An introduction to mobile data networks Use your applications provider as a sounding board and obtain a commitment regarding functions that it provides now and that it plans to provide as you define your network requirements.
Basic facts about mobile data (sometimes called digital) terminals (MDTs) can help public safety agencies to evaluate available options. Protocols In computer-science language, a protocol is a standard procedure for regulating data transmission between computers. This definition also applies to mobile communications. Each MDT network supplier has a protocol to control data flow across the network’s radio frequency (RF).
It can be important to understand the basics of, and reasons for, a network supplier’s protocol when selecting MDTs. Unfortunately, there is no universal standard protocol. Each RF services provider has its own “standard.” You may be tied to an individual supplier for many years, because each supplier’s protocol is proprietary, which means that only devices designed for use on that protocol will work.
Throughput and user control Differences in protocols can affect system functions, performance and capacity. Two potentially serious limitations are throughput and user control over allowed applications. Throughput is the number of transactions that a network can carry over a single frequency. It restricts the number of active users who can use the MDT network at the same time. An RF channel is a single-thread pipeline, and it is the protocol that regulates the flow of traffic. “At the same time” is emphasized because only one device can actually communicate at a time; nevertheless, because the data are digital, information can be transmitted in a series of regulated RF bursts.
Dedicated data systems have frequency pairs. One frequency transmits from the base to the field units; the other transmits from the field units to the base. Some protocols actually prevent more than one unit from transmitting at the same time. Others will accept one incoming transmission and reject any others, therefore requiring an automatic retransmission. These differences affect a protocol’s actual throughput. Computer capacity at both ends of the RF may influence perception of throughput; still, the protocol design imposes more serious limitations on throughput than does the hardware speed. Throughput may not be a significant issue if the number of active users per frequency is limited, or if the applications are restricted to conventional MDT functions.
Software control An MDT’s functionality is under software control. Know the origin of the software that controls the network and determine its flexibility to handle future needs. Some application vendors use third-party system control software that puts functionality in the hands of the system software vendor, who may not necessarily be the application vendor. Being locked into a vendor’s protocol can be expensive when it comes to future hardware purchases, but being locked out of any say in the system control software functionality can make the purchase obsolete before it is even installed. System use, after all, is determined in part by the number of functions provided to the officer in the vehicle.
Dumb vs. intelligent terminals The debate over dumb vs. intelligent terminals in vehicles continues, but just as there is no single computer or operating system that can meet the needs of all users, there is no simple choice between dumb and intelligent terminals, either. It really is a matter of desired functionality, durability and cost.
Selection criteria An MDT, whether dumb or intelligent, is designed for long and rugged use. A department first purchasing MDTs does not necessarily need intelligent devices that cost thousands of dollars more per vehicle than dumb ones. Evaluate the department’s desired functionality, how soon that functionality is to be achieved and the available budget to determine whether intelligent terminals are worth it. Intelligent MDTs, called mobile computer terminals (MCTs), typically cost thousands of dollars more than their “dumb” counterparts, but they have essentially the same screen display and font (character) sizes. Using dumb terminals improves productivity while reducing voice traffic by as much as 79%; yet if you will want intelligent functionality within the normal upgrade or replacement time cycle for your agency, consider initially purchasing intelligent terminals-if the budget allows.
Report writing Report writing from vehicles often is stated as the big reason for considering whether to purchase MCTs. Many agencies have successfully eliminated as much as 85% of their written reports by having officers dispose of an incident from the vehicle using so-called “dumb terminals.” The training cycle associated with an MCT that contains reporting applications can be considerably longer than that for a dumb terminal. Consider whether the productivity gain will offset the additional investment.
NCIC 2000 The future plan for National Crime Information Center operations (NCIC 2000) assumes the ability to send fingerprint information, mug shots and other complex data over RF networks. These functions require intelligent devices. Determine whether your agency must comply with NCIC 2000 standards, sooner or later. Establish whether you are limited to a single large capital investment vs. an upgrade-and-replacement cycle that will allow you to replace the initial terminals on a suitable schedule.
Undoubtedly, intelligent devices are beginning to capture many departments’ imaginations. The power of these devices provides a platform for innovation and opportunities, but it is important to consider the overall picture. Careful planning and consultation with the applications vendor can help to ensure that the capital investment is well- spent.
