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CDPD

1. INTRODUCTION

PURPOSE The Commercial Wireless Applicability Report provides a tool for public safety communications system planners to help them make informed decisions when considering the use and purchase of commercial wireless services. This report identifies key characteristics of commercial wireless services, analyzes these services, and compares and evaluates these services based on the communication requirements of the public safety community.

CDPD – CELLULAR DIGITAL PCKET DATA It is a technique used for transmitting small chunks of data, commonly referred to as packets over the cellular networks in a reliable manner. It allows users to send and receive data from anywhere in cellular coverage area at any time quickly and efficiently. CDPD is a packet switched communication network that operates as an overlay on the cellular system. Packet networks including the Internet breaks a stream of data in to independent blocks of data, called packets. Each packet is transmitted independently carries its own destination and error correction information. Thus packet networks perform extremely well in systems with variable and channel quality both of which are found in wireless environment. CDPD provides an advantage over circuit switched (dial up) data, in that we have to connect to the network once, there is no need to dial in repeatedly. CDPD technology provides extensive high speed (data can be sent over cellular air link at networks at rate 19.2kb/s), high capacity, cost effective data services to mobile users. With this technology, both voice and data can be transmitted over existing cellular channels. CDPD utilizes digital networks. Placing data conversations, photographs, and multimedia in to binary (0 & l) form and transmitting the information through a network with a large bandwidth permits more information to be sent more quickly with greater clarity.

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CDPD CDPD is a wireless mobile data network service that allows users to perform various data applications, such as remote database access and file transfer. CDPD uses channel hopping or dedicated voice channels on the cellular network to provide packet data capabilities. Packet data transmission allows CDPD service providers to charge customers based on the amount of data sent instead of the amount of time required for a transmission. Service plans vary, but costs are generally $.02 to $.10 per kilobyte. CDPD may transmit data at speeds up to 19.6 Kbps, although actual user speeds are typically from 10 to 12 Kbps, depending on the level of traffic on the network. This service uses a data format similar to the one used for Internet communications, which allows most data applications to be supported through CDPD services. CDPD response times for database inquiries average fewer than 5 seconds but can be more depending on network conditions. Because CDPD is a packetswitched cellular service, it takes minimal time for call setup. CDPD service is available in half of the geographic areas of the US and in 30 international markets. However, the service is not available in all areas that offer cellular service. In addition, CDPD coverage may be spotty in areas with poor cellular coverage. Since CDPD shares the resources of the cellular network, conflicts may occur between competing cellular voice traffic and packet data traffic. The likelihood of congestion depends on the type of CDPD network implemented. The two types of networks are channel hopping and dedicated. Channel hopping CDPD transmits information on the unused capacity of the cellular voice network. Users on a channel hopping network compete with cellular voice users. Conversely, dedicated CDPD sets aside certain channels for CDPD traffic. Therefore, CDPD capacity does not vary as cellular voice calls increase or decrease. However, users must still compete with other CDPD users for call setup and channel capacity.

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CDPD

2 CDPD NETWORK

CDPD is a data network designed to meet growing mobile data transmission needs. CDPD networks can stand-alone or overlay on the cellular system by drawing radio resources from the pool of unused or free cellular analog voice channels. Detection as to whether a channel is free or occupied is mechanized through channel sniffers. Once the CDPD network seizes a channel it continue to use the channel as long as it is free and leaves the channel within 40 ms of initiation of any voice activity on the channel. The CDPD network then hops to another free channel, if available. CDPD follows slotted nonresistant Digital Sense Multiple Access with Collision Detection (DSMA/CD). In addition, it performs channel error detection and channel recovery. Several aspects of CDPD network are, i.

Effect of CDPD on cellular voice and

ii.

Delay throughput performance due to DSMA/CD with error detection. CDPD has been designed to provide connectionless network services to

mobile end system. It is a relatively new data network. The radio resources for such a connectionless network are drawn from the pool of free or unused channels in existing analog cellular voice systems. CDPD is a connectionless network that supports Internet Protocol (IP) and connectionless network protocol (CLNP). Now CDPD is available in over 40 states. Current usage of CDPD includes credit card verification, financial transactions, and remote telemetry. CDPD service is growing rapidly. Because, in many situations, it is most cost effective than other private packet radio network services such as ardis and ram.

