# Section 12 Different Access

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No station allows, or does not allow, another station to send. ... ms before this station begins transmission and no station ought to begin sending ...

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﻿Section 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Authorization required for propagation or show.

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Figure 12.1 Data connect layer separated into two usefulness arranged sublayers

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Figure 12.2 Taxonomy of numerous get to conventions talked about in this part

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12-1 RANDOM ACCESS In arbitrary get to or dispute strategies, no station is better than another station and none is relegated the control over another. No station allows, or does not allow, another station to send. At every case, a station that has information to send utilizes a strategy characterized by the convention to settle on a choice on regardless of whether to send. Points examined in this area: ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance

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Figure 12.3 Frames in an unadulterated ALOHA organize

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Figure 12.4 Procedure for immaculate ALOHA convention

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Example 12.1 The stations on a remote ALOHA system are a most extreme of 600 km separated. On the off chance that we expect that signs engender at 3 × 10 8 m/s, we discover T p = (600 × 10 5 )/(3 × 10 8 ) = 2 ms. Presently we can discover the estimation of T B for various estimations of K . a . For K = 1, the range is {0, 1}. The station needs to| create an irregular number with an estimation of 0 or 1. This implies T B is either 0 ms (0 × 2) or 2 ms (1 × 2), in light of the result of the arbitrary variable.

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Example 12.1 (proceeded with) b. For K = 2, the range is {0, 1, 2, 3}. This implies T B can be 0, 2, 4, or 6 ms, in light of the result of the irregular variable. c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This implies T B can be 0, 2, 4, . . . , 14 ms, in view of the result of the arbitrary variable. d. We have to say that if K > 10, it is regularly set to 10.

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Figure 12.5 Vulnerable time for unadulterated ALOHA convention

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Example 12.2 An immaculate ALOHA arrange transmits 200-piece outlines on a mutual channel of 200 kbps. What is the necessity to make this casing impact free? Arrangement Average casing transmission time T fr is 200 bits/200 kbps or 1 ms. The defenseless time is 2 × 1 ms = 2 ms. This implies no station ought to send later than 1 ms before this station begins transmission and no station ought to begin sending amid the one 1-ms period that this station is sending.

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Note The throughput for immaculate ALOHA is S = G × e −2G . The greatest throughput S max = 0.184 when G= (1/2).

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Example 12.3 An unadulterated ALOHA organize transmits 200-piece outlines on a mutual channel of 200 kbps. What is the throughput if the framework (all stations together) produces a. 1000 edges for every second b. 500 edges for every second c. 250 casings for each second. Arrangement The casing transmission time is 200/200 kbps or 1 ms. a. On the off chance that the framework makes 1000 edges for each second, this is 1 outline for every millisecond. The heap is 1. For this situation S = G× e −2 G or S = 0.135 (13.5 percent). This implies the throughput is 1000 × 0.135 = 135 edges. Just 135 edges out of 1000 will likely survive.

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Example 12.3 (proceeded with) b. In the event that the framework makes 500 edges for every second, this is (1/2) outline per millisecond. The heap is (1/2). For this situation S = G × e −2G or S = 0.184 (18.4 percent). This implies the throughput is 500 × 0.184 = 92 and that exclusive 92 outlines out of 500 will likely survive. Take note of this is the most extreme throughput case, percentagewise. c. In the event that the framework makes 250 edges for every second, this is (1/4) outline per millisecond. The heap is (1/4). For this situation S = G × e − 2G or S = 0.152 (15.2 percent). This implies the throughput is 250 × 0.152 = 38. Just 38 outlines out of 250 will most likely survive.

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Figure 12.6 Frames in an opened ALOHA arrange

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Note The throughput for opened ALOHA is S = G × e −G . The greatest throughput S max = 0.368 when G = 1.

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Figure 12.7 Vulnerable time for opened ALOHA convention

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Example 12.4 An opened ALOHA organize transmits 200-piece outlines on a common channel of 200 kbps. What is the throughput if the framework (all stations together) produces a. 1000 edges for each second b. 500 edges for each second c. 250 casings for every second. Arrangement The casing transmission time is 200/200 kbps or 1 ms. a. On the off chance that the framework makes 1000 casings for every second, this is 1 outline for every millisecond. The heap is 1. For this situation S = G× e −G or S = 0.368 (36.8 percent). This implies the throughput is 1000 × 0.0368 = 368 casings. Just 386 edges out of 1000 will presumably survive.

