Wireless LAN
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A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers without using wires. WLAN utilizes spread-spectrum technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network.
This technology is becoming more and more popular, especially with the rapid emergence of small portable devices such as PDAs (personal digital assistants). For the home user, wireless has become popular due to ease of installation, and location freedom with the gaining popularity of laptops. For the business, public businesses such as coffee shops or malls have begun to offer wireless access to their customers, some are even provided as a free service. Large wireless network projects are being put up in many major cities, Google is providing a free service to Mountain View, California[1] and has entered a bid to do the same for San Francisco[2], New York City has also begun a pilot program to cover all five boroughs of the city with wireless internet.
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[edit] History
In 1971, researchers at the University of Hawaii developed the world’s first WLAN, or wireless local area network, it was named ALOHAnet. The bi-directional star topology of the system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines [3]
Originally WLAN hardware was so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible. Early development included industry-specific solutions and proprietary protocols, but at the end of the 1990s these were replaced by standards, primarily the various versions of IEEE 802.11 (Wi-Fi). An alternative ATM-like 5 GHz standardized technology, HIPERLAN, has so far not succeeded in the market, and with the release of the faster 54 Mbit/s 802.11a (5 GHz) and 802.11g (2.4 GHz) standards, almost certainly never will.
The Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) claims to have first developed wireless networking, and as of 2006 they are engaged in a controversial battle with software and hardware giants looking for royalties on the alleged patents held by CSIRO regarding these technologies.[4]
[edit] Benefits of Wireless LANs
The popularity of wireless LANs is a testament primarily to their convenience, cost efficiency, and ease of integration with other networks and network components. The majority of computers sold to consumers today come pre-equipped with all necessary wireless LAN technology.
The benefits wireless LANs include:
- Convenience: The wireless nature of such networks allows users to access network resources from nearly any convenient location within their primary networking environment (a home or office). With the increasing saturation of laptop-style computers, this is particularly relevant.
- Mobility: With the emergence of public wireless networks, users can access the internet even outside their normal work environment. Most chain coffee shops, for example, offer their customers a wireless connection to the internet at little or no cost.
- Productivity: Users connected to a wireless network can maintain a nearly constant affiliation with their desired network as they move from place to place. For a business, this implies that an employee can potentially be more productive as his or her work can be accomplished from any convenient location.
- Deployment: Initial setup of an infastructure-based wireless network requires little more than a single access point. Wired networks, on the other hand, have the additional cost and complexity of actual physical cables being run to numerous locations (which can even be impossible for hard-to-reach locations within a building).
- Expandability: Wireless networks can serve an suddenly-increased number of clients with the existing equipment. In a wired network, additional clients would require additional wiring.
- Cost: Wireless networking hardware is at worst a modest increase from wired counterparts. This potentially increased cost is almost always more than outweighed by the savings in cost and labor associated to running physical cables.
[edit] Disadvantages of Wireless LANs
Wireless LAN technology, while replete with the conveniences and advantages described above, has its share of downfalls. For a given networking situation, wireless LANs may not be desirable for a number of reasons; most of these have to do with the inherent limitations of the technology.
- Security: Wireless LAN transceivers are designed to serve computers throughout a structure with uninterrupted service using radio frequencies. Furthermore, because of space and cost, the "antennas" typically present on wireless networking cards in the end computers are generally nothing more than the most naive of reception devices. In order to properly receive signals using such limited antennas throughout even a modest area, the wireless LAN transceiver utilizes a fairly considerable amount of power. What this means is that not only can the wireless packets be intercepted by a nearby adversary's poorly-equipped computer, but more importantly, a user willing to spend a small amount of money on a good quality antenna can pick up packets at a remarkable distance; perhaps hundreds of times the radius as the typical user. In fact, those who engage in the activity of locating (and sometimes, exploiting for profit) wireless LAN networks are known as wardrivers. On a wired network, any adversary would first have to overcome the physical limitation of tapping into the actual wires; this is not an issue with wireless packets. To combat this consideration, wireless networks may choose to utilize some of the various encryption technologies available. Some of the more commonly utilized encryption methods, however, are known to have weaknesses that a dedicated adversary can compromise. (See main article Wireless security)
- Range: The typical range of a common 802.11g network with standard equipment is on the order of tens of meters. While sufficient for a typical home, it will be insufficient in a larger structure. To obtain additional range, repeaters or additional access points will have to be purchased. Costs for these items can add up quickly. Other technologies such as WiMAX offer ranges up to a hundred or more kilometers.
