Technical note on IN Prepaid-paid Telephone System and Capacity planning.
Author. P. Abraham Paul.
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 grace periods,
– delay in identifying and recycling of dormant MSIDN/IMSI etc.
Therefore, for a defined volume of Prepaid business, considerably higher Pre-paid Subscriber base is needed both in the IN system and the HLR. Or looking at 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 the IN and HLR.
So this aspect has to be taken into consideration while planning the IN system.
Dynamic Capacity and Static Capacity:
Since the IN prepaid business volume depends on the volume of IN subscribers who have Prepaid Card in activated state, this represent the Dynamic Subscriber base.
Dynamic Capacity calculations based on the Traffic:
Dynamic load of the IN system is calculated based on the Dynamic Subscriber base and the BHCA/BHCC of the Dynamic subscribers.
The dynamic load of IN system is expressed in Kilo BHCA or Million BHCA
Busy Hour Transactions. Quantum of INAP/CAP signaling Traffic between Switch and SCP is based on the Transactions per Busy Hour.
The Transactions per Call Attempt depends on the type of Service/feature and duration of the call. For prepaid service, there will about 4 to 5 transactions per call attempts. There can be additional/abnormal transactions for double triggering, additional ‘In-call Tones’, Announcements, USSD, SMS handling, IN based call routing etc..
Therefore, design of IN is to be done for the number of transaction due to the incidental BHCA in the Busy Hour in Switch and between SSP(Switch) and SCP.
Static Capacity of IN System is the number of Subscribers that can be created in the IN system database which is called the licensed Capacity of the IN System.
Static Capacity of HLR is number of Subscribers that can be created in the HLR depends on the licensed Capacity of the HLR.
Static Capacity calculations based on Active Ratio:
For generating the full revenue for the forecasted business volume of dynamic capacity, the Static Capacity need to be provided based on the Active Ratio.
The Active Ratio related to IN Prepaid subscribers in IN System is the proportion of Number of IN subscribers who have Prepaid Cards in activated state, to the total number of IN Subscriber Base in the SCP/SMP.
The Active Ratio related to IN Prepaid subscribers in Switch/HLR is the proportion of the Number of IN subscribers who have Prepaid Cards in activated state, to total Number of IN Subscriber Base in the HLR.
Static Capacity Planning:
Since Revenue generated is 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 IN Prepaid in a well administered network is between 0.6 and 0.5. It means that about 60% to 100% additional subscriber base is required as Static Capacity in IN system and HLR.
For example: for generating a business volume based on ARPU of 800K prepaid subscribers, the subscriber Static Subscriber base required would be around 1300 K to 1600K.
Top-down or Bottom-up planning:
In Top-Down planning, how much subscriber base is required to generate certain volume of business and accordingly Dynamic Capacity is decided.
Dynamic load of the system (BHCA Capacity), is calculated based on BHCA per active subscriber.
The Static Capacity needed is then calculated from the Dynamic Capacity and Active Ratio. The average BHCA per subscriber in the system will be the Dynamic load of the IN divided by the Static subscriber base in the IN.
Example of Top-down planning:
Active subscriber base needed to generate the business volume = 800 K lines
BHCA per active subscriber = 1 BHCA
Dynamic Load of the system = 800 K BHCA
The Static capacity needed for Active Ratio of 0.6 is about 1330 K lines.
The overall IN BHCA will be 0.6 BHCA.
The revenue generated will be equal to Active Subscriber base x ARPU.
Bottom -Up planning:
This is done by the Companies who do IN capacity planning according to forecast of Prepaid subscriber growth. The Dynamic Subscriber based possible in the system is calculated based on the Active Ratio. Dynamic BHCA load to be purchased is calculated 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.
Dynamic subscriber base to generate the budgeted revenue is calculated based on the ARPU.
Static Capacity is calculated based on the Dynamic Capacity and Active Ratio as experienced in the network. Investment needed for the Static and Dynamic capacity is calculated accordingly.
Example of Bottom – Up planning:
IN Prepaid Subscribers according to subscriber growth forecast = 1 million
BHCA per active subscriber = 0.8 BHCA
Dynamic Load of the system = 0.8 million BHCA
The Static capacity needed according to Active Ratio of 0.6 = 1.66 million
The overall IN BHCA will be 0.48 BHCA.
The revenue generated will be equal to Active 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 normal and sporadic overload situations.
Currently, IN systems of major vendors are capable of handling a Dynamic load of about 1.2 to 1.5 million BHCA depending on the type of services and its proportion and Static Capacity of about 2.5 to 3 million subscribers. The dynamic load handling depends on the processor speed. Multiple SCPs systems with higher capacity are not yet available for field use.
It is desirable to do a Bottom – Up planning based on the growth forecast and then decide on the quantum of Static Capacity and Dynamic Capacity needed. The system shall be ordered for Static Capacity and the Dynamic load 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 in a system is dimensioned taking the full BHCA value for the Static subscriber base, resources the cost increases because of the idle capacity, and the optimum capacity of the system has to be rated down.
Ex: Forecasted IN subscriber base. = 1 Million.
BHCA per active subscriber for the pre- repaid Call traffic = 1 BHCA.
Dynamic load needed in the IN system = 1 million. BHCA
Active Ratio anticipated = 0.6
Static Subscriber base needed applying active ratio = 1.66 million
Average BHCA per IN prepaid subscriber = 0.6 BHCA
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. (Pl. see SS7 link calculation given in the end )
Planning of number of IN systems required is done taking into consideration the modularity and scalability of the system. As far it is not possible to have load sharing with multiple SCPs, it is necessary to plan scaling to additional system. 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.