1 white paper High-Density Wi-Fi Design Principles
2 Introduction Wi-Fi networks are seeing increased utilization and load across organizations in most industries. High-density wireless networks are typically considered to be environments where the number of client devices and required application throughput exceed the available capacity of a traditional coverage-oriented Wi-Fi network design. In these situations, even a well-designed wireless network that provides coverage at good signal strength and SNR (signal-to-noise ratio) throughout the desired coverage area is insufficient to ensure high performance due to inadequate capacity. The limiting factor is available airtime. Since Wi-Fi relies on a shared and unbounded medium (that is, the air), clients and access points must contend for available airtime to transmit data. The Wi-Fi link between a client and access point is shared among all clients within range of one another on a particular frequency. Therefore, signal strength as an indication of link quality is not analogous to a wired link connection indicator. Instead, aggregate capacity demands must be determined across the entire client population and the network must be designed to distribute the load effectively across the available spectrum. Spectral capacity, channel utilization, interference, frequency reuse, and regulatory requirements become the critical design variables in addition to coverage and signal strength. This requires a detailed analysis of client capabilities, application requirements, facility characteristics, and the appropriate use of sufficient access points. The goal is to segment clients into the smallest possible collision domains (different radio frequencies) while maximizing the use of available spectral capacity. This whitepaper outlines a set of simple principles that you can use as a basic guide when designing your high-density network. This list of principles is not meant to be comprehensive but rather a quick overview of the most important items to take into account. This document presents a condensed overview of the design recommendations in the High-Density Wi-Fi Design and Configuration Guide, which is available on the Aerohive.com website. 2 Copyright 2012, Aerohive Networks, Inc.
3 Principle #1: Wi-Fi Coverage Does Not Guarantee Adequate Capacity Historically, many Wi-Fi networks have been designed to provide basic coverage throughout a desired service area. While a coverage-oriented design can meet basic Wi-Fi network access needs, supporting only a few users and light workload, a different approach is required to support a growing Wi-Fi client population and use as the primary network access method. Modern Wi-Fi networks should be designed to accommodate the current and anticipated capacity and performance levels within the organization rather than focusing on providing basic coverage. Significant capability differences exist between Wi-Fi networks designed for coverage versus those designed for capacity. The nuances between these two different approaches are often not well understood by organizational decision-makers. The use of wireless signal bars as a representation of network viability and a satisfactory user experience has proven to be a poor indicator of network success. A coverage-oriented design often does not factor in other critical variables required to meet performance and capacity needs, such as the following: Minimization of co-channel interference Maximization spectral capacity by co-locating radios on different channels for example Client band steering to optimize use of available spectral capacity Client load balancing between access points based on available airtime and load, application bandwidth and latency requirements End-to-end quality-of-service design Attempting to augment a coverage-oriented Wi-Fi network to increase capacity and support dense user populations is an appealing approach for organizations looking to minimize workload and expense. However, such an approach should be met with caution, as existing constraints of the coverage-oriented network implementation can limit proper high-capacity Wi-Fi network design and ultimately fail to adequately meet performance needs. Many organizations are quickly realizing that existing WLAN deployments, which were designed for basic coverage requirements in common areas, are not adequate to meet these growing demands, and that simply adding more access points is ineffective without proper network planning and RF design. Principle #2: Place Adequate Focus on Network Planning and Design When working with high-density WLANs, it is important to place sufficient focus on network planning and design. It is also important