7.9 小结

7.9 小结#

7.9 Summary

无线与移动网络彻底改变了电话通信，并且在计算机网络领域也正产生越来越深远的影响。它们通过随时随地、无需有线连接的方式接入全球网络基础设施，不仅使网络访问更加普及，也促使了一系列令人兴奋的基于位置的服务成为可能。鉴于无线与移动网络日益增长的重要性，本章重点介绍了支持无线与移动通信的基本原理、常见的链路技术及网络体系结构。

我们从介绍无线与移动网络开始本章的内容，并明确区分了这类网络中由通信链路的无线特性带来的挑战，与由于这些无线链路所支持的移动性带来的挑战。这使我们能够更清晰地分离、识别并掌握每个领域中的关键概念。我们首先关注无线通信，在 第7.2节 中探讨了无线链路的特性。在 第7.3节 与 第7.4节 中，我们研究了IEEE 802.11（WiFi）无线局域网标准的链路层方面、两个IEEE 802.15个人区域网标准（蓝牙与Zigbee）、以及3G与4G蜂窝因特网接入技术。

接着我们将注意力转向移动性问题。在 第7.5节 中，我们识别了几种形式的移动性，这些移动性在不同程度上带来不同的挑战，并需采取不同的解决方案。我们讨论了定位与路由到移动用户的问题，以及在用户从一个网络连接点动态移动到另一个连接点时的切换方案。在 第7.6节 与 第7.7节 中，我们研究了移动IP标准与GSM系统中如何应对这些问题。最后，在 第7.8节 中，我们讨论了无线链路与移动性对传输层协议及网络应用的影响。

尽管我们用整整一章来研究无线与移动网络，但若要全面探讨这一令人兴奋且快速发展的领域，则至少需要一本完整的书（甚至不止一本）。我们鼓励你通过查阅本章提供的众多参考文献，进一步深入探索这一领域。

Wireless and mobile networks have revolutionized telephony and are having an increasingly profound impact in the world of computer networks as well. With their anytime, anywhere, untethered access into the global network infrastructure, they are not only making network access more ubiquitous, they are also enabling an exciting new set of location-dependent services. Given the growing importance of wireless and mobile networks, this chapter has focused on the principles, common link technologies, and network architectures for supporting wireless and mobile communication.

We began this chapter with an introduction to wireless and mobile networks, drawing an important distinction between the challenges posed by the wireless nature of the communication links in such networks, and by the mobility that these wireless links enable. This allowed us to better isolate, identify, and master the key concepts in each area. We focused first on wireless communication, considering the characteristics of a wireless link in Section 7.2. In Sections 7.3 and 7.4, we examined the link-level aspects of the IEEE 802.11 (WiFi) wireless LAN standard, two IEEE 802.15 personal area networks (Bluetooth and Zigbee), and 3G and 4G cellular Internet access. We then turned our attention to the issue of mobility. In Section 7.5, we identified several forms of mobility, with points along this spectrum posing different challenges and admitting different solutions. We considered the problems of locating and routing to a mobile user, as well as approaches for handing off the mobile user who dynamically moves from one point of attachment to the network to another. We examined how these issues were addressed in the mobile IP standard and in GSM, in Sections 7.6 and 7.7, respectively. Finally, we considered the impact of wireless links and mobility on transport-layer protocols and networked applications in ­Section 7.8.

Although we have devoted an entire chapter to the study of wireless and mobile networks, an entire book (or more) would be required to fully explore this exciting and rapidly expanding field. We encourage you to delve more deeply into this field by consulting the many references provided in this chapter.