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CDPD is a secure, proven, and reliable protocol that has been used for several years by law enforcement, public safety, and mobile professionals to securely access critical, private information. CDPD has several features to enhance the security of the mobile end user's data and these are discussed below.
OPERATION OF CDPD
A brief overview of the operation of the CDPD network is as follows:
A wireless modem (or Mobile End System-M-ES) communicates by radio with the Mobile Data Base Station (MDBS). The MDBS transfers this data by landline and microwave to the Mobile Data Intermediate Systems (MD-IS), which processes and sends the information, by Intermediate System gateways (routers), to the appropriate destination.Over the years semiconductor devices have set a new trend in the field of electronics and created waves in the existing technology of memory storage. Flash memorywhich has been in use for long now , is now being replaced by new advents in technology of memory such as Ferro Electric Memory, Magneto resistive Type Memory and Ovonic Unified Memory. This paper contrasts the working of flash kind of memory used in Dynamic - RAMS with that of the other types mentioned above . In brief , advantages over the flash kind of memory are evaluated and justified.
In a world where "nanoseconds " matter a lot , 'the new Technology ' awaits recognition and exposure
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Cellular Digital Packet Data (Cdpd)
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PRESENTED BY
K.S.BHARATH
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CDPD: Cellular Digital Packet Data
ABSTRACT
cellular digital packet data allows high data rate of transfer in a free medium and it allows long distance transmission
INTRODUCTION
Cellular digital packet data is a data transmission technique used to transfer the data in the form of packet in air or free space and limited bandwidth is used for the transfer of data where the mobile unit is served by the MSC
What is CDPD?
required
Analog providers have AMPS allocation.
Use AMPS channelMotivation: Packet data over AMPS
AMPS is unsuited for packet data
Long call setup times
Modem handshaking s to provide data service.
“Cellular digital packet data”
Can’t interfere with existing analog service.
CDPD is cheap: no new spectrum license needed!
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 History
Standard released Jan, 1995 (v1.1)
Initially used by police (~1996)
Wide service availability around 2000
Omnisky, Verizon Wireless, others.
Covers most US population centers
Champaign-Urbana now covered.
Rural area coverage poor.
CDPD Market
CDPD is used primarily for
Law enforcement
Handheld/laptop IP access
Main competition: “Wireless Web” phones.
CDPD costs:
Wireless modems: ~$300 (Omnisky Palm V)
Service: $30-$40 per month (handheld)
$40-$80 per month (laptop)
CDPD Infrastructure
CDPD - Layering
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.
AMPS and CDPD
CDPD runs alongside AMPS
AMPS system is unaware of CDPD system
CDPD system watches AMPS behavior
AMPS generally has unused channels.
Blocked calls when all channels are allocated.
1% block probability => all channels used only 1% of the time.
CDPD Channel Usage
CDPD uses unused AMPS channels.
Usually are several available.
Each 30KHz channel = 19.2kbps up and down
CDPD channel hopping.
Forced: AMPS must be vacated within 40ms of allocation for voice use.
Planned: Regular hops prevent AMPS system from identifying channel as unusable.
Channel Scanning
1. Check signal levels from nearby cells.
Use a list of reference channels distributed by the CDPD infrastructure to find levels.
2. Select cell with best signal.
If non-critical and no cell is significantly better than current, no handoff is done (hysteresis)
3. Scan RF channels in cell for CDPD.
Stop when an acceptable channel is found.
Handoff in CDPD
Critical handoffs: Must choose new channel
High error rate is observed or BS signal lost.
Received signal strength below a threshold.
Base station does not receive data from mobile.
Noncritical handoffs
Channel rescan interval expires.
Signal strength changes significantly.
CDPD effects on AMPS
CDPD logically transparent to AMPS
Can reduce AMPS service quality
More channel usage => increased interference.
If AMPS system is close to SIR margin, CDPD can push it below.
Full CDPD usage can push SIR down ~2dB
19 channels/cell, Pblock = 0.02, 12.3 Erlangs
Limiting channels used reduces SIR cost..
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 page link activity indicated by BS.
Forward Link Structure
Reverse Link Structure
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.
Reverse Link MAC
Reverse Link MAC
Reverse page 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 page link busy:
Delay for a random number of slots.
Check busy status again.
Mobile Data Link Protocol
APPLICATION
Transportation: Ambulance, dispatch system.
Public security: Policemen can access the latest database in real time to. E.g. Check suspected person, Verify identity. Check the car current status.
Remote measurement: Remote control.
Commerce. Wireless ATM. Wireless Credits Card Authentication. Mobile Payment. Adv of CDPD used in commerce field.
Implementing easily.
High speed. CDPD modem set up time is much faster than wire dial up modem. Even CDPD is 19.2kbps, wire dial up modem is 33.6kbps.
Reliable and Safety with strong encryption algorithm.
Personal use which covers the main part of the CDPD usage.
CONCLUSION
Thus by using CDPD the data transmission
Cost will be reduced for the longer distance.
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