Medium access control facts for kids

Imagine your computer or phone needs to send a message across a network. The Medium Access Control (MAC) sublayer is like a traffic cop for your network hardware. It makes sure your device can talk to the internet or other devices using wires, fiber optics, or Wi-Fi.

The MAC sublayer works closely with another part called the logical link control (LLC) sublayer. Together, they form the data link layer, which is like Layer 2 in the OSI model (a way to understand how networks work). The LLC helps manage the flow of data for the "logical" connection, while the MAC handles the actual physical connection.

The MAC sublayer hides the complicated details of how data travels physically. This means different types of network connections can use the same MAC sublayer. When your device sends data, the MAC sublayer adds special information, like a starting signal and error checks. It then waits for the right moment to send the data, making sure it doesn't crash into other data on the network. If a collision happens, the MAC sublayer helps fix it by trying to send the data again. When receiving data, it checks for errors and removes the extra information before passing the data to your computer.

What MAC Does

The MAC layer has several important jobs. It helps organize data into "frames" (small packets of information) and recognizes them.

Here are the main things the MAC layer does:

• It marks the start and end of data frames.
• It helps send data to the right computer or group of computers.
• It shows where the data came from.
• It moves data from higher layers without changing it.
• It checks for errors in the data to keep it safe.
• It controls when devices can send data on the network.

For Ethernet networks, the MAC layer also:

• Sends and receives normal data frames.
• Manages sending data again if there's a problem (in half-duplex mode).
• Adds and checks special codes to find errors.
• Makes sure there's a small pause between data frames.
• Throws away damaged data frames.
• Adds or removes special starting and ending signals for data.
• Helps with half-duplex connections, where devices take turns sending data.

MAC Addresses

Every device that connects to a network has a special ID number called a MAC address. Think of it like a unique serial number for your network card. These addresses are usually built into the hardware when it's made.

The first part of a MAC address tells you who made the device. The rest of the address is unique to that specific device. This system helps data find its way to the correct device on a local network, even if it goes through hubs or switches.

For example, when an IP packet (a piece of internet data) reaches its final local network, its IP address (which is like a street address on the internet) is turned into a MAC address. This allows the data to be delivered to the exact device on that local network.

Both Ethernet and Wi-Fi networks use these 48-bit MAC addresses.

Sharing the Network

The MAC layer also has ways to control how multiple devices share the same network connection. This is called a multiple access method. It's needed when many devices are connected to the same physical medium, like in bus networks, ring networks, or wireless networks.

These methods help prevent data "collisions" when multiple devices try to send data at the same time. They can also help reserve space for data if needed.

One common method is called CSMA/CD, used in Ethernet networks. It helps devices listen to the network before sending data to avoid collisions. If a collision happens, devices wait a random amount of time and try again. This method is mainly used in parts of a network where collisions can occur, like older Ethernet setups.

In modern networks, like switched Ethernet, where devices have dedicated connections, a multiple access method isn't always strictly needed. However, it's often still built into the equipment for compatibility.

MAC in Phone Networks

Cellular networks, like those for your mobile phone (GSM, UMTS, or LTE), also use a MAC layer. In these networks, the MAC protocol is designed to use the valuable radio spectrum as efficiently as possible.

The base station (the cell tower) has full control over how devices send and receive data. The MAC protocol helps the base station manage when all the phones in its area can talk.

See also

In Spanish: Control de acceso al medio para niños

• Isochronous media access controller
• List of channel access methods
• MAC-Forced Forwarding
• MACsec (IEEE 802.1AE)