Media Access Control (MAC) refers to protocols that determine which computer on a shared-medium environment (collision domain) is allowed to transmit the data. MAC, with LLC, comprises the IEEE version of Layer 2. MAC and LLC are both sublayers of Layer 2. There are two broad categories of Media Access Control: deterministic (taking turns); and non-deterministic (first come, first served).

Tollbooth Analogy
Consider how a tollbooth controls multiple lanes of vehicles crossing a bridge. Vehicles gain access to the bridge by paying a toll. In this analogy, the vehicle is the frame, the bridge is the shared medium, and paying the fee at the tollbooth is the protocol that allows access to the bridge.

Ticket Line Analogy
Picture yourself waiting in line to ride a roller coaster at an amusement park. The line is necessary to ensure order; there are a specified maximum number of people that can fit into the roller coaster car at one time. Eventually, as the line moves, you pay for your ticket, and sit in the car. In this analogy, the people are the data, the cars are the frames, the roller coaster tracks are the shared medium, and the protocol is the waiting in line and presentation of the ticket.

Meeting Analogy
Imagine yourself at a meeting table, along with the other members of a large talkative group. There is one shared medium - the space above the meeting table (air)- through which signals (spoken words) are communicated. The protocol for determining access to the medium is that the first person that speaks, when everyone quiets down, can talk as long as he/she wishes, until finished. In this analogy, the words of the individual members are the packets; the air above the meeting table is the medium; and the first person to speak in the meeting is the protocol.

Deterministic MAC protocols use a form of "taking your turn". Some Native American tribes used the custom of passing a "talking stick" during gatherings. Whoever held the talking stick was allowed to speak. When that person finished, he/she passed it to another person. In this analogy, the shared media is the air, the data are the words of the speaker, and the protocol is possession of the talking stick. The stick might even be called a "token."

This situation is similar to a data link protocol called a Token Ring. In a Token Ring network, individual hosts are arranged in a ring. A special data token circulates around the ring. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then places the token back in the ring, where it can be, passed along, or seized, by another host.

Non-deterministic MAC protocols use a first-come, first-served (FCFS) approach. In the late 1970s, the University of Hawaii developed and used a radio communication system (ALOHA) that connected the various Hawaiian Islands. The protocol they used allowed anyone to transmit at will. This led to radio wave collisions that could be detected by listeners during transmissions. However, what started as ALOHA, eventually became a modern MAC protocol called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). 

CSMA/CD is a simple system. Everyone on the system listens for quiet, at which time it’s OK to transmit. However, if two people talk at the same time, a collision occurs, and neither person can transmit. Everyone else on the system also hears the collision, waits for silence, and then tries to transmit.

Three common Layer 2 technologies are Token Ring, FDDI, and Ethernet. All three specify Layer 2 issues (e.g. LLC, naming, framing, and MAC), as well as Layer 1 signaling components and media issues. The specific technologies for each are as follows:

• Ethernet - logical bus topology (information flow is on a linear bus) and physical star or extended star (wired as a star)
• Token Ring - logical ring topology (in other words, information flow is controlled in a ring) and a physical star topology (in other words, it is wired as a star) 
• FDDI - logical ring topology (information flow is controlled in a ring) and physical dual-ring topology (wired as a dual-ring)

In this chapter, you learned that the Institute of Electrical and Electronic Engineers (IEEE) is a professional organization that defines network standards. You should know that IEEE LAN standards (including IEEE 802.3 and IEEE 802.5) are the best-known IEEE communication standards and are the predominant LAN standards in the world today. The IEEE divides the OSI link layer into two separate sublayers:

• Media Access Control (MAC)
• Logical Link Control (LLC)

This chapter explained how Layer 2 of the OSI model provides access to the networking media and physical transmission across the media, which enables the data to locate its intended destination on a network. With this in mind, you should understand how:

• Layer 2 provides reliable transit of data across a physical link
• Layer 2 uses a system called Media Access Control (MAC)
• Layer 2 uses the MAC address, which is the physical address located on a NIC
• Layer 2 uses framing to organize or group the bits

Now that you have a firm understanding of Layer 2 concepts, you are ready to learn about the Layer 2 technologies, which are discussed in the next chapter.