Ch. 12 Routing in Switched Networks

12.1 Routing in Circuit Switched Networks 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.

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.

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.

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.

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.

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.

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.)

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.

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.)

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.)

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.)

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

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.

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.

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.

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)

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.

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.

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.

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)].

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.