GSM capacity planning

GSM Network Capacity Planning

Trunking

Traffic Theory
-- Traffic Intensity
-- Grade of Service

Traffic Channels Dimensioning

SDCCH Channels Dimensioning

Company Confidential 15/7/05 1

Trunking

LOCAL GATEWAY
SWITCH SWITCH

So, What is the objective behind Capacity Planning ?
Estimating the optimum number of resources required in a
system to meet the desired performance requirements.


Traffic Theory

Terminologies
Traffic Intensity
Busy Hour
Request Rate ( BHCA )
Set-up Time
Holding Time
Blocked Call
Grade of Service (GoS)


Traffic Theory
Traffic Intensity
TRAFFIC INTENSITY IS MEASURED ON 1 CALL
PER-HOUR BASIS OR 1 CALL PER MINUTE BASIS

THE UNIT OF MEASUREMENT IS ERLANGS

Au = uH
Au : Traffic in Erlang generated by each user
H : Average duration of call / 60 (per hour basis)
u : Average no of calls per hour

A = UAu
A : Total traffic offered by the system
U : Total number of users


Traffic Theory

Traffic Intensity ... Contd.

In GSM, we have two types of Traffic Intensities

TCH Traffic Intensity = Avg no of calls x Avg duration of call

Average duration of call = 120 secs

Average number of calls = 0.75 -- 1.5 ( range )

Traffic generated on TCH will range between 0.025 -- 0.05 erlang


Traffic Theory
Traffic Intensity ... contd

and ...
SDCCH Traffic Intensity = Avg no of SDCCH usages x Avg usage time

Avg no of SDCCH usage = 1(for a TCH call) + 3 updates = 4

Average usage time = 4 secs

Traffic generated on SDCCH will be typically 0.0044 erlang


Traffic Theory
Busy Hour
1 Hour of the day in which Traffic is maximum
Also referred to as Peak Hour.
Busy Hour is not a fixed hour, its timing will vary in
different locations

Busy Hour may also be different for different resources
SDCCH busy hour
-- typically morning hours ( frequent on/offs and updates)

TCH busy hour
-- heavy call traffic hour ( could be back-home hours )


Traffic Theory
Request Rate ( BHCA )

No of requests(or attempts) for a resource in the busy hour

SDCCH Request Rate
-- No of RACH's + No of Handover Requests for SDCCH

TCH Request Rate
-- No of RACH's in a cell with cause as MOC or MTC
+ No of Handover Request for TCH


Traffic Theory
Set up Time

Average time spent on a resource before getting response
from the called end.
Typically 3 - 5 secs for GSM ( up to POI setup)

Holding Time

Average time spent on any dedicated resource.

SDCCH Holding time ( typically 3 - 4 secs)
TCH Holding time ( actuall call duration + Alerting )


Traffic Theory

Blocked Call

A call request rejected due to unavailability of resource.
Indication of Congestion

In GSM a call can be blocked due to unavailability of :

AGCH
SDCCH
TCH

How many blocked calls can you tolerate ?


Traffic Theory
Grade of Service

Percentage requests blocked in an hour

Ability of the user to access the system
during busiest hour
Benchmark to define desired system
performance

GOS and blocking are same.
A network is non-blocking if the communication resources
equals the number of users.

Conventionally used value of GOS is 2 %


TYPES OF TRUNKING SYSTEM
Blocked Calls Cleared System

Requested is immediately cleared (forgotten) at blocking
Erlang B table is used to estimate traffic for a GOS
No. of Capacity (Erlangs) for GOS
channels C = 1% = 1.5 % =2% = 5%
2 .153 .190 .223 .381
7 2.50 2.74 2.94 3.74
8 3.13 3.40 3.63 4.54
14 7.35 7.82 8.20 9.73
15 8.11 8.61 9.01 10.6
16 8.88 9.41 9.83 11.5
22 13.7 14.3 14.9 17.1
30 20.3 21.2 21.9 24.8
37 26.4 27.4 29.6 31.6


There are two types of trunked systems which are commonly used.

The first type offers no queuing for call requests.That is, for every
user who request service, it is assumed there is no setup time and the
user is given immediate access to a channel if one is available.

If no channels are available, the requesting user is blocked without
access and is free to try again later.

This type of trunking is called blocked calls cleared.

It is assumed that there are infinite number of users and there are
memory less arrivals of requests, there are finite number of channels
available.The capacity of a radio adopting this concept is tabulated for
various values of GOS.


