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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.



                    Company Confidential 15/7/05         2
Traffic Theory

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



                     Company Confidential 15/7/05   3
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


                         Company Confidential 15/7/05                    4
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



                           Company Confidential 15/7/05        5
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



                            Company Confidential 15/7/05            6
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 )

                          Company Confidential 15/7/05           7
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




                        Company Confidential 15/7/05      8
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 )


                         Company Confidential 15/7/05        9
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 ?

                          Company Confidential 15/7/05        10
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 %

                        Company Confidential 15/7/05     11
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



                           Company Confidential 15/7/05            12
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.


                             Company Confidential 15/7/05                13
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.




                                Company Confidential 15/7/05                  14
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


                         Company Confidential 15/7/05   15
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.


                           Company Confidential 15/7/05                 16
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



                                  Company Confidential 15/7/05   17
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

                            Company Confidential 15/7/05           18
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 .



                        Company Confidential 15/7/05    19
Cell Configuration




          1L         1L
                                          1L
                           2L

          2L
     1L                   4L
                                     2L
                                    1L


               Company Confidential 15/7/05    20
CONNECTIVITY PLANNING




        Company Confidential 15/7/05   21
WHAT TO CONNECT ?



     MSC -----   PSTN

     MSC -----   BSC

     MSC -----   TRANSCODER *

     BSC -----    TRANSCODER *

     BSC -----    BTS


            Company Confidential 15/7/05   22
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




                             Company Confidential 15/7/05                     23
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

                 Company Confidential 15/7/05           24
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




                                 Company Confidential 15/7/05                     25
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.

                            Company Confidential 15/7/05                26
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
                         64 kbps      4     TRX Signaling
                         64 kbps      5     4 Traffic Channels
                         64 kbps      6     4 Traffic Channels   } 1 TRX
                         64 kbps      7     TRX Signaling
                         64 kbps      8     4 Traffic Channels
                         64 kbps      9     4 Traffic Channels   } 1 TRX

                               Company Confidential 15/7/05                27
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.



                           Company Confidential 15/7/05              28
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
                   64 kbps       2     4 Traffic Channels
                   64 kbps       3     4 Traffic Channels
                                                            } 1 TRX
                   64 kbps       4     4 Traffic Channels
                   64 kbps       5     4 Traffic Channels   } 1 TRX
                   64 kbps       6     4 Traffic Channels
                   64 kbps       7     4 Traffic Channels   } 1 TRX
                   64 kbps       8     4 Traffic Channels
                   64 kbps       9     4 Traffic Channels   } 1 TRX
                   64kbps        10    4 x TRX Signaling



                             Company Confidential 15/7/05              29
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.




                       Company Confidential 15/7/05             30
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        2    4 Traffic Channels
                    64 kbps        3
                                                         } 1 TRX
                                        4 Traffic Channels
                    64 kbps        4    4 Traffic Channels
                    64 kbps        5
                                                         } 1 TRX
                                        4 Traffic Channels
                    64 kbps        6    4 Traffic Channels
                    64 kbps        7
                                                         } 1 TRX
                                        4 Traffic Channels
                    64 kbps        8    4 Traffic Channels
                    64 kbps        9    4 Traffic Channels
                                                             } 1 TRX
                    64 kbps       10    4 Traffic Channels




                              Company Confidential 15/7/05                31
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.




                          Company Confidential 15/7/05           32
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      2     4 Traffic Channels
                    64 kbps      3     TRX Signaling/ 3 Traffic Channels
                    64 kbps      4     4 Traffic Channels
                                                                           } 1 TRX
                    64 kbps      5     TRX Signaling/ 3 Traffic Channels
                    64 kbps      6     4 Traffic Channels
                                                                           } 1 TRX
                    64 kbps      7     TRX Signaling/ 3 Traffic Channels
                    64 kbps      8     4 Traffic Channels
                                                                           } 1 TRX
                    64 kbps      9     TRX Signaling/ 3 Traffic Channels
                    64 kbps     10     4 Traffic Channels
                                                                           } 1 TRX

                           Company Confidential 15/7/05                         33
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.




                   Company Confidential 15/7/05     34
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 )




                             Company Confidential 15/7/05          35
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


                                Company Confidential 15/7/05                 36
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.


                           Company Confidential 15/7/05              37
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.




                      Company Confidential 15/7/05      38
BSC Capacity




Maximum BTS's         Capacity on "A" interface



                Company Confidential 15/7/05      39
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




                              Company Confidential 15/7/05           40
BSC Capacity
BTS configuration

 Star Configuration                                   BTS

         BTS                         BTS



                      BSC



                                      BTS
         BTS


                       Company Confidential 15/7/05         41
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




                         Company Confidential 15/7/05             42
BSC Capacity
BTS configuration
Loop configuration


                                        BTS
                         A               A
                          B

            BSC                       B      A

                          B
                                        BTS
                          A              B



                     Company Confidential 15/7/05   43
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 ?


                                 Company Confidential 15/7/05                         44
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


                               Company Confidential 15/7/05            45
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




                                  Company Confidential 15/7/05                    46
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



            Company Confidential 15/7/05         47
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.

