1.0
Introduction
1.1
Requirements Analysis
2.0
Introduction To Wireless Data Communications
2.1
Packet Data Technology
3.0
Wireless Design Considerations/Technical Constraints
3.1
Wireless Middleware
4.0
Wireless Data Communication Options
4.1
Circuit-Swiched Cellular Data
4.2
Cellular Digital Packet Data (CDPD)
4.3
Public Two-Way Wide-Area Packet Data Networks
4.4
Satellite Networks
4.41
The IRIDIUM Network
4.42
The Teledesic Network
5.0 Comparison of Wireless Communication Options
5.1 Network Architectures
5.2 Speed/Capacity
5.3 Protocols
5.4 Costs
6.0 Recommendation
1.0 Introduction
The City of Tucson Police Department (TPD),
like many city law enforcement agencies, faces manpower and budgetary challenges
as demand for public safety services continue to increase. For this reason,
ease of information sharing and accessibility has become an even more critical
aspect to the prevention of crime and the apprehension of criminals. The
Tucson Police Department and the University of Arizona, Department of Management
Information Systems, Artificial Intelligence Group have teamed to investigate
information technology solutions to the problems of data incompatibility,
interagency data sharing, and information retrieval which plague the Department.
The collaboration will initially focus on a needs assessment and requirements
analysis, local database integration, and data retrieval through the use
of user friendly Intranet-based technology. The information technology
project is referred to as COPLINK [1].
Furthermore, giving police officers access
to information from their patrol cars is another important issue. Technological
advances in mobile computing and wireless data communication has enabled
mobile users to remain connected to the services and information available
on their wired network from virtually anywhere. However, mobile computing
offers unique challenges when applied to the public safety arena due to
its increased demands for responsiveness, accuracy, security, and reliability.
Remote database access and messaging are the most relevant aspects of mobile
computing that are applicable to the COPLINK project and will be discussed
in this paper.
Mobile computing and wireless communication
technologies are important elements that should be considered early in
the overall research and development associated with the COPLINK project.
A thorough understanding of the technological constraints and available
options associated with developing a wireless Intranet solution for the
Tucson Police Department is imperative to providing a complete solution
to meet its operational requirements.
1.1 Requirements Analysis
A needs assessment and requirements analysis
for the Tucson Police Department was conducted to identify issues relating
to database integration, Intranet development, and remote access mobile
computing technology. Currently, various information databases are implemented
within TPD to assist in crime fighting. One such database is the Computer
Aided Dispatching (CAD) system which stores incident histories and times
from service calls for up to 30 days. The system uses Mobile Data Terminals
(MDTs) installed in patrol cars to provide dispatching functions and database
access over radio frequencies at 4800 bps. The system, while initially
very capable when first implemented in the mid-1980’s, has now become overloaded
and inefficient. The Records Management System (RMS) is another TPD database
which holds information from the CAD system as well as other criminal case
information, particularly involving stolen property and wanted persons.
Various units within the Department also have databases which are accessible
only within their section. An example is the ELVIS mug shot system comprised
of images and other demographic information of arrested persons. Other
criminal databases that are available to TPD are the Arizona Crime Information
Center (ACIC) and the National Crime Information Center (NCIC). Eventually,
it is envisioned that these databases as well as other City systems will
be integrated and information accessible to all officers from their patrol
cars. The COPLINK project group has identified the following TPD requirements:
Near-Term
- Database integration of RMS and ELVIS
- Development of a user-friendly Web-based browser interface for database access
- Remote database access for mobile computers using wireless communication technology
- Database integration of CAD, County Sheriff systems, City systems, ACIC, and NCIC
- Dispatching/messaging capabilities to mobile computing terminals (replacement of CAD system)
Advances in digital wireless communications
has made data accessibility to mobile computers a reality. Wireless data
communications involves the transmission of information in digital form
from a source that generates the information to one or more destinations.
