Network Topologies

1
Network Topologies
2
Fundamentals

• A LAN (Local Area Network) is a Transmission
  system intended primarily for linking computers
  and associated devices within a restricted
  geographical area.
• It covers an area of moderate size, such as an
  office block, factory or campus.
• In practice, its size may range from a few meters
  to, in rare cases, tens of kilometers.

3
Speed of LANs

• The raw transmission rate of LANs are high,
  typically being in the 1-1000 Mbps range.
• On some LANs (eg Open System LANs) every device
  has the potential of connecting to any other
  device on the LAN.
• Smaller LANs typically operate on a Slave/Master
  basis with slave PCs clustered around a shared
  master filestore system.

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Network Sharing

• A fundamental feature of many LANs is the network
  itself is shared.
• The physical network medium is shared by many
  machines.
• In a traditional network, each machine is usually
  wired into a switching device.
• For example, a telephone is connected to a local
  switching office.

5
Low Error Rates/Low Cost

• On LANs, network errors are expected to be
  relatively few when compared with larger
  networks.
• LANs are relatively inexpensive when compared to
  the cost of the equipment that connects to it.
• However, Each Network Interface Unit (NIU) still
  costs in the region of 10-50.

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WAN

• A WAN (Wide Area Network) is a network that is
  spread over multiple sites (gt30Km).
• WANs are not limited in size (they can even cross
  the world).
• Public facilities (such as the public switched
  telephone network) are extensively used.
• However, this means that the rate at which data
  is sent is limited by the bandwidth of these
  facilities.

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WAN vs. LAN

• When comparing WANs with LANs, the main
  difference is in the data transmission rates.
• Delay and error rate parameters are also relevant
  to some applications.
• We can view the technical facilities offered by a
  WAN as a subset of those offered by a LAN.
• What a WAN offers is long distance connectivity.

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Implementation of a Network

• How easy a network is to use depends on the
  sophistication of the software provided.
• The most basic level of netware provision is to
  only have programs designed for specific tasks
  such as file transfer.
• More sophisticated systems incorporate network
  facilities in the operating system of the
  computer (thus network operations become a
  coherent part of the user interface).

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Network Specifications

• A network implies that all the computer can
  communicate with each other.
• This requirement can be met in a number of ways
  but there are certain basic principles common in
  most networks.

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Network Structure

• Each host computer communicates via a Network
  Interface Unit (NIU).
• By host computer we mean computers on which users
  can run applications.
• The term node is often used for the intelligent
  interface that is part of each host computer (or
  sometimes for the host computer itself).
• The term node can also be used to mean another
  computer with which the host communicates (such
  as a server or a router).

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Network Topology

• The term topology refers to the way in which the
  nodes of a network are connected.
• The topology of a network will effect its
  performance (it terms of speed) and its cost
  (both short and long term).
• Cost/resource considerations and the environment
  in which the network is to be used often
  determines the choice of topology.

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• A network topology is the way in which a network
  is connected up.
• The simplest network topology you can have is a
  single link (i.e. cable, optical fibre, radio or
  any other means of transmitting data) between two
  computers.
• It may not be a very big network, but technically
  it is still a network. The computers can
  exchange data and they are autonomous.

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Switching Broadcast Networks

• We can distinguish networks by the way in which
  they transmit data.
• WAN usually use switching networks to send data
  from source to destination nodes.
• LANs, however, often use broadcast networks
  because they are cheaper to build and maintain.
• Broadcast networks send all the data to all nodes
  (which must out listen for the data meant for
  them).

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Common Topologies

• Some common topologies include
• MESH
• STAR
• BUS
• TREE
• RING
• BACKBONE

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Mesh Topology

• A mesh topology is a network in which the
  connections between nodes is random.
• Mesh topologies include fully connected networks
  and random networks (e.g. Internet).
• Redundant connections in random networks ensure
  that alternative routes exist for data.

Fully connected Network with 5 nodes
Random Network with 7 nodes
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Fully Connected Networks

• Fully connected networks are the fastest types of
  networks since each device directly connects to
  every other device.
• There is no time delay due to switching.
• If there are N hosts in the network, we need
  N(N-1)/2 bi-directional connections (e.g. 20
  hosts needs 190 connections).
• This is far too many connections (most of which
  will be idle most of the time).

