Network Capacity Planning
IACT 418 IACT 918
Corporate Network Planning
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
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


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Cable capacity
End-to-end blocking probabilities
Delay and reliability requirements
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

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Node location
Links connecting backbone nodes
Links between end users
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
The Importance of Planning
Or more specifically:

Given
 Node locations
 Edge locations
 Traffic requirements
between 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
Working It Out
These models generally end up being
combinatorially explosive or NP
complete
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)
Working It Out
Modern network based applications
don’t follow these assumptions

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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)
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assumptions are no longer valid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
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.
2.
3.
facilities,
equipment and
evaluation
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)
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)
Facilities
For practical purposes, we can estimate the
effective capacity as about 60-70% of the
bandwidth



(See Terplan P 558-561)
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
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
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.
Evaluation
When major upgrades are considered,
there is always a choice among several
alternatives

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The different possibilities must be
developed and evaluated
Development requires thinking of the
alternatives, discussion among those
involved and may include brainstorming
Evaluation
Evaluation requires computer modelling
or simulation

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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