Design Requirements

IACT424/924 Corporate Network Design and

Implementation

Written by Gene Awyzio September 2002

Review

Requirements Analysis Network Requirements User Requirements Application Requirements Host Requirements

Determining New Customer Requirements

Written by Gene Awyzio September 2002

Overview

Gathering and Listing Requirements Working With Users Service Metrics Characterising Behaviour Developing Performance Metrics Estimating Data Rates Comparing Application Characteristics

Written by Gene Awyzio September 2002

Gathering and Listing Requirements

Determine Initial Conditions These are the basis for the start off

any design project Initial conditions include

Type of project Initial design goals Outside forces

Written by Gene Awyzio September 2002

Gathering and Listing Requirements

Common initial constraints Funding limitations Organizational constraints Existing components

User inertia Customised applications Performance and functional limitations

Knowing initial conditions allows us to make informed design choices

Written by Gene Awyzio September 2002

Working With Users

This allows us to understand user behaviour patterns and environments Applications Usage patterns Requirements

Written by Gene Awyzio September 2002

Service Metrics

Measurable network variables Availability

% uptime or downtime Recoverability

MTBF MTBSO MTTR

Error and loss rates BER CLR CMR Frame and packet loss

Written by Gene Awyzio September 2002

Service Metrics

Capacity metrics Data Rates

Peak Data Rates (PDR) Sustained Data Rate (SDR) Minimum data rate

Data size Burst size Duration

Written by Gene Awyzio September 2002

Service Metrics

Delay metrics End-to-end, round trip, system delay Latency Delay variation (jitter)

Written by Gene Awyzio September 2002

Service Metrics

These metrics are configured and measured using network management platforms SNMP CMIP PING Pathchar

We also need to consider where in the network we want to measure each metric and potential mechanisms

Written by Gene Awyzio September 2002

Characterising Behaviour

Goal: To estimate network performance by gaining understanding of how users and their applications function across the network

Usage patterns Total number of users Frequency of use (sessions/day) Average session length (seconds) Estimated simultaneous sessions

Written by Gene Awyzio September 2002

Characterising Behaviour

Session 1

Session 2

Session 3

Session 4

Application

Sessions

Active Active Active

Active Active Active Active Active Active

Active Active Active

Active Active

Time

Number of Simultaneous sessions

FrequencyDuration

Written by Gene Awyzio September 2002

Characterising Behaviour

Application behaviour considerations Data sizes application will be processing Frequency and time duration of data

passing Traffic flow characteristics

Direction Flow pairs Multicasting

Written by Gene Awyzio September 2002

Developing Performance Metrics

Reliability/availabilityAvailability (% uptime)

AMOUNT OF ALLOWED DOWNTIME

Yearly Monthly Weekly Daily

95% 438h 36.5h 8.4h 1.2h

99.5% 43.8h 3.7h 50.5m 7.2m

99.95% 4.38h 21.9m 5.05m 43.2s

99.98% 1.75h 8.75m 2.0m 17.3s

99.99% 0.88h 4.4m 1.0m 8.7s

Written by Gene Awyzio September 2002

Developing Performance Metrics

Most systems operate at 99.95% 5 minutes downtime per week

Transients (a few seconds) such as rerouting or congestion

One minor interruption per month

Written by Gene Awyzio September 2002

Developing Performance Metrics

Effort and costs to support higher availability can skyrocket Some applications cannot tolerate any

downtime during session Remote control of vehicles

Times of high availability are known and planned for in advance

Many system outages are brief Applications stall for a few seconds These still must be accounted for in overall

availability

Written by Gene Awyzio September 2002

Developing Performance Metrics

Two guidelines for availability measurements Availability is measured end-to-end A loss of availability in any part of the

system is counted in overall availability Availability may be measured

selectively between particular users, hosts or networks

Written by Gene Awyzio September 2002

Developing Performance Metrics

General reference thresholds

99.0 99.5 99.95 99.9899.9

Testbed

Low Performance

High Performance

Written by Gene Awyzio September 2002

Developing Performance Metrics

Thresholds for delay Interaction Delay (INTD)

How long is the user willing to wait for a response Aim for 10 –30 seconds

Human Response Time (HRT) Time boundary when users begin to perceive delay INTD < HRT : Users do not perceive delay Approximately 100ms

Network Propagation Delay Depends on distance and technology

Written by Gene Awyzio September 2002

Developing Performance Metrics

Thresholds for delay

0.01 0.1 1.0 10 100

Delay (Seconds)

Human Response Time

Network Propagation Delay

Interaction Delay

Written by Gene Awyzio September 2002

Estimating Data Rates

Based upon How much you know about the

transmission characteristics of application

Accuracy of estimation Types of estimations

Peak data rate Minimum data rate Sustained data rate

Written by Gene Awyzio September 2002

Estimating Data Rates

Consideration must be given to applications with Large capacity requirements Specific capacity requirements Task completion times (TCT) for

applications May be based upon user expectations or

be set by the application

Written by Gene Awyzio September 2002

Comparing Application Characteristics

If application characteristics can be grouped then we can compare to determine thresholds

High Performance

Low Performance

Written by Gene Awyzio September 2002

Comparing Application Characteristics

The threshold settings may be arbitrary Particularly if applications form a

continuous range of delay