1-持久性、p-持久性和非持久性 CSMA 之间的区别

先决条件 – 载波侦听多路访问 (CSMA)

1. 1-持久CSMA：
在 1-persistent CSMA 中，站点不断地侦听信道以检查其状态，即空闲或忙碌，以便它可以传输数据或不传输数据。在信道忙的情况下，站将等待信道变为空闲。当站点发现空闲信道时，它立即将帧发送到信道。它以概率 1 传输帧。由于概率 1，它被称为 1-persistent CSMA。

这种方法的问题是有很多发生冲突的机会，这是因为有可能两个或多个站同时发现空闲状态的信道和传输帧。在发生冲突时，站点必须等待信道空闲的随机时间并重新开始。

图 – 1 持久性 CSMA 2. p 持久性 CSMA ：
这是当信道具有时隙并且时隙持续时间等于或大于最大传播延迟时间时使用的方法。当站准备好发送帧时，它将检测信道。如果发现通道忙，通道将等待下一个时隙。如果发现信道空闲，它以概率 p 发送帧，因此对于左概率，即 q 等于 1-p，站将等待下一个时隙的开始。万一，当发现下一个时隙也空闲时，它将以概率 p 和 q 再次传输或等待。重复此过程，直到传输帧或另一个站已开始传输。
图 – p 持久性 CSMA 3. 非持久性 CSMA：
在这种方法中，有帧要发送的站点，只有该站点才能感知信道。在空闲通道的情况下，它将立即向该通道发送帧。如果发现信道忙，它将等待随机时间，并再次感知站点的状态是空闲还是忙。在这种方法中，当站点检测到前一次传输结束时，它不会为了捕获信道而立即感知信道。使用这种方法的主要优点是它减少了碰撞的机会。这样做的问题是它降低了网络的效率。
图 –非持久性 CSMA 1-persistent, p-persistent 和 Non-persistent CSMA 的区别：

Parameter	1-persistent CSMA	p-persistent CSMA	Non-persistent CSMA
Carrier Sense	It sends with the probability of 1 when channel is idle.	It sends with the probability of p when channel is idle.	It send when channel is idle.
Waiting	It continuously senses the channel or carrier.	It waits for the next time slot.	It will wait for the random amount of time to check the carrier.
Chances of Collision	There is highest chances of collision in this.	Less chances as compared to 1-persistence and p-persistence.	Less chances as compared to 1-persistence but more than the p-persistence.
Utilization	It’s utilization is above ALOHA as frames are only sent when the channel is idle.	It’s utilization is depend upon the probability p.	It’s utilization is above 1-persistent as not all the stations constantly check the channel at the same time.
Delay Low Load	It is low as frames are send when the channel become idle.	It is large when p is small as station will not always send when channel is idle.	It is small as station will send whenever channel is found idle but longer than 1-persistent since it checks for the random time when busy.
Delay High Load	It is high due to collision.	It is large when the probability p of sending is small when channel is idle and channel is rarely idle.	It is longer than 1-persistent as channel is checked randomly when busy.