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ABSTRACT
Cellular Digital Packet Data (CDPD) is a specification for supporting wireless access to the internet and other public data networks. CDPD transmits digital packet data at 19.2 Kbps, using idle times between cellular voice calls on the cellular telephone network. CDPD technology represent a way for law enforcement agencies to improve how they manage their communications and information system.
CDPD technology represent a way for law enforcement agencies to improve how they manage their communications and information systems data transmitted on the CDPD systems travel several times faster than data send using analog networks.
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 page link transmissions have a 19,200 bps raw data rate. As a tool for transmitting data CDPD utilizes digital networks.
The Raven is a rugged, full duplex Cellular Digital Packet Data (CDPD) modem that provides wireless transport capabilities for fixed and mobile applications. The Raven is an efficient and secure wireless packet data technology that is ideal for un-tethered applications.

1. INTRODUCTION

Cellular Digital Packet Data (CDPD) 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.
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 represent 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 MDTâ„¢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 MDTâ„¢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-1990â„¢s, 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 represent 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.
2. 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 Internetâ„¢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, CDPDâ„¢s support for packet switching means that a persistent page link isnâ„¢t needed. The same broadcast channel can be shared among a number of users at the same time. The userâ„¢s recognizes the packets intended for itâ„¢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 page link transmissions have a 19,200 bps raw data rate.
The Raven is a rugged, full duplex Cellular Digital Packet Data (CDPD) modem that provides wireless transport capabilities for fixed and mobile applications. The Raven is an efficient and secures wireless packet data technology that is ideal for un-tethered applications. The CDPD Raven is ideally suited for all fixed and mobile Industrial and Commercial Applications, including Telemetry, SCADA, Public Safety, Dispatch, Field Service, Financial Transaction Processing, and Security. The Ravenâ„¢s embedded TCP/IP stack enables virtually any type of remote device to access the CDPD network.
The Raven is currently installed with many different types of Remote Terminal Units (RTUs), which perform remote metering, and monitoring functions in the oil, gas, and water industries. The CDPD Raven can be a low-cost replacement for existing landline, private radio and circuit-switched cellular installations.
Features
¢ 19.2 kbps raw data transfer rate
¢ Full duplex transceiver
¢ 600 mW transmit power
¢ Integrated TCP/IP protocol stack
¢ Proven technology
¢ Compact size
¢ Rugged aluminum case
¢ LEDs show status of CDPD operation
¢ Mounting brackets
Application Interfaces
Standard interfaces include:
¢ AT command serial character streams (uses embedded TCP/IP stack)
¢ Host TCP/IP stack communicates with Raven using SLIP
¢ Windows 95/98/NT Dial-Up Networking communicates with Raven using PPP
In addition, several protocol conversions (Modbus, Opto22, BSAP) have been implemented to allow hosts to communicate with RTUs over CDPD without modifying the host or remote software.

3. 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 page 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.
4. INTERFACES AND COMPONENTS OF CDPD NETWORK
The CDPD network consists of three interfaces and five network components. They are as follows:
INTERFACES
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 Userâ„¢s fixed end system (F-ES) to a CDPD network provider.
COMPONENTS
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 page 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-ESâ„¢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.

5. WHAT MAKE CDPD RELIABLE?
5.1 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.
5.2 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.
¢ User Authentication
The procedure which checks if the identity of the subscriber transferred over the radio path corresponds with the details held in the network.
¢ User Anonymity
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.
¢ Fraud Prevention
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.

