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I have two Windows 7 machines on my local network. Both use wireless LAN adapters to connect to the AP/router.

Now if I download from Internet, my speed is 1800KB/s but if I copy files from one Windows computer to the other using shared folders, my speed is 500KB/s.

Why is the local transfer so much slower? How can I fix this?

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Your description and title are a little confusing. It would help us all if you could clearify this a little bit. Anyway, why would you expect wireless speed to be as good as that on your local LAN? – mdpc Jan 5 '13 at 4:36
up vote 2 down vote accepted

There's probably no problem, this is expected behavior. When both clients are wireless, the data has to be sent twice over the wireless link, halving the bandwidth. In addition, the link has to be "turned around" twice as much, increasing the dead time. If you want higher wireless speeds, upgrade your wireless hardware.

Consider these two cases. Each example shows two data packets being sent and one acknowledgement being sent in reply.

Case 1: Internet to LAN. Packets are being received over the Internet link by the access point bound for machine A.

  1. AP -> A: Here's a data packet for you.

  2. A: Got it.

  3. AP -> A: Here's a data packet for you.

  4. A: Got it, and here's a reply.

Notice all requests send useful data and each data packet is sent once.

Care 2: LAN to LAN. A wants to send data to B.

  1. A: I need the link.

  2. AP->A: It's yours.

  3. A: Here's some data for B.

  4. AP->A: Got it.

  5. AP->B: Here's some data for you.

  6. B: Got it.

  7. A: I need the link.

  8. AP->A: It's yours.

  9. A: Here's some data for B.

  10. AP->A: Got it.

  11. AP->B: Here's some data for you.

  12. B: Got it. And I have a reply for A.

  13. AP->A: Here's some data.

This is clearly much worse. A must frequently arbitrate with the AP to get the link to send a packet. Each bit of data must be sent twice. There are far more handoffs of control of the link. Yuck.

Upgrading to 802.11n and an access point with multiple simultaneous stream capability can make a huge difference.

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