2.9 Summary

This chapter introduced the many and varied types of links that are used to connect users to existing networks and to construct large networks from scratch. While links vary enormously in their detailed characteristics, there are many problems and techniques for solving them that are We looked at the five key problems that must be solved so that two or more nodes connected by some medium can exchange messages with each other.

The first problem is to encode the bits that make up a binary message into the signal at the source node and then to recover the bits from the signal at the receiving node. This is the encoding problem, and it is made challenging by the need to keep the sender's and receiver's clocks synchronized. We discussed four different encoding techniques—NRZ, NRZI, Manchester, and 4B/5B—which differ largely in how they encode clock information along with the data being transmitted. One of the key attributes of an encoding scheme is its efficiency, the ratio of signal pulses to encoded bits.

Once it is possible to transmit bits between nodes, the next step is to figure out how to package these bits into frames. This is the framing problem, and it boils down to being able to recognize the beginning and end of each frame. Again, we looked at several different techniques, including byte-oriented protocols, bit-oriented protocols, and clock-based protocols.

Assuming that each node is able to recognize the collection of bits that make up a frame, the third problem is to determine if those bits are in fact correct or if they have possibly been corrupted in transit. This is the error detection problem, and we looked at three different approaches: cyclic redundancy check, two-dimensional parity, and checksums. Of these, the CRC approach gives the strongest guarantees and is the most widely used at the link level.

Given that some frames will arrive at the destination node containing errors and thus will have to be discarded, the next problem is how to recover from such losses. The goal is to make the link appear reliable. The general approach to this problem is called ARQ and involves using a combination of acknowledgments and timeouts. We looked at three specific ARQ algorithms: stop-and-wait, sliding window, and concurrent channels. What makes these algorithms interesting is how effectively they use the link, with the goal being to keep the pipe full.

The final problem is not relevant to point-to-point links, but it is the central issue in multiple-access links: how to mediate access to a shared link so that all nodes eventually have a chance to transmit their data. In this case, we looked at a variety of media access protocols—Ethernet and several wireless protocols—that have been put to practical use in building local area networks. Media access in wireless networks is made more complicated by the fact that some nodes may be hidden from each other due to range limitations of radio transmission. Most of the common wireless protocols today designate some nodes as wired or base-station nodes, while the other mobile nodes communicate with a base station. Wireless standards and technologies are rapidly evolving, with mesh networks, in which all nodes communicate as peers, now beginning to emerge.

Further Reading

One of the most important contributions in computer networking over the last 20 years is the original paper by Metcalf and Boggs (1976) introducing the Ethernet. Many years later, Boggs, Mogul, and Kent (1988) reported their practical experiences with Ethernet, debunking many of the myths that had found their way into the literature over the years. Both papers are must reading. The third paper laid much of the groundwork for the development of wireless networks including 802.11.

  • Metcalf, R., and D. Boggs. Ethernet: Distributed packet switching for local computer networks. Communications of the ACM 19(7):395-403, July 1976.

  • Boggs, D., J. Mogul, and C. Kent. Measured capacity of an Ethernet. Proceedings of the SIGCOMM '88 Symposium, pages 222-234, August 1988.

  • Bharghavan, V., A. Demers, S. Shenker, and L. Zhang. MACAW: A media access protocol for wireless LANs. Proceedings of the SIGCOMM '94 Symposium, pages 212-225, August 1994.

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