Is a WLAN Site Survey Needed?

The answer to that question is almost always a resounding yes. If an owner of a small retail flower shop desires a wireless network, the site survey that is conducted may be as simple as placing a residential wireless gateway in the middle of the shop, turning the transmit power to a lower setting, and making sure you have connectivity.

Performing a site survey in a medium-size to large-size business entails much more physical work and time. Before the actual survey is conducted, a proper site survey interview should occur to both educate the customer and properly determine their needs.

Asking the correct questions during a site survey interview will not only ensure that the proper tools are used during the survey, it will also make the survey more productive.

Most important, the end result of a thorough interview and thorough survey will be a WLAN that meets all the intended mobility, coverage, and capacity needs. In the following, we will discuss the questions that should be thoughtfully considered during the site survey interview.

Customer Briefing

Even though 802.11 technologies have been around since 1997, much misunderstanding and misinformation about wireless networking still exists. Many businesses and individuals are familiar with Ethernet networks, therefore a “just plug it in and turn it on” mentality is prevalent.

If a wireless network is being planned for your company or for a prospective client, it is highly recommended that you sit management down and give them a quick overview of 802.11 wireless networking and talk with them about how and why site surveys are conducted.

You do not need to explain the inner workings of Orthogonal Frequency Division Multiplexing or Distributed Coordination Function; however, a conversation about the advantages of Wi-Fi as well as the limitations of a WLAN is a good idea.

For example, a brief explanation about the advantages of mobility would be an excellent start. Chances are that a wireless network is already being considered because the company’s end users have requested mobility or a specific application such as Voice over Wi-Fi (VoWiFi) is being contemplated.

Just as important is a discussion about the bandwidth and throughput limitations of current 802.11a/b/g technology. Enterprise users are accustomed to 100 Mbps full-duplex or better speeds on the wired network.

Because of vendor hype, people often might believe that a Wi-Fi network will provide them with similar bandwidth and throughput. Management will need to be educated that because of overhead, the aggregate throughput is usually one half or less of the advertised data rate.

The aggregate throughput of a 54 Mbps data rate is 20 Mbps or less. It should also be explained that the medium is a half-duplex shared medium and not full-duplex.

Chances are that an 802.11b/g network is being considered and it might be necessary to briefly explain the a effect on throughput as a result of the 802.11g protection mechanism.

In the future, 802.11n WLAN equipment will address greater throughput needs, thus making the bandwidth/throughput conversation less painful. However, with the demand for faster networks, in the future we are sure we will be explaining why 802.11n is so much slower than Gigabit Ethernet.

Another appropriate discussion is why a site survey is needed. A very brief explanation on how RF signals propagate and attenuate will provide management with a better understanding of why an RF site survey is needed to ensure the proper coverage and enhance performance.

A discussion and comparison of a 2.4 GHz versus a 5 GHz WLAN might also be necessary. If management is properly briefed on the basics of Wi-Fi as well as the importance of a site survey, the forthcoming technical questions will be answered in a more suitable fashion.

Business Requirements

The first question that should be proposed is, What is the purpose of the WLAN? If you have a complete understanding of what is the intended use of a wireless network, the result will be a better-designed WLAN.

For example, a VoWiFi has very different requirements than a heavily used data network. If the purpose of the WLAN is only to provide users a gateway to the Internet, security and segmentation recommendations will be different.

A warehouse environment with 200 handheld scanners is very different than an office environment. A hospital’s wireless network will have different business requirements than an airport’s wireless network.

Here are some of the business requirement questions that should be asked:

  • What applications will be used over the WLAN?

This question could have both capacity and Quality of Service (QoS) implications. A wireless network for graphic designers moving huge graphics files across a WLAN network would obviously need more bandwidth than a wireless network for nothing but wireless bar code scanners.

If time-sensitive applications such as voice or video are required, proprietary QoS needs might have to be addressed. 802.11e/WMM will address these QoS needs in the future.

  • Who will be using the WLAN?

Different types of users have different capacity and performance needs. Groups of users might be segmented into VLANs or even segmented by different frequencies. This is also an important consideration for security roles.

  • What types of devices will be connecting to the WLAN?

Handheld devices may also be segmented into separate VLANs or by frequency. VoWiFi phones are always put in a separate VLAN than data users with laptops.

Also, most handheld devices currently only operate in the 2.4 GHz ISM band. The capabilities of the devices may also force decisions in security, frequency, technology, and data rates.

