Lecture 13: Medium Access Control, the ALOHA protocol

So far we have looked at the application, transport, and routing layers of the networking stack. These are the layers of networking that are traditionally associated with computer scientists and/or computer engineers. Over the next two weeks, we’ll cover the two lowest layers of the stack: the medium access control (MAC) layer (also called the link layer) and the physical layer. The lowest two layers have traditionally been the domain of electrical engineers, though there has been much recent research work in these two layers that sits squarely in the middle of electrical engineering and computer science. These two layers get much closer to physical realities that constrain communication: interference between laptops that are close to each other, the loss in intensity of an electromagnetic wave as it propagates from one point in space to another, and how bits are represented by voltages so that they can be transmitted within a cable. The flip side of getting close to reality is that we can no longer paper over problems using abstractions and must confront them head on. This week, we’ll look at the medium-access control (or MAC) layer. In particular, we will look at the problem of arbitrating access to a shared communication medium among several different users. Examples of shared communication media are all around us. WiFi networks are one example, where a electromagnetic (EM) waves belonging to a certain frequency range (2.4 to 2.48 GHz) are reserved for WiFi communication. Regardless of how many users are in a particular room, the same 80 MHz range (2.4–2.48 GHz) must be shared across all these users in some manner. Another example is the variety of cellular network bands that are available: e.g., there is an LTE band from 2.11 to 2.17 GHz and a GSM band from 390.2 to 399.8 MHz. Again, regardless of the number of users, the same frequencey range must be shared across all users. Decisions regarding which link-layer technology gets to use which frequency range are handled in the U.S. by the Federal Communications Commission (FCC). The frequency range of a shared communication medium is also called the bandwidth of the medium (e.g., for WiFi this is 80 MHz because it spans the range 2.4–2.48 GHz). In general, the total available capacity in bits/second in a shared communication medium increases with the bandwidth of the medium. This relationship between bandwidth and capacity is partly why the term bandwidth is used interchangeably with capacity in many situations. In reality though, they are different concepts: bandwidth is an analog quantity measured in Hz, while capacity is a digital quantity measured in bits/sec. Not all media are shared. An Ethernet cable between a desktop and the wall socket is an example of a dedicated medium because only one desktop can use any given cable at an instant. This desktop can use up the full capacity of its own cable. Further, a second desktop can be added with its own cable, and can transmit at the full capacity of the second cable. In other words, there isn’t a shared pool of networking resources that is being divided up among the desktops. The next two lectures will deal with the problem of arbitrating access to a shared medium such as WiFi or a cellular network like GSM, WiMAX, or LTE. There are a few different ways to do this, and we’ll consider each in turn over this and the next lecture. 1For the very specific case of a point-to-point communication medium with a single link connecting a single sender and receiver, the relationship between bandwidth in Hz and capacity in bits/sec is captured by an equation known as the ShannonHartley theorem. For more complicated communication media, such as multiple senders and a single receiver (e.g., multiple laptops and a WiFi AP) a mathematical characterization of the capacity still remains an open problem. 2At least at the link layer, i.e., at the level of the connection between the desktop and the wall socket. It is possible that both desktops share a common link deep inside the Internet if they are transmitting to the same destination.