An Application of the Five
Fundamental Rules of
Wide Area Network Design
Henry Jacobsen
November 1996
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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
Digital Access Cross-connect System
DS-3s
DS-3s
DS-1s
DS-1s
A 3/1 DACS allows DS-1s to be rearranged and
redistributed among various DS-3s. A 3/1 DACS
has both DS-3 and DS-1 interfaces.
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Typical 1/0 DACS Locations
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Alternative 1: Mesh
Networking
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Minimizes Backhaul
Hub-and-Spoke Design
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Maximizes individual link
efficiency
Spanning Tree Network
Design
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Minimizes network spans
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
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
Relative cost/mile
and price/mile tends
to decrease as a
function of
bandwidth
»
»
equipment costs
general pricing
Network Cost Considerations

Network Hubbing
»
»

Number of ports
Type of ports
Cost Elements
»
»
»

Fixed costs
Recurring costs
Reconfiguration
charges
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Mileage Elements
»
»

Route miles
Cost per channel
Cost Elements
»
»
»
Fixed costs
Recurring costs
Reconfiguration
charges
Top-Down Design
Major nodes and major routes
tend to account for the majority
of 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
»
»

4-Intermediate
Routes
»
»
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Simple spanning tree
Community of
interest, costs
May cause design
iteration
Avoid over designing
the network
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
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
Cost Factors
»
The cost of facilities
from node to hub can
over-ride traffic
issues
Joe Weber’s Law of
Networking
All networks cost the same
Weber’s
law presumes a good basic design
There are many designs having a similar cost
There is a law of diminishing return in
network design
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Jacobsen Law of Networking
Network administration is
more 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
3-Excessive overflow
analyzed on the
routing is an
basis of point-toindication of poor
point demand
administration
2-Augments are
4-Intermediate routes
always made to firstwill be added as a
choice routes
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

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