Understanding Media Access Control (MAC) methods is crucial for optimizing network performance and ensuring efficient data transmission.

Whether you're setting up a home network or managing an enterprise system, knowing how devices communicate - whether through half or full duplex modes, hubs, or Ethernet switches - can make a significant difference. This text describes the core concepts of MAC methods, including contention-based access with CSMA/CD and CSMA/CA, and controlled access techniques like Token Ring and ARCNET.

Duplex Communication: Half and Full Duplex Explained

Half and Full Duplex Communication refers to the capability to send and receive data. The data link layer assumes that the media is available for transmission for both nodes at any time.

Half Duplex Communication: Devices can transmit and receive on the media but cannot do so simultaneously.

Half-duplex communications restrict the exchange of data in one direction at a time.

Full Duplex Communication: Devices can simultaneously transmit and receive on the shared media.

Modern network cards and the devices they connect to support auto-sensing. If the device on the other side requires half duplex, it will select half duplex. If the device on the other side supports full duplex, it will select full duplex.

The Role of Hubs in Networking

It’s possible to connect two devices with a wire or wireless connection. However, networks usually have more than two devices.

In Ethernet networks, the network typically uses a central device to connect all the nodes. This redistribution point takes the incoming data and sends it to the receiving nodes.
Early networks used devices called hubs. Hubs, also known as repeaters (Layer 1 devices), take the incoming signals and send them to all the ports on the hub.

That means if any other node were about to transmit, there would be a collision. Then both nodes will have to wait for a random time delay. The more devices connected to the hub, the more collisions occur. More collisions slow down the network.

Ethernet Switches: The Modern Solution

Modern networks don’t use hubs; they use switches. Ethernet switches operate in full-duplex mode by default.

An Ethernet switch is a device that operates at Layer 2.

It builds a MAC address table of MAC addresses and associated port numbers by examining the source MAC address found in inbound frames and makes forwarding decisions based on the destination MAC address.

If a switch has no entries in the MAC address table or the destination MAC address is not in the switch table, the switch will forward the frame out of all ports except the port that brought the frame into the switch (unknown unicast).

The MAC address table is sometimes referred to as a content addressable memory (CAM) table. Ethernet switches usually keep an entry in the table for five minutes.

Ethernet LANs today use switches that operate in full-duplex mode. Full-duplex communications with Ethernet switches do not require access control through CSMA/CD.

It is important that two interconnected interfaces, such as a host NIC and an interface on an Ethernet switch, operate using the same duplex mode.

Understanding Media Access Control (MAC) Methods

Ethernet (local area networks) LANs and wireless local-area networks (WLANs) are examples of multi-access networks.

A multi-access network can have two or more end devices attempting to access the network simultaneously.

Multi-access networks require rules to govern how devices share the physical media.

There are two basic access control methods:

  1. Contention-based access
  2. Controlled access

Contention-Based Access: CSMA/CD and CSMA/CA

In contention-based multi-access networks, all nodes are operating in half-duplex, competing for the use of the medium.

There are two primary contention-based access methods:

Carrier Sense Multiple Access with Collision Detection (CSMA/CD): Used on legacy bus-topology Ethernet LANs.

CSMA/CD ensures that only one device is transmitting at a time.

CSMA/CD allows multiple devices to share the same half-duplex medium, detecting a collision when more than one device attempts to transmit simultaneously. It also provides a back-off algorithm for retransmission.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA): Used on Wireless LANs.

CSMA/CA initially transmits the intent to send data (a request to send (RTS) message). Once an acknowledgment (a clear to send (CTS) message) is received, the sender sends the data.

Each device that transmits includes the time duration needed for the transmission.

This method is used by 802.11 wireless networking technologies. Wireless LANs (WLANs) only support half-duplex because only one device can access the media at a time.

Controlled Access: Token Ring and ARCNET

In a controlled-based multi-access network, each node has its own time to use the medium.

Controlled access methods include:

Legacy Token Ring: Operates by passing a special three-byte frame (a token) around a logical ring of workstations or servers. Each device on the network waits for its turn to transmit data, eliminating collisions.

Legacy ARCNET (Attached Resource Computer Network): A legacy local area network (LAN) technology introduced in the late 1970s by Datapoint Corporation. It provided high-speed communication and easy expandability at a low cost.