What is CSMA?

Carrier sense multiple access (CSMA) is a protocol that senses when an Ethernet network transmits data and ensures only one signal is transmitted at a time, which prevents data collisions during network communications. CSMA listens to networks trying to send information, known as data packets, to do this. When networks try to send packets simultaneously, CSMA holds one data packet until the other sends before permitting the subsequent transmission.

If a data collision does occur over a network, the information might not arrive at the intended location. Essentially, by preventing data collisions within the Ethernet network, CSMA helps to ensure information reaches its proper destination. Explore how CSMA works, why it’s important, and job roles to consider in the field to grasp this networking protocol and the careers that may require its familiarity. 

What does CSMA stand for?

CSMA is an acronym for carrier sense multiple access. The “carrier sense” refers to how the network detects, or senses, if data is being sent over the channel and prevents additional data attempts to transmit until the channel is free. “Multiple access” means that multiple devices share the network.

How does CSMA work?

The CSMA protocol limits the potential for collisions when two or more devices share information packets over a network. CSMA observes the transmission lines within the network and requires the individual devices to check if the lines are clear before transmitting their packets. The device waits for its turn to transmit if the line is unclear. Similar to a traffic light, the CSMA determines when it is safe for information to move forward and when it is not.

3 types of CSMA modes

You can choose from three main types of CSMA access modes. While all three have differences, each has the same goal of avoiding a collision during data transmission over a network. These access modes include:

1-persistent CSMA

As discussed, CSMA’s purpose is to ensure the medium is idle before transmitting a packet. If it is in use, the 1-persistent CSMA protocol monitors the network and waits until it senses it is idle. Once it is idle, this access mode immediately transmits the data packet. This method works well with lightly loaded systems. Still, it is unreliable with heavy loads because all data immediately transmits once the mode senses the channel is idle, thereby increasing the chances of collisions.

Non-persistent CSMA

This method is similar to 1-persistent CSMA, except that in this mode, the protocol waits a random amount of time if the system is busy before sending the data. With systems experiencing a lot of data trying to transmit simultaneously, non-persistent CSMA, by selecting random amounts of time to reschedule the transmission, lessens the likelihood that more than one data packet is attempting to transmit simultaneously.  Because of this, the chances of a collision are less likely than they are with 1-persistent CSMA.

P-persistent CSMA

Typically, Wi-Fi networks utilize this mode of CSMA to avoid collisions. Additionally, p-persistent CSMA uses similar methods to the previously discussed modes. Like 1-persistent CSMA, it waits for the network to be idle, but rather than a probability of one (meaning it continuously transmits once the network is idle), this mode transmits its packet with a probability p. When compared to the other two CSMA modes, the p-persistent algorithm operates in a space between 1-persistent and non-persistent. You’ll likely use this protocol infrequently (if at all) because of the difficulty of estimating the number of busy nodes at a given time.  

Read more: What Is the CCNA? An Entry-Level Networking Certification

CSMA/CD vs. CSMA/CA

Collision detection (CD) and collision avoidance (CA) are two CSMA protocols that can manage data collisions. CD protocols detect collisions after they occur and manage the aftermath, while CA protocols help to avoid collisions in the first place. Typically, the type of computer network you use (wired versus wireless), and the efficiency level you need determine which one will deploy.

CSMA/CD

The CSMA/CD protocol is a modification of the CSMA system that improves efficiency. The CD, or collision detection, listens to transmissions and detects collisions simultaneously. Additionally, CSMA/CD decides how long the device within the network waits before resending information if a data collision occurs.

CSMA/CD prevents a second collision by waiting for a random period before resending their packets. While helpful, the CSMA/CD protocol seems to work best in small businesses with 30 computers or fewer. When companies have over 30 computers, collisions happen more frequently because users send too many packets at any given time. 

CSMA/CA

While CSMA/CD detects collisions that have occurred, CSMA/CA (the CA stands for collision avoidance) attempts to prevent the possibility of a data collision occurring. This system, which works with wireless networks, avoids collisions by recognizing the probability of a collision and planning accordingly. It avoids collisions by having devices back off for a random amount of time if the network is busy, ensuring two or more devices on the wireless network do not transmit information packets simultaneously.

This technique assigns a time slot for each device participating on the network. Once the network is idle, the system tells the next device in line to send its data packet and then the following one, updating the complex schedule to keep track of what to send next. CSMA/CA helps prevent collisions, but because of the protocol’s setup, wait times for transmission can be longer. If it receives no acknowledgment of receipt, the sending node arranges to resubmit the data. 

Why is CSMA important?

CSMA reduces network collisions by checking the network before sending a data packet to ensure no other participant is sending information simultaneously. This protocol ensures the safe, reliable transmission of data.

CSMA helps prevent data collisions and the possibility of data loss, which makes it an essential aspect of the modern interconnected world.  When you transmit data over a network, CSMA ensures no other information travels on that channel. By doing this, CSMA and its various iterations significantly improve the chances that your data will reach its destination. 

Job roles that use CSMA

Computer networking professionals will likely work with or become familiar with CSMA protocols. If you're interested in one of the following roles, consider learning more about this technology:

1. Computer support specialist

Average median US salary (BLS.gov): $60,810 [1]

Job outlook (projected growth from 2022 to 2032): 6 percent [1]

Requirements: Computer support specialist requirements vary by position, with some requiring an associate degree or some college coursework. Candidates may qualify with a high school diploma and relevant certifications.

As a computer support specialist, you troubleshoot and solve computer issues for customers, often providing support for various maintenance and update functions. You may diagnose issues, repair devices, document updates and procedures, and communicate with your team.

2. Software developers, quality assurance analysts, and testers

Average median US salary (BLS.gov): $130,160 [2]

Job outlook (projected growth from 2022 to 2032): 17 percent [2]

Requirements: Software developers, quality assurance analysts, and testers typically hold a bachelor’s degree in computer and information technology or a related field. Sometimes, employers may require programming experience or a master’s degree.

As a software developer, you will take user needs and develop software to meet those needs, including updating and maintaining existing programs. As a quality assurance analyst, you will create test plans, document program failures, and provide usability feedback.

3. Computer network architect

Average annual US salary (BLS.gov): $129,840 [3]

Job outlook (projected growth from 2022 to 2032): 13 percent [3]

Requirements: While not required, many candidates have a bachelor’s or master’s degree in computer and information technology, engineering, or a related field. Sometimes, employers may accept experience as an alternative to a formal degree. 

As a computer network architect, you will design and implement computer and information networks. This role involves creating network plans, communicating design ideas, implementing new structures, maintaining existing architectures, and troubleshooting errors. 

Read more: What Is a Data Engineer? A Guide to This In-Demand Career

Learning more about CSMA on Coursera

CSMA is a network protocol that prevents data collisions and ensures your system reliably sends data to its intended recipient. Discover more about CSMA and other computer networking and programming topics by taking courses on Coursera. Two options to explore include Fundamentals of Network Communication, which can help you better understand computer network fundamentals, and IoT (Internet of Things) Wireless & Cloud Computing Emerging Technologies, where you can explore cloud computing and computer networking.