Effective Data Rate Of Sas Technology - Compaq BL10e - HP ProLiant - 512 MB RAM Update Manual

ISS Technology Update

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-First-
Out (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:
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
1.5 Gbps SAS-1, SAS-1.1
3 Gbps SAS-1, SAS-1.1
6 Gbps SAS-2, SAS-2.1
*Effective Data Rate shown is converted to bytes.
Effective data rate = Physical link rate × 0.8
Effective Data Rate*
150 MB/s
300 MB/s
600 MB/s
4x links
600 MB/s
1200 MB/s
2400 MB/s
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