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TL-WR703N-based OpenWRT 3G WLAN hotspot

There is a variety of cheap networking equipment available on the Chinese market, some exceptions being even useful. The TL-WR703N by TP-Link is one of them, a router that includes an USB port for a 3G modem, a Wifi radio and an ethernet port. Priced at ~22e, it comes with a Chinese firmware and UI. This document describes steps necessary to turn this tiny router into a full-featured OpenWRT hotspot.

New firmware

Several images of OpenWRT exist, and luckily someone has made an image that suits our application perfectly. Download:

http://dl.dropbox.com/u/17850330/openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-sysupgrade.bin

Mirrored here: openwrt-ar71xx-generic-tl-wr703n-v1-squashfs-sysupgrade.bin.

Connect the USB power cord and ethernet cable to your computer. The default address of the unit should be 192.168.1.1. Enter using your favorite Web browser and you will be asked for an username and a password. Enter admin / admin.

The WebUI is strongly encrypted. Choose the last item on the list, and choose the third item on the sublist. There should be a file input field and a submit button. Choose the previously downloaded firmware file and press the lower button. Programming takes a few minutes, and it is the time to pray for not bricking the router!

If everything went well (or better than expected), OpenWRT interface should be at the same 192.168.1.1, and you can begin the configuration.

If you missed something, OpenWRT documentation may help you.

Configuration

Our target is to make the unit to operate as an DHCP server that can be accessed via both LAN and WLAN. The default GW is the 3G modem. Feel free to configure subnet parameters as you prefer. Remember to set your password as we shouldn't provide too easy targets for script kiddies.

On Network-tab, choose Add new interface. We will name our mobile broadband connection as 'wan'. Choose UMTS/GPRS/EV-DO-protocol and click Submit. You should get more fields to fill.

The tricky part is to choose correct virtual serial port. usb-modeswitch produces with my MF668A four (4) ttyUSB ports, out of only one functions, ttyUSB1 in my case. You should try different settings until the modem establishes a connection. Service type is UMTS/GPRS and APN according to your ISP. In my case (Elisa/Saunapalvi) it is internet.saunalahti . Some sources state that disabled PIN codes are problematic, which hasn't been tested by me. Enter your PIN code and leave PAP/CHAP-fields empty unless required by your ISP. Click Save & Apply to save changes.

If everything went better than expected, the modem should establish a broadband connection automatically. If not, you are on your own to troubleshoot it. Default GW etc. are configured automatically, so you should reach the Interwebs now. Accommodate other settings to suit your needs and have fun!

External antenna connector

I guess that numerous hams are willing to use their own antennas with TL-WR703N. An external antenna connection was implemented and will be documented later. While waiting, please look at pictures.

Contact information: callsign at sral dot fi, where the callsign is OH2FLO.