Sometimes you have to face the fact that it is necessary to evaluate bandwidth of SAS 2.0 connection. This article will help you to understand how to make needed calculations.
Let’s consider briefly the basic things we need to know:
- Bidirectional serial connections (lanes) are used for data transmission.
- Each lane in the physical layer consists of 2 low-voltage differential signaling pairs (LVDS pairs). One pair of wires is used for receiving data and the other for transmission. The voltage difference between the wires in the pair is used for signal coding. The using of low voltages and strong electromagnetic coupling between the wires significantly reduces noise level and power dissipation. Each pair of wires is a current loop with a set of characteristics that are described the Texas Instruments TIA-644, TIA-889 standards.
- Max theoretical data rate in 1 direction for x1 lane is 6Gbps for SAS 2.0 standard.
- Information about synchronization is included in the transmitted signal and there is used channel coding 8bit / 10bit for the standard SAS 2.0. I.e. a redundancy of data transmission is just 20%.
- The parameter is between the theoretical maximum bandwidth and the real value we denote [Real / Theory Bandwidth SAS] = 0,91667. For its calculation we used the following data:
In general, we have everything you need for the calculation.
- Max theoretical data rate in 1 direction for x1 lane is 6Gbps.
- The parameter of channel codding is 8/10 = 0,8.
- The ratio between real and theoretical bandwidth is 0,91667.
The calculation can be represented in a table form:
|SAS 2.0 (lanes)||x1||x2||x4||x8||x16||x32|
|Receive Bandwidth, Gbps||6||12||24||48||96||192|
|Transmit Bandwidth, Gbps||6||12||24||48||96||192|
|Determine the aggregate bandwidth|
|1 Direction Bandwidth, Gbps||6||12||24||48||96||192|
|Aggregate Bandwidth, Gbps||12||24||48||96||192||384|
|Multiply by a factor that takes into account the redundancy of channel coding|
|1 Direction Bandwidth w 8bit/10bit, Gbps||4,8||9,6||19,2||38,4||76,8||153,6|
|Aggregate Bandwidth w 8bit/10bit, Gbps||9,6||19,2||38,4||76,8||153,6||307,2|
|Translate Gbps into GBps (div into 8)|
|1 Direction Bandwidth w 8bit/10bit, GBps||0,6||1,2||2,4||4,8||9,6||19,2|
|Aggregate Bandwidth w 8bit/10bit, GBps||1,2||2,4||4,8||9,6||19,2||38,4|
|Multiply by the parameter that takes into account the ratio between the real and the theoretical bandwidth|
|1 Direction Bandwidth Real, GBps||0,55||1,1||2,2||4,4||8,8||17,6|
|Aggregate Bandwidth Real, GBps||1,10||2,2||4,4||8,8||17,6||35,2|
|Translate GBps into Gbps (multiply by 8)|
|1 Direction Bandwidth Real, Gbps||4,4||8,8||17,6||35,2||70,4||140,8|
|Aggregate Bandwidth Real, Gbps||8,8||17,6||35,2||70,4||140,8||281,6|