5.3 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.2 Typical mobile data schematic including mobile data terminal.
You can't always get what you want -- but if you try, sometimes, you just might find you get what you need. Reed and Solomon managed to get a coding system that was based on groups of bits (bytes) rather than individual 0s and 1s.That feature makes Reed-Solomon code particularly good at dealing with "bursts" of errors.
Mathematically, Reed-Solomon codes are based on the arithmetic of finite fields. Indeed, the 1960 paper begins by defining a code as "a mapping from a vector space of dimension m over a finite field K into a vector space of higher dimension over the same field." Starting from a "message" $(a_0, a_1, . . .a_{m-1})$, where each $a_k$ is an element of the field K, a Reed-Solomon code produces $(P (0),P(g), P(g^2), . . ., P(g^{N-1}))$, where N is the number of elements in K, g is a generator of the (cyclic) group of nonzero elements in K, and P(x) is the polynomial $a_0 + a_1x + . . . + a_{m-1} x^{m-1}$. If N is greater than m, then the values of P over determine the polynomial, and the properties of finite fields guarantee that the coefficients of P--i.e., the original message--can be recovered from any m of the values.
Conceptually, the Reed-Solomon code specifies a polynomial by "plotting" a large number of points. And just as the eye can recognize and correct for a couple of "bad" points in what is otherwise clearly a smooth parabola, the Reed-Solomon code can spot incorrect values of P and still recover the original message. A modicum of combinatorial reasoning (and a bit of linear algebra) establishes that this approach can cope with up to s errors, as long as m, the message length, is strictly less than N - 2s.
In today's byte-sized world, for example, it might make sense to let K be the field of degree 8 over $Z_2$, so that each element of K corresponds to a single byte (in computerese, there are four bits to a nibble and two nibbles to a byte). In that case, $N = 2^8 = 256$, and hence messages up to 251 bytes long can be recovered even if two errors occur in transmitting the values $P(0), P(g), . . ., P(g^{255})$. That's a lot better than the 1255 bytes required by the say-everything-five- times approach.
5.4 CDPD vs. OTHER TECHNOLOGIES
It is crucial to develop a secure personal mobile communication environment as secure information flow and privacy are amongst users' top priorities today. CDPD , at 19.2 kbps is not only the fastest available technology, but also the most secure due to its built in RC4 encryption algorithm.
Cellular Digital Packet Data (CDPD) Circuit Switched Cellular Specialized Mobile Radio (Extended) Proprietary Wireless Data Networks
Speed best best good good
Security best better good better
Ubiquity best best good better
Cost of Service best better better good
Cost of Deployment best best better good
Mobility best good better good
Interoperability best good good better
6. CIRCUIT SWITCHED CDPD (CSCDPD)
6.1 INTRODUCTION

The astute reader may object to the title of this section. It seems inappropriate to fashion a title that contains circuit switch concepts with a packet data network! Furthermore, what is this topic doing in a section of network access? This strange marriage of the two technologies is discussed in this section.
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 6.1: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.
The data page 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:
Â¥ Reliable delivery of data frames
Â¥ Sequenced delivery of data frames
Â¥ 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 page 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 concensus was reached that such gains were small and may make implementation of dual mode devices more complex. SNDCP is not changed.
Since the SNDCP is not changed, protocols at the Network Layer and above are also unchanged. This ensures transparency of application operation between CDPD and CSCDPD .
6.2 Circuit Switch CDPD Control Protocol
The purpose of the Circuit Switch CDPD Control Protocol is to provide the services necessary to maintain efficient CDPD mobility management function on circuit switched data modem technology. The goals for this protocol are:
Â¥ Use of circuit switched connection
Â¥ Efficient use of circuit switched technology
Â¥ Continual connection to network
Â¥ Efficient circuit switched backbone connections
Â¥ Robust connection
To address these design goals, the specification team developed the following CS CDPD messages:
Â¥ Connection Request
Â¥ Connection Response
Â¥ Reconnection Request
Â¥ Reconnection Response
Â¥ SNDCP Data Packet
Â¥ SNDCP Unit data Packet
Â¥ Link Reset
Â¥ Link Reset Acknowledge
The use of these message to achieve the design goals may best be illustrated through examples of connection events. The events presented include the following:
Â¥ Initial connection (by the mobile)
Â¥ CS CDPD M-ES initiated reconnection
Â¥ CS CDPD MD-IS initiated reconnection
Â¥ Redirection
Â¥ Redirection with override
Â¥ Link Reset
6.3 Working
Initial Connection
The CS CDPD connection begins with the connection request by the mobile device. Within the CS CDPD specifications, the mobile device is called the CS CDPD Mobile End System or CM-ES. Just as in CDPD, the CM-ES must initiate the connection.
The CM-ES starts by selecting a dial code from the list programmed into the device by the service provider. Using an appropriate dial code, the CM-ES establishes a circuit switched data connection. The peer end point of this circuit is the CS CDPD MD-IS or CMD-IS. The CM-ES then sends a Connection Request message carrying the following parameters:
Â¥ CM-ES Equipment Identifier
Â¥ V.42bis data compression parameters
Â¥ Duration time
Â¥ Cellular System Identifier (AMPS System ID)
Â¥ Dial code
The CMD-IS responds with a Connection Response message containing the following parameters:
Â¥ CMD-IS Identifier
Â¥ Service Provider Network Identifier (SPNI)
Â¥ Wide Area Service Identifier (WASI)
Â¥ V.42bis compression parameter response
Â¥ Result code
This exchange of messages (see Figure 6.2) allows the CM-ES and the CMD-IS to identify themselves to each other and establish compression negotiation parameters. In addition, the CM-ES informs the CMD-IS of the dial code to use in order to contact the CM-ES.

