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Another name for flexible data communication is WLAN system, used as an extension against wired LAN. Using radio frequency (RF) technology, WLANs are used to receive and transmit data over the channel, which minimizes the want for wired connections. Thus, WLAN is the combination of connectivity of data with mobile user.
The importance of WLAN technology, however, is much more than just the absence of wires. The advent of WLAN tells us new criteria of what an infrastructure of network can be. There is no need of the infrastructure to be solid and fixed, non-mobile, and costly to make amendments. It should move with the demand and amend as the changes are required.
Recently, manufacturers have developed WLANs for process and control applications. Retail applications have expanded to include wireless point of sale (WPOS). The healthcare and education industries are also fast growing markets for WLANs. WLANs provide high-speed, reliable data communications in a building or campus environment as well as coverage in rural areas. WLANs are simple to install and do not incur monthly user fees or data transmission charges.
In WLANs the client and the user connection is established by the use of wireless medium which can be referred as RF or infrared (IR) communications replacing a cable. This type of connection gives a remote user the access to stay in connection to the network via mobile state and not needed to physically attach to the network. The connection to wireless network is usually accomplished by the user having a handheld terminal or laptop that has a RF interface card installed inside the terminal or through the pc card slot of the laptop. The client connection from the wired LAN to the user is made through an access point (AP) that can support multiple users simultaneously. The AP can reside at a node on the wired network.
The range of a WLAN depends on the actual usage and environment of the system and can vary from 100 feet inside a solid walled building to several thousand feet outdoors, with a direct line of sight. Much like cellular phone systems, the WLAN is capable of roaming from the AP and reconnecting to the network through other APs residing at the other points in the network. This can allow the wired LAN to be extended to cover a much larger are than the existing coverage by the use of multiple APs.
An important feature of the WLAN is that it can be used independently of a wired network. It can be used anywhere as a stand-alone network to link multiple computers together without having build or extent a wired network. The costing of WLANs is based on the installation cost of the equipment; which once installed at place; no more charges are there for the usage of the network. The network communications take place in the license free spectrum designed for the purpose. In the band of 2.4-2.5 GHz, which is a license free band so the users can operate as long as they use the equipment of the type approved for it. The 2.4-GHz band has been designed as license free by the ITU and is available as such in most countries of the world.
A WLAN is capable of operating at speeds in the range of 1-2 Mbps depending on the actual system; both of these speeds are supported by the standard for WLAN networks defined by the Institute of Electrical and Electronic Engineers (IEEE). The faster WLANs use a 802.11 b high-rate standard to move data through air at maximum speed of 11 Mbps. The IEEE established the 802.11 b standard for wireless networks and the Wireless Ethernet Compatibility Alliance (WECA) to assure that WLAN products are interoperable from manufacturer to manufacturer. Any LAN application, network operating system, or protocol, including TCP/IP, will run on an 802.11b-compliant WLAN as easily as they run over Ethernet.
2.2 WLAN REQUIREMENTS
Throughput: defining the most efficient usage of the wireless medium by the medium access control protocol for maximum capacity.
Number of nodes: there should be accommodation and support for thousands of nodes in multiple cells.
Connection to backbone LAN: Requirement of the backbone LAN is there with stations. This is easily achieved through the use of control modules which connect to both types of LANs.
Service Area: A diameter of 100 to 300 m is termed as typical range for a wireless LAN or the service area.
Battery Power Consumption: The implementation of Wireless LAN is such that it reduces the power consumption when the network is not used, just like being in sleep mode.
Transmission Robustness and Security: The Problem with wireless LAN is the interference of noise and easy network drop. Henceforth, the design of wireless LAN should be less prone to transmission drop in noisy atmosphere and should provide security from eavesdropping.
Collocated Network Operations: It is quite likely for two or more wireless LANs to operate in the same area where interference between the LANs is possible. This type of interference can be a threat to the normal operation of MAC algorithm and can result in unauthorized access to particular LAN.
