An Application of the Five Fundamental Rules of Wide Area Network Design

1. An Application of the Five Fundamental Rules of Wide Area Network Design Henry Jacobsen November 1996

2. 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

3. 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)

4. 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.

5. Typical 1/0 DACS Locations

6. Alternative 1: Mesh Networking Minimizes Backhaul

7. Hub-and-Spoke Design Maximizes individual link efficiency

8. Spanning Tree Network Design Minimizes network spans

9. 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

10. 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 Two Axioms of Network Design

11. 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 Network Cost Considerations

12. 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.

13. 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 Top - Down Design

14. 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)

15. 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.

16. 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 Node-to-Hub Homing

17. 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

18. 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

19. 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 Bottom-Up Administration

20. 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

21. 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