1. © 2006, Monash University, Australia CSE4884 Network Design and Management Lecturer: Dr Carlo Kopp, MIEEE, MAIAA, PEng Lecture 4 Design Strategies

2. © 2006, Monash University, Australia Design Strategies What is the basic networking technology used? What is the characteristic topology? What is the cabling or wireless link technology? Limitations vs Strengths? How do I design a network? 1. Identify constraints – user needs, budgets, technology 2. What legacy infrastructure can I exploit – cables etc? 3. Top down approach – model loads vs performance to define link / switch / router capacities 4. Design for evolution – project future growth and upgrades. 5. Design for maintainability, reliability, supportability

3. © 2006, Monash University, Australia What is Design? Wikipedia: Design, usually considered in the context of the applied arts, engineering, architecture, and other such creative endeavours, is used as both a noun and a verb. "Design" as a verb refers to the process of originating and developing a plan for a new object (machine, building, product, etc.). As a noun, "design" is used both for the final plan or proposal (a drawing, model, or other description), or the result of implementing that plan or proposal (the object produced).applied artsengineeringarchitecturecreativenoun verb Designing normally requires considering aesthetic, functional, and many other aspects of an object, which usually requires considerable research, thought, modeling, iterative adjustment, and re-design.aesthetic functionalresearchthought modeling Design as a process can take many forms depending on the object being designed and the individual or individuals participating.process

4. © 2006, Monash University, Australia Design In systems engineering and networking, design is a very systematic process, performed step by step. The technology in use will usually impose limits on ‘creativity’ for the designer. In many engineering disciplines, design is codified by standards or design rules, which impose safety margins and ‘headroom’ in the design. This is to protect against poor design and failure. A designer must often invest considerable time, effort and resources to establish a good design for a system. The difference between a good design and a poor design can be dramatic. Poor design usually results from poor understanding or attempts to do more than the budget permits.

5. © 2006, Monash University, Australia The Design Process (1) The first step in any design is to establish and understand what the system must achieve in terms of capability and performance. Demand for network performance usually grows over time; Available optical fibre capacity grows over time (Bandwidth Law – ‘doubling time of approximately two years ‘ for optical fibre). Understanding capacity needs and performance limits extends to understanding how they will also grow over time for the network being designed. Good design allows for low cost incremental growth in capacity as the load evolves over time, and technology evolves over time. Often testing and measurement is required to establish or validate capability and performance needs.

6. © 2006, Monash University, Australia The Design Process (2) The second step in the design process is to survey the technology available which can be used to implement a design. This means collecting information on the performance, capabilities and costs of all of the components which might be used in the design. For a network this means routers, switches, cables, management software, and other equipment. Knowing the strengths and weaknesses of specific products is critical to success. Vendors will inflate strengths and conceal weaknesses or limitations. Often testing is necessary to validate survey analysis, as limitations of components may not be obvious from product documentation or marketing literature.

7. © 2006, Monash University, Australia The Design Process (3) Once we have understood the demands / needs / requirements for the product, and the capabilities of the components available for use in a design, we are in the position to consider specific designs. The third step in the design process is to define a ‘conceptual design’ which accounts for the performance / capability needs, and the available technology. The conceptual design lacks the detail of a final design, but is detailed enough to construct a performance model for the product. The performance model reconciles demands / needs / requirements against components put together in a fashion reflecting a design.

8. © 2006, Monash University, Australia The Design Process (4) Once we have a performance model we can perform simulations to establish whether the conceptual design is good enough, or too good. Design is usually iterative, since the mathematics of modelling such systems are usually intractable and finding a perfect solution in one pass impossible. The conceptual design is repeatedly adjusted, and tested by simulation, until an acceptable result is found. Often a conceptual design cannot meet needs, and must be replaced completely. In most designs, performance must be reconciled against cost. Bad conceptual design results in a bad final design which is often impossible to correct easily.

9. © 2006, Monash University, Australia The Design Process (5) Once an acceptable conceptual design is established, we can proceed to the detail design. In a detail design, assumed system components, such as generic routers, switches etc, are replaced with real components. Simulations and testing are then required to validate the detail design, and confirm that it can fit the performance model which was met by the conceptual design. Often the largest amount of effort expended in the design process is in the detail design. When the detail design is completed and costed, we have a product which can be implemented.

