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    WIRELESS LAN

    A fast-growing market introducing the flexibility of wireless access into office, home, or productionenvironments.

    WLANs are typically restricted in their diameter to buildings, a campus, single rooms etc. and are operatedby individuals, not by large-scale network providers.

    The global goal of WLANs is to replace office cabling, to enable join less access to theinternet and, to introduce a higher flexibility for ad-hoc communication in, e.g., groupmeetings.

    Advantages of WLANs are:

    Flexibility:

    o Within radio coverage, nodes can communicate without further restriction. Radio wavescan penetrate walls, senders and receivers can be placed anywhere (also non-visible,e.g., within devices, in walls etc.).

    Planning:

    o No prior planning is required for connectivity as long as devices followstandard convention.

    Design:

    o Wireless networks allow for the design of small, independent devices which canfor example be put into a pocket.

    o Wireless senders and receivers can be hidden in historic buildings, i.e., currentnetworking technology can be introduced without being visible.

    Robustness:

    o Wireless networks can survive disasters, e.g., earthquakes or users pulling a plug. If thewireless devices survive, people can still communicate.

    Cost:

    o After providing wireless access to the infrastructure via an access point for the first user,adding additional users to a wireless network will not increase the cost. This is, importantfor e.g., lecture halls, hotel lobbies or gate areas in airports.

    Disadvantages of WLANs are:

    Quality of service:

    o WLANs typically offer lower quality than their wired counterparts.

    o The main reasons for this are the lower bandwidth due to limitations in radio transmission

    (e.g., only 110 Mbit/s user data rate instead of 1001,000 Mbit/s).o Higher delay in error correction and detection mechanisms.

    Proprietary solutions:

    o Slow standardization procedures.

    IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

    Network)

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    Restrictions:

    o All wireless products have to obey with national regulations. Several government andnon-government institutions restrict frequencies to minimize interference.

    Safety and security:o Using radio waves for data transmission might interfere with other high-tech equipment,

    e.g., hospitals.

    Competing Requirement

    Global operation:

    o WLAN products should sell in all countries so, national and international frequencyregulations have to be considered.

    o Wireless LAN equipment may be carried from one country into another theoperation should still be legal in this case.

    Low power:o Wireless devices running on battery power. The LAN design should take this into account

    and implement special power-saving modes and power management functions.

    License-free operation:

    o Should be able to operate the WLAN without license.

    Robust transmission technology:

    o Compared to their wired counterparts, WLANs operate under difficult conditions. If theyuse radio transmission, many other electrical devices can interfere with them (vacuumcleaners, hairdryers, train engines etc.).

    Easy to use:

    o Must be easy to use by a common man without complicated procedure

    Protection of investment:

    o A lot of money has already been invested into wired LANs. The new WLANs shouldprotect this investment by being interoperable with the existing networks.

    Safety and security:

    o Wireless LANs should be safe to operate, especially regarding low radiation if used, e.g.,in hospitals. Users cannot keep safety distances to antennas. The equipment has to besafe for leader.

    o Users should not be able to read personal data during transmission, i.e., encryptionmechanisms should be integrated.

    Transparency for applications:

    o Existing applications should continue to run over WLANs, the only difference beinghigher delay and lower bandwidth.

    IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

    Network)

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    Infra red Vs Radio Transmission

    Now days we have used 2 Basic Transmission Technology. The transmission Technologiesare Infra red and Radio Transmission.

    Infrastructure and ad-hoc network

    The two basic alternative of wireless networks are infrastructure-based and ad-hoc.

    Infrastructure

    Normally infrastructure is used to access other network.

    Infrastructure networks not only provide access to other networks, but also include forwardingfunctions, medium access control etc.

    In these infrastructure-based wireless networks, communication is done between the wireless nodesand the access point.

    The access point does not just control medium access, but also acts as a bridge to other wireless orwired networks.

    The design of infrastructure-based wireless networks is simpler because most of the networkfunctionality lies within the access point, whereas the wireless clients can remain quitesimple.

    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 only the access point controls medium access, no collisions arepossible.

    IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

    Network)

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    This setting may be useful for quality of service guarantees such as minimum bandwidth for certainnodes.

    Typical cellular phone networks are infrastructure-based networks for a wide area.

    Ad-hoc network

    Each node can communicate directly with other nodes, so no access point controlling medium accessis necessary.

    The below figure shows two ad-hoc networks with three nodes each. Nodes within an ad-hocnetwork can only communicate if they can reach each other physically, i.e., if they are withineach others radio range.

    . In ad-hoc networks, the complexity of each node is higher because every node has to implement

    medium access mechanisms.

    The mechanisms to handle hidden or exposed terminal problems, and perhaps priority mechanisms, toprovide a certain quality of service.

    This type of wireless network display the greatest possible flexibility as it is, for example, needed forunexpected meetings, quick replacements of infrastructure.

    However, ad-hoc networks might only have selected nodes with the capabilities offorwarding data. Most of the nodes have to connect to such a special node first totransmit data if the receiver is out of their range.

    The three WLANs presented, IEEE 802.11 and HiperLAN2 are typically infrastructure-based networks,which additionally support ad-hoc networking.

    The third WLAN, Bluetooth is a typical wireless ad-hoc network. Bluetooth focusesprecisely on unplanned ad-hoc meetings or on the simple connection of two or moredevices without requiring the setup of an infrastructure.

    IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

    Network)

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    IEEE 802.11 ARCHITECUTRE AND SERVICES

    In 1990, the IEEE 802 Committee formed a new working group, IEEE 802.11, specificallydedicated to wireless LANs.

    Its agreement to develop a MAC protocol and physical medium specification.

    The initial interest was in developing a wireless LAN operating in the ISM (industrial, scientific, andmedical) band.

    At the same time, increasing the WLAN needed. So the team expanding the list of standards.

    Wi-Fi Alliance The first 802.11 standard, the industry accepted 802.11b. Although 802.11b products are all based on the same standard.

    This organization, subsequently renamed the Wi-Fi (Wireless Fidelity) Alliance, created a test suite tocertify interoperability for 802.11b products.

    IT2402 Mobile CommunicationYear-IV Semester-VII UNIT 2 (Wireless

    Network)

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    IEEE 802.11 System Architecture:

    The smallest building block of a wireless LAN is a basic service set (BSS).

    BSS consists of some number of stations executing the same MAC protocol and access to the same sharedwireless medium.

    A BSS may be connecting to a backbone distribution system (DS) through an access point (AP).

    The AP functions as a bridge and a relay point. In a BSS, client stations do not communicate directly with one another. If one station in the BSS wants to communicate with another station in the same BSS.

    The MAC frame is first sent from the beginning station to the AP, and then from the AP to thedestination station.

    Similarly, a MAC frame from a station in the BSS to a remote station is sent from the

    local station to the AP and then relayed by the AP over the DS on its way to thedestination station. The DS can be a switch, a wired network, or a wireless network.

    When all the stations in the BSS are mobile stations, with no connection to other BSSs, the BSS iscalled an independent BSS (IBSS). It also called ad hoc network.

    In an IBSS, the stations all communicate directly, and no AP is involved.

    An extended service set (ESS) consists of two or more basic service sets interconnected by a distributionsystem.

    Normally, the distribution system is a wired or wireless.

    The extended service set (ESS) appears as a single logical LAN to the logical link control (LLC) le

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