Mapping your wireless local area network: How to make your WLAN shine
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From IPTV streaming to FaceTime conferencing, enterprise video use is growing fast. Often, this video is sent and...
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received from mobile devices that connect over the enterprise WLAN. Now, network managers must implement a new kind of design for the WLAN in order to optimize wireless video delivery and avoid performance degradation.
Challenges in WLAN design for video go well beyond bandwidth
Capacity is clearly a consideration when building a wireless LAN (WLAN) to support video applications, but the answer isn't simply bandwidth. Video application bandwidth needs are influenced by resolution, frame rate and codec. For example, a 720p or 1080p surveillance camera feed recorded at 6 to 10 frames per second (FPS) and encoded with H.264 generates 1 to 2 Mbps. However, watching an HD video at 120 to 240 FPS on a 72-inch screen could require as much as 30 Mbps for a satisfactory viewing experience.
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Codec compression can reduce throughput at the expense of quality, but supporting multiple IPTV channels or camera feeds can still max out all available AP or RF capacity. Competition for airtime is aggravated by video popularity in high-density environments such as classrooms and dorms. Worse, many video streams utilize multicast to reduce wired network load, but over Wi-Fi, multicast reduces the data rate to the weakest (oldest or most distant) client. In an auditorium filled with diverse consumer electronic devices, poor quality is almost guaranteed unless further steps are taken to optimize and prioritize video delivery.
Defining wireless video application capacity requirements
The first step to ensuring wireless video performance is to establish network capacity and performance requirements for the video and multimedia applications on your WLAN. Wireless networking vendor Aruba Networks recommends breaking video into the following classes:
- Broadcast IPTV over Wi-Fi. This requires one-way downlink delivery of latency-insensitive traffic over many high-bandwidth (1 to 4 Mbps SD or 6 to 10 Mbps HD) channels to few users per channel.
- Live event video streaming (webcast) over Wi-Fi. This requires one-way downlink delivery of latency-insensitive traffic, typically over a single high-bandwidth (1-4 Mbps) channel viewed by all users.
- IP surveillance video over Wi-Fi. This requires uplink or downlink delivery of latency-insensitive traffic, with many channels feeding video of varied quality (500 Kbps to 2 Mbps) to a small set of viewers.
- Interactive video conferencing. This requires two-way delivery of interactive traffic impacted by latency and jitter (maximum of 150 to 200 msec), but requires low symmetric bandwidth (1 Mbps) between peers.
- On-demand video (training, pre-recorded programs). This requires one-way downlink delivery of latency-insensitive traffic, with many high-bandwidth (1 to 4 Mbps SD or 6 to 10 Mbps HD) channels consumed by few simultaneous users per channel.
These examples illustrate the variables that should be quantified for all video and multimedia WLAN deployments: directionality, throughput, latency and jitter tolerance, as well as the number of channels and users. Another key metric is error tolerance; low packet error rate is usually required but can be a problem for User Datagram Protocol multicast.
802.11n WLAN design considerations for delivering wireless video
Fortunately, enterprise WLAN products have matured enough to provide a solid foundation for video and other multimedia applications. Video WLANs should use 802.11n to increase capacity, density and reliability. Important 802.11n features and design considerations include the following:
- Channel bonding and spatial multiplexing via multiple-input multiple-output (MIMO) antennas can increase each radio's capacity to 450 Mbps (3x3) or 600 Mbps (4x4), supporting more video users per AP.
- Throughput available to each user will be constrained by Wi-Fi client capabilities, especially smartphones and tablets with 1x1 MIMO (max 65 Mbps, declining with distance). This may be sufficient for a single video stream, but keep in mind that slower clients can drag down video performance for everyone -- design your WLAN with those lower data rates in mind.
- Utilize 802.11n standard options such as A-MPDUs (MAC Protocol Data Unit) and block acknowledgements to further boost throughput for one-way video streams that are latency-insensitive.
- 802.11n devices may support 2.4 GHz and/or 5 GHz. When higher throughput is needed, maximize use of 5 GHz to enable bonding. For example, use Use a <a href=" target="_blank">predictive WLAN planner to design coverage areas and suggest AP placement to meet downlink and uplink goals for throughput, user density, and per user rate. However, don't assume a strong signal will result in good quality video; always verify performance.
- Use Wi-Fi Multimedia (WMM) prioritization to give video applications a greater share of airtime than data, but without consuming all available bandwidth. Give applications sensitive to latency and jitter (e.g., VoIP, video conferencing) higher priority than one-way streaming. WMM Admission Control can also be helpful to avoid over-loading a given AP.
In the second part of this series on WLAN design for wireless video delivery, read about measuring video performance on enterprise Wi-Fi networks.
ABOUT THE AUTHOR: Lisa A. Phifer is president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment and the use of emerging technologies and best practices.