Topologies, Routing and Deadlock by siz19146

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									Topologies, Routing
   and Deadlock
Various speeds of networks
      1Gb


    100Mbb   BUS           LAN

     10Mb
                                     MAN
      1Mb                                          WAN

     100Kb


      10Kb
                                EXCH
      1Kb




             0.1m   1m   10m   100m 1Km   10Km   100Km1000Km



              Figure – Recap of speeds
    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.
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.
         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.
         WAN
   A WAN (Wide Area Network) is a network
    that is spread over multiple sites (>30Km).
   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.
          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.
         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).
         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.
         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.
          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).
           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.
          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).
          Common Topologies

   Common topologies are:
    –   MESH
    –   STAR
    –   BUS
    –   TREE
    –   RING
    –   BACKBONE
         Mesh Topology
   A mesh topology is a network in which the
    connections between nodes is random.



           Fully connected Network   Random Network with 7
                 with 5 nodes               nodes


   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 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).
         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.
         Star Networks
   A star network consists of a special central
    node (or hub node) to which host computers
    or terminals are connected.


                                 Special Central
                                 Node




   Any host computer can connect to any other
    host computer via the hub.
         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.
              Bus Networks
   A bus network consists of a single medium
    (typically 5 pair twisted-wire cable) to
    which all the host computers are connected.
                             Host
                           Computers   Bi-directional
                                       medium




       Network                              Terminator
       Interface
           Units



   Packets are broadcasted on the medium to
    all nodes on the network.
         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.
         Tree Networks
   A tree network (as used in LANs) is a
    variant of the Bus topology.
                            Root
                            Node



               Host
           Computers



                                   Communications
                                   Link




   Nodes are connected in a tree structure and
    messages are broadcast across whole tree.
         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.
           Ring Topology
   A Ring network consists of nodes
    connected to each other to forma a closed
    loop.
         Host
       Computers      Data Flow




                     ACTIVE CABLE




   Nodes accept data from neighbouring nodes
    in the form of packets.
           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.
         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.
           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.
         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.
         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.
         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.
           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)
         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.
         Time Sequence Diagram for
         Circuit Switching
   We can use a time sequence diagram to see
    the order of events in circuit switching.
                   node            switch           switch           node
                    A Establish      1                2               B
                        Connection        Establish
                                         Connection        Establish
                                                          Connection


               t                                        Acknowledge
                                       Acknowledge
               i                                         Connection
                       Acknowledge      Connection
               m
               e        Connection

                        Send Data
                                         Relay Data
                                                         Relay Data

                                                         Send Data
                        Relay Data      Relay Data
                         Close
                       Connection         Close
                                        Connection        Close
                                                        Connection
         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.
         Time Sequence Diagram for
         Message Switching
   The events in message switching occur as
    follows:          node              switch             switch             node
                       A                  1                  2                 B
                               Send
             Send small       Message             Relay
               Message                           Message             Relay
                                                                    Message
         t
         i
         m                                                                       Send large
         e                                                                       Message
                                                                     Send
                                                                    Message
                                                  Relay
                                                 Message
                              Relay
                             Message
         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.
                 Time Sequence Diagram for
                 Packet Switching
                    Host A                   node               switch              switch               node                   Host B
                                              A                   1                   2                   B


                                                    Transmit
       Send Message broken Packet 1                 Packet 1
      Small into packets with Packet 2                                   Packet 1
            distinct sequence                       Packet 2                                 Packet 1
    Message      numbers      Packet 3                               Packet 2                                                   Packets
                                                                                                                Packet 1
                                                    Packet 3                                 Packet 2                      reassembled into
                                                                                                                Packet 2
                                                                                                                                               Received
                                                                     Packet 3                                               message using
                                                                                             Packet 3                                          Small
t                                                                                                               Packet 3       sequence
i                                                                                                                          numbers to order
                                                                                                                                               Message
m                                                                                            Transmit           Packet 1
e                                                                                            Packet 1
                                                                     Packet 1                                   Packet 2
                                                    Packet 1                                 Packet 2                      Message broken      Send
                                  Packet 1                               Packet 2                               Packet 3
                                                                                              Packet 3                     into packets with   Large
                                                     Packet 2                                                              distinct sequence
                    Packets       Packet 2                               Packet 3                               Packet 4                       Message
                                                    Packet 3                                 Packet 4                           numbers
    Received   reassembled into   Packet 3                           Packet 4                                   Packet 5
       Large    message using                       Packet 4                                 Packet 5
                   sequence       Packet 4                               Packet 5                               Packet 6
    Message    numbers to order                     Packet 5                                  Packet 6
                                  Packet 5                               Packet 6
                                                    Packet 6
                                  Packet 6




