A Technical seminar on CDPD: Cellular Digital Packet Data BY K.S.BHARATH 06751A0411

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

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

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 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”

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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