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• Network design principles/issues

• Network topology

• Minimum Spanning Tree algorithms

• Pricing networks

Network Design

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Design Requirements

What are the nodes to be connected?

What is the data traffic volume on each line (by application)?

What are the peak traffic volume requirements?

What is the response time required to the user, average/max?

What is the error percentage? Compression ratio?

Anticipated growth rate, yearly over next 3 to 5 years?

Security issues?

How reliable must the network be?

Any new applications likely to come on stream?

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Network Design steps

• Message Analysis / Traffic Data Analysis

• Point to Point Traffic Table

• Study service options for each pair of nodes

(e.g. Acunet [AT&T], leased line, VPN)

• Generate alternative network topologies

• Evaluate each alternative configuration in terms of cost, response time, reliability, etc.

• Select the best one.

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Price versus Usage

Usage

Price

Leased

Switched

Frame relay

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Computing Bits/sec on each link

Start with average characters per day from link traffic table.

Then add the effect of following additional factors:

1) Future growth

2) Protocol overhead

3) Transmission errors

4) Turnpike effect (usage is more than anticipated!)

5) Number of hours available per day

6) Periods of unusually high activity.

Options for periods of peak activity:

1) Tolerate some slowdown.

2) Plan based on peak characters per day.

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Message Analysis Application Name: Hotel Reservation.

Fields in the Input Message:

Field Name Average Length Max Length

Guest Name 15 chars 30 chars

Address 50 100

Arrival date 6 6

Dep. date 6 6

Payment mode 4 4

Guaranteed 1 1

Type of room 5 5 87 152

Fields in reply message:

Confirmation # 10 chars 10 chars

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Example

Say, 10,000 transactions per hour and each transaction is 1000 bytes long.

Characters per hour = 10,000,000

Account for Growth rate of 10% per year (5 year horizon):

= 16,100,000 characters per hour (over 5 year horizon)

Protocol overhead (10 bits per character): 16,100,000 X 10

= 161,000,000 bits per hour

Transmission errors (1%): 161,000,000 X 1.01

= 162,600,000 bits per hour

= 45,167 bits per sec.

If line utilization is 67%, then we need a line speed = 45,167/0.67

= 67,413 bits per sec.

51.1000,000,10

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Formulating Network Design Problems

Minimize Communications costsuch that:

1) Average response time < 1 secs2) 90% response time < 1.5 secs.3) Reliability requirements: e.g. there must be at least two paths between

any 2 points, or any X link failures should not cause any

disruption

Formulation 2

Formulation 1 (simplified version)

Minimize Total circuit mileage

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Network Topology

MULTIPOINT: connect all pairs of nodes in a minimum spanning tree. Simple design, but not very reliable. Usually least expensive. Minimizes total mileage.

RING: connect all nodes in a loop.

STAR: connect all nodes to central computer.

MESH: Multiple paths between nodes.

Topology refers to arrangement of nodes in a network

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Multipoint Topology

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RING Topology

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Ring-Based WAN Design

A ring-based WAN design connects all computers in a closed loop, with each computer linked to the next, usually with a series of point-to-point dedicated circuits.

One disadvantage is of the ring topology is that messages can take a long time to travel from the sender to the receiver.

In general, the failure of one circuit or computer in the network means that the network can continue to function. More reliable than multipoint.

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STAR Topology

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STAR Topology

STAR: connect all nodes to central computer.

• Great for centrally controlled networks• Easy to manage and fix problems• Predictable performance• Easy to expand.• Failure prone at central point• High circuit cost• Not scalable• Recommended for small networks,voice networks and

switched networks accessing backbone networks.

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MESH Topolog

y

MESH: Multiple paths between nodes.

• Resistant to nodal and link failures - highly reliable.• Relatively easy to expand and modify• However, more complex to design and manage.• Recommended for host-to-host connectivity, large multi-

application networks.• Employed by banks, financial and insurance companies,

retail operations.

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Centralized versus Distributed Network design

• Centralized network:

One central computer. Where to locate the central computer?

Location of concentrators? Multi-point line layout?

• Distributed network:

1. Develop network topology.

2. Assign traffic flows over network links.

3. Size the line capacities based on topology and traffic flows.

Need to consider:

How many computers? Where? Routing Paths? Data allocation to nodes?

Distributed network design is much harder than centralized design.

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Backbone networks

Backbone networks consists of high speed links and nodes that consolidate traffic from smaller access networks.Networks can be structured in three ways:

•Private access networks connecting to private backbone networks.•Private access networks connecting to public backbone networks.•Combination of the above.

(SMDS [Switched Multimegabit Data Service] and ATM are good public backbone options. Frame relay is a good option for both public and private backbones.)

Private backbones integrate the company's subnetworks and provide a reliable, secure and high speed means of transmission for distributed voice, data and time-sensitive applications. Public backbones are also becoming popular with VPNs (or virtual private networks).

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Network design techniques

Examples of network design techniques:

1. Minimum spanning tree to minimize circuit mileage

2. Queuing models for response time computations

3. ADD heuristic for solving location problems

4. Linear programming techniques

5. Other heuristics

Medium or large size problems cannot be solved optimally. Therefore, we often use heuristics. Sometimes, the problem must be simplified to make it tractable.

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Minimum Spanning Tree

Prim's Algorithm:

1. Start with any single site or node in the tree and add nodes one at a time.

2. At each stage, add the node nearest to the whole tree (in case of a tie choose either one randomly).

3. Stop when all nodes are included.

Problem: Connect several nodes to a central node such that total length of links is minimized.

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Minimum Spanning Tree

Kruskal's Algorithm:

1. Sort all inter-node distances in ascending order.

2. Check each potential link starting with the shortest.

3. If that link can be added without forming a circuit, add it.

4. Stop when all nodes are connected in the tree.

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Source: http://www.nwfusion.com/research/2000/0508feattele.html

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Automate

d Net

work Design

Tools

• Requirements analysis: collecting data on line types and costs, node types and costs, sources and traffic volumes.

• Topological design: Using heuristics and other techniques to produce network topology.

• Performance analysis: Assessing cost, reliability and delay for each design.

• There are several tools available in the market. Prices range from few $1000 to $100,000.

• Examples: Autonet Advisor, Performance-3,WinMIND, etc.

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Summar

y•Network design is both an art and a science.•Need to combine several techniques.•Tools of increasing sophistication available in the

market at reasonable price.•Hard to do optimal design for more than 50 node

network.

Bottom line: Good design can save a company millions of dollars per year in cost and also increase revenues.