to ensure the network equipment that is purchased and deployed is capable of delivering both high-performance and intelligent features to optimize the use of limited spectrum. Only through proper requirements gathering, network design, configuration, and continual optimization can a Wi-Fi network be deployed that meets the demands of a dense user population. Through the process of requirements gathering, you can identify network design objectives so that the Wi-Fi network will align with business objectives and meet desired performance levels. Proper planning requires a thorough understanding of the client devices that will be supported and their capabilities, client density in each Copyright 2012, Aerohive Networks, Inc. 3
4 coverage area, and the applications that rely on the WLAN and their associated throughput requirements. Network design involves the synthesis of identified requirements into a network architecture that meets established capacity and performance requirements. This is accomplished by understanding and adopting design principles for a high-capacity Wi-Fi network rather than a coverage-oriented network and by following three broad steps. First, identify client capabilities and application throughput requirements. Second, determine the access point and radio capacity necessary to support the identified client and application requirements. Third, determine the appropriate type and quantity of access point equipment and accessories you need and their placement on site for optimal coverage based on facility characteristics. Conducting site surveys to collect much of the information you need and to verify design decisions is critical for creating a successful high-density network design. Principle #3: Identify Client Device Capabilities The process of requirements gathering should be the first step in designing and implementing a high-density WLAN. Through the process of requirements gathering, you can identify network design characteristics that will help the Wi-Fi network align with business objectives and meet desired performance levels. First, identify the client device types that will be supported within the environment, their quantities, and their wireless radio capabilities. The wireless network must serve all client devices simultaneously. Because Wi-Fi leverages a shared RF medium, the network design and configuration must balance requirements between all clients. Identify the type of radio in each device and the Wi-Fi Alliance certifications they have passed (802.11b/g/a/n). This will aid in network design by determining the data rates and frequency bands supported, application throughput capabilities, and in determining if b support (HR/DSSS modulation) is required on the network. Once you have identified the radio type, including the number of n spatial streams supported, determine the maximum Wi-Fi data rate that the device is capable of achieving and on what channels and bands. The maximum data rate is important for determining how fast a client can transmit and receive data, which impacts the amount of airtime utilized to achieve the target application throughput level and overall network capacity planning. The channel and band support is important for determining whether or not DFS channels can be used and for understanding the effectiveness of band-steering. Next, determine the maximum application throughput that each type of wireless client device that will be on the network can achieve. Due to various sources of network overhead, the maximum Wi-Fi data rate does not represent the actual application throughput that can be achieved. To estimate the maximum amount of TCP/IP throughput that a client is capable of achieving, the amount of network overhead must be determined either through live network testing under load or through an educated assumption. It is common for Wi-Fi networks to have between 40-60% overhead. The best and most accurate method of determining throughput capabilities of a device is to perform live network testing on client devices that will be used in the network and the same network equipment and configuration settings that will be used in production, especially with regards to available data rates and channel width. 4 Copyright 2012, Aerohive Networks, Inc.
