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Capacity Planning of GSM Data Service

Timo Virtanen

Department of Computer Science, University of Helsinki

February 26, 1999

Abstract

New mobile data services are being introduced to GSM system. High Speed Circuit Switched

Data (HSCSD) and General Packet Radio Service (GPRS) are the first steps towards higher

data speeds over GSM. Standardisation of HSCSD began during 1994, and first networks

supporting HSCSD were commercially available at the end of 1998. HSCSD enables circuit

switched data transfer over GSM system. The standardisation work on the GPRS Phase1 was

officially finalised in the first quarter of 1998 at ETSI and the standardisation work on the

GPRS Phase2 is still going. GPRS enables packet switched data transfer over GSM system.

This paper presents first generally a preliminary network planning process for GSM speech.

After that the characteristics of HSCSD and GPRS services are presented and the things related

to their capacity calculation are discussed.

1. Introduction

1.1 Dimensioning GSM speech

The network dimensioning is a process of analysis and comparison of different alternatives for

building a cellular network in order to satisfy given requirements for coverage, quality and

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capacity. There are several parameters that are used as input for dimensioning and that should

be known or estimated, such as network area size, distribution of the subscribers, expected

traffic load, available frequency band, etc. The output of the process is an estimate of the

equipment needed for the network development.

In dimensioning the whole geographical area is divided into the regions based on radio wave

propagation and traffic distribution. Traffic distribution should be flat in each region and

propagation environment should not vary too much. If these two assumptions are not valid in a

region, the region should be divided into smaller regions to fulfil the assumptions.

The three main concepts that are used to describe the composition of the cellular network are

site, cell, sector and transceiver-receiver (TRX). Usually a network consists of more than one

sites. One site can be divided into several sectors. Each sector can contain one or several TRXs

and each TRX operates on a specific frequency. A cell is more like a logical concept that is

used to divide the total geographical area into location areas. If mentioned in this document, a

cell equals to a sector. A site with three sectors (cells), one with three TRXs and two with two

TRXs is depicted in Figure 1.

TRX 1TRX 2TRX 3

TRX 1TRX 2

TRX 1TRX 2

Figure 1. A site with three sectors

In order to be able to estimate the transmission capacity need of a network, the number of sites

and the transmission capacity of one individual site (i.e. the number of TRXs per site) should

be known. This is done by coverage and capacity calculations. Coverage calculation calculates

the number of sites that are needed to cover the geographical area. Capacity calculation

calculates the number of sites that are needed for the estimated traffic load in the area. The total

number of sites required is then the number that is bigger, either the number of sites for

capacity (capacity limited network) or the number of sites for coverage (coverage limited

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network). A new network is usually capacity limited in urban areas and coverage limited

outside of urban areas.

1.2 Capacity calculation

Network traffic is not evenly spread. There are traffic peaks, "busy hours", usually around 10

a.m. and 3 p.m. but as the number of non-business subscribers increase, traffic gets more and

more evenly spread over time. A network has to be dimensioned according to the "busy hour"

traffic.

In order to simplify the capacity calculation some assumptions should be made. Firstly, traffic

is evenly distributed over the target area. Secondly, all the subscribers in the target area share

the same quality targets. This means that the base stations have some common parameters (e.g.

location probability, blocking probability, bandwidth, frequency reuse number). In addition to

that there are some parameters that should be defined individually for different type of base

stations (e.g. propagation model, output power and antenna height).

The purpose of the capacity calculation is to define the number of TRXs needed to handle

certain traffic load with given blocking probability. The number of carriers, and hence the

number of channels, that are available depends on the available bandwidth. In GSM system

each channel is allowed to use 200 kHz. For example if the bandwidth is 5 MHz, the number of

channels is 25. The number of carriers that can be used per sector depends on the frequency

reuse number. Each TRX operates on certain carrier. Again, if the bandwidth is 5 MHz and

reuse is 12, the number of TRXs per sector is 2.08 according to the equation below.

