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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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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 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.