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Example 12.4 (proceeded with) b. On the off chance that the framework makes 500 edges for each second, this is (1/2) outline per millisecond. The heap is (1/2). For this situation S = G × e −G or S = 0.303 (30.3 percent). This implies the throughput is 500 × 0.0303 = 151. Just 151 casings out of 500 will likely survive. c. On the off chance that the framework makes 250 casings for every second, this is (1/4) outline per millisecond. The heap is (1/4). For this situation S = G × e −G or S = 0.195 (19.5 percent). This implies the throughput is 250 × 0.195 = 49. Just 49 outlines out of 250 will most likely survive.

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Figure 12.8 Space/time model of the impact in CSMA

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Figure 12.9 Vulnerable time in CSMA

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Figure 12.10 Behavior of three determination techniques

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Figure 12.11 Flow outline for three constancy strategies

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Figure 12.12 Collision of the primary piece in CSMA/CD

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Figure 12.13 Collision and premature birth in CSMA/CD

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Example 12.5 A system utilizing CSMA/CD has a data transmission of 10 Mbps. On the off chance that the greatest spread time (counting the deferrals in the gadgets and disregarding the time expected to send a sticking sign, as we see later) is 25.6 μs, what is the base size of the casing? Arrangement The edge transmission time is T fr = 2 × T p = 51.2 μs. This implies, in the most pessimistic scenario, a station needs to transmit for a time of 51.2 μs to distinguish the crash. The base size of the casing is 10 Mbps × 51.2 μs = 512 bits or 64 bytes. This is really the base size of the casing for Standard Ethernet.

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Figure 12.14 Flow chart for the CSMA/CD

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Figure 12.15 Energy level amid transmission, inertness, or impact

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Figure 12.16 Timing in CSMA/CA

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Note In CSMA/CA, the IFS can likewise be utilized to characterize the need of a station or an edge.

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Note In CSMA/CA, if the station finds the channel occupied, it doesn't restart the clock of the dispute window; it stops the clock and restarts it when the channel gets to be sit without moving.

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Figure 12.17 Flow outline for CSMA/CA

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12-2 CONTROLLED ACCESS In controlled get to , the stations counsel each other to discover which station has the privilege to send. A station can't send unless it has been approved by different stations. We examine three mainstream controlled-get to strategies. Themes talked about in this segment: Reservation Polling Token Passing

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Figure 12.18 Reservation get to strategy

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Figure 12.19 Select and survey works in surveying access technique

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Figure 12.20 Logical ring and physical topology in token-passing access strategy

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12-3 Channelization is a numerous get to technique in which the accessible data transmission of a connection is partaken in time, recurrence, or through code, between various stations. In this segment, we talk about three channelization conventions. Subjects examined in this segment: Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA)

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Note We see the use of every one of these techniques in Chapter 16 when we talk about PDA frameworks.

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Figure 12.21 Frequency-division different get to (FDMA)

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Note In FDMA, the accessible data transmission of the basic channel is isolated into groups that are isolated by monitor groups.

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Figure 12.22 Time-division numerous get to (TDMA)

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Note In TDMA, the transmission capacity is only one channel that is timeshared between various stations.

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Note In CDMA, one channel conveys all transmissions at the same time.

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Figure 12.23 Simple thought of correspondence with code

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Figure 12.24 Chip successions

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Figure 12.25 Data representation in CDMA

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Figure 12.26 Sharing divert in CDMA

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Figure 12.27 Digital flag made by four stations in CDMA

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Figure 12.28 Decoding of the composite flag for one in CDMA

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Figure 12.29 General lead and cases of making Walsh tables

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Note The quantity of groupings in a Walsh table should be N = 2 m .

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Example 12.6 Find the chips for a system with a. Two stations b. Four stations Solution We can utilize the columns of W 2 and W 4 in Figure 12.29: a. For a two-station organize, we have [+1 +1] and [+1 −1]. b . For a four-station arrange we have [+1 +1 +1], [+1 −1 +1 −1], [+1 +1 −1 −1], and [+1 −1 +1].

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Example 12.7 What is the quantity of arrangements in the event that we have 90 stations in our system? Arrangement The quantity of groupings should be 2 m . We have to pick m = 7 and N = 2 7 or 128. We can then utilize 90 of the groupings as the chips.

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Example 12.8 Prove that a getting station can get the information sent by a particular sender on the off chance that it increases the whole information on the channel by the sender's chip code and after that partitions it by the quantity of stations. Arrangement Let us demonstrate this for the primary station, utilizing our past four-station case. We can say that the information on the chann