Computer Networking: A Top-Down Approach by James F. Kurose & Keith W. Ross

Author:James F. Kurose & Keith W. Ross
Language: eng
Format: azw3, pdf
Publisher: Pearson Education
Published: 2012-06-01T04:00:00+00:00

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CHAPTER 5 • THE LINK LAYER: LINKS, ACCESS NETWORKS, AND LANS

access networks. Since many hosts are connected to the same broadcast communica-

tion channel, a so-called medium access protocol is needed to coordinate frame

transmission. In some cases, a central controller may be used to coordinate transmis-

sions; in other cases, the hosts themselves coordinate transmissions. The second type

of link-layer channel is the point-to-point communication link, such as that often

found between two routers connected by a long-distance link, or between a user’s

office computer and the nearby Ethernet switch to which it is connected. Coordinating

access to a point-to-point link is simpler; the reference material on this book’s web site has a detailed discussion of the Point-to-Point Protocol (PPP), which is used in settings ranging from dial-up service over a telephone line to high-speed point-to-point

frame transport over fiber-optic links.

We’ll explore several important link-layer concepts and technologies in this chap-

ter. We’ll dive deeper into error detection and correction, a topic we touched on briefly in Chapter 3. We’ll consider multiple access networks and switched LANs, including

Ethernet—by far the most prevalent wired LAN technology. We’ll also look at virtual

LANs, and data center networks. Although WiFi, and more generally wireless LANs,

are link-layer topics, we’ll postpone our study of these important topics until Chapter 6.

5.1 Introduction to the Link Layer

Let’s begin with some important terminology. We’ll find it convenient in this chapter to refer to any device that runs a link-layer (i.e., layer 2) protocol as a node. Nodes include hosts, routers, switches, and WiFi access points (discussed in Chapter 6). We will

also refer to the communication channels that connect adjacent nodes along the com-

munication path as links. In order for a datagram to be transferred from source host to destination host, it must be moved over each of the individual links in the end-to-end path. As an example, in the company network shown at the bottom of Figure 5.1, consider sending a datagram from one of the wireless hosts to one of the servers. This datagram will actually pass through six links: a WiFi link between sending host and WiFi

access point, an Ethernet link between the access point and a link-layer switch; a link between the link-layer switch and the router, a link between the two routers; an

Ethernet link between the router and a link-layer switch; and finally an Ethernet link

between the switch and the server. Over a given link, a transmitting node encapsulates

the datagram in a link-layer frame and transmits the frame into the link.

In order to gain further insight into the link layer and how it relates to the network

layer, let’s consider a transportation analogy. Consider a travel agent who is planning a trip for a tourist traveling from Princeton, New Jersey, to Lausanne, Switzerland. The

travel agent decides that it is most convenient for the tourist to take a limousine from Princeton to JFK airport, then a plane from JFK airport to Geneva’s airport, and finally a train from Geneva’s airport to Lausanne’s train station. Once the travel agent makes

the three reservations, it is the responsibility

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