IEEE 802.11 Wireless LAN


19.5.2000

Marja-Leena Lahti

Department of Electrical Engineering
Helsinki University of Technology
marja-leena.lahti@sonera.com

Abstract

This document introduces the IEEE standard 802.11 for Wireless Local Area Network (WLAN). The document also makes a comparison to GSM cellular network, where the cell size could be much larger. The paper discusses the basis outlining the network planning process, such as frequency, scale of mobility, transmission capacity needs and population variation. Finally the total network planning process of a cellular network is introduced. The question is how does the use of unlicensed frequency band affect the WLAN network planning. Finally the study suggest what are the relevant requirements for wireless communication with short range mobility. It also suggests what GSM planning criteria can be omitted in the WLAN environment.


Table of Contents

1 Introduction
 

2 The Standards
2.1 Characteristics of GSM and WLAN
2.1.1 Extent of Coverage
2.1.2 Services
2.1.3 Cellular and Non Cellular -Networks
2.2 GSM Physical Layer
2.3 IEEE 802.11 Protocol Layers

3 Basis for the Network Planning Process
3.1 Frequency Range
3.2 Mobility Aspects
3.2.1 Scale of Mobility
3.2.2 Speed of the Mobile
3.3 Transmission Capacity Needs
3.4 Power Control
3.5 Population Variation
3.6 Timing Advance

Network Planning Process
4.1 Targets for the Network Planning
4.2 Coverage Planning
4.3 Capacity Planning
4.4 Frequency Planning
4.5 Parameter Planning and the Follow Up
4.6 Relevant Requirements for IEEE 802.11
4.6.1 Infrastructure WLAN
4.6.2 Ad Hoc WLAN

5 Conclusion

6 References

7 Further Information
 

1 Introduction

The purpose of this document is to introduce the standard IEEE 802.11 for Wireless Local Area Network (WLAN). Also the document compares the different mobility scenarios of short range WLAN users with long range GSM users. Also an important issue is the network planning process, due to the use of unlicensed frequency band of WLAN.

In the first chapter the technologies, the cellular standard GSM and WLAN are introduced. Then the aspects outlining the network planning process - such as frequency, scale of mobility, transmission capacity needs, population variation - are discussed. Finally the total network planning process of a cellular network is introduced. An important question is how does the use of unlicensed frequency band affect network planning in the WLAN environment. Finally the study suggests what are the relevant requirements for wireless communication with short range mobility. It also suggests what GSM planning criteria can be omitted in the WLAN environment.
 

2 The Standards

2.1 Characteristics of GSM and WLAN

GSM and IEEE 802.11 wireless LAN have very different characteristics. The main differences between the two technologies can be found in extent of coverage, services and therefore users and scale of mobility. The mobility issues are discussed further in the section 3.2. These two wireless networks can also be categorized as a cellular and a non-cellular network.
 

2.1.1 Extent of Coverage

GSM is a cellular network typically offering a nation wide coverage depending on the operator. With the possibility of international roaming GSM user can use mobile services also globally. Also national roaming is supported by GSM.

The GSM network is consisted of cells. The coverage area of each cell is different in different environments. Macro cells can be regarded as cells where the base station antenna is installed in a mast or a building above the average roof top level. However, small cells or micro cells are cells where the antenna height is under the average roof top level. Thus the cell radius can vary depending on the antenna height, antenna gain and propagation conditions from couple of hundred meters to several tens of kilometers. Officially 35 km is the longest distance GSM specification supports, though the specifications define an extended cell, where the cell radius could be double [8]. Also indoor coverage is supported by GSM. Indoor coverage can be built by using power splitters to deliver RF signal from the antenna outdoors to separate indoor antenna distribution system. When all the capacity of the cell is needed indoors, e.g. in shopping centers or airports etc., the indoor coverage can be built by using antennas only inside the building. In suburban areas the indoor coverage is usually origin by the inbuilding penetration of radio signal, not by a separate indoor antenna system.

WLAN is a standard offering a limited coverage for LAN users. Cell radius is usually from few tens of meters to some hundred meters. IEEE 802.11 wireless LAN is a locally situated network - a local area network. The coverage area is consisted of small islands and the purpose is certainly not to offer a large coverage network like GSM. The coverage area is often tailored according to the users own need and can also be temporary.

The specifications of wireless LAN outlines two possible modes of operations: client/server and ad hoc mode WLAN. [4] In the client/server WLAN - also often called as an infrastructure configuration - terminals communicate with base stations or access points (AP), which form the coverage area. The access points are further connected to the wired network. This document concentrates mainly on the infrastructure WLAN when discussing network planning issues. This is because a WLAN access point can comparable to a GSM base station, but in the ad hoc mode the same station acts both as an access point and as a station and needs therefore a different point of view.

The coverage area of the client/server type WLAN network is usually bordered upon a building or a campus and can therefore be comparable to a single GSM inside cell. The main difference between the GSM and WLAN technologies are, that to cover the whole building with the WLAN technology, there has to be several access points depending on the building architecture, wall materials etc. In the GSM solution the coverage area can be built with a single base station by distributing the signal into antennas locating in different rooms by using power splitters.

