Transcript Presentation by Svetoslav Naydenov

CS 556
Packet Data Over Cellular Networks:
The CDPD Approach
Svet Naydenov
Overview
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What is CDPD? CDPD Overview
The Physical Layer
Medium Access Control
Forward Link Structure
Reverse Link Structure
The Medium Access Procedure
Logical Link Control
What is CDPD?
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CDPD (Cellular Digital Packet Data) is a
specification for supporting wireless access
to the Internet and other public packetswitched 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)
CDPD Overview
Overview – Elements of CDPD
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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
Overview – Elements of CDPD
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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.
The Physical Layer in CDPD
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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. The
reverse channel is shared among all M-ESs
communicating with the same MDBS.
The Physical Layer in CDPD
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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.
Medium Access Control (MAC)
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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
Forward Link Strucrure
Reverse Link Structure
Medium Access Procedure
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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.
Medium Access Procedure
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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.
Logical Link Control (LLC)
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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 it’s serving MD-IS.
- broadcast data links are used for point-tomultipoint or multipoint-to-multipoint
communications on a CDPD channel stream.
Logical Link Control (LLC)
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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.
References
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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
URL:http://www.leapforum.org/published/internetworkMobil
ity/one/main.html