The tiff about Switch & Prepaid subscriber capacity in BSNL GSM systems.

Reffered again in view of "The Tiff about Switch and Prepaid subscriber capacity in BSNL GSM Systems"

In any telecom system the equipped capacity base shall be more than planned subscriber capacity to take care of the  inevitable redundacy due to delay in recycling dormant subscribers and various other factors. 

Capacity planning of Mobile Switching Systems are more complex compared to the land line systems. Basically in any land line network the total capacity in a network is the arithmetic total of the subscribers in that network where as in the Mobile network the capacity need to be worked out keeping in mind the number of susbscribers who are moving across the network. In the mobile network everybody is in roaming condition whether inside the house or outside in the same locality or in other locations. While the Home Location registers shall have capacity of at least 25% more due to redundant subcriber data base that is inevitable due to  various factors, the Visited Location Registers (VLR) also should have have enough redudant capacity to take care of the subscribers registered in own Home Location Register as well the visiting subscribers from other HLR areas. 

For example: In Mumbai, large percent of residents move to city during day and back in the suburbs by evening.  Therefore, the City and subsurbs should have enough capacity to take care of the subscribers to log in and use the system. It may be noted that, even though the mobile phone is not used, VLR should have enough capacity to keep the subscriber in logged in condition. This will become more comlex with Number Portability.   

In case of Pre-paid subscribers the capacity need are much more as explained in deatail below.  I wonder whether these factors are taken into consideration while blaming the BSNL systems keeping excess of 27% capacity as idle.

In this context I am reffering article posted by me in TooStep  some time back again for the technology oriented members from telecom area, who are interested to have a look.    

Pre-paid Telecom service - System Capacity and business planning.

In Prepaid Systems, a sizable portion of the Subscriber base remain either unused or dormant at any point of time due to:

-          Internal administrative delays in marketing SIMs,

-          Latency in logistics,

-          Different normal and extended grace periods,

-          Delay inevitable in identifying, removing / recycling dormant, discontinued  subscriptions.

Therefore, to realize a defined amount of Prepaid Business Volume, considerably higher volume of Pre-paid Subscriber base is needed IN system and in the HLR. Looking it the other way around; at any point of time, the number of Prepaid subscribers contributing to the business volume will be only a portion of the Prepaid subscriber base in IN and HLR.

It is necessary to consider this aspect while planning the Prepaid Telecom Business and the systems needed to support it.

Dynamic Capacity and Static Capacity:

Dynamic Capacity:

Revenue in Prepaid telecom business is generated from the subscribers who have their Prepaid Cards in activated state. Therefore, this number of such subscribers form the Dynamic Subscriber base.

Dynamic load calculations based on the Traffic:

Dynamic load of the IN system is the product of the Number of Dynamic Subscribers and the average busy hour call Completion (BHCC) per Dynamic subscriber.  

The dynamic load of IN system is expressed in Kilo BHCC or Million BHCC.  

Busy Hour Transactions.  Quantum of INAP/CAP signaling Traffic between Switch and IN SCP is based on the Transactions per Busy Hour. Busy Hour Call Attempts (BHCA) are to be taken into consideration for calculation of nework elements related to busy hour transactions in IN, Switch and the interface.

The Transactions per Call Attempt depends on the type of Service/feature and duration of the call.  For prepaid service, there will be an averge of about 4 to 5 transactions per call attempts. There can be additional/abnormal transactions for double triggering, additional ‘In-call Tones’, Announcements, USSD, SMS / USSD handling, IN based call routing etc.

Busy hour traffic load due to dynamic subscriber base is taken into consideration for dimensioning the quantity of network elements and devices in switching system and network.

Calculation of signaling links depends on number of  transactions due to the incidental BHCA in the Busy Hour in Switch and between SSP(Switch) and SCP.  

Static Capacity: 

Static Capacity of IN System is the number of Subscribers created in the IN system database which is called the licensed Capacity of the IN System.

Static Capacity needed in HLR for prepaid service will be the static capacity of prepared subcibers in IN system and the dormant subscriber numbers, which are yet to be recycled and reused.   

The Static Capacity is relevant only for deciding mainly the quantum of data base liecense and not for other net work elements.  

Active Ratio: 

The Active Ratio related to IN Prepaid subscribers in IN System is the proportion of Number of subscribers who have Prepaid Cards in 'activated' state, to the total number of prepaid Subscriber in the data base in the SCP/SMP.  

The Active Ratio related to Prepaid subscribers in HLR is the proportion of the Number of IN subscribers who have Prepaid Cards in activated state, to total Number of  prepaid Subscribers created in the database in the HLR. The IN subscribers in VLR will be more or less same as the number of dynamic subscribers in the IN SCP/SMP. 

