The IP protocol based on GSM / GPRS provides the possibility of "always online" with the Internet, making it a cost-saving option. In order to develop applications based on the IP protocol, system designers need to be familiar with both wireless networks and TCP / IP protocols. This article will introduce Sony Ericsson modules and their application examples that support TCP / IP protocol and embedded applications in detail.
Various access technologies are used in wireless networks. In FDMA technology, each user is assigned a frequency, while TDMA technology separates users by time. In CDMA technology, all users use the same frequency band to send signals at the same time, and separate users by their unique codes. In the 2G system, data transmission is mainly through circuit switching, that is, a transmission channel allocated to users. When a user completes the transmission task and gives up the channel, the transmission channel is only provided to other users for use. Since burstiness is a basic feature of data services, when a user no longer has data transmission, broadband is in a state of waste. In a packet-switched system, all users share the same channel, and users can implement multiplexing. When a user has no data to transmit, the channel will be used by other users. The transmission method of the information packet can be arranged through the media access control (MAC) protocol.
The transmission of information packets on the Internet is all based on the IP protocol. The wireless network will be integrated with the Internet, and the information packets will be seamlessly transmitted to the network based on the IP protocol through the wireless medium. Before transmission, in order to directly reach the specified destination, the data packets must be encapsulated according to the TCP / IP protocol. IP protocol is also adopted for transmission in GPRS network.
In terms of service flexibility, almost all GSM modules can be connected to a GPRS network. Normally, the information packaging function according to the TCP / IP protocol stack is not part of the module's features. Before sending data through the module, the TCP / IP protocol stack must be developed in advance, however, only a few modules have an embedded TCP / IP protocol stack, and data can be sent directly through the module. As a result, the application development cycle is shortened, and its execution capabilities are also greatly improved.
This article will briefly introduce the TCP / IP protocol and the necessary conditions for implementing TCP / IP in the GPRS network, and describe in detail the Sony Ericsson module and its application examples that support the TCP / IP protocol and embedded applications.
TCP / IP protocol based on GPRS
TCP / IP is a set of protocols for cooperatively operating computers and sharing network resources. The protocol members include IP, TCP, UDP, etc. TCP ensures the correct transmission of data. If the amount of data is too large for a packet, TCP can divide it into several groups. IP is used to find a channel for the packet to reach the other end. Each workstation connected to the network has a unique IP address. The IP address is a 32-digit number. It is usually written as four decimal digits, each representing 8 For example, 165.221.32.70 is an IP address of the Sony Ericsson website server.
TCP / IP is a connectionless protocol, information is transmitted in packets, and the maximum capacity of the packet is negotiated during the initialization process. If the information exceeds its maximum capacity, it will be divided into several groups and transmitted separately. There is the possibility that the packets are not transmitted in sequence or even lost and forwarded again. Therefore, TCP must add the sequence number, the source and destination channel number, and other information to the data packet, which is called the TCP header. The receiver will use this information to detect whether the data packet is received correctly, and then sort it out. To directly send the information packet to the correct workstation, only the TCP packet header is insufficient. An IP header must be added to the TCP header, and the source and destination addresses are included in the IP header. When sending a packet to a destination, the computer will first check its own routing list. If the destination is within its own network, it will send the packet directly. Otherwise, it will send packets to the gateway, each gateway has a routing list, and the packet will reach the destination address through several separate routing nodes.
The GPRS network provides network functions to transfer IP packets from the mobile user point to the external network. GPRS can provide multiple choices for IP addresses, including public addresses, private addresses, and customer agent addresses. The GPRS mechanism that specifies the right to choose an IP address is called the access point domain name (APN).
Figure 1 shows the structure of a TCP / IP system based on GSM / GPRS. Because the overall supply of IP addresses is limited, a GPRS network provider usually uses a dedicated, dynamically assignable address. However, the choice of public website can be provided by a dedicated customer, and the dedicated website cannot be operated across the public network. To connect to the public network, the dedicated website is converted into a public website. The translation occurs through a node, which is a dedicated network address translation (NAT). Multiple private addresses can be mapped to a public address. For applications based on the IP protocol, like wired networks, they work in the same way in wireless networks. When the GPRS conversation starts, most service providers will also monitor the data transmission flow, if there is no data transmission within a certain period of time, the connection will be disconnected.
Because TCP / IP packets are transmitted through the GSM network, some parameter configuration of the TCP / IP protocol stack must be considered. TCP based on GPRS should support the appropriate window size to optimize the bandwidth delay of the end-to-end connection. The default receive and transmit buffer ratio should be increased to accommodate a sufficiently large window. The maximum value of IP packets supported by the connection layer is the largest Transmission Unit (MTU). On a link with a high bit error rate, the smaller MTU size increases the chance of successful transmission. The default IP maximum transmission unit value is 576 bytes. Depending on the link conditions and ARQ configuration, it is possible to increase the MTU size to a larger value. A larger MTU allows TCP to increase the congestion window faster. For the design of TCP / IP protocol based on GPRS, the designer must consider these issues.
Sony Ericsson machine-to-machine communication module
Many GSM modules can directly access the Internet through GSM / GPRS. If there is no TCP / IP protocol stack on the module, it must be reflected in its application or peripheral microprocessor. The Sony Ericsson GR47 / 48 module is one of the modules with a built-in TCP / IP protocol stack. To develop TCP / IP applications based on the GSM / GPRS network, Sony Ericsson provides a complete set of tools. Its machine-to-machine communication business solution (M2mpower Business SoluTIon) is a powerful support environment designed to promote higher cost-effectiveness ratios. Designed for the development of wireless machine-to-machine applications. M2mpower enables developers to use specific development tools to more easily embed wireless applications directly into compatible Sony Ericsson machine-to-machine products. M2mpower was introduced together with the programmable wireless device GR47 / 48. Its scripting language is based on the industrial-grade ANSI C language, enabling developers to smoothly convert existing applications to M2mpower.
Figure 2 shows the structure of the GR47 real-time operating system. The new programmable wireless module GR47 / 48 provides complete and comprehensive functions. The module includes a built-in TCP / TP protocol stack to use GPRS more efficiently. It allows a TCP / UDP transmission mechanism to be used with minimal pre-configuration and operation. Its embedded controller facilitates the integration of customer applications and reduces the need for external controllers, such as in applications such as alarms, instrument CC and automatic vending. GR47 and GR48 support dual-band GSM broadband 900MHz / 1800MHz and 850MHz / 1900MHz respectively, they can send or receive data through SMS short message service, CSD, HSCSD or GPRS, and can handle voice and fax. Its TCP / IP protocol stack can also be accessed through AT commands or embedded applications.
The GR47 / 48 module has many powerful features. Its TCP / UDP session can be started by a command; it also provides the function of querying the IP address for a uniform resource locator (URL); one of the most useful features is the IP server listening function, which allows the communication module to start up with the network Dialogue, for example, an assigned IP address does not need to connect any part, as long as the unit waits for an IP-based connection introduction request, it can be achieved.
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