What does GT/s mean, anyway?

-March 01, 2007

When announcing version 2.0 of the PCI Express (PCIe) standard in January, the PCI Special Interest Group (PCI-SIG) said that the new version “doubles the interconnect bit rate from 2.5 GT/s to 5 GT/s.” Most of us are used to seeing bus speeds specified in Gbps, or gigabits per second, but GT/s stands for gigatransfers per second. What’s the difference?

The difference has to do with the encoding of the data. Because PCIe is a serial bus with the clock embedded in the data, it needs to ensure that enough level transitions (1 to 0 and 0 to 1) occur for a receiver to recover the clock. To increase level transitions, PCIe uses “8b/10b” encoding, where every eight bits are encoded into a 10-bit symbol that is then decoded at the receiver. Thus, the bus needs to transfer 10 bits to send 8 bits of encoded data.

Looking at a single PCIe 1.1 lane, the bidirectional bus can transfer 2.5 Gbps in each direction, or 5 Gbps in total. Because the bus needs to send 10 bits of encoded data for every 8 bits of unencoded data, the effective bit rate is

5 Gbps • (8/10), or 4 Gbps

A 16-lane PCIe 1.1 bus can transfer 80 Gbps of encoded data or 64 Gbps of unencoded data. Because PCIe 2.0 doubles the transfer rate, a single lane can transfer 5 Gbps of encoded data in each direction, or 10 Gbps of encoded data in total. That’s 8 Gbps unencoded data. Thus, a 16-lane PCIe 2.0 bus transfers 160 Gbps encoded, which is 128 Gbps of unencoded data. That’s 16 Gbytes/s of unencoded data.

So, when the PCI-SIG announced the new rate of 5 GT/s, it was referring to raw data rate—the number of bps that the bus can move, or transfer. The encoding process reduces the rate of useful data transferred over the bus to 80% of the bus’s raw speed.

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