Wireless Local Area Networks
What is an Access Point (AP)?
An AP is a network-bridging device, providing the basic connectivity functions between a wireless and a wired network. All APs have an 802.11 wireless port, and at least one wired Ethernet (802.3) port. They provide the basic bridging functionality through bridging tables, and store MAC associations for both the wireless and wired interfaces. APs that act as wireless routers or gateways, also provide basic IP services to the wireless clients, such as DHCP and NAT. To accommodate wireless client roaming between APs without interrupting the link layer connectivity, an Inter Access Point Protocol (IAPP) may be used.
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How many concurrent clients can an Access Point (AP) handle?
The number of wireless clients that can be handled by an AP depends on the size of the client association table in the AP and the rate at which the AP can associate and authenticate new clients as they sign on. A client association database that is too small, will limit the amount of users that be serviced by the AP, even though capacity is available, causing them to unnecessarily roam in search of other APs, or denying them service altogether. Similarly, if the AP is not capable to performing the management functions required to associate and authenticate clients in a timely manner, clients will be denied service, or service will be delayed. In installations supporting high mobility users, or in recovery situations from network or power outages, the APs ability to efficiently associate and disassociate clients has a major effect on quality of service. Although these two metrics are very important to overall quality of service, they are difficult to measure. VeriWave's WaveApps addresses this problem and provides precise measurement of these parameters, as well as, the APs ability to buffer information for clients in power save mode, handling of roaming clients, and recovery from power and network outage situations. Obviously, the number of clients that can be handled is only one part of the service equation. The quality of service provided to each client, depends on the data throughput awarded to it and the overall delay its information will encounter.
To address this problem, VeriWave’s WaveTest generates full air-rate traffic, allowing the AP performance to be measured in light, moderate, high and extreme traffic loading conditions, while providing full control over the amount and characteristics of the clients attempting to use the service.
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What is Roaming?
The ability of a wireless client to communicate through the wireless LAN is based on its association with an Access Point (AP) which serves as the gateway for a connection to other wireless or wired devices. As long as the signal quality received by the wireless client remains unchanged, the client will continue to communicate through the same AP. As the client moves away from the AP, or as changes occur in the physical medium between the client and the AP (e.g. an office environment in which a door is being closed or a file cabinet is being moved) and the signal quality degrades, the AP steps down the transmission speed (also known as rate adaptation) to maintain the connection. The client can measure the signal quality from all the APs servicing its area by utilizing the Beacon Frames generated by the APs. At some point the quality of the link will degrade significantly and the client will be forced to use a different AP for continued connection. This process is called ROAMING. When a wireless client moves to a different AP, re-authentication is required by the AP.
The 802.11i security standard, ratified in June 2004, adds a few provisions that allow for seamless transition between APs without the need for complete re-authentication, while maintaining a secure connection.
The first provision, known as Pair-wise Master Key (PMK) Caching, allows the client to store a master key with the AP, and in the case the client roams away from the AP, and back again, that same key will be used by the AP, making it unnecessary to re-authenticate.
A second provision allows for advance authentication, or “pre-authentication”. This enables a client to authenticate with one AP, and use a wired network to convey the authentication to another AP, thus pre-authenticating with the second AP before actually coming into its range.
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Do clients sometime roam unnecessarily between APs, and how does roaming affect overall network performance?
Unnecessary roaming between APs occurs more often than one would expect. APs maintain connections with clients based on association databases that register every client requesting to use the AP's services. If the association database is not properly maintained or if the AP cannot associate clients in a timely fashion, clients will attempt to roam to other APs. Overall network performance is adversely affected by unnecessary roaming. Every time a client roams between APs, client throughput is affected. In addition, the APs involved are unnecessarily busy disassociating and re-associating clients which should not be roaming, thus decreasing the APs ability to handle new "legitimate" clients.
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What are the challenges facing mobile WiFi clients relative to fixed clients?
Fixed WiFi devices, such as desktop computers, associate with the strongest signal from a nearby AP and establish a connection. Mobile WiFi devices, such as portable inventory devices used in warehouses, tablet PCs used by doctors in hospitals and VoIP handsets, face the constant challenge of moving between APs and utilizing the one providing the best signal at any given time. As the client moves away from the AP and the signal quality degrades, the AP steps down the transmission speed (also known as rate adaptation) to maintain the connection. The client can measure the signal quality from all the APs servicing its area by utilizing the Beacon Frames generated by the APs. At some point the quality of the link will degrade significantly for the client to use a different AP for continued connection. This process is called Roaming. The client will disassociate from the AP and attempt a connection to another AP with a higher quality signal. This process of switching from one AP to another, is rarely instantaneous and an interruption in transmission usually occurs. This interruption can cause frame losses, which may result in increased delay due to errors and re-transmissions. While many protocols can easily recover from such events, other, such as VoIP are more sensitive and this interruption can cause abnormal noises to be heard by the user, or even the call being disconnected.
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