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Transcript of Mobile Communication



    Dr.A.Kathirvel, Professor and Head, Dept of IT Anand Institute of Higher Technology, Chennai

  • Unit - II


    Wireless LAN IEEE 802.11 Standards Architecture Services Mobile Ad hoc Networks- WiFi and WiMAX - Wireless Local Loop

  • LAN/WLAN World

    LANs provide connectivity for interconnecting computing resources at the local levels of an organization

    Wired LANs

    Limitations because of physical, hard-wired infrastructure

    Wireless LANs provide




    Ease of Installation

  • LAN Components

  • Repeater

    A repeater receives a signal, regenerates it, and passes it on.

    It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media.

    It operates at Physical Layer of OSI

    The Four Repeater Rule for 10-Mbps Ethernet should be used as a standard when extending LAN segments.

    This rule states that no more than four repeaters can be used between hosts on a LAN.

    This rule is used to limit latency added to frame travel by each repeater.

  • Hub

    Hubs are used to connect multiple nodes to a single physical device, which connects to the network.

    Hubs are actually multiport repeaters.

    Using a hub changes the network

    topology from a linear bus, to a


    With hubs, data arriving over the

    cables to a hub port is electrically

    repeated on all the other ports

    connected to the same network

    segment, except for the port on

    which the data was sent.

  • Bridge

    Bridges are used to logically separate network segments within the same network.

    They operate at the OSI data link layer (Layer 2) and are independent of higher-layer protocols.

    The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.

    When a bridge receives a frame on the

    network, the destination MAC address is

    looked up in the bridge table to determine

    whether to filter, flood, or copy the frame onto

    another segment

    Broadcast Packets are forwarded

  • Switch

    Switches are Multiport Bridges.

    Switches provide a unique network segment on each port, thereby separating collision domains.

    Today, network designers are replacing hubs in their wiring closets with switches to increase their network performance and bandwidth while protecting their existing wiring investments.

    Like bridges, switches learn certain information about the data

    packets that are received from various computers on the


    Switches use this information to build forwarding tables to

    determine the destination of data being sent by one computer to

    another computer on the network.

  • Switches: Dedicated Access

    Hosts have direct connection to switch

    Full Duplex: No collisions

    Switching: A-to-A and B-to-B simultaneously, no collisions

    Switches can be cascaded to expand the network








  • 10

    Wireless networks

    Wireless PANs (Bluetooth IEEE 802.15) very low range

    wireless connection to printers etc

    Wireless LANs (WiFi IEEE 802.11) infrastructure as well as ad-hoc

    networks possible

    home/office networking

    Multihop Ad hoc Networks useful when infrastructure not

    available, impractical, or expensive

    military applications, emergencies

    Wireless MANs (WiMAX-802.16)

    Similar to cellular networks

    traditional base station infrastructure systems

  • 802.11 Wireless LAN

    Provides network connectivity over wireless media

    An Access Point (AP) is installed to act as Bridge between Wireless and Wired Network

    The AP is connected to wired network and is equipped with antennae to provide wireless connectivity



    to the


    wired LAN


    with PCI 802.11 LAN card


    with PCMCIA 802.11 LAN card Access Point

  • 802.11 Wireless LAN Range ( Distance between Access Point and WLAN client)

    depends on structural hindrances and RF gain of the antenna at the Access Point

    To service larger areas, multiple APs may be installed with a 20-30% overlap

    A client is always associated with one AP and when the client moves closer to another AP, it associates with the new AP (Hand-Off)

    Three flavors:




    LAN Technologies

  • Multiple Access with Collision Avoidance (MACA)

    Before every data transmission Sender sends a Request to Send (RTS) frame containing

    the length of the transmission Receiver respond with a Clear to Send (CTS) frame Sender sends data Receiver sends an ACK; now another sender can send data

    When sender doesnt get a CTS back, it assumes collision

    sender receiver other node in

    senders range RTS




    other node in

    receivers range

  • WLAN : 802.11b

    The most popular 802.11 standard currently in deployment.

