Carrier Sense Multiple Access (CSMA)
This method was developed to decrease the chances of collisions when two or more stations start sending their signals over the data link layer. Carrier Sense multiple access requires that each station first check the state of the medium before sending.
Prerequisite - Multiple Access Protocols
Vulnerable time = Propagation time (Tp)
The persistence methods can be applied to help the station take action when the channel is busy/idle.
1. Carrier Sense Multiple Access with Collision Detection (CSMA/CD):
In this method, a station monitors the medium after it sends a frame to see if the transmission was successful. If successful, the transmission is finished, if not, the frame is sent again.
In the diagram, starts sending the first bit of its frame at t1 and since C sees the channel idle at t2, starts sending its frame at t2. C detects A’s frame at t3 and aborts transmission. A detects C’s frame at t4 and aborts its transmission. Transmission time for C’s frame is, therefore, t3-t2 and for A’s frame is t4-t1
So, the frame transmission time (Tfr) should be at least twice the maximum propagation time (Tp). This can be deduced when the two stations involved in a collision are a maximum distance apart.
Process: The entire process of collision detection can be explained as follows:
Throughput and Efficiency: The throughput of CSMA/CD is much greater than pure or slotted ALOHA.
- For the 1-persistent method, throughput is 50% when G=1.
- For the non-persistent method, throughput can go up to 90%.
2. Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) –
The basic idea behind CSMA/CA is that the station should be able to receive while transmitting to detect a collision from different stations. In wired networks, if a collision has occurred then the energy of the received signal almost doubles, and the station can sense the possibility of collision. In the case of wireless networks, most of the energy is used for transmission, and the energy of the received signal increases by only 5-10% if a collision occurs. It can’t be used by the station to sense collision. Therefore CSMA/CA has been specially designed for wireless networks.
These are three types of strategies:
- InterFrame Space (IFS): When a station finds the channel busy it senses the channel again, when the station finds a channel to be idle it waits for a period of time called IFS time. IFS can also be used to define the priority of a station or a frame. Higher the IFS lower is the priority.
- Contention Window: It is the amount of time divided into slots. A station that is ready to send frames chooses a random number of slots as wait time.
- Acknowledgments: The positive acknowledgments and time-out timer can help guarantee a successful transmission of the frame.
Process: The entire process of collision avoidance can be explained as follows:
Types of CSMA Access Modes:
There are 4 types of access modes available in CSMA. It is also referred as 4 different types of CSMA protocols which decides time to start sending data across a shared media.
- 1-Persistent: It senses the shared channel first and delivers the data right away if the channel is idle. If not, it must wait and continuously track for the channel to become idle and then broadcast the frame without condition as soon as it does. It is an aggressive transmission algorithm.
- Non-Persistent: It first assesses the channel before transmitting data; if the channel is idle, the node transmits data right away. If not, the station must wait for an arbitrary amount of time (not continuously), and when it discovers the channel is empty, it sends the frames.
- P-Persistent: It consists of the 1-Persistent and Non-Persistent modes combined. Each node observes the channel in the P-Persistent mode, and if the channel is idle, it sends a frame with a P probability. If the data is not transferred, the frame restarts with the following time slot after waiting for a (q = 1-p probability) random period.
- O-Persistent: A supervisory node gives each node a transmission order. Nodes wait for their time slot according to their allocated transmission sequence when the transmission medium is idle.