The specifications of WLAN defines also an ad hoc mode. In this mode mobile terminals by themselves build the network. The coverage area is built by the help of wireless adapters and is limited. [5, 13] In the ad hoc mode the whole network is seen as movable, and it is independent of any infrastructure unlike GSM or client/server type WLAN. It is also isolated, because it has no interface to the wired network.
 

2.1.2 Services

GSM has been defined with the main purpose of voice services, operators offer also data services at speeds of 9,6 and 14,4 kbit/s [8]. Although a cellular network can never be regarded as completed, today the operators can less and less compete with coverage area or quality of the network. However, in these days the data services start to play a big role in operator business. GSM service called High Speed Circuit Switch Data Service (HSCSD) offers data services at speed up to 57,6 kbit/s, depending on the multislot usage. Also this year some advanced operators will launch a new data service named General Packet Radio Service (GPRS), which enables data communication in the first phase at speed of 9,1-40,2 kbit/s, depending on the possible multislot usage and the coding scheme used. Also short message service and a set of value added services are specified in GSM system [8].

The services and therefore the users of WLAN are different from those in GSM network. Due to the versatile set of GSM services voice, data, short message services etc. and the possibility of the large scale mobility GSM can be regarded as a competitor to the wired public network. However, WLAN supports only data communication and therefore can be seen as a potential alternative or an extension to the wired LAN. [11]

The standardization process of IEEE 802.11 begun from the need to connect wirelessly to the wired Ethernet based data communications network and thus to offer mobility for the LAN users within a rather small area. One of the main advantages of WLAN is that it provides LAN users an access to real time information anywhere in their organization. Further one aim was to reduce installation costs and thus achieve short and long term cost savings. Installing a wireless system can be fast and the need to pull cable through walls and ceilings can be eliminated. Wireless technology allows also the network go where the wire cannot go.

The specification work of the standard IEEE 802.11 was started in 1990 and completed in 1997 by the Institute of Electrical and Electronic Engineering (IEEE) [13]. The first phase of the standard IEEE 802.11 supports only 1 Mbit/s and 2 Mbit/s data rates. The first phase standard was followed by an extension IEEE 802.11b, which supports data rates up to 11 Mbit/s with the radio frequency technology direct sequence spread spectrum. [6, 12] However, the user does not have to purchase the radio operator's license in order to use the frequency band 2,4-2,483 GHz, which is dedicated to WLAN use, but it is also known as an ISM band (band for the industrial, scientific and medical use) [14]. This means that anyone who have access to a WLAN can buy and install an access point. This is a very problematic basis from the network planning point of view.
 

2.1.3 Cellular and Non Cellular -Networks

GSM and WLAN network can also be categorized into cellular and non-cellular communication systems. [2] In a cellular system like GSM, the network is consisted of cells. Each cell has it's own serving area and the user can move over the cell boundaries by handovers. This can mean a very high infrastructure cost, because several cells need to exist, before the user can move within the network. Also very complex mobility protocols and signalling is needed [9].

The non-cellular WLAN do not require an infrastructure when using ad hoc mode of operation. Thus the complete system can be seen as movable.
 

2.2 GSM Physical Layer

The operation frequency of GSM lies in the 900 MHz band. The reserved bandwidth can be divided into three areas [8]. Each of these frequency areas is located in the 900 MHz band and uses separate frequencies for uplink and downlink directions. However, there are also extra frequencies defined for the GSM use in 1800 MHz band. The bandwidth specified for GSM use in 1800 MHz is three times wider than the bandwidth of primary GSM in the 900 MHz frequency band. The purpose of reserving frequencies also in the 1800 MHz band was to fulfill the requirements of the increasing capacity need also in the future [10]. Today e.g. in Finland there are several mobile operators which operates either on both bands or in the 1800 MHz band only. The specifications allow a handover between these two bands. Also there are several dual mode cellular phones available in the market, which in the last resort enables the mobility in the dual band network.

The physical layer of GSM defines the modulation scheme used, which can be considered as a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA) [8]. The spectrum available for GSM use is first according to FDMA technique divided into channels, each channel 200 kHz of bandwidth. One or more frequencies are then assigned to each base station (BTS). Every 200 kHz channel is further divided into 8 time slots. Each user is then assigned to a time slot or to a set of time slots. Transmission is possible only during this time slot, after that the user has to wait until he receives another time slot.
 

2.3 IEEE 802.11 Protocol Layers

The specifications of IEEE 802.11 define two layers: layer one is called Physical Layer (PHY) and layer two is called Media Access Control (MAC) layer. Layer one specifies the modulation scheme used and signalling characteristics for the transmission through the radio frequencies, whereas the layer two defines a way accessing the physical layer [7]. The specifications of the IEEE 802.11 layer two, called MAC layer, defines also services related to the radio resource and the mobility management [13].

