Voice telecommunication has already been introduced by the mobile phone and is spreading around all over the world like a contagious virus. Data communication started in the early 80's and has been improving ever since, but with a great drawback. Using the PSDN (Public Switched Data Network), it does not allow mobility at all.
Can the two be combined to offer mobile data communications and meet the biggest need of the business-people on the move?
MOBILE PHONES
The fixed line PSTN (Public Switched Telephone Network) severely limits communications by
forcing people to be present at a particular location for connection to be established. For many years people have longed for a truly mobile system that would allow them to move where they pleased, and yet still be able to make calls, and more challengingly, receive calls. The above vision has become a reality and mobile phone is now becoming a necessity rather than luxury.
The need of new networks that will provided the ability of mobile communication is met by the introduction of cellular networks.
A cellular radio system consists of mobile units linked via a radio network to an infrastructure of switching equipment interconnection the different parts of the system and allowing access to the fixed PSTN. The technology is hidden from view-incorporated in numerous transceivers called Base Stations (BS) which are located at selected strategic places, and covering a given area or cell - hence the term cellular communications. A number of cells grouped together form the "area" and the corresponding BS are connected to a Mobile Switching Centre (MSC) which stores information and directs calls to phones within its area. The various MSC's communicate with each other.
The frequencies used by GSM are:
DSC uses the 1800 MHz while PCS the 1900 MHz spectrum.
Each cell has a number of channels assigned to it which can then be used by a subscriber. When a Mobile Station (MS) becomes "active" it registers with the nearest BS. Information regarding the MS and its position is then stored in the area MSC.
This is then used to direct incoming calls to the MS. If during a call the MS moves to another cell then a change of frequency will necessarily occur - as adjacent cells never use the same channels. This procedure of transferring control from one BS to the next is called hand over and is the key to Mobile communications. During the switch the line is lost for about 400ms, but this is not detectable by the user.
Data communications have been achieved using a variety of networks such as PSTN , leased-lines, Packet switching, and more recently ISDN and ATM/Frame Relay. These networks are partly or totally analogue or digital using technologies such as circuit switching, packet switching etc. Circuit switching, implies that data has to follow a prespecified path from user A (sender) to user B (receiver). Packet-switching tries to make a better utilisation of the existing network by splitting the message into packets each containing information on:
Recently fax and data services have been introduced on mobile digital cellular networks at speeds varying from 2400 bits/s to 9600 bits/s. These speeds are much higher than those used for speech.
The introduction of mobility in data communications implies a move from the existing PSDN to other networks like the ones used by mobile phones. PCSI has come up with an idea called CDPD networks (Cellular Digital Packet Data) (see Omar Iqbal's article on CDPD) , which uses the existing mobile network (frequencies used for mobile telephony). The CDPD packets are inserted in the idle time on the cellular telephone channels.
Mobility in data communications has a significant difference from the mobility on voice communications. Using mobile phones, one can move around and talk at the same time. Packets of data, though, have to be sent from the same place, that is one can take his/her mobile computer wherever he/she likes and send or receive information from there, but once connected must remain stable until the end of the exchanging of data. As stated at the end of the previous section, during hand over between cells, the line is lost for about 400ms. If it is voice telecommunications this is undetectable but for data communication not only it is detectable but delay is caused since the message will have to be sent again. Furthermore, the channels used for data are of higher speed capability which presents the hand over to other channels either within the same cell or in another cell.
Estate agents can work either at home or out in the field. With mobile computers they can be more productive. They can obtain current real estate information by accessing multiple listing services, which they can do from home, office or car when out with clients. They can provide clients with immediate feedback regarding specific homes or neighborhoods, and with faster loan approvals, since applications can be submitted on the spot. Therefore, mobile computers allow them to devote more time to clients.
Defense counsels can take mobile computers in court. When the opposing counsel references a case which they are not familiar, they can use the computer to get direct, real-time access to on-line legal database services, where they can gather information on the case and related precedents. Therefore mobile computers allow immediate access to a wealth of information, making people better informed and prepared.
Managers can use mobile computers in, say, critical presentations to major customers. They can access the latest market share information. At a small recess, they can revise the presentation to take advantage of this information. They can communicate with the office about possible new offers and call meetings for discussing responds to the new proposals. Therefore, mobile computers can leverage competitive advantages.
Uses like the above are endless. People find one that serves their needs so more and more are subscribing for mobile computers.
Visions of a cellular future
http://www.ericsson.se/EPI/BR-vision.html
An overview of cellular techology
http://www.ericsson.se/EPI/BR-overview.html
The CDPD Network
John Gallant, Technical Editor, PCSI