A central problem for business individuals on the move, concerns the ability to communicate data between the work base and remote locations. A new technology has evolved over the past few years which allows the transmission of digital data across existing air link analogue cellular voice channels as well as across existing Circuit Switched telephone networks. This technology is known as Cellular Digital Packet Data (CDPD).
This technology allows data from a portable computer or a Personal Digital Assistant (PDA) to be transmitted to Wide Area Computer Networks (WAN) and Public Switched Telephone Networks (PSTN) as well as other mobile units ie: mobile phones or portable computers. This allows individuals total freedom to transmit data back to the office from remote locations. This article will attempt to explain the principles and implementation techniques involved in this relatively new phenomenon which has become one of the hottest issues in mobile data communications.
Principles of CDPD
Cellular Digital Packet Data networks allow both mobile and fixed end user devices to be used. This means that the data can be transmitted from a portable device ie: portable computer, or a computer which has a fixed link to the CDPD computer network. In the case of a fixed end user system, a server or host computer has a fixed connection to the CDPD network. The system's advantages become clear when it is considered for a mobile end user device. The end user in this case can be moving around the cellular reception area. The existing Cellular Telephone Network is made up of a number of cells which cover the reception area. Each of these cells are used to pinpoint the position of the mobile device ie: the mobile phone, within the reception area and then calls to and from the particular mobile device can be routed appropriately.
CDPD uses a Packet Switched System , where data is split up into small packets, and then sent across the communication channel. This is available in both Virtual Circuit Switching and Datagram Switching form. The data packets are cleverly interleaved with the existing voice signals on momentarily unoccupied voice frequencies, during idle time in the voice signals. This detection of unoccupied frequencies and sending of packets is done by a Mobile Data Base station (MDBS). There is (at least) one of these in each cell of the cellular network. Multiple MDBS may be used to increase reliability of the system.
CDPD networks can allow a mobile user to transmit data using either cellular channels or existing Circuit Switched telephone channels as shown above, which means that even outside the bounds of the cellular reception area, data transmission can be achieved. This data can then be sent to either another mobile end user or a fixed connection computer on the CDPD network. Below is a breakdown of the major components of the system in detail. The diagram shows how these components link together.
The end user can transmit either voice, fax or data across the CDPD network, using just a single device which is a Personal Digital Assistant (personal computer) connected to an Radio Frequency (RF) modem. This device can send voice signals via a cellular phone unit. The RF modems are specially designed with the antennae necessary to transmit data packets across the cellular network.
The MDBS are responsible for relaying data between the mobile units and the computer and telephone networks. This involves detection of idle time on the voice channels, and sending packets of digital data onto the appropriate unoccupied voice frequencies. These stations are situated in each cell in the reception area. The MDBS use the same antenna as the existing cellular network to recieve signals. Preceding a data tranmission session, the MDBS creates two air link RF channels to each mobile unit for bi-directional communication. These are forward and reverse channels. Forward channels are unique to each mobile unit and hence contentionless, whereas reverse channels, for communication from the mobile units to the MDBS are shared.
This means that two mobile units using the same frequency reverse channel can't communicate with each other. Mobile units access the reverse channel using Digital Sense Multiple Access with Collision Detection (DSMA-CD) protocol which is similar to the protocol used in Ethernet communication which utilises Carrier Sense Multiple Access with Collision Detection (CSMA-CD) . This protocol allows the collision of two data packets on the common channel to be detected so that the Mobile unit can be alerted by the MDBS to retry transmission at a later time, in case of failure.
Detection of idle time is done using a scanning reciever (known as a "sniffer") that scans the voice channels for signal strength hence telling whether a channel is idle or not. If the MDBS can find two channels free (for bi-directional communication) then the MDBS establishes the air-link required. Once a link has been established, the MDBS can detect if an analogue voice channel is about to use this link. On detection, it releases the link, establishing a link on another unoccupied frequency. This takes much less time than the analogue voice channels do to request and set up a communication link.
This ensures that the digital data transmission never interferes with the voice transmissions and is, therefore, completely transparent to the existing analogue voice network. This process of establishing and releasing channel links is known as channel hopping. When the voice channels are at capacity, then extra frequencies specifically set aside for CDPD data can be utilised. This is a very unlikely scenario however, as each cell within the reception area has typically 57 channels, each of which has an average of 25-30% of idle time.
The data packets are transmitted at speeds of typically 19.2 Kilobits/second to the MDBS, but actual throughput may be as low as 9.6 Kilobits/second due to the extra redundant data that is added to transmitted packets. This is information includes sender address, reciever address and in the case of Datagram Switching, a packet ordering number. Check data is also added to allow error correction if bits are incorrectly recieved. Each data packet is encoded with the check data using a Reed-Solomon forward error correction code. The encoded sequence is then logically OR'ed with a pseudo-random sequence, to assist the MDBS and mobile units in synchronisation of bits. The transmitted data is also encrypted to maintain system security.
Groups of MDBS that service a particular cell can be grouped together and connected to a Mobile Data Intermediate System (MDIS). These MDIS units, together, form the backbone of the CDPD network.
Each mobile unit has a fixed home area but may be located in any area where reception is available. So, if a MDIS unit recieves a data packet addressed to a mobile unit that resides in its domain, it sends the data packet to the appropriate MDBS in its domain which will forward it as required. If the data packet is addressed to a mobile unit in another group of cells, then the MDIS forwards the data packet to the appropriate MDIS. The MDIS units hide all mobility issues from systems in higher levels of the network hierarchy.
The units above this level are called Intermediate Systems (IS) and these are unaware of the mobility of units, which is taken care of by the MDIS. It is at this level that computer network interfacing and interfacing to other cellular carrier networks can be done using Datagram switching.
The CDPD technology has the following advantages:
There are disadvantages to the system.
| CDPD | Cellular Digital Packet Data |
| PDA | Personal Digital Assistant |
| PSTN | Public Switched Telephone Network |
| WAN | Wide Area Network |
| MDBS | Mobile Data Base Station |
| CS-CDPD | Circuit Switched Cellular Digital Packet Data |
| RF | Radio Frequency |
| CSMA-CD | Carrier Sense Multiple Access with Collision Detect |
| DSMA-CD | Digital Sense Multiple Access with Collision Detect |
| MDIS | Mobile Data Intermediate Station |
| IS | Intermediate Station |