5

0 2 122 08

MHz

MHz..

⋅≈

The number of TRXs per sector and blocking probability define the amount of traffic that one

sector can handle. The number of the timeslots per TRX that are allocated for traffic depends

on the planning solution and should be noticed. Traffic per one sector with given blocking

probability can be calculated by using Erlang B formula, see Table 1 below for some example

values.

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Blocking probability means the probability that a call generated by MS is blocked because all

the possible traffic channels are reserved. GSM mobiles generate call requests for circuit

switched calls as in ordinary telephony by a Poisson process. These GSM calls have an

exponentially distributed holding time with a mean value of 50 seconds. Blocking probability

for one timeslot speech call is estimated from Poisson distribution. The unit of traffic is called

Erland (Erl) and 1 Erl equals to the amount of traffic that is carried by one channel during one

hour. The number of the signaling timeslots per TRX, and thus the number of the traffic

channels per TRX, depend on the planning solution. Blocking values used for the air interface

vary normally been between 1% and 5%. As a network evolves, the number of subscribers will

grow, while the average traffic intensity per subscriber will gradually decline over time. At

network startup, the traffic per subscriber is typically around 18-20 mErlangs, and this declines

to 12-13 mErlangs as the network matures [GSC98].

TRXs TimeSlots

TrafficTSs*

SignallingTSs*

Traffic(1%)

Traffic(2%)

Traffic(5%)

1 8 7 1 2.5 2.9 3.7

2 16 15 1 8.1 9.0 10.6

3 24 22 2 13.7 14.9 17.14 32 30 2 20.3 21.9 24.8

5 40 38 2 27.3 29.2 32.6

6 48 45 3 33.4 35.6 39.5

7 56 53 3 40.6 43.1 47.5

8 64 61 3 47.9 50.6 55.69 72 69 3 55.2 58.2 63.7

10 80 76 4 61.7 64.9 70.8*TS = time slot

Table 1. Mapping TRXs, timeslots and traffic (in Erlangs).

If the number of sectors per site is not known some estimate (e.g. weighed average, see Table 2

below) can be used and the traffic per sector and thus per site can be calculated. Then, knowing

the traffic per site and the total traffic, the minimum number of sites for capacity can be

calculated.

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Site configuration (number of sectors per site) 1 2 3

Percentage of this type of sites 10% 30% 60%

Weighed number of sectors per site 1*0.1+2*0.3+3*0.6 = 2.5

Table 2. Weighed number of sectors per site

After the total number of sites is calculated and hence the total number of TRXs and channels

are known, the number of needed upper level controllers (BSCs) and switches (MSCs) can be

calculated. The number of needed BSCs depends on how many TRXs one BSC can handle.

The capacity of the BSCs should be decided, thus should there be fewer high capacity BSCs or

several low capacity BSCs. This decision depends naturally on the possible locations of the

network elements. The number of MSCs is mainly dependent on the number of subscribers in

its serving area but also on the services it is expected to provide.

2. GSM Data Services

2.1 High speed circuit switched data

With the idea with HSCSD is to introduce data services in GSM with some software updates,

but without any major hardware changes. Higher data throughput is achieved in two ways:

using higher coding efficiency over air interface and using multiple channels (TDMA

timeslots) for one connection. Thus the data is carried through multiple channels within circuit

switched speech traffic.

Figure 2 illustrates the concept of HSCSD. In the figure seven vertical timeslots correspond to

traffic channels in a TDMA frame (carrier). The content of the TDMA frame changes with

time. White boxes represent speech traffic and blue boxes represent HSCSD traffic. If there are

four timeslots allocated for HSCSD traffic this could mean for example that there are two

connections having two timeslots each or there could also be four timeslots allocated to one

connection.

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7

6

5

4

3

2

1

HSCSD Traffic

Speech Traffic

Time

Figure 2. The concept of HSCSD.