License Free Operation: Because there is no license for the frequency band used in the WLAN, the users prefer buying the products associated with it and use the WLAN in day to day life.
Handoff or Roaming: Roaming allows the mobile stations for movement from one cell to another through the MAC protocol incorporated in the wireless LAN.
Dynamic Configuration: The MAC network and addressing aspects of the LAN should not disrupt the user and thus it gives permission for addition, deletion and dynamic allocation of the end points or hubs on the wireless.
2.3 WLAN Equipment
The basis of the wireless LAN is formed by three main things. These are:
LAN adapter- wireless adapters are made in the same way as PCMCIA, USB, Cardbus, and PCI are made which are wired. They also provide the same functions and enables end users to access the network. In a wired LAN, an interface is provided between the network operating system and the wire by the adapters. But in a WLAN, adapters provide the interface between an antenna and the network operating system which creates a transparent connection to the network.
Access point-abbreviated as AP is the equivalent to a LAN hub in wireless. Its work is to receive, buffer, and transmit the data between wired network and WLAN, gieving support to a group of wireless user devices. Typically, an AP is connected with the backbone through a standard Ethernet cable, and communicates with wireless devices by means of an antenna. The AP or antenna connected to it generally mounted on a high wall or on the ceiling. Like cells in a cellular phone network, multiple APs can support handoff from one AP to another as the user moves from area to area. APs have range from 20 to 500 meters. A single AP can support between 15 and 250 users, depending on technology, configuration, and use. It is relatively easy scale WLANs by adding more APs to reduce network congestion and enlarge the coverage area. Large networks that require multiple APs can monitor movement of clients across it domain and permit or deny specific traffic or clients from communicating through it.
Outdoor LAN bridges- are used to make connections to the LANs in different buildings. When the cost of buying a fiber optic cable between buildings is considered, particularly if there are barriers such as highways or bodies of water in the way, a WLAN can be an economical alternative. An outdoor bridge can provide a less expensive alternative to recurring leased line charges. WLAN bridge products support fairly high data rated and ranges of several miles with the use of line-of-sight directional antennas. Some APs can also be used as a bridge between buildings of relatively close proximity.
2.3 Wireless LAN classification
The broad classification of Wireless LANs can be done into two categories: ad hoc wireless LANs and wireless LANs with infrastructure.
Ad hoc network is made up of several wireless nodes joined together to establish a peer-to-peer communication. There is no permission granted to talk. This is obtained using the Independent Basic Service Set (IBSS).Every node has the capacity to communicate directly with other nodes, so no access point controlling medium access is necessary. Nodes within an ad-hoc network can make the communication possible if the reach to each other is physically possible i.e., if they are within each other's radio range or if other nodes can forward the message. Nodes from the two networks shown in figure 2.2 cannot, therefore, communicate with each other if they are not within the same radio range. In ad-hoc networks, the complexity of each node is higher because every node has to implement medium access mechanism, mechanism to handle hidden or exposed terminal problems, and perhaps priority mechanism, to provide a certain quality of service. This type of wireless network exhibits the greatest possible flexibility as it is, for example, needed for unexpected meetings, quick replacement of infrastructure or communication scenario far away from any infrastructure.
Figure 3.2 ad-hoc network with three nodes [ ]
To set up an ad-hoc wireless network, each wireless adapter must be configured for ad-hoc mode versus the alternative infrastructure mode. In addition, all wireless adapters on the ad-hoc network must use the same SSID and the same channel number. Clearly the two basic variants of wireless LANs, infrastructure based and ad-hoc, do not always come in their pure form. There are networks that rely on access points and infrastructure for basic services (e.g.: authentication of access, control of medium access for data with associated quality of service, management functions), but that also allow for direct communication between the wireless modes.
However, ad-hoc networks might only have selected nodes with the capabilities of forwarding data. Most of the nodes have to connect to such a special node first to transmit data if the receiver is out of their range.
