Multiple Access Protocols and LANs in Java

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slot's time even when it is the only node with frames to send A second drawback is that a node must always wait for its turn in the transmission sequence - again, even when it is the only node with a frame to send Imagine the partygoer who is the only one with anything to say (and imagine that this is the even rarer circumstance where everyone at the party wants to hear what that one person has to say) Clearly, TDM would be a poor choice for a multiple access protocol for this particular party While TDM shares the broadcast channel in time, FDM divides the R bps channel into different frequencies (each with a bandwidth of R/N) and assigns each frequency to one of the N nodes FDM thus creates N "smaller" channels of R/N bps out of the single, "larger" R bps channel FDM shares both the advantages and drawbacks of TDM It avoids collisions and divides the bandwidth fairly among the N nodes However, FDM also shares a principal disadvantage with TDM - a node is limited to a bandwidth of R/N, even when it is the only node with frames to send A third channel partitioning protocol is Code Division Multiple Access (CDMA) While TDM and FDM assign times slots and frequencies, respectively, to the nodes, CDMA assigns a different code to each node Each node then uses its unique code to encode the data bits it sends, as discussed below We'll see that CDMA allows different nodes to transmit simultaneously and yet have their respective receivers correctly receive a sender's encoded data bits (assuming the receiver knows the sender's code) in spite of "interfering" transmissions by other nodes CDMA has been used in military systems for some time (due its anti jamming properties) and is now beginning to find widespread civilian use, particularly for use in wireless multiple access channels In a CDMA protocol, each bit being sent by the sender is encoded by multiplying the bit by a signal (the code) that changes at a much faster rate (known as the chipping rate) than the original sequence of data bits Figure 53-4 shows a simple, idealized CDMA encoding/decoding scenario Suppose that the rate at which original data bits reach the CDMA encoder defines the unit of time; that is, each original data bit to be transmitted requires one bit-slot time Let di be the value of the data bit for the ith bit slot Each bit slot is further subdivided into M mini-slots; in Figure 53-4, M=8, although in practice M is much larger The CDMA code used by the sender consists of a sequence of M values, cm, m = 1,,M, each taking a +1 or -1 value In the example in Figure 53-4, the M-bit CDMA code being used by the sender is (1, 1, 1, -1, 1, -1, -1, -1)
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Multiple Access Protocols and LANs
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Figure 53-4: A simple CDMA example: sender encoding, receiver decoding To illustrate how CDMA works, let us focus on the ith data bit, di For the mth mini-slot of the bit-transmission time of di, the output of the CDMA encoder, Zi,m, is the value of di multiplied by the mth bit in the assigned CDMA code, c m: Zi,m = di
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(Equation 53-1)
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In a simple world, with no interfering senders, the receiver would receive the encoded bits, Zi,m, and recover the original data bit, di, by computing: di = (1/M) m=1,M Zi,m cm (Equation 53-2)
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The reader might want to work through the details of the example in Figure 53-4 to see that the original data bits are indeed correctly recovered at the receiver using Equation 53-2
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