TABLE OF CONTENTS The Link layer is the portion of the interface that is responsible for encoding transmitted data, for decoding received data, and for basic communications and protocol. Because the Link layer performs both encoding/decoding and provides the basic communications protocols, the Link layer description is divided between two chapters, with this chapter focusing on the encoding/decoding of data. Figure 9.1 indicates the layer of the architecture covered in this chapter.
Although the bit rate of the Serial ATA interface is 1.5 gigabits per second, the data rate is 150 megabytes per second rather than 187.5 megabytes per second (1,500/8) as you d expect. This apparent discrepancy is a result of the encoding scheme used in Serial ATA.While 8 bits usually represent a byte, for the purposes of transmission across the serial interconnect, a byte in Serial ATA is encoded as 10 bits. This approach is common in high-speed serial communications, and it is usually referred to as 8b/10b encoding. Although this would seem wasteful because it reduces the efficiency by 20 percent, other benefits compensate for the loss of efficiency. The 8b/10b encoding scheme is fairly common and is covered by a well-known patent held by IBM. [1] Although a lot of information on the 8b/10b encoding scheme is available from various sources, the topic is so central to the Serial ATA protocol we have provided a brief introduction here for completeness.
An 8-bit byte has 256 different values or...
