; wlan
Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

wlan

VIEWS: 12 PAGES: 35

  • pg 1
									802.11 – Introduction, Updates and
   Business cases/applications



  Presented by Ashutosh Dutta
                                  Contents
   Introduction to 802.11
           Spectrum View with respect to Unlicensed Band
           Network Topology of 802.11
           802.11 Layers Description (PHY and MAC Layers)
           Features of the standard
   Update
           Standard Bodies/Forums/Alliance (3GPP, MWIF, WECA)
           Industry
   Business cases and Applications
INTRODUCTION to 802.11
Spectrum View (Unlicensed Band)
                                        Spectrum View
           Unlicensed Bands                Spectrum               Typical Applications


ISM: Industry Science and Medicine
902 - 928 MHz,                             234.5 MHz   Cordless Phones,
2.4 - 2.4835 GHz                           83.5 MHz    Wireless LANs (WLAN)
5.725 - 5.85 GHz                           125 MHz     and Wireless PBXs (WPBX)

UPCS: Unlicensed PCS
Asynchronous:1910-1920, 2390-2400 MHz      20 MHz      WLAN
Isochronous: 1920-1930 MHz                 10 MHz      WPBX


UNII: Unlicensed National Information
Infrastructure
UNII (5.15-5.25 GHz)                       100 MHz     Indoor applications WLAN,WPBX
UNII (5.25-5.35 GHz)                       100 MHz     Short outdoor links, campus applications
UNII (5.725-5.825 GHz)                     100 MHz     Long outdoor links, Point-To-Point links

Millimeter Wave (59-64 GHz)                5 GHz       Home networking applications
            Unlicensed Band and Types of 802.11 Stds
   Unlicensed Band
            It is free, Huge and Nationwide Spectrum without License from FCC
            Any device can transmit in this band, with some rules like power limits
   It can be used for:
            Fixed and mobile services, WLANs, Wireless Private Branch Exchange,
             Spectrum Sharing, Experimentation and innovation
   Types of IEEE 802.11 standard
            IEEE 802.11a:-Operate in unlicensed 5GHz band, supports 6-54 Mbps speed
            IEEE 802.11b (also called WiFi):- Operates in unlicensed 2.4GHz band,
             supports up to 11Mbps (Oddly enough, the latter came before the former)
            IEEE 802.11d - LAN/MAN standard
            IEEE 802.11e - Working on QoS (quality of service) issue in LANs
            IEEE 802.11g - Objective is to double the speed of 802.11b (22 Mbps in 2.4
             GHz band. Still not approved as of mid 2001.
            IEEE 802.16 - A draft Wireless LAN standard for Metro Area Networks based
             on OFDM and using IEEE 802.11a as a foundation
                  Network Topology of 802.11 (1/2)
     Standard defines two Configurations
             Cellular and Ad-hoc
     Cellular
             System is sub-divided into cells (A cell called Basic Service Set BSS)
             Each cell is controlled by a base station (called Access Point or AP)
             APs are connected through some kind of backbone called distributed
              System (DS), typically Ethernet and in some cases wireless itself
             The whole interconnected wireless LAN including different cells, their
              respective access points and the distribution system is called ESS.
                                                      Distribution System
                                                      AP                AP




    ESS is seen to the upper layer of                BSS               BSS
    OSI model, as a single NW


                                                   ESS = Extended Service Set
                   Network Topology of 802.11 (2/2)
   Generally
               WLAN may consist of single or collection of cells
               10-30 stations per AP can be supported (depends on NW traffic)
               Typical coverage is 200-1000 ft (Depends on type of walls, height of
                ceiling, physical obstruction
                                     Distribution System

                                     AP            AP



                                     BSS           BSS



                                   ESS = Extended Service Set                AP    AP


                                                                             BSS   BSS

   Standard also defines the Concept of Portal
              It is a device that interconnects two different 802.11 LANS
              Even though standard does not requests so, typical installation will have
               AP and the portal on a single physical entity
                                   Physical Layer (1/3)
        Multiple Physical Layers supported by MAC layer
                     Direct Sequence Spread Spectrum in 2.4 GHz Band
                     Frequency Hopping (FH) Spread Spectrum in 2.4 GHz Band
                     Infrared in 850-to-950nM
        PHYSICAL LAYER Convergence Protocol (PLCP)
                     Supports common PHY SAP
                     Provides Clear Channel Assessment signal (carrier sense)
                      802.2
                                    Data Link
                                     Layer
                 802.11 MAC