- Reliability: Like any radio frequency transmission, wireless networking signals are subject to a wide variety of interference, as well as complex propogation effects (such as multipath, or especially in this case Rician fading) that are beyond the control of the network administrator. In the case of typical networks, modulation is achieved by complicated forms of phase shift keying or quadrature amplitude modulation, making interference and propogation effects all the more disturbing. As a result, important network resources such as servers are rarely connected wirelessly.
- Speed: The speed on most wireless networks (typically 1-54 Mbps) is far slower than even the slowest common wired networks (100Mbps up to several Gbps). For most users, however, this observation is irrelevant since the speed bottleneck is not in the wireless routing but rather in the outside network connectivity itself. For example, the maximum ADSL throughput (usually 8Mbps or less) offered by telecommunications companies to general-purpose customers is already far slower than the slowest wireless network to which it is typically connected. That is to say, in most environments, your wireless network running at its slowest speed is still dozens of times faster than the DSL serving it in the first place. However, in specialized environments, the throughput of a wired network might be necessary.
[edit] Architecture of a Wireless LAN
[edit] Stations
All components that can connect into a wireless medium in a network are referred to as stations. All stations are equipped with wireless network interface cards (WNICs). Stations fall into one of two categories: Wireless Clients and Access Points.
[edit] Access Points (AP’s)
Access Points are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with.
[edit] Wireless Clients
Wireless clients can be mobile devices such as laptops, personal digital assistants (PDAs), IP phones or fixed devices such as desktops and workstations that are equipped with a wireless network interface card.
[edit] Basic Service Set
The Basic Service Set (BSS) is a set of all stations that can communicate with each other. There are two types of BSS: Independent BSS and Infrastructure BSS. Every BSS has an id called the BSSID, it is the MAC address of the access point servicing the BSS.
[edit] Independent Basic Service Set
Independent BSS are an ad-hoc network that contain no Access Points. Since they do not use Access Points they can not connect to any other basic service set
[edit] Infrastructure Basic Service Set
An Infrastructure BSS can communicate with other stations not in the same basic service set by communicating to each other through Access Points.
[edit] Extended Service Set
An Extended Service Set (ESS) is a set of connected BSS. Access Points in an extended service set are connected by a distribution system. Each ESS has an ID called the SSID which is a 32 byte (maximum) character string. Example: linksys (the default SSID for Linksys routers).
[edit] Distribution System
A distribution system connects Access Points in an extended service set. A distribution system is usually a wired LAN but can be a wireless LAN.
[edit] Types of Wireless Local Area Networks
[edit] Peer-to-peer or ad-hoc
This type of network allows wireless devices to directly communicate with each other. Wireless devices within range of each other can discover and communicate directly without involving central access points. This method is typically used by two computers so that they can connect to each other to form a network.
If a signal strength meter is used in this situation, it may not read the strength accurately and can be misleading, because it registers the strength of the strongest signal, which may be the closest computer.
[edit] Wireless distribution system
When it is difficult to connect all of the Access Points in a network by wires, it is also possible to put up access points as repeaters.
[edit] Monitoring station
Good wireless network cards can be set up to monitor a network without connecting to an access point or revealing itself. This can be used for NIDS (network intrusion detection systems), diagnostics, and troubleshooting, or more malevolently, to sniff clear-text activity or crack encryption.
[edit] See also
- Local area network
- IEEE 802.11
- Wireless network
- Exposed terminal problem
- Hidden terminal problem
- Fixed wireless data
