| Digital trunked systems provide secure, reliable and high-speed voice and data communications. They are being developed rapidly across the whole world. Nowadays, the technology of oversea digital trunking systems is more mature, but the price is very high, and the systems lack security and confidentiality. Digital trunking systems of China started fairly late. They are not mature enough and also need more time to perfect themselves. So it is significant to develop digital trunking systems with independent intellectual property rights of our own.Logic Link Control (LLC) protocol layer is one of the most important parts of digital trunked system protocol stack, and it shall handle establishment and disconnection of logical links, the sending and receiving of acknowledgement data and unacknowledgement data, retransmissions of data and error control. LLC Advanced Link is used to transmit packet data in digital trunked system. Therefore, the research and realization of LLC protocol layer for Advanced Link is the key of the development of protocol stack in digital trunked system.First of all, the development of digital trunked system and the developing situation of LLC at home and abroad are summarized in this paper. Based on analysing digital trunked air interface protocol stack, the structure, functions and services of Advanced Link in LLC protocol layer are researched. Then, segmentation and reassembly technology,feedback and retransmission technology and window technology to be researched in this paper are described respectively. After that, the developing languages, tools and methods of protocol stack are given after analyzing and comparing several protocol stack developing methods that are used recently. Based on Telelogic Tau G2 and Tornado integrated development environments, the above three technologies of Advanced Link in LLC protocol layer are implemented by using UML, SDL and C languages, and the related protocol consistent tests are executed. At last, the programs are transplanted and debuged in CVT_PXA270 platform loading VxWorks embedded system, and the results satisfy the design requirements. |
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