An Application of the Five Fundamental Rules of Wide Area

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

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 2 LDDS WorldCom

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) 3 LDDS WorldCom

7 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. LDDS WorldCom

Typical 1/0 DACS Locations 9 LDDS WorldCom

Alternative 1: Mesh Networking 10 LDDS WorldCom Minimizes Backhaul

Hub-and-Spoke Design 11 LDDS WorldCom Maximizes individual link efficiency

Spanning Tree Network Design 12 LDDS WorldCom 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 13 LDDS WorldCom

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 16 LDDS WorldCom 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 17 LDDS WorldCom 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. 21 LDDS WorldCom

Top - Down Design 1-Hub Selection » Simple spanning tree » Community of interest, costs » (Heuristic) » Size, location » Connection costs 2-Hub Meshing » Dictated policy » Essential for reliable routing 22 LDDS WorldCom 3-Homing 4-Intermediate Routes » 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) 23 LDDS WorldCom

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. 25 LDDS WorldCom

Node-to-Hub Homing Traffic Volume » Community of Interest » Intra-regional Overflow Server » Inter-regional Traffic Aggregator 26 LDDS WorldCom 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 35 LDDS WorldCom

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 36 LDDS WorldCom

Bottom-Up Administration 1-Growth is always analyzed on the basis of point-topoint demand 2-Augments are always made to firstchoice routes 38 LDDS WorldCom 3-Excessive overflow routing is an indication of poor administration 4-Intermediate routes will be added as a network grows in size

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 40 LDDS WorldCom

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 42 LDDS WorldCom