2.1

NETWORK BASED COMPNENTS

The CDPD overlay network is made up of a combination of key components that operate together to provision the overall service. These components are described below:

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CDPD

2.1.1 Mobile End System (M-ES) It is defined as any mobile computing device, which is equipped with a CDPD modem (e.g. a PC). Unlike voice cellular phones, the decision to initiate a transfer, or hand-off from one cell to another cell is under the control of the CDPD M-ES itself, as it is the M-ES, which is responsible for monitoring the received signal strength of the cellular channels being used. 2.1.2

Fixed End System (F-ES)

It is defined as a stationary computing device, such as a host computer or an on-line information service. 2.1.3

Mobile Data Intermediate System (MD-IS)

It is a stationary network component with similar responsibilities to the cellular voice switch. It is responsible for keeping track of the M-ES's location and routing data packets to and from the CDPD network and M-ES appropriately. It has been referred to as the "brain" of the network, because of its functionality. Not only is it responsible for ensuring that an M-ES is valid to log on to the network, but it also stores information on the M-ES's last known location, traffic statistics and billing information. 2.2

MOBILE DATA BASE STATION (MDBS)

It is primarily responsible for RF channel management. It is located at the voice cell sites and is responsible to instruct the M-ES to "hop" to new channels for continued communication in the event voice communication (which is the priority traffic) is detected. It also handles the legwork for the M-ES in locating new channels when a hand off is required between cell sites.

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CDPD 2.3

INTERMEDIATE SYSTEM (IS)

It is made up of (off the shelf) routers, which are CDPD compatible with the primary responsibility for relaying the data packets. The way these components interact with each other can be seen from the graphic depiction below:

Fig 2.1 Network

2.4

CELLULAR Cellular service is similar to the wireline voice service provided by local and

long distance carriers. Commercial cellular networks provide one-to one voice and enhanced services to wireless subscribers. Enhanced services include voice mail, call waiting, caller ID, and call forwarding. Cellular service is offered in the 800 MHz frequency band. Cellular service providers cover more territory than any other terrestrial wireless system. Cellular networks provide coverage to almost 95 percent of the US population, covering 70 percent of the US land mass. There are typically two cellular providers per region. Although providers have defined areas in which they can offer service, users can get almost nationwide service with roaming agreements negotiated

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CDPD between providers. However, some coverage gaps exist in the network due to low demand in certain areas or difficult geographic terrain. Cellular is the most widely used commercial wireless service. This is due to the fact that it has appealed to and been marketed to a wider range of people than any other commercial wireless service, and costs for the unit and service have decreased substantially. Because cellular service is widely utilized by the general public, a large number of subscribers compete with one another for capacity on the network. When demand exceeds capacity, congestion may occur in the network. This may happen during emergencies in which public safety users may be responding. During congestion, subscribers may experience a delay in call setup or may be unable to place a call. Satellite networks may serve as a secondary network when the cellular network is congested. Dual mode cellular telephones have been recently developed that automatically use a terrestrial wireless cellular connection when available, but switch to a satellite connection when in remote areas.

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CDPD

3. WORKING 3.1

HOW DOES CDPD WORK?

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.

3.1.1 CDPD channel model In a CDPD network, multiple Mobile end stations share the channel medium with a single mobile data base station (MDBS). Direct communication is possible only between an MES and MDBS, but not between two MES’s in same cell. The voice signals are transmitted via a mobile phone connected to RF modem unit. RF transfers data both in forward and reverse channel. The medium consists of forward channel from MDBS to MES and reverse channel from MES to MDBS.