The wireless LANs having the infrastructure has a very high-speed either wired or wireless backbone. Many WLANs of today need an infrastructure networks. Infrastructure networks not only provide access to other networks, but it also provides function forwarding and control to medium access. In these infrastructure-based wireless networks, communication typically takes place only between the wireless nodes and the access point (see figure 2.3), but not directly between the wireless nodes. For infrastructure mode wireless networking, a wireless access point (AP) is required . To join the WLAN, the AP and all wireless clients must be configured to use the same SSID. Through access points the wireless nodes backbone are accessed. The access points enable the wireless nodes to make use of the available network resources in a proper and reliable way. The AP is then cabled to the wired network to allow wireless clients access to, for example, Internet connections or printers. Additional APs can be added to the WLAN to increase the reach of the infrastructure and support any number of wireless clients.
Figure 2.3 infrastructure-based wireless LAN
Typically, the design of infrastructure-based wireless networks is simpler because most of the network functionality lies within the access point, whereas the wireless clients can remain quite simple. This type of network can use different access schemes with or without collision. Collisions may occur if medium access of the wireless nodes and the access point is not coordinated. However, if the control of medium access is only by the access points, then there are no collisions possible. Infrastructure-based networks lose some of the flexibility wireless networks can offer, e.g., they cannot be used for disaster relief in cases where no infrastructure is left. Typical cellular phone networks are infrastructure-based networks for a wide area. Also satellite-based cellular phones have an infrastructure-the satellites.
Before the data is communicated, the clients having wireless access and access points should establish a connection, or an association. Only after the successful connection is established then only the two wireless stations can exchange data. Compared to ad-hoc wireless networks, infrastructure mode networks offer the advantage of scalability, centralized security management and improved reach. Its disadvantage is simply the additional cost to purchase AP hardware. All home wireless routers feature a built-in AP to support infrastructure mode.
2.4 STANDARDS OF WLAN
The IEEE standard 802.11 (IEEE, 1999) is the major family of WLAN specifying the most extensive available products. As the standard's number indicates, this standard belongs to the group of 802.x LAN standards, e.g., 802.3 Ethernet or 802.5 Token Ring. This means that the standard defines the physical and medium access layer adapted to the special requirements of wireless LANs, but it also offers the same interface as the others to higher layers to maintain interoperability.
Original 802.11 is the reference to the term, which sometimes called "802.11 legacies" falling under the category of WLAN (Wireless Local Area Network).
IEEE 802.11 defines the physical layer and Media Access Control layers based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The 802.11 specification has provisions incorporated for the minimization of collisions when two mobile units are in range of a common access point, but when out of range to each other.
There are six over the air modulation techniques in the 802.11 family which uses similar protocol, most used techniques are defined by the a, g, and b amendments against the original standard. Standards there in the family except these(c-f, h-j, n) are service enhancement and extensions, and also amendments to previous specifications. The first widely accepted wireless networking standard was the 802.11b, followed by 802.11a and 802.11g. 2.4 gigahertz (GHz) band uses the 802.11b and 802.11g standards. The 5 GHz band is used by 802.11a. Thus 802.11a is over b and g.802.11a, 802.11b, 802.11g are the ones specifying three different PHY layers though MAC is same in three of them.
Main technical characteristics of IEEE 802.11 are as follows:
1. Bandwidth originally 1, 2 Mbps (BPSK and QPSK), then CCK 5.5 and 11 Mbps.
2. Asynchronous, connectionless service.
3. Supports both ad-hoc and infrastructure mode operation.
4. Spread Spectrum without requiring licensing.
5. Diffused Infrared (850 - 900 nm) bands (see more below).
6. Multiple priorities supports.
7. Time-critical and data traffic support.
8. Power management allows a node to doze off.
This is the original version, a standard of IEEE 802.11 which was released in 1997 defines two raw data rates of 1 and 2 megabits per second (Mbit/s) to be transmitted via infrared (IR) signals or can be also in the Scientific Industrial Medical frequency band operating at 2.4 GHz. IR roles to be a part of the standard but have no actual implementations.802.11b has a chipping rate of 11 Mbit/s.