Capacity and Coverage Requirements

Once the purpose of the WLAN has been clearly defined, the next step is to begin asking all the necessary questions for planning the site survey and designing the wireless network.

While the final design of a WLAN is completed after the site survey is completed, some preliminary design based on the capacity and coverage needs of the customer is recommended.

You will need to sit down with a copy of the building’s floor plan and ask the customer where they want RF coverage. The answer will almost always be everywhere. If a VoWiFi deployment is planned, that answer is probably legitimate because Wi-Fi VoIP phones will need mobility and connectivity throughout the building.

If the WLAN is strictly a data network, the need for blanket coverage might not be necessary. Do laptop data users need access in a storage area? Do they need connectivity in the outdoor courtyard? Do handheld bar code scanners used in a warehouse area need access in the front office?

The answer to these questions will often vary depending on the earlier questions that were asked about the purpose of the WLAN. However, if you can determine that certain areas of the facility do not require coverage, you will save the customer money and yourself time when conducting the physical survey.

Depending on the layout and the materials used inside the building, some preplanning might need to be done as to what type of antennas to use in certain areas of the facility. A long hallway or corridor will most likely need an indoor semi-directional antenna for coverage as opposed to an omni-directional antenna.

When the survey is performed, this will be confirmed or adjusted accordingly. The most often neglected aspect prior to the site survey is determining capacity needs of the WLAN. As mentioned earlier, you must not just consider coverage; you must also plan for capacity.

Cell sizing and/or co-location might be necessary to properly address your capacity requirements. In order for the wireless end user to experience acceptable performance, a ratio of average amount of users per access point must be established.

The answer to the capacity question depends on a host of variables, including answers from earlier questions about the purpose of the WLAN. Capacity will not be as big of a concern in a warehouse environment using mostly handheld data scanners.

However, if the WLAN has average to heavy data requirements, capacity will absolutely be a concern. The following are among the many factors that need to be considered when planning for capacity:

  • Data applications - The applications that are used will have a direct impact on how many users should be communicating on average through an access point.

So the next question is, What is a good average number of data users per access point? Once again, it depends entirely on the purpose of the WLAN and the applications in use. However, in an average 802.11b/g network, 12 to 15 data users per access point is an often-quoted figure.

  • User density - Three important questions need to be asked with regard to users. First, how many users currently will need wireless access? Second, how many users may need wireless access in the future?

These first two questions will help you to begin adequately planning for a good ratio of users per access point while allowing for future growth. The third question of great significance is, Where are the users?

Sit down with network management and indicate on the floor plan of the building any areas of high user density. For example, one company might have offices with only 1 or 2 people per room, while another company might have 30 or more people in a common area separated by cubicle walls.

Other examples of areas with high user density are call centers, classrooms, and lecture halls. Also plan to conduct the physical survey when the users are present and not during off hours. A high concentration of human bodies can attenuate the RF signal due to absorption.

  • Peak on/off use Be sure to ask what the peak times are, that is, when access to the WLAN is heaviest. For example, a conference room might be used only once a day or once a month.

Also, certain applications might be heavily accessed through the WLAN at specified times. Another peak period could be when one shift leaves and another arrives.

  • Existing transmitters - This does not refer just to previously installed 802.11 networks. Rather, it is referring to interfering devices such as microwaves, cordless headsets, cordless phones, wireless machinery mechanisms, and so on. Often this is severely overlooked.

If a large open area will house the help desk once the wireless is installed, you may be thinking of capacity. However, if you don’t know that they are using 2.4 GHz cordless headsets or Bluetooth keyboard and mouse, then you may be designing a network destined for failure.

  • Mobile vs. mobility - There are two types of mobility. The first is related to being mobile and the other is true mobility. To help explain this, think of the marketing manager working on a presentation and saving it on a network share. He later wants to give that presentation in the boardroom.

If he picks up his laptop, closes the lid, and walks to the conference room where he opens the laptop, connects to the wireless network, and gives his presentation, that is being mobile. He may have disconnected in between points and that is OK.

However, having true mobility means that a user must remain connected 100 percent of the time while traveling through the facility.

This would be indicative of VoWiFi or warehouse scanning applications. Determining which type is necessary can be key for not only troubleshooting an existing network but also designing a new one.

  • 802.11g protection mechanism - It should be understood in advance that if there is any requirement for backward compatibility with 802.11b HR-DSSS clients, the 802.11g protection mechanism will always adversely affect throughput.

