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Wireless Data Networks

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					   First generation cellular systems that provide data
    communications using circuit switching have
    difficulty passing modem signals through the audio
    filters of receivers designed for analog, FM,
    common air-interfaces.
   Inevitably, voice filtering must be deactivated
    when data is transmitted over first genera-tion
    cellular networks.
   Wireless data networks are mainly divided into two
    categories:
   Individual mobile data
   Shared/overlay mobile data
   CDPD is a data service for first and second
    generation U.S. cellular systems and uses a full 30
    kHz AMPS channel on a shared basis.
   CDPD provides mobile packet data connectivity to
    existing data networks and other cellular systems
    without any additional bandwidth requirements.
   CDPD directly overlays with existing cellular
    infrastructure and uses existing base station
    equipment, making it simple and inexpensive to
    install.
   CDPD does not use the MSC, but rather has its own
    traffic routing capabilities.
   CDPD occupies voice channels purely on a
    secondary, non interfering basis, and packet
    channels are dynamically assigned (hopped) to
    different cellular voice channels as they become
    vacant, so the CDPD radio channel varies with
    time.
   As with conventional, first generation AMPS, each
    CDPD channel is duplex in nature.
   The forward channel serves as a beacon and
    transmits data from the PSTN side of the network,
    while the reverse channel links all mobile users to
    the CDPD network and serves as the access
    channel for each subscriber
 Collisions may result when many mobile users
  attempt to access the network simultaneously.
 Each CDPD simplex link occupies a 30 kHz RF
  channel, and data is sent at 19,200 bps.
 Since CDPD is packet-switched, s large number of
  modems are able to access the same channel as
  needed, packet-by-packet basis.
 GMSK BT=0.5 modulation is used in CDPD
CDPD Transmission
 CDPD transmissions are carried out using fixed-
  length blocks.
  User data is protected using a Reed Solomon (63,47)
   block code with 6-bit symbols.
 For each packet, 282 user bits are coded into 378 bit
   blocks, which provide correction for up to eight
   symbols.
Protocols used in CDPD
 Two lower layer protocols are used in CDPD.
 The mobile data link protocol (MDLP) is used to convey
   information between data link layer entities (layer 2
   devices) across the CDPD air interface.
 The MDLP provides logical data link connections on a
   radio channel by using an address contained in each
   packet frame.
   The MDLP also provides sequence control to
    maintain the sequential order of frames across a
    data link connection, as well as error detection
    and flow control.
   The radio resource management protocol (RRMTh)
    is a higher, layer 3 protocol used to manage the
    radio channel resources of the CDPD system and
    enables an M-ES (the mobile and system) to find
    and utilize a duplex radio channel without
    interfering with standard voice services.
   The RRMP also handles channel hopping
    commands, cell hand- offs, and M-ES change of
    power commands.
Protocols                    MDLP, RRMP, X.25
Channel Data Rate 'bps;      19,200
Channel Bandwidth (kHz)      30
Spectrum Efficiency (b/Hz)   0.64
Random Error Strategy        cover with burst protect
Burst Error Strategy         RS 63,47(6 bits per symbol)
Fading Performance           withstands 2.2 ms fade
Channel Access               slotted DSMA/CD .
   Advance Radio Data Information Systems (ARDIS) is
    a private network service provided by Motorola
    and IBM.
   It is based on MDC 4800 and RD-LAP (Radio Data
    Link Access Procedure) protocols developed at
    Motorola
   ARDIS provides 800 MHz two-way mobile data
    communications for short-length radio messages in
    urban and in-building environments, and for users
    traveling at low speeds.
   ARDIS has been deployed to provide excellent in-
    building penetration.
   When a mobile sends a packet, many base
    stations which are tuned to the transmission
    frequency attempt to detect and decode the
    transmission, in order to provide diversity reception
    for the case when multiple mobiles contend for the
    reverse link.
        Protocol                  MDC 4800                    RD-LAP

       Speed (bps)                   4800                      19,200

Channel Bandwidth (kHz)               25                         25

Spectrum Efficiency (b/Hz)            0.19                      0.77
  Random Error Strategy      convolutional 1/2, k=7   trellis coded modulation,
                                                               rate = 3/4

   Burst Error Strategy        interleave 16 hits        interleave 32 bits

   Fading Performance        withstands 3.3 ms fade    withstands 1.7ms fade

     Channel Access                 CSMA                     slot CSMA
 General Packet Radio Service (GPRS) is a
  packet-based data network.
 GPRS is well suited for non-real time
  internet usage, including the retrieval of
  email, faxes, and asymmetrical web
  browsing.
 GPRS supports multi-user network sharing
  of individual radio channels and time
  slots.
 Similar to the CDPD the GPRS standard
  provides a packet network on
  dedicated GSM or IS-136 radio channels.
 GPRS retains the original modulation
  formats specified in the original 2G TDMA
  standards
 But it uses a completely redefined air
  interface in order to better handle
  packet data access.
 GPRS subscriber units are automatically
  instructed to tune to dedicated GPRS
  radio channels and particular time slots
  for always on access to the network.
 When all eight time slots of a GSM radio
  channel are dedicated to GPRS, an
  individual user is able to achieve as
  much as 171.2 kbps
 So per slot data rate is 21.4 kbps
 As is case of any packet network , the
  data throughput experience by as
  individual GPRS user decreases
  substantially as more users attempt to
  use the network
 Or as propagation conditions become
  poor for particular use
 No new base station RF hardware is
  required.
 Implementation  of GPRS just requires
 the GSM operator to install new
 routers and internet gateways at the
 base station, along with new software
 that redefines the base station air
 interface standard for GPRS channels
 and time slots.
   EDGE is a more advanced upgrade to the
    GSM standard and requires the addition of
    new hardware and software at existing base
    stations.
   EDGE introduces a new digital modulation
    format , 8-PSK (octal phase shift keying), which
    is used in addition to GSM’s standard GMSK
    modulation.
   EDGE allows for nine different air interface
    formats , known as multiple modulation and
    coding schemes (MCS), with varying degrees
    of error control protection.
   Each MCS state may use either GMSK (low data
    rate) or 8-PSK (high data rate) modulation for
    network access, depending on the operating
    conditions.
   EDGE uses the higher order 8-PSK modulation and
    a family of MCSs for each GSM radio channel time
    slot so that each user connection may adaptively
    determines the best MCS setting for the particular
    radio propagation conditions and data access
    requirements of the user
   This adaptive capability to select the best air
    interface is called incremental redundancy.
 In Incremental redundancy packets are
  transmitted first with maximum error protection and
  maximum data rate throughput, and then the
  subsequent packets are transmitted with less error
  protection and less throughput, until the link has an
  unacceptable outage or delay.
 Rapid feedback between the base station and the
  subscriber unit then restore the previous
  acceptable air interface state.
 Incremental redundancy insures that the radio link
  for each user will quickly reach a condition which
  uses a minimum overhead as a result user capacity
  on network is maximized.
   When EDGE uses 8-PSK modulation without any
    error protection, and all 8 time slots of a GSM radio
    channel are dedicated to a single user, a raw
    peak throughput data rate of 547.2 kbps can be
    provided.
   In practice , the slotted schemes used in EDGE
    when combine with practical network contention
    issues and error control coding requirements, limits
    practical raw data rates to bout 384 kbps for a
    single dedicated user on a single GSM channel.

				
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