Fig. 3.1 CDPD channel model

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CDPD The forward channel is a connectionless broadcast channel carrying transmission from only MDBS. Information is received and decoded by all MES’S on the channel simultaneously. The reverse channel is shared among all MES’s. Access to the resolution of contention is controlled by each MES assisted by reverse channel status returned by the MDBS on the forward channel. CDPD extends only to the network layer, whose packets before transmission from an MES undergo several transformations, namely packet-header compression, packet encryption, segmentation and framing. For forward channel these packets undergo transmission before they are transmitted by MDBS on forward channel. The basic unit of information transfer is a variable length ordered sequence of octets called frame. These frames are transmitted over the radio channels. Any frame that contains a block having uncorrectable errors is discarded and transmitted.

The reverse channel-signaling format is,

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CDPD

Fig. 3.2 Revere channel-signaling format A reverse channel transmission burst consists of a dotting sequence during transmitted ramp up, a reverse channel synchronization word, followed by one or more blocks. Each block is interleaved with a continuity indicator, which signals termination or continuation of the burst. The termination of a burst is followed by the transmitter ramp down. The reverse channels are shared between numbers of mobile units sharing a reverse link cannot communicate each other.

Fig 3.3 Forward-channel signaling format

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CDPD

The channel for forward communication is from MDBS to the mobile units. This channel is unique to each mobile unit and hence connectionless. Forward channel data consists of a continuous stream of RS blocks interleaved with control flags. Communication between MDBS and MES take place over a pair of RF channel, each channel having a bandwidth of 30 kHz and transmission rate of 19.2 kbps. The MAC function used over the air interface is unique to CDPD. It creates a bit stream by taking the its within the MDLP frames and “blocking” them in to a format that incorporates a forward error correction scheme known as Reed Soloman (RS) block. Within the forward channel, there is channel Busy/Idle status and BlockDecode status.

3.1.2 Channel Busy/Idle status The Busy/Idle status indicates whether the reverse channel is busy or idle. An n MES wishing to transmit defers until the channel is signaled idle.

3.1.3 Block-Decode status The decode status indicates whether the previous RS block was decoded successfully or unsuccessfully by MDBS. Since, the MDBS cannot distinguish between errors due to collision and channel impairment. The decode status flag effectively acts as a collision detection signal. Decode status timing relationship is as, BIB [1]

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CDPD Fig 3.4 Decode-status timing relationship Busy/Idle and decode status flags together constitute the control flag. The control flags allow the ensemble of MES’s to “Digitally Sense” the state of the reverse channel, providing a multiple access capability. Hence, the access mechanism is named Digital Sense Multiple Access (DSMA).

3.1.4 DSMA/CD Reverse channels are accessed using a Digital Sense Multiple Access with Collision Detection (DSMA/CD) protocol, which utilizes carrier sense multiple accesses with collision detection. This protocol allows the collision of two data packets on a common channel to be detected so that the mobile unit can be alerted by the MDBS to retry transmission at a later time. When the channel is available for CDPD, the MDBS has permanent receive access to the reverse channel. When the MDBS detects the presence of a reverse channel transmission, it sets the Busy/Idle flag to busy state. Otherwise this flag continues to indicate the idle state. A terminal with a message ready for transmission senses the forward channel. If the channel is idle, MES initiates transmission. If the channel is sensed busy, the MES goes in to defer mode and waits for a random interval before sensing the channel again. Once an MES is successful in gaining access to the channel, it continues to transmit until there is a decoding failure indicated in the control flags on the forward channel. Upon detection of decoding failure, MES ceases transmission and goes in to back off mode. In the back off mode, MES waits for a random interval before it senses the channel again. Upon sensing the channel idle, if two or more MES transmit in the slot, a collision results, which in turn causes decoding failure. The decode status flag thus effectively acts as a collision detection signal. Thus, MES goes in to differ mode when the channel is sensed busy, and it goes in to back off mode when there is a decoding failure.

3.1.5 Effect on Cellular Voice CDPD network is overlaid on a cellular voice network and draws the channel resources from the pool of free voice channels. There are two ways one can overlay CDPD on cellular voice.

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CDPD 1. Channel hopping 2. Dedicating voice channels to CDPD In first, channel snuffers are deployed to detect whether the channel is free or occupied. From the free voice channels, may allocate some channels to CDPD use. Once an MES gets access to an allocated CDPD channel, it continues to use it until the channel is assigned to a voice customer. And if quits the channel within 40 ms of initiation of any voice activity on the channel and then hops to another free channel. An alternative to sniffing is dedicating some voice channels to CDPD use, but such dedication will reduce the number of channels available for regular voice use and will result in increased blocking probability and hence poorer service quality.

