摘要
随着Internet上话音、视频等宽带服务的发展，目前的窄带接入方式已经不能满足人们对实时性的要求，提升接入网的速度是形成信息高速公路的关键所在。
本文从协议工程的角度研究了电缆调制解调器中协议的实现。作者首先研究了DOCSIS协议规范物理层的电气特征、传输汇聚子层MPEG包的处理以及MAC层的信道共享机制，并绘制出了Cable Modem初始化的SDL／GR。然后分析了硬件环境，论文中给出了利用一种高档专用CPU和一块超大规模ASIC为基础设计出Cable Modem的框图。然后加上FLASH、DRAM、EPROM、E2PROM存储器，以太网、RS232接口，以及高频头电路，最后形成Cable Modem的完整硬件系统。在此基础上，作者探讨了Cable Modem的软件设计。
作者利用pSOSytem嵌入式实时操作系统为软件平台，把软件模块和接口驱动程序以“软总线”上的“插件”形式，与操作系统紧密结合在一起，使得系统软件不对硬件作假设。在这个操作系统的基础上作者采用多进程编程的原理，根据独立性原则划分了系统的软件模块，并描述了各个模块的功能及其实现。论文以DOCSIS应用软件模块为例详细设计了它所包含的几个任务进程，在设计这个模块时，作者采用了消息驱动的设计思想，并通过创建消息队列来进行进程间通信。在编制控制软件过程中，解决协议上下层之间的数据交换问题时，作者采用了缓冲区技术，即利用缓冲器的指针传递来实现高效率的“零拷贝”技术，并讨论了不合理的分配算法对缓冲器造成的影响和解决办法。在课题研究中作者还提出了用软件方法控制CPE数量的方案，提出了利用上行带宽限制参数来处理下行带宽达到共享代码目的的方案，提出了防止FLASH代码被意外更新的方法，现这些方案已经在产品中使用，运行状况良好。
最后，作者在论文中提出了一些改进方案，例如对DOCSIS附录D的改进建议，软件功能代码的精确划分等等。
Following the broadband service of audio and video to be provided,
the recent method of narrowband access does not adapt to the
requirement of the real-time service. So increasing the access speed
will be emphasis to build information superhighways.
This thesis included the studying of implementing DOCSIS
specification in view of Protocol Engineering. At first, author
researched into the electric characteristic of physical layer of the
protocol and how to process the MPEG packet in its transmission
convergence sublayer, and protracted the SDLIGR of Cable Mode
initialization. Then the paper designed for Cable Modem frame by
using a kind of top grade special CPU and an ASIC, when analyzing
the hardware architecture, and finally formed the entire hardware
system by integrating FLASH, DRAM, EPROM, and E2PROM memories,
Ethernet, RS232 interfaces, and high frequency circuit. Basing on
those, author probed into software design in Cable Modem.
The author used pSOSytem, a embedded real-time operating system,
as a software base, to combine the software modules and interface
drivers, in form of plug-in, and OS, and then the system software
does not assume the hardware. The system was divided into modules
and the thesis described their function and implement according to
multi-process programming and module independency. The thesis also
exemplified the DOCSIS applications module implement and designed
for tasks included by the module, using the thought of message
driving and inter-process communication by message queue. When
designing for the control software, author resolved the data
exchange between two layers by using buffer technique which
implements the efficient “zero-copy”method by passing buffer
pointer, discussed the infection, derived from illogicality
allocation algorithmic, and the way. In the course of studying,
author put forward the method which controls CPE number by software,
the processing method which shares the codes by using upstream
bandwidth parameters to restrict downstream bandwidth, and the
method which avoids changing FLASH codes by chance. The product has
used these methods and is running normally.
In the end, author brought forward some blue point to advance
Cable Modem design, for example, advancing DOCSIS appendix D, the
more accurate divide of software functions and so on.
引文
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