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Sharing Connections

• Without a fully connected network, connections
  between nodes must be shared.
• One way to do this is to allow nodes to switch
  data through a random network.
• We can view a random network as interconnecting
  star networks.
• Data is passed through this network, via
  intermediate nodes, until it arrives at its
  destination.

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Mesh Topology - Recap

• A randomly connected network is called a mesh
  topology. Mesh topologies are most suited to
  networks that are built up over time.
• The Internet is a meshtopology. It has
  beenadded to over time withno central control.
• Mesh topologies areeasily extended.
• Often there is more than one possible route for
  messages to follow. If one path fails then
  another can be used.
• There is no simple algorithm for routing
  messages. Messages are often sent over
  sub-optimal paths.
• Control and security can be difficult to enforce
  effectively.

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Star Networks

• A star network consists of a special central node
  (or hub node) to which host computers or
  terminals are connected.
• Any host computer can connect to any other host
  computer via the hub.

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Star Topology

• If all the hosts are connected to a central hub
  (which can be a switching device or a central
  server also acting as a router) then we have a
  star topology.
• Hosts communicate by sending their messages to
  the hub. It then forwards the messages to the
  destination host.
• Star topologies are popular with financial
  institutions who keep their latest records on a
  central server. Any changes to the records are
  updated centrally.
• Its easy to control and monitor access to a
  central server.
• Cabling costs can still be high if network is
  spread out.
• The central hub may be a bottleneck in busy
  networks.

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Star Networks

• The hub switches messages through to the
  appropriate destination.
• The hub may also provide a translation service
  for devices with different protocols.
• Star Networks are vulnerable, however. If the
  hub fails then the network fails.
• Star Networks may require a lot of cabling and
  can be expensive to install.

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Bus Networks

• A bus network consists of a single medium
  (typically 5 pair twisted-wire cable) to which
  all the host computers are connected.
• Packets are broadcasted on the medium to all
  nodes on the network.

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Contention

• The is an obvious danger that two host computers
  may attempt to use the network medium
  simultaneously.
• This problem is called contention and is a
  problem with all Bus topologies.
• The nodes must employ Medium Access Protocols,
  which function in conjunction with other nodes,
  to permit access only at times when the medium is
  free.

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Bus Topology Recap

• We can make a cheap network by connecting not
  just two but a number of hosts to a single link.
  Such a network is said to have a bus topology.
  It is also called a multi-drop link.
• Although physically simple, bus topologies need a
  complicated protocol in order to ensure that
  hosts get fair use of the link and do not attempt
  to use the link at the same time. This can slow
  things down in a busy network.

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Fully Connected Network

• At the other extreme, we do not have to share
  links at all. Each pair of hosts could have
  their own exclusive link. This would ensure the
  fastest possible communication between any two
  hosts. Such a network is called a fully
  connected network.
• Of course there is one smalldrawback to this
  type of networkthe number of links.
• If N is the number ofhosts then we needN(N-1)/2
  links.
• For 100 hosts, we need10099/2 4950 links.
• The cabling and maintenance costs wouldbe
  enormous and there would be no room for computers.

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Tree Networks

• A tree network (as used in LANs) is a variant of
  the Bus topology.
• Nodes are connected in a tree structure and
  messages are broadcast across whole tree.

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Tree Networks

• Tree topologies have the advantage that they are
  easy to expand.
• Furthermore, if a fault occurs, the effected
  branch can be easily isolated so that the rest of
  the network is not effected.
• The disadvantage is that signals can be reflected
  from the ends of branches and cause interference.
  For this reason, Tree Networks are usually run
  at lower speeds.

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Tree Topology - Recap

• A tree topology has a root and branches that
  gives it a distinctive tree shape.
• Messages pass up the tree untilthey reach a
  branching point incommon with the
  destinationhost. They are then passeddown the
  tree to thedestination.
• Tree topology is easily extended.
• There is a simple algorithm for routing messages.
• A large number messages pass through the root,
  which may become a communications bottleneck.

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Ring Topology

• A Ring network consists of nodes connected to
  each other to form a closed loop.
• Nodes accept data from neighbouring nodes in the
  form of packets.

