capacity planning

Company Confidential 15/7/05 1

Trunking

Traffic Theory-- Traffic Intensity-- Grade of Service

Traffic Channels Dimensioning

SDCCH Channels Dimensioning

GSM Network Capacity Planning


GATEWAY 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.

SWITCHLOCAL

Trunking


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

Traffic Theory


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

THE UNIT OF MEASUREMENT IS ERLANGS

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

A = UAuA : Total traffic offered by the systemU : Total number of users

Traffic Intensity

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 resourcesSDCCH 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 :

AGCHSDCCHTCH

How many blocked calls can you tolerate ?

Traffic Theory


Ability of the user to access the system during busiest hour

Benchmark to define desired system performance

Grade of Service

Percentage requests blocked in an hour

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

Conventionally used value of GOS is 2 %

Traffic Theory


Blocked Calls Cleared System

2 .153 .190 .223 .3817 2.50 2.74 2.94 3.748 3.13 3.40 3.63 4.54

14 7.35 7.82 8.20 9.7315 8.11 8.61 9.01 10.616 8.88 9.41 9.83 11.522 13.7 14.3 14.9 17.130 20.3 21.2 21.9 24.837 26.4 27.4 29.6 31.6

No. of channels C

Capacity (Erlangs) for GOS = 1% = 1.5 % = 2 % = 5 %

Requested is immediately cleared (forgotten) at blocking Erlang B table is used to estimate traffic for a GOS

TYPES OF TRUNKING SYSTEM


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.


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 delayedH = Average duration of calls

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

GOS = Targetted GOS

Types of Trunking Systems


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.


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

Traffic Channel Dimensioning


Example : Car usage distribution

1L

1L 4L

2L

2L

1L

2L

1L

1L

4L streets = 1.1 Km2L streets = 2.1 km1L streets = 6.4 km

Avg Spacing between vehicles = 10mTotal vehicles in 100% street congestion case= 1500For 50% penetration= 750 users

Traffic = 750 x 0.025 = 18.5 erl;corresponds to 27 TCH's

Estimating No of users and Traffic


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 and14 TCH in each sectored cell .

Estimating Channels from last case


1L

1L 4L

2L

2L

1L

2L

1L

1L

Cell Configuration


CONNECTIVITY PLANNING


MSC ----- PSTN

MSC ----- BSC

MSC ----- TRANSCODER *

BSC ----- TRANSCODER *

BSC ----- BTS

WHAT TO CONNECT ?


S

Abis

A

A

BSC Transcoder

MSC

BTS

13 Kbps16 Kbps 16 Kbps

64 Kbps 0123

0 1 2 3S S 0 1 2 3

Speech on Terrestrial circuit


BTS13 Kbps

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

Air 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 Interface


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.


LAPD ModesLAPDSignaling for each TRX is on a dedicated 64 Kbps circuitMaximum Signalling for 10 Transceivers on 1 E1 link

64 kbps 0 Sync64 kbps 1 TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 TRX Signaling64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 TRX Signaling64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels

} 1 TRX

} 1 TRX

} 1 TRX

Abis Interface


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.


LAPD ModesLAPD Concentrated mode 1Signaling for 4 TRX's is on a dedicated 64 Kbps ciruitMaximum Signalling for 13 Transceivers on 1 E1 link

64 kbps 0 Sync64 kbps 1 4 x TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 4 Traffic Channels64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 4 Traffic Channels64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels

64kbps 10 4 x TRX Signaling

} 1 TRX

} 1 TRX

} 1 TRX

} 1 TRX

Abis Interface


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.


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

64 kbps 0 Sync64 kbps 1 ALL TRX Signaling64 kbps 2 4 Traffic Channels64 kbps 3 4 Traffic Channels64 kbps 4 4 Traffic Channels64 kbps 5 4 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 4 Traffic Channels64 kbps 8 4 Traffic Channels64 kbps 9 4 Traffic Channels

64 kbps 10 4 Traffic Channels

} 1 TRX

} 1 TRX

} 1 TRX

} 1 TRX

Abis Interface


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.


LAPD ModesLAPD MultiplexedSignaling for each TRX is on 16kbps subchannel.Maximum signaling for 15 TRX's on

64 kbps 0 Sync64 kbps 1 TRX Signaling/ 3 Traffic Channels64 kbps 2 4 Traffic Channels64 kbps 3 TRX Signaling/ 3 Traffic Channels64 kbps 4 4 Traffic Channels64 kbps 5 TRX Signaling/ 3 Traffic Channels64 kbps 6 4 Traffic Channels64 kbps 7 TRX Signaling/ 3 Traffic Channels64 kbps 8 4 Traffic Channels64 kbps 9 TRX Signaling/ 3 Traffic Channels

64 kbps 10 4 Traffic Channels

} 1 TRX

} 1 TRX

} 1 TRX

} 1 TRX

} 1 TRX

Abis Interface


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.


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

For Local Transcoding

Capacity on Abis is the number of 64 kbps circuits required

For Remote Transcoding

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

Capacity = Number of 64 kbps circuitsNo of TCH = Sum of all TCH's in each sector at the BTSNo of LAPD circuits = Depends on LAPD modeOML = optional ( vendor dependent )

Abis Interface Capacity


15 speech chC1

C2C3

15 speech ch 15 speech ch

1 cell = 15 x 16 kbps speech channels3 cells = 45 x 16 kbps speech channels = 12 x 64 kbps speech channels1 BTS = 12 + 1 ( RSL ) = 13 x 64 kbps channels

BSC

Example

Abis Interface Capacity


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.


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.

Exercise !!!


Maximum BTS's Capacity on "A" interface

BSC Capacity


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

Maximum BTS's

BSC Capacity


BTS BTS

BTSBTS

BSC

Star Configuration BTS

BTS configuration

BSC Capacity


Chain Configuration

BTSBSC BTS13 x 64 kbps ch

26 x 64 kbps ch

1 x E1 1 x E1

BTS configuration

BSC Capacity


Loop configuration

A

B

B

A

A

B

BSC

BTS

BTS

A

B

BTS configuration

BSC Capacity


Each BTS needs 13 x 64 kbps circuits

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

BTS

BTS

BSC

BTS

A

BC

D

E

F

BTS

BTS

BTSG

BTSHBTS I

BTS

BTS

BTS

BTS

BTSBTS

JK

L

M

N

O

Exercise !


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.

Capacity = No of Speech Circuits + Signaling CircuitsFor Local Transcoding

For Remote Transcoding

Capacity = No of Speech Circuits/4 + Signaling Circuits

BSC Capacity


Signalling Circuit Capacity on A interface

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

Signaling circuits

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

Capacity for SS7 link

BSC Capacity


= 1 x E1 = 112 x 16 kbps chs

= 1 x E1 = 30 x 64 kbps chs

MSC BSCTRANS CODER

4 x E1 = 120 x 64 kbps chs

Transcoder - MSC Cpacity


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 signaling1 for OMC signaling1 for Transcoder signalling1 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 = 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

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

MSC Capacity = Population Penetration x Traffic per subscriber

MSC 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

Network Elements Capacity

capacity planning

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