fig 6.2:CM-ES Initial connection
Once a successful connection has been established, the CMD-IS initiates the exchange of encryption keys. From this point forward the communications process proceeds as in standard CDPD.
This interchange of messages achieves the first goal of establishing a circuit switched connection.
CM-ES Initiated Reconnection
After the connection has been established, data transfer between the CM-ES and the network proceeds. For most connections, there are periods of inactivity. On a circuit switched connection, these periods are wasteful since the page link cannot be shared by other devices. To account for this data traffic characteristic, the CM-ES disconnects after a predetermined idle period and suspends the data page link connection.
When the CM-ES has data to send after having disconnected, it must initiate reconnection procedures. This is accomplished by the CM-ES selecting an appropriate dial conde and establishing a circuit connection. However, unlike the initial connection, the CM-ES sends a Reconnection Request message (see Figure6.3) which contains only the CM-ES Equipment Identifier (EID). This EID allows the CMD-IS to quickly ascertain this to be a reconnection by a previously connected device. There is therefore no need to repeat the exchange of data compression parameters and AMPS system ID. The CMD-IS responds with a Reconnection Response carrying the CMD-IS ID. This allows the CM-ES to quickly confirm that it has reconnected to the same CMD-IS.

fig 6.3 CM-ES Initiated Reconnection
Once the reconnection message exchange has been successful, the two peer entities resume the suspended data page link connection.
The only other deviation from the CDPD system is the use of an End System Query message to force the exchange of registration data and authentication credentials. This is an added precaution to avoid fraudulent access.
This procedure achieves the second goal of efficient use of the circuit switch technology. There is no need to keep the circuit switched connection active when there is no data to transfer.
CMD-IS Initiated Reconnection
After the data page link is disconnected due to an extended idle period, it is possible that the network needs to deliver data to the CM-ES. In the usual circuit switch connection scenario, this is not possible. The mobile user must periodically "check-in" for data. However, the CS CDPD designers wanted to offer connection service similar to CDPD. In which case, the network must be able to initiate reconnection to the CM-ES.
One of the optional parameters provided by the CM-ES during initial connection is a dial code. This dial code is to be used when the network wishes to initiate reconnection. Therefore, when the CMD-IS has data to send to the CM-ES, it establishes a circuit switch connection using the earlier supplied dial code. Once connected, the CMD-IS sends a Reconnection Request message (see Figure6.4) containing the CMD-IS ID. If the CM-ES finds the CMD-IS ID acceptable, it responds with a Reconnection Response containing the CM-ES ID. Once again, the two peer entities resume the suspended data page link connection. The End System Query is sent by the CMD-IS to cause registration and authentication.

