Chapter 5 - Wireless and Mobile Networks

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Chapter 5 - Wireless and Mobile Networks Powered By Docstoc
					CSNB314: Advanced
 Computer Networks
                    Chapter 5
 Wireless and Mobile Networks
   Wireless and mobile devices have been proliferating
    for the last 15 years.
       More such devices will come in the future.
   Wireless and mobile are actually two different
       Wireless: connection between device and network is done
        through wireless medium.
       Mobile: device can move around.
       Both have their own technical issues that need to be
   Wireless technology is implemented in data link and
    physical layers.
   Two modes of wireless network:
       Infrastructure mode: hosts are associated with a base
       Ad hoc mode: hosts communicate with themselves, without
        the use of a base station.
   When a wireless device is also mobile, it may need
    to associate / disassociate with different base
       This process is called handoff.
       Performing handoff can cause various technical issues.
   Different wireless standards are designed for
    different usage and environment.
               Personal              Local Area            Metropolitan Wide Area
               Area                  Network               Area         Network
               Network               (LAN)                 Network      (WAN)
               (PAN)                                       (MAN)
               Bluetooth             802.11a/b/g/n         802.16               GSM
               Ultra-wideband        (a.k.a. Wi-Fi)        (a.k.a. WiMAX)       GPRS
               (UWB)                 WiGig                                      W-CDMA
               Low data rates (1 –   High data rates (up   Medium data rates    Low to high data
Data           2 Mbps)               to 600 Mbps)          (up to 37 Mbps)      rates (10 Kbps –
Transfer                                                                        300 Mbps)

               Very short range (3   Short range (100      Medium range (50     Long range
Range          meters)               meters)               km)                  (Global)

