There are many routes that lead from the identification of a user requirement for wireless networking to the operation and support of an installed WLAN, and the best approach to be taken will depend on the nature and scale of the project. In this tutorial a five-step process is described that is scalable from a simple ad-hoc home network to a large scale corporate WLAN, linking multiple buildings.
In small scale projects, such as in implementing a typical home or small office WLAN, some of these steps will be very short or may be eliminated altogether. Nevertheless, an awareness of the issues addressed in these steps will contribute to the successful implementation of even the smallest project.
The five key steps in the planning and implementation of a wireless LAN area are as follows:
1. Evaluating requirements and choosing the right technology.
- Establish the user requirements; what is it that the users want to be able to achieve and what are their expectations of performance?
- Establish the technical requirements; what attributes does the technological solution need to possess in order to deliver these user requirements?
- Evaluate the available technologies; how do each of the available or emerging wireless LAN technologies rank against the technical requirements?
- Selecting network hardware components; should a single or multi-vendor strategy be followed? What are the advantages and disadvantages?
2. Planning and designing the wireless LAN.
- Surveying the RF environment; what other sources of RF energy or potential barriers to RF propagation are present in the target area of the wireless LAN?
- Designing the physical architecture; which architecture is right for the specific setting of the network?
3. Pilot testing
- Testing the chosen technology and architecture; does the chosen solution deliver the expected performance?
4. Implementation and configuration.
- Putting the final wireless LAN in place and introducing it to the user group.
- Configuring the appropriate security measures
5. Operation and support.
- Keeping the wireless LAN operating efficiently and providing user support.
Evaluating Wireless LAN Requirements
Establishing User Requirements
If the wireless LAN is being implemented to support a large user group it will be important to gather a wide range of views on user requirements, perhaps by using a questionnaire or by interview. As a first step it may be necessary to raise awareness by demonstrating the technology to the prospective user group, so that they are better able to give an informed view on requirements.
User requirements should be expressed in terms of the user experience rather than any particular solution or technical attribute, as they are independent of specific technologies. For example, in relation to performance expectations, a PHY layer data rate is a technical attribute, whereas the transfer time for a specified large file size is what the user is really concerned about.
Common categories of user requirement are listed and discussed below:
- Usage model - What user activities does the WLAN have to support? Are users routinely transferring large files over the network, such as Internet downloads or video editing? Is the WLAN required to support applications such as voice or video streaming, either now or in the future?
- Performance expectations - What are the user’s performance expectations? If large data files are commonly used, what are the required transfer times?
- Areal coverage - What is the operating area in which users will need wireless network coverage? Do usage requirements vary at different locations within this area? Is future growth of the required coverage area expected?
- Mobility - If users will move within the operating area while working, will they need to access the WLAN from several fixed locations (roaming) or will they need continuous service while in motion (mobility) — for example to support voice services?
- Device interoperability - What types of user devices will need to connect to the network?
- User population - What is the total number of users and user devices that are required to be supported? How many users will typically require concurrent service? How much future growth is the network expected to cater for?
- Security - How confidential is the information transferred across the network? What level of protection is required against unauthorised access?
- Battery life - If mobile devices will be used in the network, how often will the user need to recharge battery operated devices?
- Economic - What budget is available to implement the WLAN? Are there specific requirements that deliver high value and may justify a higher cost solution?
Establishing Technical Requirements
Technical requirements follow from user requirements, by translating these into the specific technical attributes that are needed to deliver the user requirements:
- Effective data rate - The required data rate for a single user will be dictated by the usage model, for example by the typical file size and upload/download time, or by the requirements for voice or video streaming. Effective data rates can be significantly lower than a standard PHY layer data rate, and will be further affected by adverse environmental factors such as RF interference.
- Network capacity - What is the overall network capacity needed to provide the required level of service, given the current and future expected size of the user group and number of user devices? Required capacity will be a key factor both in the technology selection and in determining the appropriate physical architecture for the WLAN.
- Quality of service - If the usage model includes applications such as VoWLAN, then guaranteed quality of service will be an important attribute to ensure performance expectations are met.
- Application support - Are there specific technical attributes required to support particular usage models?