5 Common Client Device Categories and Capabilities Device Category Feature Phones Smart Phones Tablets Netbooks Low-End Laptops Mid-Range Laptops Wi-Fi Radio Type Channel Support* Channel Width Transmit Power Output Maximum Data Rate 20 MHz/40 MHz b/g MHz Only 11dBm 54 Mbps n 1x1: n 1x1: n 1x2: n 2x2: n 2x3: MHz Only 11 dbm Mbps 1-11, 36-48, , 36-48, MHz Only dbm Mbps 20/40 MHz dbm 72 Mbps (Up) 144/300 Mbps (Down) MHz Only dbm 144 Mbps 1-11, 36-48, /40 MHz dbm 144/300 Mbps 1-11, 36-48, 52- High-End n 64, , Laptops 3x3: /40 MHz dbm 216/450 Mbps VoIP Handsets a/b/g 1-11, 36-48, MHz Only dbm 54 Mbps * This chart references channels from the FCC regulatory domain only. Principle #4: Identify the Target Application Throughput Level for Each Device Type Once the client devices and radio capabilities have been identified, determine the applications that will be used and the application performance requirements (throughput and QoS) for each device. A thorough understanding of the application requirements will provide the necessary information to design a network that meets organizational needs. Through this process, the organization can identify the critical and non-critical applications used on each device and establish the target application throughput level per device that is required for successful operation. Common application classes and typical bandwidth requirements are listed in the table below. Additionally, the recommended QoS class for both Layer 2 and Layer 3 is provided. The QoS classes are based on common network best practices, but should be tailored to each environment to ensure integration with existing network policies and consistent end-to-end QoS implementation. Copyright 2012, Aerohive Networks, Inc. 5
6 Common application classes, throughput requirements, and recommended QoS classes: Application Class Required Throughput QoS Class (Layer 2/Layer 3) 1 Web-browsing/ 500 Kbps 1 Mbps WMM 0 (BE)/DSCP 0 Video Conferencing (example: WebEx 2 ) SD video streaming (example: Netflix 3, Hulu 4 ) HD video streaming (example: Netflix, Hulu) Apple TV 5 streaming Apple FaceTime Kbps 1 Mbps Mbps 2 5 Mbps Mbps 900 Kbps WMM 5 (VI)/DSCP AF41 (34) WMM 4 (VI)/DSCP CS4 (32) WMM 4 (VI)/DSCP CS4 (32) WMM 4 (VI)/DSCP CS4 (32) WMM 5 (VI)/DSCP AF41 (34) YouTube 7 video streaming 500 Kbps WMM 0 (BE)/DSCP 0 Printing 1 Mbps WMM 0 (BE)/DSCP 0 File Sharing 8 5 Mbps WMM 0 (BE)/DSCP 0 E-Learning and Online Testing 2 4 Mbps WMM 4 (VI)/DSCP CS4 (32) Thin-client (example: RDP 9, XenDesktop 10 ) Kbps WMM 4 (VI)/DSCP CS4 (32) Thin-client (with video or printing) 600 1,800 Kbps WMM 4 (VI)/DSCP CS4 (32) Thin-apps (example: XenApp 11 ) 20 Kbps (per App) WMM 4 (VI)/DSCP CS4 (32) Device Backups (example: cloud services) Uses available WMM 1(BK)/DSCP bandwidth CS1 (8) VoIP Call Signaling (example: SIP) 5 Kbps WMM 3 (BE)/DSCP CS3 (24) VoIP Call Stream (codec 12 dependent) Kbps WMM 6 (VO)/DSCP EF (46) These application classes are provided as a generic reference only. Network administrators should research specific application requirements from the product developer. 1 The QoS Baseline At-A-Glance (Cisco Systems, Inc., 2005) 2 WebEx Network Bandwidth White Paper (Cisco Systems, Inc., 2010) 3 Hunt, Neil, Encoding for streaming (Netflix, 2008) 4 Hulu Plus System Requirements (Hulu, 2012) 5 Apple TV (2 nd and 3 rd Generation): Troubleshooting playback performance (Apple, Inc., 2012) 6 FaceTime for Mac: About HD video calling (Apple, Inc., 2011) 7 YouTube Help, Watching Videos, System Requirements (Google, Inc., 2012) 8 Zimmerman, Tim, Best Practices for WLAN Site Surveys That Save Money (Gartner, Inc., 2012) 9 Remote Desktop Protocol Performance Improvements in Windows Server 2008 R2 (Microsoft, 2010) 10 XenDesktop Planning Guide: User Bandwidth Requirements (Citrix Systems, Inc., 2010) 11 ICA Client Bandwidth Analysis (Citrix Systems, Inc., 2001) 12 Enterprise QoS Solution Reference Network Design (Cisco Systems, Inc., 2005) 6 Copyright 2012, Aerohive Networks, Inc.
7 Principle #5: Forecast AP and Channel Capacity Once the client device and application requirements have been identified, the network administrator can forecast the required access point radio capacity and subsequently estimate the number of APs. The forecast is derived by estimating the network load, or airtime, required for each client device to achieve the target application throughput level. This is represented as a percentage of airtime that will be consumed per client device on an AP radio, with the aggregate sum being used to forecast the required AP capacity to support all clients concurrently. First, determine how much airtime the target application will consume when used on each device type by dividing the required throughput by the maximum TCP or UDP throughput the device is capable of achieving. For example, a 1-Mbps standard definition TCP video application running on an Apple ipad 2 that is capable of achieving a maximum 30 Mbps of TCP throughput yields an airtime consumption of 3.33% per device on a particular channel. This means that every ipad 2 device running the video application requires 3.33% of the capacity