User functionality This discussion covers some of the more common MDT functions and assumes the use of an MDT network connected to a computer-aided dispatching (CAD) system. The combination of MDTs and CAD provides the most powerful potential for increasing the productivity of an agency’s patrol division since the introduction of vehicular two-way radios.
Basic functions The use of dedicated computer keyboard keys to notify the dispatcher about the mobile unit’s status, to make state and national crime information center inquiries and to send messages is generally accepted as a basic MDT function. Using these keys can significantly reduce voice traffic, which is a recognized bottleneck in any agency.
Advanced functions MDT functionality is enhanced by adding: (1) unit and incident inquiry capability. (2) incident disposition (report writing). (3) access to local intelligence files, general data files and shift notes. (4) access to local and county computer data. (5) electronic mail.
Imagine the usefulness of sending a message to an officer who is coming on duty later by having the message automatically downloaded to the officer’s MDT when he signs on in the vehicle. Automatic “hit” identification
As a matter of officer safety, the system should be able to notify the dispatcher when an officer initiates a traffic stop and receives a warning of a possible “hit” on the license plate that has been checked. In some systems, MDT functions are isolated from dispatch, in which case the officer must send redundant messages to keep the dispatcher informed of his activity. Obviously, a system of this type puts the officer in danger and should be avoided. Network options
Rather than investing in a so-called “private” network, consider a semi-private network that your agency can join for a subscription fee, and then purchase the in-car devices. In all cases, these networks have limited functionality because the system’s control is defined by the application software, not the hardware. One or more of the following network options may be available.
Subscription network – Several subscription networks are available. For example, the Illinois Criminal Justice Authority (ICJA) recognized the need for a subscription network for smaller agencies. With this type of network, member agencies need not be concerned with infrastructure. ICJA installed base stations in selected populated areas and a central system control at its Chicago headquarters. ICJA has the software that provides system functions. Member agencies pay a modest user fee and purchase one or more in-car devices. ICJA provides the agencies with operating manuals and an access protocol that allows them to connect their private CAD system via telephone line to ICJA’s central control computer.
Subscription networks such as ICJA’s are subject to performance variations because of their design or intended use and therefore must be regulated; nevertheless, they provide a valuable service to many agencies that otherwise might be deprived of MDT functionality.
Shared infrastructure – A controller can be used to allow multiple agencies using a CAD system to share a single MDT infrastructure. Each agency “sees” the system as private, but the controller allows inter-agency communication. It also allows the infrastructure cost to be shared. Because the infrastructure is shared, participating agencies must use the same manufacturer’s hardware; still, the shared infrastructure can be a cost-effective alternative and a more efficient use of available frequencies.
Statewide network – A CAD system and the control network for the first statewide mobile data network in the United States was provided in 1992. When completed, this system will permit any public safety agency in that state to subscribe to MDT services.
Two of the most significant advantages of this distributed processing design are statewide roaming and frequency balancing. With roaming, a vehicle can travel anywhere in the state and automatically communicate with its home base. Frequency balancing provides maximum system efficiency. Each district has at least two frequencies. Vehicles are assigned automatically to one or the other frequency to balance the load. In the event of a hardware failure, the second frequency also serves as a redundant path to provide uninterrupted communications.
Automatic vehicle location and MDT In 1988, MDT and automatic vehicle location (AVL) functions were integrated for the first time. Previously, individual RF networks were used for each function. AVL data were “piggybacked” onto MDT messages, saving the cost of using separate systems. AVL installed with this technology, together with the reduced cost of the locators, makes AVL an attractive function for providing additional officer safety. When the “piggyback” technique is used, AVL can add overhead to an RF infrastructure, depending on the system’s polling frequency. This technique can add further sensitivity to the protocol efficiency and the number of active vehicles that can be supported.
When considering an MDT network, make sure that the AVL is available from the vendor, and know its cost and implementation method before making a commitment.
Summary As you contemplate whether to implement an MDT network, use your applications provider as a sounding board and obtain its commitment regarding functions that it provides now and that it plans to provide in the future. Get answers to the following questions: (1) Is the vendor the primary supplier of the interface software? (2) Does the vendor have the experience to demonstrate its competence? (3) Is the vendor capable of supporting your current and future applications needs? Most importantly, check references. Talk with similar agencies that have bought MDTs and use their experience to help you to plan and to implement a system that will serve your agency and your community successfully for years to come.