- Network topology - What types of connections are required to meet user requirements? For example, peer-to-peer for local data sharing, point-to-point for linking buildings, etc.
- Security - If users’ confidentiality requirements are high, then data encryption, network access monitoring and other security measures will be required.
- Interference and coexistence - If the WLAN will have to operate in an environment with other wireless networks, such as Bluetooth, or alongside cordless phones, then coexistence will need to be a consideration.
- Technology maturity - Before standards have been agreed early products have an interoperability risk, while a fully mature technology may have limited scope for future development and risk early obsolescence as new usage models arise. The significance of this attribute will depend on whether the user requirements are within the proven capabilities of existing technology or require a leading edge solution.
- Operating range - The required range will be determined by the physical extent and nature of the operating area, as well as the layout of components such as access points. The overall link budget will be important in implementing point-to-point connections (wireless bridges between buildings).
- Network scalability - If the WLAN is likely to require more than a few access points, or significant future growth is anticipated, then ease of initial configuration and ongoing network management tasks will be a requirement, at least for the network manager.
For example, if there is a user requirement for rapid transfer of large files, for example for video editing applications, this will translate into a technical requirement for a high effective data rate. Some technical attributes, such as operating range and those relating to interference and coexistence, will be clarified following site surveying and initial planning of the physical layout of the network hardware.
Evaluating Available Technologies
Having established the technical attributes necessary to meet user requirements, the available technologies can then be directly assessed against these attributes. A simple table can be used to display the assessment, resulting in a transparent and objective comparison of the available solutions.
More sophisticated evaluation methods can also be applied, for example, by assigning a weighting factor to each requirement and a score to each technical solution depending on the extent to which it meets the requirements.
The total required network capacity will be determined by the sum of the bandwidth requirements of the maximum number of concurrent users expected on the network, with some allowance being made for the fact that this maximum will occur infrequently and some limited degradation of performance may be acceptable during brief periods of high usage.
If this requirement exceeds the capacity of a single access point then multiple access points will be required, up to the limit imposed by the number of available non-overlapping channels. The 802.11h enhancements open up an additional 12 OFDM channels in the 5 GHz band, doubling the achievable network capacity for 802.11a networks.
The operating range of a wireless network link is influenced by a wide range of factors, from the modulation and coding scheme being used to the nature of the materials used in the construction of the building in which the network operates. The key factors are summarised below:
- Frequency band - As described earlier, free space loss is proportional to the logarithm of operating frequency, and increases by 6.7 dB with the increase in frequency from 2.4 to 5.8 GHz.
- Transmitter power and receiver sensitivity - These two factors are grouped together since they determine the end points of the link budget.
- Modulation and coding scheme - Higher data rate modulation and coding schemes are less robust in that they require correspondingly higher received signal strength to assure accurate decoding. Other things being in equal range therefore decreases for higher data rates.
- Environmental factors - Construction materials, particularly metal objects, have a major influence on path loss if RF signals have to pass through walls, ceilings, floors or other obstructions. Path loss is also highly frequency dependent and for all practical purposes a line-of-sight is required for communication in the 5 GHz band.
Selecting the Technical Solution
With the demise of competing standards such as HiperLAN/2 and HomeRF, the essence of the technical choice is simply — which 802.11 flavour best fits the bill?
While the requirements analysis may point to a clear winner among the available technical options, if a selection needs to be made between operating in the 2.4 or 5 GHz bands then an RF site survey, described in the next section, should be conducted as an input to that decision.
Similarly, if the requirements dictate that network capacity needs to be stretched beyond the throughput of a single channel, for example with multiple access points fully exploiting non-overlapping channels, then an initial physical layout may need to be made for both 2.4 and 5 GHz options.
Some on-site physical testing may also be valuable prior to making the final decision, for example to confirm the achievable range if a 5 GHz network is envisaged in a constricted indoor environment. Future hardware developments may soon make the choice between 2.4 and 5 GHz operation irrelevant.
As increasing volumes of dual band radios are shipped, supporting both 802.11g/b and 802.11a, and prices fall to parity with single band products, it will be cost-effective to implement a dual band WLAN that makes the best use of the characteristics of both RF bands.