The standard defines three different physical layer characteristics for WLAN: one infrared, and two RF transmission methods. This document concentrates only on the two RF methods: direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). [12] However, due to the operation in an unlicensed RF band, the spread spectrum modulation must fulfill the requirements set by each country.

The chosen modulation techniques for the DSSS is Differential Bi and Quadrature Phase Shift Keying (DBPSK and DQPSK). However, the FHSS uses 2-4 level Gaussian Frequency Shift Keying (GFSK) as the modulation schemes. Depending on the modulation scheme used both direct sequence and frequency hopping spread spectrum supports data rates of 1 Mbit/s and 2 Mbit/s. As mentioned in chapter 2.1.2 also the data rates of 8 Mbit/s and 11 Mbit/s with the DSSS are possible to achieve while using WLAN that supports the standard IEEE 802.11b [6, 12]. The operation frequency of both RF methods is 2,4 GHz. [5, 13]

In order to wireless LAN devices to be interoperable, they have to have the same physical layer standard. Therefore a DSSS equipment is not capable of communicating with a FHSS based equipment.
 

3 Basis for the Network Planning Process

The section four describes the major points outlining the network planning process, both from the GSM and the WLAN technology point of view. The WLAN part has been written from the infrastructure WLAN point of view unless otherwise stated. However, some problems concerning planning an ad hoc WLAN has been discussed in the section 4.6.2.
 

3.1 Frequency Range

The frequency used in wireless communication is strongly affecting the range of coverage. In different frequency band there are different circumstances to the RF signal to propagate. However, the used spectrum makes anyway the environment of GSM and WLAN different, therefore propagation issues of different frequency band is not compared in this document. This section states the allocated frequencies for WLAN use, as well as gives an example of a frequency plan of a DSSS WLAN.

As mentioned earlier the specified frequencies for GSM use lie in the 900 and 1800 MHz. However, both WLAN RF methods are planned to operate in the unlicensed 2,4 GHz frequency band occupying typically a bandwidth of 83 MHz between 2,4-2,483 GHz. The maximum allowed antenna gain is 6 dBi. Also different frequencies are approved in different countries. [4, 12]

The direct sequence spread spectrum generates a redundant bit pattern, length of eleven bits, which then multiplies each data bit to be transmitted. The bit pattern is also often referred to a chipping code. Only one chipping code is used [12] unlike in CDMA system, which has a set of codes. The receiver must be tuned into the right frequency in order to be able to receive the information being broadcast. Direct sequence spread spectrum is at the moment the one that most wireless spread spectrum WLANs uses [14].

In Europe there are 13 frequencies for DSSS use, the center frequencies and corresponding channel numbers are listed in the table 3.1. The channels are overlapping and the receiver adjacent channel rejection demand is 35 dB, which needs 30 MHz gap between the neighbouring channels. [4] The center frequencies are located in 5 MHz interval and according to the adjacent channel rejection requirement, there has to be five channels between, in order to avoid interference caused by neighbouring access points. This frequency reuse plan means also that an access point having six neighbours, alternate neighbour uses the same frequency as illustrated in figure 3.1. This frequency design demand has to be taken into account, when choosing operating frequency to the DSSS equipment. The problem is that there is no controlling organisation that follows the reuse of frequencies, like regulator and operators in the case of GSM.

Table 3.1. DSSS frequency plan used in Europe.[4]

 
Channel number
Frequency
1
2412 MHz
2
2417 MHz
3
2422 MHz
4
2427 MHz
5
2432 MHz
6
2437 MHz
7
2442 MHz
8
2447 MHz
9
2452 MHz
10
2457 MHz
11
2462 MHz
12
2467 MHz
13
2472 MHz
Figure 3.1. An example of a frequency plan of an infrastructure network using DSSS.

In the frequency hopping spread spectrum the carrier changes the frequency used according to a pattern known both by the transmitter and the receiver. There are 79 non-overlapping channels specified, each channel occupying 1 MHz. 26 different pseudorandom hopping pattern are specified. [4] The hopping rate is specified by the regulator of each country. Therefore WLAN equipment must meet the requirements of the country where it is sold. [12] The user can choose the frequency hopping pattern used by a FHSS equipment.

The frequency hopping spread spectrum is a technique that enables coexistence of multiple networks or other devices in the same area. This is due to the resistant to multipath fading through the inherent frequency diversity system. Also the frequency hopping design criteria described in [1] have the desirable effect on the multipath fading resistant.

Certain differences can be found between the direct sequence and the frequency hopping spread spectrum. DSSS supports bigger data rates (up to 11 Mbit/s) and FHSS equipment are usually inexpensive compared to DSSS equipment. [6, 12, 14]. In a case where high throughput is needed and the interference is not a problem DSSS is the better choice.

The defined frequency band or the frequency channels available sets the practical limit for the capacity of a GSM network. However, this is rarely the case, typically the defined bandwidth is divided between numerous operators by the regulator of each country. Thus the regulator defines the maximum capacity of the network.