fig 6.4 CMD-IS Initiated Reconnection
The procedure achieves the third goal of allowing the CM-ES to be logically continually connected to the network without the need to maintain the circuit switch link.
Redirection
Due to the mobile nature inherent in CDPD devices, it is possible that a CM-ES relocates to an area where the dial code normally used is not the optimal path through the infrastructure. This can result if the service provider has a local point of presence through a local modem bank (see Figure6.5), or that the service provider has an alternate CMD-IS at the local system (see Figure6.6).

fig 6.5 Redirect To Local Modem Bank


fig 6.6 Redirect To Local CMD-IS
In these cases, it may be more efficient for the mobile to use the a different set of dial codes to access the network. This is provided for in the CSCCP through the Redirect result code.
The procedure occurs on the initial connection request. The CM-ES proceeds with an initial connection request but the CMD-IS responds with a Connection Response message containing the optional parameter to indicate a Redirect directive. Along with that directive, a list of alternate dial codes is provided.
The CM-ES, barring other problems, disconnects from the CMD-IS and attempts to re-initiate connection requests with one of the new dial codes. If for some reason the new dial codes are not operational, the CM-ES may retry the connection with the original dial code and issue a Redirect Override indicator. If the CMD-IS cannot accept any connection requests, it may issue a Forced Redirection command.
This procedure allows the service provider to instruct the mobile device to access the network at the most efficient point of presence.

Robust Connections
Even though the CS CDPD system has been built on using reliable data transfer mechanisms available from current modem technologies, errors may rise from various internal connection points. To address these errors, the CS CDPD specification included an error recovery mechanism.
The mechanism is achieved in two steps. First, the data transfered is contained in the CSCCP SNDCP Data Packets. These messages contain both a simple checksum and a sequence number. The receiver of each message verifies the checksum. If a checksum failure is detected, the page link is reset by the receiver issuing a Link Reset message (see Figure6.7). This Link Reset message contains the sequence number of the failed packet. The peer entity then responds with a Link Reset Acknowledge packet containing its next expected sequence number. Once the Link Reset message and the Link Reset Acknowledge messages are exchanged, the two entities reset their sequence numbers to 0 and restart the exchange of SNDCP Data packets from the point of the failure.

fig 6.7 CSCCP Link Reset Procedure
This procedure corrects the small residual error probability of the link.
7.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.
The following two tables demonstrate the effectiveness of CDPD technology as compared to Circuit Switched:
CDPD Circuit Switched
Efficient for short Efficient for very large
to large burst transmissions
transmissions
No call set-up or Call set-up and take down
take down delays required
Broadcast Point to point
capabilities connections
One log-on at power Log -on (call) for every
up transmission
Power ramping for Power ramping for entire
short bursts connection
Airlink Security Airlink unsecured

Application CDPD Circuit
Switched
E-Mail Good Fair
Remote Terminal to Host Poor Good
Comm.
FAX Poor Good
Remote Equipment Good Fair
Monitoring
Remote Control of Good Fair
Machinery
Small File Transfers Good Poor
(1-1000 bytes)
Large File Transfers Poor Good
(5000+ bytes)

