Capacity Planning

IACT 918 July 2004

Gene Awyzio

SITACS University of Wollongong

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The Importance of Planning

• Network capacity planning lies at the heart of all network management functions

• The design of networks to route data from one location to another with the least cost is of great concern to network designers

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The Importance of Planning

• The objective of network designers is to minimise the cost of installing and maintaining the network

• These problems are complicated by the need to consider many factors such as

– Cable capacity

– End-to-end blocking probabilities

– Delay and reliability requirements

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The Importance of Planning

• The process of designing a network requires that many variables are determined simultaneously

• In designing and upgrading a network, it is necessary to simultaneously select

– Node location

– Links connecting backbone nodes

– Links between end users

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The Importance of Planning

• Mathematically we can say that

– The objective of designing and maintaining a network is to:

• Minimise the cost of installation and maintenance

• Whilst meeting some given performance criteria

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The Importance of Planning…or more specifically:• Given

– Node locations

– Edge locations

– Traffic requirementsbetween node pairs

– Cost of transmission capacity

– Cost of node installation

• Minimise

– Total network costs

• Determine

– Network topology

– Edge capacities

– Routing policy

• Subject to

– Performance constraints

– Capacity constraints

– Reliability constraints

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Working It Out

• These models generally end up being combinatorially explosive or NP complete

– NP = Non-deterministic Polynomial-time

• To simplify the problem of solving these models several assumptions are used

– Packet (data) arrival is independent of other traffic (Poissonian arrival)

– Packet size is independent of other traffic (exponential packet size)

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Working It Out

• Modern network based applications don’t follow these assumptions

– Most applications have a real time component

– These types of applications tend to create a traffic stream that creates packets:

• At fixed time intervals (deterministic)

• Have a fixed size (deterministic)

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Working It Out

• This means that all the simplifying assumptions are no longer valid

• Also,

– potential savings can be achieved by installing excess capacity to meet future requirements

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Developing the Network Capacity Plan

• We start with a knowledge of the present network and its traffic loads etc, and our prediction of the future demands of the organisation

• We will develop the network capacity plan in three steps,

1. facilities,

2. equipment and

3. evaluation

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1. Facilities

• “A facility is a transmission path between two or more locations, not including terminating or signalling equipment. The addition of terminating equipment would produce either a channel, a central office line, or a trunk.”

(Terplan P 556)

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1. Facilities

• A facility has a theoretical maximum capacity which is given by the bandwidth

• In practice it is found to be impossible to use all the bandwidth because of:

– Overheads

– Downtime

– Utilisation ceiling (depends on burstiness)

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1. Facilities

• Capacity of facilities:– The utilization level beyond which the facility is

unable to perform work in a timely manner within service-level limits

– The facility has reached its capacity when when its utilization has attained a level at which targeted service levels cannot be met.

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1. Facilities• Utilization ceiling:

– The theoretical capacity of the facilities

• Identified by the speed of the resource• How many bits can be transmitted per second

– Bits per second on average

– Bits per second at peak loading times

• There must be no compromise on service-levels for the average load

– Some violations at peak load may be tolerated

– Depends on the patterns of usage and the culture of the organisation

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1. Facilities

• Terplan shows a graph as follows:

• We will run out of capacity when we reach the usable limit. Then we are in trouble on this facility

Reserve

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1. Facilities

• Utilization = arrival rate / service time

This yields a ratio (sometimes expressed as a percentage)

>1 = unstable, queue will just grow bigger

<1 = stable

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1. Facilities

Utilization

Tim

e

There is a ‘knee’often at around 60~70%

• As utilization increases in a stable system ( <1), if the system were purely deterministic, then time would increase linearly

• But as this assumption does not hold, there is a ‘knee’ point beyond which delays increase

idealized

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• Congestion collapse

1. Facilities

Packets sent

Pack

ets

deliv

ered

Maximum carrying capacity of subnet

perfect

desirable

congested

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1. Facilities• Response time = service time + waiting time

• Service time = transmission time + poll transmit time + select transmit time + poll/select wait times

• Waiting time = (service time x utilization) / [2 x (1 – utilization)]

• If response time < response time objective, then the utilization ceiling is acceptableotherwise, the utilization ceiling must be reduced, by installing larger capacity lines, or more lines.

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1. Facilities

• For practical purposes, we can estimate the effective capacity as about 60-70% of the bandwidth

– As the traffic approaches this maximum, the performance drops. Response time and packet loss increase

– This effective capacity is the upper limit available to the network, and we can use our estimates of future traffic to predict when we will run out of capacity

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2. Equipment

• Equipment is everything not included in the facility

• Equipment capacity can be computed in the same way as facility capacity, using overhead, downtime and utilisation ceiling.

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3. Evaluation

• When major upgrades are considered, there is always a choice among several alternatives

– The different possibilities must be developed and evaluated

– Development requires thinking of the alternatives, discussion among those involved and may include brainstorming

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3. Evaluation

• Evaluation requires computer modelling or simulation

– It can be done on a spreadsheet or a simulation package designed for the purpose (eg Arena, nm)

– The value of any of these exercises depends on the accuracy of the original information on present and future loads