1753.7.1.2.3Blocking Probability
3.7.2 Network Dimensioning Process
The network dimensioning process is described below in detail. As the whole dimensioning process is in fact a chain of small processes that have there respective inputs and outputs, with outputs of a preceding one being the inputs to the next, the best method is to detail each process individually in form of steps of the global dimensioning process.
3.7.2.1 Network Dimensioning Engine
During the dimensioning process, Atoll first computes the number of timeslots required to accommodate the circuit switched traffic. Then it calculates the number of timeslots to add in order to satisfy the demand of packet switched traffic. This is performed using the quality curves entered in the dimensioning model used. If the dimensioning model has been indicated to take all three KPIs in to account (throughput reduction factor, delay and blocking probability), the number of timeslots to be added is calculated such that:
• The throughput reduction factor is greater than the minimum throughput reduction factor,
• Delay is less than the maximum permissible delay defined in the service properties, and
• The blocking probability is less than the maximum allowable blocking probability defined in the service properties.
The figure below depicts a simplified flowchart of the dimensioning engine in Atoll.
Figure 3.10: Blocking Probability for Different Packet Switched Traffic Loads (Lp, X-axis)
Reference: T. Halonen, J. Romero, J. Melero; GSM, GPRS and EDGE performance – Evolution towards 3G/UMTS, John Wiley and Sons Ltd.
Figure 3.11: Network Dimensioning Process
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On the whole, following are the inputs and outputs of the network dimensioning process:
3.7.2.1.1 Inputs
• Circuit switched traffic demand
• Packet switched traffic demand
• Timeslot configurations defined for each subcell
• Target traffic overflow rate and Half-rate traffic ratio for each subcell
• Service availability criteria: minimum required throughput per user, maximum permissible delay, maximum allowable blocking probability etc.
• Dimensioning model parameters: Maximum number of TRXs per transmitter, dimensioning model for circuit switched traffic, number of minimum dedicated packet switched timeslots per transmitter, maximum number of TRXs added for packet switched services, KPIs to consider, and their quality curves.
3.7.2.1.2 Outputs
• Number of required TRXs per transmitter
• Number of required shared, circuit switched and packet switched timeslots
• Traffic load
• Served circuit switched traffic
• Served packet switched traffic
• Effective rate of traffic overflow
• Actual KPI values: throughput reduction factor, delay and blocking probability
3.7.2.2 Network Dimensioning Steps
This section describes the entire process step by step as it is actually performed in Atoll. Details of the calculations of the parameters that are calculated during each step are described as well.
3.7.2.2.1 Step 1: Timeslots Required for CS Traffic
Atoll computes the number of timeslots required to accommodate the circuit switched traffic assigned to each subcell. Atoll takes the circuit switched traffic demand (Erlangs) either user-defined or calculated in the traffic analysis and assigned to the current subcell and the maximum blocking probability defined for the circuit switched service, and computes the required number of timeslots to satisfy this demand using the Erlang B or Erlang C formula (as defined by the user).
If the user-defined target rate of traffic overflow per subcell, OTarget, is greater than the maximum blocking rate defined in the services properties, it is going to be taken as the Grade of Service required for that subcell instead of the maximum blocking rate of the service.
For the blocking probability GoS and circuit switched traffic demand TDC, Atoll determines the required number of timeslots TSreq. C for each subcell using formulas described below. In fact, Atoll searches for TSreq. C value until the defined grade of service is reached.
For Erlang B, we have:
For Erlang C, we have:
Atoll considers the effect of half-rate circuit switched traffic by taking into account a user-defined percentage of half-rate traffic. Atoll computes the effective equivalent number of full-rate timeslots that will be required to carry the total traffic with the defined percentage of half-rate traffic.
If the number of timeslots required to accommodate the full-rate circuit switched traffic is TSreq. FR, and the percentage of half-rate traffic within the subcell is defined by HR, then the effective number of equivalent full-rate circuit switched timeslots TSeff. that can carry this traffic mix is calculated by:
GoS
Atoll employs this simplified approach to integrating half-rate circuit switched traffic, which provides approximately the same results as obtained by using the half-rate traffic charts.
3.7.2.2.2 Step 2: TRXs Required for CS Traffic and Dedicated PS Timeslots
This stage of the network dimensioning process computes the number of TRXs required to carry the circuit switched traffic demand through the number of required timeslots calculated above and the timeslot configuration defined by the user in the network settings. Atoll distributes the number of required circuit switched timeslots calculated in Step 1 taking into account the presence of dedicated packet switched timeslots in each TRX according to the timeslot configurations.