               Notebook to PC to     Computer to           LAN or computer to   Smartphones and
Connectivity   peripheral devices    computer and the      high-speed wire      other mobile
               to systems            Internet              line Internet        devices to WANs
                                                                                and the Internet
Wireless Transmission Media
   Wireless transmission is done by radiating
    electromagnetic energy.
   Frequencies of interest:
       Microwave frequency: 1Ghz – 40GHz
       Radio frequency: 30Mhz – 1Ghz
       Infrared: 3x1011 Hz – 2x1014 Hz
   Antenna is required to transmit and receive
Wireless Transmission Media
   An electrical conductor used to radiate or
    collect electromagnetic energy.
       Electrical energy is converted into
        electromagnetic energy and vice versa.
   Different types of antenna is used for
    transmitting / receiving electromagnetic
    energy of different frequencies.
       Parabolic reflective antenna for microwave.
       A simple conductor for radio.
Terrestrial Microwave
   Use parabolic dish antenna.
   Transmitting and receiving antenna needs to
    face each other.
       Both antennas need to have line of sight.
       Normally located at substantial heights above
   To achieve long distance transmission, a
    series of antennas are used.
Terrestrial Microwave -
   Long haul telecommunications service.
       An alternative to coaxial cable or optical fiber.
       Commonly used for voice and television
   Short point-to-point links between buildings.
   Cellular systems.
Terrestrial Microwave –
Transmission Characteristics
   The higher the frequency used, the higher the
    potential data rate.
   Higher frequency has higher attenuation.
       Less useful for long distance transmission.
   Attenuation increases with rainfall.
   With increased number of users, signals may
    overlap and cause interference.
Satellite Microwave
   A satellite is a microwave relay station.
   Used to link two earth stations that are
    located far apart.
   Satellite receives transmission on one
    frequency (uplink), amplifies or repeats it, and
    transmits it on another frequency (downlink).
   Satellite can both be used for point-to-point
    communication or broadcast communication.
Satellite Microwave
Satellite Microwave
Satellite Microwave
   Communication satellites are normally
    geosynchronous orbit (GEO) satellites.
       Satellite remains stationary with respect to its position on
       This is achieved by placing the satellite at the height of
        35,863 km at the equator.
   To avoid interference, two satellites using the same
    frequency cannot be put close together.
       This limits the number of satellites that can be put in the
Satellite Microwave -
   Television distribution.
       Very suitable due to broadcast nature of satellite.
       Example: ASTRO.
   Long-distance telephone transmission.
   Private business network.
       Satellite provider can lease some channels to
        business users.
Satellite Microwave –
Transmission Characteristics
   Frequency bands used in satellite
       4/6 GHz band
       12/14 GHz band
       20/30 GHz band
   There is a propagation delay of about ¼
    second from the transmission from one earth
    station to the reception by another.
   Satellite microwave is a broadcast facility.
Radio Wave
   Omnidirectional. Antennas need not be aligned.
   Does not require dish-shaped antennas.
   Suffers less attenuation than microwave.
   Applications:
       AM and FM radio.
       UHF and VHF television.
       Mobile phones.
       Walkie-talkie.
       Wireless networks (WiFi, WiMAX).
   Use transmitters / receivers that modulate
    noncoherent infrared light.
   Requires line of sight.
   Does not penetrate walls.
   Used in electronic devices:
       TV remote control.
       Device-to-device communication. For example:
        computer to mobile phone.
Air Interface Techniques
   Refers to the way the radio spectrum is shared by
    multiple communications.
       A particular wireless technology is normally allocated a
        range of frequency (frequency spectrum).
       Need to share this frequency spectrum between multiple
        users at the same time.
       But need to prevent different communications from
        overlapping each other.
   Common techniques:
       FDM (frequency division multiplexing)
       TDM (time division multiplexing)
       CDMA (code division multiple access)
   The frequency spectrum is divided into smaller
    frequency bands.
       Each band will be used by one communication
   Example: Say that there is a radio spectrum
    from 20 to 32 KHz and that each channel
    uses 4 KHz.
       Channel 1: Use spectrum from 20 to 24 KHz.
       Channel 2: Use spectrum from 24 to 28 KHz.
       Channel 3: Use spectrum from 28 to 32 KHz.
   Time is divided into slots.
       Each communication channel is assigned one or more
   Example: Say that a radio spectrum has a
    capacity of 1 Mbps and that there are 5
    communication channels.
       Channel 1 transmits data during t = 0s until t = 0.2s.
       Channel 2 transmits data during t = 0.2s until t = 0.4s.
       Etc…
       Each channel effectively will have a data rate of 0.2
        Mbps (200 Kbps).
   All users share the same frequency spectrum at the
    same time.
   Each user is given a distinct sequence of bits called the
    chipping sequence.
   Data sent by the sender is encoded with the chipping
   The receiver can read the data sent by a particular
    sender by decoding the received signal (this signal
    contains data from multiple simultaneous senders) with
    the same chipping sequence used by the sender.
WiFi: 802.11 Wireless LANs
   Several wireless LAN technologies were developed
    in the 1990s.
       Today, the only one being used is IEEE 802.11 wireless
        LAN (commonly known as WiFi).
   There are several 802.11 standards.
       802.11a, 802.11b, 802.11g, 802.11n.
       They vary in terms of frequency range and data rate.
       Most WiFi devices support more than one standards.
       They all use the same medium access protocol
        (CSMA/CA) and the same frame structure.
       They all support both infrastructure and ad hoc modes.
WiFi: 802.11 Wireless LANs

  Standard      Frequency Band      Data Rate

IEEE 802.11b        2.4 GHz      Up to 11 Mbps
IEEE 802.11a         5 GHz       Up to 54 Mbps
IEEE 802.11g        2.4 GHz      Up to 54 Mbps