HSCSD offers two different kinds of connections, transparent and non-transparent.

Transparent connection means that the number of the timeslots is fixed during the entire

connection. This gives a constant bitrate and transmission delay. There is no error correction

made by the network so the end application must take care of error correction.

Non-transparent connection allows the number of allocated timeslots to be changed during the

call. More timeslots can be allocated if some resources are released from other calls. The

number of timeslots can be decreased for example if current traffic load is high and there are

not enough free timeslots left for speech traffic (speech traffic should have priority over data

traffic). Increasing and decreasing the number of timeslots are called resource upgrading and

resource downgrading procedures, respectively. Error correction (i.e. retransmission of the

frames) is done by the network so the data rate seen by the end application can vary.

Maximum user data rates depend on used service. With non-transparent service, one connection

can have up to four timeslots simultaneously. If 14.4 kbps timeslots are used, the four timeslot

configuration results 57.6 kbps data rate. The maximum data rate for transparent service is 64

kbps, which also yields maximum four timeslots. The maximum amount for used timeslots is

not limited in transparent service by the specification. The limit for 64 kbps is set by the current

infrastructure, since BSC is connected to MSC with 64 kbps transmission link.

The intention of using multiple timeslots is to keep the access, signaling and transmission

mostly unchanged and simply split the data into several parallel streams for transmission and

combine them at the other end. For physical constraints, all times slots associated to an HSCSD

connection must belong to the same carrier. Where the timeslot are allocated (in consecutive or

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non-consecutive timeslots) is dependent on the timeslot allocation procedure. The data rates of

the different coding schemes are shown in Table 3. [GSM 02.34] [GSM 03.34]

Timeslotsused

9.6 kbpschannel coding

14.4 kbpschannel coding

1 9.6 kbps 14.4 kbps 4 38.4 kbps 57.6 kbps

Table 3. Data rates with 1-4 timeslots for transparent service.

2.2 GPRS

GPRS provides packet switched connections between GSM system and external packet

switched networks. With GPRS a user can access the standard data networks, such as TCP/IP

and X.25, directly using their standard protocol addresses, which can be activated when the MS

is attached to the GPRS network. GPRS has four different air interface coding schemes which

have different throughput and error correction capabilities.

GPRS MS can use from one to eight timeslots for transferring the data, depending on the

capacity of the MS. Packet switched GPRS traffic is transferred by using the timeslots that are

not used by circuit switched traffic at the moment. Circuit switched traffic load can consist of

GSM speech and data traffic and has always priority over GPRS traffic. This means that

introducing GPRS will not reduce the quality of service given for the subscribers that are using

circuit switched services. However, in order to guarantee some minimum quality of service for

GPRS users it is possible to allocate a number of timeslots per cell that can be used only for

GPRS traffic. Figure 3 below illustrates the concept of GPRS. In the figure there are seven

traffic channels per carrier and one traffic channel is permanently allocated for GPRS traffic.

[GSM 03.60]

8

7

6

5

4

3

2

1

1 TS reserved for GPRS

GPRS Traffic

Circuit Switched Traffic

Time

Figure 3. The concept of GPRS.

GPRS supports both symmetric and asymmetric connections, which means that timeslots for

uplink and downlink are allocated separately making it possible that they have different

amounts of timeslots in use. For example, if MS is connected to the Internet more capacity

(timeslots) is needed on downlink direction.

GPRS standard determines four different air-interface coding schemes, CS-1, CS-2, CS-3 and

CS-4. CS-1 has the highest error correction and the lowest throughput, while CS-4 has no error

correction and the highest throughput. The data rates of the four coding schemes for one and

eight timeslots are shown in Table 4. [GSM 03.60]

Channel coding scheme CS1 CS2 CS3 CS4

1 timeslot 9.05 kbps 13.4 kbps 15.6 kbps 21.4 kbps

8 timeslots 72.04 kbps 107.2 kbps 124.8 kbps 171.2 kbps

Table 4. The data rates for different coding schemes.

