WiFi and WiMax - Break through in Wireless Access technologies by ashi7790


									           WiFi and WiMax                    -   Break through in Wireless Access
                                           Author R.V. DATAR
                                        and Vice Chairman IETE Pune Centre,                            India
          CGM BSNL         (RETD.)
                                            Contact no:       09422526363

                                                                 -oncepts in wireless communication. Ship to shore and
Keywords: WiFi, Spread spectrum, HRDSSS technique,               satellite communications were further enriching wireless data
FHHS,DSSS, OFDM and WIMAX.                                       communication techniques albeit at speeds much slower than
                                                                 in the wired media. But the real motivation for wireless
                                                                 access came with the development of laptops and notebook
Abstract                                                         type PCs. Restless24D7H generation wanted to remain
                                                                 connected to internet whether in office or in transit.
Data in the good old days was only the stepchild of telephone All this led to wireless data communication technologies like
network, the speech being the main child. This scenario Wi-Fi and WiMax which combine the techniques developed
gradually changed with data acquiring important place and for modems, LANs, WANs, MANs with the wireless
this gave rise to access technologies. Data volumes to be technologies. The purpose of this paper is to discuss the
transported were constantly on the rise due to computers various technologies that have gone into high speed wireless
becoming more and more sophisticated and graduating from data communication and led to significantly higher data
mainframe to PCs and necessitating ever growing need for speeds and to look at the IEEE WiFi and WiMax standards.
computer- to- computer communication at higher and higher Incidentally, doing away with jumble of wires at short
bit rates. All this led to wireless data communication distances is resulting in wireless technology being used in
technologies like Wi-Fi and WiMax which combine the other exciting networking applications such as PANS (Blue
techniques developed for modems, LANs, WANs, MANs Tooth and Zigbee), BANS, RFID and HANS. The WiFi and
with the wireless technologies. The purpose of this paper is to WiMax are very ambitious standards aiming at providing
 discuss the various technologies that have gone into high triple play real time services.
 speed wireless data communication and led to significantly
 higher data speeds and to look at the IEEE WiFi and WiMax Wi-Fl:
 standards.                                                       Essentially, Wi-Fi is wireless LAN. It is enabling laptop and
                                                                  notebook users to access internet while in transit, from such
 1 Introduction                                                   places as Airports, Railway stations, university campuses,
                                                                  hotels, shopping malls etc.. once these places are equipped
 Access technologies    or the technologies for accessing a PSTN with WiFi hotspots. A number of companies started making
 or an ISP node have traditionally been of the wired variety, wireless LANS simultaneously with the introduction of
 copper or optical fibre. Data in the good old days was only notebook computers. This created a compatibility problem.
 the stepchild of telephone network, the speech being the main So the IEEE committee which created wired LAN standards
  child. This scenario gradually changed with data acquiring (802.3 Ethernet, 802.5 EDDI etc. ) was entrusted with
  important place and this gave rise to access technologies. creating a wireless LAN standard. In middle of the decade
  Data volumes to be transported were constantly on the rise 1990-2000 the standardization process started. At that time
  due to computers becoming more and more sophisticated and the most widespread wired LAN was Ethernet. The
  graduating from mainframe to PCs and necessitating ever committee therefore decided to make the wireless standard
  growing need for computer- to- computer communication at 802.11 compatible with Ethernet above data link layer. The
  higher and higher bit rates. Modems were introduced to adapt objective was to be able to send an IP packet over the wireless
   digital computer data to analogue speech band so that the data LAN just as in a wired LAN from one computer to another.
   could be taken to the nearest telephone exchange. Modems The standard however had to necessarily differ from Ethernet
   with evermore sophistications were developed. And as the in the lower layers, namely the data link layer and physical
   cable arteries and routes in the large cities became more and layer due to different types of issues faced in wireless
   more congested, the need was felt for wireless access. As if communication.
   this was not sufficient, there was demand for higher data bit The initial standard IEEE 802.11 was at 1Mbps and came in
   rates to carry triple play services, streaming videos due to 1997. By 1999, 802.11 a (54Mbps at wider frequency band),
   competition.                                                     802.11 b (11I Mbps, same frequency band but a different
   In parallel with the above, the wireless communication was modulation technique) and 802.11 g (using modulation
   developing on its own. Defence and the two World Wars had technique of 802.11 a but frequency band of 802.1 lb). Table 1
   contributed to techniques of encryption and spread spectrum

