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L10-11 - MIT 6.02 DRAFT Lecture Notes Fall 2010(Last update...

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MIT 6.02 DRAFT Lecture Notes Fall 2010 (Last update: October 18, 2010) Comments, questions or bug reports? Please contact [email protected] L ECTURE 10 Sharing a Common Medium: Media Access Protocols These are the lecture notes for Lectures 10 and 11 in Fall 2010. In this course so far, we have studied various techniques to develop a point-to-point link between two nodes communicating over a channel. The link includes techniques to: synchronize the receiver with the sender; ensure that there are enough transitions between voltage levels (e.g., using 8b/10b encoding); to cope with inter-symbol interference and noise; and to use channel coding to correct and detect bit errors. There are many communication channels, notably radio (wireless) and certain kinds of wired links (coaxial cables), where multiple nodes can all be connected and hear each other’s transmissions (either perfectly or to varying degrees). The next few lectures ad- dress the fundamental question of how such a common communication channel—also called a shared medium —can be shared between the different nodes. We will study two fundamental ways of sharing a medium: time sharing and frequency sharing . The idea in time sharing is to have the nodes coordinate with each other to divide up the access to the medium one at a time, in some fashion. The idea in frequency sharing is to divide up the frequency range available between the different transmitting nodes in a way that there is little or no interference between concurrently transmitting nodes. This lecture and the next one focus on approaches to time sharing. We will investigate two common ways: time division multiple access , or TDMA , and contention protocols , a fully distributed solution to the problem that is commonly used in many wireless networks today. The subsequent lectures discuss the technical ideas behind frequency sharing, par- ticularly frequency division muliplexing . These schemes are usually implemented as communication protocols . The term protocol refers to the rules that govern what each node is allowed to do and how it should oper- ate. Protocols capture the “rules of engagement” that nodes must follow, so that they can collectively obtain good performance. Because these sharing schemes define how multiple nodes should control their access to a shared medium, they are termed media access (MAC) protocols or multiple access protocols . Of particular interest to us are contention protocols, so called because the nodes contend 1
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2 LECTURE 10. SHARING A COMMON MEDIUM: MEDIA ACCESS PROTOCOLS Figure 10-1: The locations of some of the Alohanet’s original ground stations are shown in light blue markers. with each other for the medium without pre-arranging a schedule that determines who should transmit when, or a frequency reservation that guarantees little or no interference.
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