When the vendors are initially introducing GPRS they want, most probably, offer a cost-

effective GPRS solution without large-scale investment for operators. Because the coding

schemes CS-3 and CS-4 do not fit into 16 kbps Abis-interface, only the coding schemes CS-1

and CS-2 will probably be implemented in the first GPRS versions.

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2.3 Capacity calculation

When introducing GSM-data services, the capacity of the underlying GSM network has to be

studied. It has to be determined if the GSM network is able to provide sufficient capacity also

for data services. The total data traffic load should be estimated based on the percentage of data

users and the traffic load per subscriber during peak hour. Perhaps in the beginning, when the

number of data users is low, it is possible to introduce data services without having to add new

sites for capacity. But later on as the number of data users increase, some new capacity

enhancements have to be made to the network.

As mentioned before the speech traffic load can be defined in Erlangs per subscriber. One

Erlang equals to the amount of traffic that one timeslot can carry during one hour with given

blocking probability. When introducing data services, there will be speech traffic and data

traffic in the network simultaneously. The most comprehensible way to express data traffic load

is kbps per subscriber, but in order to be able to estimate the transmission load of speech and

data together, the kbps value should be changed to the corresponding value in Erlangs. For this

conversion the average throughput of one timeslot for each coding scheme should be estimated.

The data throughput is mainly dependent on the air interface quality (i.e. C/I ratio). Network

congestion also decreases data throughput, especially in case of non-transparent HSCSD and

GPRS since GSM speech traffic has priority over them. In interference limited network, which

is the case in urban area where also the data users are mostly expected to be, the level of

frequency reuse and the surrounding environment determine the C/I ratio, which determines the

throughput. The level of throughput in respect of certain C/I ratio should be estimated by

simulations.

After defining the estimated throughput per timeslot, the data traffic amount in Erlangs can be

calculated by dividing the total data traffic load by the estimated average throughput per

timeslot. This can be done because in a very short time period one Erlang takes exactly one

timeslot.

2.3.1 HSCSD

The non-transparent service, which allows the number of radio timeslots to vary during a call,

can be introduced into the network without any capacity planning. HSCSD statistics are used to

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monitor service usage, and in places where HSCSD users are not served according to the

operator's requirement, the TRX capacity may need upgrading. However, with HSCSD-specific

parameter planning, the service quality between HSCSD and normal users can be tuned.

The transparent HSCSD service, with a fixed number of allocated timeslots during the entire

call, can cause more congestion. This depends on the structure and configuration of the

network, so in some cases transparent HSCSD service can be introduced without new network

planning. However, when transparent services are launched on a larger scale, new capacity

planning methods and algorithms are required.

For HSCSD the throughput is 14,4 kbps per timeslot in the best case but decreases as the C/I

ratio decreases. Throughput for transparent service depends directly on the blocking rate since

there is no upgrade or downgrade procedures, whereas throughput for non-transparent service

can vary a lot because of the downgrade and upgrade procedures. The Erlang B formula is not

applicable as it is for HSCSD capacity calculation. Capacity planning algorithms are needed for

non-transparent and transparent services in order to estimate more precisely the capacity also

with different timeslot connections and with different amounts of TRXs. One possibility is to

investigate by simulations if it is possible to define Erlang B based algorithms for non-

transparent and transparent services.

2.3.2 GPRS

It is possible to introduce GPRS in the network without any capacity planning. This is because

in the early state of GPRS the existing capacity of the network is probably sufficient to provide

good quality service for all GPRS users, due to the low usage of GPRS. But as the number of

GPRS users increases, also the capacity demand increases and has to be recalculated. An

example how circuit switched traffic and packet switched traffic could be distributed over a day

is presented in the Figure 4.

