Introduction
Multi-access links, some textbooks are also called media access control. I prefer the first name because it is clearer.
First, in the link layer, we have two types of networks: point-to-point links and broadcast links ( point-to-point is not the subject of this article ). However, there will be a problem with the broadcast link, that is, the problem of multiple access. What does it mean?
For example, if two teams are debating (the medium is air at this time), if both sides talk about their own and both talk at the same time, then the audience below may not be able to understand what the two sides are debating at all (why can't they hear? This is collision, which causes distortion of the data received by the audience), just like scolding on the street.
So in a realistic debate, we will let one party speak first, and then the other party, and then there are still rules for the time when each side speaks, and the same debate is divided into one round, two rounds, etc., after these debate rules ( this is Agreement, agreement is regulation, network agreement is network regulation, the same as traffic rules and regulations ), we can understand what everyone is debating.
In the case of multiple access protocol , the literal translation of multiple access in English means multiple point access, just like multiple people talking together, we also have to have an agreement, otherwise we will talk about each other, noisy and noisy. How are the rules?
Although there are many multiple access protocols, they are generally divided into three categories:
- Channel Division Protocol
- Random access protocol
- Rotation Agreement
Channel Division Protocol
classification:
- Time Division Multiplexing (TDM)
- Frequency division multiplexing
- Code division multiple access
What is reuse and division. For
example, I have a road (single lane), and then, cars, trucks, tanks, all running on this road, it can be said that this single lane has been reused (shared) , Used together).
But the reality is often that small cars go on regular roads, large trucks and large trucks arrive, and tanks tend to go on more specialized roads (otherwise it is easy to crush the road), so that different roads go on different cars, which is called Separate use (use separately)
Time division multiplexing
Official definition is time division multiplexed in the same channel (the same road a) the time into a time frame ( data frame and where the data link layer frame is not the meaning of a ), in the time frame into time slots, then what Each node in the channel is allocated a time slot, and in each cycle, the node transmits data in its own time slot.
Let's use the example of the debate just now. Assuming that our debate will last for one hour and there are two parties to the debate, then I will give each party 30 minutes (time frame). Each team has six players, so I will divide 30 minutes among six people, and each person will get 5 minutes (time slot) to speak.
Of course TDM is flawed. For example, if the teammates and opponents are all rookies in the debate, and there is no word, I alone can dominate the battle, but everyone still has 5 minutes to speak (although everyone else has a sentence) Don't say anything), but I still wait for their time to pass before I can speak.
Frequency division multiplexing
Frequency division multiplexing is to divide the frequency of the same channel into different divisions, and different frequencies transmit different data. To make an analogy, there is now the same road. On top of this road, there is a viaduct parallel to it. The overlook is the same lane, but the two lanes are at different heights (different frequencies). Division multiplexing.
Code division multiple access
Random access protocol
The so-called random access means that you talk whenever you want to talk. Unlike the above protocol, everyone has a prescribed time for talking.
For details, see Random Access Protocol
Rotation Agreement
Rotation agreement means taking turns to speak, playing drums and passing flowers when I was young. Now, whoever passes the flowers can speak. In computer networks, the flowers are "tokens."
For details, please refer to: Rotation Agreement