    summarizes the various WiFi versions. These versions were
    developed by different groups, not necessarily                       I 6bit grey code. infrared signal cannot penetrate walls and
                                                                 in      gets swamped by sunlight and hence not very popular.
    chronological order, but, more or less concurrently.
    In the physical layer various modulation methods                     HRDSSS technique (used in 802.11lb to achieve 11IMbps)
                                                                or       uses 1 Imillion chips/sec in the 2.4 GHz ISM band. 802.11 b
    transmission techniques are specified. The transmission
    techniques specified in 802.11 are infrared (.85 or.95               also provides lower rates like 1,2 and 5.5 apart from 11IMbps.
    microns), FHSS (frequency hopping spread spectrum)                   Lower rates use I and 2 bits per baud where as, higher rates
                                                              and        use 4 and 8 bits/baud together with Walsh! Hadamard codes.
    DSSS (direct sequence spread spectrum).Whereas 802.11
                                                                  a     The same multiple bits per baud concepts with further
   and8O2.l1lg use OFDM (orthogonal frequency division
   multiplexing), 802.11lb uses HRDSSS (high rate direct                encoding refinements seem to have evolved from wired
   sequence spread spectrum). Let us briefly discuss                    technologies to wireless technologies.
                                                              the       It may be recalled that link layer in 802 protocols unlike
   techniques mentioned above.                                                                                                           in
   Spread spectrum is an increasingly important form                    OSI protocols is split into two or more sub layers. The MAC
                                                               of       sub layer protocol in wireless domain is much more complex
   communication. Multi path fading, security, noise immunity
   and encryption are very essential issues in wireless                 than that of Ethemnet.802. II.It does not use CSMA/CD ,
                                                             data                                                                     but
   communication and these are effectively addressed by using           uses CSMA/CS in DCF(Distributed Coordination Function)
   either FHSS (Frequency Hopping Spread Spectrum) or DSSS              and optionally PCF(point coordination function). When DCF
   (Direct Sequence Spread Spectrum).                                   is employed it uses a protocol known as CSMA/CS while
    In FHSS, the signal is broadcast over nearly random series          PCF, the base station controls everything using a polling
  of radio frequencies hopping from frequency to frequency              procedure in which a beacon frame is sent periodically (10
                                                               at                                                                       to
  fixed intervals (several times in a second). Receiver hops            100 times per second) containing system parameters and asks
                                                               in       new stations to sign up polling service.
  synchronism with transmitter. The dwell time at
                                                            each       To deal with the problem of noisy channels, where the
  frequency and the random hopping sequence are variable and
  unless the hawker knows both, the hacking is impossible. The         probability of one full frame being received is very low
  effect of multi path fading is minimal because direct signal         802.11 allows fragmented frames with their own check sum.
  always arrives earlier and by the time the reflected signals         DCF and PCF can coexist due to well-defined frame intervals.
  from other paths arrive the receiver would have hopped to            The upper sub layer of the link layer, namely the logical link
 other frequency.                                                      control sub layer in 802.11, essentially hides the differences
 In DSSS, each bit in original signal is represented by multiple       between different types of physical layers, and makes them
 bits in the transmission signal using a spreading code.               indistinguishable as far as the network layer is concerned.
 Spreading code spreads the signal across a wider frequency            FigI shows the topography of Wi-Fi implementation. A no.
 band, in direct proportion to the number of bits used in              WAPs (wireless Access Points) can be connected by OFC
                                                             the                                                                       to
 spreading code. Spreading code is a bit stream generated             the base stations. The base station can be then connected
                                                           by a                                                                       to
 PN generator which is combined with data stream using XOR            the backhaul network. Fig2 shows a step by step
 gate. The spreading of signal reduces the effect of multi path       authentication process with the help of a RADIUS (Remote
 fading. This form of spread spectrum has good spectral               Authentication Dial In User Services) server which
 efficiency and immunity from noise.                                  provides an encryption key to the user for the duration of
 OFDM spreads the data to be transmitted over a large number          802.11In: is a very recent version of WiFi. It promises
 of carriers in the available bandwidth. These carriers               carry data farther and faster than its predecessors and suited
                                                         are          for multimedia transmission.
 orthogonal to each other (that is perpendicular in
                                                            a               a Works on.2.4 and 5GHz
 mathematical sense) and hence non interfering. The
 orthogonality allows compact spacing and therefore high                    a Speeds up to 540Mbps
 spectral efficiency as compared to the FDM. Using multiple                 0 Covers 200 feet or 60 meters(indoors)
 narrow bands for signal provides immunity to narrow band                   * Backward compatible with WiFi standards
 interference and makes it possible to use non contiguous                   * Supports all major platforms from consumer
 bands. OFDM provides efficient spectrum utilization, as the                     electronics to personal computing
 frequency bands can be overlapping. Combats Inter Symbol                   * 802.11In envisages powerful MIMO (i.e. multiple
 Interference and provides robustness against fading.                           input multiple output, smart antenna technology)
 The OFDM in 802.11Ia uses 52 frequencies. A complex                            radio links with multiple antennas at the transmitter
encoding system uses phase shift modulation up to 18Mbps,                       and receiver side. MIMO transmits two or more
and QAM above that. Incidentally OFDM is also used                              unique data streams In the same radio channel at the
ADSL broadband and is emerging as a very effective                              same time whereby the system delivers two or more
technology for broadband transmissions, wired or wireless.                      times the data rate per channel.
DSP techniques using IIFFT and FFT chips has simplified the
generation of the OFDM signals.                                          Wi-Fl vs. Blue Tooth: they are not exactly competitors
                                                                         but they share the same frequency band and can interfere
The infrared option uses diffused infrared transmission                  with each other if not carefully designed. Wifi uses one
1Mbps and 2Mbps. It encodes in groups of 4 bits (or 2bits)               of the 12 overlapping channels of 22 MHz each in the
                                                                         2.4Ghz ISM band, and Bluetooth frequency-hops among

