Industrial wireless tutorials: WLAN devices are generally classified as being either infrastructure or client devices, and understanding how access points work and what their purpose is regardless of your WLAN device is important when working from a fixed or mobile location.
Devices on a WLAN fall into two broad categories: infrastructure and client. Access points, or APs, can serve a variety of functions, depending on where in the system they reside, but are generally considered an infrastructure device. These APs serve as a portal or bridge into a wired network; this is true for the vast majority of APs installed today. To fully integrate a WLAN, it must have access to network resources located on the wired side, including servers, routers, and access to the Internet.
A client device, on the other hand, is the device you are currently reading this article on. It can be a wired or wireless computer, smartphone, tablet, or wireless barcode scanner. It is important to consider the types of client devices to be used on a WLAN when designing or expanding a wireless network. An effort should be made to accommodate client behavior and performance in the physical space being covered. It should also be considered that client devices can be fixed or mobile, depending on deployment. In most cases, but not all, the client device will be mobile. This requires the wireless network to be capable of providing adequate wireless coverage over the entire range of movement of the device.
The term "infrastructure" refers to the APs connection to the wired network and is thus considered to be part of that infrastructure. It is a portal to the bounded, wired segment of a larger network. APs also operate in other modes, depending on network design. In the next segment, which deals with WLAN architecture, we will see the different AP roles needed to facilitate different configurations.
An AP can be autonomous, controller-based, or controller-less (also called cooperative). An AP, regardless of configuration, is the basis for a wireless network; a wireless network begins its existence when the AP begins transmitting RF, creating a basic service area (BSA), or if you like, a cell. An autonomous AP has all the intelligence required to authenticate wireless clients to the network and then properly process and route data to its final destination, such as a server. A controller-based AP relies on a dedicated hardware controller for control of all the functions required for authentication and routing. A cooperative AP contains all the intelligence of an autonomous AP and more: the AP also needs to cooperatively manage the network, and this requires the additional processing capability needed to do so.
Another distinction that can be made is that of SOHO (small office, home office) vs. enterprise infrastructure devices. A SOHO AP is typically an autonomous AP, and contains all the features required to authenticate clients and process traffic for distribution over the wired network. SOHO APs often have features such as channel selection, network address translation (NAT), dynamic host configuration protocol (DHCP) servers, bridging, and repeater capability. Most SOHO devices also support a variety of basic security options. Enterprise devices have vastly more capability and flexibility. These features include power output control, advanced security options, AP power output control, multiple operating modes, and graphical user interfaces. Enterprise devices are used in larger installations requiring greater coverage and capacity.
While client devices are simply required to adhere to 802.11 standards in order to access and participate in network resources, APs can perform a variety of functions. Not all APs are wireless routers. Controller-based APs are often radio transceivers with no other intelligence. However, as explained below, APs can be used in several useful ways depending on the requirement of the installation.
The most common role for a SOHO AP is as a wireless residential gateway. In this role, the AP provides all the functions of a wireless router and gateway to all network resources, including the Internet. Another term used for this type of device is "wireless broadband router." An AP can also be used as a branch router in an extended service set (ESS) arrangement where several APs are configured as separate nodes on a larger wireless network, providing another gateway into the wired infrastructure at another location. This allows a client device to roam from the BSA of one AP to the BSA of another AP. In a well-designed network, this roaming is seamless and the client will not experience any error.
Bridge mode is used to connect two wired network segments together wirelessly. This is done to connect two physically separated networks, such as those residing in two buildings on the same campus. Directional antennas are used to provide the required gain and reduce interference; the link becomes a dedicated point-to-point connection. Security is of paramount concern because in an unbounded medium, anyone with the correct equipment can intercept the signal anywhere in the path. The link can be as short as a few hundred feet, or can be many miles. It should be noted that the cost savings of using a bridge as opposed to installing a wired connection is nothing short of astounding.
Another mode an AP can operate in is repeater mode. In repeater mode, the AP extends the parent APs BSA and simply forwards the traffic without any other action. This is sufficient for smaller networks, but in larger networks it can present a problem. Recall that wireless LANs communicate half-duplex. This will present throughput problems. A repeater is configured with the same channel, SSID, and security information as the main AP. For a simple installation, the root AP will transmit data and then the repeater will receive the transmission and repeat it on the same channel. This will cause delay and possibly interference within that channel. Another downside to using repeaters is the fact that each repeat, or hop, will diminish the signal by an average of 3dB, or half of the original signal. There is also significant transmission overhead added to the signal, which adds latency. The use of repeaters is discouraged unless there is no other choice. Typically, the coverage gains achieved are canceled out by the loss of speed and quality. A much better solution would be to add additional APs using a multi-channel arrangement if possible. Yet another solution is to set up a wireless mesh network, which will be discussed in the next segment.
- Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Chris Vavra, production editor, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
Wireless propagation fundamentals
Wi-Fi acronyms, wireless buzzwords, WLAN nomenclature, wireless terms
Wi-Fi and the OSI model
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.
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