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Bandwidth Inside Your Computer

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MattSlagle View Drop Down
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Joined: 03 Apr 2008
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  Quote MattSlagle Quote  Post ReplyReply Direct Link To This Post Topic: Bandwidth Inside Your Computer
    Posted: 21 May 2008 at 11:50am

Bandwidth Inside Your Computer

In this article, I will explain all about the bandwidth inside your computer.  Now I will be talking much about the different speeds and frequencies as well, so it would help to read this article that I wrote all about them.

All About The Different Frequencies and Speeds Inside A Computer

There are so many different buses where bandwidth can play a concern.  Let me display a list of the different buses I will be going over.

  • Front Side Bus (FSB)
  • Memory Bus
  • PCI-Express
  • SATA
  • Video Card

But first, what exactly is bandwidth?  Bandwidth is amount of data that can travel over a bus (array of wires) in a given amount of time.  Bandwidth is commonly given as how much data can pass through the bus in one second.  To calculate bandwidth, you take the frequency of the bus times how wide (amount of wires) the bus is.

Front Side Bus

The FSB is the link between CPU and Northbridge of the chipset.  FSB speeds are always rated on total amount of transfers but in reality is much lower.  Because Intel currently uses quad-pumping (sends 4 bits of data on every clock), FSB speeds are rated at 4x the core clock rate of the motherboard.

Data Path Width:  64 bits (8 bytes)

Common FSB Speeds and Bandwidths

  • 800 MHz x 64 bits = 6.4 GB/s
  • 1066 MHz x 64 bits = 8.5 GB/s
  • 1333 MHz x 64 bits = 10.6 GB/s
  • 1600 MHz x 64 bits = 12.8 GB/s

Memory Bus

The memory bus is the link between memory and chipset.  Memory bus speeds are always rated on total amount of transfers, but in reality is half the quoted speed.  That is because DDR (Double Data Rate) memory sends data twice on every clock cycle.  Now with the advent of dual-channel ram, we can double the width of the bus by accessing two memory dimms at the same time.  You can think of this as having the memory in a Raid 0 combination.

Data Path Width:  64 bits (8 bytes) or 128 bits (16 bytes) dual-channel

Common Memory Bus Speeds and Bandwidths

  • 667 MHz x 64 bits = 5.3 GB/s or 10.6 GB/s
  • 800 MHz x 64 bits = 6.4 GB/s or 12.8 GB/s
  • 1066 MHz x 64 bits = 8.5 GB/s or 17 GB/s
  • 1333 MHz x 64 bits = 10.6 GB/s or 21.2 GB/s
  • 1600 MHz x 64 bits = 12.8 GB/s or 26.5 GB/s
  • 1800 MHz x 64 bits = 14.4 GB/s or 28.8 GB/s

PCI-Express

PCI-Express is different from the older technologies of the FSB and memory buses.  It instead uses a point to point serial connection (lane) between device and controller.  This means that this connection does not need to be shared like the older IDE and AGP buses that used to dominate computers.  There recently has been an upgrade from version 1.1 to 2.0 that doubled the bandwidth of the PCI-Express lanes.  You will also notice that different devices like video cards use multiple lanes (Newer cards use x16) to achieve higher bandwidths.

Data Path Width:  1 bit at 250 MB/s (version 1.1) or 500 MB/s (version 2.0)

Common PCI-Express Speeds and Bandwidths

  • Version 1 x1 = 250 MB/s
  • Version 1 x4 = 1 GB/s
  • Version 1 x8 = 2 GB/s
  • Version 1 x16 = 4 GB/s
  • Version 2 x1 = 500 MB/s
  • Version 2 x4 = 2 GB/s
  • Version 2 x8 = 4 GB/s
  • Version 2 x16 = 8 GB/s
  • Version 2 x32 = 16 GB/s

SATA

Serial ATA (SATA) is the serialized version of PATA, which was more commonly called IDE.  It replaced the wide 40 and 80 bit wide ribbon cables with a more streamlined cable which is only a few wires wide.  It is just like the PCI-Express as each connection is a direct point to point connection.  There are currently two versions (SATA 150 and SATA 300) where SATA 300 is currently replacing SATA 150.  You will notice that the bandwidth is stated in Gb/s which is 8 times or more slower than GB/s.  Even though a byte is 8 bits, to transfer a full byte, the protocol needs to send 10 bits.  So to find the GB/s a second, you divide the Gb/s rating by 10.

***Note about hard drives.  Current generation hard drives are only able to deliver between 80 MB/s to 120 MB/s transfer rates from disk to buffer.  This means that SATA 300 currently has large unused bandwidth.

Data Path Width:  1 bit at 1.5 Gb/s or 3.0 Gb/s

Common SATA Speeds and Bandwidths After Protocol Overhead

  • SATA 150 = 150 MB/s
  • SATA 300 = 300 MB/s

Video Cards

Now, I already explained the bus from system to the video card through PCI-Express.  But I thought you would like to know what kind of data transfer rates happen inside your video card.  You will notice that these rates are much higher than able to be sustained by the PCI-Express bus I mentioned before.  This is because a simple command such as drawing a 3D square may become hundreds of smaller commands inside the video card.  Depending on the video card, it may use DDR2, GDDR3, or even GDDR5 memory.  GDDR3 memory is much like DDR2, but runs at the frequencies of DDR3 memory found inside computers.  It is better suited at handling the different graphics data than regular computer memory.  Also notice that depending upon the type of video card, the data path wide could be anywhere from 128 bits to a super wide 512 bits.

Data Path Width:  64 bit ( 8 bytes), 128 bit (16 bytes), 192 bits(24), 256 bits (32 bytes), 384 bits (48 bytes), and 512 bits (64 bytes)

Common Video Card Speeds and Bandwidths

DDR2

  • 64 bit x 800 MHz = 6.4 GB/s
  • 128 bit x 800 MHz = 12.8 GB/s

GDDR3

  • 128 bit x 1400 MHz = 22.4 GB/s
  • 128 bit x 2000 MHz = 32.0 GB/s
  • 192 bit x 1600 MHz = 38.4 GB/s
  • 192 bit x 1900 MHz = 45.6 GB/s
  • 256 bit x 1800 MHz = 57.6 GB/s
  • 256 bit x 1940 MHz = 62.0 GB/s
  • 256 bit x 2200 MHz = 70.4 GB/s
  • 348 bit x 1800 MHz = 86.4 GB/s
  • 348 bit x 2160 MHz = 103.6 GB/s
  • 512 bit x 2000 MHz = 128.0 GB/s


Edited by MattSlagle - 21 May 2008 at 2:24pm
Matt Slagle
AVADirect Research and Developement
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