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Medium Access Control |
Point-to-Multipoint systems use two distinct modes of transmissions, one in the downstream direction and the other in the upstream direction. The Medium Access Control (MAC) layer is responsible for making sure that these transmissions get to their proper destination in an error free manner. The operation of the MAC in the downstream direction is quite straightforward, since a single transmitter at the BSU can be received by each of the SUs tuned to the channel
(see Figure 1). |
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| Figure 1: Downstream Transmissions in a Point-to-Multipoint System |
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| Upstream transmissions on the other hand (Figure 2), are more difficult to handle, since multiple SUs transmitters are now sending to a single BSU receiver. In the absence of any co-ordination between SU transmissions, there will be packet collisions at the BSU and resulting loss in performance. |
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| Figure 2: Upstream Transmissions in a Point-to-Multipoint System |
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| The MAC protocol has a number of techniques at its disposal to co-ordinate upstream transmissions, including Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Polling and Code Division Multiple Access (CDMA). |
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