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Time

00:00 12:00 23:5906:00 18:00

GPRS Traffic Load

Circuit Switched Traffic Load

Total Traffic Load

Figure 4. Traffic distribution example.

For dimensioning it should be possible to define the capacity need separately for circuit

switched and packet switched traffic. Depending on the number of subscribers and on the

traffic per subscriber it is necessary to define the following figures in the GPRS network:

• Peak circuit switched traffic load (in Erlangs)

• Peak GPRS traffic load (in kbps)

Also depending on the percentage of subscribers attached during total traffic peak hour and also

on the peak hour traffic demand per subscriber, the following figures should be defined:

• Circuit switched traffic load during total traffic peak hour (in Erlangs)

• GPRS traffic load during total traffic peak hour (in kbps)

If coding schemes CS-1 and CS-2 are available, their data rates being 9.05 kbps and 13.4 kbps

respectively, the network data throughput can be estimated to be something between these

values. During off peak hour time, when the traffic and interference levels are low, the

throughput will increase towards 13,4 kbps. Some simulations were conducted by ETSI to find

out more about the single timeslot throughput with respect to C/I ratio, see Figure 5 [GSM

05.50]. From the figure it can be seen that with C/I values of 11-15 dB, the corresponding

average throughput for CS-1 and CS-2 is roughly 10 kbps (8*1.25 koctet/s). Actual throughput

experienced by the end-user is probably even lower because several users share the timeslot

resources.

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maximum throughput in TU 50 no FH

0

0,5

1

1,5

2

2,5

3dB 7dB 11dB 15dB 19dB 23dB 27dB

C/I

koctet/s

CS1

CS2

CS3

CS4

CS-1 and CS-2average throughput

Figure 5. GPRS throughput performance [GSM 05.50].

It should be noted that for multislot MS, for example if MS is using three timeslots, the average

throughput is less than three times the one timeslot throughput. This is because:

• Under high load conditions three consecutive timeslots are not often available.

• There is a set up overhead required to transmit each packet (average packet sizes will be

probably be small). The setup time starts to be a larger proportion of the total transmission

time for multislot connections. Hence average throughput is less.

• The retransmission of erroneous blocks is not so efficient for higher data rates, due to

mobile delays in acknowledging reception or requesting retransmission.

The simulations made by Nokia showed that after certain point of network congestion the data

throughput starts to decrease considerably. For dimensioning it should be possible to define this

point, but the situation is very complicated because in general there is no such point but an

interval where this situation occurs. However, a variable, loading factor, should be defined to

estimate the cell loading in percentages before end-user GPRS data throughput starts to

decrease considerably. The value of loading factor is dependent on the configuration and the

system parameters, thus some configuration-specific simulations should be made to find out

more about the actual value of loading factor.

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The following cases could be considered when calculating capacity for GPRS:

1. Circuit switched traffic (e.g. speech and HSCSD) has priority over GPRS data traffic. First

the number of sites for circuit switched traffic is calculated with certain blocking. Then it

must be calculated if the network provides enough throughput for given GPRS data traffic

load. If more sites have to be added for capacity because of GPRS data, the increased

number of sites will also decrease the blocking of circuit switched traffic so the blocking

value should be calculated once more.

2. Fixed number of timeslots allocated for GPRS data (e.g. 1 TS/carrier). The throughput that

the network provides for GPRS data is the throughput provided by the fixed and varying

timeslots together. If this throughput is not enough and more sites have to be added because

of GPRS data, the increased number of sites will also decrease the blocking of circuit

switched traffic and thus the blocking value should be calculated once more.

3. GPRS data traffic has equal priority compared to circuit switched traffic (this could be the

case in the future). This means that if there is some amount of timeslots allocated for GPRS

connection and if the network gets so loaded that there aren’t any more free timeslots for

new circuit switched calls, the timeslots allocated for GPRS are not released for speech

calls (nor incoming GPRS calls) and the incoming calls are blocked.

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