    Supports 1, 2, 5.5 and 11 Mbps data rates in the 2.4 GHz ISM (Industrial-Scientific-Medical) band

  • WLAN : 802.11a

    Operates in the 5 GHz UNII (Unlicensed National Information Infrastructure) band

    Incompatible with devices operating in 2.4GHz

    Supports Data rates up to 54 Mbps.

  • WLAN : 802.11g

    Supports data rates as high as 54 Mbps on the 2.4 GHz band

    Provides backward compatibility with 802.11b equipment

  • Medical Professionals


    Temporary Situations


    Security Staff

    Emergency Centers

    Wireless LAN Applications

  • 18

    Wireless Local Area Networks

    The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking.

    Companies jumped in, quickly developing incompatible wireless products in the 1990s.

    Industry decided to entrust standardization to IEEE committee that dealt with wired LANS namely, the IEEE 802 committee!!

  • In response to lacking standards, IEEE developed the first internationally recognized wireless LAN standard IEEE 802.11

    IEEE published 802.11 in 1997, after seven years of work

    Most prominent specification for WLANs

    Scope of IEEE 802.11 is limited to Physical and Data Link Layers.

    IEEE 802.11 Wireless LAN Standard

  • Appliance Interoperability

    Fast Product Development

    Stable Future Migration

    Price Reductions

    The 802.11 standard takes into account the following significant differences between wireless and wired LANs:

    Power Management



    Benefits of 802.11 Standard

  • 21

    IEEE 802 Standards Working Groups

    Figure 1-38. The important ones are marked with *. The ones marked with

    are hibernating. The one marked with gave up.

  • 22

    Categories of Wireless Networks

    Base Station :: all communication through an access point {note hub topology}. Other nodes can be fixed or mobile.

    Infrastructure Wireless :: base station network is connected to the wired Internet.

    Ad hoc Wireless :: wireless nodes communicate directly with one another.

    MANETs (Mobile Ad Hoc Networks) :: ad hoc nodes are mobile.

  • 23

    Wireless LANs

    Figure 1-36.(a) Wireless networking with a base station. (b) Ad hoc networking.

  • IEEE 802 LAN Standards Family

    IEEE 802.3



    IEEE 802.4



    IEEE 802.5



    IEEE 802.11


    IEEE 802.2

    Logical Link Control (LLC)

    PHY OSI Layer 1



    OSI Layer 2

    (Data Link)

  • 25

    The 802.11 Protocol Stack

    Note ordinary 802.11 products are no longer being manufactured.

    Figure 4-25. Part of the 802.11 protocol stack.

  • 26

    Wireless Physical Layer

    Physical layer conforms to OSI (five options) 1997: 802.11 infrared, FHSS, DHSS 1999: 802.11a OFDM and 802.11b HR-DSSS 2001: 802.11g OFDM

    802.11 Infrared Two capacities 1 Mbps or 2 Mbps. Range is 10 to 20 meters and cannot penetrate walls. Does not work outdoors.

    802.11 FHSS (Frequence Hopping Spread Spectrum) The main issue is multipath fading. 79 non-overlapping channels, each 1 Mhz wide at low end of

    2.4 GHz ISM band. Same pseudo-random number generator used by all stations. Dwell time: min. time on channel before hopping (400msec).

  • 27

    Wireless Physical Layer

    802.11 DSSS (Direct Sequence Spread Spectrum) Spreads signal over entire spectrum using pseudo-random sequence (similar

    to CDMA see Tanenbaum sec. 2.6.2).

    Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud.

    1 or 2 Mbps.

    802.11a OFDM (Orthogonal Frequency Divisional Multiplexing) Compatible with European HiperLan2.

    54Mbps in wider 5.5 GHz band transmission range is limited.

    Uses 52 FDM channels (48 for data; 4 for synchronization).

    Encoding is complex ( PSM up to 18 Mbps and QAM above this capacity).

    E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols.