| 13 | | The entire purpose of a MAC is to figure out a way that a shared wireless medium can be split up amount multiple competing devices. The standard 802.11 DCF employs two classic techniques to achieve this goal: |
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| 15 | | 1. '''[https://en.wikipedia.org/wiki/Exponential_backoff Binary Exponential Backoff] - '''Devices should randomly dither their transmission times to reduce the likelihood they transmit at the same time as another device. In the event of a failed transmission, a device should increase the average wait time before the next transmission. The result is that a large network of many competitors inherently reduces the rate at which each competitor contends for the medium. |
| 16 | | 1. '''[https://en.wikipedia.org/wiki/Carrier_sense_multiple_access_with_collision_avoidance Carrier Sense Multiple Access] - '''Devices should listen for the transmissions of competitors and defer their own transmissions to any ongoing transmissions of others. |
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| 18 | | These two simple ideas form the crux of the entire 802.11 DCF. It's worth noting that these two concepts are often conflated but they are logically distinct. [https://en.wikipedia.org/wiki/ALOHAnet ALOHAnet] employed a backoff mechanism in 1971 prior to the development of CSMA. The basic idea of the DCF is to employ randomness to resolve collisions. Devices will never deadlock and continually collide. The random access structure of the DCF ensures that one device will eventually win. Additionally, the protocol tweaks the parameters of the random variables as a function of its estimation on how much contention is present in the network at any point in time. When a network load is busy, each device actively reduces the likelihood of collisions by transmitting less often. When the network load is light, devices become more aggressive in the access of the shared channel in order to reduce overhead. |
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| 20 | | In this tutorial, we will implement a completely different kind of MAC based on [https://en.wikipedia.org/wiki/Token_ring the token ring]. This MAC does not use randomness to avoid collisions but rather explicitly allows only a single transmitter at a time by passing around a token that authorizes transmissions. This is a form a scheduled access MAC (as opposed to a random access MAC). |
| | 13 | The entire purpose of a MAC is to figure out a way that a shared wireless medium can be split up amount multiple competing devices. The standard 802.11 DCF employs randomness to achieve this goal and avoid deadlocks where multiple nodes continually collide when accessing the medium. In this tutorial, we will implement a completely different kind of MAC based on [https://en.wikipedia.org/wiki/Token_ring the token ring]. This MAC does not use randomness to avoid collisions but rather explicitly allows only a single transmitter at a time by passing around a token that authorizes transmissions. This is a form a scheduled access MAC (as opposed to a random access MAC). |