A wireless data communication system is composed of several elements such
as the source encoder/decoder, channel encoder/decoder, digital modulator/demodulator,
and the physical channel [2]. The process of converting the output of either
an analog or digital source into a sequence of binary digits is accomplished
by the source encoder. The information sequence produced by the source
encoder is passed to the channel encoder where redundant information is
added to the binary sequence. This is called Forward Error Correction (FEC)
and serves to increase the reliability of received data that can become
corrupted by the effects of noise and interference encountered by the transmission
of the signal through the physical channel. The digital modulator then
accepts the binary encoded information sequence and maps it into signal
waveforms to be transmitted over the communication channel. In the case
of wireless communications, the physical channel that is used to send the
signal from the transmitter to the receiver is the atmosphere (free space).
At the receiver the reverse process occurs to decode and reconstruct the
original binary information sequence. A measure of the fidelity of a digital
communications system is expressed by the probability of bit error. This
is a function of the coding characteristics, the types of waveforms used
to transmit the information, the transmitter power, channel characteristics,
ie., the amount of noise and interference, and the modulation/demodulation
method used.
2.1 Packet Data Technology
Wireless data communications has become cost
effective primarily because of packet data technology. Sending wireless
data over channels dedicated to individual users are too expensive and
inefficient for multiuser networks [3]. Packet data networks, like the
Internet, are well suited for a multiple user environment where data transmissions
are bursty in nature and last only a few seconds at a time. A data sequence
from an information source is broken into packets ranging from anywhere
between 240 to 2048 bytes. Each packet includes the source and destination
address, allowing multiple users to share a single transmission channel.
Packet switches use the destination address to route the information to
the next appropriate transmission link and eventually to the destination.
The actual route isn’t specified and can change in the middle of the process
to accommodate a change in network load. Packets from an information stream
can take different routes and may even arrive out of sequence. Packet data
technology is used in two-way public packet data networks such as Advanced
Radio Data Information Services (ARDIS), and in cellular networks with
the Cellular Digital Packet Data (CDPD) service.
3.0 Wireless Design Considerations/Technical Constraints
A thorough understanding of the technological
constraints and characteristics of wireless communications must be realized
when considering the design of a wireless data application. The most important
factors to consider that will affect the performance and feasibility of
a mobile computing application in a wireless environment are:
- Limited Bandwidth/Low Data Throughput
- Network Unreliability
- Data Security
- High Costs
The radio spectrum available for wireless communications
is strictly controlled by the Federal Communications Commission and is
allocated for everything from garage door transmitters and cordless phones
to television and radio broadcasts. In order to make most efficient use
of limited spectrum for multi-user wireless data communications, the available
spectrum is segmented into narrow-bandwidth channels with limited information
carrying capacity. Therefore, it is important that applications are optimized
to be more efficient in a wireless environment and to make the most out
of each packet transmitted or received. Furthermore, data compression techniques
such as v.42bis and image compression like JPEG should be used to limit
the quantity of information transmitted over the radio channel. Another
wireless communication characteristic to be aware of is that wireless data
networks are inherently much less reliable than a wired network. Wireless
data communications typically utilize low power radio transmissions to
limit the number of devices competing for media access to a confined geographic
area and to reduce interference between users. However, radio waves may
still be adversely affected by environmental obstructions and other interference,
resulting in signal fading, intermittent connections, and transmission
errors. For this reason, wireless communications implement forward error
correction (FEC) and protocols requiring packet acknowledgements.
Data security is another important concern
when using a public wireless communication network. This is especially
true in a law enforcement environment where interception of sensitive information
might compromise the effectiveness of the police force and the safety of
police officers. A wireless data network should provide for data encryption
using industry or government standard algorithms such as DES-56 or RC4-256.
User authentication using is also very important to verify the identity
of devices requesting information. Implementation of these measures will
provide a secure mobile computing environment.
Finally, the cost of sending data over a wireless
network is significantly higher than traditional wireline (LAN/WAN) networks.
Most public packet data networks charge per kilobyte of data transmitted.
Costs can typically be a dollar or more for sending 10 Kbytes of information.
Thus, it is important that the application not be "chatty" and send only
necessary, non-redundant information over the wireless link.
3.1 Wireless Middleware
The design and implementation of a wireless
application can be simplified and made more feasible with the use of a
middleware software toolkits. Middleware serves to insulated the application
from the basic nature of wireless data communications and the problems
of performance, reliability, security, and cost associated with these networks.
