The
network capacity is a keyword for efficient network and operator profit. But
there is a very heavy tradeoff that should be solved in order to achieve those
profit targets – from one hand this is a traffic losses and customer
satisfaction due to Quality of Service and from other hand there are
investments for network infrastructure expansion (Capital Expenses – CapEx) and
deployment limitations.
Correctly
designed network capacity will prevent traffic losses, will provide high
Quality of Service for end users and will minimize investments into network
expansion once network will grow.
There are few different techniques that providing
an ability to estimate network capacity. But prior to network capacity
calculation some important criteria should be defined in order to provide
correct results.
CAPACITY
AND QUALITY OF SERVICE
The
Quality of Service definition becomes very complicate for modern GSM networks
due to various traffics' types. A mix of signaling, voice and data traffics
causing headache for network operation departments due to fact that end user
Quality of Service cannot be measured efficiently.
The most common approach to measure Quality of
Service in the network is to define a certain set of Key Performance Indicators
(KPIs) and monitor them over the time. Once some KPIs will exceed predefined
thresholds, then a certain action should be performed in the network in order
to bring this KPI back to normal behavior. This process very often called
"Optimization".
Some
of well known KPIs are representing a circuit switched network capacity issues:
·
SDCCH
Congestion Rate
·
TCH
Congestion Rate
It
is very important to note that in order to estimate an end user perceived
Quality of Service, mentioned above KPIs should represent only congestion
events that affecting the end user experience and not a condition of "no
available resources" at specific network element. In other words, the end
user related congestion is a ratio between blocked and initiated call attempts
(Call Congestion), while often many infrastructure vendors are suggesting usage
of "Time Congestion" KPIs that are representing only network resources
conditions and not an end user experience.
Mentioned above KPIs may point out on specific
bottle neck in the circuit switched network and initiate a troubleshooting
activity. In case of capacity related KPIs a typical troubleshooting activity
is a network resource expansion i.e. new equipment installation.
The
Quality of Service definition for packet switched data services is much more
complicated. This is due to reason that different data services have different
requirements for Quality of Service. The end user perceived experience for data
services can be represented by two measures:
·
Service
end - data throughput
NETWORK UTILIZATION
In
order to provide a short cycle for Return of Investments (ROI), each operator
should closely evaluate the efficiency of each network element. It is very
important that already deployed infrastructure will be fully utilized for
revenue generating services.
For cell efficiency evaluation the cell
utilization KPIs should be monitored. The definition of utilization KPI is
pretty simple and described as ratio between amount of served traffic and
available cells' resources (timeslots).
SIGNALING CAPACITY
The
signaling processes in GSM network are responsible for the following
activities:
·
Mobility
Management
·
SMS
sending
·
Call
Setups
It is possible to differentiate signaling traffic
according to revenues, where SMS and Call Setups are revenue generators, while
Mobility Management signaling traffic is consuming network resources without
any revenue generation. It is very important that Mobility Management signaling
traffic will be as lower as possible and will not occupy network resources that
can be used for revenue generating traffic types. Excessive Mobility Management
signaling traffic will lead to increase of network resources utilization and
may affect Quality of Service for end users or even lead to higher requirements
for new hardware.
CIRCUIT SWITCH CAPACITY
Circuit switched cell capacity typically estimated
by Erlang B calculation, where each combination of timeslot configuration and
given Grade of Service provides the amount of offered traffic:
There
is a relation between Offered traffic and Served traffic:
Served
Traffic = Offered Traffic/(1+GoS)
Since Erlang B is just a mathematical model, it
does not represent an instant traffic load and end user behavior.
PACKET SWITCH CAPACITY
By
definition the packet switching services in GSM network performing according
Best Effort approach, where circuit switched traffic has priority over packet
switched. Due to instant voice traffic behavior there is always can be a
situation where circuit switched traffic occupies all available cell's resources
and packet switched traffic cannot be served.
The following graph shows typical instant traffic
behavior and resources availability for packet switched (PS) traffic: