• Abstract and Figures
• Public Full-text

Although the IEEE 802.11 standard has been around since 1997, work continues to make it more adaptable to the demand for higher data rates and true wireless flexibility. William Stallings IEEE 802.11: Moving Closer to Practical Wireless LANs ireless LANs have quickly be- began relying more on inexpensive twisted-pair come a significant niche in the cabling for LANs—in particular Category 3 and LAN market. As adjuncts to Category 5 unshielded twisted pair. Category 3 traditional wired LANs, they sat- wiring is the traditional telephone wiring found Wisfy mobility, relocation, and ad hoc networking in every office building; category 5 wiring is requirements and provide a way to cover loca- higher-performance wiring able to carry higher tions that are difficult to wire. data rates. Many older buildings are prewired As the name suggests, a wireless LAN uses a with an abundance of Category 3 cable, and many wireless transmission medium. Until relatively newer buildings are prewired with Category 5. recently, few organizations used wireless LANs Thus, there was little motivation to replace wired because they cost too much, their data rates were LANs with wireless. too low,they posed occupational safety problems This is not true of all environments, how- because of concerns about the health effects of ever. For some, the motivation to use wireless electromagnetic radiation, and the spectrum used LANs is much higher. Buildings with large open required a license. Today, however, these prob- areas, such as manufacturing plants, stock lems have largely diminished, and wireless LAN exchange trading floors, and warehouses, make popularity is skyrocketing. wired LANs awkward to install because of lim- ited choices for cable placement. Historical build- WHEN WIRELESS LANs MAKE SENSE ings often have insufficient twisted-pair cabling Wireless LAN products first appeared in the and prohibit drilling holes for new wiring. Finally, late 1980s, marketed as substitutes for traditional small offices often find it uneconomical to install wired LANs.The idea was to use a wireless LAN and maintain wired LANs. to avoid the cost of installing LAN cabling and ease the task of relocating or otherwise modify- POSSIBLE CONFIGURATIONS ing the network’s structure. In most cases, an organization already has a As events unfolded, however, wired LAN to support servers and some station- organizations began to rethink ary workstations. For example, a manufacturing Inside this substitution strategy.LANs facility typically has an office area that is physi- had become more popular, and cally separate from the factory floor but must be What Is the architects were designing new linked to it for networking. Therefore, organiza- MAC Protocol? buildings to include extensive tions will commonly link a wireless LAN into a prewiring for data applications. wired LAN on the same premises. This kind of Resources Also, as data transmission tech- application, or LAN extension, can take several nology advanced, organizations forms. 1520-9202/01/$10.00 © 2001 IEEE May ❘ June 2001 IT Pro 17 NETWORKS portable computers and access the servers on Figure 1. Single-cell wireless a wired LAN from various locations. LAN configuration. Ad hoc network This network is set up temporarily to Server meet some immediate need. It has no cen- tralized server. Thus, in meetings, a group of Ethernet employees, each with a laptop or palmtop UM computer, can link their computers in a net- 10-Mbps CM work that lasts just as long as the meeting. Ethernet switch WIRELESS LAN REQUIREMENTS As these configurations show, wireless LANs must meet requirements typical of any LAN,including high capacity,ability to cover short distances, full connectivity among 100-Mbps attached stations, and broadcast capability. Ethernet switch They must also meet requirements specific to their intended environment. Bridge or router • Throughput. The medium access control (MAC) protocol should use the wireless A backbone wired LAN, such as Ethernet, supports servers, work- medium as efficiently as possible to maxi- stations, and one or more bridges or routers to link with other mize capacity. The “What is the MAC networks. A control module (CM) acts as an interface to a wire- Protocol?” sidebar describes this protocol less LAN. The module includes either bridge or router function- in more detail. ality to link the wireless LAN to the backbone and some sort of • Number of nodes. Wireless LANs may access control logic, such as a polling or token-passing scheme, to need to support hundreds of nodes across regulate access from the end systems. Some of the end systems multiple cells. are stand-alone devices, such as a workstation or a server. Hubs • Connection to backbone LAN. Most or other user modules (UMs) that control several stations off a applications require interconnection with wired LAN may also be part of the configuration. stations on a wired backbone LAN.Wire- less LANs easily satisfy this requirement by using control modules that connect to Single and