To achieve a higher data rate in the same bandwidth at the same chipping rate, a modulation scheme known as complementary code keying (CCK) is used. The CCK modulation scheme is quite complex and is not examined in detail here.
IEEE 802.11b defines two physical-layer frame formats, which differ only in the length of the preamble.
It is a correction to the original standard which was approved in 1999. The 802.11a standard uses frequency band called the Universal Information Infrastructure (UNNI), which is divided into three parts.
IEEE 802.11a utilizes more available bandwidth than 802.11b/g.
IEEE 802.11a provides much higher data rates than 802.11b and the same maximum data rate rate as 802.11g
IEEE 802.11a uses a uncommon, relatively uncluttered frequency spectrum(5 GHz)
Each channel in this protocol is 20MHz wide. It is intended for both indoor and outdoor use.
A third modulation standard was approved in June 2003 as 802.11g. The working of the standard is in the 2.4 GHz band (like that of 802.11b) but its operations are at a maximum chipping rate of 54 Mbit/s, talking more about calculations then a net throughput of 24.7 Mbit/s like that of 802.11a. It's backward consistency is with b and uses the same frequencies. The combination details of b and g gel very well together and thus occupy much of the technical processes.
802.11g gives the reliability of higher throughput, actually the results have been composed by several factors:, exposing to the same interference sources as 802.11b, limited channelization (these are the 3 complete non-overlapping channels present like that of 802.11b), conflicting with 802.11b-which claims for the devices and fact that the data rates of 802.11g are higher than 802.11b maximum record, creating the 802.11g device reduction in the data rate and thus finally bringing it to the same rates used by 802.11b.
â€¢ IEEE 802.11 - The main and the original 1 and 2 Mbit/s, IR standard with 2.4 GHz RF
â€¢ IEEE 802.11a - 5 GHz standard (1999) of 54 Mbit/s
â€¢ IEEE 802.11b - Enhancements and amendments to 802.11 for the support of 5.5 and 11 Mbit/s (1999)
â€¢ IEEE 802.11d - Used as International roaming extensions
â€¢ IEEE 802.11e - Enhancements: QoS, this includes bursting of packets
â€¢ IEEE 802.11F - Defining Inter-Access Point Protocol (IAPP)
â€¢ IEEE 802.11g - 2.4 GHz standard 54 Mbit/s (backward compatibility with b) (2003)
â€¢ IEEE 802.11h - A spectrum of 5GHz
â€¢ IEEE 802.11i - Security Enhancement
â€¢ IEEE 802.11j - Japanese Extension
â€¢ IEEE 802.11k - Enhanced resource for radio measurement
â€¢ IEEE 802.11n - Improvements in higher throughput
â€¢ IEEE 802.11p - Giving non-wired access to Vehicular Environment(WAVE) e.g. ambulances
â€¢ IEEE 802.11r - Better and reliable roaming
â€¢ IEEE 802.11s - Mesh networking in wireless
â€¢ IEEE 802.11T - WPP - Wireless Performance Prediction, a test method
â€¢ IEEE 802.11u - Interco ordination with non-802 networks (e.g., cellular)
â€¢ IEEE 802.11v - Management of wireless network
â€¢ IEEE 802.11w - Protected Management Frames
Several other related wireless network technologies exist aside from these four general-purpose Wi-Fi standards. Other IEEE 802.11 working group standards like 802.11h and 802.11j are extensions of Wi-Fi technology that serve a very precise purpose.
Bluetooth is a wireless network technology that followed a different development path than the 802.11 family. Bluetooth technology aims at so-called ad-hoc piconets, which are local area networks with a very limited coverage and without the need for an infrastructure. Bluetooth is a wireless LAN technology designed to operate in an environment of many users. It connects devices of various functions such as home stereos, telephones, notebooks, cameras, computers, printers, and so on. Like IEEE 802.11b, Bluetooth operates in the 2.4 GHz ISM band.
WiMax was developed separately from Wi-Fi. For long-range networking (spanning miles or kilometers) WiMax is designed as opposed to local area wireless networking.