3331 Ch.12 Routing in Switched Networks

8/9/2019 3331 Ch.12 Routing in Switched Networks

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Ch. 12 Routing in Switched

Networks


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12.1 Routing in Circuit Switched

Networks Routing

The process of selecting the path throughthe 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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12.2Elements 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 bedistributed 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 directorybecomes 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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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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Adaptive Routing(cont.)

Schemes can be characterized by

Source ofNetwork 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 congestionand 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 (directmeasurements).

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 transformedinto estimates of utilization.

The link "costs" were calculated as a functionof utilization--this helped to preventoscillations.

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 twonodes 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 thealgorithm.

w(i,j)=link cost from node i to node j; w(i,i)=0

and w(i,j)=g 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) whichhas 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 thanh links.


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Comparison of the Algorithms Dijkstras

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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3331 Ch.12 Routing in Switched Networks