Cellular Digital Packet Data systems offer what is currently one of the most advanced means of wireless data transmission technology. Generally used as a tool for business, CDPD holds promises for improving law enforcement communications and operations. As technologies improve, CDPD may represent a major step toward making our nation a wireless information society. While CDPD technology is more complex than most of us care to understand, its potential benefits are obvious even to technological novices.
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In this so-called age of information, no one need to be reminded of speed but also accuracy in the storage, retrieval and transmission of data. The CDPD network is a little one year old and already is proving to be a hot digital enhancement to the existing phone network. CDPD transmits digital packet data at 19.2 Kbps, using idle times between cellular voice calls on the cellular telephone network.
CDPD technology represents a way for law enforcement agencies to improve how they manage their communications and information systems. For over a decade, agencies around the world have been experimenting with placing Mobile Data Terminals(MDT) in their vehicles to enhance officer safety and efficiency.
Early MDT’s transmits their information using radio modems. In this case data could be lost in transmission during bad weather or when mobile units are not properly located in relation to transmission towers. More recently MDT’s have transmitted data using analog cellular telephone modems. This shift represented an improvement in mobile data communications, but systems still had flaws which limited their utility.
Since the mid-1990, computer manufacturers and the telecommunication industry have been experimenting with the use of digital cellular telecommunications as a wireless means to transmit data. The result of their effort is CDPD systems. These systems allow users to transmit data with a higher degree of accuracy, few service interruptions, and strong security. In addition CDPD technology represents a way for law enforcement agencies to improve how they manage their communications and information systems. This results in the capacity for mobile users to enjoy almost instantaneous access to information.
WHAT IS CDPD?
CDPD is a specification for supporting wireless access to the Internet and other public packet-switched networks. Data transmitted on the CDPD systems travel several times faster than data send using analog networks
Cellular telephones and modem providers that offer CDPD support make it possible for mobile users to get access to the Internet at up to 19,2 Kbps. Because CDPD is an open specification that adheres to the layered structure of the Open Systems Interconnection (OSI) model, it has the ability to be extended in the future. CDPD supports both the Internet’s Connectionless Network Protocol (CLNP).
CDPD also supports IP multicast (one-to-many) service. With multicast, a company can periodically broadcast company updates to sales and service people on the road or a news subscription service can transmit its issues as they are published. It will also support the next level of IP, IPV6. With CDPD we are assigned our very own address. With this address, we are virtually always connected to our host without having to keep a constant connection.
There are currently two methods for sending data over cellular networks: cellular digital packet data (CDPD) and cellular switched-circuit data (CSCD). Each has distinct advantages depending on the type of application, amount of data to send or receive, and geographic coverage needs. CDPD is currently available to roughly 50 percent of the population base. Two methods to transmit data are used, depending upon the service provider’s network architecture. Some providers have radio channels dedicated to data transmission installed at existing voice cellular sites. Others use voice cellular channels and interleave data messages within the unused portion of voice radio signals. To use a CDPD data service, users require a laptop computer, a connector cable and a CDPD radio modem. Radio modems come in a PC-card format or connect to the user device with a serial cable. Regardless of the method used, messages are broken up into discrete packets of data and transmitted continuously over the network. Messages are then “reassembled” into the original message at the receiving device. This technology supports roaming and is especially attractive for multicast (e.g., one-to-many) service, allowing updates to be periodically broadcast to all users. Users log on once per day to register on the network. Messages and transmissions automatically locate them. Major CDPD providers generally have roaming agreements to allow users to access the service when outside their home coverage area. For the mobile users, CDPD’s support for packet switching means that a persistent link isn’t needed. The same broadcast channel can be shared among a number of users at the same time. The user’s recognizes the packets intended for it’s user. As data such as e-mail arrives, it is forwarded immediately to the user without a circuit connection having to be established. There is circuit switched version, called CS CDPD that can be used where traffic is expected to be heavy enough to warrant a dedicated connection. As a tool for transmitting data CDPD utilizes digital networks. Placing data, conversions, photographs, and multimedia into digital form and transmitting the information through a network with a large bandwidth permits more information to be sent more quickly with greater clarity. Thus, data send using CDPD is received in a quick, secure, and accurate fashion. Data send using CDPD systems is less likely to be lost between senders and receivers due to the position of mobile units, weather conditions, or other anomalies.
