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Henry Jacobsen

November 1996

An Application of the Five Fundamental Rules of

Wide Area Network Design

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The Five Fundamental Rules...

All networks become hierarchies Networks are designed top down Administration is more important than

design Networks are administered bottom up Routing rules must be defined and

followed

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Types of Networks

Mux and 3/1 DACS Network (DS-3s) 1/0 and 3/0 DACS Network (DS-1s) Telephone Network (Circuit Switched) Store & Forward (Message Switched) X.25 and Frame Relay (Packet Switched) ATM (Cell Switched)

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3/1 DACS

DS-3s DS-3s

DS-1s DS-1s

A 3/1 DACS allows DS-1s to be rearranged andredistributed among various DS-3s. A 3/1 DACS

has both DS-3 and DS-1 interfaces.

Digital Access Cross-connect System

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Typical 1/0 DACS Locations

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Alternative 1: Mesh Networking

Minimizes Backhaul

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Hub-and-Spoke Design

Maximizes individual linkefficiency

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Spanning Tree Network Design

Minimizes network spans

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The Five Fundamental Rules...

All networks become hierarchies Networks are designed top down Administration is more important than

design Networks are administered bottom up Routing rules must be defined and

followed

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Two Axioms of Network Design

The efficiency of a path increases with traffic intensity» Random traffic» Stochastic serving

processes» This is sometimes

referred to as the fundamental rule of traffic engineering

Relative cost/mile and price/mile tends to decrease as a function of bandwidth» equipment costs» general pricing

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Network Cost Considerations

Network Hubbing» Number of ports» Type of ports

Cost Elements» Fixed costs» Recurring costs» Reconfiguration

charges

Mileage Elements» Route miles» Cost per channel

Cost Elements» Fixed costs» Recurring costs» Reconfiguration

charges

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Top-Down Design

Major nodes and major routestend to account for the majorityof network costs. An optimal design is dominated by how

this traffic is served.

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Top - Down Design

1-Hub Selection» (Heuristic)» Size, location» Connection costs

2-Hub Meshing» Dictated policy» Essential for reliable

routing

3-Homing » Simple spanning tree» Community of

interest, costs

4-Intermediate Routes» May cause design

iteration» Avoid over designing

the network

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Hub Selection

Dominant (largest) nodes in the network High connectivity to other nodes Good geographic coverage to reduce

network backhaul The number hubs is important but not

critical (See Weber’s Law)

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Hub Meshing

Rule: All high-level nodes are meshed.» Proper hub selection should guarantee these

to be the most efficient routes in the network.» Mesh serves as primary or overflow routes for

subtending nodes on different hubs.» Mesh routing eliminates excessive switching.» Mesh is essential for robust (fault tolerant)

routing.

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Node-to-Hub Homing

Traffic Volume» Community of

Interest» Intra-regional

Overflow Server» Inter-regional Traffic

Aggregator

Cost Factors» The cost of facilities

from node to hub can over-ride traffic issues

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Joe Weber’s Law of Networking

All networks cost the same

Weber’s law presumes a good basic designThere are many designs having a similar costThere is a law of diminishing return in network design

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Jacobsen Law of Networking

Network administration ismore important than design Design data will always be lacking or in error Sensitivity analysis to mis-administration Ease of administration and redesign is critical

to controlling cost and performance

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Bottom-Up Administration

1-Growth is always analyzed on the basis of point-to-point demand

2-Augments are always made to first-choice routes

3-Excessive overflow routing is an indication of poor administration

4-Intermediate routes will be added as a network grows in size

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Network Routing Rules

Rules preserve network economics Rules define network robustness Rules provide accurate growth planning Rules prevent networking difficulties

» e.g.. Circular Routing» Avoid overly-complex routing rules» How deep??? Typically four or less

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The Five Fundamental Rules...

All networks become hierarchies Networks are designed top down Administration is more important than

design Networks are administered bottom up Routing rules must be defined and

followed