Whereas bandwidth and latency considerations are important in wired networks, they are critical in wireless environments. The focus in this case is edge connectivity because backhaul connectivity is assumed the same irrespective of the wired or wireless connectivity provided to the end user.

The contention that occurs in wired VoIP environments for network access is between different applications on a client that require network access. The client is connected via a dedicated connection to an access switch, and applications compete for access to the network. QoS mechanisms can be used to classify, mark, and provide priority queuing. This alleviates many of the bandwidth and latency challenges in wired networks.

Unfortunately, this is not the case for WLANs because they share access to the transport medium among all connected stations. As such, contention for airspace access occurs not only inside of the client, but also between the clients. Even though a client could provide a higher priority queue for its time-sensitive applications such as voice (and video), the client must still compete with neighboring clients for airspace access.

Because of the aforementioned situation, it is difficult to guarantee timely access to the medium for voice traffic. 802.11e extends QoS mechanisms to the WLAN Media Access Control (MAC) layer to increase the probability of gaining access to the network. The simpler method, EDCF, is a “best effort” QoS method where high priority traffic is given a slightly higher transmission probability than lower priority traffic.

The more complex type is HCF, which is the 802.11e version that provides features like bandwidth control, fairness between stations, classes of traffic, jitter management, and so on. As such, WLAN QoS can be configured with much greater precision with HCF. Even though this method provides more granularity, it has not been widely implemented yet due to its complexity.

If voice is indeed a critical application for your environment, ensure that both QoS as well as sufficient bandwidth is provisioned. Avoid situations where many clients are part of the same cell because this will prevent excessive access contention. Chapter 5 provides strategies for determining the appropriate number of access points for your environment.

A challenge that is unique to WLANs is that of roaming clients. Stations that are on the move transfer their association from one access point to another. When this transfer occurs, the device must re-authenticate with the authentication, authorization, and accounting (AAA) infrastructure, thereby introducing a possible momentary interruption in service. This is especially noticeable in voice applications where any interruption can result in lost packets and a corresponding impact on voice quality.

WLAN vendors have addressed this by introducing fast Layer 2 roaming, which reduces the time to re-authenticate (usually to less than 100 ms or so) as the station moves its association from one access point to another. This fast roaming capability limits the disruption of the voice stream as packet loss is minimized.

An additional challenge caused by roaming occurs when a station crosses an Extended Service Set (ESS) boundary. When a client transfers its association to an AP in a different ESS, it effectively moves into a different IP subnet. This is known as Layer 3 roaming, as the client device has moved from one IP subnet to another. Routing issues arise as the station’s old IP address is invalid and VoIP sessions can terminate under these circumstances. If no additional measures are taken, the active call will be dropped. Figure 4-2 illustrates the effect of mobile VoIP handsets roaming across ESS boundaries and the resulting invalid IP address in the new ESS.

Figure 4-2. Limitations in WLAN Layer 3 Roaming

Given that most organizations have far more than a single ESS, roaming with mobile voice devices can become a significant challenge if not addressed in the architecture. Multisite or campus locations will have a hard time maintaining voice sessions throughout the campus. Various solutions do exist to address this roaming challenge, and each has its benefits and challenges. These include Mobile IP Protocol and “predictive” tunneling solutions. Almost all vendors have moved to some type of tunneling technology to solve the ESS roaming problem. The tunneling solutions are essentially the same as those discussed later in this chapter for constructing guest networking solutions. Mobile IP is typically deployed only in difficult environments such as moving vehicles.

Voice on WLANs brings along a number of considerations that are not necessarily related to the network, but rather to the physical attributes of the devices. These considerations include battery life and physical security of the clients.

Battery life is important no matter what kind of mobile device you use. It is especially significant when contemplating voice because voice is a real-time form of communication. It is certainly more annoying to lose an active voice session due to battery power depletion than having your laptop warn you of an impending shutdown.

As always, remain sensitive to security of the device. As opposed to a laptop or desktop, mobile voice-enabled devices are typically smaller and harder to physically control and secure. Security of mobile voice devices should not necessarily be thought of in terms of compromise of your network. Authentication information could indeed be extracted from the device facilitating other attacks. A greater liability is the information that is stored on the device such as phone books or the ability to make calls from an active phone, which can result in financial or legal burdens. Use security features such as PIN codes and policy administration tools to protect access to the devices. For a more in-depth treatise on security in wireless environments, refer to Chapter 7, “Security and Wireless LANs.”

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

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.

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.

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.

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.

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.

Guest networking is made available to nonemployees as an amenity. By ensuring your users can access the Internet, you improve their experience when at your site. This can be important in industries that have a high degree of public interaction or organizations that have many visitors.

A guest portal is often used, so the visitor is greeted with a Web page when they first use the service. Typically, this will include a welcome page, perhaps a legal disclaimer, and maybe an authorization or check box for them to acknowledge.

After guests successfully obtain Internet access, they can use their own remote access solution to connect to their corporate infrastructure. Guests thus effectively extend their organization’s Intranet to your own site making their full suite of productivity applications available to them. For example, they can download their e-mail, browse their internal website, and retrieve voicemail.

A particularly useful application of guest networks can be found in product demonstrations. When a sales representative visits your office, he can access all applications and information that would be available to him if he were at his own corporate offices. As such, a full-featured demonstration can be delivered without being encumbered by the potential unavailability of tools and data.