multiple cells both types of LANs.Applications may also require accom- Figure 1 shows the single-cell configuration—a simple modating mobile users and ad hoc wireless networks. wireless LAN strategy typical of many environments. It is • Service area. A typical coverage area for a wireless LAN so named because all the wireless end systems are within has a diameter of 100 to 300 meters. range of a single control module. Another common con- • Battery life. Mobile workers use battery-powered work- figuration is a multiple-cell wireless LAN,in which a wired stations that must have a long battery life when used LAN connects multiple control modules. Each control with wireless adapters.Thus, the wireless LAN’s MAC module supports wireless end systems within its trans- protocol typically should not require mobile nodes to mission range.An infrared LAN,for example, limits trans- monitor access points constantly or engage in frequent mission to a single room, so each room in an office building handshakes with a base station. Typical wireless LAN would need one cell. implementations have features to reduce power con- sumption when the network is not being used, such as Nomadic access sleep mode. In this configuration, the wireless LAN links a LAN hub • Transmission robustness and security. If not properly and a mobile data terminal equipped with an antenna, designed, a wireless LAN may be prone to interference, such as a laptop or notepad computer.Thus, for example, making it easy for intruders to eavesdrop. A properly an employee returning from a trip can transfer data from designed wireless LAN permits reliable transmission, a personal portable computer to an office server. Nomadic even in a noisy environment, and provides some level of access is also useful in an extended environment such as security from eavesdropping. a campus or a business operating from a cluster of build- • Collocated network operation. As wireless LANs ings. In both cases, users can move around with their become more popular, multiple wireless LANs are 18 IT Pro May ❘ June 2001 likely to operate in close proximity. Consequently, a device assigned to one LAN What Is the MAC Protocol? may be able to transmit or receive without authorization on a nearby LAN. To prevent Every LAN consists of devices that must share its transmis- this, the wireless LAN scheme must use sion capacity.Thus, an individual LAN needs some way to con- addressing and access control techniques. trol access to the transmission medium so that devices will use • License-free operation. Users would prefer to that capacity in an orderly and efficient fashion. This responsi- buy and operate wireless LAN products bility falls to the medium access control protocol, without having to secure a license for the which ensures that all the end systems on a LAN frequency band the LAN uses. cooperate. The MAC protocol requires that only • Handoff/roaming. The MAC protocol one station transmit at a time, and it specifies that the wireless LAN uses should let mobile data be transmitted in blocks, or MAC frames. Each stations move from one cell to another. frame includes user data, a destination and source • Dynamic configuration. The MAC pro- address, error-detection code, and MAC control tocol’s provision for addressing and net- bits. work management should let Every LAN architecture includes a MAC layer, organizations dynamically and automati- which is responsible for detecting errors and discarding any cally add, delete, or relocate end systems with- erroneous frames. Each end system monitors the shared out disrupting other network users. medium for frames whose destination address is a match with its address and copies those frames. BALANCING STANDARDIZATION The MAC layer for IEEE 802.11 is rather complex (see AND FLEXIBILITY Figure 3 in the main text). Unlike MAC in Ethernet, for exam- With so many possible applications and wire- ple, it includes a distributed coordination function with a rudi- less configurations and the need to meet the spe- mentary priority scheme, in which all stations cooperate for cific requirements of wireless environments, medium access. It also includes a point coordination function, some standardized approach to product creation implemented in a central controller, to accommodate urgent is imperative. Without it, equipment from vari- ous vendors would not work together. But flexi- Introducing the IEEE Computer Society Career Service Center Career Advance your career Service Center Search for jobs • Certification Post a resume • Educational Activities List a job opportunity • Career Information Post your company’s profile • Career Resources • Student Activities Link to career services • Activities Board computer.org/careers/ computer.org NETWORKS ically within wireless range of only the stations Figure 2. IEEE’s 802.11’s that also belong to that set. The exception is extended service set architecture. when two basic service sets overlap geo- graphically,making the range narrow enough for a single station to participate in both sets.

Load more

  7

Copy Link