As stated the use of the ISM band requires no operator license. The technology choice of spread spectrum is different from the one used in GSM. Therefore the actual capacity limit is not defined according the available bandwidth but the interference level. Further packet mode traffic, which is used in WLAN, does not know the specification of congestion. Therefore the available transmission capacity depends strongly on the interference level. The problem of a WLAN is that there could be interference not only caused by the other access points located in the own network, but also caused by neighbouring WLAN or other equipment using the same frequency band. There is not much to do with the interference level in the latter case.
 

3.2 Mobility Aspects

In this chapter the mobility aspects scale of mobility and speed of a mobile of GSM and WLAN are discussed.

The main difference is that GSM network is consisted of cells and therefore requires a handover mechanism. However, the specifications of WLAN do not define a handover mechanism [13], therefore the roaming is transparent to the MAC layer of IEEE 802.11. This means that in case of an Infrastructure WLAN from the wired network point of view there seems to be two node's having different IP addresses, but the IP addresses share the same domain network name. In case of ac hoc WLAN the concepts of handover or adjacent access point can not be found, because the stations act both as an access point and as a station. As pointed out the whole system can then be seen as movable.
 

3.2.1 Scale of Mobility

GSM is a network consisted of cells. The purpose of the operator usually is to offer a nation wide coverage and with international roaming the user can use mobile services also globally. The mission for several operators and vendors is that the user can use his phone where ever and whenever. The need to move within the network is essential otherwise the concept of mobile services would not exist.

In order to ensure the large scale of mobility, several functions has had to be designed. The mobility management of GSM is defined in the layer three specifications, and it is very complicated compared to that of a WLAN user. As stated the smallest unit of the radio network is a cell. Several cells build up a location area (LA). The concept of LA has been a necessity to define, in order to handle large scale mobility of the user. If a powered mobile station (MS) is having an incoming call, it is paged through the pacing (PAGCH) channel of a cell. It would be a waste of bandwidth if the paging has to be done in every incoming call through every cell of the network. Thus the network knows situation of each user in an accuracy of a location area. [9] The operator decides the size of the location areas in order to balance the signalling traffic of the network.

Several cells are connected to Base Station Controllers (BSC) and further BSCs are connected to Mobile Switching Centers (MSC). The number of BSC and MSC is a matter of the vendor but also a matter of optimisation and maximizing the reliability of the network and therefore not discussed here. When the MS moves from cell to cell, and from a location area to another area, additionally two subsequent registers HLR (home location register) and VLR (visitor location register) are required.

Radio Resource Management, which controls the call setup, maintenance and termination, as well as it also handles the signalling used in handovers. Different types of handovers are supported. Intra cell handover, which can occur within a cell or within a frequency from time slot to another time slot. This is normally done in case of interference. The other type is a handover between different cells. The cells can be situated in different BSCs and/or different MSC. This of course affects the signalling need in handover. This type of a handover is the way to move within the network. Despite of the ability to move within the network by handovers, handovers can be used to control the traffic load of the cells.

The MS measures the RX level of the surrounding BTSs according to the neighbouring list and reports the measurements to the BSC, where the handover decision is done. The suitable choices of handover and power control parameters makes the BSC to keep the connection alive by the help of power control. The handover attempt is not done until the user really is in the border of the cell.

---

On the other hand the mobility concept of WLAN is slightly simplier. Typical users of WLAN infrastructure mode can be e.g. doctors seeing his patient, the patient information can be written straight to the patient register in the laptop by the help of WLAN. Also students at the universities or people participating conferences could use their laptops as a notebook by the help WLAN. Ac hoc WLAN offers no access to the wired network, instead of that the stations can share files, which could be useful e.g. in confrerences. These are indoor solutions, but the specifications tell no reason why WLAN coverage can not be built outdoors as well. Anyway in cases like this it could be supposed that the WLAN users do not need to move within the network in that extent what the user is expected to move in GSM network. This is perhaps because WLAN does not support speech transmission and data service users are not expected to move in a large scale. Thus it could be assumed that a typical WLAN user would be a user that moves from location to location, but uses the WLAN equipment only at a fixed location. Therefore no complex mobility scenario is needed. Next the basic services supporting mobility are shortly described.

Association is a basic service that enables the connection between the station (STA) and the access point (AP) in an infrastructure WLAN. An access point is basically a radio base station that covers an area of about 30300 m depending on the environment. An access point and its associated clients form all together a Basic Service Set (BSS). [13] Though no handover mechanism is specified in the standard, the standard introduces a service called reassociation, which is related to the roaming from one BSS to another. Two adjoining BSS form together an Extended Service Set (ESS) if they share the same ESS identity (ESSID). This is the case when roaming is possible. [4] Thus the parameter ESSID can be referred to the concept of neighbouring cell in GSM network.

An Independent Basic Service Set (ad hoc mode, IBSS) is the most basic type of IEEE 802.11 WLAN. At the minimum it is consisted of two stations. The network is often formed without pre-planning is alive until either station is moved away from the others coverage area. [4]
 

3.2.2 Speed of the Mobile

According to the mobility theorem presented in [3] the speed of the mobile is a crucial fact affecting the time the mobile station stays in the coverage area of a cell. If the user reaches the border of an individual cell by the velocity v, the velocity v' is the time the user reachers the border of N cells. In the [3] it is certified that the ratio of v/v' follows the square root of N.

