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Ch. 12 Routing in Switched Networks. 12.1 Routing in Circuit Switched Networks. Routing The process of selecting the path through the switched network. Two Requirements Efficiency --ability to handle expected load of traffic using the smallest amount of equipment.

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Ch. 12 Routing in Switched Networks

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Ch 12 routing in switched networks l.jpg

Ch. 12 Routing in Switched Networks


12 1 routing in circuit switched networks l.jpg

12.1 Routing in Circuit Switched Networks

  • Routing

    • The process of selecting the path through the switched network.

  • Two Requirements

    • Efficiency --ability to handle expected load of traffic using the smallest amount of equipment.

    • Resilience--ability to handle surges of traffic that exceed the expected load of traffic.


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12.1 Routing in Circuit Switched Networks (p.2)

  • Traditionally has been static hierarchical tree structure with additional high usage trunks.

  • Today, a dynamic approach is used, to adjust to current traffic conditions.


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12.1 Routing in Circuit Switched Networks (p.3)

  • Alternate Routing

    • Approach where possible routes between end offices are predefined.

    • The alternate routes are sequentially tried, in order of preference, until a call is completed.

  • Fixed Alternate Routing--only one set of paths provided.

  • Dynamic Alternate Routing--different sets of preplanned routes are used for different time periods--Fig. 12.1.


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12.2 Routing in Packet Switched Networks

  • Routing Algorithm Requirements

    • Correctness

    • Simplicity

    • Robustness--the ability of the network to deliver packets via some route in the face of localized failures and overloads.

    • Stability--does not “over react” to network changes.

    • Fairness--as related to all other users.

    • Optimality--as related to some criterion.

    • Efficiency--as related to processing overhead.


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12.2 Elements of Routing Techniques

  • Performance Criteria

    • Number of hops, cost, delay, & throughput.

    • See Fig. 12.2

  • Decision Time

    • Virtual Circuit--at connection establishment.

    • Datagram--before packet is placed in outgoing buffer.

  • Decision Place

    • Each node, central node, originating node.


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12.2 Elements of Routing Techniques (cont.)

  • Network Information Source

    • None, local, adjacent nodes, nodes along the route, or all nodes.

  • Network Information Update Timing

    • Continuous, periodic, major load change, topology change.


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12.2 Routing Strategies

  • Fixed Routing

    • A route is selected for each source-destination pair of nodes.

    • A central routing directory can then be distributed to the various nodes.

    • Routes are not changed unless topology changes.

    • Simple (advantage) but inflexible (disadvantage.)


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12.2 Routing Strategies

  • Fixed Routing Example (Fig. 12.3)

    • Refer back to the network in Fig. 12.2.

    • Central directory lists all the routing information.

    • Each column of the central directory becomes the Next Node columns in the nodal directories.


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12.2 Routing Strategies (p.2)

  • Flooding (Fig. 12.4)

    • A packet is sent out on every outgoing link except the link that it arrived on.

    • Duplicates must be discarded.

      • Hop counter could be used.

    • Very robust (advantage.)

    • High traffic loads are generated (disadvantage.)


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12.2 Routing Strategies (p.3)

  • Random Routing

    • An outgoing link is selected at random (based on a probability distribution.)

    • Requires no use of network information (advantage.)

    • Actual route will not be least-cost or minimum-hop route (disadvantage.)


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12.2 Routing Strategies(p.4)

  • Adaptive Routing

    • These algorithms react to changing conditions of the network, for example failures and congestion.

    • Advantages--can improve performance and aid in congestion control.

    • Disadvantages--complex, requires extra "overhead" traffic to collect information, and may react too quickly (unstable.)


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12.2 Routing Strategies (p.5)

  • Adaptive Routing(cont.)

    • Schemes can be characterized by

      • Source of Network Information

        • Local--Fig. 12.5 Isolated Adaptive Routing

          • Minimize Queue Length + Bias

        • Adjacent Nodes

        • All Nodes

      • Distributed or Centralized Control


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12.2 Routing Strategy Examples

  • First Generation ARPANET (1969)

    • Distributed adaptive algorithm.

    • Performance criteria--estimated delay (from queue length).

    • Version of the Bellman-Ford Algorithm.

    • Problems: did not consider line speed, queue length is not an accurate measure of delay, and the algorithm responded slowly to congestion and delay increases.

    • See Fig. 12.6, 12.7 and discussion--page380.


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12.2 Routing Strategy Examples (p.2)

  • Second Generation ARPANET (1979)

    • Distributed adaptive algorithm.

    • Performance criteria--delay (direct measurements).

    • Version of Dijkstra's Algorithm.

    • Problem: did not work well for heavy loads.


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10.2 Routing Strategy Examples (p.3)

  • Third Generation ARPANET (1987)

    • The average delay is measured and transformed into estimates of utilization.

    • The link "costs" were calculated as a function of utilization--this helped to prevent oscillations.

    • Fig. 12.8--traffic could oscillate from link A to link B and back.


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12.3 Least-Cost Algorithms

  • The Problem

    • Given a network of nodes connected by bi-directional links, where each link has a cost associated with it in each direction, define the cost of a path between two nodes as the sum of the costs of the links traversed. For each pair of nodes find the path with least cost.

  • Solutions

    • Dijkstra's Algorithm (1959)

    • Bellman-Ford Algorithm (1962)


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Dijkstra's Algorithm

  • Define:

    • N=set of nodes in the network.

    • s=source node.

    • T=set of nodes so far incorporated by the algorithm.

    • w(i,j)=link cost from node i to node j; w(i,i)=0 and w(i,j)= if the nodes are not directly connected.

    • L(n)= cost of the least-cost path from node s to node n that is currently known to the algorithm.


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Dijkstra's Algorithm (p.2)

  • Three Steps (repeated until M=N)

    • Step 1: Initialize Variables

      • T= {s}.

      • L(n)=w(s,n) for n s.

    • Step 2: Find the neighboring node (x) which has the least-cost path from node s and incorporate that node into T.

    • Step 3: Update the least cost-paths.

      • L(n)= min[ L(n), L(x) + w(x,n)] for all n  T.

      • See Table 12.2 and Fig. 12.10.


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Bellman-Ford Algorithm

  • Define:

    • s = the source node.

    • w(i,j)=link cost from node i to node j.

    • h=maximum number of links in a path at the current stage of the algorithm.

    • Lh(n)= cost of the least-cost path from node s to node n under the constraint of no more than h links.


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Bellman-Ford Algorithm (p.2)

  • Step 1: Initialize

    • L0(n)=, for all n not equal to s.

    • Lh(s) =0, for all h.

  • Step 2: For each successive h,

    • L h+1(n) = Minj [Lh(j) + w(j,n)].


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Comparison of the Algorithms

  • Dijkstra’s

    • Complete topology information is needed.

  • Bellman-Ford

    • Knowledge of link costs to each neighbor, and the current “distance-vector” of each neighbor is required.


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