tailieunhanh - Lecture Data communications and networks: Chapter 12 - Forouzan 

Although the previous chapters in this part are issues related to the physical layer or transmission media, Chapter 8 discusses switching, a topic that can be related to several layers. We have included this topic in this part of the book to avoid repeating the discussion for each layer. | Chapter 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12. Figure Data link layer divided into two functionality-oriented sublayers 12. Figure Taxonomy of multiple-access protocols discussed in this chapter 12. 12-1 RANDOM ACCESS In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance Topics discussed in this section: 12. Figure Frames in a pure ALOHA network 12. Figure Procedure for pure ALOHA protocol 12. The stations on a wireless ALOHA network are a maximum of 600 km apart. If we assume that signals propagate at 3 × 108 m/s, we find Tp = (600 × 105 ) / (3 × 108 ) = 2 ms. Now we can find the value of TB for different values of K . a. For K = 1, the range is {0, 1}. The station needs to| generate a random number with a value of 0 or 1. This means that TB is either 0 ms (0 × 2) or 2 ms (1 × 2), based on the outcome of the random variable. Example 12. b. For K = 2, the range is {0, 1, 2, 3}. This means that TB can be 0, 2, 4, or 6 ms, based on the outcome of the random variable. c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This means that TB can be 0, 2, 4, . . . , 14 ms, based on the outcome of the random variable. d. We need to mention that if K > 10, it is normally set to 10. Example (continued) 12. Figure Vulnerable time for pure ALOHA protocol 12. A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free? Example . | Chapter 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12. Figure Data link layer divided into two functionality-oriented sublayers 12. Figure Taxonomy of multiple-access protocols discussed in this chapter 12. 12-1 RANDOM ACCESS In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance Topics discussed in this section: 12. Figure Frames in a pure ALOHA network 12. Figure Procedure for pure ALOHA protocol 12. The stations on a wireless ALOHA network are a maximum of