It is assumed the N to be 100. The velocity v' can now be calculated to be one tenth of the velocity v. Thus the speed of the mobile is more crucial issue to the user of a wireless local area network than to a mobile phone moving within a network covering larger entities. Thus the aim of the WLAN coverage planning is often to maximise the coverage area of an individual WLAN access point.
 

3.3 Transmission Capacity Needs

A good question is how much transmission capacity does a single application need? Of course this depends very much on the application. In case of GSM voice service, we know exactly what the voice transmission over the air needs. [8] Further if we know, how often the user does take a call, we can calculate from Erlangs formula the needed capacity.

In case of data services the estimated need of capacity is not as straight forward. It is not possible to measure the amount of capacity needed from the used application. If we know that the www-application would demand say 40 kbit/s from the GPRS service, this does not take into account how unexpectedly the user can behave. He can suddenly found some interesting link transmitting e.g. video picture and thus the demand for transmission capacity still increases. However, this kind of a phenomena does not take into account that the behave of one user affects also to the behaves of the other users. Let us think about the www-surfer, that has found an interesting video picture. As his transmission capacity need increases, the transmission capacity of other www-surfers in the same cell decreases. So the behaving scheme of a data user can be regarded as a very complex mathematical model and thus this theme is suggested to a subject of further study.

It can also be stated the packet data user - e.g. GPRS or WLAN user - does not experience the congestion in the way that the circuit switched user (e.g. GSM voice) does. The congestion of a packet data transmission is experienced as the transmission rate of the service becomes slower. So practically there is always room for another user using packet service. Only the speed of the service can not be quaranteed.
 

3.4 Power Control

The power consumption of the mobile station is a crucial issue when using wireless services. Both standards GSM and WLAN have taken this point into account. The closer the user is to the base station or access point the smaller transmission power is needed. The aim of the power control is simply to minimise the power used, but still to keep the quality of the connection good enough [10].

The GSM specifications specifies both downlink and uplink power control. The downlink power control mainly decreases the interference level of the network and thus improves the quality of the network. BTS power control is not obligatory and thus it is up to the decision of the operator. On the other hand the uplink power control improves the battery lifetime of a mobile phone. The GSM power control is controlled by BSC and it is based on the measurements done by the BTS and MS. The measurements are done in 480 ms interval. If the average result of the measurement exceeds the threshold, the power change is done in 2 dB steps, each step takes 60 ms. The power control range of a GSM mobile phone in the 900 MHz band is from the 13 dBm (20 mW) to the maximum specified 39 dBm (8 W). [8]

WLAN is a symmetrical system, which can be noticed from the stations ability to act as an access point in the ad hoc mode. Thus the power control functions similarly from the stations and the access points of view. In the DSSS WLAN the maximum transmission power is specified to be 100 mW (EIRP), whereas the minimum power level shall be no less than 1 mW. Four different power levels are defined. For example in an infrastructure network the station selects it's power management mode by a parameter, by which the station informs the access point through a successful frame exchange. In a case when there is nothing to be received or transmitted the station is able to fall into a sleep mode and then consumes very low power. In this mode the station is not able to transmit or receive any data. However, in a low power mode the station listens periodically beacons sent by the access point. In this way the access point is announcing the stations of it's existence. [4]

In the FHSS WLAN the power level are specified as follows. The EIRP of the maximum transmission power is 100 mW and the smallest transmission level is 10 mW. [4] The specified power levels are regulated by local regulators and are different from continent to continent. The values presented here are the values used in Europe.

It could be assumed that the speed of the mobile also affects the users ability to recover from the fading point. Thus the mobile station moving at high speed recovers from the fading caused by the multipath propagation more quickly than the slow moving mobile. Thus the power control can regarded more critical to the slow moving user.
 

3.5 Population Variation

One of the corner stones of the network planning is to define the capacity of the network according to the peak traffic. However, there are times when capacity needed is remarkable bigger. Exhibitions, rock concerts, sport happenings, even a shopping day before christmas could cause a remarkable variation in offered traffic of the network. The GSM operators usually takes the biggest happenings into account by building extra temporary capacity to the network. This can be done e.g. using movable base stations or simply adding the extra capacity to the specific cells covering the happening area.

In case of WLAN user or packet data user, the capacity planning can be omitted, because there is always room for another packet data user, only the offered transmission rate decreases when the amount of users increases. So the document suggests, that the population variation does not have to take into account when planning a WLAN network. On the other hand in a case where are several hundreds of concurrent users and a certain level of data rate must be quaranteed, the use of several access point instead of one should be considered, although sufficient coverage area could be created with one access point.
 

