Network Dimensioning

Atoll is capable of dimensioning a GSM GPRS EDGE network with a mixture of circuit and package switched services. This section describes the technical details of Atoll’s dimensioning engine.

3.7.1 Dimensioning Models and Quality Graphs

In Atoll, a dimensioning model is an entity utilized by the dimensioning engine along with other inputs (traffic, limitations, criteria, etc.) in the process of dimensioning. A dimensioning model defines the QoS KPIs to be taken into account when dimensioning a network for both circuit and packet switched traffic. The user can define either to use Erlang B or Erlang C queuing model for circuit switched traffic and can define which KPIs to consider when dimensioning the network for packet switched traffic. The dimensioning engine will only utilize the quality curves of the KPI selected. The KPIs not selected are supposed to be either already satisfactory or not relatively important.

3.7.1.1 Circuit Switched Traffic

The network dimensioning for circuit switched traffic is performed using the universally accepted and adopted Erlang B and Erlang C formulas. The dimensioning criterion in these formulas is the Grade of Service or the allowed blocking probability of the circuit switched traffic.

In the Erlang B approach, this Grade of Service is defined as the percentage of incoming circuit switched calls that are blocked due to lack of resources or timeslots. This formula implies a loss system. The blocked calls are supposed to be lost and the caller has to reinitiate it.

In the Erlang C approach, the Grade of Service is the percentage of incoming calls that are placed in a waiting queue when there are no resources available, until some resources or timeslots are liberated. This queuing system has no lost calls. As the load on the system increases, the average waiting time in the queue also increases.

These formulas and their details are available in many books. For example, Wireless Communications Principles and Practice by Theodore S. Rappaport, Prentice Hall.

Following the common practice, network dimensioning in Atoll is based on the principle that a voice or GSM call has priority over data transmission. Therefore, as explained later in the network dimensioning steps, Atoll first performs network dimensioning according to the circuit switched traffic present in the subcell in order to ensure the higher priority service availability before performing the same for the packet switched traffic.

O_maxTxj,TCH_INNER O_maxTxi TCH  O_maxTxi,TCH_INNER

S^microTxi,TCH S^microTxi,TCH_INNER

D_{p t m}^macro_{ }

S^microTxi,TCH S^microTxi,TCH_INNER

3.7.1.2 Packet Switched Traffic

Since packet switched traffic does not occupy an entire timeslot the whole time, it is much more complicated to study than circuit switched traffic. Packet traffic is intermittent and bursty. Whenever there is packet data to be transferred, a Temporary Block Flow (TBF) is initiated for transferring these packets. Multiple TBFs can be multiplexed on the same timeslot. This implies that there can be many packet switched service users that have the same timeslots assigned for packet data transfer but at different intervals of time.

This multiplexing of a number of packet switched service users over the same timeslots incurs a certain reduction in the throughput (data transfer rate) for each multiplexed user. This reduction in the throughput is more perceivable when the system traffic load is high. The following parts describe the three most important Key Performance Indicators in GPRS/EDGE networks and how they are modelled in Atoll.

3.7.1.2.1 Throughput

Throughput is defined as the amount of data delivered to the Logical Link Control Layer in a given unit of time. Each temporary block flow (TBF), and hence each user, has an associated measured throughput sample in a given network. Each network will have a different throughput probability distribution depending on the load and network configuration. Instead of using the precise probability distributions, it is more practical to compute the average and percentile throughput values.

In GPRS, the resources are shared between the users being served, and consequently, the throughput is reduced as the number of active users increases. This reduction in user perceived throughput is modelled through a reduction factor. The throughput experienced by a user accessing a particular service can be calculated as:

User throughput = Number of allocated timeslots x Timeslot capacity x Reduction Factor Or

User throughput per allocated timeslot = Timeslot capacity x Reduction Factor Timeslot Capacity

The timeslot capacity is the average throughput per fully utilized timeslot. It represents the average throughput from the network point of view. It mainly depends on the network’s propagation conditions and criteria in the coverage area of a transmitter (carrier power, carrier-to-interference distribution, etc.). It is a measure of how much data the network is able to transfer with 1 data Erlang, or in other words, how efficiently the hardware resources are being utilized by the network. It may also depend on the RLC protocol efficiency.

Atoll computes the average timeslot capacity during the traffic analysis and is used to determine the minimum throughput reduction factor. But since this information is displayed in the network dimensioning results (only due to relevance), this information has been considered as a part of the network dimensioning process in this document.

Timeslot Utilisation

Timeslot utilization takes into account the average number of timeslots that are available for packet switched traffic. It is a measure of how much the network is loaded with data services. Networks with timeslot utilisation close to 100% are close to saturation and the end-user performance is likely to be very poor.

In Atoll this parameter is termed as the Load (Traffic load for circuit switched traffic and packet switched traffic load for packet switched traffic). It is described in more detail in the Network dimensioning steps section.

Reduction Factor

Reduction factor takes into account the user throughput reduction due to timeslot sharing among many users. The figure below shows how the peak throughput available per timeslot is reduced by interference and sharing.Reduction factor is a function of the number of timeslots assigned to a user (N_u), number of timeslots available in the system (N_s) and the average system packet switched traffic load (L_p) (utilization of resources in the system). Data Erlangs or data traffic is given by:

Data Erlangs = L_PN_S

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More precisely, the reduction factor is a function of the ratio N_s/N_u (N_p). N_p models the equivalent timeslots that are available for the packet switched traffic in the system. For example, a 24-timeslot system with each user assigned 3 timeslots per connection can be modelled by a single timeslot connection system with 8 timeslots in total.

The formula for reduction factor can be derived following the same hypotheses followed by Erlang in the derivation of the blocking probability formulas (Erlang B and Erlang C).

Let X be a random variable that measures the reduction factor in a certain system state:

n is the instantaneous number of connections in the system. The throughput reduction factor is defined as:

Or,

Here, P(X=n) is the probability function of having n connections in the system. Under the same assumptions as those of the Erlang formulas, the probability function can be written as:

Hence the reduction factor can finally be written as:

Figure 3.8: Reduction of Throughput per Timeslot

This formula is not directly applicable in any software application due to the summations up to infinity. Atoll uses the following version of this formula that is exactly the same formula without the summation overflow problem.

The default quality curves for the Reduction Factor have been derived using the above formula. Each curve is for a fixed number of timeslots available for packet switched traffic (N_p) describing the reduction factor at different values of packet switched traffic load (L_p). The figure below contains all the reduction factor quality curves in Atoll. The Maximum reduction factor can be 1, implying a maximum throughput, and the minimum can be 0, implying a saturated system with no data throughput.

Each curve in the above figure represents an equivalent number of packet switched timeslots, N_P.

3.7.1.2.2 Delay

Delay is the time required for an LLC PDU to be completely transferred from the SGSN to the MS, or vice versa. As the delay is a function of the delays and the losses incurred at the packet level, the network parameters, such as the packet queue length, and different protocol properties, such as the size of the LLC PDU, become important. It is also quite dependent upon the radio access round trip time (RA RTT) and has a considerable impact on the application level performance viewed by the user.

The delay parameter is a user level parameter rather than being a network level quantity, like throughput per cell, timeslot capacity, TBF blocking and reduction factor, hence it is difficult to model and is currently under study. Hence, no default curve is presently available for delay in Atoll.

Figure 3.9: Reduction Factor for Different Packet Switched Traffic Loads (L_p, X-axis) RF

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