What does GT/s mean, anyway?
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/1/2007 2:00:00 AM
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 unencoded data in each direction, or 10 Gbps of unencoded data in total. That’s 8 Gbps encoded. Thus, a 16-lane PCIe 2.0 bus transfers 160 Gbps unencoded, which is 128 Gbps of encoded data. That’s 16 Gbytes/s of encoded 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.
Links to more about PCI Express, data encoding, and transfer rates:
The PCI-SIG Web site contains the announcement of PCI Express 2.0 specification. www.pcisig.com/specifications/pciexpress/base2.
For an explanation of 8b/10b encoding, see “Encoding Dictionary, terms, and definitions,” www.interfacebus.com/Definitions1.html.
For an overview of the PCI Express bus, see “PCI Express Technology,” a white paper from Dell Computer, www.dell.com/content/topics/global.aspx/vectors/en/2004_pciexpress.
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www.tmworld.com/index.asp?
layout=talkBackPost&articleid=CA6419189&talk_back_header_id=641
9189
First time user. Just wanted info. But though I'm a casual user, I am a
careful reader. Interesting that, though it's been more than two years
since you posted this article, no one has noticed (or perhaps just not
bothered to call to your attention) the obvious inadvertent reversal
[below] of the words "encoded" and "unencoded."
That’s 8 Gbps encoded. Thus, a 16-lane PCIe 2.0 bus transfers 160
Gbps unencoded, which is 128 Gbps of encoded data. That’s 16
Gbytes/s of encoded data.
ssr8n
Fred Cole - 2009-29-8 02:55:00 EDT -
Thats all well and impressive but my question revolves around this
idea: Where is all that data going to flow to? The CPU bus cannot
compete or handle all that data flow - can it? The typical FSB of a
Core 2 Duo is around 1066 therefore providing around something like
6 to 8.5 GB/sec depending on the memory used and whatnot.
If the GPU is pounding the motherboard with data and the CPU cannot
handle it all ..whats the point of worrying about all the potential of the
PCI-e x16 v2.0 spec?
Furthermore, if the GPU renders the graphics internally then what good
does a PCI bus that can deliver or send more data than the CPU can
spit out? Sorta like using a shovel to eat your cereal with. Your
mouth (unless you're Mike Moore) is only so big after all.
Based on these assumptions like these In April of 2006 , I built a
gaming RIG that has - Tforce590SLI with 7600GTs in SLI and a FX-62
at 2.8 (no O'clock) with COrsiar XMS2 DDR2800 and a 7200.11 sata2
HD on XPSP2 and when I ran the benchmark PCMark05 I scored
11,600. Recently this score was still in the top 15% of the modders
who submitted a score. Imsure it cant compete now but I can run
Crysis to some level of enjoyment on the system which I think is
impressive as many are paying $3000.00 to Falcon and Crysis runs at 4
FPS while Im getting 25 to 30 on a rig worth $500 today. (was $2000
then)
Again the question is - exactly how much data can a Core2 Duo E8400
actiually handle from the PCI bus at any one time and whats the point
of exceeding that figure?
Shawn Sewell - 2008-18-12 10:20:00 EST
Make way for PCI Express
10/01/2003PCI Express analyzer runs at 5 Gbps
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