of a single access point radio (assuming no outside noise or interference). Perform this calculation separately for every device and application type that will be supported in the environment. Second, multiply the total client device quantity for each device type by the required airtime per client device to determine the number of AP radios required to support those devices in the environment. For example, if there are 40 total Apple ipads, and each one consumes 3.33% airtime, then a total of 1.33 AP radios are required to support all 40 devices concurrently (40 x 3.33%). Perform this calculation individually for every device type that will be supported in the environment. Finally, to forecast the aggregate required AP capacity, add the number of AP radios required to support each device type at the target application throughput level together to determine the total number of AP radios required. Adjust the number of radios if only a percentage of clients will be connected to and transmitting on the WLAN concurrently. If all client devices will be online concurrently, then no adjustment is necessary. Since most deployments utilize dual-radio APs, divide the adjusted number of AP radios by two to forecast the number of dual-radio access points required to support the identified devices and applications. If a fractional number results, round up. It is important to understand that the forecasted AP capacity is only an estimate; the final capacity will likely be slightly different. The forecast is useful as an initial starting point for the Wi-Fi network design with a predictive site survey. Through the site survey process, verification of the forecasted AP capacity occurs and will likely result in modifications due to facility characteristics that require higher capacity by colocating APs or coverage in less commonly used areas such as hallways, stairwells, and elevators. Copyright 2012, Aerohive Networks, Inc. 7
8 Principle #6: The 5 GHz Frequency Band Offers Greater Capacity than the 2.4 GHz Band The amount of available unlicensed spectrum bounds the total capacity of a Wi-Fi network composed of multiple co-located and adjacent cells. A cohesive wireless network requires coverage overlap between adjacent cells to facilitate uninterrupted service for mobile clients as they roam through the environment. A high-density wireless network also requires co-located Wi-Fi radios serving the same coverage area to meet the capacity demands of a large client population. All Wi-Fi stations, including access points that operate on the same frequency, must contend for airtime with one another. Therefore, placing multiple access points that are operating on the same channel within the same physical service area does not increase capacity. It has the opposite effect of increasing medium contention and effectively reducing capacity. It is critical that co-located and adjacent Wi-Fi cells operate on different frequencies to reduce medium contention (co-channel interference) and increase network capacity. When APs contend for airtime with one another, wireless professionals often refer to it as co-channel interference (CCI) because it indicates improper network design and can often be avoided. However, it is not truly interference and would be more accurately described as co-channel contention. When Wi-Fi stations operate on nonoverlapping frequencies, they cannot receive transmissions from one another, and it less likely that their transmissions will interfere with one another. In effect, by segmenting groups of Wi-Fi stations onto different frequencies, the collision domain is made smaller. This serves to reduce medium contention to a smaller group of stations and thereby increases capacity within each cell. The total amount of spectrum available in the unlicensed frequency bands serves as an upper bound for aggregate Wi-Fi network capacity. Sufficient spectral capacity is required to provide both sufficient network capacity within a single coverage area through the use of co-located APs and to provide enough nonoverlapping channels for a channel reuse plan that minimizes co-channel interference between adjacent coverage areas. Significant differences in the amount of available unlicensed spectrum between the 2.4 GHz ISM band and the 5 GHz UNII bands influences network design. The 2.4 GHz ISM band is comprised of only three nonoverlapping 20 MHz channels that can be used for Wi-Fi networks (22 MHz-wide channels in the case of HR-DSSS used with b). In Japan, four nonoverlapping channels are available, but channel 14 is restricted to DSSS/CCK modulation (802.11b) and OFDM modulation (802.11g/n) cannot be used. 8 Copyright 2012, Aerohive Networks, Inc.