Middleware software, a connectivity tool, gets its name from the fact that
it operates at the middle of the OSI protocol stack, shielding the application
layer from the peculiarities of the network’s data-link and physical layers.
Figure courtesy of Oracle Corporation[4]
Wireless middleware optimizes the wireless
link and enables applications to handle intermittent connections. It provides
a software piece for the mobile system and a matching software piece on
a server on the wired enterprise network. Applications on the mobile system
make simple calls to the local middleware layer, which exchanges messages
with the middleware residing on the network. There, the middleware acts
as an "agent" on behalf of the mobile client to conduct transactions, such
as sending e-mail or doing database queries. This is called a client-agent-server
architecture, which utilizes software agents that are active even when
the client is disconnected. Oracle Mobile Agents is an example of this
kind of wireless middleware.
Oracle Mobile Agents is comprised of the Message
Manager, Message Gateway, and Agent Event Manager which act as mediators
between the client application and the server. There is no direct connection
between the client and the server, instead the client issues requests to
the agent to execute a task. The frequent disconnects which plague mobile
computing devices in a wireless network are solved, because once a request
is received by the agent, the client does not have to remain connected
to the network. Data traffic over the low-bandwidth wireless link is minimized
resulting in an improved performance of the network. Oracle Mobile Agents
uses an open architecture and supports a variety of wireless data networks.
It also provide data encryption, user authentication, and data compression.
The Message Manager manages the communications
of the applications and hides the network complexities. The Message Gateway
is responsible for routing messages to appropriate servers and for managing
the message queues for registered clients and agents. It also implements
system-wide security and authentication and system management. The Agent
Event Manager manages the communications between the server agents and
the Message Gateway.
Figure courtesy of Oracle Corporation[4]
4.2 Cellular Digital Packet Data (CDPD) IBM’s Advanced Radio Communications on Tour
(ARTour) WebExpress is also a middleware software technology that makes
it possible to run Internet World Wide Web applications in wide area wireless
networks. It significantly reduces user cost and response time of wireless
communications by intercepting the Hyper-Text Transport Protocol (HTTP)
data stream and performing various optimizations including: file caching,
forms differencing, protocol reduction, and the elimination of redundant
HTTP header transmission.
4.0 Wireless Data Communication Options
There are various wireless data communication
options available for mobile computing applications. Some technologies
make use of existing cellular voice networks while others are either public
or private networks devoted exclusively to wireless data communications.
Networks vary in message routing technologies, in protocols used to connect
to the network and to translate and transmit the data, in transmission
speeds, and in coverage. The wireless data networks that will be discussed
include circuit-switched cellular data, Cellular Digital Packet Data (CDPD),
two-way wide-area public packet data, and the IRIDIUM and Teledesic Low-Earth-Orbiting
(LEO) satellite networks.
4.1 Circuit-Swiched Cellular Data
Wireless circuit-switched data is a technology
offered by many cellular service providers for sending and receiving data
over the cellular voice network at transmission rates up to 14.4 kilobits
per second. Data can be sent using circuit switched technology by attaching
a modem to a mobile computing device, like a PC laptop. The modem translates
the digital data into an analog signal and transmits the data over the
cellular network Cellular modems include specific protocols, such as ETC
or MNP10, which improve reliability and throughput for wireless data communications.
Circuit switching provides a constant connection between the communication
endpoints by "locking" in a cellular channel for the duration of the data
transmission. The cellular data connections are session-based which means
that once a session is established, users pay for the connection time,
even when no data is being transmitted. It is considered the most effective
and economical solution for transferring large amounts of data in batch
operations. For short, bursty data transfers, circuit switched cellular
data is not as efficient or economical due to the overhead of call set-up
and tear-down delays and the high cost and connection time per message.
For these types of data transfers, Cellular Digital Packet Data (CDPD)
is more suitable.
CDPD technology utilizes the same analog cellular
network infrastructure that is used for voice communications with mobile
telephones, but adapts it for packet switched data. Data is transmitted
over radio channels during idle times between voice transmissions, making
efficient use of bandwidth that would otherwise be wasted. However, voice
users always have priority over data in accessing the shared radio channels.
