The references in this article are unclear because of a lack of inline citations. Help Wikipedia improve by adding precise citations! Speeds up to Developed in the early s, CDPD was large on the horizon as a future technology.
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GSM is basically a circuit-switched network. Using GSM services, a mobile computer with a special modem can place a call the same way it would place one on a hardwired telephone. Would you like to write for us? However, this strategy is problematic, as the technology suffers from a few drawbacks. They are as follows: The first problem is handoffs between base stations are frequent, sometimes even with stationary users, and each handoff results in losing ca.
Second, GSM is associated with a higher error rate. Finally, wireless calls are more expensive, and costs mount rapidly because the charge is per minute, and not per byte sent.
Packet-switched digital datagram services are used to solve these problems. Basically any idle kHz channel can be temporarily available for delivering data frames at a gross rate of Because CDPD consists of quite a bit of overhead, the net data rate is closer to bps.
Still, a connectionless, wireless datagram system for delivering IP packet using the existing cellular phone system is an interesting proposition for many users so its use is growing rapidly. The physical layer is related with the details of modulation and radio transmission. Data link, network, and transport protocols are also present but are not used. A CDPD system includes 3 kinds of stations: mobile hosts, base stations, and base interface stations.
These stations communicate with stationary hosts and standard routers of the kind found in any WAN. The base stations are the transmitters that talk to the mobile hosts. CDPD defines three kinds of interfaces. This interface must be well specified to permit CDPD to connect to a number of different networks. The A-interface is called the air interface, which is used between the base station and mobile hosts.
Using compression, encryption and error correction data are sent over the air interface. Units of bits, compressed using error correcting code and encrypted data bits are wrapped in bit blocks. To each block, seven 6-bit flag words are added to form a total of bit blocks.
Each bit block is split into seven bit microblocks, which are delivered consecutively. Each microblock consists of 6-bit flag word, which is used for indicating channel status. In full-duplex mode, these microblocks go over a As a result, both downlink and uplink channel are allocated in time as a sequence of bit microblocks.
Each microblock ends after 3. The downlink channel is easy to handle, since there is only one sender per cell: the base station. All frames delivered on it are broadcast with each mobile host selecting out those destined for it or for everyone. The tricky part in CDPD technology is the uplink channel, for which all mobile hosts wishing to transmit must compete. When a mobile host has a frame to deliver, it checks a downlink channel for a flag bit telling whether the current uplink slot is busy or idle.
If it finds that the uplink channel is still busy, then its wait time doubles with each unsuccessful attempt. Finally, when it finds the channel idle, it starts transmitting its microblock. The point of this algorithm is known as Digital Sense Multiple Access.
It is used to prevent all the mobile hosts from jumping on the uplink channel as soon it goes idle. It somewhat looks like the slotted p-persistence CSMA protocol, which makes use of discrete item slots on both channels. An additional asset of CDPD is that data users are second-class users. When a new voice call is about to be allocated to a channel currently in use for CDPD, the base station transmits a special signal on the downlink, closing down the channel.
If the base station already knows the number of the new CDPD channel, it declares it. Otherwise mobile hosts have to find among a designed set of potential CDPD channels to find it. Related Posts.
Cellular digital packet data
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. In this age of information, no one needs 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 CDPD technology represents a way for law enforcement agencies to improve how they manage their communications and information systems.
CDPD (Cellular Digital Packet Data)