Henry Jacobsen

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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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Transmission SystemsDS-0 and DS-1 Relationship
Channel Bank
1
1.544 mb/sec
64 kb/sec
DS-1 (signal) T-1 (carrier system)
24
8 kHz sampling 8 bits/sample Mu-law companding
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Transmission SystemsDS-1 and DS-3 Relationship
M13 Multiplex
1
1.544 mb/sec
44.736 mb/sec
DS-1
DS-3 (signal) T-3 (carrier system)
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1/0 DACS
Digital Access Cross-connect System
1/0
DS-1s
DS-1s
3/0
DS-3s
DS-3s
A 1/0 DACS allows DS-0s to be rearranged and
distributed among various DS-1s. A larger 1/0
DACS may have a DS-3 interface, in which case it
is referred to as a 3/0 DACS.
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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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Multiplexed PCM NetworksTypical Facility Layout
3/0
1/0
F/R
3/1
Fiber Optics Transmission Systems
Local Access
Telephone Switch
Telephone Switch
Local Access
C/B
ATM
3/1
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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 link efficiency
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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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Hub Meshing
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Meshed-Hub Design
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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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1/0 DACS -- Reduced Problem
A
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DS-0 Point-to-Point Demand
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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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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
• 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 Webers Law)

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Hub Selection
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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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Traffic-Based Homing
A
102 206 81 161 118 185 245 162 139 306
A-C B-C E-D F-D G-H I-H J-L K-L M-N O-N
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Meshed Hubs and Homing
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Economic Alternate Routing
Assume multiple DS1s required a-c and c-b...
b
c
2.60/DS1
Economic Cross-over a-b
2.20/DS1
2.40/DS1
a

12 DS0s

4.80 buys me 24 DS0s on the route a-c-b. There
must be at least 12 circuits on the route a-b to
break even.
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Economic Alternate Routing
Assume multiple DS1s between a-c and b-c. Assume
30 DS0s required between a-b.
b
c
2.60/DS1
2.20/DS1
2.40/DS1
a
Design alternatives a-c-b only (30/24 x
4.80) 6.00 a-b only (2x2.40) 4.80 a-b
(1) and a-c-b (6/24) 2.40 1.20 3.60
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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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First 10 Intermediate Routes
A
116 106 102 71 67 63 62 61 55 53
L-O J-N H-O M-O K-O D-O H-J A-B I-N E-H
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Second 10 Intermediate Routes
A
49 44 43 42 41 40 38 34 31 31 31
B-N I-L D-G B-D F-N J-O B-H C-O A-D A-N J-K
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Savings Vs. Complexity
30
35
40
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Groups
1.22
1.13
1.09
1.06
1.04
1.02
1.00
Base
Next 5
Next 5
Next 5
Next 5
Next 5
Optimal
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Joe Webers Law of Networking

• All networks cost the same
• Webers 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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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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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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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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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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Hierarchical Routing
c
b
a
d
For traffic from a to b Choice 1 a -
d Choice 2 a - c - d Choice 3 a - b -
d Choice 4 a - b - c - d
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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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Isaac Asimov
I have seen the future.... and it computes.
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Henry Jacobsen
I have seen the future.... and it networks.