3.6 Timing Advance

The delay of radio propagation time from transmitter to receiver is limiting the distance of GSM connection. Therefore the MS  transmission point of time has be changed depending on the distance between the MS and the BTS. This is done by the parameter timing advance. The MS transmission point of time has to adjusted, in order the MS transmission to hit the right time window assumed by BTS. The range of the value is between 0...63. One unit corresponds to 0,55 km. Thus the maximum distance from the transmitter is limited to 35 km. [8]

The resolution of the parameter timing advance (0,55 km) is bigger than the specified maximum range of Wireless LAN. Therefore no timing advance parameter is needed in short range communication system like WLAN.
 

4 Network Planning Process

This section describes the corner stones for the network planning process of a cellular network, which is typically a land wide network, planned for both outdoor and indoor use. It is further discussed, if a planning process for a WLAN network is needed at all, and if needed, what GSM network design principles can be omitted in the WLAN environment.
 

4.1 Targets for the Network Planning

The targets of the network planning are depending on the offered services, the user needs and therefore expected traffic load. Also the frequency band available and the chosen infrastructure outlines the structure of the network. The main targets of the network planning is to quarantee service with a quality good enough and to offer capacity with a sufficient low congestion. The good quality could mean a low dropped call percentage, low quality figures (quality level can be measured between 0...5, 0 being the best class), but also it could mean a good indoor coverage. The demand for coverage depends strongly on what the customers are used to. If the customers expects as good as coverage as the fixed network offers indoors, the mobile station should be able to take a call also everywhere indoors.

Network planning is a ongoing process, which has to be optimised economically. However, one target is also to take into account the possible capacity need in the future right from the beginning. The different steps of the planning process are illustrated in the figure 4.1.
 


Figure 4.1. The network planning is a ongoing process, which is consisted of various steps and affected by various parameters.
 

4.2 Coverage Planning

The coverage of a cell means an area, where the user can make a call without problems. The coverage planning is based on the wanted coverage level and its demanded amount of base stations. In the coverage planning the topography of the serving area is good to take into account. Also indoor coverage target in the cities affects the amount of needed base stations. As a result of the coverage planning will be the predicted coverage area and the transmission powers of the cells. Different propagation models and planning software are needed in order to predict the radiowave propagation and therefore coverage.
 

4.3 Capacity Planning

The coverage and capacity planning go hand in hand in GSM network planning process. Seldom, hardly ever the network is planned and built to be ready right in the beginning. Usually the operator plans and builds the network in an incremental way. This means that the operator may start to offer service with not a very dense network, perhaps offering no indoor coverage at all. Usually first the network is consisted of big cells, in order to offer as wide as coverage as possible. The capacity is seldom a problem at first. As the amount of customers increases also the need for better coverage area and capacity increases. Then usually bigger cells will be divided into several small and micro cells in places where high capacity is needed. This leads to a network, where micro and small cells are used in the cities and big cells or smaller cities in the countryside. Therefore the goal of GSM coverage planning is not necessary to maximise the coverage of an individual cell. The denser the network, the bigger the offered capacity.

In the capacity planning two different schemes can be distinguished: capacity planning of a certain area entity and capacity planning of an individual cell. The capacity of a GSM cell is consisted of TRX's. One TRX contains 8 channels or timeslots, and TRX can carry 4 Erlangs traffic with 2 % time congestion. Thus accepted congestion level affects the capacity planning. Some level of congestion can be acceptable e.g. in the city areas where cell overlapping can be found in a great extent. However, the offered capacity is not only a matter of excisting channels, the operator can affect the amount of needed channels by the help of various parameters by which the operator can share the offered load between neigbouring cells.
 

4.4 Frequency Planning

The capacity plan gives as a result the number of TRXs, whereas the frequency plan gives the frequency in used in every TRX. Frequency planning step is also affected by coverage plan. The basis to plan the frequencies are the same and neighbour channel rejections, which are defined in the specification. Also the frequencies allocated to the operators use are essential to know. In GSM network the frequencies are repeated in a sufficient distance. The distance depends on the specified channel rejection levels and it is also further affected by the topography and obstacles of the environment.
 

4.5 Parameter Planning and the Follow Up

In addition to the steps of coverage, capacity and frequency planning, there are still a lot to be optimised. Once the planned BTSs functions in the network, the important phase to the planner is to investigate how successful was the plan. This can be done e.g. by making test drives around the measured area with a test mobile phone and/or following cell specific measurements reported by the network. Then the possible changes needed in antenna configuration, capacity or frequency plan is now to be done. Also the following up as well as the parameter planning of the network is important. By the parameters the operator can affect e.g. the handovers, BTS and MS power control.
 

4.6 Relevant Requirements for IEEE 802.11

4.6.1 Infrastructure WLAN

The question is that does the WLAN network need a multi phased planning process like used in GSM cellular system. The costs of an access point and a wireless adapter needed in the e.g. laptop connected wirelessly are probably lower than one cellular planners monthly salary costs. Further we can ask who would be the planner, if a user can buy a WLAN access point and install one by himself. Also there is no organisator who would supervise the frequency reuse as the frequencies lie in the unlicenced frequency band.

