Die Luftbrücke in Offenbach: WLAN Bridging

Die Luftbrücke in Offenbach: WLAN Bridging

Hi Blog, as this technical blog entry might be interesting for a larger audience, i'll write in English this time. We run a Wireless-Lan "WiFi" bridge over a distance of about 100 Meters without line-of-sight contact for serveal years now. I switched active equipment and antennas several times and now we set up a solution with accesspoint reapeters. The setup is of that kind:

(Please note that all accesspoints are only "connected" via air-interface, so I was able to remove the ugly ethernet cable through my living room - so those D-Link boxes are not WLAN-to-Ethernet bridges but real AP repeaters on layer 2!)

All WLAN users have the Linksys router as default gateway, so all share the same default gateway IP number. The network name is also the same, no matter at which repeater the user's device "sees" physically. The only difference to them is the MAC address of the accesspoint - on ISO/OSI layer 2 they use the MAC of the particular repeater. All devices share the same collision domain. The "AP Repeater" mode is not the same as Wireless Distribution System (WDS).

The D-Link DWL-2100 has some quirks, so ATTOR you should upgrade to firmware version 2.20. Besides a Web-Interface and telnet it also offers ssh access to the box and you can access those interfaces in-band via WLAN since the 2.20. But with that firmware you still dont have a DHCP server in the box, so you have to configure your PC with an appropriate IP manually for configuration (The box' IP is 192.168.0.50/24). Ah, and don't wonder why the 3-page manual tells to try DHCP first - I think D-Link uses that "manual" for several types of accesspoints without adapting the content. Another quirk is, that it won't repeat for every type of accesspoint: I do not understand what the difference in layer 2 packets is, but I was not able to repeat for a Draytek Vigor 2200, nor for a ZyXEL Prestige WLAN router. With the Linksys Router I switched the the open-source OpenWRT firmware and I can report that combination works. I'll try an ASUS 500G deluxe with OpenWRT soon.

There is some alternative firmware out, as the D-Link DWL-2100 is only a OEM from a differnt company. In former times the people had good results with reflashing other compatible firmware, it seems not to make sense in times of D-Link's firmware version 2.20.(It works ;))

I read somewhere that the DWL-2100 also supports "XR Mode", which is a special low-speed mode that drops the WLAN bitrate to 256 KBits/s instead of the normal minimum of 1 MBit/s, but reaches a longer distance that way. I guess XR is an abbreviation for "eXtended Range". If you set the DWL-2100 to auto-detect the best bitrate (that's default) it may need some time to switch from 54MBit/s down to the lower speeds. So you have to give it some time when you try to ping through first: If pings get lost after starting the repeaters, wait a minute or two before crying.

We made a strange observation with range-extension by antenna: With a very large external antenna (+14 DB, 120° degree sectoral antenna from Lucent) the box simply crashed, while the Huber/Suhner +8.5 DB antenna works fine with the device. After the crash we had to reset the box and by pressing the reset button in a long and mystical order we deleted the firmware from the device (no joke). It was not obvious that it went into a "gimme new firmware" mode. In that mode it's only possible to upload a new firmware. Nothing else will work and the DWL-2100 will look like broken. The firmware-upload is done via a crippled-down web-interface is reachable via 10.0.0.1/24. Without knowing that IP you are lost and it would be a warranty case. I fetched that information somewhere from the net and avoided sending back the device to the manufacturer.

YMMV,

derjohn