Packet Data Over Cellular Networks: The CDPD Approach Svet Naydenov

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Packet Data Over Cellular NetworksThe CDPD
ApproachSvet Naydenov

• CS 556

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Overview

• What is CDPD? CDPD Overview
• The Physical Layer
• Medium Access Control
• Forward Link Structure
• Reverse Link Structure
• The Medium Access Procedure
• Logical Link Control

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What is CDPD?

• CDPD (Cellular Digital Packet Data) is a
  specification for supporting wireless access to
  the Internet and other public packet-switched
  networks. Cellular telephone and modem providers
  that offer CDPD support make it possible for
  mobile users to get access to the Internet at up
  to 19.2 Kbps .
• CDPD support both IP and CLNP (Connectionless
  Network Protocol)

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CDPD Overview
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Overview Elements of CDPD

• M-ES consists
• - mobile terminal (PC, PDA, etc)
• - CDPD radio modem attached to the mobile
  terminal and manages the radio link and protocols
• IS (Intermediate Systems) are two types
• - generic IS simple router without knowledge of
  CDPD and mobility issues
• - MD-IS mobile data IS is a specialized IS that
  provides switching, accounting, registration,
  authentication, encryption, and mobility
  management functions

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Overview Elements of CDPD

• MDBS-mobile data base stations
• - passes data between M-ESs and MD-IS and does
  not perform networking functions
• - creates and manages the air interface the
  M-ESs and the CDPD backbone.

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The Physical Layer in CDPD

• The Physical Layer in CDPD is a functional entity
  that accepts a sequence of bits from the Medium
  Access Control (MAC) layer and transforms them
  into a modulated waveform onto a physical 30kHz
  RF channel
• Communication between an MDBS and a M-ES take
  place over a pair of RF channels - forward and
  reverse channels.
• The forward channel accommodates transmissions
  from the MDBS to M-ESs.
• The reverse channel accommodates transmissions
  from the M-ESs to MDBS. Thereverse channel is
  shared among all M-ESs communicating with the
  same MDBS.

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The Physical Layer in CDPD

• The physical layer communicates with Radio
  Resource Management Entity, which tunes the
  physical layer to a specific RF channel pair,
  sets the transmission power level to a desired
  level, measures the received signal level of an
  RF channel and estimates its potential to offer
  acceptable communication.

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Medium Access Control (MAC)

• MAC is a functional entity logically operating
  between the physical layer and link layer control
• Purpose of MAC to transport information (link
  protocol data units - LPDU) between peer link
  layer control entities across the CDPD air
  interface.
• MAC provides the following services
• - encapsulates LPDUs into frame structures to
  establish the LPDU boundaries
• - encodes LPDUs to provide error protection
• - detects and corrects bit errors within
  received frames

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Forward Link Strucrure
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Reverse Link Structure
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Medium Access Procedure

• An M-ES can access the reverse channel using
  digital sense multiple access with collision
  detection (DSMA/CD) algorithm.
• DSMS/CD make use of two flags
• - busy/idle flag is a 5-bit sequence transmitted
  on the forward channel once every 60 bits,
  providing binary info whether the reverse channel
  is busy/idle.
• - decode status flag is a 5-bit sequence
  indicating whether the MDBS has decoded the
  preceding block.
• On successful decoding (no decoding errers) the
  flag is 00000, on unsuccessful decoding 11111.

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Medium Access Procedure

• Assume M-ES wants to transmit data. It first
  senses the busy/idle flag, which needs to be idle
  so transition can take place. If the channel is
  found busy, the E-MS waits a random number of
  seconds and than repeats the sensing of the flag.
• After an M-ES has started a transmission, it
  checks the decode status flag in every forward
  channel back it receives and resumes or suspends
  transmission depending on the value of this flag.

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Logical Link Control (LLC)

• The purpose of the LLC layer is to transport
  information between network layer entities across
  the CDPD air interface.
• Data links connection may be either
  point-to-point or broadcast.
• - point-to-point data links are used to
  transport info between single M-ES and its
  serving MD-IS.
• - broadcast data links are used for
  point-to-multipoint or multipoint-to-multipoint
  communications on a CDPD channel stream.

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Logical Link Control (LLC)

• There are two operation modes for information
  transfer within a data link connection
• - unacknowledged mode is used for information
  transfer on a broadcast channel, which makes
  transmission unreliable (no error or flow control
  mechanisms are used).
• - acknowledged mode is used for information
  transfer on a point-to-point data link channel.
  This channel also supports unacknowledged mode
  for information transfer.

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References

• A. Salkintzis, Packet Data over Cellular
  Networks The CDPD Approach, IEEE Communication
  Magazine, June 1999, pp. 152-159.
• Mark S. Taylor, William Waung, Mohsen Banan,
  Internetwork Mobility The CDPD Approach, June
  11, 1996 URLhttp//www.leapforum.org/published/i
  nternetworkMobility/one/main.html