Types of Trunking Systems

Assumptions deciding Erlang B table :

A request for channel may come at any time.
All free channels are fully available for servicing calls until all
channels are occupied.
Call durations are exponentially distributed. Longer calls are less
likely to happen.
Traffic requests also follows exponentially distribution of inter-arrival
times. Mulitple requests will not occur at regular intervals.
Inter-arrival times of call requests from different users are
independent of each other.
There are finite number of channels available in the trunking pool.


Types of Trunking Systems
Blocked Calls Delayed System

GOS ( delay calls) = exp ( - ( C - A ) t / H )
C = No of channels,
A = Traffic Intensity obtained from chart,
t = Time (secs ) for which call is delayed
H = Average duration of calls

GOS ( blocked delayed calls ) = GOS x GOS (delay calls)

GOS = Targetted GOS


The second type of trunked system is one in which a queue is
provided to hold calls which are blocked.If a channel is not
available immediately,the call request may be delayed until a
channel becomes available. This type of trunking is called Blocked
Calls Delayed,and the GOS in this case is defined as the
probability that a call is blocked after waiting a specific length of
time in the queue.If no channels are immediately available the call
is delayed .The GOS for delayed system is calculated by the
formula shown above.

The first formula calculates the percentage of calls that will be
delayed for a period (t) for a traffic intensity A (which is
calculated from the chart keeping a target GOS) .
The second formula calculates the percentage of blocking after a
delay of t seconds, that is percentage of attempts denied after
being queued for t seconds.


Traffic Channel Dimensioning
Calculation of no of TCH required in a cell* depends on :
GOS & Traffic Intensity
Traffic Intensity = No of users x Traffic Intensity per user
No of users depends on demographic data as :
Population Distribution
Car usage distribution
Income
Fixed Line data
Service cost
Mobile Phone cost

* Cell area depends on propagation factors


Estimating No of users and Traffic
Example : Car usage distribution

4L streets = 1.1 Km
1L 2L streets = 2.1 km
1L 1L 1L streets = 6.4 km
2L Avg Spacing between
vehicles = 10m
2L Total vehicles in 100%
street congestion case
1L 4L = 1500
2L For 50% penetration
= 750 users
1L
Traffic = 750 x 0.025 = 18.5 erl;
corresponds to 27 TCH's


Estimating Channels from last case
Traffic Intensity = 750 x 0.025 = 18.5 erlangs

At GOS of 2 %, we need 27 TCH's

& 9 SDCCH's.

A cell configured with 4 ARFCN with B+D & 1 D config,
will provide 12 SDCCH's and 30 TCH's which satisfies.

Another method of achieving is with 2 sectors, each having
2 ARFCN's , with B & D config, which will give 8 SDCCH and
14 TCH in each sectored cell .


Cell Configuration

1L 1L
1L
2L

2L
1L 4L
2L
1L


CONNECTIVITY PLANNING


WHAT TO CONNECT ?

MSC ----- PSTN

MSC ----- BSC

MSC ----- TRANSCODER *

BSC ----- TRANSCODER *

BSC ----- BTS


Speech on Terrestrial circuit

BSC Transcoder
BTS
Abis A
S 0 1 2 3 S 0 1 2 3

16 Kbps 16 Kbps
S
13 Kbps

64 Kbps 0
1 A
2
3

MSC


Air Interface

13 Kbps
BTS

TCH/SDCCH are the traffic resources

8 PCHN on 1 ARFCN

Minimum 1 PCHN required for CCCH / and SDCCH

1 ARFCN gives 7 TCH max and 4 SDCCH min.

TCH's and SDCCH's can be altered by adding carriers
and channel configurations


Abis Interface

E1 / T1

Abis is a G.703 interface. It could be E1 or T1
Abis carriers Traffic information of all the mobiles in the cells controlled by the
BTS.
Abis also carries signaling information between BTS and BSC
Signaling over Abis is done by LAPD protocols
LAPD has several modes of implementation
--- LAPD
--- LAPD Concentrated
--- LAPD Multiplexed
Each TCH/F on Air Interface requires 16kbps sub-channel on Abis.
16 kbps subchannel on Abis is a nailed connection also known as RTF


Abis is the PCM interface between the BSC and MSC. Physically
this is a G.703 interface and could be E1 or T1. in all our further
discussions we will consider E1.

Abis carrier the traffic and signaling information for all the
transceivers inside the BTS. It also carries O&M information
between the BSC and BTS, like the control commands coming from
the BSC and traffic reports originated by the BTS.

Abis used the HDLC protocol for signaling which is LAPD ( Link
Access Protocol on D channel ).