Many enterprises do not allow nonemployees to access the network. This simple security policy avoids the risk associated with visitors introducing viruses to the network, snooping, hacking, and other undesirable activity. However, visitors can benefit from Internet connectivity to gain access to their own enterprise networks (to check e-mail, access files, and so on). A policy decision to altogether prohibit access therefore negatively impacts the productivity of your visitors.

A guest networking solution addresses this conflict. You can provide visitors, contractors, and vendors access to the Internet, while avoiding the ability to access your enterprise network. Guest traffic is separated and tunneled securely on your network and to the Internet; thus creating an isolated and secure environment for your visitors to work in.

Internet traffic can typically be tracked to its source. Therefore, all Internet traffic that originates from an enterprise can easily be identified as having come from that enterprise’s network. Employees usually sign an acceptable use policy when hired. They agree not to undertake malicious or illegal behavior, such as hacking or deliberately spreading viruses. However, guests are not required to sign such employee agreements.

Protect yourself from legal liability by implementing a portal in which users need to read and explicitly accept a policy for acceptable use prior to connecting to your wireless guest network. In the unfortunate case of a crime or unacceptable behavior, you can audit records, identify the offending guest, and take appropriate action.

Develop a guest portal to be the public face of your guest network. Make it aesthetically pleasing; include your corporate identity; and, depending upon your security policy, require the guest user to record their name, acknowledge a legal disclaimer, or sign an acceptable use policy.

Include a legal disclaimer with your guest network. You should engage independent legal counsel to ensure that the disclaimer conforms with local legislation and that you are protecting your enterprise from any legal liability that might accrue from misuse by your guests. Display the acceptable use policy on the guest portal and require that guests agree to comply with the policy prior to granting access to the Internet. Having users select an I Accept box or type their name into a Signature field works well for this purpose.

Make the guest networking solution easy to use. When providing guests with access to a guest network, you should not require specific software or configuration changes to their laptops.

Implement the guest networking solution with its own SSID configured with OPEN authentication and no security settings. Ensure that the SSID is broadcast. Because the guest network is logically isolated from your enterprise network, and only provides access to the Internet, this should not present any security concerns. As always, ensure that your Information Security department review and approve your design prior to making it available to visitors.

Minimize the support burden of your guest networking solution. Because the users will primarily be guests, you do not want to expend operational cycles on supporting them. Keep the system easy to use and produce some basic guidelines for your guests to lighten the support burden. Frequently Asked Questions (FAQ) sheets can be produced that tell the guest what SSID to use, how to navigate the guest portal, and to help with basic connectivity troubleshooting.

Log the activity of your guest users. Your enterprise might already log Internet activity by your own staff, which should make it easier if you decide to log guests also. At the minimum, keep records of the number of sessions, the identity and IP address of the guest users, and the time and date of their session.

Hospitals spend millions of dollars on the latest technology to provide the best level of care to their patients. This often results in the use of very expensive, but mobile, assets, such as electronic and automated IV pumps, vital signs monitors, and even gurneys. Not only can the loss or misplacement of these devices create a financial burden for the hospital, but the lack of the device can also prohibit timely patient care. The ability to track, locate, and recover these mobile assets is, therefore, absolutely critical for the hospital or health center.

Some hospitals combine WLAN voice with location services. This allows hospital staff to carry WLAN-based VoIP handsets that include a “panic button” or key-code that, when pressed, will page all appropriate staff that are located nearby.

In some instances, patients themselves have been provided with location tags. The WLAN can then be used to locate the patient, and even provide an automatic link to the patient information system. The synergy of an existing WLAN infrastructure, location services, WLAN voice, and back-end hospital systems thus enables a faster response time and improved patient care.

Location services can offer improved business knowledge by automating and simplifying supply chain management. The ability to identify exactly how many items are in production, where they are located in the assembly line, and the current rate of manufacture is critical for operations managers who rely on timely and accurate information to finely tune the production process. Intimate knowledge of the goods and their whereabouts is, therefore, essential.

Location services can also be used by robotic delivery mechanisms and warehousing vehicles to automatically store and retrieve equipment, and monitor stock levels in real time. The use of a WLAN-based solution avoids the enterprise from having to deploy a proprietary, nonstandard RFID solution instead.

Personal security for customers is very important in the entertainment and leisure industries. In Europe, some theme parks have used WLAN location services to provide an online, active, and real-time positioning solution for visitors to track the location of children. Children are provided with active location tags embedded in wristbands or name badges. This provides additional safeguards for security staff and park management, and peace of mind for parents.

The most common logistics operation performed with handheld wireless devices is inventory taking. Almost every large retail chain and distribution center use some type of wireless network to assist with the mundane, but necessary, task of counting things.

WLAN location services can provide incremental value in these environments by providing online, active, and real-time information on asset location. In the car rental business, identifying whether vehicles have been returned and whether they are in the garage, workshop, or cleaning bay improves the response time and productivity of the business. The business can improve its operating margins by ensuring quick turnaround of its vehicles.

Transport and shipping companies are in the business of timely delivery of packages on time. This is only possible by ensuring that distribution and dispatch centers operate smoothly, accurately, and expeditiously. Location services assist in this by making sure that pallets and crates can be tracked and identified accurately. By attaching asset tags to the pallets and crates, the transport and shipping business can guarantee that the data provided to its back-office system is accurate.