Requirements Capture and Specification

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Requirements Capture and Specification

• IACT424/924 Corporate Network Design and
  Implementation

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Overview

• Network Services
• Requirements Analysis
• Network Requirements
• User Requirements
• Application Requirements
• Host Requirements
• Determining New Customer Requirements

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Definition

• Requirement
• That which is required or needed a want, need
• That which is called for or demanded a condition
  which must be complied with.

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Network Services

• Sets of network capabilities that can be
  configured and managed within the network
• Levels of performance and function offered
• Sets of requirements expected
• For services to be useful and effective they need
  to be provisioned end-to-end

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Network Services

• Services need to be
• Configurable
• Measurable
• Verifiable
• Ensure end users are getting the services they
  what they requested
• Accounting

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Network Services

• Services are generally hierarchical
• General services in the backbone
• Specific services close to users
• Network services are derived from the
  requirements of all entities within the network
• They describe what is expected by/from each entity

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Network Services

• Network service requirements include
• User requirements
• Application requirements
• Host requirements
• Network requirements

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Network Services

• Service offerings need to be configured
  end-to-end
• Requirements add to each other filtering from
  user to network
• In addition to providing performance and function
  to users they support design and operation of the
  network

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Network Services

• Mismatches in services can result in network
  bottlenecks

T1
Ethernet
Ethernet
FW
10 Mb/s
1.5 Mb/s
10 Mb/s
200 kb/s
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Requirements Analysis

• The identification of bottlenecks like this is
  one of the primary goals of network design
• After identifying service requirements the next
  stage is to quantify what we want from the
  network
• To do this we need to analyse and define the
  requirements of the network

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Requirements Analysis

• Without adequate requirement analysis networks
  tend to be designed on factors other than users
  needs
• Requirement analysis assists in understanding the
  probable behaviour of the network

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Requirements Analysis

• Payoffs include
• Objective, informed choices of technologies and
  services
• Match of interconnection strategies to networks
• Networks and components that are properly sized
  to users and applications
• Better understanding of of where and how to apply
  services in the network

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Network Requirements

• Network designers have traditionally focused on
  providing connectivity between hosts
• Typically users and applications were not
  considered

Host
Host
Network
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Network Requirements

• This view is not complete enough for todays
  networks
• Users and applications need to be considered

Network
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User Requirements

• Users generally have the following requirements
• Timeliness
• Interactivity
• Reliability
• Quality
• Adaptability
• Security
• Affordability

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User Requirements

• We also need to know how many users are expected
  to use the system and their locations

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Application Requirements

• Whilst many applications are still best-effort
  users requirements and performance requirements
  (delay, capacity, reliability) are now being
  emphasized
• These requirements lead to a distinction between
  application that need specific service levels and
  those that dont
• Descriptions of performance requirements for
  applications will separate specified services
  from best effort

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Types of Specified Service applications

• Mission Critical
• Specified reliability
• Controlled-Rate
• Specified capacity
• Real-Time
• Specified delay

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User vs Application Requirements

• User Service Requirement
• Timeliness
• Interactivity
• Reliability
• Quality
• Adaptability
• Security
• Affordability
• User numbers
• User locations
• Expected growth

• Performance Requirement
• Delay
• Reliability
• Capacity

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Host Requirements

• Types of hosts and equipment
• Generic computing devices
• Desktop PCs
• Interface between application and network
• Servers
• Provide service to one or more users
• Impact on information flow
• Specialised equipment
• Supercomputers, mainframe, data gathering
  equipment
• Location Dependant

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Host Requirements

• Performance characteristics include
• Storage performance
• Processor performance
• Memory performance (access times)
• Bus performance

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Determining New Customer Requirements

• Identify business constraints
• Identify security requirements
• Identify manageability requirements
• Determine application requirements
• Characterise new network traffic
• Identify performance requirements
• Create a customer needs specification document

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Determining New Customer Requirements

• Identify business constraints
• Document budget and available resources
• Document project timeline
• Identify staffing requirements such as training
  or hiring

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Determining New Customer Requirements

• Identify security requirements
• Appraise security risks and determine how much
  security will be needed and of what type
• Determine requirements for for outsiders to
  access data
• Determine the authorisation and authentication
  requirements for
• Corporate branch offices
• Mobile users
• Telecommuters

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Determining New Customer Requirements

• Identify security requirements
• Identify requirements for authenticating routes
  received from access routers or other routers
• Identify requirements for host security
• Physical security of hosts
• User accounts
• Dated software
• Access rights on data

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Determining New Customer Requirements

• Identify manageability requirements
• Fault
• Accounting
• Configuration
• Performance
• Security

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Determining New Customer Requirements

• Determine application requirements
• Document names and types of new applications
• Document names and types of new protocols
• Document the number of users who will be using
  new applications and protocols
• Diagram the flow of information when new
  applications are introduced
• Identify peak hours of usage for new applications

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Determining New Customer Requirements

• Characterise new network traffic
• Characterise traffic load
• Characterise traffic behaviour including
• Broadcast/multicast behaviour
• Frame size(s) supported
• Windowing and Flow control
• Error recovery mechanisms

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Characterising Traffic Loads and Behaviour

• Approximate sizes of objects transferred across
  networks

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Determining New Customer Requirements

• Identify performance requirements
• Response time
• Accuracy
• Availability
• Maximum network utilisation
• Throughput
• Efficiency
• Latency

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Determining New Customer Requirements

• Create a customer needs specification document
• Record
• The customers requirements and constraints
• Characteristics of the existing network

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References

• Teare, D. 1999, Designing Cisco Networks, Cisco
  Press Indianapolis
• McCabe, J. 1998, Practical Computer Network
  Analysis and Design, Morgan Kaufman, San Francisco