9 Figure 1: 2.4 GHz frequency band capacity Due to the relatively sparse amount of spectrum available in the 2.4 GHz band, there are typically not enough channels for multiple co-located APs to serve a high-density client population without negatively affecting the nonoverlapping channel reuse plan and introducing significant levels of co-channel interference. Therefore, in a high-density network design, only a single 2.4 GHz AP radio should provide service within a physical coverage area. Minimizing CCI between adjacent 2.4 GHz cells is a primary high-density WLAN design consideration, yet even with sound design practices, it is often not possible to completely eliminate CCI in the 2.4 GHz frequency band due to inadequate spectral capacity. The 5 GHz UNII bands comprises four unique frequency bands and a total of 23 nonoverlapping 20-MHz channels (24 channels when including UNII-2 Extended channel 144, which was added with the ac amendment, and 25 channels when including ISM channel 165). In contrast to the 2.4 GHz ISM band, the 5 GHz bands offer much more spectral capacity for Wi-Fi networks. This facilitates greater separation between access points operating on the same channel and allows for a better frequency reuse plan. In high-density environments, multiple 5 GHz AP radios can be physically co-located, using different channels, in order to increase capacity. These factors make 5 GHz operation essential for successful high-density Wi-Fi network deployments. Copyright 2012, Aerohive Networks, Inc. 9
10 Figure 2: 5 GHz frequency band capacity Two of the four 5 GHz bands come with regulatory restrictions that can make their use unappealing and can significantly reduce available capacity. Two of these bands, UNII-2 and UNII-2 Extended (UNII-2e), require Dynamic Frequency Selection (DFS) certification in most regulatory domains to detect and prevent interference with military and weather radar systems. The regulations are strict in requiring transmitters to cease operation immediately when radar is detected. This can cause service outages in Wi-Fi networks as APs issue a Channel Switch Announcement (CSA) to inform connected clients of a newly selected channel and then abruptly change to that channel. Wi-Fi networks in areas without military or weather radar systems can operate in the DFS bands with relative confidence, but both infrastructure and clientside equipment must support UNII-2 and UNII-2e channels for such a deployment to be effective. In areas where radar is present or Wi-Fi networks are leveraged for mission-critical operations, the risk might be too great to use these frequency bands safely. In these circumstances, administrators should disable the use of the UNII-2 and UNII-2e bands, which leaves only nine nonoverlapping 20-MHz channels remaining in the 5 GHz UNII-1, UNII-3, and ISM bands in the USA, and just four nonoverlapping 20- MHz channels in the UNII-1 band in Europe. Overall, the 5 GHz bands offer much more capacity than the 2.4 GHz band. However, network designers must still be cognizant of the impact that disabling DFS channels will have on network capacity and the channel reuse plan. 10 Copyright 2012, Aerohive Networks, Inc.
11 Principle #7: Use 20-MHz Channel Width The available spectral capacity is also influenced by the operating channel width. This is of primary consideration in the 5 GHz band, where more channels are available and the bonding of channels is a common technique to increase individual cell capacity n introduces wider channels of 40 MHz to increase peak bandwidth and throughput within a single Wi-Fi cell. The forthcoming 80211ac amendment will introduce even wider channel widths of 80 MHz and 160 MHz as shown below (for simplicity, noncontiguous channels are not shown). Figure 3: Spectral capacity versus channel width Increasing channel width is attractive because it increases the peak data rate and throughput for individual clients capable of wide-channel operation, which is highly visible to end-users and forms a perception of a high-performing wireless network. However, using wide channels in a high-density network can have the opposite effect by reducing the total number of channels available for reuse among colocated and adjacent APs and can reduce overall network capacity. In high-density Wi-Fi networks, channel reuse and segmentation of client devices into separate collision domains is of greater importance than wide channels and the peak throughput per client device due to efficiency of spectrum use. Limiting the number of available channels in your frequency reuse plan by using wide channels can reduce overall network capacity. It is recommended that 20-MHz-wide channels be used in high-density environments and 40-MHz wide channels be used when 5 GHz DFS frequency bands are enabled and in physical areas where high-throughput client devices will be used. A 20-MHz channel width should always be used in the 2.4 GHz frequency band. Principle #8: Provide High Quality Signal throughout the Coverage Areas When designing a network for capacity, clients should connect to access points at the highest data rates possible to maximize application throughput. This improves overall network capacity by reducing individual client airtime utilization. Since airtime is shared among all clients operating on the same channel and within range of one another, this allows either additional clients to be supported or additional throughput for existing clients. Copyright 2012, Aerohive Networks, Inc. 11
12 With the process of Adaptive Rate Selection, Wi-Fi clients achieve the highest data rates when they have the strongest signal to and from the access point with minimal interference. Client device specifications for Receive Sensitivity determine the signal strength required to achieve various data rates and vary between devices. Many device manufacturers recommend maintaining a minimum signal strength of -65 to - 67 dbm (-67 dbm is used in this document) with an SNR (signal-to-noise ratio) of db to achieve the best performance, especially for multimedia (voice and video) applications. Therefore, high-density networks should be designed to ensure that clients always have a strong signal to two APs on nonoverlapping channels (for roaming purposes), high SNR, and are able to maintain high data rates. Figure 4: Data rate versus coverage range This effectively reduces the Wi-Fi cell size at which clients should be associated to individual APs. As clients move farther from the AP and signal strength declines toward the -67 dbm threshold, they should be able to discover and roam to an alternate AP that can continue to provide a sufficiently high-strength signal. This requires that network administrators design the Wi-Fi network so that AP coverage areas provide overlap at or above -67 dbm. When compared to a coverageoriented network, designed to provide signal in all locations at much lower levels (- 72 dbm for example), this effectively reduces cell sizing and distance between APs. Access points will be located in closer proximity to one another in capacityoriented Wi-Fi networks. 12 Copyright 2012, Aerohive Networks, Inc.
