Read this summary of Chapter 6. In this chapter, you explored how Local Area Networks pose a problem for transporting frames since several devices share the same transmission channel. A variety of Medium Access Control algorithms have been necessary to regulate the access to the transmission channel by reducing collisions: ALOHA, CSMA, CSMA/CD, and CSMA/CA. Review the key technologies discussed in this section.

6.4 Summary

In this chapter, we first explained the principles of the datalink layer. There are two types of datalink layers: those used over point-to-point links and those used over Local Area Networks. On point-to-point links, the datalink layer must at least provide a framing technique, but some datalink layer protocols also include reliability mechanisms such as those used in the transport layer. We have described the Point-to-Point Protocol that is often used over point-to-point links in the Internet.

Local Area Networks pose a different problem since several devices share the same transmission channel. In this case, a Medium Access Control algorithm is necessary to regulate the access to the transmission channel because whenever two devices transmit at the same time a collision occurs and none of these frames can be decoded by their recipients. There are two families of MAC algorithms. The statistical or optimistic MAC algorithms reduce the probability of collisions but do not completely prevent them. With such algorithms, when a collision occurs, the collided frames must be retransmitted. We have described the operation of the ALOHA, CSMA, CSMA/CD and CSMA/CA MAC algorithms. Deterministic or pessimistic MAC algorithms avoid all collisions. We have described the Token Ring MAC where stations exchange a token to regulate the access to the transmission channel.

Finally, we have described in more detail two successful Local Area Network technologies: Ethernet and WiFi. Ethernet is now the de facto LAN technology. We have analysed the evolution of Ethernet including the operation of hubs and switches. We have also described the Spanning Tree Protocol that must be used when switches are interconnected. For the last few years, WiFi became the de facto wireless technology at home and inside enterprises. We have explained the operation of WiFi networks and described the main 802.11 frames.

Source: Olivier Bonaventure,
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