Mobile Computing : A technology that allows transmission
of data, via a computer, without having to be connected to a fixed physical
link.
Mobile voice communication is widely established throughout the world
and has had a very rapid increase in the number of subscribers to the various
cellular networks over the last few years. An extension of this technology is
the ability to send and receive data across these cellular networks. This is
the principle of mobile computing.
Mobile data communication has become a very important and rapidly
evolving technology as it allows users to transmit data from remote locations
to other remote or fixed locations. This proves to be the solution to the
biggest problem of business people on the move - mobility.
In this article we give an overview of existing cellular networks and
describe in detail the CDPD technology which allows data communications across
these networks. Finally, we look at the applications of Mobile Computing in the
real world.
The question that always arises when a business is thinking of buying a
mobile computer is "Will it be worth it?"
In many fields of work, the ability to keep on the move is vital in
order to utilise time efficiently. Efficient utilisation of resources (ie:
staff) can mean substantial savings in transportation costs and other non
quantifyable costs such as increased customer attention, impact of on site
maintenance and improved intercommunication within the business.
The importance of Mobile Computers has been highlighted in many fields
of which a few are described below:
- For Estate Agents
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.
- Emergency Services
Ability to recieve information on the
move is vital where the emergency services are involved. Information regarding
the address, type and other details of an incident can be dispatched quickly,
via a CDPD system using mobile computers, to one or several appropriate mobile
units which are in the vicinity of the incident.
Here the reliability and security
implemented in the CDPD system would be of great advantage.
- In courts
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.
- In companies
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.
- Stock Information Collation/Control
In environments where access to stock
is very limited ie: factory warehouses. The use of small portable electronic
databases accessed via a mobile computer would be ideal.
Data collated could be directly written
to a central database, via a CDPD network, which holds all stock information
hence the need for transfer of data to the central computer at a later date is
not necessary. This ensures that from the time that a stock count is completed,
there is no inconsistency between the data input on the portable computers and
the central database.
- Credit Card Verification
At Point of Sale (POS) terminals in
shops and supermarkets, when customers use credit cards for transactions, the
intercommunication required between the bank central computer and the POS
terminal, in order to effect verification of the card usage, can take place
quickly and securely over cellular channels using a mobile computer unit. This
can speed up the transaction process and relieve congestion at the POS
terminals.
- Taxi/Truck Dispatch
Using the idea of a centrally
controlled dispatcher with several mobile units (taxis), mobile computing
allows the taxis to be given full details of the dispatched job as well as
allowing the taxis to communicate information about their whereabouts back to
the central dispatch office. This system is also extremely useful in secure
deliveries ie: Securicor. This allows a central computer to be able to track
and recieve status information from all of its mobile secure delivery vans.
Again, the security and reliabilty properties of the CDPD system shine through.
Taxi Dispatch Network
- Electronic Mail/Paging
Usage of a mobile unit to send and read
emails is a very useful asset for any business individual, as it allows him/her
to keep in touch with any colleagues as well as any urgent developments that
may affect their work. Access to the Internet, using mobile computing
technology, allows the individual to have vast arrays of knowledge at his/her
fingertips.
Paging is also achievable here, giving
even more intercommunication capability between individuals, using a single
mobile computer device.
Mobile telephony took off with the introduction of cellular technology
which allowed the efficient utilisation of frequencies enabling the connection
of a large number of users. During the 1980's analogue technology was used.
Among the most well known systems were the NMT900 and 450 (Nordic Mobile
Telephone) and the AMPS (Advanced Mobile Phone Service). In the 1990's the
digital cellular technology was introduced with GSM (Global System Mobile)
being the most widely accepted system around the world. Other such systems are
the DCS1800 (Digital Communication System) and the PCS1900 (Personal
Communication System).
A cellular network consists of mobile units linked together to switching
equipment, which interconnect the different parts of the network and allow
access to the fixed Public Switched Telephone Network (PSTN). The technology is
hidden from view; it's incorporated in a number of tranceivers called Base
Stations (BS). Every BS is located at a strategically selected place and covers
a given area or cell - hence the name cellular communications. A number
of adjacent cells grouped together form an area and the corresponding BSs communicate through a so
called Mobile Switching Centre (MSC). The MSC is the heart of a cellular radio
system. It is responsible for routing, or switching, calls from the originator to the destinator. It
can be thought of managing the cell, being responsible for set-up, routing
control and termination of the call, for management of inter-MSC hand over and
supplementary services, and for collecting charging and accounting information.
The MSC may be connected to other MSCs on the same network or to the PSTN.
Mobile Switching Centre
The frequencies used vary according to the cellular network technology implemented. For GSM, 890 - 915 MHz range is used for transmission and 935 -960 MHz for reception. The DCS techology uses frequencies in the 1800MHz range while PCS in the 1900MHz range.