        FH           DS       IR    Phy layer


       Modulation:
                    DPSK Differential Phase Shift Keying
       Channel Bandwidth
                    Channel bandwidth is about 20 MHz for DSSS systems
                    Thus ISM band accommodates up to three non-overlapping channels
                    It is Regardless of data rate (1, 2, 5.5, or 11 Mbps)
                           Physical Layer (2/3)
   Principle of DSSS
           DSSS receivers employ different PN codes
           PN sequence spreads transmitted bandwidth & reduces peak power
           Upon reception, the signal is correlated with the same PN sequence to
            recover the original binary data
           This technique reduces the effect of narrowband interference
           Spreading architecture of 802.11 is slightly different from cellular CDMA
           DSSS Mode Supports three data rates 1, 2 & 11 Mbps
                           Physical Layer (3/3)
   Principle of FHSS
           Receivers employ different FH patterns, governed by certain codes
                   There are 22 hop patterns to choose from
           The frequency hops across 2.4GHz band covering 79 channels
           Each channel occupies 1Mhz of bandwidth
           Minimum hop rate for a channel is specified by regulatory bodies
                   For USA it is 2.5 hops per second
           Transmitter applies the same code to retrieve the signal
           FHSS supports two data rates 1 & 2 Mbps
   Infrared
           Operates in the 850-to-950nM band with peak power of 2 W.
           Modulation is either 4 or 16-level pulse-positioning modulation.
           Supports two data rates; 1 and 2Mbps.
                      MAC Layer (Functions)

   Beyond standard functions usually performed by MAC layers, 802.11
    MAC performs some typically upper layer functions I.e.
          Fragmentation
          Packet Retransmission
          Acknowledgments
   MAC LAYER - Regular Functions
          Provides Basic access mechanism
          Performs Encryption
   MAC LAYER - Management Functions
          Performs Synchronization
          Performs Power management                 802.2           Data Link
                                                                      Layer
          Supports Roaming                       802.11 MAC

                                             FH    DS        IR       Phy layer


                                         Each BSS has a unique 48 bit address
                                         Each ESS has a variable length address
                    MAC Layer (Access Methods)
   MAC Layer defines two different access methods
          Distribution Coordination Function
          Point Coordination function
   Distribution Coordination Function (DCF)
          It is A basic Access Method
          It is based on CSMA/CA (Carrier Sensed Multiple Access/Collision Avoidance)
          Note: Ethernet is based on CSMA/CD (Collision Detection)
   Point Coordination function (PCF)
          It is an optional function
          It may be used to implement time bounded services like voice or video
   Distinguishing Feature of PCF over DCF
          This function gives AP higher priority
          AP gains high priority because it uses Smaller IFS (inter frame space)
          By using this function, AP issues polling requests to the stations for
           data transmission. Thus it controls the medium access
                                       CSMA
   Principle of CSMA Protocol
          A station desiring to transmit senses the medium,
                  If the medium busy it defers its transmission to later time
                  If the medium free then it transmits
   Efficiency of CSMA
          It is very efficient when the medium is not heavily loaded as it allows
           stations to transmit with a minimum delay
          But there is always a chance of COLLISION, if two station happen to
           sense simultaneously, find the medium free and transmit together
   How CSMA efficiency can be improved?
          Through Collision Detection (CSMA-CD) mechanism
          If collision is detected and MAC retransmits the packet by itself
           (without involving upper layers) that would reduce significant delay
          In Wired LAN collision can be detected by a transmitting station, using
           some algorithm (Exponential Random Backoff to resolve contention)

   What about CSMA-CD in WIRELESS?                 Would it work?
                   CSMA CD and Wireless LAN
   In Wireless LAN Collision Detection can not be used because:
         CD scheme is based on the assumption that::
                        “All the Stations Can Hear Each Other”
         but in Wireless it can not be assumed because:
            If the area around the transmitting station is free, it doesn't
          necessarily mean that the area around the receiver is free as well.
         One way to overcome the problem could be:
        To use full duplex radio capable of transmitting & receiving at once.
         But it would increase the price significantly