Users log on once per day to register on the network and messages automatically
locate them. Provider fees are based on the amount of data that's transmitted
not the actual connection time. The channel data rate supported by a CDPD
network is 19.2 kilobits per second (kbps), however actual data throughput
is more like 10 kbps due to protocol overhead and radio channel contention.
Figure courtesy of Motorola, Inc .[5]
The Federal Communications Commission (FCC)
has allocated 50 megahertz (MHz) of radio spectrum beween to be used for
cellular communications. This 50 MHz is broken up into two 25 MHz sections:
one for forward communications (cell to mobile terminal) and one for reverse
communications (mobile terminal to cell). The forward frequencies are 869
MHz to 894 MHz and the reverse frequencies are 824 MHz to 849 MHz. A 20
MHz gap in the cellular ban helps to minimize interference between forward
and reverse communications. CDPD technology supports full-duplex communications
which means that data can be transmitted and received simultaneously. This
is accomplished by allowing a CDPD device to communicate in both the forward
and reverse directions using a pair of 30 kilohertz (kHz) channels.
A CDPD network is comprised of various components
which include the Mobile End-Station (M-ES), Fixed End-Station (F-ES),
Mobile Data Intermediate System (MD-IS), and Mobile Data Base Station (MD-BS).
The M-ES can be any mobile computing device (laptop PC) which uses a CDPD
modem to connect to the network. The F-ES can be any number of stationary
devices, such as a database host computer, Web-server, or other workstation.
MD-IS’s are the brains of the network and are stationary components that
are responsible for keeping track of the M-ES’s location and routing data
packets between the network and the M-ES. MD-IS’s also exchange M-ES location
information amongst other MD-IS’s providing seamless mobility for the mobile
device. The MDBS acts as a relay between the M-ES and the MD-IS using the
CDPD Mobile Data Link Protocol (MDLP). Data packets can be sent between
M-ESs and the MDBS with a full-duplex data rate of 19.2 kilobits per second
(kbps). A MD-BS is located at each cell site and is also responsible for
Radio Frequency (RF) channel management of the network.
CDPD radio channels are a shared resource
where multiple mobile devices contend for their use within a radio cell.
Each device only transmits on the radio channel when it has data to transmit,
otherwise it remains silent and listens. Mobile devices are not able to
detect the transmissions of other mobile devices because they only receive
the "forward channel" frequency from the cell. This means that a Carrier
Sense Multiple Access (CSMA) mechanism similar to that used for Ethernet,
can’t be used for cellular data networks. Instead, CDPD uses a multiple
access mechanism called Digital Sense Multiple Access (DSMA) that is very
similar.
In DSMA, the forward channel includes transmission
of "Busy/Idle" and "Decode Status" channel indicators. The "Busy/Idle"
channel status indicator is set by the Mobile Data Base Station (MD-BS)
on detection of a reverse channel CDPD transmission. Mobile devices use
this to obtain information on whether another mobile device is already
transmitting on the channel. The "Decode Status" indicator is transmitted
by the MD-BS to indicate whether it is successful in decoding the received
data transmission. The mobile devices interprets a failure indication to
indicate a possible collision and initiate the back-off and retransmission
mechanism. This is similar to the collision detection mechanism in Ethernet.
Figure courtesy
of GTE Wireless Data Services.[8]
Unlike other wireless data networks that use
proprietary networking protocols, CDPD uses industry-standard TCP/IP protocols.
CDPD in its most basic form can be used as a wireless extension of an existing
TCP/IP network. A host computer can be connected to the CDPD network with
a high speed leased circuit (eg. X.25 or T1) providing transparent data
networking service to the mobile subscriber. The data packets transmitted
on the CDPD network are called Network Protocol Data Units (NPDUs) and
they hold a maximum of 2048 bytes. Any information block larger than this
will be broken down into 2048 byte segments by the network before transmission.
CDPD incorporates forward error-correction, data compression and encryption,
and subscriber authentication providing a highly reliable, efficient, and
secure mobile computing environment.
