priority scheme - not a good one though!
• Most businesses are opting for Ethernet in one
  form or another.

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802.6 DQDB MAN

• Distributed Queue Double Bus
• A high speed broadcast bus network for the
  interconnection of LANs usually over a metro
  area. Able to interconnect with different
  networks
• B-ISDN, Fast Packet Switching and ATM are all
  compatible with DQDB.

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How DQDB works

• Implemented as two unidirectional fibre optic
  buses, constructed from a set of point to point
  links. Uses a slotted access method.
• Broadcasting means transmitting on both buses.
• Synchronous data is assigned a higher priority
  than asynchronous data.

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Structure of DQDB
BUS A
Slot Generator
Slot Generator
BUS B

• Access to the slots is controlled by a request
  counter, a request for a slot on BUS A is made on
  BUS B.

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Priority and DQDB

• Very complicated scheme giving almost constant
  and low access delay time.
• This allows synchronous applications such as TV,
  Video etc.
• Allows loading up to 90 of bus capacity

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Uses of DQDB

• This would be an unusual application

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Uses of DQDB

• More likely usage of DQDB

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Network Integration

• DQDB offers exciting new applications
• Because it deals in fixed size transfer units
  synchronous applications are possible.
• Because it can connect with almost any type of
  network it allows the maximum return on
  investment and and is an ideal integration
  mechanism. However it is neither modular nor
  scalable and so is losing market share to ATM

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Choosing A LAN

• Depends on the nature of the business
• Performance is usually more important than
  standards.
• Look for user support - single vendor solutions
  if possible.
• A good NOS ease of use
• Flexibility upgradeability
• However it is often the existing cabling that
  dictates the choice of architecture.
• Switch based systems would seem the preferred
  option
• Remember to think about compatibility with
  backbone networking strategy.

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Summary

• This is where most management jobs are to be
  found. Managements criteria for assessment are
  performance and cost effectiveness.
• However do not accept any position where you are
  expected to run things on a shoestring - if you
  dont get the resources that you need you will
  not succeed.