   The time delay between sending first packet
    and its delivery is much less.
         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.
      Time Sequence Diagram for
      Virtual Circuit (LLC-2)                                                               Host B
      Host A              node            switch            switch            node
                            A   Establish   1                 2                B
               L_connect_req   Connection         Establish
                                                 Connection         Establish
                                                                   Connection   L_connect_req

tim                                                                  Acknowledge       L_connect_
e                                                  Acknowledge        Connection         confirm
                               Acknowledge          Connection      Indicate Status    L_connect_
                L_connect_      Connection        Indicate Status                       indication
                  confirm
                               Indiate Status
                L_connect_
                 indication
                                 Send Data
                  L_data_                           Relay Data
                connect_req                                                               L_data_
                                                                       Relay Data       connect_req
                                                                                      L_connect_dat
                L_connect_dat                                         Confirm Data
                              Confirm Data                                              a_confirm
                  a_confirm                        Confirm Data
                                                                    Indicate Status     L_connect_
                 L_connect_                                                              indication
                                 Indiate Status   Indicate Status
                  indication
               L_disconnect_
                    req          Close
                               Connection             Close
                                                    Connection          Close
                                                                                      L_disconnect_
                                                                      Connection
                                                                                           req
                                                                                      L_disconnect_
               L_disconnect_                                          Disconnect
                               Disconnect                                                confirm
                  confirm                          Disconnect
         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
         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.
         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.
                                                    2
                                                                                    5
                                                                                        End Device
                               1
          End Device                                                                        B
                                                             3
              A

                       Routing Table for   4                                 6
                            Node 1                  Routing Table for
                        A5    3 2                        Node 3
                        A2    2                         A5    5   6
                                 3
                        A6    4                         A2    5   1
                        … … 3                           A6    6   5
                                 …                      …    …    …


                                        Packet
                                      Destination        Next Node
                                       Address                        Alternative
                                                                      Next Node
         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.
         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.
        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.
         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.
      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.
         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.
         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.
          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.
                                                         Delay

        Offered    Flow Control   Throughput   Routing
         Load




                  Rejected
                   Load
   Feedback can be used to dynamically
    control the number of packets (or frames)
    given to the sub-net. This is flow control.
         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.
         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.

                                                        More Flow
              Delay                                   Control Pressure
          (microseconds)


                           Poor Routing

                                                            Good Routing


                               Throughput (packets per second)
         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.
         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.
         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.     Weight = 3
                                   F
                                   F        Primary Route (3+1)

   The ratio of                   F
                                   F
                                   E

    (W1+E1):               A       Output               either route is taken   B
                                   Queues
    (W2+E2)                        E

    is used.     E=Empty
                                   E
                                   E
                                   F        Secondary Route (1+3)
                 F=Full
                                   F
                               Weight = 1
          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.
          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.
          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.
         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.
         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!
         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.
         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.
          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).
                        Time Sequence Diagrams for
                        Timer
                                                                           Host A                                           Host
    Host                                            Host                                    Node           Node
                     Node         Node                                                                                       B
     A                                               B


       Send Frame       Relay Frame                                           Send Frame       Relay Frame
                                      Relay Frame                                                            Frame Lost            2Propagatio
T                                                          2Propagation
                                                                                                                                   n delay
I                                                          delay
             Relay                      Send                                   Retransmit
m                          Relay
           ACK/NAK                    ACK/NAK                                    Frame
e                        ACK/NAK
                        Relay Frame
       Retransmit if                                                                           Relay Frame
                                      Relay Frame
          NAK                                                                                                 Relay Frame


                                                                                                               Send ACK
                                                                              Relay ACK
                                                                                               Relay ACK


             ACK (or NAK)
           arrives back in time                                               ACK (or NAK) does
                                                                                not arrive back
          Buffer Overflow
   Consider a situation where two sessions are
    passing packets through the same node.
                                         End Device
                                             X




                                           Slow Link
                                     2
                         Fast Link                         5
                                                                     End Device
                     1                                                   B
        End Device
            A                                 3


                             4                         6

                                                               End Device
                                                                   Y




   Node 3 will temporarily store the packets it
    receives in its buffers.
         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.
         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).
         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.

                P                 Q
              (Full)            (Full)



   Both are unable to receive packets so no
    packets move.
    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.

								
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