Figure courtesy of GTE Wireless Data Services.[8]
4.3 Public Two-Way Wide-Area Packet Data Networks
Public two-way wide-area packet data networks
are owned and operated by service providers offering wireless data communications
to support mobile computing applications. One such network is the Advanced
Radio Data Information System, or ARDIS. It is a wireless data network
owned by Motorola that uses an infrastructures of base stations, network
control centers, and switches to transmit data for business and government
customers. The network operates on Motorola DataTAC technology supporting
data transmission speeds from 4800 bps to 19.2 kbps, depending on the service
area. Two proprietary protocols are used; MDC-4800 (4800 bps) and RD- LAP
(19.2 kbps). ARDIS has deployed MDC-4800 service throughout its network,
but RD-LAP capability, at the higher data rate, is only available in select
markets.
The ARDIS network is dedicated to data and
is best for short, bursty messages. Charges are based on the amount of
data transmitted, not the connect time, and there is no charge for seamless
roaming of the mobile device. It offers data encryption using a proprietary
method and incorporates hardware redundancy and Uninterruptible Power Supply
(UPS) backup for increased security and fault tolerance. ARDIS utilizes
proprietary protocols, specifically the Standard Context Routing (SCR)
protocol at the data-link layer. Applications requiring TCP/IP messaging
is possible through the use of gateways.
Figure courtesy of Motorola, Inc .[6]
The ARDIS data network is comprised of the
mobile computing devices with attached Radio Packet Modems (RPM), Area
Communication Controllers (ACC), and the Network Management Host. RPM’s
are distributed throughout the radio coverage area of the network and are
registered in the Home Location Register (HLR) database of their "home"
area. The HLR database entry contains information that is needed to support
connection management and routing for the RPMs. An Area Communications
Controller (ACC) controls and coordinates the communications activities
for one or more radio subnetworks. Large, geographically dispersed radio
networks can have multiple ACCs, each responsible for controlling a group
of radio subnetworks. All connections between RPMs and application hosts
are managed by the ACC, which controls the radio coverage area where the
RPM is operating. If an RPM is operating within its home Area, the local
ACC can handle the entire transaction. However, if the RPM is operating
in a foreign area, the local ACC must register that RPM in its Visitor
Location Register (VLR) database and seek authorization from the home ACC
of the RPM before granting service. For RPMs that roam between areas, connection
responsibility is handed off from ACC to ACC. The network management host
is a network backbone facility used to configure and maintain the radio
data communications infrastructure.
Figure courtesy of Motorola, Inc .[7]
RPMs may communicate with application hosts
that are either directly interconnected to the system through an ACC, or
are indirectly interconnected using the facilities of an X.25 Public Switched
Packet Data Network (PSPDN), the Internet, or a point-to-point leased data
circuit.
Figure courtesy of Motorola, Inc .[6]
The differences in the way data is transmitted
over a dedicated circuit-switched cellular channel, a two-way wide-area
packet data channel, and a channel employing CDPD technology are clearly
depicted in the following diagram:
4.42 The Teledesic Network 5.4 Costs
Figure courtesy of Motorola, Inc.[5]
4.4 Satellite Networks
Low Earth Orbit (LEO) satellite networks also
promise to support data communications for mobile computing applications
in the near future. They offer the advantage of providing superior coverage
(global) as well as service to remote, underdeveloped areas not possessing
terrestrial communications infrastucture. However, this advantage comes
at a price, because more transmitter power is required, the mobile devices
are less-compact, there is less total channel capacity, and service costs
are typically greater than land-based communications. The most heralded
satellite networks currently being developed are the IRIDIUM and Teledesic
systems.
4.41 The IRIDIUM Network
The IRIDIUM system is a satellite-based, wireless
personal communications network designed to permit any type of transmission
-- voice, data, fax, or paging -- to reach its destination anywhere on
earth. However, the data service only supports a circuit-switched connection
of 2400 baud. The IRIDIUM constellation will consist of 66 interconnected
satellites orbiting 420 nautical miles above the earth and will be ready
for service in 1998.
The Teledesic system will use a constellation
of 288 inter-linked LEO satellites operating in the Ka frequency band to
provide global two-way, broadband connections for applications such as
voice, data, videoconferencing and high-performance Internet access. The
Teledesic network is based on a distributed architecture, and offers dynamic
routing, and robust scalability. Each satellite acts as a node in a large-scale
packet-switching network and connections to other networks are provided
via gateways. Channel rates from 16 Kbps up to 2.048 megabits per second
(Mbps) (E1) are offered, and for special applications, even OC-24 service
of 1.24416 Gigabits per second (Gbps) . The quality of service will be
comparable to terrestrial communication systems with fiber optic-like delays,
bit error rates less than one in 10 Gbits (10-10), and link
availability of 99.9% over most of the United States. The initial Teledesic
constellation will support a peak capacity of 1,000,000 full-duplex E1
connections with service availability in 2002.