Now in case of infrasructure WLAN the situation is that the user who have access to the LAN and wants to make that wirelessly, buys an access point and installes it. The user makes a coverage plan by simply installing the equipment e.g. to the wall and if the place is not optimised he probably tries another place. Further the user makes a frequency plan by selecting randomly the channel, if there are lot of interference he will probably change it. So the coverage planning and frequency planning steps can be found in WLAN planning "process". However, the planning step of capacity can be omitted in the WLAN environment. As stated previously this is from the reason, that there is always room for another WLAN packet user. The information is transmitted in packet mode, therefore the user does not experience any congestion, only the transmission rate varies. However, as stated previously if a certain transmission data rate is to be quaranteed and there are e.g. several hundreds of concurrent users there might be a need for using several access points instead of one, although sufficient coverage area could be created with one access point. Overall it can be stated that a distinctive capacity design principle can be found from the GSM world but cannot be found from the WLAN environment.

The problem of the WLAN planning is also that the user is not necessarily familiar with technical equipment. In a case the user installes an access point and tries to make a wireless connection to the LAN, but does not get it, he probably does not know what the problem is. Is the used frequency too interfered or perhaps the installation place for the access point is not good enough? Further what is making the interference? However, the writer does not have experience of testing a live connection in WLAN, but assumes that the biggest problem at the moment is the interference caused by other equipment (not WLAN) using same frequency band. Micro owens, Blue Tooth equipment or different electronical equipment using 2,4 GHz (e.g. door opening equipment) could cause interference.

Relevant requirement for the IEEE 802.11 infrastucture network planning would be that the intelligence of the frequency planning should be in the equipment itself. In this way the level of interference would be minimised. The other demand could be that there would be a larger frequency band dedicated to the WLAN use than available today, so the interference would be spread into a larger area and thus the average interference level would be lower. Both demands suggested in the document are taken into account in the next generation WLAN called HiperLAN2, which uses higher frequency (5 GHz) and supports also interfunction with third generation cellular standard UMTS. In HiperLAN2 the access point automatically selects an appropriate channel and dynamically changes it according to the level of interference [6].
 

4.6.2 Ad Hoc WLAN

In the ad hoc mode several wireless stations (e.g. laptops) can get together in a local area (e.g. conference room) and form their own wireless network. Through an ad hoc mode, the users can share documents and devices.

Generally the problems an ad hoc WLAN is facing could be the same as in the infrastructure mode. The used frequency is perhaps interferred or the coverage area is not large enough etc. However, the writer assumes that the coverage is not a usual problem in an ad hoc mode. This is because it is assumed that the users have decided to meet each other in some conference room and the only demand for the coverage is that the stations can hear each other in that specific room. So the network is typically created spontaneously and is alive until a station has moved out of the coverage area of other stations.

The question of the network planning in an ad hoc mode is irrelevant. No network planning process of an ad hoc WLAN can be addressed. So rather than discussing the problem of network planning of an ad hoc WLAN, the more suitable question is that how the stations configure themselves to act both as an access point and as a station. The ad hoc WLAN is using a certain frequency and thus a station that wants to be a member of the network should use the same frequency. So can an accidental WLAN station be a member of any ad hoc WLAN it has founded? Further, if the used frequency is too interferred and the connection between stations is poor, an interesting question is that what happens if one of the stations suddenly decides to the try another frequency? Is this somehow possible or are the frequency selection and the possibly frequency change a common decision of all the members of ad hocWLAN ? These are questions that are left under further study.
 

5 Conclusion

GSM is a cellular network standard. The standard defines a network usually covering large entities, enabling large scale of mobility and offering a versatile set of services - e.g. voice and data - and thus competing with the wired public telephone network. As the user demand for higher data rate increases, new data services has been defined. The maximum range of GSM communication is 35 km.

The standard IEEE 802.11 WLAN defines to the LAN user (e.g. laptop) a mean to connect wirelessly the Ethernet based LAN. Also wireless connection (ad hoc mode) between stations are defined. The main advantages of WLAN is to offer mobility for the LAN users though within a rather small area. Cell radius is usually from few tens of meters to some hundred meters. WLAN provides LAN users an access to real time information anywhere in their organization. Further one aim was to reduce installation costs and thus achieve short- and long-term cost savings. Installing a wireless system can be fast and the need to pull cable through walls and ceilings can be eliminated. Wireless technology allows also the network go where the wire cannot go.

The document has compared various parameters - such as frequency range,  mobility aspects, transmission capacity needs, power control, population variation and timing advance  - outlining the network planning of GSM and WLAN. The coverage area of a WLAN is usually very limited, usually bordered upon a building or a campus and can therefore be comparable to a single GSM inside cell. Because of the limited coverage the speed of the mobile is stated to be crucial to WLAN user, as well as the power control. The documents handles the transmission capacity need of a user using data communication. It is stated that the behaving of an individual data user affects the behaving of the other data users sharing the resources of the same cell. The conclusion is that it is difficult to estimate the need for transmission capacity from the used application. Because of WLAN data communication uses packet mode communication instead of GSM's circuit switch mode, the WLAN user does not experience congestion in the same way as the circuit switch user. Only the offered transmission rate varies. This is the reason why there is no need to take the population variation of WLAN into account. Further it was stated that no timing advance parameter is needed in short range wireless communication like WLAN.