LAPD has several modes of operation. What modes means is how
the signaling circuits are distributed over the E1 interface, whether
each TRX has separate signaling circuits or several TRX signaling
information is concentrated or multiplexed on limited signaling
circuits.


Abis Interface
LAPD Modes
LAPD
Signaling for each TRX is on a dedicated 64 Kbps circuit
Maximum Signalling for 10 Transceivers on 1 E1 link

64 kbps 0 Sync
64 kbps 1 TRX Signaling
64 kbps 2 4 Traffic Channels
64 kbps 3 4 Traffic Channels } 1 TRX


The first LAPD mode illustrated above is the LAPD basic mode. In
this mode each TRX has a separate signaling circuit of 64 Kbps.

Each signaling circuit has two immediate 64 Kbps Traffic circuits.

GSM used 13 kbps of speech rate on the air interface, to which
some TRAU information is added and it becomes 16 Kbps.

Four such 16 kbps traffic channels are mapped on one 64 Kbps
circuit. Each TRX has 8 traffic channels. So for each Transceiver,
two 64 kbps circuits are required, one for Traffic and one for
Signaling.

With this mode, 10 Transceivers can be accommodated on one E1.


Abis Interface
LAPD Modes
LAPD Concentrated mode 1
Signaling for 4 TRX's is on a dedicated 64 Kbps ciruit
Maximum Signalling for 13 Transceivers on 1 E1 link

64 kbps 0 Sync
64 kbps 1 4 x TRX Signaling
} 1 TRX
64kbps 10 4 x TRX Signaling


In this mode which is LAPD Concentrated Signaling
information for certain number of TRX's are concentrated on a
single 64 kbps circuit. There are two different methods of
concentration.

The above figure illustrates one method in which on one 64
kbps circuit the signaling information for 4 TRX's are
concentrated. This is typically done by creating 16 kbps
subchannels. So with this method 13 TRXs signaling as well as
speech can be accommodated on a single E1 Link.


Abis Interface
LAPD Modes LAPD Concentrated mode 2
Signaling for All TRX's is on a dedicated 64 Kbps circuit
Maximum Signaling for 15 Transceivers on 1 E1 link

64 kbps 0 Sync
64 kbps 1 ALL TRX Signaling
64 kbps 3
} 1 TRX
4 Traffic Channels
64 kbps 5
} 1 TRX
4 Traffic Channels
64 kbps 7
} 1 TRX
4 Traffic Channels
} 1 TRX


In this type of concentrated LAPD mode , signaling for all the
Transcevier are concentrated on one 64 kbps circuit. With
this, 15 TRX's signaling and Speech can be accommodated on
1xE1 link. This method is becoming very popular and is
adopted by many of the NEMS.


Abis Interface
LAPD Modes
LAPD Multiplexed
Signaling for each TRX is on 16kbps subchannel.
Maximum signaling for 15 TRX's on

64 kbps 0 Sync
64 kbps 1 TRX Signaling/ 3 Traffic Channels } 1 TRX
64 kbps 3 TRX Signaling/ 3 Traffic Channels
} 1 TRX
} 1 TRX
} 1 TRX
} 1 TRX


LAPD multiplexed is a mode in which Signaling for
each TRX is on a 16 kbps circuit which is
multiplexed with 3 speech channels of 16 Kbps. So
for each TRX two 64 kbps circuits are required.


Abis Interface Capacity

Capacity on Abis is the number of 64 kbps circuits required

For Local Transcoding

Capacity = No of TCH at BTS + No LAPD signaling circuits + OML*

For Remote Transcoding

Capacity = No of TCH at BTS / 4 + No LAPD signalling circuits + OML*

Capacity = Number of 64 kbps circuits
No of TCH = Sum of all TCH's in each sector at the BTS
No of LAPD circuits = Depends on LAPD mode
OML = optional ( vendor dependent )


Abis Interface Capacity
Example

1 cell = 15 x 16 kbps speech channels
3 cells = 45 x 16 kbps speech channels
= 12 x 64 kbps speech channels 15 speech ch
1 BTS = 12 + 1 ( RSL ) = 13 x 64 kbps
channels C1

BSC

C3 C2
15 speech ch 15 speech ch


The BSC to BTS Link is connected by E1 signaling system,which
uses a 2.048 Mbps stream with 32 x 64kbps channels.
The link between the BSC to BTC is termed as Radio Signaling Link.