3.1.6 Voice Call Activity Activity on voice channels in any sector is best illustrated by the arrival departure process of the voice calls, which is often modeled as a state independent birth death process

Fig 3.5 Arrival-Departure process of voice customer’s birth-death process

3.1.7 Propagation Generally, the cellular propagation model is assumed to comprise of a combination of small and large-scale effects. The small-scale effects are noticeable on the scale of a few inches. In our propagation model, we consider only large-scale effects. Which are due to propagation path loss and shadowing, when the area is very open and line of

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CDPD sight condition exist, propagation loss. In addition to the negative exponential power loss, the no uniform nature of the medium causes some local variations.

3.1.8 Voice Quality CDPD be overlaid on cellular voice through sniffing and hopping. There is enough CDPD demand in each cell such that whenever a channel becomes free from cellular voice activity, the channel is taken over by CDPD transmission.

3.2

ANALYSIS OF PROTOCOL

In a cell there are number of mobile end stations which share a reverse channel through DSMA. MES can be in two states active or idle. In active state it may either be transmitting or in differ or back off mode. [BIB 2].

4. DELAY ANALYSIS In circuit switching network, the data from one user (sender) to another(receiver) has to follow a prespecified path, if a link to be used is busy, the message can not be redirected, which causes many delays. While, in packet switching network, it make better utilization of existing network by splitting the message to be sent in to packets. Each packet contains the information about the sender, the receiver, and the position of the packet in the message as well as part of actual message. Packet switching requires more equipment at the receiver, where reconstruction of the message will have to be done. PCSI has come up with an idea called CDPD (cellular digital packet data) technology, which uses the existing mobile networks. CDPD is a connectionless. It sends each packet intermittently, when there is ‘space’ available there is also packet delay analysis for cellular digital packet data. To analyze the packet delay, here formulate and analyze a model of voice and data burst traffic for cellular digital packet data.

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CDPD

4.1

CDPD SYSTEM CDPD is a wireless data system that operates on a no interfacing basis with the

existing analog cellular voice systems. CDPD uses the idle cellular voice channels for data transmission. The CDPD system exploits the fact that there is significant number of idle AMPS (advanced mobile phone standard) channels on the average that can be used effectively for short data traffic bursts. The CDPD system is intended to provide all data services such as message delivery and paging.

4.2

FUNCTION OF CDPD SYSTEM

To avoid interference with the voice traffic, the CDPD traffic hops from a CDPD channel about to be preempted by voice to a free AMPS channel. Mobile units or terminals on power up will scan the cellular channels to identify the CDPD channels; it goes through a registration process. Upon successful registration, the terminal is ready to transmit and receive data over the channel. Typically, for short message applications a single burst is sent. After a predetermined amount or time or when a voice call is assigned to a CDPD channel, the MDBS instructs the terminals to switch channels or to hop to a new channel. They do not register if the channel is served by same MDBS. The MDBS periodically broadcasts a list of available CDPD channels. When a CDPD channel is preempted by a voice call, it is called force hopping. When a CDPD channel is moved to a different channel to avoid preemption by a voice call, it is called planned hopping. In CDPD system, there is random delay in the processing time for arriving data bursts, is sufficiently minimized. The amount of data burst traffic redirected toward the logic CDPD channels not preempted by voice, which increases the data burst transmission delay times for these channels. For typical CDPD systems, it is reasonable for this data burst processing rate to be considerably larger than all of the other rates. 4.3

THE MODEL DUE TO DELAY ANALYSIS

Due to stochastic nature of voice calls, the number of CDPD channels available for data burst traffic fluctuates. In order to understand the quality of service that the data burst traffic receives in the presence of the voice traffic, we construct an approximate model of a CDPD system that captures the following capture key features. •

Service times for voice calls are exponentially distributed.

•

Voice traffic has access to all of the channels.