5.0 Comparison of Wireless Communication Options
Choosing the best wireless data communication
solution to support the Tucson Police Department’s mobile computing requirements
is no easy task. The requirements for a high data rate, secure, and dedicated
law enforcement network that supports Web-based applications (TCP/IP protocols)
demands a network technology that is highly tailored to its needs. In this
case, the establishment of a private packet data network would appear to
be a perfect solution. However, the monetary investment in such a system
is a major drawback. Initially, there is a large amount of capital investment
required for the purchase of the RF infrastructure and devices. For proper
wide-area coverage in the Tucson metropolitan area, a large number of radio
base sites, radio frequency licenses, and base station radios would be
required. Beyond the initial capital investment, there are greater Operations,
Administration and Maintenance (OA&M) expenses associated with a private
data network. Knowledgeable personnel with expertise in both data communications
and RF technologies would be required to successfully operate such a complex
network.
For these reasons, it is extremely difficult
to justify the deployment of an infrastructure for a private packet data
network for the Tucson Police Department. Satellite data networks are years
off from being deployed and have yet to be evaluated. Therefore, the most
viable options are leasing airtime from either a cellular provider offering
CDPD service, or from a public packet data network provider such as ARDIS.
In the Tucson metropolitan area CDPD service is offered by Cellular One
and AT&T Wireless Services using network infrastructure installed by
Bell Atlantic Mobile Systems. The ARDIS public packet data network service
is also available in the Tucson area, but is limited to the 4800 bps data
rate.
5.1 Network Architectures
There are distinct differences between the
two wireless network architectures. The CDPD specification is "open", meaning
that multiple equipment vendors are able to provide equipment to the network
subscribers without incurring high licensing costs. This results in a higher
level of competition among CDPD modem manufacturers and lower costs to
the mobile users. In the case of the ARDIS network, a mobile data user
is restricted to utilizing the Motorola DataTAC technology. Furthermore,
the CDPD system has been adopted by multiple service providers across North
America and more specifically in the Tucson metropolitan area. This increased
choice and competition further assures that the CDPD mobile data user will
receive the best possible service and pricing.
5.2 Speed/Capacity
CDPD and ARDIS networks can also be contrasted
by the speed and data capacity that they offer. A CDPD network supports
a raw data rate of 19.2Kbps using shared channels for both voice and data
users. Data can only be transmitted when voice channels are idle, except
in some cases where special data channels have been implemented. Actual
throughput is typically about 10Kbps and depends on how many voice and
data users in the same cell are contending for a channel. In the case of
the ARDIS network available in the Tucson area, a raw data rate of only
4800 bps is offered. However, this system is dedicated to data transmission
only.
5.3 Protocols
The CDPD and ARDIS wireless data technologies
also implement different network protocols. For Web-based applications,
like the database access application being developed for the Tucson Police
Department, it is imperative that the chosen network architecture is open
and supports the Transmission Control Protocol (TCP) and Internet Protocol
(IP). The CDPD system design follows the Open System Interconnect (OSI)
layered architecture approach. This allows CDPD to support standard connectionless
network protocols such as IP and the ISO based Connectionless Network Protocol
(CLNP). An application that has been developed for use on a wired Local
Area Network (LAN) will also operate over the wireless CDPD network. The
CDPD system architecture is also designed to support the recently developed
Internet Protocol version6 (IPv6), which offers a larger address space,
enhanced security, and quality of service features.
On the other hand, the ARDIS network utilizes
proprietary protocols, including SCR at the data-link layer. In order for
TCP/IP based applications to be supported on the network, a gateway must
convert IP protocol packets to some internal addressing scheme to effect
packet delivery. This conversion function may introduce incompatibilities
that result in data transmission problems. Routing of TCP/IP packets on
the ARDIS network also requires the encapsulation of of TCP/IP messages
within the SCR protocol frame. This results in unnecessary layers of address
information and a decrease in the efficiency of the application.