The majority of enterprise deployments will always require backward compatibility to provide access to handhelds, VoIP phones, or older 802.11b radio cards.

Carefully planning coverage and capacity needs prior to the site survey will assist you in determining some of the design scenarios you may possibly need, including AP power settings, type of antennas, cell sizes, and so on.

The physical site survey will still have to be conducted to validate and further determine coverage and capacity requirements.

Existing Wireless Network

Quite often the reason you are conducting a WLAN site survey is that you have been called in as a consultant to fix an existing deployment.

Professional site survey companies have reported that as much as 40 percent of their business is being hired to troubleshoot existing WLANs, which often requires conducting a second site survey or discovering that one was never conducted to begin with.

As more corporations and individuals become educated in 802.11 technologies, the percentage will obviously drop. Sadly, many untrained customers just install the access points wherever they can mount them and leave the default power and channel settings on every AP.

Usually, site surveys must be conducted either because of performance problems or difficulty roaming. Performance problems are often caused by co-channel interference and multipath interference as well as other sources of interference.

Roaming problems may also be interference related or caused by a lack of adequate coverage and/or by a lack of proper cell overlap. Here are some of the questions that should be asked prior to the reparative site survey:

  • What are the current problems with the existing WLAN? - Ask the customer to clarify the problems. Are they throughput related? Are there frequent disconnects? Is there any difficulty roaming?

In what part of the building do the problems occur most often? How often do they occur and have there been any steps taken to duplicate the troubles?

  • Are there any known sources of RF interference? More than likely the customer will have no idea, but it does not hurt to ask. Are there any microwave ovens? Do they use cordless phones or headsets? Does anyone use Bluetooth for keyboards or mouse?

After asking these interference questions, you should always conduct a spectrum analysis. This is the only way to determine whether or not there is any RF interference in the area that may inhibit future transmissions.

Something like a new Wireless Internet Service Provider (WISP) in the area may simply be interfering with one of your channels.

  • Are there any known coverage dead zones? This is related to the roaming questions, and areas probably exist where proper coverage is not being provided. Remember, this could be too little or too much coverage. Both create roaming and connectivity problems.
  • Does prior site survey data exist? Chances are that an original site survey was not even conducted. However, if old site survey documentation exists, it may be helpful when troubleshooting existing problems.

It is important to note that unless quantifiable data was collected that shows dB strengths, the survey report should be taken with extreme caution.

  • What equipment is currently installed? Ask what type of equipment is being used, such as 802.11a or 802.11b/g and which vendor has been used. Once again, chances are the customer has no idea and it will be your job to determine what has been installed and why it is not working properly.

Also check the configurations of the devices, including SSIDs, WEP keys, channels, power levels, and firmware versions. Oftentimes issues can be as simple as all the access points are transmitting on the same channel or there is a buffer issue that is resolved with the latest firmware.

Depending upon the level of troubleshooting that is required on the existing wireless network, a second site survey consisting of coverage and spectrum analysis will usually be necessary.

After the new site survey has been conducted, adjustments to the existing WLAN equipment should be adequate; however, the worst case scenario would involve a complete redesign of the WLAN.

Keep in mind that whenever a second site survey is necessary, all the same questions that are asked as part of a survey for a new installation (Greenfield survey) should also be asked prior to the second site survey.

Infrastructure Connectivity

You have already learned that the usual purposes of a WLAN are to provide mobility and to provide access via an AP into another network infrastructure. Part of the interview process will be to ask the correct questions so that the WLAN will integrate properly into the existing wired architecture.

Asking for a copy of the wired network topology map is highly recommended. For security reasons, the customer may not want to disclose the wired topology and a nondisclosure agreement might need to be signed. It is a good idea to request that an agreement be signed to protect you legally as the integrator.

Understanding the existing topology will also be of help when planning WLAN segmentation and security proposals and recommendations. With or without a topology map, the following topics are important to ensure the desired infrastructure connectivity:

  • Roaming - Is roaming required? In most cases, the answer will always be yes because mobility is a key advantage of wireless networking. Any devices that run connection-oriented applications will need seamless roaming.

Roaming is mandatory if handheld devices and/or VoWiFi phones are deployed. Surprisingly, many customers do not require roaming capabilities. In these cases, being mobile is sufficient, as mentioned previously. Coverage may be needed only in some areas of the building and roaming may not a requirement.