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CDPD •

Voice traffic that finds all of the channels occupied by voice are assumed to be lost.

•

Voice traffic has priority over data traffic, and voice calls preempt the data traffic occupying a CDPD channel.

Our model for the CDPD system differs from existing models in the way we model the data burst traffic. We assume following •

Arriving data bursts are evenly distributed among the free CDPDchannels, and each CDPD channel transmits data bursts at an exponential rate.

•

If no CDPD channels are available, data bursts are assumed to be lost. Since, lost packets are really retransmitted.

•

When a voice call takes over a CDPD channel, the data arrival mechanism for this preempted channel is shut off, and newly arriving data bursts are distributed evenly among the remaining free CDPD channels.

E.g. when CDPD channels are free, the total traffic is uniformly distributed among the channels, i.e. the arrival rate of data bursts for one of the CDPD channels. No new arrivals of data bursts are allowed of these channels. Arrivals resume when the preempted channels become free again. [BIB 3]

4.4

ANALYSIS CONSIDERATIONS This section describes analysis considerations that can be used to compare

similar technologies or services. These analysis considerations were chosen to broadly reflect the requirements of the public safety community and provide a structure to highlight the attributes and characteristics of commercial wireless services. Public safety network planners can use them to evaluate each service in terms of their applicability and suitability for public safety communications. Exhibit 3-1 broadly defines each consideration. The following sections further describe the analysis considerations that will be used to evaluate and compare the commercial services. Section 4 describes each wireless service. They are categorized according to the type of service each provides: voice or data. Section 5 provides a set of matrices that compare and contrast each service.

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CDPD

 AVAILABILITY Availability identifies whether a service can be acquired from a provider in a given region or service area. Availability may vary by service or by service provider. For example, many commercial wireless networks are relatively new or are still being developed. Therefore, these services are not available in all areas of the United States. However, some services, such as cellular and CDPD are well developed and can be acquired in many areas throughout the United States and in selected international markets. Service providers are continually adding new service areas to their networks. In addition, many commercial wireless service providers have developed roaming agreements with other providers, allowing them to extend their service areas. Certain service providers can more easily expand their service areas and availability because they operate using the same wireless air interface protocols as other service providers.

 COVERAGE Coverage identifies whether users can send and receive wireless transmissions in a particular area. Coverage for commercial wireless services varies depending on the

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CDPD type of service and provider. Even though some wireless services have coverage that spans the nation or the globe, subscribers to these services may still experience coverage gaps in service. These gaps may occur for three primary reasons: lack of assets, terrain interference, and building interference. Service providers often deploy networks in areas with high population densities, such as metropolitan areas and along roadways. This is especially true when carriers first construct their networks. They typically construct them in areas that have high levels of anticipated demand for a service. Consequently, full coverage may not exist in rural areas or away from major roadways where public safety operations may be conducted. Coverage gaps also exist where terrain interferes with the wireless signal. This may occur in mountainous areas or in valleys where the signal cannot reach users. When a user moves back into the coverage area of a signal, service can be restored. Wireless signals may also be blocked due to buildings or structures that obstruct the signal. This may happen as users travel under bridges or operate their telephones while inside a structure, such as a parking garage or large building. In these cases, users cannot send or receive transmissions. If users are operating phones during these times, their conversation may be terminated.

 ACCESSIBILITY Accessibility measures how readily users can access and use a service, especially during peak hours or in the event of a network disruption. Each wireless network has a limited capacity. Users on wireless service networks compete with one another for access to these networks. If demand exceeds capacity, users may not be able to access the network. This may occur particularly during peak periods such as rush hour or in the event of an emergency. If public safety operations coincide with peak calling times, congestion may occur, and public safety users will not be able to access the network to place a call. Accessibility depends in great part on whether the service is widely used by the general public or whether the usage is limited to a specific community. If the general public uses a particular service, there may be a greater number of users competing for network resources. These networks are typically more susceptible to congestion than those that serve the business communities.

