What is Network Dimensioning?
The dimensioning exercise is to identify the equipment and the network type (i.e. technology employed) required in order to cater for the coverage and quality requirements, apart from seeing that capacity needs are fulfilled for the next few years (generally 3–5 years).[1] In other words, network dimensioning is based on the coverage and capacity requirements. The main objective is to optimize the network in a cost effective way.
The purpose of dimensioning a new network/service is to determine the minimum capacity requirements that will still allow the Tele-traffic Grade of Service (GoS) requirements to be met. To do this, dimensioning involves planning for peak-hour traffic, i.e. that hour during the day during which traffic intensity is at its peak.
The dimensioning process involves determining the network’s topology, routing plan, traffic matrix, and GoS requirements, and using this information to determine the maximum call handling capacity of the switches, and the maximum number of channels required between the switches. This process requires a complex model that simulates the behavior of the network equipment and routing protocols.
A dimensioning rule is that the planner must ensure that the traffic load should never approach a load of 100%. To calculate the correct dimensioning to comply with the above rule, the planner must take on-going measurements of the network’s traffic, and continuously maintain and upgrade resources to meet the changing requirements. Another reason for "over-provisioning" is to make sure that traffic can be rerouted in case a failure occurs in the network.
Because of the complexity of network dimensioning, this is typically done using specialized software tools. Whereas researches typically develop custom software to study a particular problem, network operators typically make use of commercial network planning software (e.g. OPNET Technologies, [http://www.wandl.com WANDL], [http://www.vpisystems.com VPISystems]).
System Design & Network Dimensioning
Network planning and design is an iterative process, encompassing topological design, network-synthesis, and network-realization, and is aimed at ensuring that a new network or service meets the needs of the subscriber and operator.[1] The process can be tailored according to each new network or service.[2]
This is an extremely important process which must be performed before the establishment of a new telecommunications network or service.
The network planning process consists of several phases, which can be combined at a higher level to main phases that differ depending on the logics.
The preplanning phase covers the assignments and preparation before the actual network planning is started. The network planning criteria is agreed with the customer. As specified earlier, the requirements depend on many factors, the main criteria being the coverage and quality targets. Also several limitations exist, like the limited frequency band and the budget for the investments. The priority for the planning parameters comes from the customer. Due to the fact that the network plan can not be optimized with regard to all the parameters the priorities need to be agreed with the customer throughout the whole process.
The network planning criteria is used as an input for network dimensioning. The following are listed as the basic inputs for dimensioning:
- coverage requirements, the signal level for outdoor, in-car and indoor with the coverage probabilities;
- quality requirements, drop call rate, call blocking;
- frequency spectrum, number of channels, including information about possible needed guard bands;
- subscriber information, number of users and growth figures;
- traffic per user, busy hour value;
- services
During the process of network planning and design, it is necessary to estimate the expected traffic intensity and thus the traffic load that the network must support.[2] If a network of a similar nature already exists, then it may be possible to take traffic measurements of such a network and use that data to calculate the exact traffic load.[3] However, as is more likely in most instances, if there are no similar networks to be found, then the network planner must use telecommunications forecasting methods to estimate the expected traffic intensity.[2]
The forecasting process involves several steps as follows:[2]
- Definition of problem
- Data acquisition
- Choice of forecasting method
- Analysis/Forecasting
- Documentation and analysis of results
Necessity & Importance
The importance of dimensioning a new network, is to determine the minimum capacity requirements that will still allow the Telet-raffic Grade of Service (GoS) requirements to be met.[2][3] To do this, dimensioning involves planning for peak-hour traffic, i.e. that hour during the day during which traffic intensity is at its peak.[3]
The dimensioning process involves determining the network’s topology, routing plan, traffic matrix, and GoS requirements, and using this information to determine the maximum call handling capacity of the switches, and the maximum number of channels required between the switches.[3] This process requires a complex model that simulates the behavior of the network equipment and routing protocols.
