Topologies, Routing and Deadlock

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Topologies, Routing and Deadlock
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Figure Recap of speeds
Various speeds of networks
1Gb
LAN
100Mbb
BUS
10Mb
MAN
WAN
1Mb
100Kb
10Kb
EXCH
1Kb
0.1m
1m
10m
100m
1Km
10Km
100Km
1000Km
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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, your telephone is connected to your
  local switching office.

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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 80-200.

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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 Computer 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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The Most Sophisticated type of Software Provision

• The highest level of refinement is to regard each
  computer as part of a single distributed
  operating system.
• This level of sophistication allows the user to
  access files, programs, utilities and resources
  as if they were on his or her own computer.

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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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Switching Networks/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

• Common topologies are
• 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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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 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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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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Ring Topology

• A Ring network consists of nodes connected to
  each other to forma 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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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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Housekeeping

• 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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Transmission of Data

• A network implies that we wish to transmit data
  from one node to other nodes.
• In switched networks, we can use four switching
  methods
• Circuit Switching (e.g. POTS technology)
• Message Switching
• Packet Switching
• Virtual Circuits (e.g. ATM technology)

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Circuit Switching

• We have already seen circuit switching in POTS
  and it is the same in computer networks.
• A connection from node A to node B is established
  and the full bandwidth of that connection is
  available for communication.
• The connection remains in place for the duration
  of the computer conversation.

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Time Sequence Diagram for Circuit Switching

• We can use a time sequence diagram to see the
  order of events in circuit switching.

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Message Switching

• Message switching sends complete messages from
  node to node until they reach their destination.
• Because messages are of variable length, they
  time delay for delivery is variable.
• This makes message switching unsuitable for
  interactive traffic.
• Large messages can also cause congestion and
  overflow switches with small memory.

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Time Sequence Diagram for Message Switching

• The events in message switching occur as follows

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Packet Switching

• Packet switching is similar to message switching
  except that the messages are broken up into small
  datagrams called packets (about 1Kbyte in
  length).
• Because packets are smaller than messages, they
  do not suffer the same delays as messages do.
• This makes packet switching better for
  interactive traffic.

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Time Sequence Diagram for Packet Switching

• The time delay between sending first packet and
  its delivery is much less.

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Virtual Circuit

• A Virtual Circuit is similar to packet switching
  with the exception that all the packets follow
  the same route.
• The route, or virtual circuit, is established in
  a similar way as a connection of a circuit
  switched network.
• The difference is that connections are shared
  between many virtual circuits.

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Time Sequence Diagram for Virtual Circuit (LLC-2)
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Network Control

• In a switched network, we need to have mechanisms
  for controlling the movement of data.
• There are three main aspects to Network Control
• Routing
• Flow Control
• Error Control

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Routing

• Routing is the process of directing data around
  the network.
• Ideally we want to direct as much data from the
  source to the destination with the minimum delay.
• How well this can be done depends a great deal on
  the topology of the network .
• In a simple network (such as a star topology)
  there is little difficulty.

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Routing in a Mesh Network

• Routing becomes complicated in a random mesh
  network (as widely used in WANs).
• One widely used method uses routing tables to
  decide the best direction to send data.

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Routing Packets (or Frames)

• Packets (or Frames) enter the network with a
  destination address field and a source address
  field.
• At each node, the destination address is used to
  look up an entry in a routing table.
• The routing table tells the node which is the
  preferred direction to send the data.
• A alternative direction is usually also provided
  in case the first is unavailable.

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Routing Tables

• The routing tables obviously need to be filled
  in.
• There are a number of approaches for doing this.
• There are two basic classes of routing algorithm
• shortest-path algorithms try to find the least
  cost path between source and destination.
• bifurated routing algorithms attempt to evenly
  spread out the load across the entire network.

