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Ch. 12 Routing in Switched Networks

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Ch. 12 Routing in 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.

- Traditionally has been static hierarchical tree structure with additional high usage trunks.
- Today, a dynamic approach is used, to adjust to current traffic conditions.

- 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.

- 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.

- 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.

- 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.

- 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.)

- 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.

- 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.)

- 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.)

- 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.)

- 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

- Local--Fig. 12.5 Isolated Adaptive Routing
- Distributed or Centralized Control

- Source of Network Information

- Schemes can be characterized by

- 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.

- 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.

- 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.

- 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)

- 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.

- 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.

- Step 1: Initialize Variables

- 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.

- 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)].

- 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.