IEEE 802.11n    2.4 GHz or 5 GHz Up to 600 Mbps

IEEE 802.11ac        5GHz        Up to 1 Gbps
802.11 Architecture
802.11 Architecture
   Basic service set (BSS):
       Fundamental building block of the 802.11 architecture.
       Contains one or more wireless devices and a central base
        station known as access point.
   Access point (AP):
       Connect wireless devices to an interconnection device
        (switch or router) that leads to the Internet.
       AP and router can be integrated into one unit.
   Each AP and wireless device has a 6-byte MAC
    address stored in the firmware of its network
    interface card.
802.11 Architecture
   Stations can also group themselves together
    to form an ad hoc network.
       Connecting several stations using Wi-Fi without
        using an access point.
       Will create a small, isolated network connecting
        the stations together.
       Useful when we have several mobile devices and
        need to exchange data in the absence of an
        access point.
Channels and Association
   Each wireless device need to associate with an AP
    before it can send / receive data.
   Each AP must be given an SSID (Service Set
       Wireless devices identify an AP by its SSID.
   IEEE 802.11b/g operates in the frequency range of
    2.4 GHz to 2.485 GHz.
       This 85 MHz band is divided into 11 partially overlapping
       Any two channels are non-overlapping if they are
        separated by 4 or more channels.
       Bandwidth can be increased by installing more than one
        AP that use non-overlapping channels.
Channels and Association
   Passive scanning:
       AP periodically sends beacon frames. These frames
        include the AP’s SSID and MAC address.
       Wireless device scans the 11 channels to seek for beacon
       If any AP is found, it can be selected for association.
   Active scanning:
       Wireless device broadcasts a probe frame to all APs within
       APs responds by sending a probe response frame.
       Wireless device can then choose an AP for association and
        sends an association request frame.
Channels and Association
   After association is formed, the wireless device joins
    the subnet to which the AP belongs.
       IP address is typically obtained using DHCP.
   Access for association with an AP can be open or
    require authentication.
   Authentication can be done in several ways:
       Based on the MAC address of the device
       Using username and password based on security protocols
        such as:
         WEP
         WPA / WPA2
         802.1x
IEEE 802.11 Frame
IEEE 802.11 Frame
   802.11 frame shares many similarities with
    an Ethernet frame.
   Main differences:
       There are 4 address fields
           Address 1: MAC address of receiver
           Address 2: MAC address of sender
           Address 3: MAC address of router interface connected
            to the AP
           Address 4: Used in ad hoc mode when APs forward
            frames to each other
IEEE 802.11 Frame
    Sequence number
      In wireless environment, bit corruption and frame lost can
       occur more easily.
      Each frame must be acknowledged by the receiver.
      Sequence number is used to distinguish between new
       frame and retransmitted frame.
    Duration
      802.11 protocol allows a transmitting device to reserve a
       channel for a period of time.
      The reserved duration is included in this field.
    Frame control fields
      Contain various subfields that control various aspects of the
       802.11 protocol and its security protocol.
Mobility in the Same IP Subnet
Mobility in the Same IP Subnet
   As a wireless device moves further from an AP, the
    signal gets weaker.
       The wireless device will then starts scanning for a stronger
   If another AP with stronger signal is found:
       The device would then disassociate from its current AP and
        associate with the other AP.
   This association / disassociation is only at the link
       Only possible if the two APs are connected to the same
        subnet (i.e. connected to the same switch).
       IP address and any ongoing TCP connections are
Mobility in the Same IP Subnet
   With the change in AP, the switch must also update
    its forwarding table.
       Otherwise, any frame directed to the wireless device would
        still be sent to the first AP.
   Solution: the second AP sends an Ethernet
    broadcast frame to the switch with the wireless
    device’s MAC address just after association.
       This would cause the switch to update its forwarding table.
   What happen if the wireless device moves to a BSS
    that is attached to a different subnet?
       This is a more difficult problem since the device would now
        have to change its IP address.
       Can be handled using mobility protocol such as mobile IP.
IEEE 802.15.1 – Bluetooth
   Bluetooth a cable replacement technology for
    interconnecting notebooks, peripheral devices and
    mobile phones.
   Designed to operate over a short range, at low
    power and at low cost.
   Operates in the 2.4 GHz – 2.4835 GHz unlicensed
    radio band in a TDM (time division multiplexing)
   Can provide data rate of up to 2.1 Mbps (for
    Bluetooth 2.0).
Bluetooth Classes

Class   Maximum Permitted   Range
        Power               (approximate)
Class 1 100 mW (20 dBm)     ~100 meters

