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TechKnowSurge Catalog Videos Collisions and Access Control
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Collisions and Access Control

Half-duplex network environments require media access control methods to prevent or manage collisions when multiple devices attempt to communicate simultaneously. Controlled access and contention-based access represent the two primary approaches to this problem.

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About this video

Half-duplex network hubs allow only one device to transmit at a time, meaning that when two devices send data simultaneously, the signals interfere and produce a collision. To keep network communication functional, some form of media access control must either prevent collisions from occurring or provide a reliable way to handle them when they do. Two broad categories of access control method exist for this purpose: controlled access and contention-based access. Controlled access assigns each device a dedicated time slot in which it is permitted to transmit, with an orchestrating process cycling through devices in sequence — a model exemplified by token ring, where a token passes from machine to machine around a logical ring. This approach guarantees that collisions cannot occur, but it introduces inefficiency because the system must still cycle through devices that have no data to send, and its effectiveness depends heavily on the protocol in use. Contention-based access removes the scheduling structure entirely, allowing any device to transmit whenever it needs to. This freedom reintroduces the possibility of collisions, so contention-based networks rely on specific detection or avoidance mechanisms to manage the consequences. The two primary protocols in this category — CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) — represent distinct strategies for dealing with the inherent risks of open, unscheduled network access.

What you'll learn

What's covered

Media Access Control Methods

Aligned to

CompTIA Network+
1.6 Compare and contrast network topologies, architectures, and types.
Cisco CCNA
1.13 Describe switching concepts
Cisco CCST Networking
4.5 Explain basic switching concepts
1.1 Identify the fundamental conceptual building blocks of networks

Key terms

Half Duplex
A communication mode that allows data to be transmitted in both directions but only one direction at a time.
Collision Domain
A network segment where data packets can collide with each other when transmitted simultaneously.
Media Access Control
MAC
A sublayer of the Data Link layer that controls how devices on a network gain access to a medium and transmit data.
Controlled Access
A media access method that assigns each device a designated time slot to transmit, preventing collisions by ensuring only one device speaks at a time.
Contention-Based Access
A media access method that allows any device to transmit at any time, relying on collision detection or avoidance mechanisms to handle simultaneous transmissions.
Token Ring
A network implementation in which devices share a physical bus or star layout but communicate logically in a ring, with only one device permitted to transmit at a time.

Transcript

So now, traditionally, our LANs had hubs in the middle of them. And hubs, we know, are half duplex — only one device could talk at a time. And if two devices did talk at the same time, what would happen is what's called a collision.

We need some way to be able to mitigate this, or stop it from happening altogether. So what we have to do is we have to have some sort of access control method, some sort of way that devices on a network can still be able to talk without having all these collisions, or to mitigate those collisions.

There's two different methods that we can use to have some sort of access control. Number one, we can have a controlled method, and a controlled method only allows one device at the same time, so it eliminates that there would ever be any collisions. Or we could have contention-based. Contention-based means that a collision could happen — two devices could talk at the same time — but they deal with it, or overcome it, when a collision does take place. So we're going to be talking about a controlled method to stop collisions from happening, or a contention-based method of how to deal with collisions if they happened.

Controlled access control

If your access control method is controlled, then that means that each one of these machines is given a time slot in which it can speak. You possibly have some sort of orchestrator that says, okay, it is your turn to speak, gives it a moment for that machine to speak, and if it has nothing to say goes to the next machine and says, okay, now it is your turn to speak, and then it passes it around. So Token Ring is an example of this, and one reason why it's called a ring is because it goes from machine to machine. It says it is now your turn, your opportunity to speak.

Now what we found is that a lot of machines may not necessarily have much to say during this round robin as it goes from machine to machine. And so there is some limitations that we found with this methodology of using a controlled method, but it could be effective depending on the type of protocol that's being used.

Contention-based access control

So that is the controlled access control method, versus contention-based, which just allows any machine to talk at any time. Now of course what this causes is that sometimes there's going to be collisions on the network where two devices could talk at the same time and a collision happens. So now we need some sort of mechanism to deal with that collision. So contention-based just allows any machine to talk.

And there's two methodologies that we're going to be talking about with this. And first of all is CSMA/ collision avoidance versus collision detection.

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