25-08-2012, 12:17 PM
Cellular Digital Packet Data
Cellular Digital Packet Data.doc (Size: 443.5 KB / Downloads: 44)
. 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 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.
. 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.
. 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.
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 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.
. 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.
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.
Cellular Digital Packet Data.doc (Size: 443.5 KB / Downloads: 44)
. 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 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.
. 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.
. 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.
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 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.
. 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.
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.