Class 2 2.5 mW (4 dBm)      ~10 meters

Class 3 1 mW (0 dBm)        ~1 meter
Bluetooth Applications
   Bluetooth can be used for communications
       A cell phone and a hands-free headset.
       A PC and its I/O devices (e.g. keyboard, mouse,
       Two cell phones for transferring files, address
        book, etc.
       Devices within an entertainment system.
       Many, many others…
Bluetooth Communication and
   Bluetooth networks are ad hoc networks.
       No network infrastructure (e.g. an access point).
   When Bluetooth-capable devices come within
    range of one another, an electronic
    conversation takes place to determine
    whether they have data to share or whether
    one needs to control the other.
       This happens automatically without user
Bluetooth Communication and
   The information exchanged between devices
       Device name
       Device class
       List of services
       Technical information, for example, device
        features, manufacturer, Bluetooth specification,
        clock offset.
Bluetooth Piconet
   If the devices figure out they need to
    communicate, they will create a personal
    area network called the piconet.
       In a piconet, there can be 8 devices connected
       Can also contain 255 parked devices.
       One of the devices in the piconet is designated as
        the master.
       The rest become the slaves.
Bluetooth Piconet
Bluetooth Piconet
   The master node controls the piconet.
       Its clock determines time in the piconet.
       It can transmit in each odd number slot.
       A slave can transmit only after the master has
        communicated with it in the previous slots.
       Slave can only transmit to the master.
   Parked devices can be in the piconet, but cannot
    communicate with any other node.
       To communicate, the parked devices must first has its
        status changed from parked to active by the master node.
Bluetooth Pairing
   To have a more secure communication, pairs of
    Bluetooth devices can perform pairing.
   Pairing is a trusted relationship where the
    communication between the two devices can be
    encrypted using a passkey.
       The passkey is normally entered by the user.
       The same passkey must be entered on the two devices to
        be paired.
   For a pair of device, the passkey only need to be
    entered when they are paired for the first time.
       The pairing information can be stored to be used during
        future pairing.
IEEE 802.16 – WiMAX
   WiMAX stands for Worldwide Interoperability for
    Microwave Access.
   An evolution from the WiFi technology.
   Designed to deliver last mile broadband access as
    an alternative to DSL and cable modem
   The actual data rate varies depending on the
    WiMAX standard used and several other factors:
       Distance to base station.
       Number of users sharing the same base station.
       Ground speed (speed of the user).
       The bandwidth of a channel.
WiMAX Standards
   802.16d (Fixed WiMAX)
       Does not support mobility.
       Data rate: up to 75 Mbps in 20 MHz channel.
   802.16e (Mobile WiMAX)
       The standard currently deployed by most service providers.
       Data rate: Up to 37 Mbps (downlink) in 10 MHz channel.
   802.16m
       Also known as WiMAX 2.0.
       Fulfill the IMT-Advanced (a.k.a. 4G) requirements.
       Data rate: 1Gbps for fixed users and 100 Mbps for mobile
WiMAX Architecture
   A WiMAX system consists of two parts:
       A WiMAX tower
           The WiMAX base station.
           Similar to a cell phone tower.
           Can provide a coverage of about 30 – 50 km for fixed
            stations and 5 – 15 km for mobile stations.
       A WiMAX receiver
           Also called the subscriber stations.
           Can be a small box, a USB dongle or built into the
            mobile device.
WiMAX Architecture
WiMAX Architecture
   WiMAX can provide two forms of wireless
       The non-line-of-sight service:
           Lower data rate, but can go around obstacles.
           Uses frequency between 2 GHz to 11 GHz.
       The line-of-sight service:
           Higher data rate but require line-of-sight (there should
            be no obstacles between the base station and the
            subscriber station).
           Uses frequency between 10 GHz to 66 GHz.
Cellular Wireless Network
   Cellular network was primarily developed to
    provide mobile telephone.
       Designed to transfer voice.
       Use circuit switching technology.
   Over the time, the cellular network has
    evolved in the following ways:
       Use of digital data (voice data is digitized).
       Provide data transfer capability (this allows us to
        access the Internet).
Cellular Wireless Network
   A cellular network is divided into a number of
    cells which are viewed as hexagons.
       This is the reason why a mobile phone is also
        called a cell phone.
   Each cell is allocated a band of frequencies
    and served by a base station.
       Adjacent cells are assigned different frequencies
        to avoid interference.
       However, cells sufficiently distant from each other
        can use the same frequency band.
Cellular Wireless
Cellular Wireless Network
   A base station consists of:
       An antenna.
       A controller.
       A number of transceivers for communicating on the
        channel assigned to the cell.
   Each base station is connected to a mobile
