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CDPD tutorial-CDPD features,CDPD network architecture

This CDPD tutorial covers CDPD (Cellular Digital Packet Data) features, CDPD network architecture, CDPD advantages and disadvantages.

Introduction: As we know AMPS (Advanced Mobile Phone System) cellular network has been developed for voice communication in 1980s. Due to long established setup times and modem handshaking requirement packet data communication was not supported by AMPS. In order to support packet data, CDPD has been introduced in 1993.

CDPD stands for Cellular Digital Packet Data. Though it has its own infrastructure it utilizes vacant AMPS assigned channels or gaps between the channels for packet communication. CDPD architecture is explained below. It co-exist with AMPS network architecture. Hence it supports both data and voice communication.

CDPD technology features

Following are the features of CDPD (Cellular Digital Packet Data).
• Communication between BS (Base station i.e. MDBS) and MS (i.e. M-ES) is full duplex.
• It utilizes or seizes 30 KHz channel from existing AMPS/GSM networks for transmitting data at 19.2 Kbps. The net data rate is 9.2 Kbps due to large amount of overhead.
• It utilizes same frequency band as used by AMPS i.e. 824 MHz and 894 MHz for uplink and downlink. Both uplink and downlink are separated using FDMA.
• There are two types of handsets or phones available viz. single mode and dual mode. In single mode phone can initiate data call or voice call. In dual mode phone can switch between data call and voice call due to simultaneous registration on both AMPS and CDPD networks.
• It utilizes DSMA (Digital Sense Multiple Access) technique. In this technique CDPD mobile checks for flag bit in downlink channel which informs whether uplink slot is idle or busy. If idle, it utilizes for transmission. If busy, it waits for random period instead of transmitting in the next time slot.
• It supports different types of services and has access to internet backbone.

CDPD network architecture

CDPD network architecture
Fig-1: CDPD network architecture

The figure depicts CDPD network architecture. As shown it consists three major system elements M-ES, MDBS and MD-IS. Moreover CDPD co-exist with AMPS network and hence will have fall back to AMPS for voice call. Figure shows AMPS-BS and AMPS-MSC which are part of AMPS network connected with PSTN for voice connectivity.

There are three CDPD interfaces viz. E-interface, I-interface and A-interface. E-interface exists between CDPD and fixed network external to CDPD. I-interface exists between two CDPD networks. A-interface exists between BS and MS. It is also known as Air interface.

Let us understand network elements used in CDPD architecture.
M-ES: It functions similar to subscriber or mobile unit used in any cellular system. It requires SIM for operation which can be housed in laptop, mobile or PDA. It interfaces with radio equipment at 19.2 Kbps. Each M-ES has unique NEI (Network Equipment Identifier) which is associated with its home MD-IS.
MDBS: It functions similar to Base Station. It broadcasts available channels for M-ES. It takes care of radio activities such as channel allocation, usage etc. These MDBSs co-exist with AMPS Base Stations and hence share the same antenna and site together.
MD-IS: It provides connectivity with internet and PSDN. It has functionalities of both frame relay switch and packet router. It does buffering of packets routed for M-ES. It also supports roaming management as it contains registration directory.

CDPD advantages and disadvantages

Following are the advantages of CDPD:
• Utilizes existing channels of the AMPS network and hence easy to install and start using the existing channels if not in use.
• It has cellular like architecture and hence can support larger capacity due to ease in upgrading the network.
• There is no delay in establishing data call as CDPD phone is already registered with the CDPD network.

Following are the disadvantages of CDPD:
• There is no mesh connectivity in CDPD. Hence M-ES and M-ES can not communicate directly. Communication between them occurs via MDBS.
•  CDPD cell size is limited to less than 10 miles.

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