A dimensioning rule is that the planner must ensure that the traffic load should never approach a load of 100 percent.[2][3] To calculate the correct dimensioning to comply with the above rule, the planner must take on-going measurements of the network’s traffic, and continuously maintain and upgrade resources to meet the changing requirements.[3][4]. Another reason for "over-provisioning" is to make sure that traffic can be rerouted in case a failure occurs in the network.
Because of the complexity of network dimensioning, this is typically done using specialized software tools. Whereas researchers typically develop custom software to study a particular problem, network operators typically make use of commercial network planning software. (E.g. OPNET Technologies, SevOne, WANDL, VPISystems, Cariden, Aria Networks). However, there is one notable open source network planning software available by the name of TOTEM named after Toolbox for Traffic Engineering Methods.
Impact upon Network Operators & the Network Users
Network dimensioning is basically based on the subscriber and traffic forecasts. The objective of dimensioning is to model an operator network based on subscriber and traffic forecasts to produce a technically optimum model of the network.[3] It is also typically based on a traffic matrix that contains the bandwidth demand for each source, destination pair.
The dimensioning gives a preliminary network plan as an output, which is then supplemented in coverage and parameter planning phases to create a more detailed plan. The preliminary plan includes the number of network elements that are needed to fulfill the quality of service requirements set by the operator, e.g. in GSM the number of BTSs and TRXs (transceivers). It also needs to be noted that dimensioning is repeated in the case of network extension. The result of dimensioning has two aspects; it tells the minimum number of base stations due to coverage or capacity reasons. Both of these aspects need to be analyzed against the original planning targets. It is also important to understand the forecasts for the subscriber growth and also the services that are going to be deployed. This has posed a challenge in particular to mobile network operators. The dimensioning result is an average capacity requirement per area type like urban, suburban, etc. More detailed capacity planning, capacity allocation for individual cells, can be done using a planning tool having digital maps and traffic information. The dimensioning results are an input for coverage planning, which is the next step in the network planning process. The radio network configuration plan also provides information for preplanning of the transmission network. The topology can be sketched based on the initial configuration and network design criteria.
The definition of the radio network planning criteria is done at the beginning of the network planning process. The customer requirements form the basis of the negotiations and the final radio network planning criteria is agreed between the customer and the radio network planner. The network operator has performance quality targets for the cellular network and these quality requirements are also related to how the end user experiences the network. The network planner’s main target is to build as high a quality network as possible. On the other hand, there must be cost-efficiency – how much money the operator can spend for the investments so that the business is financially profitable. The two factors – network quality and investments – are connected to profit. The link is not straight forward but, is one factor, if the network end user quality perception is good it has an impact on the profits. This explains the complexity of network planning; sufficient cellular network coverage and capacity needs to be created with as low an investment as possible.
The coverage targets include the geographical coverage, coverage thresholds for different areas and coverage probability. The range for a typical coverage probability is 90–95 %. The geographical coverage is case-specific and can be defined in steps according to network roll-out phases.
The quality targets are those agreed in association with the customer and network planning. The main quality parameters are call success or drop call rate, handover success, congestion or call attempt success and customer observed downlink (DL) quality. The DL quality is measured according to BER as defined in GSM specifications and mapped to RXQUAL values. Normally downlink RXQUAL classes 0 to 5 are considered as a sufficient call quality for the end user. The classes 6 and 7 represent poor performance and thus need to be avoided. The target value for RXQUAL can be, for example, 95% or the time equal or better than 5. Example values for network quality targets are shown in the below table.
The coverage and quality targets need to be considered in connection with the network evolution strategy. The subscriber forecast predicts the need and pace of network enlargement. Due to this obvious connection it is important to verify the subscriber forecast from time to time and keep it up to date. The coverage and quality targets need to be adjusted for the different network evolution phases. Interference probability becomes more important as the network capacity enlarges and has to be added as part of the quality targets.
The network features that are used have an effect on the dimensioning phase. The capacity and quality requirements need to be adjusted according to the features in use.
