CELLULAR DIGITAL PACKET DATA(CDPD)

ABSTRACT

Cellular Digital Packet Data (CDPD) is a specification for supporting wireless access to the internet and other public data networks. CDPD transmits digital packet data at 19.2 Kbps, using idle times between cellular voice calls on the cellular telephone network. CDPD technology represent a way for law enforcement agencies to improve how they manage their communications and information system.

CDPD technology represent a way for law enforcement agencies to improve how they manage their communications and information systems data transmitted on the CDPD systems travel several times faster than data send using analog networks.

INTRODUCTION

The Cellular Digital Packet Data (CDPD) network is a little over one year old and already is proving to be a hot digital enhancement to the existing phone network. CDPD transmits digital packet data at 19.2Kbps , using idle times between cellular voice calls on the cellular telephone network.

CDPD is an overlay to the existing cellular network, which enables users to transmit packets of data over the cellular network using a portable computing device and a CDPD modem. CDPD offers a high-speed, high-capacity, low-cost system with the greatest possible coverage. Additionally data is encrypted for security. CDPD air link transmissions have a 19,200 bps raw data rate. As a tool for transmitting data CDPD utilizes digital networks.

Design Goals

Goals: Low speed, high latency data service

Primarily intended for paging and email. Provide broadcast and multiple-access

service. Dynamically shared media, always online. Share channels with AMPS allocation Transparency to existing AMPS service

CDPD Infrastructure

CDPD - Layering

Physical

MAC

MDLP

SNDCP

IP/CLNP

Physical

Data link

Network

Transport

Application

Network layer CDPD Layer

Connectionless Network Protocol

Subnetwork Dependant Convergence Protocol

Mobile Data Link Protocol

Media Access Control

Physical

CDPD Physical Layer

30KHz BW channels, shared with AMPS Separate forward and reverse

channels Forward channel is continuous Reverse channel is multiple access.

Gaussian Minimum-Shift Keying-GMSK GMSK compromises between channel

bandwidth and decoder complexity. 19.2kbps per channel.

Physical

MAC

MDLP

SNDCP

IP/CLNP

Data Transmission Format

All links are base to mobile. Mobile to mobile goes through base station. Full-duplex; separate forward and reverse

links. Forward link

Continuous transmission by BS Reverse link

Shared multiple access for mobiles. Reverse link activity indicated by BS.

Physical

MAC

MDLP

SNDCP

IP/CLNP

Forward Link Structure

Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

Reverse Link Structure

Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

Reverse Link MAC

Near/Far problem Mobile may not detect a faraway

transmitter. Base station must report busy status.

Protocol: Digital Sense Multiple Access Nonpersistant: Checks once for busy state. Slotted: Can only start when BS reports

state. Similar to Ethernet MAC.

Physical

MAC

MDLP

SNDCP

IP/CLNP

Reverse Link MAC

Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

Reverse Link MAC

Reverse link idle => can transmit. Busy status checked before transmission

starts.. Continue burst unless error is indicated. If BS indicates error, assume collision;

exponential backoff is used. Reverse link busy:

Delay for a random number of slots. Check busy status again.

Mobile Data Link Protocol

Physical

MAC

MDLP

SNDCP

IP/CLNP

Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

CDPD - MDLP

Mobile Data Link Layer Protocol (MDLP) High-level data link control (HDLC)

Similar to ISDN HDLC. Mobile (M-ES) to Infrastructure (MD-IS)

In this layer, air link and BS become transparent

Connection oriented MDLP Frame (message structure)

Address, control field, information field No checksum; MAC discards incorrect packets.

CDPD - MDLP

Temporary equipment identifier (TEI) Identifies destination mobile - virtual

address. Assigned by infrastructure.

Packet types Unacknowledged information Sequenced information

Sequence number, ack, timeout Sliding window Selective rejection supported.

CDPD - SNDCP

Subnetwork-Dependent Convergence Protocol (SNDCP) Between IP or CLNP and MDLP In both mobile and infrastructure (MD-IS) Segmentation, compression, encryption Questions:

Where and how to segment data? Where and how to compress data?

Physical

MAC

MDLP

SNDCP

IP/CLNP

CDPD - SNDCP

Segmentation Goal: to fit the size of underlying frames Two type of headers

Sequenced headers: For compressed, encrypted, and segmented user

data. Unnumbered headers: Control information.

Efficiency consideration (similar to X.25) Which layer should segment/assemble

messages? Use “More” indicator to avoid IP fragmentation.

Problems with CDPD

Limited bandwidth 19.2kbps shared per channel Modern applications demand more

bandwidth. Security:

“Man in the middle” identity theft attack IP network attacks Denial of Service attacks easy.

References

J. Agosta and T. Russell, CDPD: Cellular Packet Data Standards and Technology, McGraw Hill, 1996.

Y. Frankel et al., “Security Issues in a CDPD Wireless Network,” IEEE Personal Communications, August 1995, pp. 16-26.

D. Saha and S. Kay, “Cellular Digital Packet Data Network,” IEEE Transactions on Vehicular Technology, August 1997, pp. 697-706.

A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach,” IEEE Communication Magazine, June 1999, pp. 152-159.

A. Salkintzis, “Radio Resource Management in Cellular Digital Packet Data Networks,” IEEE Personal Communications, December 1999, pp. 28-36

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