Each cell has a number of channels associated with it. These are
assigned to subscribers on demand. When a Mobile Station (MS) becomes 'active'
it registers with the nearest BS. The corresponding MSC stores the information
about that MS and its position. This information is used to direct incoming
calls to the MS.
If during a call the MS moves to an adjacent cell then a change of
frequency will necessarily occur - since adjacent cells never use the same
channels. This procedure is called hand over and is the key to Mobile communications. As the MS is approaching the
edge of a cell, the BS monitors the decrease in signal power. The strength of
the signal is compared with adjacent cells and the call is handed over to the
cell with the strongest signal.
During the switch, the line is lost for about 400ms. When the MS is
going from one area to another it registers itself to the new MSC. Its location
information is updated, thus allowing MSs to be used outside their 'home'
areas.
channel structure
Since radio spectrum is a limited resource shared by all users, a method
must be devised to divide up the bandwidth among as many users as possible. The
method chosen by GSM is a combination of Time- and Frequency-Division Multiple
Access (TDMA/FDMA). The FDMA part involves the division by frequency of the
(maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart.
One or more carrier frequencies are assigned to each base station. Each of
these carrier frequencies is then divided in time, using a TDMA scheme. The
fundamental unit of time in this TDMA scheme is called a burst period and
it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into
a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the
basic unit for the definition of logical channels. One physical channel is one
burst period per TDMA frame.
Channels are defined by the number and position of their corresponding
burst periods. All these definitions are cyclic, and the entire pattern repeats
approximately every 3 hours. Channels can be divided into dedicated
channels, which are allocated to a mobile station, and common
channels, which are used by mobile stations in idle mode.
Traffic channels
A traffic channel (TCH) is used to carry speech and data traffic.
Traffic channels are defined using a 26-frame multiframe, or group of 26 TDMA
frames. The length of a 26-frame multiframe is 120 ms, which is how the length
of a burst period is defined (120 ms divided by 26 frames divided by 8 burst
periods per frame). Out of the 26 frames, 24 are used for traffic, 1 is used
for the Slow Associated Control Channel (SACCH) and 1 is currently unused (see
Figure 2). TCHs for the uplink and downlink are separated in time by 3 burst
periods, so that the mobile station does not have to transmit and receive
simultaneously, thus simplifying the electronics.
In addition to these full-rate TCHs, there are
also half-rate TCHs defined, although they are not yet
implemented. Half-rate TCHs will effectively double the capacity of a system
once half-rate speech coders are specified (i.e., speech coding at around 7 kbps, instead
of 13 kbps). Eighth-rate TCHs are also specified, and are used for signalling.
In the recommendations, they are called Stand-alone Dedicated Control Channels
(SDCCH).
Figure 2.
Organization of bursts, TDMA frames, and multiframes for speech and data
Control channels
Common channels can
be accessed both by idle mode and dedicated mode mobiles. The common channels
are used by idle mode mobiles to exchange the signalling information required
to change to dedicated mode. Mobiles already in dedicated mode monitor the
surrounding base stations for handover and other information. The common channels
are defined within a 51-frame multiframe, so that dedicated mobiles using the
26-frame multiframe TCH structure can still monitor control channels. The
common channels include:
Broadcast Control
Channel (BCCH)
Continually
broadcasts, on the downlink, information including base station identity,
frequency allocations, and frequency-hopping sequences.
Frequency Correction
Channel (FCCH) and Synchronisation Channel (SCH)
Used to synchronise
the mobile to the time slot structure of a cell by defining the boundaries of
burst periods, and the time slot numbering. Every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are by
definition on time slot number 0 (within a TDMA frame).
Random Access Channel
(RACH)
Slotted Aloha channel
used by the mobile to request access to the network.
Paging Channel (PCH)
Used to alert the
mobile station of an incoming call.
Access Grant Channel
(AGCH)
Used to allocate an
SDCCH to a mobile for signalling (in order to obtain a dedicated channel),
following a request on the RACH.
Burst structure
There are four
different types of bursts used for transmission in GSM [16]. The normal burst is used to carry
data and most signalling. It has a total length of 156.25 bits, made up of two
57 bit information bits, a 26 bit training sequence used for equalization, 1
stealing bit for each information block (used for FACCH), 3 tail bits at each
end, and an 8.25 bit guard sequence, as shown in Figure 2. The 156.25 bits are
transmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.
The F burst, used on
the FCCH, and the S burst, used on the SCH, have the same length as a normal
burst, but a different internal structure, which differentiates them from
normal bursts (thus allowing synchronization). The access burst is shorter than
the normal burst, and is used only on the RACH.
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