A group of protocols known as multiple access protocols work within the Open Systems Interconnection (OSI) model’s Medium Access Control (MAC) sublayer. Multiple nodes or users can access a shared network channel due to these technologies.
In this article, we will look more into the Multiple Access Protocols according to the GATE Syllabus for (Computer Science Engineering) CSE. We will read ahead to find out more about it.
Table of Contents
- What are Multiple Access Protocols?
- Data Link Control
- Multiple Access Control
- Classification of Multiple Access Protocol
What are Multiple Access Protocols?
Data transmission between two nodes is handled by the Data Link Layer. Its primary duties include data link control and multiple access control.
Data Link Control
By utilising methods like framing, error control, and flow control, the data link control is in charge of ensuring that messages are reliably transmitted via transmission channels. To regulate a data link, use the Stop and Wait ARQ command.
Multiple Access Control
The data connection control layer is sufficient if there is a dedicated link present between the sender and the receiver, but numerous stations can access the channel at once if there isn’t. Therefore, it is necessary to use several access protocols to reduce collision and prevent crosstalk.
When a teacher asks a question in a classroom full of students and all the students start responding simultaneously (send data at the same time), a lot of chaos is created (data overlap or data lost), so the teacher’s job is to manage the students and force them to respond one at a time using multiple access protocols. Protocols are therefore necessary for data sharing over non-dedicated channels.
Classification of Multiple Access Protocol
Multiple access protocols may also be classified into: Random Access Protocol, Controlled Access and Channelization.
1. Random Access Protocol
All stations in the random access protocol have equal superiority, which means that no station has higher priority than any other station. Depending on the status of the medium, any station may send data ( idle or busy). It has two attributes:
- There is no set timing for data transmission.
- The order of the stations delivering data is not fixed.
The following are divisions of the random access protocols:
(a) ALOHA
Although ALOHA was created for wireless LAN, it can also be used for shared mediums. This allows for simultaneous data transmission from numerous stations, which might cause collisions and jumbled data.
(b) CSMA
Fewer collisions are guaranteed by carrier sensing multiple access (CSMA) since the station must first determine whether the medium is busy or idle before delivering data. If it isn’t idle, it waits for the channel to become idle before sending data. Due to propagation latency, there is still a potential for collision in CSMA.
For instance, station A will sense the medium before sending any data. It will begin sending data if it discovers that the channel is empty. However, if station B wishes to send data and senses the medium, it will also find it idle and send data at the same time the first bits of information are transferred from station A (delayed owing to propagation delay). As a result, data from stations A and B will collide.
Here are the CSMA access modes:
- Non-persistent: The node senses the channel; if it is free, it sends the data; if not, it checks the medium once or twice (not continuously) and sends the data when it is.
- 1-persistent: The node senses the channel, sends the data if it is idle, or constantly checks the medium for idleness before sending data unconditionally (with a probability of 1) when the channel becomes idle.
- P-persistent: The node senses the media and sends data with p probability if it is idle. If the data is not transferred ((1-p) probability), the system waits a while before checking the media once more. If the medium is still empty, the system sends the data with a p probability. This process will repeat until the frame is sent. It is used in packet radio and Wifi systems.
- O-persistent: Transmission takes place in the sequence determined by the superiority of nodes. The node waits because of its time slot in order to send data if the medium is not in use.
(c) CSMA/CD
CSMA/CD is an abbreviation of Carrier Sense Multiple Access/Collision Detection. It refers to the multiple access carrier with collision detection. In CSMA/CD, all the stations have the ability to stop data transmission if a collision is found anywhere.
(d) CSMA/CA
CSMA/CA is an abbreviation of Carrier Sense Multiple Access/Collision Avoidance. Multiple access with carrier awareness and collision avoidance, sender receipt of acknowledgement signals is a necessary step in the collision detection process. The data is successfully delivered if there is just one signal (its own), but a collision has occurred if there are two signals (its own and that with which it collided). The collision must significantly affect the received signal in order to discriminate between these two scenarios. However, this is not the case in wired networks, which is why CSMA/CA is employed here.
The CSMA/CA prevents collisions by:
- Interframe space – In order to prevent collisions caused by propagation delays, the station waits for the medium to become idle before sending data. This waiting period is known as the Interframe Space (IFS). Once more, it checks to see if the medium is idle after this. The priority of the station affects the IFS duration.
- Contention Window – The quantity of time has been broken up into slots. When the transmitter is prepared to send data, the number of wait slots it chooses at random doubles each time the medium is not discovered to be idle. If the medium is determined to be in use, the process is not restarted in its entirety; rather, the timer is restarted when the channel is once more found to be inactive.
- Acknowledgement – If the acknowledgement is just not received before time-out, the sender resends the data.
2. Controlled Access Protocol
In this, the station sends the data once it has received approval from all other stations. The stations under controlled access exchange information to determine which station has the authority to send. In order to prevent message collisions over a shared medium, it only permits one node to send at a time. These are the three controlled-access techniques:
- Token Passing
- Polling
- Reservation
3. Channelization
The channelization protocol allows numerous stations to access the same channel at the same time by sharing the link’s available bandwidth according to time, frequency, and code. The three types of channelization are: Frequency Division Multiple Access, Time Division Multiple Access and Code Division Multiple Access.
(a) Frequency Division Multiple Access
In order to assign each station its own band, the available bandwidth is split into equal bands. In order to prevent crosstalk and noise, guard bands are also included to ensure that no two bands overlap.
(b) Time Division Multiple Access
The bandwidth is split across several stations in this. Time is separated into slots for stations to broadcast data in order to prevent collisions. However, there is a synchronisation overhead because each station needs to be aware of its time slot. By including synchronisation bits in each slot, this problem can be fixed. Propagation delay is a problem with TDMA as well, but it can be remedied by adding guard bands.
(c) Code Division Multiple Access
All signals are broadcast simultaneously on one channel in this process. The concepts of time and bandwidth are not divided. For instance, even if only two individuals in the room speak the same language, complete data reception is still achievable when many people are speaking at once. Similar to this, data from several stations can be sent simultaneously in various code languages.
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