Tuy nhiên, như chúng ta đều biết, công nghệ chỉ liên tục thay đổi. Hiện nay, nhiều folks cần để có thể truy cập ở khắp mọi nơi, cho dù họ có làm việc hay chơi, trong văn phòng hoặc ở nhà. Để đáp ứng nhu cầu này, GSM | 82 WIRELESS DATA NETWORKS standard for Ethernet wired LANs. The Physical Layer under includes three alternatives covering all the usual forms of WLAN Diffused Infrared DFIR . Direct Sequence Spread Spectrum DSSS . Frequency Hopping Spread Spectrum FHSS . Both radio frequency spread spectrum specifications are in the GHz band. The GHz band was chosen because it is available for unlicensed operation worldwide and because it is possible to build low-cost low-power radios in this frequency range that operate at LAN speeds. Spread spectrum and low power are requirements to allow unlicensed operation and to avoid interfering with other types of devices that may use the GHz band. Data Networks and Internetworking In the same way that this book provides a foundation for understanding wireless technologies this section builds a foundation for understanding Data Networks and Internetworking which is needed in order to have a full appreciation for Wireless Data Networks. Topics in this chapter will include flow control error checking and multiplexing however this sections focus is mainly on mapping the Open System Interconnection OSI model to networking internetworking functions and also on summarizing the general nature of addressing schemes within the context of the OSI model. The OSI model represents the building blocks for internetworks regardless of whether those internetworks are wireless or wired. Understanding the conceptual model will help you understand the complex pieces that make up an internetwork. What is an Internetwork An internetwork is a collection of individual networks wired or wireless that are connected by intermediate networking devices. This internetwork functions as a single large network. Internetworking refers to the industry products and procedures that meet the challenge of creating and administering internetworks. The first networks were time-sharing networks that used mainframes and attached terminals. Local-area .