Since Revenue generated is directly related to ARPU of the ACTIVE users, it is necessary to provide enough buffer in HLR and IN system to take care of the Active Ratio according to right business plan.

Experience the world over is that the Active Ratio of Prepaid susbcibers in a well administered network is between 0.6 and 0.5.  It means that about 60% to 100% of additional subscriber base over the Dynamic Capacity is required as Static Capacity in IN system and HLR to have enough buffer capacity to take care of the dormant, discontinued and yet to be sold prepaid cards.

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There are basically two methods of prepaid business planning;

Top-down and "Bottom- top"  planning.

In Top-Down planning, a pre decided Dynamic capacity (the top) to realize the proposed Pre-paid business volume is taken as the basis and then go down and decide the Static capacity needed for it in the data base of IN and HLR according to Active Ratio. 

Example of Top-down planning:

Volume of prepaid traffic expected to generate the projected revenue. 800 K BHCC

BHCC per active subscriber = 1 BHCC

Dynamic subscriber base needed to generate the business volume =  800 K

Active ratio is assumed as 0.5

The Static capacity needed for the Active Ratio of 0.5 is about 1600 K lines. 

The overall IN BHCC will be 0.5 BHCC.

Bottom  to Top planning:

This is done by the Companies that do IN capacity planning based on the forecast of Prepaid subscribers in their network. Taking this figure of Static capacity as the bottom base, the  Dynamic Capacity for the forcasted subscriber base is arrived based on the Active Ratio. 

Dynamic BHCA load is calcutated based on the BHCA per Dynamic Subscriber. Quantum of Dynamic BHCA divided by the Static Subscriber base will be the average BHCA per subscriber of the IN system. ARPU is arrived from the forecasted Traffic per Active Subscriber.

Example of "Bottom – Up"  planning:

IN Prepaid Subscribers according to subscriber growth forecast  (Static Capacity) = 1 million

BHCA per active subscriber = 0.8 BHCA

Dynamic Load of the system = 0.8 million BHCA

Active ratio is assumed as 0.5

The Dynamic capacity expected with above  Active Ratio of 0.5 =  0.5 million

The overall IN BHCA will be 0.4 BHCA.

The revenue generated will be equal to Dynamic Subscriber base x ARPU.  

In both cases, the optimum capacity and scalability of the system related to Static and  Dynamic Capacity limits are to be kept in mind. Also there shall sufficient buffer in the system to take care of about 30% overload handling capability. 

Currently, IN systems of major vendors are capable of handling very high Dynamic load in terms of millions of BHCA depending on the type of services and its proportion and Static Capacity required as per the active ratio.

The dynamic load handling depends on the processor speed and Multiple SCP configuration whereas the Static capacity depends on the SCP data base limitation.   

 System and capacity ordering philosophy: 

It is desirable to do a Bottom to Top planning based on the growth forecast to know the quantum of Static Capacity and then calculate the Dynamic Capacity needed at various budgeting needs in the forecast period.  The initial order and expansions of IN system shall be done considering the Static Capacity and the Dynamic load time to time and based on the Active Ratio and the BHCA per Active Subscriber to optimize cost and capacity utilization.

Unless clearly specified in the Tenders/RFQs, the vendors tend to assume the Active Ratio and use the BHCA value given in the tender for Dynamic Load calculation based on the quantum of active subscribers. This is done to avoid idle capacity and to optimize the overall system capacity and cost.  This logic is valid because if a system is dimensioned taking the full BHCA value for the Static subscriber base,  the cost will be high because of the  idle capacity, and the optimum capacity of the system has to be rated down.  

SMP, database, Voucher management system in IN and additional HLR & AuC capacity for the IN in Switch are to be planned based on the Static Capacity needs.

Switching resources, Signaling link and interface elements required between the Switch and IN  need to be calculated based on the Dynamic load.  Planning of netork elements if done on the basis of Static capcity, it will result in too much of over dimensioning and added cost.

Planning of number of IN systems required is done taking into consideration the modularity and scalability of the system. As far as it is not possible to have load sharing with multiple SCPs, it is necessary to plan for additional system.  Otherwise, scalability depends on version and release. Static and dynamic capacity of the fully configured platform to be ascertained from the supplier.  

Additional platform to be planned based on the growth plans. There shall be sufficient buffer for expansion for the forecast period after implementation.

In a situation of 2% to 3% growth per month, the next platform to be planned when 80% of the max configurable capacity is reached so that the additional platform is available within next 6 to 8 months.             

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Author: Abraham Paul MD FCOMNET UAE, FUTURE GROUPS India. Ex: V.P. Tech. Sales, SPCNL (SIEMENS) India / Director SIEMENS ICN  RHQ  UAE / G.M. BPL Mobile Network / TES DOT India.