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Effects of Routing on Quality of Service (QOS)

• Routing effects two Quality of Service (QOS)
  parameters transit delay and throughput.
• Networks are often described in terms of common
  qualitative and quantitative parameters known
  collectively as QOS parameters.
• QOS parameters are worth a brief discussion in
  themselves.

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Quality of Service

• There are 8 parameters commonly used to describe
  networks (typically WANs).
• Connection establishment delay is the time it
  takes between a request for a connection from
  host A to host B and receiving confirmation.
• Connection establishment failure probability is
  the chance that a connection is not confirmed
  within a reasonable time.
• Throughput is the amount of user data (usually in
  bytes) successfully transmitted from A to B every
  second.

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Quality of Service

• Transit delay is the time between sending data
  and it being received by the destination host.
• Residual error ratio is the proportion of packets
  that are lost or garbled.
• Protection is how easy it is for a third party to
  listen to or corrupt data in the network.
• Priority is if it is possible to earmark some
  connections as having a greater priority than
  others.
• Resilience is the probability that a connection
  is terminated due to congestion or network faults.

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Quality of Service for LANs

• Although LANs can use the same parameters for
  Quality of Service, their small size and general
  reliability means that greater emphasis is placed
  on parameters such as priority.

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Measuring Throughput and Transit Delay

• We can measure throughput and transit delay by
  examining acknowledgements sent back to the
  source host from the destination host (this is
  called feedback).
• The number of acknowledgements received back in a
  second will tell us the throughput and the
  difference in the time fields of the packet (or
  frame) and the acknowledgement tells us the
  transit delay.

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Feedback in Flow Control

• To avoid congesting various parts of the network,
  the number of packets offered to any sub-net
  should match its throughput.
• Feedback can be used to dynamically control the
  number of packets (or frames) given to the
  sub-net. This is flow control.

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Transit Delay vs. Throughput

• Ideally we want the throughput to equal the
  offered load. Excessive offered loads result in
  a portion of the packets (or frames) being
  rejected (either discarded or sent via an
  alternative route).
• Achieving maximum throughput in a network often
  comes at the cost of greater transit delay. Flow
  control is usually designed to find a balance
  between them.

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Effects of Good Routing

• While precise balance between throughput and
  transit delay is determined by flow control, the
  effects of good routing within a network are to
  improve throughput and reduce transit delay.

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Deterministic Routing

• If the routing tables used by the nodes are
  fixed, then the routing method is known as fixed
  routing or deterministic routing.
• Here routing tables are change only occasionally
  (typically when there has been a change to the
  network).
• Any modifications to the tables are usually
  managed centrally or from regional centres.
• There is an advantage to this since routes can be
  assigned to users as they log on.

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Adaptive Routing

• An improved scheme is to use traffic flow data on
  neighbouring nodes in order to dynamically decide
  the best route to send packets (or frames).
• This approach reduces the chance of packets being
  rejected.
• The snag is that the traffic flow data itself has
  to be transmitted and this traffic reduces the
  amount of user data that can be sent.

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Isolated Adaptive Routing

• Despite the snag adaptive routing is used in many
  WANs. One form of adaptive routing measures
  queue sizes and (together with a weight) chooses
  to use alternative routes on a probabilistic
  basis.
• The ratio of(W1E1)(W2E2)is used.

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Distributed Adaptive Routing

• A different algorithm is used in ARPANET. Here
  the object is to find the path of least delay
  under the prevailing conditions.
• The average delay for each of the neighbouring
  nodes is measured and, every 10 seconds, this
  information in propagated to other nodes.
• In this way, a dynamic database is formed of the
  current fastest route in the local area of the
  network.

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Distributed Adaptive Routing

• The delay incurred as a result of the updating
  process can be considerable.
• If one packet is used by each node during each
  time period to update its routing table, then the
  cost can very high.
• In earlier versions of ARPANET, updates were
  performed every 0.67 seconds. This used up about
  50 of the network capacity.
• Now they are performed every 10 seconds only when
  there are significant changes.