In a sectorial cell configuration, one BTS supports 3 cells.
Take a case where each cell has 2 carriers which means there are 16
physical channels in this cell. With only one channel reserved for
control, 15 channels will be available for speech.
So for one BTS with 3 cells, will have 45 speech channels of 16 kbps,
each will in turn occupy 12 channels of 64 kbps on the RSL Link.

That is, including the RSL channel which occupies 1 x 64 kbps slot
on the link, overall 13 x 64 kbps channels are required for 1 BTS.


Exercise !!!

A BTS has 3 sectored cells.
Each cell has a subscriber capacity of 600, calculate the
number of TCH and SDCCH required at GOS 2 % and also
calculate the capacity on the Abis interface with LAPD
concentrated mode 2 signaling.


BSC Capacity

Maximum BTS's Capacity on "A" interface


BSC Capacity
Maximum BTS's

No of BTS's supported by the BSC is vendor specific

It is generally based on either or both of below :

1. Maximum number of TRX's BSC can support
(in terms of traffic)

2. Maximum number of PCM interfaces BSC can support.

Max PCM interfaces can be optmized by selecting BTS configurations


BSC Capacity
BTS configuration

Star Configuration BTS

BTS BTS

BSC

BTS
BTS


BSC Capacity
BTS configuration

Chain Configuration

BSC BTS BTS
26 x 64 kbps 13 x 64 kbps
ch ch
1 x E1 1 x E1


BSC Capacity
BTS configuration
Loop configuration

BTS
A A
B

BSC B A

B
BTS
A B


Exercise !
Each BTS needs 13 x 64 kbps circuits

H BTS
BTS BTS I BTS
B
BTS C
L BTS
N J
K A BTS D
BTS
BSC BTS
M E
BTS G
O
BTS F BTS
BTS BTS

Calculate the Number of E1 Links for each of the links ?


BSC Capacity
Capacity on "A" Interface

Capacity on A interface depends on Traffic of BSC at targeted GOS.

Traffic of BSC = No of Subscribers under BSC x Traffic per Subscriber
From calculated traffic, using Erlang B table calculate the
number of circuits required.

For Local Transcoding
Capacity = No of Speech Circuits + Signaling Circuits
For Remote Transcoding

Capacity = No of Speech Circuits/4 + Signaling Circuits


BSC Capacity
Signalling Circuit Capacity on A interface
Signaling circuits

SS7 "A" Link : Used for MSC - BSC signaling
OML : For OMC
TBL : Transcoder BSC Link

Capacity for SS7 link

Calculate the BHCA per second

BHCA : No of SDCCH attempts ( call+updates) x No of Subscribers .

On average each attempt requires 6 signaling messages
No of messages per second = 6 x BHCA per second
On average each message is of 25 octets
Capacity of Signaling circuit ( kbps ) = 25octets x No of messages per second


Transcoder - MSC Cpacity

TRANS BSC
MSC CODER

= 1 x E1 = 112 x 16 kbps chs
= 1 x E1 = 30 x 64 kbps chs

4 x E1 = 120 x 64 kbps chs


BSC to MSC link also uses E1 signaling structure.
The above figure considers the case for remote transcoding.The
capacity of the BSC to Transcoder link should be planned out on
the basis of number of BTS connected to the BSC.
The BSC to transcoder is an E1 link having 32 channels, out of
which;
1 for MSC signaling
1 for OMC signaling
1 for Transcoder signalling
1 for sync.
So 28 channels are left out for speech which are 64kbps each so
this will result into 112 x 16kbps speech channels. After
transcoding these 16 kbps channels will map on to 64 kbps
channels, so for 1 x 16 kbps channel coming to the transcoder will
become 1 x 64 kbps channel going towards MSC.
That is 112 x 16 kbps channels will require a capacity of 112 x
64kbps on the MSC link, which will result into 4 E1.


MSC Capacity
MSC Capacity = No of Subscribers x Traffic per subscriber

Long term calculation is based on Population Penetration
--- Population Penetration is the mobile population
out of total population of PLMN ( city )

Population Penetration = Total Population x Penetration rate

MSC Capacity = Population Penetration x Traffic per subscriber

Example : For a city population of 10,000000 with penetration
rate of 2 %.
Population Penetration = 200000
MSC Capacity = 10,000 Erlangs


Network Elements Capacity

MSC - PSTN Link Capacity

--- Estimate the % of PSTN calls from Total calls
--- Calculate the PSTN Traffic based on above estimation
--- Set a GOS
--- Calculate the no of channels by using Erlang B Table


GSM capacity planning

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to GSM capacity planning

Similar to GSM capacity planning (20)

Recently uploaded

Recently uploaded (20)

GSM capacity planning