The targets of the GSM network planning are depending on the offered services, the user needs and therefore expected traffic load. Also the frequency band available and the chosen infrastucture outlines the structure of the network. The main targets of the network planning is to quarantee service with a quality good enough and to offer capacity with a sufficient low congestion. The GSM network planning process is consisted of three phases: coverage, capacity and frequency planning. The phases are strongly dependent on and affected by each other.

An infrastructure WLAN does not need a multi phased network planning process like used in GSM cellular system. This can be argued with the costs of the network planning. Also there is not going to be an organisator who would supervise the frequency reuse as the frequencies lie in the unlicenced frequency band. The steps of coverage planning and frequency planning can be found in WLAN planning "process". However, the planning step of capacity can be omitted in the WLAN environment for the same reason which was mentioned when defined the rules for the population varation. It was also stated that the question of the network planning process in an ad hoc WLAN is irrelevant.

Relevant requirement for the IEEE 802.11 infrastucture WLAN planning would be that the intelligence of the frequency planning should be in the equipment itself. Also the more channels are available for WLAN use the lower the interference level would be. Both demands suggested in the document are taken into account in the next generation WLAN called HiperLAN2, it also supports interfunction with third generation cellular standard UMTS.
 

References


[1] Chayat Naftali, BreezeCom, Frequency Hopping Spread Spectrum PHY of the 802.11 Wireless LAN Standard, IEEE 802.11 Tutorials, 1996. [referred 10.3.2000]
<http://grouper.ieee.org/groups/802/11/Tutorial/FH.pdf>

[2] Glas Jacobus Petrus Franciscus, Non Cellular Wireless Communication Systems, Delft University of Technology, Delft, 1996. [referred 3.2.2000]
<http://cobalt.et.tudelft.nl/~glas/thesis/main-text.html>

[3] Hämäläinen Jari & Kari Hannu H., Proposed Operation of GSM Packet Radio Networks, PIMRC '95, September 27-29 1995.

[4] Institute of Electrical and Electronics Engineers, Inc., IEEE standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1997, IEEE, 445 p.

[5] Institute of Electrical and Electronics Engineers, Inc., Short Description of the Standard, 1999. [referred 11.3.2000]
<http://grouper.ieee.org/groups/802/11/main.html>

[6] Johnson Martin, HiperLAN/2 - The Broadband Radio Transmission Technology Operating in the 5 GHz Frequency Band, White Paper, HiperLAN/2 Global Forum, 1999. [referred 24.3.2000]
<http://www.hiperlan2.com/site/specific/whitepaper.pdf>

[7] Nations Daniel, Wireless Local Area Networks, 1999. [referred 4.2.2000]
<http://www.ac.wwu.edu/~n9649918/wlans.html>

[8] Penttinen Jyrki, GSM-tekniikka, Järjestelmän toiminta, palvelut ja suunnittelu. Porvoo 1999, WSOY. 349 pages

[9] Scourias John, Overview of the Global System for Mobile Communiactions, 1997. [referred 9.2.2000]
<http://ccnga.uwaterloo.ca/~jscouria/GSM/gsmreport.html>

[10] Weckström Marja-Leena, Solukkosuunnittelu DCS1800 -matkaviestinjärjestelmässä, Master's Thesis, Helsinki University of Technology, Department of Electrical Engineering, Espoo 1996, 105 p.

[11] The Wireless LAN Association, What is a Wireless LAN?, Introduction to Wireless LANs. [referred 9.2.2000]
<http://www.wlana.com/intro/index.html>

[12] The Wireless LAN Association, What is a Wireless LAN?, The IEEE 802.11 Wireless LAN Standard. [referred 9.2.2000]
<http://www.wlana.com/intro/standard/index.html>

[13] The Wireless LAN, Interoperability Forum, What is the IEEE 802.11 Wireless LAN Standard?, Technical Background [referred 9.2.2000]
<http://www.wlif.com/tech/wp_80211.html>

[14] Wood Joel B, The Wireless LANs Page, 1995. [referred 18.2.2000]
<ftp://ftp.netlab.ohio-state.edu/pub/jain/courses/cis788-95/wireless_lan/index.html>
 

 Further Information


Johnson Martin, Hyping Up the Wireless LAN, Telecommunications Magazine, 2000, Jan 2000

LaMaire Richard O. & Krishna Arwind & Bhagwat, Pravin & Panian James, Wireless LANs and Mobile Networking: Standards and Future Directions, 1996.
<http://www.comsoc.org/livepubs/comsoc/private/1996/aug/lamaire.html>

Skellern David J. & Lee L.H. Charles & McDermott Tom & Weste Neil H.E & Dalton John & Graham Jeffrey & Wong Tan F. & Myles Andrew F., A High-Speed Wireless LAN, IEEE micro, 1997, vol. 17, No 1 January/February 1997.