CDPD is an overlay to the existing cellular network, which enables users to transmit packets of data over the cellular network using a portable computing device and a CDPD modem. CDPD offers a high-speed, high-capacity, low-cost system with the greatest possible coverage. Additionally data is encrypted for security. CDPD air link transmissions have a 19,200 bps raw data rate.
WORKING OF CDPD
To effectively integrate voice and data traffic on the cellular system without degrading the level of service provided to the voice customer, the CDPD network implements a technique called channel hopping. The way this works is that when a CDPD mobile data unit desires to initiate data transmission, it will check for availability of a cellular channel. Once an available channel is located, the data link is established. As long as the assigned cellular channel is not needed for voice communications, the mobile data unit can continue to transmit data packet bursts on it. However, if a cellular voice customer initiates voice communication, it will take priority over the data transmission. At such time, the mobile data unit will be advised by the Mobile Data Base Station (which is the CDPD serving entity in the cell and constantly checks for potential voice communication on the channel) to “hop” to another available channel. In the event that there are no other available channels, then data transmission will be temporarily discontinued. It is important to note that these channel hops are completely transparent to the mobile data user. As far as the user can see, there is only one data stream being used to complete the entire transmission.
CDPD and current cellular voice network are essentially two separate networks that happen to share cellular airspace. During transmission across cellular telephone channels, there are moments when the channel is idle. In fact, industry research indicates that over 30 percent of the air time, even during heavy traffic times is unused. CDPD technology is able to detect and use these otherwise wasted moments, by packaging data in small packets and sending it in short “bursts” or chunks during the idle time. As a result, the cellular channel operates more efficiently while remaining transparent to the cellular voice network. Thus voice and data transmissions are unaffected. CDPD is based on the same communications protocol as the Internet; so mobile users have access to the broadest range of information.
CDPD accomplishes this amazing feat by transmitting the data in small chunks or packets. Then the chunks can be sent separately on whatever channel is available at the time of transmission. Here is an analogy to help visualize this:
Instead of channels or lines we now have pipes. The pipes can be open in the traditional way. Now no one can use the pipe while you have it open. You send your data down the pipe. Now you wait for a response. Nothing is going through the pipe but still no one else can use it. Majority of your data transmission using this method is idle time that some else could exploit. CDPD is the new way. You have little bundles of data. You find a pipe that is available and you open it. You send your data and shut the pipe again. If that pipe is available when you send more data, you use it. If not, you find a new pipe. You get a response through which ever pipe is unused when the host sends you the data. With CDPD you are assigned an IP address. This is better than a phone number. This allows you to change channels or “pipes” and still send and receive data as if you were directly connected to your host
INTERFACES AND COMPONENTS OF CDPD NETWORK
The CDPD network consists of three interfaces and five network components. They are as follows:
A Interface: A Interface is the abbreviated form of Air Interface. It is the interface between the mobile end station (M-ES) and the cell site equipment (MDBS). It consists of a new media access and control (MAC) protocol unique to CDPD, IP protocols used for routing, existing RF equipment and mobility management features such as RF “sniffing” and the channel “hopping”. The Air Interface constitutes the space between our antenna and the cell site tower.
I Interface: I Interface is the abbreviated form of Intermediate system Interface. It is the interface between routing functions within the network, or between the routing functions on a network-to-network interface. It is the interface between the “off the shelf” routers. It consists of off the shelf router equipment, such as Wellfleet, employing the IP or ISO 8473 profiles. Use of other profiles X.25 and Frame relay is considered to be optional.
E Interface: E Interface is the abbreviated form of External Interface. It is the interface connecting a non-CDPD network with a CDPD network. It is typically a leased line connecting a User’s fixed end system (F-ES) to a CDPD network provider.
Mobile End System (M-ES): The M-ES can be any mobile computing device which has a CDPD modem built -in or attached. It supports the MAC functionality required over the A interface and IP protocols that are mandatory for addressing and mobility management. The M-ES transmits data over the air link to the Mobile Data Base Station (MDBS) located in the cell site. The M-ES is also concerned with radio resource management such as discovering and keeping synchronization with RF data streams from an MDBS. The M-ES should provide a transparent interface to the user’s applications. The CDPD modem is a very good example for the M-ES.
