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I have a Broadcom wireless chip which I've managed to wrestle into working with Debian GNU/Linux (I'm on Sid, if it matters). The interface is definitely there:

612 ± ip link    
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT qlen 1000
    link/ether 3c:07:54:06:e0:86 brd ff:ff:ff:ff:ff:ff
3: wlan0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN mode DORMANT qlen 1000
    link/ether e4:ce:8f:40:ec:c4 brd ff:ff:ff:ff:ff:ff
alex-debian ओम् ~:(14h55m|git@master)
613 ±  

I have GNOME, and hence NetworkManager, up and running. When I look at GNOME Control Center in the Network pane, the Wireless tab gives information but doesn't list any wireless networks. I've tried connecting to a hidden network, just in case, but this didn't work. I know my network isn't hidden.

Edit: per this wiki page, I've added myself to the netdev group, and relogged into my session, with no result.

How can I start diagnosing the source of this problem?

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I'd double check using iwlist wlan0 scan from the terminal to see if the device wlan0 can see access points:

$ iwlist wlan0 scan | grep -v IE:
wlan0     Scan completed :
          Cell 01 - Address: 34:12:AB:12:C4:4D
                    Frequency:2.437 GHz (Channel 6)
                    Quality=58/70  Signal level=-52 dBm  
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
                              24 Mb/s; 36 Mb/s; 54 Mb/s
                    Bit Rates:6 Mb/s; 9 Mb/s; 12 Mb/s; 48 Mb/s
                    Extra: Last beacon: 71963ms ago
                        Group Cipher : TKIP
                        Pairwise Ciphers (2) : CCMP TKIP
                        Authentication Suites (1) : PSK
                        Group Cipher : TKIP
                        Pairwise Ciphers (2) : CCMP TKIP
                        Authentication Suites (1) : PSK

I would also open a terminal window and tail the /var/log/syslog while you restart NetworkManager:

$ sudo tail -f /var/log/syslog

Then restart NetworkManager:

$ service NetworkManager restart

I'd also make sure that the Linux Kernel detected the WIFI device correctly by reviewing the dmesg log:

$ dmesg
share|improve this answer
turned out it was because I was using WEP, but props for the detailed answer. upvoted. – strugee Jul 28 '13 at 23:54
up vote 0 down vote accepted

The wireless network was using WEP, which as stated by the wiki page in the question, is unsupported. I assumed that this meant "will work but have weird problems", but in fact, upgrading the firmware, then using the new options to switch to WPA2 solved my problem.

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