Section 4.7. Summary

4.7. Summary

We started this chapter by discussing wired and wireless transmission media. Data can travel on guided links, such as optical fibers, and unguided links, such as certain types of wireless links. Link capacity is further partitioned into channels . Wireless channels have several weaknesses, such as shadow fading, path loss, and interference. Methods of accessing link channels deal with how to mediate access to a shared link so that all users eventually have a chance to transmit their data. We examined frequency-division, time-division, code-division, and space-division multiple-access methods; in most cases, time-division multiple-access methods offer several benefits for channel access in local area networks.

We also looked at methods that determine whether transferred bits are in fact correct or whether they possibly were corrupted in transit. With the cyclic redundancy check (CRC) method, some frames arriving at the destination node contain errors and thus have to be discarded.

Two link-control schemes are stop and wait and sliding window . In the stop-and-wait method, a sender waits for an acknowledgment after transmitting one frame. This flow-control method is significantly improved in the sliding window method, which lets multiple frames travel on a transmission link.

In the next chapter, we use our knowledge of data links from Chapters 2, 3, and 4 to form small local area networks.