Understanding CDPD Architecture in Wireless Communication

Introduction

As we know, the AMPS (Advanced Mobile Phone System) cellular network was developed for voice communication in the 1980s. Due to long-established setup times and modem handshaking requirements, packet data communication was not natively supported by AMPS.

In order to support packet data, CDPD was introduced in 1993. Cellular Digital Packet Data (CDPD) is a technology that revolutionized wireless data communication by allowing data to be transmitted over existing cellular networks.

CDPD was developed to facilitate efficient and reliable data transmission, leveraging the infrastructure of analog cellular networks.

This tutorial delves into the key features, network architecture, applications, advantages, and disadvantages of CDPD.

What is CDPD (Cellular Digital Packet Data)?

CDPD stands for Cellular Digital Packet Data. Though it has its own infrastructure, it utilizes unused channels or gaps between the channels in an analog cellular network for packet communication.

CDPD architecture co-exists with the AMPS network architecture.

It supports both data and voice communication, making it a cost-effective solution for mobile data services. By utilizing existing cellular infrastructure, CDPD provided a significant boost to mobile data communication without requiring major network overhauls.

Key Features of CDPD

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 a 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 the large amount of overhead.
• It utilizes the 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: single mode and dual mode. A single-mode phone can initiate a data call or voice call. In contrast, a dual-mode phone can switch between data calls and voice calls due to simultaneous registration on both AMPS and CDPD networks.
• It utilizes the DSMA (Digital Sense Multiple Access) technique. In this technique, the CDPD mobile checks for a flag bit in the downlink channel, which informs whether the uplink slot is idle or busy. If idle, it utilizes it for transmission. If busy, it waits for a random period instead of transmitting in the next time slot.
• It supports different types of services and has access to the internet backbone.

CDPD Network Architecture

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

There are three CDPD interfaces: E-interface, I-interface, and A-interface.

• The E-interface exists between CDPD and a fixed network external to CDPD.
• The I-interface exists between two CDPD networks.
• The A-interface exists between BS and MS. It is also known as the Air interface.

Let us understand the network elements used in the CDPD architecture.

• M-ES: It functions similarly to a subscriber or mobile unit used in any cellular system. It requires a SIM for operation, which can be housed in a laptop, mobile, or PDA. It interfaces with radio equipment at 19.2 Kbps. Each M-ES has a unique NEI (Network Equipment Identifier) which is associated with its home MD-IS.
• MDBS: It functions similarly to a 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 the internet and PSDN. It has the functionalities of both a frame relay switch and a packet router. It does buffering of packets routed for M-ES. It also supports roaming management as it contains a registration directory.

CDPD Applications

Following are the applications of CDPD.

• Mobile Internet Access: CDPD enabled early mobile internet access, allowing users to browse the web, check emails, and access online services on the go.
• Telematics: CDPD was widely used in telematics applications, including vehicle tracking, fleet management, and remote monitoring of assets.
• Point of Sale Systems: CDPD facilitated wireless point of sale (POS) systems, allowing businesses to process transactions from virtually any location.
• Remote Monitoring and Control: CDPD was utilized for remote monitoring and control of various systems, such as utility meters, vending machines, and industrial equipment.
• Emergency Services: CDPD provided reliable data communication for emergency services, enabling quick and efficient coordination during critical situations.

Advantages of CDPD

Following are the benefits or advantages of CDPD:

• It utilizes existing channels of the AMPS network and hence is easy to install and start using the existing channels if not in use. By leveraging existing cellular infrastructure, CDPD reduces the need for additional investments in network hardware, making it a cost-effective solution for mobile data services.
• The use of packet-switched technology and idle channels enhances network efficiency, allowing more data to be transmitted without impacting voice communication.
• It has a cellular-like architecture and hence can support larger capacity due to ease in upgrading the network.
• There is no delay in establishing a data call as the CDPD phone is already registered with the CDPD network.
• CDPD incorporates error correction mechanisms to ensure data integrity and reliability during transmission. This feature is crucial for maintaining high-quality data communication in mobile environments.
• CDPD includes encryption and authentication features to protect data transmission from unauthorized access and ensure secure communication.
• CDPD supports seamless handoffs between cells, allowing uninterrupted data communication as users move across different coverage areas.

Limitations of CDPD

Following are the limitations or disadvantages of CDPD:

• There is no mesh connectivity in CDPD. Hence, M-ES and M-ES cannot communicate directly. Communication between them occurs via MDBS.
• CDPD cell size is limited to less than 10 miles.
• CDPD offered relatively low data transmission speeds compared to modern standards. The typical data rates were around 19.2 kbps, which are insufficient for many contemporary data-intensive applications.
• CDPD relied on the infrastructure of analog cellular networks (AMPS), which were phased out in favor of digital cellular technologies. As analog networks were decommissioned, the availability of CDPD services declined.
• The use of idle channels in the analog cellular network limited the available bandwidth for CDPD. This restriction impacted the overall data throughput and capacity of the system.
• Due to the nature of packet-switched technology and the dynamic allocation of idle channels, CDPD could experience higher latency compared to dedicated circuit-switched data connections.
• Operating in the analog spectrum, CDPD was susceptible to interference from various sources, which could degrade signal quality and data transmission reliability.
• While CDPD included basic encryption and authentication features, the security mechanisms were not as robust as those found in later digital mobile data technologies. This made CDPD less secure against sophisticated attacks.
• CDPD shared resources with voice communication channels. During peak voice call times, the availability of idle channels for data transmission could be significantly reduced, leading to potential data transmission delays and reduced service quality.
• The implementation of CDPD required modifications to existing cellular infrastructure, including the installation of additional equipment and upgrades to support packet-switched data. This added complexity and cost for network operators.
• The emergence of digital cellular technologies, such as GPRS, EDGE, and later, 3G, 4G, and 5G networks, offered significantly higher data rates, better efficiency, and more robust security. These newer technologies quickly outpaced CDPD in terms of performance and capabilities.

Conclusion

CDPD (Cellular Digital Packet Data) was a significant milestone in the evolution of mobile data communication, offering a cost-effective, efficient, and reliable solution for wireless data transmission.

By leveraging existing cellular infrastructure and incorporating key features such as packet-switched technology, error correction, and security, CDPD paved the way for the development of more advanced mobile data technologies.

Despite being largely replaced by newer technologies such as GPRS, EDGE, and LTE, the legacy of CDPD continues to influence the design and implementation of modern mobile data services.