Some parameters that affect network planning cannot be controlled and therefore it is important to know what they are and then to take them into account in the planning. The topology and morphology is always area specific and due to this the area related planning parameters are case specific. An accurate digital map is needed in network planning. The map is used together with the propagation model to calculate the coverage areas in the planning phase. The propagation model is customized for the planning area with propagation measurements.
Population data are needed when estimating subscriber numbers. Population data as a layer on the map are useful in the planning tasks in order to cover a dense population area and allocate needed capacity.
The available bandwidth is a critical network planning parameter. Some basic decisions are dependent on the bandwidth, BTS configuration and frequency planning.
The quality of the cellular network is highly dependent on the quality of the network plan. The network performance will be measured and analyzed to prove that it is working according to the planning requirements.
Does Network Dimensioning Pose a Challenge in Particular to Mobile Network Operators?
The network dimensioning poses a great challenge to the mobile network operators.
Competition is leading increasing numbers of operators to identify network relocation by equipment vendors as an efficient way of solving existing problems, improving network capacity and maintaining competitiveness. However, doing so creates numerous challenges for both operators and vendors.
Firstly, new equipment must enable better network performance in order to solve existing problems. Increasing subscriber numbers and restricted frequency resources mean that operators require base stations with larger capacity and higher spectrum utilization. To reduce CAPEX and OPEX, operators need to minimize the number of sites via technical support from equipment vendors. The new equipment must also provide a rich array of services so as to increase operators' profits.
Secondly, each equipment vendor must possess sufficient relocation project management and implementation capabilities. Outstanding equipment represents a precondition for relocation, while project management describes the core. Each relocation project invariably consists of three phases: preparation, cutover implementation, and network optimization. To implement large-scale network relocation, effective project management systems monitor all the involved procedures. To avoid affecting service usage, cutover occurs in the evenings, and hundreds or thousands of base stations must be cutover in a very limited timeframe, while stability and reliability guarantees remain of paramount importance. This has placed considerable demands upon the shoulders of equipment vendors.
Thirdly, given tight deadlines that involve thousands of TRXs, an equipment vendor must display strong network planning and network optimization capabilities in order to ensure relocation success. The replacement network must exhibit improved quality and solve network congestion, restricted frequency resources and coverage blind spots.
Fourthly, newly adopted equipment must enable long-term evolution, especially towards 3G and IP networks to render operators' investment long-term and sustainable, and obviate the need to significant reinvestment.
Challenges of Network Dimensioning
The network dimensioning challenges are as follows;
- Complex emission environment, severely fluctuate signal, big difference of the multi-approach emission caused by the various man-made buildings, and the difficulties in theoretic prediction of the coverage area.
- Severe interference. Except for the human noises, all adjacent frequency interference, inter-modulation interference and other wireless interference should be considered and controlled in the permitted index during the engineering design.
- Limited frequency resources. It’s getting more serious along with the big increasing of the subscriber.
- There are strict rules for the cell structure and the cell splitting behavior designed for the frequency re-use. The station address planning can hardly be carried out in the real project due to various reasons.
- The investment control is the technical and economical issue of the network construction, which can be by no means ignored.
References
[1] Second-generation Network Planning and Optimization (GSM)
[2] Penttinen A., Chapter 10 – Network Planning and Dimensioning, Lecture Notes: S-38.145 - Introduction to Teletraffic Theory, Helsinki University of Technology, Fall 1999.
[3] Farr R.E., Telecommunications Traffic, Tariffs and Costs – An Introduction For Managers, Peter Peregrinus Ltd, 1988.
[4]. Wiley%20Advanced%20Cellular%20Network%20Planning%20and%20Optimisation%20Jan%202007.pdf
[5] http://books.google.lk/books?id=ynyG9TB-tJ0C&pg=PA30&lpg=PA30&dq=network+pre-
planning+challenges&source=bl&ots=HoImfCjKmu&sig=8K6JWYiSOUSFDrmllsCChu6hHg0&hl=
en&ei=0eK5TKCJHeWP4ga2vqWLDg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBMQ6
AEwAA#v=onepage&q&f=false
[6] http://www.scribd.com/doc/22066142/Chapter1-Network-Planning-Overview
