HP ProLiant DL288 ISS Technology Update Volume 8, Number 6 - Page 4

ISS Technology Update Volume 8, Number 6 Effective data rate of SAS Technology Serial Attached SCSI (SAS) uses differential signaling to transfer data over a physical link, or phy. This reduces the effects of capacitance, inductance, and noise experienced by parallel SCSI at higher speeds. SAS communication is full duplex, which means that each phy can send and receive information simultaneously over the two wire pairs. SAS encodes (embeds) the clock signals into the data stream, which eliminates the skew problem with aligning data and clock signals experienced by parallel SCSI buses. SAS communication requires a device called a serializer/deserializer (SerDes) to convert parallel data into a serial bit stream and vice versa. The SerDes chip contains a parallel digital interface, First-In-FirstOut (FIFO) caches, an 8 bit/10 bit (8b/10b) encoder and decoder, a serializer, and a deserializer (see Figure 2-1). Figure 2-1. The SerDes chip converts 8-bit parallel data to 10-bit serial data. The 8b/10b encoder converts each 8-bit data byte to a 10-bit transmission character as it embeds the clocking information into the data stream. This adds about 20 percent embedded overhead to the data stream, which is why the effective data rate of SAS (and SATA) communications is calculated as follows: Effective data rate = Physical link rate × 0.8 As such, the effective data transfer rates for SAS technologies (per direction) are listed in Table 2-1. The SAS interface allows data transfer to be scaled by using a combination of multiple physical links to create two connections (2x, 3x, 4x, or 8x) per port. Table 2-1. Physical link rates and effective data transfer rates per direction Physical link rate Generation Effective Data Rate* 4x links 1.5 Gbps SAS-1, SAS-1.1 150 MB/s 3 Gbps SAS-1, SAS-1.1 300 MB/s 6 Gbps SAS-2, SAS-2.1 600 MB/s *Effective Data Rate shown is converted to bytes. 600 MB/s 1200 MB/s 2400 MB/s 4