Some network administrators may want to be able to restrict certain areas where a user or a group of users can roam. For example, the sales team is allowed to roam only between access points on floors one and two and not permitted to roam to APs on floors three and four.

The marketing team, however, is allowed to roam between access points on all four floors. The role-based access control (RBAC) capabilities of a wireless switch or controller will deliver the granular control needed to segment and control roaming.

This may also have to be segmented with different SSIDs and VLANs. Another important roaming consideration is whether users will need to roam across layer 3 boundaries.

A Mobile IP solution or a proprietary layer 3 roaming solution will be needed if client stations need to roam across subnets. Special considerations will have to be given to roaming with VoWiFi devices due to the issues that can arise from network latency.

With regard to the existing network, it is imperative that you determine whether or not the network infrastructure will support all the new wireless features.

For instance, if you want to roll out five SSIDs with different VLANs but haven’t checked to see if the customer’s network switches can be configured with VLANs, you may have a serious problem.

  • Wiring closets - Where are the wiring closets located? Will the locations that are being considered for AP installation be within a 100-meter (328-foot) cable drop from the wiring closets?
  • Antenna structure - If an outdoor network or point-to-point bridging application is requested, then there may be some additional structure that will need to be built to mount the antennas.
  • Hubs/switches - Will the access points be connected by CAT5 cabling to hubs or managed switches? A managed switch will be needed if VLANs are required. Connecting access points to hubs is not a recommended practice because of security and performance reasons.

All traffic is broadcast to every port on a hub, and any traffic that traverses through an access point connected to a hub port can be heard on any of the other ports. Are there enough switch ports? Who will be responsible for programming the VLANs?

  • PoE - How will the access points be powered? Because APs are often mounted in the ceiling, Power over Ethernet will be required to remotely power the access points. Very often the customer will not yet have a PoE solution in place and further investment will be needed.

If the customer already does have a PoE solution installed, it must be determined if the PoE solution is 802.3af compliant or a proprietary PoE solution. Also, is the solution an Endspan or Midspan solution?

Regardless of what they have, it is important to make sure that it is compatible with the system you are installing. If PoE injectors need to be installed, you will need to make sure there are sufficient power outlets. If not, who will be responsible for installing those?

  • Segmentation - How will the WLAN and/or users of the WLAN be segmented from the wired network? Will the entire wireless network be on a separate IP subnet? Will VLANs be used and is a guest VLAN necessary? Will firewalls or VPNs be used for segmentation?

Or will the wireless be a natural extension to the wired network and follow the same wiring, numbering, and design schemes as the wired infrastructure? All these questions are also directly related to security expectations.

  • Naming convention - Does the customer already have a naming convention for cabling and network infrastructure equipment and will one need to be created for the WLAN?
  • User management - Considerations regarding role-based access control (RBAC), bandwidth throttling, and load balancing should be discussed.
  • Infrastructure management - How will the WLAN remote access points be managed? Is a central management solution a requirement? Will devices be managed using SSH2, SNMP, HTTP/ HTTPS, and so on?

A detailed site interview that provides detailed feedback about infrastructure connectivity requirements will result in a more thorough site survey and a well-designed wireless network. Seventy-five percent of the work for a good wireless network is in the pre-engineering. It creates the road map for all the other pieces.

Security Expectations

Network management should absolutely be interviewed about security expectations. All segmentation and encryption needs should be discussed. All authorization, authentication, and accounting (AAA) requirements must also be documented.

It should also be determined whether the customer has plans to implement a wireless intrusion detection system (WIDS) solution for protection against rogue APs and the many other types of wireless attacks. Some security solutions, such as layer 3 VPNs, may put extra overhead on the WLAN because of the type of encryption that is used.

Overhead caused by encryption should be accounted for during the capacity planning stages. Special considerations will have to be given to VoWiFi devices due to the latency issues that might result from EAP authentication.

A comprehensive interview regarding security expectations will provide the necessary information to make competent security recommendations after the site survey has been conducted and prior to deployment.

Industry-specific regulations such as HIPAA, Gramm-Leach-Bliley, and Sarbanes-Oxley may have to be taken into consideration when making security recommendations.

U.S. government installations may have to abide by the strict FIPS 140-2 regulations and all security solutions may need to be FIPS compliant.

All of these answers should also assist in determining if the necessary hardware and software exists to perform these functions. If not, it will be your job to consider the requirements and recommendations that may be necessary.