Mobile Data Base Station (MDBS): MDBS is located at the cell site. It acts as a relay station between M-ES and MD-IS systems. The MDBS relays packets of data to the MD-IS (Mobile Data Intermediate System) located at the MTSO (Mobile Telephone Switching Office). The MDBS is primarily responsible for radio frequency management; such as making sure that M-ES does not transmit on a frequency that is currently being used by the cellular voice, channel hopping, and aiding the M-ES to transfer from one cell to another by assisting in the location of a new channel. It provides a hop-to-hop control over the air interface. It controls the hop and RF segment between the M-ES and the CDPD network. Each cellular geographic service area (CGSA) is controlled by MBDS.
Mobile Data Intermediate System (MD-IS): The MD-IS keeps track of an M-ES’s location and routes data packets to and from the CDPD Network and the M-ES appropriately. In addition, the MD-IS is responsible for validating an M-ES on the network, and exchanging the encryption keys with the M-ES that allows for secure transmission of data over the air link. It is responsible for IP routing. MD-ISs are the only routers that are aware of mobility of the M-ESs. These devices support a CDPD specific mobility network location protocol (MNLP) which allows the exchange of mobility information. An MD-IS may serve single or multiple CGSAs (cells). MD-IS will also determine where an M-ES is and routes between the F-ES and M-ES. An MD-IS can provide “mobile home” and “mobile serving” functions.
Intermediate System (IS): The IS is a router and an off-the-shelf device. The IS routes the data through the IP and the CLNP network. The Intermediate System is a standard IP router with the primary responsibility of relaying data packets. It consists of the off-the-shelf commercially available router equipment, such as manufactured by Wellfleet. Most ISs are unaware of mobility. The IS components are the backbone of the CDPD mesh.
Fixed-End System (F-ES): The F-ES is whatever network environment/resource the User is attempting to make available to mobile access via CDPD. The F-ES is the final destination of the message sent from an M-ES. The Fixed-End System receives the data and processes it appropriately. The F-ES can be one of many stationary-computing devices, such as a host computer, a UNIX workstation, an online information service, or another Mobile-End System. CDPD subscribers administrate external F-ES. The CDPD operator such as Ameritech or US West administrates internal F-ES. The F-ES could be the directory services database. It is important to realize that the F-ES is not required to be aware of mobility issues in anyway whatsoever. As such, an F-ES should be able to connect to a CDPD network with absolutely no modifications. This is probably be a router connected to a token ring or other LAN.
Mobile Access Control (MAC): The MAC function used over the air interface is unique to CDPD. It creates a bit stream by taking the bits within the MDLP frames and “blocking” them into a format that incorporates a sophisticated forward Error Correction scheme known as Reed/Solomon. The MAC defines a “back-off” procedure in the event of suspected collisions with data streams from other M-ES.
WHAT MAKE CDPD RELIABLE?
SOURCES OF ERRORS
Machines do make errors, and their non-man-made mistakes can turn otherwise flawless programming into worthless, even dangerous, trash. Just as architects design buildings that will remain standing even through an earthquake, their computer counterparts have come up with sophisticated techniques capable of counteracting the digital manifestations of Murphy’s Law.
Digital information, by definition, consists of strings of “bits” — 0’s and 1’s — and a physical device, no matter how capably manufactured, may occasionally confuse the two. The most probable reason for this is the low powered transmission of data over long distances.
Mobile communications use radio signals which are subject to eavesdropping. The mobile network is also vulnerable to other unwanted security breaches.
WHAT HAS TO BE DONE TO MAKE A NETWORK RELIABLE?
Fig.1 Data over GSM network
There are some actions that are necessary in order to obtain reliability over a network.
The procedure which checks if the identity of the subscriber transferred over the radio path corresponds with the details held in the network.
Instead of the actual directory telephone number, the International Mobile Subscriber Identity (IMSI) number is used within the network to uniquely identify a mobile subscriber.
Protection against impersonation of authorized users and fraudulent use of the network is required.
Protection of user data
All the signals within the network are encrypted and the identification key is never transmitted through the air. This ensures maximum network and data security.