If a timeslot configuration defines a certain number of dedicated packet switched timeslots pre-allocated in certain TRXs, those timeslots will not be considered capable of carrying circuit switched traffic and hence will not be allocated. For example, if 4 timeslots have been marked as packet switched timeslots in the first TRX and Atoll computes 8 timeslots for carrying a certain circuit switched traffic demand, then the number of TRXs to be allocated cannot be 1 even if there is no packet switched traffic considered yet.
The total numbers of timeslots that carry circuit switched and packet switched traffic respectively are the sums of respective dedicated and shared timeslots:
and
3.7.2.2.3 Step 3: Effective CS Blocking, Effective CS Traffic Overflow and Served CS Traffic
In this step, the previously calculated number of required TRXs is used to compute the effective blocking rate for the circuit switched traffic. This is performed by using the Erlang B or Erlang C formula with the circuit switched traffic demand and the number of required TRXs as inputs and computing the Grade of Service (or blocking probability). It then calculates the effective traffic overflow rate, Oeff..
In case of Erlang B formula, the effective rate of traffic overflow for the circuit switched traffic is the same as the circuit switched blocking rate. While in case of the Erlang C model, the circuit switched traffic is supposed to be placed in an infinite-length waiting queue. This implies that there is no overflow in this case.
From this data, it also computes the served circuit switched traffic. This is the difference of the circuit switched traffic demand and the percentage of traffic that overflows from the subcell to other subcells calculated above. Hence, for an effective traffic overflow rate of Oeff. and the circuit switched traffic demand of TDC, the served circuit switched traffic STC is computed as:
3.7.2.2.4 Step 4: TRXs to Add for PS Traffic
This step is the core of the dimensioning process for packet switched services. First of all, Atoll computes the number of TRXs to be added to carry the packet switched traffic demand. This is the number of TRXs that contain dedicated packet switched and shared timeslots.
To determine this number of TRXs, Atoll calculates the equivalent average packet switched traffic demand in timeslots by studying each pixel covered by the transmitter. This calculation is in fact performed in the traffic analysis process or is user-defined in the subcells table. Knowing the traffic demand per pixel of the covered area in terms of kbps and the maximum attainable throughput per pixel (according to the C and/or C/I conditions and the coding scheme curves in the GPRS/EDGE configuration), Atoll calculates the average traffic demand in packet switched timeslots by:
The average timeslot capacity of a transmitter is calculated by dividing the packet switched traffic demand over the entire coverage area (in kbps) by the packet switched traffic demand in timeslots calculated above.
With the number of timeslots required to serve the circuit switched traffic, the timeslots required for packet switched traffic and their respective distributions according to the timeslot configurations being known, Atoll calculates the number of timeslots available for carrying the packet switched traffic demand. These timeslots can be dedicated packet switched timeslots and the shared ones. So, following the principle that shared timeslots are potential carriers of both traffic types,
The packet switched traffic load is calculated by the formula:
The second important parameter for the calculation of Reduction Factor, Delay and Blocking Probability is the equivalent number of available timeslots for packet switched traffic, i.e. NP. This is computed by dividing the total number of timeslots TSP = TSS+TSP dedicated TSC = TSS+TSC dedicated
STC = TDC1 O– eff
TDP
Timeslots
Traffic demand per pixel (kbps) Throughput per pixel (kbps)
---pixel
=
TSP = TSS+TSP dedicated
TSC = TSS+TSC dedicated
LP STC–TSC dedicated TDP
Timeslots
+
TSP
---=
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available for carrying packet switched traffic by the number of downlink timeslots defined in the mobile terminal properties.
So, NP is calculated at this stage as:
, TSTerminal is the number of timeslots that a terminal will use in packet switched calls.
The number timeslots that a terminal can use in packet switched calls is the product of the number of available DL timeslots for packet-switched services (on a frame) and the number of simultaneous carriers (in case of EDGE evolution).
The number of timeslots that a terminal will use in packet switched calls is determined by taking the lower of the maximum number of timeslots on a carrier for packet switched service defined in the service properties and the maximum number of timeslots that a mobile terminal can use for packet switched services (see above) on acarrier.
and
Here, the min(X,Y) function yields the lower value among X and Y as result.
Now, knowing the packet switched traffic load, LP, and the equivalent number of available timeslots, NP, Atoll finds out the KPIs that have been selected before launching the dimensioning process using the quality curves stored in the dimensioning model.
This particular part of this step can be iterative if the KPIs to consider in dimensioning are not satisfied in the first try. If the KPIs calculated above are within acceptable limits as defined by the user, it means that the dimensioning process has acceptable results. If these KPIs are not satisfied, then Atoll increases the number of TRXs calculated for carrying packet switched traffic by 1 (each increment adding 8 more timeslots for carrying packet switched traffic as the least unit that can be physically added or removed is a TRX) and resumes the computations from Step 3. It then recalculates the packet switched traffic load, LP, and the equivalent number of available timeslots, NP. Then it recomputes the KPIs with these new values of LP and NP. If the KPIs are within satisfactory limits the results are considered to be acceptable. Otherwise, Atoll performs another iteration to find the best possible results.