13 Principle #9: Minimize Co-Channel Interference (CCI) between APs A capacity-oriented design introduces a significant differential between the distance at which clients will associate to an AP (association range > -67 dbm) and the distance at which the AP signal can create co-channel interference with other APs operating on the same channel (contention range -67 to -85 dbm). Therefore, APs will cause co-channel interference at much farther distances than the clients will typically associate to the APs. Co-channel interference is more aptly referred to as medium contention between Wi-Fi stations because they are able to de-modulate the frame preamble from one another and defer concurrent transmission to prevent frame corruption. Co-channel interference is typically the most significant cause of limited performance and capacity in a Wi-Fi network. Figure 5: Association versus contention range The result of co-channel interference is that the Wi-Fi access points are effectively sharing airtime and capacity with one another. For this reason, network administrators cannot add capacity by simply installing more access points. Doing so without careful consideration of the existing channel plan could result in access points operating on the same channel within contention range of one another, which results in capacity being shared between the APs rather than providing additional capacity. Instead, additional access points should be integrated into the Wi-Fi network design to operate on nonoverlapping channels to increase overall network capacity. Network administrators should design Wi-Fi cell overlap between neighboring APs on different channels at -67dBm or greater and develop a channel plan that minimizes co-channel interference between APs on the same channel below -85 dbm. Copyright 2012, Aerohive Networks, Inc. 13
14 Figure 6: Co-Channel Interference Designing a channel plan that reduces co-channel interference is significantly easier for the 5 GHz frequency band than the 2.4 GHz frequency band due to the amount of available spectral capacity. The greater number of available nonoverlapping channels allows greater physical separation between access points operating on the same channel. In most environments, it is not possible to eliminate co-channel interference in the 2.4 GHz band with only three or four nonoverlapping channels, but this is largely dependent on the characteristics of the facility where wireless service is installed. Principle #10: Adapt the Design to Facility Characteristics The Wi-Fi network design must take into account the characteristics of the facility where wireless service is required. Materials used in building construction will have a significant impact on RF signal propagation and attenuation, and designs will be different among various areas, such as small enclosed spaces, areas with large open spaces, and outdoor areas. Building construction materials and other sources of RF signal attenuation help create separate RF collision domains, which allows greater flexibility in channel reuse and increases network capacity. Facilities with small, enclosed spaces, especially those that are enclosed with building materials that readily absorb RF signals and provide much greater signal attenuation, can allow access points to reuse the same channel in isolation from one another. On the other hand, facilities with large, open spaces often lack sufficient RF signal attenuation, which makes channel reuse, especially in the 2.4 GHz band, very difficult without creating co-channel interference. Using facility construction to isolate APs from one another can be especially useful when designing a channel plan for the 2.4 GHz band. Having only three nonoverlapping channels typically does not provide enough spectral capacity to properly isolate neighbors from co-channel interference unless sufficient RF signal attenuation exists. 14 Copyright 2012, Aerohive Networks, Inc.