The information needed for the above actions are stored in databases. The Home Location Register (HLR) stores information relating the subscriber to its network. This includes information for each subscriber on subscription levels, supplementary services and the current or most recently used network and location area. The Authentication Center (AUC) provides the information to authenticate subscribers using the network, in order to guard against possible fraud, stolen sub scriber cards, or unpaid bills. The Visitor Location Register (VLR) stores information about subscription levels, supplementary services and location for a subscriber who is currently in, or has very recently been, in that area. It may also record whether a subscriber is currently active, thus avoiding delay and unnecessary use of the network in trying to call a switched off terminal.
OVERHEAD ENSURES RELIABILITY
Although the raw data rate for CDPD is 19.2 kbps the actual throughput rate is more in the vicinity of 9.6 kbps. The reason is that CDPD adds a large amount of overhead to each block of transmitted data for reliability. The CDPD encodes each block using a systematic Reed-Solomon forward-error-correcting code. In addition, the information and parity bits in each block are exclusive-ORed with a pseudorandom sequence after Reed-Solomon encoding to assist the MDBS (Mobile Data Base Station) and M-ES (Mobile End System) modems in maintaining bit synchronization. A color code is added to each block to detect co channel interference from a remote MDBS or cell site. Encryption and decryption are also part of the specification, which ensures that a customer’s data is private and which protects the service from fraud.
Fig. Typical mobile data schematic including mobile data terminal.
CDPD vs. OTHER TECHNOLOGIES
Cellular Digital Packet Data (CDPD)
Circuit Switched Cellular
Specialized Mobile Radio (Extended)
Proprietary Wireless Data Networks
Cost of Service
Cost of Deployment
CIRCUIT SWITCHED CDPD (CSCDPD)
In 1995, a few members of the CDPD Forum saw an opportunity to add to the CDPD System Specification through the definition of a new complementary service. The thought was that if CDPD services could be available through the existing cellular voice telephone connections, the requirement for nationwide coverage would be instantly realized.
With the goal of developing a complementary standard to allow mobile devices to access the CDPD network through cellular voice telephone circuit switched connections, the group examined the CDPD system architecture. It became quickly obvious that the layered communications architecture has provided a great flexibility to accomplish this.
The development team examined the CDPD system architecture on a layer by layer basis. The resultant architecture shown in Figure 6.1 is based on the following considerations.
fig :cscdpd components
The physical layer was first examined. There really wasn’t much to decide here. Since the intent is to make use of the cellular telephone voice channel, the GMSK modulation scheme cannot be used without changes. Furthermore, since there are already cellular modem devices available, with mass manufactured chip sets, it makes much more sense to rely on that technology. All through the design of CDPD, the philosophy has been to define new technology only when necessary. The development team wisely chose to use the developments of the cellular telephone modem industry.
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The data link layer in CDPD is divided into two sub-layers, the Medium Access Control sub-layer and the Logical Link Control sub-layer. In the Circuit Switch CDPD system, the use of individual circuits for each mobile means that there is no sharing of the RF channel in use. As such, there is no need for a Medium Access Control function.
The Logical Link Control function is responsible for establishment and management of a point-to-point connection between the CS CDPD mobile device and the CS CDPD network. The basic requirements of this layer include:
¥ Reliable delivery of data frames
¥ Sequenced delivery of data frames
¥ Link establishment and disconnection
Much of these requirements are already satisfied by the typical modern day modem equipment. Current modem technologies typically include end to end protocols and procedures to provide error detection and correction, call establishment and disconnection. However, the development team identified additional control parameters that are necessary for efficient operation within the CDPD network. These additional functions are used to ensure transparent operation to the mobile user and efficient use of the RF channel. The requirements were extensive enough to require the establishment of the Circuit Switch CDPD Control Protocol (CSCCP) to be used on top of widely available reliable modem protocols.
Above the data link layer, there is the Network Layer. The lower sub-layer of the Network Layer is the Sub network Dependent Convergence Function. The SNDCF is specifically defined to address mismatch in service requirements of the Network Layer and service characteristics of the Data Link Layer. In the early stages of CS CDPD system design, there were considerations to alter the CDPD SNDCP. However, as the system design progressed, it became obvious that the SNDCP need not be modified. Some minor adjustments in terms of maximum frame size may have provided some efficiency gains, but general consensus was reached that such gains were small and may make implementation of dual mode devices more complex. SNDCP is not changed.
