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Throughput PCIE V3

Written on:September 2, 2014
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Sometimes you have to face the fact that it is necessary to evaluate bandwidth of PCIE V3 connection. This article will help you to understand how to make needed calculations.

Let’s consider briefly the basic things we need to know:

  1. Bidirectional serial connections (lanes) are used for data transmission.
  2. 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.
  3. The frequency at which the differential pair operates is 8 GHz. for the PCIE V3 standard.
  4. Information about synchronization is included in the transmitted signal and there is used channel coding 128bit / 130bit for the standard PCIE V3. I.e. a redundancy of data transmission is just 1.54%.
  5. The parameter is between the theoretical maximum bandwidth and the real value we denote [Real / Theory Bandwidth PCIE V3] = 0,8125. For its calculation we used the following data:

10242014_pcie_v3_real

In general, we have everything you need for the calculation.

  1. The frequency of LVDS pair is 8 GHz.
  2. The parameter of channel codding is 128/130 ~ 0,985.
  3. The ratio between real and theoretical bandwidth is 0,8125.

The calculation can be represented in a table form:

PCIE V3 (lanes) x1 x2 x4 x8 x16 x32
Simultaneous Transmission in 1 Direction, bit 1 2 4 8 16 32
Multiply the number of simultaneously transmitted data on the frequency
Receive Bandwidth (bit x clock), Gbps 8 16 32 64 128 256
Transmit Bandwidth (bit x clock), Gbps 8 16 32 64 128 256
Determine the aggregate bandwidth
1 Direction Bandwidth, Gbps 8 16 32 64 128 256
Aggregate Bandwidth, Gbps 16 32 64 128 256 512
Multiply by a factor that takes into account the redundancy of channel coding
1 Direction Bandwidth w 128bit/130bit, Gbps 7,877 15,754 31,508 63,015 126,031 252,062
Aggregate Bandwidth w 128bit/130bit, Gbps 15,754 31,508 63,015 126,031 252,062 504,123
Translate Gbps into GBps (div into 8)
1 Direction Bandwidth w 128bit/130bit, GBps 0,985 1,969 3,938 7,877 15,754 31,508
Aggregate Bandwidth w 128bit/130bit, GBps 1,969 3,938 7,877 15,754 31,508 63,015
Multiply by the parameter that takes into account the ratio between the real and the theoretical bandwidth
1 Direction Bandwidth Real, GBps 0,800 1,600 3,200 6,400 12,800 25,600
Aggregate Bandwidth Real, GBps 1,600 3,200 6,400 12,800 25,600 51,200
Translate GBps into Gbps (multiply by 8)
1 Direction Bandwidth Real, Gbps 6,400 12,800 25,600 51,200 102,400 204,800
Aggregate Bandwidth Real, Gbps 12,800 25,600 51,200 102,400 204,800 409,600
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