 SECURITY AND PRIVACY Security and privacy describe the level of system vulnerability to transmission

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CDPD and computer-based electronic intrusion. There are different levels of security and privacy in commercial wireless networks. Each physical component and the radio frequency (RF) link of a commercial wireless service architecture represents a potential security vulnerability. Many providers offer privacy features, such as automatic identification, authentication, and passwords to limit unauthorized, fraudulent access and data interception. In some commercial wireless networks, encryption can be applied to end user devices. Communications security is an important issue for public safety wireless users, particularly law enforcement.

 TRANSMISSION SPEED Transmission speed is the rate at which data is transmitted through the network. The overall time required to transmit data is equal to the call setup time plus the transmission duration. Call setup time is the time it takes to access a transmission channel. While each wireless service is engineered to support certain data speeds, various factors may slow the rate of transmission. Overhead information, such as error detection and correction, can reduce the effective data transmission speed. In addition, the modem a user chooses may not support the full data rate offered by a particular service. Transmission speed can also be affected by the level of traffic on the network. packet-switched network is designed to support only a certain level of traffic. When traffic exceeds the engineered capacity, the network cannot support the additional traffic. As a result, the rate at which the transmitted data is accepted is reduced, and information is transmitted at a slower rate.

 ADDRESSING FUNCTIONALITY Addressing functionality describes the method by which a service is accessed from the user point of view. Commercial wireless voice services can be accessed two ways: push-to-talk (PTT) and touch-tone dialing. Certain wireless services such as specialized mobile radio (SMR) offer PTT addressing functionality, which is how LMR communication systems are accessed. Users push the transmit button on their radios, which initiates connectivity to other users on the network. Other services, such as cellular telephones, require users to dial a series of numbers on a multi-key pad in

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CDPD order to place a call. Dialing takes considerably more time than PTT to establish communications with other users. A user must either memorize a recipient’s number or take additional time to look up a telephone number. In addition, dialing can be cumbersome to execute in time critical situations. It requires the user to see, locate, and push a series of numbers to execute a call. In data communications, a user may access the network using a circuit or packet switched connection. In a circuit switched connection, a connection must be established every time information is sent. This can take up to a minute, depending on the type of network. Packet switched data involves a very small delay because a user may maintain a connection with the network rather than having to “dial in” every time to send data. The delay associated with packet data transmission may be imperceptible to users.

 COST The costs for commercial services include user or subscriber equipment and service costs. Service costs for commercial wireless services vary by service provider and pricing plan. There are two major types of pricing plans for data and voice services, flat rate and usage based. For flat rate pricing, users pay a set amount for unlimited usage. This is advantageous to those who are heavy users of a particular service. Usage based pricing typically has users pay a set monthly price for a fixed level of usage and then pay incrementally beyond the fixed limit. Other fees may also apply depending on the service, such as long distance and roaming fees. Some service providers charge a fee for each call that connects to the wireline network. This fee can add a substantial amount to a user’s monthly phone bill. Organizations with high usage patterns may typically negotiate bulk rates with carriers. The cost for subscriber equipment varies. Service providers frequently subsidize the cost of the end user devices to attract more users to the service. Often, end user devices can be acquired for free, depending on the type of service, or they can be leased from a service provider. The subsidized subscriber equipment is often combined with a long term service agreement. Pricing of wireless services is also based largely on whether the service is viewed as a commodity or a specialized service. Specialized services are typically services that have recently been introduced to the market. They provide specialized functionality and features, such as PTT or talk groups. They are mainly adopted by the business community and government organizations rather than the general public.

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CDPD Therefore, the network is designed to support business use only. To some degree, this may leave these services less vulnerable to congestion during emergencies because there are fewer users to compete for network resources. In contrast, commodity services are widely available and considered highly mature. Their costs are generally lower and they offer less functionality than specialized services. They are typically used by both business and general public sectors. Specialized services often evolve to commodity services as they mature and proliferate.