WHY IS CDPD TECHNOLOGY BETTER THAN CIRCUIT SWITCHED?
CDPD is not better than Circuit Switch for transmitting data, but rather it is different. They both have their place in the cellular wireless solution, and it may take the combination of both services to provide the customer with the optimal solution.
As stated above, the two technologies are different. CDPD is connection-less. It sends each packet intermittently, when there is “space” available. Circuit Switching on the other hand, sends the data over a continuous connection. For this reason, CDPD would be the optimal solution for a customer who is sending information which is both “short” and “bursty”, the circuit switch solution would be optimal when sending a large data transmission. Another important difference is that CDPD uses less power than circuit switching. Since the information is sent in short bursts, the device only has to be at high levels of power for short intervals. Additionally, CDPD has a “sleep” mode which allows the device to conserve power when not in use, without logging off of the network. Therefore, a mobile computing device will have longer battery life using the CDPD technology rather than circuit switched connections. Lastly, CDPD uses an encryption technique to prevent an outside source from receiving the transmitted data.
Advantages & Disadvantages of CDPD
Theoretical maximum speeds of up to 19.2kbps are achievable with CDPD, which is faster than any other national wireless service offering available today. Immediacy
CDPD facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to cellular coverage. No dial-up modem connection is necessary.
Since CDPD is based on one of the most widely accepted networking protocols, it provides unparalleled ease of use with existing IP-based applications in a wireless environment.
Reliability and Security
CDPD packets use forward error correction methods that reduce the effects of noise and interference on the air link. CDPD also incorporates authentication and encryption to all packet transmissions over the air link. With the use of standard network protocols, users can add application-based end-to-end transmissions.
Since CDPD utilizes existing cellular networks, carriers can deploy CDPD services in a short period of time to meet customers’ unique coverage requirements. Cost Effectiveness
When transmitting appropriately sized data messages, CDPD can be substantially less expensive than making the same connection during a data-over-cellular session.
From full size vehicle mounted PC to CE and PalmPilot
Significantly reduce hardware cost
Accommodate multitude users
Limited availability for wide area
Must be quasi-stationary to use
Recurring monthly costs
There may be no coverage available.
Ongoing cost, and packet-based billing may limit the amount of data that can be economically transferred.
Not yet fully deployed.
Coverage not available in less-populated areas.
Priority access not available for government users.
Potentially significant ongoing costs.
CDPD technology, coupled with specialized software and advances in MDT systems, is moving police organizations toward the day when it will be routine to operate as a “paperless” organization.
Using MDT systems, officers can file virtually any report from their patrol vehicle. Accident reports, parking tickets, court citations, and traffic citations can be prepared using the MDT, printed in the officer’s vehicle, and transmitted via CDPD to headquarters. Every report is legible, cannot be lost, and instantly appears in the agency’s computer system. This reduces the time officers spend in police facilities and increases time spent in the community.
Supervisors can monitor the status of all the units in heir patrol area at the touch of a button. A British police force recently equipped a horse-mounted officer with a wireless, handheld MDT unit operating with CDPD technology. As technologies improve (and costs decline) it may be possible to equip all officers with such handheld units (just as most officers now are issued handheld two-way radios).
The wireless networks we have now are not perfect. They are slow and vulnerable compared to wired networks, and exist as a community of devices – laptops, PDAs, and mobile phones — that we can only hope will one day be integrated into a single, lightweight, easily portable unit. Still, it is already easy to see the advantages of wireless. The mobility that comes with even a simple wireless network seems to lend itself to desirable enhancements to education such as collaborative learning, extending the classroom into nontraditional spaces, increasing the ease with which handicapped students can use campus resources, and providing the entire campus population with unprecedented access to information. These assets combined with the relatively affordable cost of outfitting even old, quirky buildings with the technology make wireless networks and colleges a natural match Mobile data services are here today and provide solid business that are quantifiable. Users of traditional data networks should be aware of the capabilities wireless data communications offer, and must consider how these applications can be used effectively as business tools.
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