    switching center (MSC).
       This link can either be wired or wireless (normally wired).
   One MSC may be serving multiple base stations.
   The MSC is then connected to the public telephone
Cellular Wireless Network
   An MSC performs the following tasks:
       Assign voice channel to each call.
       Perform handoffs.
       Monitors the call for billing information.
   There are two types of channels available between
    a mobile unit and the base station.
       Control channels: used to setup and maintain calls and
        also establish relationship between the base station and
        the mobile unit.
       Traffic channels: used to carry voice or data connection.
Cellular Standards and
   Cellular technologies are classified into several
    “generations”: 1G, 2G, 3G, etc.
   These generations may differ in terms of:
       Data rate
       Capacity
       Signal quality
       Applications and services that can be offered
   1G systems are pretty much extinct.
   Most cellular networks today use 2G technologies
    and above.
First Generation (1G)
   Designed to carry only analog voice.
       The data is analog (voice), similar to the public telephone
   Uses FDMA (Frequency Division Multiple Access)
    technology for allocating channels to users. Each
    user is given one channel.
   The most popular 1G technology is AMPS
    (Advanced Mobile Phone Service)
       Used in the early 1980s.
   Other 1G technologies: NMT, C-Nets, TACS.
Second Generation (2G)
   The second generation systems are developed to
       Higher quality signals.
       Higher data rates to support digital services.
       Higher capacity.
   The main difference between 1G and 2G is that 2G
    systems send digital data.
       Voice is digitized before it is sent.
       But transmission is done using analog signal (all wireless
        signal is analog).
       Allows for services such as SMS.
2G Technologies
   IS-136 TDMA (Time Division Multiple Access)
       A combined FDM/TDM system that evolved from 1G FDMA
       Widely deployed in North America.
   GSM (Global System for Mobile Communications).
       Uses combined FDM/TDM.
       Started off in Europe in the early 1990s.
       Also used in Asia and North America.
   IS-95A CDMA (Code Division Multiple Access).
       Also knows as CDMAOne.
       As the name suggests, it uses the CDMA technology.
       Widely deployed in North America and Korea.
2G Technologies
   Since 2G technologies convert voice to digital data
    before transmission, they can also be used to carry
    data (i.e. application data).
       They can act as a modem.
   Although it works, this is not an effective method for
    data transmission.
       Circuit switching is used – highly inefficient for bursty
       Data rate is very slow.
         GSM – 9.6 kbps
         CDMA – 14.4 kbps
Transition from Second to
Third Generation (2.5G)
   The 2G systems are optimized for voice
    service and not well adapted for data
   In the 1990s, standard organizations have
    started developing 3G cellular technology
    targeted to carry both voice and data.
   Since 3G deployment may take many years,
    companies developed interim standards that
    enable data transmission over existing 2G
2.5G Technologies
   GPRS (General Packet Radio Services)
       Evolved from GSM and uses the underlying GSM network.
       A number of slots are set aside for data communications.
       Mobile device can use more than one time slot within a
        given channel in an on-demand basis.
       Slots are dynamically allocated to mobile device when
        there is data to send.
       Maximum data rate: 115 kbps.
       Typical data rate: 40 to 60 kbps.
2.5G Technologies
   EDGE (Enhanced Data Rates for Global
       Increase the capacity of GSM/GPRS network.
       Achieve higher data rates by replacing GSM’s
        modulation scheme with a more powerful scheme.
       Maximum data rate: 385 kbps.
       Typical data rate: 144 kbps
       Some books / web sites categorize EDGE as 3G,
        and some categorize it as 2.75G.
2.5G Technologies
   CDMAOne (IS-95B)
       This is basically an enhanced version of
        CDMAOne (IS-95A) to support 2.5G capabilities.
       Requires very minor upgrades in CDMAOne (IS-
        95A) networks.
       Maximum data rate: 115 kbps.
       Average data rate: < 64 kbps.
Third Generation (3G)
   3G wireless communication technologies are
    designed to provide fairly high-speed wireless
    communication to support multimedia, data and
    video in addition to voice.
   Referred to as IMT-2000 by ITU-R.
   Among the required 3G capabilities include:
       Voice quality comparable to public-switched telephone
       144 kbps at driving speeds.
       384 kbps for outside stationary or walking speeds.
       2 Mbps for office use (indoor).
3G Technologies
   W-CDMA (Wideband CDMA)
       Also known as UMTS (Universal Mobile
        Telecommunications Service).
       Used by GSM networks to upgrade to 3G.
       Uses a CDMA technique called Direct Sequence
        Wide CDMA (DS-WCDMA).
       Maximum data rate: 2 Mbps
       Typical data rate: 144 to 384 kbps.
3G Technologies
   CDMA2000
       Used by CDMAOne networks to upgrade to 3G.
       There are several variants of CDMA2000:
         1xRTT (Radio Transmission Technology) and 3xRTT.
         1xEV-DO (Evolution – Data Only)
         1xEV-DV (Evolution – Data Voice).