10. © 2006, Monash University, Australia Implementation As future network managers you are likely to end up not only designing a netowrk, but also building it and performing commissioning for service entry. Implementation involves procurement of the required hardware, software, services required to construct the product, in this instance a network. Once the network is constructed, it must be ‘acceptance tested’ to confirm performance and compliance. The commissioning process involves such testing, and progressive ‘sign off’ of network components. Once the network has been commissioned, it can be handed over to end users. Acceptance testing usually exposes mistakes in design and implementation, and is absolutely critical to success.

11. © 2006, Monash University, Australia Constraints – Needs, Budgets, Technology The first two steps in the design process amount to collecting and analysing information on constraints upon the design. Constraints are ‘hard limits’ you cannot cross without penalty. Constraints might be the performance of available equipment, rental costs on high speed links, user performance limits such as response time or loads. The task of the designer is to find some configuration of components which fits inside the envelope of the constraints. Most often the budget is the biggest constraint and the cause of most design failures. Designers are often asked to produce designs which cannot be implemented within the available budget.

12. © 2006, Monash University, Australia Legacy Infrastructure More than often a new network design is a replacement for an existing design. This may present opportunities to save a lot of money, or force significant additional expenditure. Where existing cabling, racks, air conditioning, and rooms may be reused, new replacements are not needed and money is saved. Often legacy infrastructure is not adequate and must be replaced partly or completely, increasing costs. Sometimes reuse of legacy infrastructure is mandated as a constraint, but that infrastructure is not good enough and must be replaced. This can be the cause of a design or project failure.

13. © 2006, Monash University, Australia Top Down Strategy vs Bottom Up Strategy A top down strategy in a design means that the designer starts with a system level model, and works down into the detail progressively. A bottom up strategy is where the designer first concentrates on the detail design and then attempts to construct the system level design. Some design problems force a bottom up strategy, but network design is usually not such a problem. Bottom up design is risky since required system level performance may be impossible to achieve. Inexperienced or poorly taught designers often prusue bottom up design strategies since this defers difficult modelling tasks.

14. © 2006, Monash University, Australia Design for Evolution (1) Evolution is a fact of life in the technological world. Technology evolves continuously, typically becoming cheaper and more capable over time. Moore’s Law - http://www.ausairpower.net/OSR- 0700.html ; http://www.ausairpower.net/AC-0700.htmlhttp://www.ausairpower.net/OSR- 0700.htmlhttp://www.ausairpower.net/AC-0700.html Bandwidth Law - http://www.useit.com/alertbox/980405.html ; http://www.ntia.doc.gov/top/conferenceworkshops/2001_ outreach_workshops/three_laws.html http://www.useit.com/alertbox/980405.html http://www.ntia.doc.gov/top/conferenceworkshops/2001_ outreach_workshops/three_laws.html A good design must account for evolving user demand and evolving technology. Failure to account for evolution results in poor designs and expensive fixes.

15. © 2006, Monash University, Australia Design for Evolution (2) User demand in any growing organisation will grow in proportion to the number of users. The applications users employ will usually demand more capacity over time – eg web servers. Any website on a local network visible to the W3 will usually experience increasing traffic loads over time. A designer cannot take today’s load statistics as a measure of future performance needs. It is necessary to model future traffic loads, accounting for past growth history, and likely changes in user needs and public demand, where applicable. A example model would be a non-linear regression fit to statistical load data for the last 3 years, to estimate for coming years.

16. © 2006, Monash University, Australia Maintainability, Reliability, Supportability Maintainability is a measure of how easily components can be repaired or replaced. Maintainability impacts running costs via manhours expended per maintenance action. Reliability is a measure of how frequently (and severely) the network breaks down requiring repair. Reliability impacts running costs via the frequency and severity of maintenance actions, and customer satisfaction. Supportability is a measure of how easily replacement components can be acquired to maintain a network, and becomes increasingly difficult as equipment become obsolete and parts difficult to source. These three items are critical design constraints, but are often absent in statements of performance and capability need. A good designer must account for them.

17. © 2006, Monash University, Australia Questions Designers Must Ask What is the basic networking technology used? What is the characteristic topology? What is the cabling or wireless link technology? Do I have choices between technologies and topologies? Does the client have preconceived ideas in terms of basic technology, topology and links to be used? How big is the budget relative to the demand/need? How sensitive is the client’s application to performance shortfalls? How sensitive is the client’s budget to cost overruns? How technically literate is the client? How much risk is involved in the design?

18. © 2006, Monash University, Australia Tutorial Discussion / Reading Designing a point to point communications links Bandwidth issues vs data rate Power budgets vs link distance Copper links – pairs, coaxial links Optical Fibre links – multimode vs single mode Microwave links - propagation