                          Company Confidential 15/7/05               48
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




                           Company Confidential 15/7/05          49
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




                          Company Confidential 15/7/05          50

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GSM capacity planning

  • 1. GSM Network Capacity Planning Trunking Traffic Theory -- Traffic Intensity -- Grade of Service Traffic Channels Dimensioning SDCCH Channels Dimensioning Company Confidential 15/7/05 1
  • 2. 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. Company Confidential 15/7/05 2
  • 3. Traffic Theory Terminologies Traffic Intensity Busy Hour Request Rate ( BHCA ) Set-up Time Holding Time Blocked Call Grade of Service (GoS) Company Confidential 15/7/05 3
  • 4. 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 Company Confidential 15/7/05 4
  • 5. 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 Company Confidential 15/7/05 5
  • 6. 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 Company Confidential 15/7/05 6
  • 7. 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 ) Company Confidential 15/7/05 7
  • 8. 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 Company Confidential 15/7/05 8
  • 9. 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 ) Company Confidential 15/7/05 9
  • 10. 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 ? Company Confidential 15/7/05 10
  • 11. 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 % Company Confidential 15/7/05 11
  • 12. 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 Company Confidential 15/7/05 12
  • 13. 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. Company Confidential 15/7/05 13
  • 14. 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. Company Confidential 15/7/05 14
  • 15. 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 Company Confidential 15/7/05 15
  • 16. 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. Company Confidential 15/7/05 16
  • 17. 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 Company Confidential 15/7/05 17
  • 18. 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 Company Confidential 15/7/05 18
  • 19. 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 . Company Confidential 15/7/05 19
  • 20. Cell Configuration 1L 1L 1L 2L 2L 1L 4L 2L 1L Company Confidential 15/7/05 20
  • 21. CONNECTIVITY PLANNING Company Confidential 15/7/05 21
  • 22. WHAT TO CONNECT ? MSC ----- PSTN MSC ----- BSC MSC ----- TRANSCODER * BSC ----- TRANSCODER * BSC ----- BTS Company Confidential 15/7/05 22
  • 23. 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 Company Confidential 15/7/05 23
  • 24. 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 Company Confidential 15/7/05 24
  • 25. 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 Company Confidential 15/7/05 25
  • 26. 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. Company Confidential 15/7/05 26
  • 27. 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 64 kbps 4 TRX Signaling 64 kbps 5 4 Traffic Channels 64 kbps 6 4 Traffic Channels } 1 TRX 64 kbps 7 TRX Signaling 64 kbps 8 4 Traffic Channels 64 kbps 9 4 Traffic Channels } 1 TRX Company Confidential 15/7/05 27
  • 28. 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. Company Confidential 15/7/05 28
  • 29. 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 64 kbps 2 4 Traffic Channels 64 kbps 3 4 Traffic Channels } 1 TRX 64 kbps 4 4 Traffic Channels 64 kbps 5 4 Traffic Channels } 1 TRX 64 kbps 6 4 Traffic Channels 64 kbps 7 4 Traffic Channels } 1 TRX 64 kbps 8 4 Traffic Channels 64 kbps 9 4 Traffic Channels } 1 TRX 64kbps 10 4 x TRX Signaling Company Confidential 15/7/05 29
  • 30. 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. Company Confidential 15/7/05 30
  • 31. 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 2 4 Traffic Channels 64 kbps 3 } 1 TRX 4 Traffic Channels 64 kbps 4 4 Traffic Channels 64 kbps 5 } 1 TRX 4 Traffic Channels 64 kbps 6 4 Traffic Channels 64 kbps 7 } 1 TRX 4 Traffic Channels 64 kbps 8 4 Traffic Channels 64 kbps 9 4 Traffic Channels } 1 TRX 64 kbps 10 4 Traffic Channels Company Confidential 15/7/05 31
  • 32. 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. Company Confidential 15/7/05 32
  • 33. 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 2 4 Traffic Channels 64 kbps 3 TRX Signaling/ 3 Traffic Channels 64 kbps 4 4 Traffic Channels } 1 TRX 64 kbps 5 TRX Signaling/ 3 Traffic Channels 64 kbps 6 4 Traffic Channels } 1 TRX 64 kbps 7 TRX Signaling/ 3 Traffic Channels 64 kbps 8 4 Traffic Channels } 1 TRX 64 kbps 9 TRX Signaling/ 3 Traffic Channels 64 kbps 10 4 Traffic Channels } 1 TRX Company Confidential 15/7/05 33
  • 34. 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. Company Confidential 15/7/05 34
  • 35. 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 ) Company Confidential 15/7/05 35
  • 36. 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 Company Confidential 15/7/05 36
  • 37. 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. Company Confidential 15/7/05 37
  • 38. 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. Company Confidential 15/7/05 38
  • 39. BSC Capacity Maximum BTS's Capacity on "A" interface Company Confidential 15/7/05 39
  • 40. 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 Company Confidential 15/7/05 40
  • 41. BSC Capacity BTS configuration Star Configuration BTS BTS BTS BSC BTS BTS Company Confidential 15/7/05 41
  • 42. 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 Company Confidential 15/7/05 42
  • 43. BSC Capacity BTS configuration Loop configuration BTS A A B BSC B A B BTS A B Company Confidential 15/7/05 43
  • 44. 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 ? Company Confidential 15/7/05 44
  • 45. 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 Company Confidential 15/7/05 45
  • 46. 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 Company Confidential 15/7/05 46
  • 47. 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 Company Confidential 15/7/05 47
  • 48. 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. Company Confidential 15/7/05 48
  • 49. 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 Company Confidential 15/7/05 49
  • 50. 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 Company Confidential 15/7/05 50