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I have a simple confusion about these two kinds of interfaces: As far as Fast Ethernet is faster, then why it is not used over WAN connections. Typically, network traffic nowadays between any workstation and the internet is more than that between two workstations on the same network. This is not true always but it is more common. Moreover, the serial interface of a router generally hold the traffic of all internal PCs towards the internet. So it has to be faster than internal Ethernet as far as using switches makes any communication between two PCs a one to one communication. Can any body clarify this point to me?

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Fast Ethernet over cooper is limited to a max distance of 100 meters over Cat5 or higher category cabling. It makes it impossible to be used over typical WAN lines.

However you can provide a long distance fast ethernet using fiber optics:

  • 100 BASE-FX can give up to 2 kms for full-duplex over multi-mode optical fiber.
  • 100 BASE-SX (not an 802.3 standard) goes 550m on multi-mode fiber.
  • 100 BASE-LX10 can reach 10km using single-mode fiber.
    [Wikipedia Fast_Ethernet]

Besides fiber optics, the other predominant WAN media is air. Typically you use RF, microwaves, wiMAX,etc.

The main problem is that on air dedicated bandwith is expensive. Having 100 Mbps dedicated could be costly in equipments and licenses. So most systems use a shared spectrum where you can have good speed, but it's a system prone to interferences.

Serial interfaces as RS-232 or V.35 were designed having in mind speeds below 2 Mbps. They aren't useful for today speeds.

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