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Flooding

• We can dispense with a formal systematic routing
  algorithm altogether by using flooding or random
  routing techniques.
• In flooding, every packet is sent down every
  connection to a node except the one on which the
  packet was received.
• This ensures that the data gets through to every
  node and will arrive in the shortest time.
• This type of routing produces an excessive amount
  of traffic, however.

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Random Routing

• In random routing, the choice of output path is
  any path on which the packet (or frame) did not
  arrive.
• The choice is made randomly and so ensures that
  traffic is evenly spread across the network.
• The only problem is that the packet may take a
  long time to find the destination node.

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Flow Control and Congestion

• Flow Control simply means the methods used to
  keep network traffic moving.
• This means taking measures to avoid sending
  packets (or frames) towards bottlenecks in the
  network.
• If more traffic offered to part of the network
  that it can handle, then buffers in it will
  overflow and packets will be lost.
• Congestion is when part of a network has so many
  packets, none of them can move at all!

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Flow Control and Transit Delays

• Flow control is often used on host machines to
  hold back the transmission of packets until the
  network is able to accept them without risking
  congestion or overflow.
• This keeps the traffic on the network down but
  does not reduce the transit delay since the delay
  is simply shifted to the host machine rather than
  the network.
• The only way to avoid dissatisfaction is to
  ensure there is sufficient network capacity.

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Keeping Users Out

• Another way to avoid overflowing the network is
  to prevent user from logging in when the network
  is near to full capacity.
• This approach tends only to cause another type of
  dissatisfaction.
• When delays do become to long, things just get
  worse
• packets (or frames) get discarded
• the timers on hosts run out and they retransmit.

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A Brief Word about Timers

• In many data link protocols, the source host
  expects to receive acknowledgements (ACK or NAK
  signals) from the destination host when packets
  (or frames) arrive.
• If this does not happen within a certain time
  then the source host assumes that the packet was
  lost or corrupted in transit and retransmits the
  packet.
• Note ACK means that the packet was received
  intact and NAK means that it was not (please
  retransmit the packet).

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Time Sequence Diagrams for Timer
ACK (or NAK) arrives back in time
ACK (or NAK) does not arrive back
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Buffer Overflow

• Consider a situation where two sessions are
  passing packets through the same node.
• Node 3 will temporarily store the packets it
  receives in its buffers.

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Buffer Overflow

• Imagine now that the buffer is full. The node
  will only be able to accept a new packet once it
  has sent an old one.
• Now imagine that one of the incoming links is 10
  times faster than the other one (not an uncommon
  situation).
• Every time a space is made in the butter, the
  node is 10 times more likely to accept a new
  packet from the faster link.

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Connection Blocked by Buffer Overflow

• The effect is to almost block the other
  connection since most of its packets will be
  rejected by the central node.
• This problem can be overcome by using more
  complex nodes that have separate buffers for all
  the incoming links and sends packets out with
  equal priority (say by using a round-robin
  selection from each buffer).

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Deadlock

• Another problem that can cause congestion in a
  network is that of deadlock.
• Imagine two nodes, P and Q, are transmitting
  packets to one another.
• Now imagine that both their buffers are full.
• Both are unable to receive packets so no packets
  move.

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Finally..Overcoming Deadlock

• This is a simple example of deadlock. There may
  be many nodes involved (typically in cycles)
  where each is unable to transmit packets to the
  next.
• We can overcome this by having separate buffers
  for different priority packets.
• The more links a packet travels over, the greater
  its priority.
• Deadlocks do not occur because it is unlikely
  that packets will overflow all the buffers.
• If a packet gets a priority greater than n-1 then
  the packet is discarded (it is assumed to have
  travelled in a loop). This also ensures that a
  deadlock cannot occur.
• Another (less certain method) is to allow
  deadlocks to occur and regularly discard very old
  packets to try to clear them.