Understanding data capacity and client density requirements

  • 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
    VoWiFi phones will need mobility and connectivity throughout the building.
  • 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 regarding the purpose of the WLAN.
  • 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 high-density area may require semidirectional patch antennas for sectorized coverage as opposed to using omnidirectional antennas.
  • 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.
  • Cell sizing for a high-density WLAN design might be necessary to properly address
    your capacity requirements.
  • In order for the wireless end user to experience acceptable performance, a ratio of average number of users per access point must be established.
  • The answer to the capacity question depends on a host of variables, including answers to
    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 the number of Wi-Fi devices that should be communicating on average through an access point.
  • So the next question is, what is a good average number of connected devices
    per access point? Once again, it depends entirely on the purpose of the WLAN and the applications being used. However, in an 802.11a/b/g/n network, 35–50 data WLAN devices per radio is an often-quoted figure for typical WLAN applications such as web browsing and email.
  • Many WLAN vendors’ marketing material states that 100 or 200 devices can connect to an 802.11n access point at the same time.
  • Although more than a hundred devices might be able to connect to an AP radio, these numbers are not realistic for active devices due to the nature of the half-duplex shared medium.
  • 35 to 50 active Wi-Fi devices per radio on a dual frequency 802.11n access point is realistic with average application use, such as web browsing. 802.11ac access points can provide for a greater number of active connections and might be a viable solution in areas with a high density of users and devices.

User and Device Density

Three important questions need to be asked with regard to users.

  1. First, how many users currently need wireless access and how many Wi-Fi devices will they be using?
  2. Second, how many users and devices may need wireless access in the future? These first two questions will help you to begin adequately planning for a good ratio of devices per access point while allowing for future growth.
  3. 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, whereas 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.
  • 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 because of 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.
  • 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 refers to interfering devices such as microwaves, cordless headsets, cordless phones, wireless machinery, and so on. Often, this is severely overlooked.
  • If a large open area will house the help desk after the wireless is installed,you may be thinking of capacity. However, if you don’t know that the employees are using 2.4 GHz cordless headsets or Bluetooth keyboards and mice, you may be designing a network destined for failure.

Portability vs. Mobility

  • There are two types of mobility. The first is related to being portable and the other is true mobility.
  • To help explain this, think of a 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 portable. He may have disconnected in between access points, and that is okay.
  • However, having true mobility means that a user remains connected 100 percent of the time while traveling through the facility.
    This would be indicative of VoWiFi or warehouse scanning applications. As mentioned earlier, most users now carry some sort of personal mobile device, such as smartphone;
    therefore, true mobility is almost always an understood requirement. Determining which type of connectivity is necessary can be key for not only troubleshooting an existing network but also for designing a new one.

Backward Compatibility for Legacy Devices

  • It should be understood in advance that if there is any requirement for backward compatibility with legacy clients, the 802.11 protection mechanisms will always adversely affect throughput.
  • The impact of protection mechanisms is not as severe on an 802.11n network, but it can be significant on a legacy 802.11b/g network.
  • Enterprise deployments will almost always require some level of backward compatibility to provide access for older 802.11a/b/g radios found in handhelds, VoWiFi phones, or older laptops.
  • Many handheld mobile devices, such as older barcode scanners, still do not have 802.11n radios and backward compatibility will be required.
  • Carefully planning coverage and capacity needs prior to the site survey will help you determine some of the design scenarios you might need, including AP power settings, types of antennas, and cell sizes.
  • The physical site survey will still have to be conducted to validate and further determine coverage and capacity requirements.
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