   How to overcome these problem?
          802.11 overcomes these problems by a mechanism called:
          COLLISION AVOIDANCE (CA) with POSITIVE ACKNOWLEDGMENT
   CSMA-CA has two steps
          Step-1:-Physical Carrier Sensing (PCS)
          Step-2:- Virtual Carrier Sensing (VCS)
                     CSMA/CA Mechanism With PCS
   Transmitting Station Senses the Medium.
           If the medium busy, it defers
           If the medium free, for a Specified Time, it transmits.
        (Specified time is called DIFS, Distributed Inter Frame Space, in the standard),

   Receiving Station Receives the packet,
           Checks its CRC, &
           Sends an Acknowledgment to the Transmitting Station
   Transmitting Station waits for the Acknowledgement:
           If it is receives ACK, it means no collision occurred,
           If Not, it keeps on retransmitting until
                    Either it gets the ACK
                    Or thrown away after a given number of trials

   But in this process, how the probability of collisions is avoided?
           Lets look at Step-2: Virtual Carrier Sensing (VCS) mechanism
                           CSMA/CA Mechanism With VCS
        VCS is based on the fact that wireless stations can‟t hear each other,
                Transmitting/Source Station
                          First transmits a short control packet RTS Request To Send
                          RTS includes source, destination & duration of the following transaction
                Receiving/Destination station
                          If medium free, responds with response control packet CTS Clear To Send
                          CTS includes the same Duration information
                All other Stations receiving either RTS or CTS will
                          Set their VCS indicator “NAV”, Network Allocation Vector, for a given
                           duration & Use this information (NAV) together with Physical carrier sense
                           when sensing medium
                                                                                          SIFS:- Short Inter frame Space
                                                                                          PIFS:- Point coordinated IFS
                    G1         G1                  G1              G1= SIFS
           G3 RTS                                             G3                          DIFS:- Distributed IFS
Source                                   Data                      G3= DIFS
                                                                                          EIFS:- Extended IFS
Dest                     CTS                            Ack
                                                                        Next MPDU
others                                NAV (RTS)
                                       NAV (CTS)
                                                                    Backoff after Defer
                                    Defer Access
                         CSMA/CA Mechanism
   This mechanism reduces probability of collision
          On Receiver Area
                  because it will hear CTS & reserve the medium as busy until the end
                   of the transaction
          On Transmitter Area
                  because of the the contained duration information
   One more advantage
          RTS/CTS being short frames also reduce the overhead of collisions
           since these are recognized faster (compared to big data packet)




NOTE:-
Standard allows transmission of packets smaller than RTS, without RTS/CTS
   transaction. It is controlled per station by a parameter RTS Threshold.
                    Fragmentation and Reassembly
     Wired LANS handle packets of several hundreds of bytes
             (e.g. Ethernet longest packet could be 1518 bytes long)
     WLANs do not prefer to handle long packets because of:
             Higher Bit Error Rate of radio link is responsible for packet corruption
             Packet corruption probability increases with packet size
             In case of Packet corruption (either because of noise or collision), the
              smallest the packet, the less overhead it causes to retransmit it.
     Thus fragmentation and reassembly is performed
     This is done at MAC layer
             It is considered as a better solution to deal big packets

                                              MSDU




MAC   HDR Frame body CRC         MAC   HDR Frame body CRC         MAC     HDR Frame body CRC
                           Steps to Access the AP
   Access: Station Can Access BSS
           When it Powers Up
           When it awakes from the Sleep Mode
           When it Just enters the BSS area
   Synchronization: Station can synchronize to AP either.
           By Passive Scanning
                    Station waits to receive a Beacon Frame from AP. It is a periodic frame sent by
                     the AP with synchronization information
           OR By Active Scanning
                    Station Sends Probe Request Frames and waits for Probe Response
           Either can be chosen according to Performance & Power Consumption tradeoff
   Beacon Frames
           These frames contain value of AP‟s clock on the moment of transmission. The
            Receiving stations check the value of their clock and correct it accordingly