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Ring Topology Operation

• The NIUs (or, in some cases, the hosts
  themselves) act as repeaters for the packets
  being forwarded.
• This means that the Ring can be expanded to any
  size (although more hops will be required to get
  the packets to their destinations).
• One big advantage of Ring Topologies is that
  contention is avoided since each repeater knows
  if it has to forward an existing packet or is
  free to accept a new one.

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Ring Topology - Recap

• In a Ring Topology hosts are connected to their
  neighbours to form a loop.
• Messages are passed fromone host to the next
  untilthey reach the destinationhost.
• Typically messages passthe whole way around
  thering and are checked andremoved by the hosts
  thatsent them.
• Less cabling is required because neighbouring
  hosts are not usually far apart.
• A break in one of the links will stop the network
  from working (but failures can be quickly
  detected and fixed).

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Dealing with Contention

• Contention is dealt with by either using a
  slotted ring system or a token-passing ring.
• With the slotted ring system, blank fixed sized
  frames are passed around the network.
• These frames get filled in with data as they pass
  a node that wishes to transmit.
• The frame goes around the entire network and is
  copied by the destination node as it passes.

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Token-Passing Ring

• A token-passing ring is similar to a slotted ring
  except that a token frame is passed around the
  ring.
• If a token arrives at a node, it can be
  exchanged for a data packet.
• The data packet is sent around the entire network
  and is copied by the destination node as it
  passes.
• When the packet comes back to the sender, the
  sender puts the token back on the network.

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Removing Packets/Frames

• A frame, or packet, will circulate around the
  network until removed.
• In some networks it is the destination node that
  removes it.
• In other networks it is the sender that removes
  it.
• The advantage of getting the sender to remove the
  frame is that it allows data to be broadcast to
  any number of nodes.

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Circulating Frames

• If, for some reason, a frame is not removed by
  the sender it will circulate forever and reduced
  the efficiency of the Ring network.
• This is not a problem with Bus networks since
  terminators (or Head Ends) absorb unwanted
  packets.
• Devices called monitors are responsible for
  housekeeping by marking frames as they pass. If
  a marked frame comes back round to the monitor,
  it is then removed.

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Backbone Network

• A backbone network connects many smaller networks
  via devices called bridges.
• This type of network is easy to expand and
  isolates local traffic.

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Backbone Network - Recap

• Rather than connecting hosts directly, a network
  can be used to connect other networks. Such a
  network is called a backbone network.
• Most messages are sent between nearby hosts which
  are usually connected to the same local networks.
• A message for a host on another network is sent
  (via bridges) over the backbone to the
  destination network.
• Backbone networks are often used by institutions
  to connect legacy networks.
• Small networks of networks, such as those formed
  with a backbone network, are often called
  intranets.

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Finally .. The Spanning Tree Algorithm

• The spanning tree algorithm was developed for
  backbone mesh networks. By mapping a tree
  topology onto a mesh topology, routing is greatly
  simplified.
• Each network (lettered A to E above) is connected
  to other networks by bridges (numbered B1 to B3
  above).
• The spanning tree algorithm dynamically
  determines a suitable tree and maps it onto the
  mesh topology.

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The Spanning Tree Algorithm

• When a bridge is manufactured, it is programmed
  with a unique serial number (there will be only
  one bridge in the world with that serial number).
• Let us say our bridges have serial numbers 1, 2
  and 3 (usually serial numbers are much bigger,
  e.g. 5321293).
• The first step is to elect a root bridge
• When the spanning tree is initialised, each
  bridge broadcasts its serial number.
• If a bridge receives a higher serial number from
  another bridge it will stand down from the
  election.
• Eventually only one bridge will remain - the
  bridge with the lowest serial number (in our
  case, bridge number 1).

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The Spanning Tree Algorithm

• The second step is to determine which networks
  are directly connected to the root bridge. The
  root bridge is then set as the parent bridge to
  these networks.
• Any new bridges connected to these networks are
  added to the tree.

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The Spanning Tree Algorithm

• Each new bridge checks to see what networks are
  directly connected to them.
• Any network that does not already have a parent
  bridge is connected to the new bridge.
• If a network can connected to more than one
  newbridge then it will be connected to the
  newbridge that is nearestthe root.
• If both new bridges are equally near the root
  then the new bridge with the lowest serial number
  is selected.
• Any new bridges on these new networks are then
  added and the above process is repeated until
  there are no more networks to add to the spanning
  tree.