Chapter 2: Direct Connections
It is a mistake to look too far ahead. Only one link in the chain of destiny can be handled at a time. —Winston Churchill
Problem: Connecting to a Network
In Chapter 1 we saw that networks consist of links interconnecting nodes. One of the fundamental problems we face is how to connect two nodes together. We also introduced the "cloud" abstraction to represent a network without revealing all of its internal complexities. So we also need to address the similar problem of connecting a host to a cloud. This, in effect, is the problem every Internet Service Provider (ISP) faces when it wants to connect a new customer to its network.
Whether we want to construct a trivial two-node network with one link or connect the one-billionth host to an existing network like the Internet, we need to address a common set of issues. First, we need some physical medium over which to make the connection. The medium may be a length of wire, a piece of optical fiber, or some less tangible medium (such as air) through which electromagnetic radiation (e.g., radio waves) can be transmitted. It may cover a small area (e.g., an office building) or a wide area (e.g., transcontinental).
Connecting two nodes with a suitable medium is only the first step, however. Five additional problems must be addressed before the nodes can successfully exchange packets, and once addressed, we will have provided Layer 2 (L2) connectivity (using terminology from the OSI architecture).
The first is encoding bits onto the transmission medium so that they can be understood by a receiving node. Second is the matter of delineating the sequence of bits transmitted over the link into complete messages that can be delivered to the end node. This is the framing problem, and the messages delivered to the end hosts are often called frames (or sometimes packets). Third, because frames are sometimes corrupted during transmission, it is necessary to detect these errors and take the appropriate action; this is the error detection problem. The fourth issue is making a link appear reliable in spite of the fact that it corrupts frames from time to time. Finally, in those cases where the link is shared by multiple hosts—as is often the case with wireless links, for example—it is necessary to mediate access to this link. This is the media access control problem.
Although these five issues—encoding, framing, error detection, reliable delivery, and access mediation—can be discussed in the abstract, they are very real problems that are addressed in different ways by different networking technologies. This chapter considers these issues in the context of specific network technologies: point-to-point fiber links (for which SONET is the prevalent example); Carrier Sense Multiple Access (CSMA) networks (of which Ethernet is the most famous example); wireless networks (for which 802.11 is the most widespread standard); fiber-to-the home (for which PON is the dominant standard); and mobile wireless (where 4G is rapidly morphing into 5G).
The goal of this chapter is simultaneously to survey the available link-level technology and to explore these five fundamental issues. We will examine what it takes to make a wide variety of different physical media and link technologies useful as building blocks for the construction of robust, scalable networks.