Media Access Control (MAC) - Multiple Access Protocols

 

Multiple Access Protocols

The Data-Link Layer is divided into two sublayers:

Data-Link Layer
       │
       ├── Data Link Control (DLC)
       │
       └── Media Access Control (MAC)

The previous section discussed DLC. This section deals with MAC, particularly Multiple Access Protocols.

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1. Why Do We Need Multiple Access Protocols?

When a link is dedicated, only two devices use the link. For example, a dedicated telephone line connects two endpoints.

In this situation, we mainly need a data-link-control protocol, such as PPP, to manage data transfer.

But consider a shared link:

             Shared Medium
          ───────────────────
          │        │        │
         Node A   Node B   Node C

Several nodes use the same communication medium.

If Node A and Node B transmit at the same time, their signals may interfere.

This is called a:

Collision

Therefore, when several nodes share a common link, we need a multiple-access protocol to coordinate access to the medium.

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2. What is a Multiple-Access Protocol?

A multiple-access protocol is a protocol used to coordinate the access of multiple nodes to a shared or broadcast link.

We can compares this situation with people speaking in an assembly.

The rules should:

• Give each node an opportunity to access the link.
• Prevent two nodes from transmitting at the same time.
• Handle collisions if they occur.
• Prevent one node from monopolizing the medium.

Thus, there are two important goals:

Goal 1: Prevent collisions

Node A ──┐
         ├── Shared Link
Node B ──┘

A and B should not transmit simultaneously.

Goal 2: Handle collisions

If two nodes transmit simultaneously and a collision occurs, the protocol must provide a method to deal with it.

These are the fundamental problems addressed by MAC protocols.

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3. Dedicated Link vs Shared Link

Dedicated Link	Shared Link
Link is used by two endpoints	Link is shared by multiple nodes
No competition for the medium	Nodes compete for access
DLC protocol is sufficient	Multiple-access protocol is required
Example: dedicated telephone connection	Example: shared wireless/cellular medium

Consider the example of a cellular phone system where the communication band is shared by different users.

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4. Types of Multiple-Access Protocols

The multiple-access protocols are divided into three groups.

             Multiple-Access Protocols
                       │
          ┌────────────┼────────────┐
          │            │            │
          ▼            ▼            ▼
      Group 1       Group 2      Group 3

The three groups are:

1. Random Access Protocols
2. Controlled Access Protocols
3. Channelization Protocols

These groups provide different ways of coordinating access to a shared medium.

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5. Basic Idea of the Three Groups

1. Random Access

In random-access methods, nodes compete for access to the shared medium.

A node can attempt transmission when it has data to send.

Because multiple nodes may attempt to transmit, collisions can occur. The protocol therefore also provides a way to handle collisions.

        Shared Medium
       /      |      \
      A       B       C
      │       │       │
      └── compete ────┘

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2. Controlled Access

In controlled-access methods, nodes do not simply compete randomly.

Access to the medium is controlled or coordinated so that nodes take turns using the shared medium.

The objective is to avoid collisions by controlling who can transmit.

Conceptually:

Node A → transmit
Node B → wait
Node C → wait

Node A finishes

Node B → transmit
Node A → wait
Node C → wait

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3. Channelization

In channelization, the shared medium is divided so that different users can be given separate portions of the available communication resource.

Conceptually:

             Shared Medium
                   │
        ┌──────────┼──────────┐
        │          │          │
      Part 1     Part 2     Part 3
        │          │          │
      Node A     Node B     Node C

This allows multiple nodes to use the medium without directly competing for exactly the same resource at the same time.

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6. Overall Concept

The main idea of the entire MAC section can be summarized as:

              Shared Link
                   │
                   ▼
       Multiple nodes want to
          transmit data
                   │
                   ▼
       Multiple Access Protocol
                   │
       ┌───────────┼───────────┐
       ▼           ▼           ▼
    Random      Controlled   Channelization
    Access       Access
       │           │           │
    Compete      Take turns   Divide the
    for medium   / controlled  medium
       │           access
       ▼
    Collisions
    possible

Key point 

Multiple-access protocols are MAC protocols used when several nodes share a common or broadcast link. Their purpose is to coordinate access to the shared medium, prevent collisions, and handle collisions when they occur.