The calculated values of all the KPIs are compared with the ones defined in the service properties. The values for maximum Delay and Blocking probability are defined directly in the properties but the minimum throughput reduction factor is calculated by Atoll using the user’s inputs: minimum throughput per user and required availability. This calculation is in fact performed during the traffic analysis process, but since it is relevant to the dimensioning procedure, it is displayed in a column in the dimensioning results so that the user can easily compare the minimum requirement on the reduction factor KPI with the resulting one. If dimensioning is not based on a traffic analysis, the minimum throughput reduction factor is a user-defined parameter.
Minimum Throughput Reduction Factor Calculation
The minimum throughput reduction factor is computed using the input data: minimum required throughput per user defined in the service properties, the average throughput per timeslot deduced from the throughput curves stored in the GPRS/EDGE configuration properties for each coding scheme, the total number of downlink timeslots defined in the properties of the mobile terminal (See defintion above) and the required availability defined in the service properties.
It is at the stage of calculating the average timeslot capacity per transmitter that Atoll studies each covered pixel for carrier power or carrier-to-interference ratio. According to the measured carrier power or carrier-to-interference ratio, Atoll deduces the maximum throughput available on that pixel through the throughput vs. C or throughput vs. C/I curves of the GPRS/EDGE configuration.
The throughput per timeslot per pixel TPTS, Pixel can be either a function of carrier power C, or carrier power C and the carrier-to-interference ratio C/I, depending on the user-defined traffic analysis RF conditions criteria. Therefore,
Or
and
The required availability parameter defines the percentage of pixels within the coverage area of the transmitter that must satisfy the minimum throughput condition. This parameter renders user-manageable flexibility to the throughput requirement constraint.
To calculate the minimum throughput reduction factor for the transmitter, Atoll computes the minimum throughput reduction factor for each pixel using the formula:
NP TSP TSTerminal
---=
TSTerminal = min TS Max Service TSMax TerminalType
TSMax TerminalType = TSDL TerminalType CarriersDL TerminalType
TSMax TerminalType
TPTS Pixel = f C
TPTS Pixel = f C TPTS Pixel f C ---i
=
Once the minimum reduction factor for each pixel is known, Atoll calculates the global minimum reduction factor that is satisfied by the percentage of covered pixels defined in the required availability. The following example may help in understanding the concept and calculation method.
Example: Let the total number of pixels, covered by a subcell S, be 1050. The reliability level set to 90%. This implies that the required minimum throughput for the given service will be available at 90% of the pixels covered. This, in turn, implies that there will be a certain limit on the reduction factor, i.e. if the actual reduction factor in that subcell becomes less than a minimum required, the service will not be satisfactory.
Atoll computes the minimum reduction factor at each pixel using the formula mentioned above, and outputs the following results:
So for a reliability level of 90%, the corresponding RFmin will be the one provided at least 90% of the pixels covered, i.e. 945 pixels. The corresponding value of the resulting RFmin in this example hence turns out to be 0.9, since this value covers 962 pixels in total. Only 87 of the covered pixels imply an RFmin of 0.98. These will be the pixels that do not provide satisfactory service.
This calculation is performed for each service type available in the subcell coverage area. The final minimum throughput reduction factor is the highest one amongst all calculated for each service separately.
The minimum throughput reduction factor RFmin value is a minimum requirement that must be fulfilled by the network dimensioning process when the Reduction Factor KPI is selected in the dimensioning model.
3.7.2.2.5 Step 5: Served PS Traffic
Atoll calculates the served packet switched traffic using the number of timeslots available to carry the packet switched traffic demand. As the result of the above iterative step, Atoll always finds the best possible answer in terms of number of timeslots required to carry the packet switched traffic demand unless the requirement exceeds the maximum limit on the number of the packet switched traffic timeslots defined in the dimensioning model properties. Hence, there is no packet traffic overflow unless the packet switched traffic demand requires more TRXs than the maximum allowed
3.7.2.2.6 Step 6: Total Traffic Load
This step calculates the final result of the dimensioning process, i.e. the total traffic load. The total traffic load L is calculated as:
RFmin Number of pixels
0.3 189
0.36 57
0.5 20
0.6 200
0.72 473
0.9 23
0.98 87
Figure 3.12: Minimum Throughput Reduction Factor RFmin Pixel TPuser min
TPTS Pixel TSTerminal
---=
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,
• STC is the served circuit switched traffic
• STP is the served packet switched traffic
• TSC, dedicated is the number of dedicated circuit switched timeslots
• TSP, dedicated is the number of dedicated packet switched timeslots
• TSS is the number of shared timeslots