                                                                     iariable bit rate non real time service for heavy file
   79-1 MHz channels, evenly spaced across the frequency             ransfers -polling type, but with contention in multicast
   bands. As a result, no matter which channel of wifi is int         ~roup. And lastly the -
   use, a risk exists of interference between the two and of         3est efforts service for every thing else. This service is non
   lowering data throughput of both.                                  )olling type and allocated to subscribers on contention basis
   Bluetooth has reduced the problem by introducing AFH              inlike the above two types.
   (adaptive frequency hopping), to some extent, by                  The security sub layer uses the cryptography techniques to
   skipping the active wifi channels in the hopping                  encrypt the frame payload but not the headers.
   sequence. But in emerging applications like voice over            rhe 802.16 utilizes the millimetre waves, in the 10-66 GHz
   WLAN, this solution may not prove adequate esp. over              band and is strictly line of sight. This was the initial version.
   short distances. At distances over l0meters the effect is          But the subsequent versions, developed due to competition,
                                                                     added 2 to 11GHz range (because this range includes a
   (Ref: Electronics Today Tech News Oct. 2005).                     number of unlicensed bands, the most well known being the
                                                                     ISM 2.4 GHz band). 802-16a Extni. is NLOS . 802-2004 is
                                                                     mobile and supports chl sizes between 1.5 MHz and 20 MHz
Wireless Inter operability for Microwave Access (WIMAX) capable of transmit power control and flexible channel
is a wireless MAN. While WiFi successfully provides bandwidths as well as re use of channels. A futuristic
wireless access to computers over a short range, it can not topography of WIMAX deployment is given in fig3.
serve over long distances. IEEE 802.16 provides a standard Present status in India: WIFI is already being commercially
for a wireless MAN which can be used over several provided with several hundred hotspots and WIMAX is being
kilometres in remote as well as urban areas. It was initially tasted on trial basis and its use in urban as well as rural
developed to provide a fixed wireless service to users having scenarios is being studied. The claims of distances covered in
space for line of sight antennae and wish to access internet various 802.16 WIMAX versions and its capacity to
and avail triple play services on wireless with QOS simultaneously serve a number of end users needs to be
 guarantees. But the latest versions have come up with non carefully evaluated, as the technology is of a very recent
 line of sight capabilities. The service is under trial in different origin.
 parts of the world including India.
 Table2 lists the various 802.16 versions.                             Future: in the short range wireless LAN arena other
 As against Wil'i the WIMAX standard is required to make applications like PAN (Blue tooth or 802.15 and Zigbee) and
 special provision for fading problems over longer distances BAN (body area networks for medical applications in human
 and take care of privacy and security requirements which body as well as, for use along with wearable computers) are
 become more vulnerable in wireless communication over being developed. These applications will eliminate the
  longer distances. It also provides for seamless convergence jumbled up wired connections apart from adding to human
  between connection oriented and connectionless protocols. comforts. HANS (home area networks) is a new branch that
  For this purpose a special security sub layer has been may enable remote and integrated control of all domestic
  provided as a part of link layer and a transmission appliances ranging from refrigerators to washing machines
  convergence sub layer has been incorporated in physical and microwave ovens, apart from providing baby monitoring
  layer. One special feature of 802.16 is the capability for through web cameras.
  flexible need based allocation of downstream channels to the A company has developed a Wi-Fi positioning system WPS
  WIMAX subscriber.
                                                                       that uses wifi instead of gps and claims it will be simpler and
  Hamming codes are used to do forward error correction in less expensive in certain environments. A WiMax standard
  physical layer apart from checksumns in the higher layers. IEEE 802.1 6e has recently been approved that will enable
   802.16 provide 3 different modulation schemes depending on subscribers to stay connected to Internet when moving
  the distance. These are, QAM64 with 6 bits per baud for between cells. A 3.5G mobile broadband service based on this
   nearby subscribers, QAM 16 at medium distance with standard is being developed for use by travelling subscribers
   4bits/baud and QPSK with 2bits per baud for longer on train etc., by two telecom giants and being tested on a
   distances. Here again we sec the bits/baud technique European train route. The technology called Wi Bro claims
   developed for wired media being gainfully utilized in the theoretical download speeds of 10 Mbps. While travelling at
   wireless world.
                                                                        75 miles an hour. (Ref: Electronics Today, Feb. 2006, Tech
   The three MAC sub layers, starting with the lowest, are, news)
   security, common part and service oriented sub layers A WIMAX standard 802.16-2004 based integrated WIMAX
   respectively. A major difference from 802.11 is that 802.16 is system has been developed that uses OFDM, and supports
   connection oriented and each time it establishes a connection, channels from 2 to 20 MHz band in the 2 to 11I GHz licensed
   it associates one of the following four types of quality of and unlicensed bands. When applying 64-QAM modulation in
                                                                        20 MHz channel and using all 192 sub carriers, peak data
    Constant bit rate service -used forT 1 channel (UN speeds of 100Mbps are claimed to have been achieved. A 256
    compressed voice) fixed time slots automatically allotted.          point FFT is used for broadband transmission. (Ref.
    Variable bit rate real time service used for compressed Electronics Today Aug 2005).
    multimedia service and other soft real time applications -
    polling type

