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When I plug my laptop via LAN cable and check speed on speedtest.net I have over 32 Mb/sec. When I unplug the cable and connect to the same router I have less than 15 Mb/sec. I tried to change channels already but this didn't help. I'm sitting just near the router.

What can it be? The router is a Dlink 400.

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How many other wifi networks can you see? Do both your computer and router support 802.11n? Are they using it? –  idbrii Sep 11 '11 at 19:24
    
No, it is 802.11g –  Eugene Sep 11 '11 at 19:27
    
Encryption overhead, near channel interference. –  Fiasco Labs Nov 22 at 22:23

5 Answers 5

up vote 4 down vote accepted

The wireless connection is the bottleneck. Even 802.11g, with theoretical bandwidth of 54Mbit/s, can typically only achieve 22Mbit/s average throughput under ideal conditions. It is likely that the laptop only supports 802.11g, rather than the higher speed 802.11n.

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Wireless access speeds are known to be slower than wired connections, for now at least. However, your router is capable of more than 15Mbps.

Sitting right next to the router means you might not actually get the best reception. Try sitting a couple yards away with direct line of sight and running speed test again. Also, make sure your router isn't next to large metal objects or electrical items as they can interfere with the wireless signal and turn off WEP and enable WPA2/AES instead, WEP can kill your wireless speed.

I strongly suggest you look through this guide for possible causes once you have done those basic things.

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802.11a/b/g routers aren't capable of transmitting and receiving data at the same time (i.e., they operate in a half-duplex mode). This will have an impact on your speeds as well, since the router has to back-off occasionally to allow the laptop to talk and vice versa. The more clients you have on your WLAN, the less time each has to talk. This is to avoid collisions 'in the air' - if they could all transmit at the same time, the WiFi channel would probably be a noisy mess.

The wired interface on your router is more than likely FastEthernet 10/100Mbps (or possibly GigabitEthernet 10/100/1000Mbps); FE (and GE) are capable of operating in a full-duplex mode - i.e., the connected devices can send and receive at the same time with no need for either to back-off to avoid collisions. This is because the ethernet cable that you use to connect them contains 4 pairs of wires and the router can use two pairs (FE) or four pairs (GE) to transmit and receive. (Note that because most devices negotiate the connection mode and auto-negotiation tends to be crap, your interfaces may actually be operating in half-duplex mode but will still be faster than the wireless connection).

For this - and the other reasons posted - your wired connection will always be faster than wireless, even under ideal conditions for WiFi.

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To add to what has already been said, your connection speed is also at the mercy of whatever else is going on in the transmission channel you're using. If there are other access points around on the same channel, that will affect your signal-to-noise ratio, and ultimately, your speed. Not to mention, a lot of technologies rely on that same 2.4GHz band, such as Bluetooth. Take a look at this reference:

http://en.wikipedia.org/wiki/Electromagnetic_interference_at_2.4_GHz

The bottom line is that wireless is great for convenience and suitable for the vast majority of home networking purposes, but if you need to transmit a large amount of data, wired is almost always going to give you better throughput.

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You probably don't have any 802.11b devices anymore. Turn off 802.11b in your router, and you should see a performance boost of a couple megabits per second. However, it seems that there is no benefit to disabling 802.11g - even if you have only 802.11n equipment.

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