15 High-Density Wi-Fi Design Principles Figure 7: Leverage facility construction for channel reuse In specific instances, it might be necessary to co-locate multiple APs in a single physical area to provide the required capacity. A lecture hall, gymnasium, or large presentation room might require more capacity than three dual-radio access points operating on nonoverlapping 2.4 GHz and 5 GHz channels can provide. Areas where large numbers of users congregate might require adding additional capacity by colocating multiple APs to serve the same physical coverage area. You can use several techniques when co-locating APs to increase network capacity: Use semi-directional antennas and low transmit power to limit the coverage area of individual APs and to create separate RF collision domains. Supplement 5 GHz capacity through the use of additional access points on 5 GHz channels that are not in use. This can be accomplished by using dualradio APs with client access restricted to the 5 GHz radio or by using singleradio APs configured for 5 GHz operation. Supplement 2.4 GHz capacity through the use of access points installed in locations where RF signal attenuation reduces signal propagation into only a portion of the coverage area such as on the opposite side of a wall or underneath the floor. Copyright 2012, Aerohive Networks, Inc. 15
16 Figure 8: Use additional APs to supplement 5 GHz capacity When any of these techniques are used, ensure that all co-located access points are mounted a minimum of 10 feet apart from one another to reduce adjacent channel interference (ACI). Additionally, co-located access points should be configured to use non-adjacent channels with appropriate output power and adaptive CCA (clear channel assessment) settings to reduce the effects of ACI further. The facility might also require additional access points to provide coverage in less common areas that will be lightly used, such as hallways, stairwells, and elevator shafts. Coverage in these areas will depend on user behavior with the supported applications in the environment. For example, in environments where Voice over IP (VoIP) is used, it is common for users to expect coverage in hallways and stairwells so that voice conversations can continue uninterrupted while they move throughout the facility. Principle #11: Always Perform a Site Survey Wi-Fi site surveying is a critical component of deploying a successful high-density network that meets user expectations and the needs of the organization. The site survey process allows a network administrator to understand the unique RF propagation characteristics of the facility and environment into which the WLAN will be installed. Network administrators can validate previously gathered design parameters through predictive modeling and live network measurements to ensure that the deployed WLAN will meet the established coverage, capacity, and performance goals. Three types of site surveys exist: predictive site surveys, pre-deployment ( AP-on-astick ) site surveys, and post-deployment site surveys. Aerohive recommends that all three types of site surveys be performed for high-density environments. Of the three types, the post-deployment site survey is the most critical and should always be performed. Predictive site surveys use computer-based software applications to model the facility and RF environment. These applications allow a network administrator to do the following: 16 Copyright 2012, Aerohive Networks, Inc.
17 Outline the required coverage areas using facility blueprints Define facility structures to aid in estimating RF signal attenuation Establish thresholds for minimum signal strength and application throughput that clients must achieve Predict the quantity, location, and type of access points that should be installed Provide channel and power settings that maximize spectral capacity while minimizing co-channel and adjacent-channel interference (CCI/ACI). One of the major benefits of predictive modeling is the ability to quickly simulate various deployment scenarios and narrow down design alternatives. A predictive site survey will never be 100% accurate, and though it cannot replace pre-deployment or post-deployment site surveys, it can help expedite them. Figure 9: Aerohive Planner A pre-deployment site survey, often called an AP-on-a-stick survey, is performed prior to WLAN network installation to determine the actual RF signal propagation characteristics of the environment. Measuring and recording the RF behavior in a facility results in a better WLAN design, one uniquely tailored to the physical properties of the environment. It is also used to verify and adjust a preliminary Wi-Fi network design created using a predictive site survey and can reduce the need for network change orders once WLAN equipment is procured and installed. Spectrum Copyright 2012, Aerohive Networks, Inc. 17
18 analysis is an integral part of a pre-deployment site survey because it can help you find sources of RF interference that could cause WLAN performance issues. You can then take steps to remediate them before deployment. A post-deployment site survey is performed after the WLAN equipment has been installed and configured and reflects the actual RF signal propagation characteristics of the network design used for the deployed WLAN. At this point, the network equipment has already been installed, and the focus of the survey is to validate that the installation matches the final network design. Principle #12: Disable Low Wi-Fi Data Rates High-density networks require that clients transmit frames at the highest possible data rates. The use of higher data rates enables faster client data transmission, reduces client airtime utilization, and provides greater overall network capacity and throughput. Disabling lower data rates ensures that clients are only able to connect using higher data rates, encourages roaming to optimal APs, and also helps avoid sticky client roaming situations that can reduce overall network capacity. Reducing the number of data rates supported by an AP can also help minimize data rate shifting. This can increase network performance because data rate shifting can cause excessive frame retransmissions. The use of any b data rates should be avoided unless absolutely necessary because these slower transmissions use older DSSS and HR/DSSS modulation. These older modulation methods consume significantly more airtime than the newer OFDM method that g and n use and require protection mechanisms such as RTS/CTS and CTS-to-Self that can induce up to 40% protocol overhead. Therefore, eliminating the DSSS and HR/DSSS data rates (1, 2, 5.5, & 11 Mbps) that b clients use will increase the overall capacity of the channel by reducing the airtime utilization of each client. However, if even a single b client must be supported on the network, then the network and all clients must accommodate this requirement and allow use of at least one b data rate. To ensure clients only use higher data rates, customize the data rate support for the 2.4 GHz and 5 GHz frequency bands. Configure a fairly high minimum basic data rate in most high-density deployments (18 Mbps for example) and disable all lower data rates. This encourages clients to roam more aggressively to APs with sufficiently strong signal strength to maintain use of their highest data rates, which conserves airtime and increases overall channel and network capacity. If client compatibility issues exist, set 6, 12, and 24 Mbps rates as basic (that is, required); set 36, 48, and 54 Mbps as optional; and disable all other rates. If legacy b clients must be supported, set 11 Mbps as the only basic rate; set 18, 24, 36, 48, and 54 Mbps as optional; and disable all other rates. 18 Copyright 2012, Aerohive Networks, Inc.