 It is also thought that WIMAX may prove to be an alternative
way to achieve 4G goals. Competition has driven WIMAX to
develop mobile in addition to fixed wireless and a
combination of OFDM with WIMAX may provide a smooth
path to mobility that is required by service operators.(Ref:
Article by Dr. MV Pitke in Electronics Today
Oct. 2006).
Wireless Access technologies have thus not only arrived but
are already moving towards an era of convergence in a not                                                      45k t.,,i   NINES,

too distant future.
                                                                                           ~                                        WSNS.SS -


                        TABLE 1: WI-Fl                                            .5.5*. b..N 5N~N
                                                                                                                                       M.~Efr5 51.5.

      *    Used as a wireless LAN essentially meant for-
           mobile laptops accessing via hotspots.                                                                  NI       *NPflIVITI
                                                                                                    555W NIN PASSPWI IINNSNNNT
                                                                                                        Ii     N

      o    Hotels, airports, campuses, office bldgs.,            Figi: Network Diagram for WI-Fl Internet Connectivity
           restaurants. More than 500 hotspots in India.
      o    IEEE 802.1llb, 802.1 la and 802.1lg
      o    2.4 Ghz./IlJMb-ps! 30feet/DSSS: 802.11lb
      o    5 GHz/54Mbps/lI feet/OFDM. 8-02.11 a
      o    2.4 GHz/54Mbps/2OfeetIOFDM: 802.11g

                       Table 2: WI-MAX

      o    Wireless Inter Operability for Miicrowave
          Access.                                                          SS.5.,5.sNp N       N

   o       Used as MAN, essentially        ýmeant
                                             for fixed
                                                                 4.55 I 5..I.SAN.,4 1555.5
                                                                                  WE?    55~WsI5.

          wireless       applications      (Millimetre                                              NtIS RADIUS
          wavelength). But recent version has added                                                                   N

          mobility due to cornpetition.
   "      Range 50 kms. Convergence & security sub
          layers take care of spl. Requirements of
                                                                                      Fig2: RADIOUS Authentication
          MAN. Employs OFDM for modulation, data
                                                                                            TABLE 2: Wi-MAX
          rates up to 75Mbps.per base stn.

   o        Seamlessly integrates Datagram protocols
           (PPP, IP, and Ethernet) and Connection
           oriented protocols (e.g. ATM).
  o        Provides for FDD and TDD (in a flexible-
  o        IEEE 802.16a (2tol6Ghz) and 802.16b (5
  o        IEEE 802.16: 10 to 66GHz, LOS
  o        IEEE8O2.16a: added 2tol I GHz range of
           which      many     parts  are   unlicensed
           internationally                                                                                -f--------------

  o        IEEE 8 02.16a Extn: NLOS.
  o       IEEE8O2. 16-2004:Flexible       RF        chl
          bandwidths, Re use of frequency chls,
          Transmit power control, channel sizes
          between 1.5MHz and 20MHz.
  o       50kms represents theoretical maximum
          ideal circumstances.                   under]                        Fig 3: WI-MAX Last mile Broadband

    1. Computer Networks by Andrew S. Tannenbaum
    2. Telecommunications Journal Special issue on
        Access Technologies, BSNL
    3. NCBCS 2006 Tutorial Article on OFDM by Dr.
        M.S. Sutaone, College of Engineering Pune
    4. i.t. magazine feb 2007
    5. NCBCS 2006 Tutorial Article on Modulation and
        Access     Techniques     by    R.V.Datar CGM


To top