   Stations need to keep Synchronization
           To keep hopping synchronized        and to     Save power during sleep mode
                                       Security
   Security Holds paramount importance to prevent intruders from
           Accessing the network resources (Authentication)
           Hijack the WLAN traffic (Eavesdropping)
   Authentication Process (performed after synchronization)
           Each (TX & RX) Proves the knowledge of a given password
   WEP (Wired Equivalent Privacy) Security
           Data security is accomplished by a complex encryption technique called WEP
           WEP shared key is based on RC4 algorithm
           It protects transmitted data over the air using 64-bit seed key & RC4 algorithm.
           WEP only protects the data packet information and not the physical layer
            header so that other stations on the network can listen to the control data
            needed to manage the network. However, the other stations cannot decrypt the
            data portions of the packet
   Association Process (performed after authentication)
           Each communicates their capabilities
                    Power Saving – Sleep Mode
   Stations
          In this mode a station sleeps to save its power
          Stations inform AP before entering sleep mode
   Access Point
          Maintains the updated record of the sleeping terminals
          Buffers the packet addressed to them
          Sends periodically (as apart of its Beacon Frames) indication about
           waiting messages
   Stations
          Also Wake up periodically to check these Frames
          If they find any indication of waiting /buffered frame, then they stay
           awake and send a pole message to AP
   Multicast and broadcasts are stored by the AP and transmitted at pre
    known time (DTIM), where all sleeping station (who wish to receive
    this kind of information) should be awake.
                                    Roaming
   Moving from one cell to another without loosing the connection
          WLAN roaming is easier than Cellular roaming because:
                  Transition from cell to cell may be performed during packet
                   transmission (In cellular it is during the conversation)
          WLAN Roaming is more difficult than Cellular roaming because:
                  On voice a temporary disconnection is not noticeable, but in WLAN
                   Performance is effected as upper layer protocols initiate retransmission
          802.11doesn‟t define roaming but basic tools/message formats only I.e.
                  1. Active Scanning, 2. Passive Scanning and 3. Re-association Process
          Everything else is left up to network vendors
   Inter-Access Point Protocol (IAPP)
          IAPP was jointly developed when everything else was left to vendors.
          IAPP was developed by Aironet, Lucent & Digital Ocean.
          IAPP extends multi-vendor interoperability to the roaming function.
          It addresses roaming within a single ESS, between 2/or more ESSs
                          Ad-Hoc Networking
   What is Ad-hoc Network?
          It is a LAN without an Access Point


   How does it work?
          Part of AP‟s functionality is performed by the end-user stations
                  Like Beacon Generation
                  Synchronization
   Functions supported by 802.11 ad-hoc Mode of operation?
          File transfer between two notebooks users
          Coworkers meeting outside the office


