ICT A2 ICT5

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ICT A2 ICT5

• Chapter 59
• Network Security and Accounting

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Star network

• Each node connected to a central computer
• the hub

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Advantages

• Each computer is independent of the rest
• Data transmission rate can vary on each link
• Allows a high degree of security since it is
  centrally controlled.
• Very suitable for a WAN where most of the
  processing is done at Head Office.

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Disadvantages

• A hub failure affects all users
• Control of communications at hub requires
  expensive hardware

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Bus network

• All workstations linked by single cable of lt500m.
  Suitable only for LANs. No central computer,
  but PCs used as file server and print server.

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Advantages

• Easy and inexpensive to install and requires
  least amount of cable
• Additional devices easily added without
  disrupting the network
• Failure of one workstation or its connection to
  the main cable (bus) does not affect others

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Disadvantages

• If part of main cable faulty, whole network down
• Cable failure is difficult to isolate
• Network performance degrades with heavy load

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Ring network

• Not suitable for a WAN no host computer and none
  of the nodes need have overall control of access
  to the network although in practice one node may
  do so.
• In the token ring system, a message token (a
  unique character sequence) is passed from node to
  node.
• Repeaters attached to each node allow a ring
  network to cover larger distances than other
  types.

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Advantages

• There is no dependence on a central computer
• Very high transmission rates are possible
• Transmission of messages around the ring is
  relatively simple, in 1 direction only

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Disadvantage

• If one node breaks down, transmission between the
  other nodes is disrupted

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Access privileges

• Typical access levels include
• No access
• Read only
• Read and copy
• Read and update.

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Access control

• May be based on
• What you know
• password or PIN number - many shortcomings
• What you have
• ID card - smart card or magnetic stripe card
• Where you are
• access may be only from a specified location or
  phone number - callback system         
• Who you are
• biometric identification by handprint, retinal
  image or voice print
• All methods are less effective if users are
  careless.

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Firewalls

• Software that prevents unauthorised communication
  into or out of the network.
• The firewall is generally placed between internal
  LANs and WANs and external networks such as the
  Internet.

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Firewalls

• Various different types are available, such as
• Router - examines each incoming packet of data,
  checking its source or destination address.
• Access rules must identify every type of packet
  that the organisation does not want to admit
• Special software that restricts traffic to a
  particular application
• such as e-mail or Lotus Notes groupware
• A proxy server
• that maintains replicated copies of Web pages for
  easy access by a designated class of users.

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Audit controls

• Track all activity on a network for example
• What programs have been used
• What files have been opened
• How many reads and writes have been executed
• How many times a server has been accessed.

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Performance management

• Network monitoring software will collect data on
• Network availability (i.e. switched on and not
  working)
• Response time (time between making a query and
  receiving a response)
• Utilisation of hardware resources (CPU, disks,
  bridges, repeaters, clients and servers)
• Utilisation of software
• Traffic density in each segment of a network.

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Data encryption

• Prevents confidential data from being read by
  unauthorised hackers. Makes it incomprehensible
  to anyone who does not hold the key to decode
  it.
• Methods include
• transposition
• characters switched around
• substitution
• characters replaced by other characters

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Communications software

• Communications software enables computers to
  communicate with each other, controlling
  transmission by specifying
• Speed of transmission
• Direction of data flow
• Method of transmission
• Which computer code is used
• (e.g. ASCII, EBCDIC)
• Type of parity
• (e.g. odd or even).
• The rules and procedures for allowing computers
  to communicate with each other are called the
  Communications protocol.

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Speed of transmission

• Related to communications channel
• telephone line
• Fibre-optic cable
• coaxial cable
• microwave or satellite link.
• Channel Bandwidth
• Baseband carries one signal at a time0 or 1
  indicated by presence / absence of voltage very
  fast but only over short distances.
• Broadband multiple signals on a fixed carrier
  wave0 and 1 sent as variations on this
  wavedata, audio and video transmission
  simultaneously

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Speed

• expressed as baud rate, in bits per second (bps).
  
• Framing bits
• each data byte uses 7 or 8 bits,
• may have start and stop bit and/or parity bit
  added
• Thus each character may need 10 bits, so a speed
  of 56,000bps means about 5,600 characters a
  second.

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Direction of transmission

• Communications software sets the transmission
  protocols that indicate the way that data will
  flow over the communications channel
• Simplex - data can flow in one direction only.
• Electronic notice-boards that receive and
  display information about, for example, train
  arrival times could use a simplex line, since
  communication is one-way only
• Half-duplex - data can flow in both directions,
  but not at the same time,
• like a CB radio
• Full-duplex - data can flow in both directions at
  the same time
• like a telephone line where two people can both
  be talking at once.

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Serial transmission

• Bits are transmitted one at a time over single
  channel. Two types of serial mode for sending
  data
• Asynchronous transmission
• one character at a time, with a start and stop
  bit sent with each character
• generally for lower speed transmission e.g. LAN
• Synchronous transmission    
• enables whole blocks of data to be sent in timed
  sequences
• much faster

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Parallel transmission

• All the bits making up a character, together with
  a parity bit (if used) transmitted simultaneously
• limited to short distances of a few metres, e.g.
  between a computer and printer.

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Telecommunications - standards and protocols

• To allow machines from different manufacturers to
  communicate with each other over local or wide
  area networks, it is necessary to define standards

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Advantages of standards

• can create networks containing hardware from
  different vendors (such as Apple and IBM)

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Disadvantages of standards

• may disallow certain features or capabilities
  that are valuable in a particular situation but
  inconsistent with the standard.
• may also prohibit features which certain vendors
  have built into their hardware in order to steal
  a march on the competition something that is
  better than other architectures but will not work
  with other hardware.
• In spite of this, most people would rather have
  hardware that has less than the maximum
  capabilities but is compatible with their other
  hardware.

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The development of de facto standards

• Discussion Why do all keyboards have the same
  layout of letters, i.e. QWERTY along the top
  line?
• Would it be a good idea to adopt a new standard?
  Some countries use AZERTY ..
• In England we push a light switch down to turn
  the light on.
• In the U.S. Up is On.
• These are de facto standards standards that
  have evolved.
• Think of some de facto standards applicable to
  PCs.