 TYPE OF APPLICATIONS SUPPORTED Type of applications supported denotes the range of communications that are offered by a particular service. Each wireless service can support different types of applications; these applications are classified as either voice or data. Voice services support point-to-point, dispatch, or point-to-multipoint communications. Data service applications support short message service (SMS), small and large file transfer, and PSN access. For this report, SMS includes email and text or alphanumeric paging services. The applications that a particular service can support depends highly on the amount of spectrum allocated to a service. Each service provider is licensed by the Federal Communications Commission (FCC) to operate using a particular amount of spectrum. Each spectrum slot can hold a limited amount of information. Higher bandwidth services require more spectrum to send information through the network. High bandwidth services include imaging, video, and large file transfers. Low bandwidth services do not require such a large portion of spectrum. These include SMS and small file transfers. For example, paging network operators are licensed by the FCC to operate using a limited amount of spectrum. Therefore, they are limited in the types of services they can offer. Paging service providers can typically provide numeric or text paging services. Additional applications such as SMS (alphanumeric messaging) can be added to the network as service providers upgrade to digital technologies and make more efficient use of the spectrum.

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CDPD

5. APPLICATIONS 5.1 COMPARING WITH CIRCUIT SWITCHED NETWORK Today, the mobile data communications market is becoming dominated by a technology called CDPD. It is more advantageous as compared to circuit switched cellular network. CDPD

CIRCUIT

SWITCHED

CELLULAR

NETWORK SPEED

Best

Best

SECURUTY

Best

Better

UBUQUITY

Best

Best

COST OF SERVICE

Best

Better

MOBILITY

Best

Good

INTEROPERABILITY

Best

Good

5.2

APPLICATIONS OF CDPD

Since, CDPD is an integrated technology, it can be used in variety of applications.

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CDPD

5.2.1 Credit Card Verification A credit card number is entered at the point of sale and an enquiry is sent to authorize the purchase. E.g. Yellow cab, located in San Francisco, California, is a customer who has started outfitting cabs with small terminals, which allow the customers to automatically pay by credit card. The terminals use the CDPD technology to validate credit card information and receive payment verification. It takes less than five seconds, which is a significant savings than the traditional approach they hadpreviously employed of validating customer credit card information through radio dispatch.

5.2.2 ATM Network The benefits of CDPD in ATM’s are, 5.2.2.1

Fast Transaction

•

Customers won’t have to wait for their money.

•

More people can use the ATM during busy times

•

Less time in front of the ATM means more security of customers.

5.2.2.2 •

Secure Data Transfer Data is transmitted in digital form (unlike most cellular phones, which send information in analog form) and is encrypted automatically by the CDPD modem.

•

5.2.2.3 •

It uses private data networks to route the transactions to the host.

Cost Effective CDPD is priced by the amount of data sent through the system , not by the time it takes to send it .

5.2.2.4 •

Easy To Install Since CDPD is wireless, it works almost anywhere there is cellular coverage.

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CDPD •

No installation is required just turn it on. On power up, the CDPD modem automatically registers itself.

5.2.3 Emergency Services Ability to receive information on the move is vital, where the emergency services are involved. Information regarding the address, type and other details of an incident can be dispatched quickly via a CDPD system using mobile computers. It is also used in wide area wireless network data systems. E.g. package pick up delivery and electronic mail notifications

5.2.4 Fleet Management Messages could be sent to specific mobile computing devices in to a vehicle to direct it toward destination for a pick up or next job. The technology allows the officers to communicate car to dispatch through e-mail messages. It is more secure than the traditional voice communication.

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CDPD

CONCLUSION As CDPD performs important role in its network it is concluded that, it can stand alone but only when it is overlaid on the cellular system. As it transfers the data quick and in an efficient manner, it increases effective capacity of cellular system. It analyzes the delay throughput characteristic.

BIBILOGRAPHY http://www.google.com http://www.wikipedia.com BIB [1]

Chris Cole, Joe Ford, Ned Montlag, Jerry, Kelvin ”CELLULAR

DIGITAL PACKET DATA” BIB [2]

Debabrata Saha, Stanely E. Kay “CELLULAR DIGITAL PACKET DATA NETWORK“ IEEE transaction vol. no. 26, 3rd august 1997

BIB [3]

William A. Massey, Raj Srinivasan “A PACKET DELAY ANALYSIS FOR CELLULAR DIGITAL PACKET DATA IEEE journal vol. no.15, 7th September 1997

BIB [4]

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