19 Figure 10: Data rate customization Principle #13: Use WPA2 (CCMP/AES) or Open Security, and Enable QoS To use n data rates, the use of either WPA2-CCMP/AES or Open security is required. Avoid selecting TKIP or WEP, which would result in limiting client operation to legacy (802.11a/b/g) data rates per Wi-Fi Alliance certification requirements. Use one of the following network security options: WPA X (Enterprise) WPA2 PSK (Personal) Private PSK (PPSK) Open When enabling Layer 2 security, ensure WPA2-Enterprise or WPA2-Personal key management and CCMP/AES encryption is selected. Avoid selecting Auto (WPA or WPA2) or WPA for the key management and TKIP for the encryption cipher, which could result in clients using WPA (TKIP) instead of WPA2 (CCMP/AES) and being limited to legacy (802.11a/b/g) data rates. Use of Quality of Service (QoS) mechanisms through Wi-Fi Multimedia (WMM) to prioritize latency and jitter-sensitive traffic such as voice and video. WMM is required to use n data rates; provides Layer 2 frame prioritization and frame aggregation techniques, which significantly improves client throughput; reduces airtime utilization; and enables greater overall network capacity. Copyright 2012, Aerohive Networks, Inc. 19
20 Principle #14: Enable Performance Optimization Features Band steering enables greater use of the 5 GHz frequency band which offers more spectrum and better channel reuse. The result is less co-channel interference and greater overall network capacity. In these environments, a higher ratio of clients on 5 GHz can provide greater capacity than urging all clients onto 5 GHz or evenly splitting clients between the 2.4 and 5 GHz frequency bands. A balanced bandsteering approach enables administrators to tailor frequency band utilization based on their unique network design. Figure 11: Client band-steering Load balancing can be used to optimize client associations across a group of AP radios. This is useful when AP radios become overloaded with traffic. By sharing information about client load, airtime utilization, CRC error rates, and RF interference conditions with one another, they build a shared database of radio and network conditions. Based on these conditions, some APs can selectively respond to probe and association requests from new clients so that they will associate with APs having the most favorable performance conditions. Airtime-based load balancing, which determines radio load based on the amount of traffic and channel utilization, provides a better indication of available radio capacity than load balancing based on the number of associated clients. Using the number of clients as the basis for load balancing is inadvisable because many clients might be associated with a radio but are relatively idle. Additionally, interference might be present on the channel, which could limit available airtime and capacity. Dynamic airtime scheduling improves network performance for high-speed clients by reducing airtime monopolization by slow-speed clients. In a traditional wireless network, medium contention procedures provide an equal opportunity of transmission for all packets of the same QoS class across all stations. This is often referred to as "packet-level fairness" because every packet of the same QoS class has a statistically equivalent probability of transmission. Slower clients (802.11a/b/g or 1SS n, where SS = spatial stream) use significantly more airtime to transmit the same amount of data as compared to much faster clients (2SS or 3SS n) due to use of lower data rates. This default "packet-level fairness" mechanism results in slower clients monopolizing airtime and significantly decreasing network capacity as a whole and performance of faster clients. 20 Copyright 2012, Aerohive Networks, Inc.
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