GPRS wireless DDN System and Application

1. Overview

General Packet Radio Service (GPRS) is short for universal grouping wireless Service. It is a GSM-based mobile grouping data Service that provides users with IP addresses or X.25 connections for mobile grouping. GPRS adopts the same wireless modulation standard as GSM, the same frequency band, the same burst structure, the same frequency hopping rules, and the same TDMA frame structure) A comprehensive GPRS coverage can be provided by adding function boards and upgrading related software.

GPRS uses the group switching technology. Each user can occupy multiple wireless channels at the same time. The same wireless channel can be shared by multiple users, and resources can be effectively used. The GPRS theoretical bandwidth can reach 171.2 Kbps, the actual application bandwidth is about 20 ~ 40 Kbps. GPRS technology is used to send and receive data in groups. Users are always online and pay by traffic, which quickly reduces service costs.

The GPRS-based wireless DDN service is based on China Mobile's GSM/GPRS network as the communication platform, using GPRS, SMS, CSD, USSD and other bearer methods, through the wireless DDN terminal equipment, data transmission services for industrial users.
2. Composition of GPRS wireless DDN Network

2.1 Network Structure

The GPRS wireless DDN communication system provides industry users with a virtual private data communication network that is always online and transparent. Supports point-to-point communication between peripheral devices and central nodes. It is suitable for industrial applications such as information transfer and remote data collection. The following are some typical network structures of the GPRS wireless DDN system.

Point ~ Point data transmission:

Meets the data transmission requirements between two points.

Screen. width-500) this. style. width = screen. width-500; ">

-Application business data

... Data transmission over the mobile network (application business data + control data)

Figure 1a Point ~ Point Data Transmission Network Structure

Center ~ Multi-point data transmission:

Supports data transmission from the center to multiple points.

Screen. width-500) this. style. width = screen. width-500; ">

-Application business data

... Data transmission over the mobile network (application business data + control data)

Figure 1b center ~ Point Data Transmission Network Structure

Multi-point network transmission:

Data transmission between multiple points and multiple points.

Screen. width-500) this. style. width = screen. width-500; ">

-Application business data

... Data transmission over the mobile network (application business data + control data)

Figure 1c center of Figure 1b ~ Point Data Transmission Network Structure
2.2 Network Access Method

Data Terminal (DTU) access:

Currently, China Mobile GPRS network users can select CMNET and APN to access the Internet. CMNET is usually used by public users to access the Internet. An APN user is usually in a group and needs to set the user name and password for better security and is suitable for industrial users. In addition, you can apply to bind a fixed IP address of the mobile intranet.

Data Center (DSC) access:

The data center can be accessed in multiple ways, and the network is established based on the characteristics of the GPRS bearer network and the specific requirements of the GPRS application field. The choice of networking mode is mainly determined by the business data volume, latency, reliability requirements, data confidentiality, and network conditions.

Screen. width-500) this. style. width = screen. width-500; ">
Figure 2 Data Center access method

Solution 1: GPRS access, suitable for applications with low total bandwidth requirements, such as data collection systems using the Polling transmission protocol; all data runs in the GPRS network; all GPRS terminals are required to communicate directly, and all terminals are connected to the same proprietary APN; the GPRS data terminal access point of the data center is bound to a fixed IP address (that is, at least one SIM card bound to a fixed IP address of the mobile Intranet ).

Solution 2: Internet access mode. GRE tunnel and RADIUS are optional. The address at both ends of GRE, the radius server address, and the enterprise router port address must be public. Considering IP address broadcast, enterprise router port address, GRE tunnel address, RADIUS Server address should be provided by the ISP that provides Internet connection for the enterprise; there is no cross-region problem, the enterprise data center server and China Mobile GGSN Server do not need to be in the same city.

Solution 3: leased line access is suitable for power, finance, securities, industrial monitoring, and other industries. Enterprises access China Mobile's routers through leased lines, and mobile provides public or mobile private IP addresses for users. Good data security, stability and reliability, and low transmission latency.

Solution 4: direct access, applicable to enterprises that do not have access to the Internet and have extremely high security requirements. Suitable for power, finance, securities, and other industries. Enterprises place GRE routers at one end of the operator, and the routers directly access the internal GPRS network without passing through the GPRS firewall.

Data center and user application interface:

Data Centers and application system application software interfaces can be dynamically connected to libraries (DLL) and virtual serial ports. The dynamic connection library method requires programming and development of interface programs. For applications that do not want to modify the original system application software, you can use the virtual serial port (VSP) connection mode:

Screen. width-500) this. style. width = screen. width-500; ">

Figure 3 virtual serial port interface connection
3. transmission efficiency and stability

I. Data Link Transmission Mode

After actual tests on China Mobile's GPRS network, the network's data transmission quality and stability are good. Considering the redundancy and efficiency of data transmission, we recommend using UDP to transmit business data, you can use TCP for file data transmission without redundancy.

Ii. Transmission Efficiency

1) user data transmission efficiency (proportion of user data to total network layer data ):

The transmission efficiency varies with the network mode and data packet length. The following assumes that the maximum length of user data packets currently supported is 1024 bytes.

I. Only supports data transmission from DTU to the data center: Maximum Transmission Efficiency (w) = 97.3%
Ii. Support for two-way active mutual transmission: Maximum Transmission Efficiency (w) = 95.9%

2) transmission latency (from the time between sending and receiving data ):

The transmission latency varies depending on the packet length. Based on the current mobile network conditions, data packets with a packet length of 100 to 200 bytes can be uploaded to each other within one second. Data Packets larger than 200 bytes can be uploaded between 1 second and 3 seconds on average.

Iii. Stability

1) Efficiency: Percentage of successfully sent data to the total sent data

According to the current Beijing Mobile GPRS network test results, under normal network conditions, the success rate of data packets is generally more than 99.9%.

2) Overall Stability Parameter: Efficiency x Transmission Efficiency

There is an extremely important relationship between the effective network bandwidth and the maximum transmission unit (MTU). Generally, increasing MTU to more than 200 bytes does not significantly increase the bandwidth, but will increase the average latency, the test results show that the optimal MTU size is about 250 bytes.

3) link detection and Activation

For data transmission that requires frequent transmission and high real-time performance (such as power automation information), you usually need to set the heartbeat function to ensure that the circuit is always online and activated, the heartbeat interval is set based on the network quality and application requirements.

4. Typical applications of GPRS wireless DDN System

China Mobile's GPRS wireless DDN system provides users with transparent data channels between data terminals and data centers. In the power system, it can be used for Meter Reading of major accounts, transmission of automated data information of distribution networks and rural networks.
5. Conclusion

This paper briefly introduces the network structure, networking and related technical points of the GPRS wireless DDN system, and aims to promote the application and promotion of the GPRS wireless DDN System in the power system through technical exchange, it provides economic, convenient, effective, and reliable data transmission channels for comprehensive monitoring of distribution networks and Meter Reading of major accounts, and complements and extends the communication networks of power systems.