What is a cellular data network

Cellular data from the transmission of data to the exchange are grouped technology, the client Configuration wireless packet modem, through a dedicated packet base station into the packet network, you can access the packet on the network host, database, can also call another mobile data terminal. This technology is mainly used in the special mobile data communication system, which can only provide the users of the packet switched data network for mobile users.

I. Overview

Cellular Digital Packet Data (CDPD) is a mobile packet data technology used in the spectrum of cellular telephone networks (such as Amps). The feature of CDPD is that it can be used to jump through channel to use any idle channel in the cellular community without affecting the voice service of the lower cellular system.

The basic components of CDPD are terminal systems (ES) and intermediate systems (IS).

In Internet terminal Technology, ES is the equivalent of a host, and is the equivalent of a router. ES is the actual physical and logical end node for exchanging information, while IS is the basic structure of CDPD storage and transmission of information.

ES have 2 kinds of forms: a mobile es (m-es), is the user through the wireless interface access to the CDPD network equipment, the other is fixed ES (f-es), for the general host, server or gateway to connect to the CDPD backbone parts and for special applications and data to provide access.

A m-es consists of a mobile terminal (PC or other standard device) and a CDPD wireless modem that is connected to the mobile terminal and manages the Wireless Link and protocol. Typically, communication between a wireless modem and a mobile terminal is supported by a standard serial port protocol, such as a serial linear Internet Protocol (SLIP) or Point-to-Point Protocol (PPP).

is also has 2 different forms: One is the general is, a simple router that does not contain CDPD and moving components (in most cases an IP router), and a mobile data is (Md-is), which is a private is that can pass messages based on the status of M-es in the current position.

Md-is is composed of a set of hardware, which provides exchange, charge, registration, authentication, encryption and mobile management functions. Mobile management software allows switching systems to track m-es, allowing M-es to use a unique network address.

Ii. Overview of the various layers

　　1. Physical Layer

The physical layer of the CDPD receives the bit streams of the MAC (media access control) layer and converts them to modulated waveforms that can be transmitted over the 30KHZ RF channel. The communication between a MDBs and a m-es is based on a pair of RF channels (with a fixed frequency interval). The transmitting direction of the forward channel is from MDBs to M-es, which is either dedicated by the CDPD or CDPD and voice cellular networks are shared. In any case, as long as the forward channel is used for CD-PD, the transmission in the channel is continuous. The transmission direction of the reverse channel is from m-es to MDBs, and is shared for all m-es that communicate with the same MDBs. A pair of correlated forward and reverse channels constitute a CDPD channel flow. The modulation mode of the RF channel flow is GMSK (bt=0.5), and a frequency larger than the central carrier is represented by the ~ logic 1, whereas the logic 0 is represented. The modulation rate of both forward and reverse RF channels is 19.2kb/s.

　　2. Media Access Layer

1. The media access control MAC layer simulates a functional unit that can be logically operated between the PHY layer and the Logical Link Control (LLC) layer.

　　2. Media Access Process

Using open seam DSMA/CD (digital recognition multiple access) algorithm with competitive detection, a m-es can be connected to the reverse channel. DSMA/CD uses the free/busy and decoding status flags. The free/busy flag indicates that the reverse channel is busy or idle.

　　3. Logical Link Control layer

The purpose of the LLC layer is to transmit information between the network layers through the CDPD air interface. The protocol applied to this layer is called the Mobile Data Link Protocol (MDLP). The main function that MDLP provides to the upper SNDCP (associated Subnet aggregation protocol) is to set and control one or more logical data link connections in a CDPD channel stream. Above the LLC layer, each data link connection is treated as a separate bit channel that can be used to transmit messages between a md-is and a m-es or between a md-is and multiple m-es. One or more network traffic flows can be accommodated in each data link connection through SNDCP. Data link connections are identified by an address label (temporary device identification-tei) that is included in each message (also known as a frame). Tei is the concept of a pure LLC layer that is used only within the LLC layer. Data-link connections can be point-to-point or multicast. A multicast data link in a CDPD channel stream to a point of multiple or more point-to-point communication.

The transmission of information in a data link connection supports two modes of operation: Acknowledgement mode and non-acknowledgement mode. The information transmitted in the same-broadcast channel uses only the unacknowledged mode, and does not use error correction and streaming system. Therefore, non-acknowledgement mode transfer is unreliable. According to the quality of the eye service required by the network layer, the information transmitted on the point-to-point data-link channel adopts the unacknowledged mode or the confirmation mode. For each confirmation mode data link connection, the MDLP establishes the control mechanism to guarantee the given service quality.

　　4. Channel Jump

Since the CDPD was proposed after the cellular phone network began to operate, its design and implementation were limited by the inability to alter the view-stored cellular system. So the CDPD is designed to be completely transparent to the underlying cellular system. At this point, the CDPD channel will occupy the other free channel as preferentially as possible and continue the CDPD operation. CDPD operation in the context of channel jump, its downlink performance will have an impact. The MDBs uses the appropriate communication protocol between it and the cellular base station, or applies a monitoring device (such as a sampler) to find the channel available for transmission. In the latter case, the MDBs detects the transmitting signal of the cellular base station by detecting the power to enter the transmitting antenna. When the power rise is found, the voice service begins, and if the channel is CDPD used, a forced channel jump is made. Before any voice transmission, the MDBs closes the current forward channel end data transfer, jumps to a new free cellular channel, and continues to transmit its forward data service on the channel. The data transfer requires a m-ess trace channel jump and a pair of specified set of possible cdpd channels to be searched before the channel is found.

　　5. Conclusion

CDPD is characterized by the ability to implement independent cellular or packet switching functions on existing cellular phone networks. In packet switching, CDPD can use the existing honeycomb structure to rapidly develop with minimal installation investment.