8. Advantages & Disadvantages of CDPD
ADVANTAGES
Speed
Theoretical maximum speeds of up to 19.2kbps are achievable with CDPD, which is faster than any other national wireless service offering available today. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, CDPD is a less costly mobile data service compared to Circuit Switched Data. Circuit Switch is generally charged on a time basis (like a voice call) where as CDPD is a fixed fee per month per vehicle.
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. This is why CDPD users are sometimes referred to as being "always connected". Immediacy is one of the advantages of CDPD when compared to conventional radio or circuit switched data. High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds.
Open Specification
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. Additional software is also available which optimizes the performance of the IP applications for CDPD networks. This ensures economical wireless data transmissions via a CDPD network.
Reliability and Security
CDPD packets use forward error correction methods that reduce the effects of noise and interference on the airlink. CDPD also incorporates authentication and encryption to all packet transmissions over the airlink. With the use of standard network protocols, users can add application-based end-to-end transmissions.
Availability
Since CDPD utilizes existing cellular networks, carriers can deploy CDPD services in a short period of time to meet customers' unique coverage requirements. The base technology platform allows for easy integration with existing applications, and a significant capability to provide wireless Internet accessibility. This also means there are many companies offering many hardware, software and network options.
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.
Device Flexibility
o From full size vehicle mounted PC to CE and PalmPilot
o Significantly reduce hardware cost
o Accommodate multitude users
DISADVANTAGES
? 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.
? Newer technology.
? Does not support sustained data transfers well.
9. THE FUTURE
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. Several agencies throughout the United States have already adapted existing technologies to allow them to make such a transformation.
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 officerâ„¢s vehicle, and transmitted via CDPD to headquarters. Every report is legible, cannot be lost, and instantly appears in the agencyâ„¢s computer system. This reduces the time officers spend in police facilities and increases time spent in the community.
As telecommunications technologies improve, the two-way radio may become obsolete except as a tool for priority traffic. Information once transmitted via radio can be transmitted via computer on a secure digital cellular frequency. Police communications personnel can dispatch officers, provide updated information on calls, and monitor the status of patrol units without touching a conventional radio. Officers in the field can receive calls for service, perform background checks, and complete reports without tying up a radio channel.
Supervisors can monitor the status of all the units in their patrol area at the touch of a button. Personnel needing to use their two-way radio for a priority transmission may do so without having to wait for another unit to report on the status of a barking dog complaint.
A paperless department could significantly increase the operating efficiency of a department covering a large jurisdiction with few officers. Field personnel would no longer have to travel out of their patrol zone to drop off paperwork at the end of their shift. It would no longer take hours or days for a report to make its way from a field officer to the records division. Officers working in the most remote recesses of an agencyâ„¢s jurisdiction can quickly and easily submit reports. Time once spend relaying paperwork can be spent on other priorities.
MDT options are quickly expanding beyond the confines of patrol cars. 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). Officers on patrol may take their computer unit out of their vehicle on calls for service. Investigators at the scene of a crime might be able to operate more effectively by accessing departmental records in order to check for similar offenses. Even officers working on a footbeat might be able to access databases and complete reports without leaving their assigned post.

As other technologies and the Internet continue to develop, CDPD may be used to support a wide variety of other police applications. Real-time digital cameras could page link an investigator in the field with experts around the world. A detective in a small town could page link with a state arson investigator to receive instant assistance in processing a crime scene.
Investigators in different countries could instantly share information as they track an international organized crime group or terrorist organization. Given the remarkable advances which have taken place in the past 15-20 years, it is hard to imagine the changes policing will undergo in the next two decades.

10. CONCLUSION
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.
? Novel idea of using voice channels to carry data.
? Future: Data transport overwhelms voice
? Crossover will occur next year
? Eclipse will occur within another year
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.

11. REFERENCES
1. Google.com
2. cdpd.org
3. rrm.org
4. gpr.org


CONTENTS
1. INTRODUCTION
2. WHAT IS CDPD?
3. WORKING OF CDPD
4. INTERFACES AND COMPONENTS OF CDPD NETWORK
5. WHAT MAKE CDPD RELIABLE?
5.1 SOURECES OF ERRORS
5.2 WHAT MAKE A NETWORK RELIABLE?
5.3 OVERHEAD ENSURES RELIABILITY
5.4 CDPD VS OTHER TECHNOLOGIES
6. CSCDPD
6.1 INTRODUCTION
6.2 CSCDPD PROTOCOLS
6.3 WORKING
7 WHY IS CDPD BETTER THAN CIRCUIT SWITCHED?
8 ADVANTAGES AND DISADVANTAGES OF CDPD
9 THE FUTURE
10 CONLUSION
11 REFERENCES



ACKNOWLEDGEMENTS

I express my sincere thanks to Prof. M.N Agnisarman Namboothiri (Head of the Department, Computer Science and Engineering, MESCE), Mr. Zainul Abid (Staff incharge) for their kind co-operation for presenting the seminars.
I also extend my sincere thanks to all other members of the faculty of Computer Science and Engineering Department and my friends for their co-operation and encouragement.
Nima Nazarulla K