Figure courtesy of Motorola, Inc .[7]
The service and equipment costs for using
a CDPD or ARDIS network are comparable. Connection to a CDPD network requires
a CDPD wireless modem which runs between $500 and $1,300. Monthly service
costs are between $10 - $120. A wireless modem for use on an ARDIS network
can be had for between $700 and $1500. Monthly service costs on the ARDIS
network are between $25 and $150. Transmission costs per kilobyte of data
on both a CDPD and ARDIS network are approximately 0.08 cents. However,
lower prices may be negotiated by high volume users.
Comparison Of CDPD & ARDIS Wireless Data Networks
|
|
|
|
| Multivendor subscriber equipment |
|
|
| Multiple service providers |
|
|
| Open system standard |
|
|
| Standard network protocol support |
|
|
| TCP/IP header compression |
|
|
| Data compression |
|
|
| Radio channel data rate |
|
|
| Full duplex/half duplex radio link |
|
|
| Power save mode |
|
|
| Authentication |
|
|
| Encryption over radio link |
|
|
6.0 Recommendation
Based on the Tucson Police Department’s mobile
computing requirements, the most viable wireless data communication solution
is Cellular Digital Packet Data (CDPD) technology. The service is provided
in the Tucson metropolitan area by cellular providers such as Cellular
One and AT&T Wireless Services. A CDPD wireless modem, like the Motorola
Personal Messenger 100C will interface to a laptop PC through a Type II
PCMCIA slot, providing it with a physical layer link to the wireless network.
The most important reason why CDPD technology is recommended is because
of its support for native TCP/IP protocols, upon which Web-based applications
are developed. This is crucial for maximizing the efficiency of the Web
browser-like database access application being developed for TPD. It is
also highly recommended that wireless middleware, such as Oracle Mobile
Agents, be incorporated into the technology solution. Since the TPD’s Records
Management System (RMS) is based on an Oracle database, use of this middleware
will optimize data retrieval over the wireless link, resulting in increased
efficiency, data security, and cost savings. Mobile computing devices will
have access to database information on the TPD Web-server by connecting
the CDPD network to the TPD LAN via a point-to-point leased circuit or
the Internet. The CDPD network and the mobile computing devices will in
effect become a wireless extension of the TPD Intranet.
Deficiencies in the CDPD technology are high
costs for bulk data transmissions and voice priority on shared radio channels.
The high cost per kilobyte of data transmitted is a serious concern when
it comes to transmission of mug shot images over the wireless network.
Costs can be reduced by using state-of-the-art image compression algorithms
to minimize the size of the mug shot images. Monthly service and data transmission
costs should be negotiated among the cellular providers to obtain the best
value. In the event of an emergency in a cell region, the Tucson Police
Department would like priority given for data use over voice. Competition
among the CDPD service providers in the Tucson area may encourage the implementation
of data-only radio channels and priority access to public safety users.
In conclusion, CDPD technology is not a perfect
solution for the demanding mobile computing requirements of the Tucson
Police Department. However, it is a technology with many good features
that deserves to be evaluated during the prototype development phase of
the COPLINK project.
References
[1] The City of Tucson Police Department, "Request for Proposal- COPLINK: Database Integration And Access For A Law Enforcement Intranet, 1996.
[2] J. Proakis, Digital Communications, McGraw-Hill Inc., New York, NY, (1995).
[3] A. Tanenbaum, Computer Networks, Prentice-Hall, Inc., Upper Saddle River, NJ, (1996).
[4] Oracle Corporation, Oracle Mobile Agents Whitepaper, 1997.
[5] Motorola, Inc., Wireless Data Communications: An Overview, 1995.
[6] Motorola, Inc., Wireless Data Communications: The Choices,
1995.
[7] Motorola, Inc., DataTAC Open Protocol Specifications: User’s Guide, 1995.
[8] GTE Wireless Data Services, Cellular Digital Packet Data (CDPD) Service Overview, 1995.
[9] F. Fluckiger, Understanding Networked Multimedia-Applications and Technology, Prentice-Hall, Inc., London, U.K., (1995).