Video

Video is another commonly used application. It is used for broadcasting news, hosting video conferences, and distributing learning modules. Just like voice, video over IP is a complex and challenging topic on its own. Therefore, this section does not provide an in-depth technical overview of all the challenges and solutions that are related to enabling IP video, but instead serves to familiarize you with key concepts of video as it applies to production enterprise-class WLANs. This section introduces the different types of video traffic as well as the challenges that are specific to implementing video in WLAN environments. Refer to the "Additional Resources" section at the end of this chapter for resources that cover Video over IP in more detail.

Types of Video Traffic

You need to consider the following three parameters when evaluating video over IP-based WLANs:

• Distribution mechanism

• Timing of the distribution

• Quality of the video stream

This section briefly describes each parameter.

Distribution Mechanism

The distribution mechanism refers to the manner in which video is transported across the communications infrastructure and how stations tune into respective viewing sessions. Generally speaking, data can be transmitted as broadcast, multicast, or unicast. The differentiation is based on the number of stations that receive the data, and it is independent of the semantics of the underlying data.

A broadcast transmission sends the data to everybody. Broadcast is one-to-all. In multicast, data is sent only to stations that have explicitly requested to be sent the data. In this case, the network creates copies of the transmission when, and only when, different paths are needed to reach the subscribers. Multicast is thus one-to-many, and its advantage is that it makes more optimal use of network resources by creating copies of data only when required. Finally, unicast sessions transport data between a single sender and receiver. Unicast is one-to-one. Figure 4-3 illustrates that unicast is a subset of multicast, which is a subset of broadcast.

Figure 4-3. Broadcast, Multicast, and Unicast Communications

Timing of the Distribution

Video is an application that permits different timings of transmissions. Users can either retrieve and view video when they want to, which is known as on-demand viewing, or they can tune into sessions that are broadcast at predetermined times. This is identical to the options for viewing television. You either tune into a particular broadcast and subordinate time to content, or you use on-demand to view programs when it is most convenient for you, in which case you subordinate content to time.

Real-time streaming video applications typically use multicast because it is a more efficient distribution mechanism. Viewers subscribe to a particular stream, and the network ensures that only the relevant streams are branched to subscribed viewers. In this manner, redundant copies of the video streams are avoided. Streaming video is, therefore, ideal for distributing the same information to a large numbers of viewers at predetermined times. Company meetings or earnings updates are prime examples.

On-demand video is retrieved at the discretion of the viewer. These types of video applications employ unicast transport for distribution because the probability that multiple viewers would retrieve exactly the same content at exactly the same time is extremely low. Examples of content that is ideally suited for on-demand are online education modules or archived videos.

Quality of the Video Stream

A third distinguishing factor in video is the quality of the video. Video is a data-intensive application that not only needs to send image data, but also audio information. Video formats such as MPEG-4 and AVI attempt to reduce the volume of data by compressing the stream before storage or transmission, and decompressing the information during playback. However, even with this compression, video remains data intensive.

A strategy for easing the burden of video on communications networks is to make the same content available in different degrees of quality. Image size and quality can be tailored to best match the available bandwidth. Users can be presented with the choice between a high-bandwidth or low-bandwidth stream to try to ensure a more consistent video experience.

WLAN Video Implementation Challenges

Many of the considerations that are true for voice are directly applicable to video. Video too is sensitive to network latency, and appropriate QoS measures should be implemented to construct a more deterministic network environment. Video, however, is more challenging than voice because it compounds some of the challenges that are encountered with VoIP.

Quality of Service

Not only is video much more data intensive than voice, but it is also truly continuous in nature. Voice communications typically have some breaks as people pause between sentences to take a breath. This is not the case for video. As such, limiting latency and jitter is critical. Use QoS classification, marking, queuing, and traffic engineering techniques to ensure that video is given preferential treatment over less time-sensitive information but avoid scenarios in which video could drown out all other communication.

Remain sensitive to the fact that video is usually less mission critical than voice. Make use of a tiered classification and marking strategy for applications. Assign network control traffic the highest priority. Follow it with voice, then video, and finally best-effort data traffic. Note that this classification scheme is highly simplified and that you should use more granular tiers if this better suits your needs.

Broadcast Transmission Medium

Another challenge that you face when porting video applications to WLANs is that all video sessions automatically become broadcast communications. This is irrespective of whether the sessions were originally broadcast, multicast, or unicast. The reason is found in the nature of RF communications. As is the case for any communication, access points broadcast data across the airwaves to all attached stations. Even though a single station might be tuned into the video stream, all stations receive the video data. Stations that are not tuned in will disregard the video data. Not only does this force the clients to perform redundant work, but it also ties up the airwaves.

Conversely, the broadcast nature of WLANs can work to your advantage as well. Because broadcast is a superset of multicast communication, WLANs are ideally suited for multicast video sessions. Clients need only to not disregard the video data that is broadcasted from the access points to subscribe to a particular session. However, there is a particular problem with multicast and WLANs. Because all stations have to be able to receive the multicast stream, the network has to enter a "lowest common denominator" mode. For example, if a single station operates at 1 Mbps, even if all others are operating at 54 Mbps, all multicast traffic will be transmitted at 1 Mbps.

Because video is a data-intensive application, the broadcasting thereof can lead to a significant increase in medium access contention. Carefully plan the architecture and design of your WLAN if you intend to support video. Pay considerable attention to bandwidth capacity planning and client-to-access point ratios. Chapter 5 provides recommended strategies and tactics for tackling this challenge.

Managing User Expectations

A final consideration in enabling video applications on WLANs is setting the correct user expectations. QoS is not a substitute for bandwidth, and it is also not the saving grace for a multi-access medium such as WLANs. Video is ultimately best served by dedicated high-bandwidth connections. This is especially true for videoconferencing applications that tie together audio, video, and web applications. Videoconferencing is not as forgiving as some other video types and also doesn't offer the ability to make efficient use of bandwidth. Set the proper expectations with the user. Emphasize that the capability of the WLAN should not be compared to the current capabilities of the wired network, especially with regard to multimedia applications.