   Functions not supported by 802.11 ad-hoc Mode of operation?
          Frame-relaying between two stations not in range
          Power Saving features etc
UPDATES
    WECA (Wireless Ethernet Compatibility Alliance)
   Mission
          To provide certification of compliance with the IEEE 802.11 Standard
          To ensure cross vendor interoperability
   WECA & Security
          The most recent concern about WLAN are related to its SECURITY
          Several security solutions including 802.1x, VPNs and RADIUS have
           been examined. (Report commissioned by IBM 19th October „01)
   NextComm also announced key hopping technique using MD-5
    algorithm (Sept ‟01)
   Companies like ORiNOCO and Agere have deployed some new
    security techniques using DIAMETER, VPN and RADIUS
   A latest survey about 802.11 adoption in corporations
          40% have already implemented & 30% will do so within 18 months
          BT usage is negligible today, expected to raise to 17 % in 18 months
                                    3GPP
   Evolution Workshop (17-18 October 2001)
          Telia, Nortel, DoCoMo, AT&T Wireless, and Nokia, presented different
           proposals
          Nortel Presented the Stand alone data cell concept based on 802.11
          NOKIA said that make AN fully independent to make complementary
           access systems (802.11, HiperLAN 2, BT, ADSL) as a part of 3GPP
          Other companies stressed on the interworking of UMTS and WLANS
          Decision was that 3GPP will focus on completion of existing releases.
   3GPP Plenary Meeting (September 2001)
          Approved a work item on interworking of UMTS and Hiperlan, and
           agreed to send a liaison to IEEE 802.11 and Home link for the same
   HiperLAN (European version of 802.11) developed by ETSI
   The general features
          High-speed transmission (54 Mbit/s) through OFDM
          Automatic frequency allocation
          Quality-of-Service (QoS) support, Security support, Mobility support
          Network & application independence
          Power saving features
                                     3GPP2
   3GPP2
          It seems that no activity is going-on on this issue.
          However 3GPP2 All IP requirement document includes support for
           inter-technology hand-off (3G-WLAN)
   MWIF
          WG-4 will develop architectural requirements to support 802.11,
           cdma2000 and WCDMA interoperability
                                   Industry
   Nokia achieved WLAN roaming for Sonera (Finnish GSM operator)
          No GSM infrastructure is used as a part of the wireless access
          Carrier will setup hotspot in a public areas with std WLAN equipment
           802.11b APs & gateways
          Users would use their SIM card in WLAN card to access hotspot
   Lucent has also announced the solution for 802.11 interworking with
    cellular systems cdma2000, WCDMA, GPRS
          The architecture introduces a protocol gateway
          It will translate various protocols into a single common protocol
          Thus various networks will maintain/use single subscriber profile
   Wayport successfully tested Windows XP in conjunction with 802.11
BUSINESS CASES & APPLICATIONS
               WLAN Applications/Business Cases
   802.11 will be the 1st generation of standard for WLANs
   It will set the pace for the next generation standard, addressing
           The demands for higher performance
           The demand for higher data rates
           The demand for higher frequency bands
   Interoperability between products is another important factor
   These products will be implemented on ISA, or PCMCIA cards for use
    in handheld PCs, PDAs, laptops or desktop applications.
   WLAN applications
           Are currently mostly in vertical markets
           Many horizontal applications will follow as 802.11 infrastructure is
            installed
           It will become more competitive & economical for virtually all
            applications requiring wireless connectivity
   On the horizon, is a need for wireless connectivity at 10Mbps & higher
           High speed & interoperability will give birth to numerous applications
               WLAN Applications/Business Cases
   General Applications
          Streaming video, rapid images transfer, Internet/intranet terminals,
           customer self-scanning services, presentations, high-bandwidth,
           immediate communications for data-intensive applications, and
           Various applications providing Mobility to the users.
   Cost Saving
          Cost saving in wiring offices, factories, educational campuses
   Mobile computing solutions
          Can be provided in Hospitals, Factories, Restaurants, Goods
           Distribution in warehouses, Stock Exchanges, Retail Business
   Tracking
          Monitoring Cards carried by students and Scanner at the bus door can
           allow parent to track school buses on Web site to see if their child
           headed home from school. Student identification number, scanned
           could be transmitted to some cite via WLAN
   Fleet Management
          If used for Public Buses, passengers can find the location/waiting time
           of the bus on their handheld
              WLAN Applications/Business Cases
   Cost Saving in T1-Connection
          WLAN bridging to connect a school, a medical building and a
           distributor to an ISP, bypassing T-1 connection
          In the same way Networking Schools for Distance Learning




                         Graphic from Aironet site
                Can Bluetooth compete with WLAN
   BT is a Cable replacement technology not network technology.
   It Holds limited ability to do handoffs.
        (Some vendors are trying to achieve this ability)
   It faces interoperability issues on PHY and application layer.
        (only few profiles agreed upon between some vendors)
   Communication flow could potentially go directly through the laptop without
    going through AP & corporate firewall, necessitating an increased need for
    "personal firewalls" residing on Laptop.
   Its Security protocols only authenticate the device, not the user. Thus
    additional application level security is needed
   It gives a max speed of 721kbps for data, compared to 11Mbps for 802.11b
    predict a move to 5GHz by 2004 and further higher. Thus not desirable for
    streaming video, or downloading big files from the Internet to a laptop or PDA


   From these arguments it can be inferred that it can not compete with WLAN

								
To top