   TD-CDMA (Time Division CDMA)
       Intended to be used by TDMA networks to upgrade to 3G.
       Use unpaired spectrum.
         A single channel is used for both uplink and downlink, but
           each uses different slots.
         Very suitable for Internet data.
       Maximum data rate: 3.3 Mbps
3G Technologies
What Can You Do With a 3G-
enabled Devices?
   A 3G-enabled devices can act like a PC
    connected to the Internet. You can:
       Browse the Web using a Web browser.
       Read and reply emails.
       Chat using instant messaging application.
       Perform file transfer (FTP).
       Play online games.
       Watch live TV (or other streaming videos).
       Make a video call.
   Refers to intermediate technologies between 3G
    and 4G.
   Sometimes marketed as 3GX, 3G+, Turbo 3G.
   Provides higher data rates compared to 3G
       Done by improving the modulation scheme and refining the
        protocols between mobile phones and base stations.
       Some of the standards may also provide all-IP network and
        QoS support.
   Standards categorized under this category:
       HSPA (High Speed Packet Access), LTE (3GPP Long
        Term Evolution ), WiMAX 802.16e.
Pre-4G Technology: HSPA
   An upgrade to W-CDMA.
   HSDPA (High-speed Downlink Packet Access)
       Designed to have faster downlink compared to uplink.
       Maximum downlink data rate: 14.4 Mbps.
       Maximum uplink data rate: 384 Kbps.
   HSUPA (High-speed Uplink Packet Access)
       Designed to have faster uplink compared to downlink.
       Maximum uplink data rate: 5.76 Mbps
   HSPA+ (Evolved HSPA)
       The latest revision of HSPA+ can achieve a data rate of up
        to 672 Mbps (downlink) and 168 Mbps (uplink).
Pre-4G Technology: LTE
   Provides an all-IP network.
   Peak data rate of 300 Mbps (downlink) and
    75 Mbps (uplink).
   Provides QoS provisioning that allows for
    RTT less than 10 milliseconds.
   Provides seamless handover between older
    technologies such as GSM and W-CDMA.
       For many LTE implementations, GSM or W-
        CDMA is still used for voice transmission.
Fourth Generation (4G) –
Beyond 3G
   Referred to as IMT-Advanced by ITU-R.
   According to ITU-R, among others, a 4G network
    must include the following features:
       A data rate of 100 Mbps when client is moving at high
        speed in relative to the base station.
       A data rate of 1 Gbps when device and base station are in
        relatively static position.
       Smooth handoff across heterogeneous network.
       Seamless roaming / connectivity across multiple networks.
       High QoS support for next generation multimedia (e.g. real
        time audio, HDTV video content, mobile TV, etc).
       An all IP, packet-switched network.
Fourth Generation (4G) –
Beyond 3G
   Standards that fully comply with the ITU-R specified
    4G definition are yet to be released.
   However, the following pre-4G standards are
    commonly advertised as 4G:
       LTE (3GPP Long Term Evolution)
       IEEE 802.16e (Mobile WiMAX)
   The following standards are currently being
    developed to fully with comply with the 4G definition:
       LTE Advanced (Long Term Evolution Advanced)
       IEEE 802.16m
Ad-hoc Networks
   In addition to infrastructure-based network
    technologies, ad-hoc network technology is gaining
    more interest these days.
   There are many situations where the use of
    infrastructure-based network is not possible.
       After natural disaster.
       During war.
       When network infrastructure is controlled by an opposing
   Ad-hoc networks will make communication possible
    in these situations.
Ad-hoc Networks
   In an ad-hoc network, a host will also take the role
    of a routing node.
       Hosts need to have routing capability.
       Data is transmitted by being forwarded from one host to
   A host may join or leave the network at any time.
       This makes routing slightly more difficult.
       This can also cause a number of security issues.
   Examples of routing protocols designed for ad-hoc
       Ad-hoc On-demand Distance Vector (AODV)
       Optimized Link-state Routing Protocol (OLSR)
Ad-hoc Networks
   There are several different types of ad-hoc
    networks. Examples:
       Mobile ad-hoc network (MANET)
       Vehicular ad-hoc network (VANET)
   Many aspects of ad-hoc networks are being widely
    studied by researchers.
   However, a number of ad-hoc networks have
    already been deployed and seen real use.
       Search and rescue mission after natural disaster.
       Military operations.

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