(NOTE: edited version once I realized I could modify the original description, some of the following comments of mine may no longer apply)
I have an Acer Aspire 9815 which has an integrated Intel 3945ABG networking chipset.
I only have *one* attempt at connecting to a wireless network per "load" (modprobe) of the ipw3945 module. network-manager and/or knetworkmanager seems to have some additional steps in its connection procedure rendering it impossible to connect unless I use DHCP; likely something early in the procedure "uses up" my one golden connection try, leaving the interface dumb and mute once it's time to actually connect. I replaced them with wicd which does the trick for me, managing to connect using either static or dynamic.
It works flawlessly in Windows. It worked marvelously and without hitches prior to sending the machine in for service, where they replaced the motherboard. Many others (and myself two months ago) could effortly use this networking chipset out-of-the-box, while the I of today and some others (going by the replies I've encountered on ubuntuforums.org) have to fight to keep it working at all, with varying degree of success. Perhaps there's a revision n+1 of the chipset, and/or a new firmware, incompatible to the current ipw3945?
zorael@azrael:~$ lspci | grep 3945
07:00.0 Network controller: Intel Corporation PRO/Wireless 3945ABG Network Connection (rev 02)
zorael@azrael:~$ sudo lshw -C network
*-network
description: Ethernet interface
product: 88E8055 PCI-E Gigabit Ethernet Controller
vendor: Marvell Technology Group Ltd.
physical id: 0
bus info: pci@02:00.0
logical name: eth0
version: 12
serial: 00:a0:d1:a0:ec:6b
capacity: 1GB/s
width: 64 bits
clock: 33MHz
capabilities: bus_master cap_list ethernet physical tp 10bt 10bt-fd 100bt 100bt-fd 1000bt 1000bt-fd autonegotiation
configuration: autonegotiation=on broadcast=yes driver=sky2 driverversion=1.13 firmware=N/A latency=0 link=no multicast=yes port=twisted pair
resources: iomemory:cc000000-cc003fff ioport:3000-30ff irq:16
*-network
description: Wireless interface
product: PRO/Wireless 3945ABG Network Connection
vendor: Intel Corporation
physical id: 0
bus info: pci@07:00.0
logical name: eth1
version: 02
serial: 00:18:de:6e:39:9a
width: 32 bits
clock: 33MHz
capabilities: bus_master cap_list ethernet physical wireless
configuration: broadcast=yes driver=ipw3945 driverversion=1.2.0mp firmware=14.2 1:0 () ip=192.168.0.2 latency=0 link=yes multicast=yes wireless=IEEE 802.11b
resources: iomemory:ce000000-ce000fff irq:19
zorael@azrael:~$ iwconfig
lo no wireless extensions.
eth0 no wireless extensions.
eth1 IEEE 802.11b ESSID:"LappskoleB"
Mode:Managed Frequency:2.437 GHz Access Point: 00:11:95:1F:68:EB
Bit Rate:11 Mb/s Tx-Power:15 dBm
Retry limit:15 RTS thr:off Fragment thr:off
Power Management:off
Link Quality=71/100 Signal level=-65 dBm Noise level=-66 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:798 Missed beacon:0
zorael@azrael:~$ iwlist eth1 scan
eth1 Scan completed :
Cell 01 - Address: 00:11:95:1F:68:EB
ESSID:"LappskoleB"
Protocol:IEEE 802.11b
Mode:Master
Channel:6
Encryption key:on
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s
Quality=67/100 Signal level=-66 dBm Noise level=-66 dBm
Extra: Last beacon: 180ms ago
--------- When broken ---------
zorael@azrael:~$ iwconfig
lo no wireless extensions.
eth0 no wireless extensions.
eth1 unassociated ESSID:"LappskoleB"
Mode:Managed Frequency=2.437 GHz Access Point: 00:11:95:1F:68:EB
Bit Rate:11 Mb/s Tx-Power:16 dBm
Retry limit:15 RTS thr:off Fragment thr:off
Power Management:off
Link Quality:0 Signal level:0 Noise level:0
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:1162 Missed beacon:0
zorael@azrael:~$ sudo iwlist eth1 scan
eth1 No scan results
zorael@azrael:~$ sudo iwlist eth1 scan
eth1 Interface doesn't support scanning.
zorael@azrael:~$ sudo iwlist eth1 scan
eth1 No scan results
At times, it also says "Frequency=nan kHz" when it's in its non-working state.
Even when borked, it is correctly set up through ifconfig and iwconfig; it just doesn't see any available networks. And if I enter a "hidden ESSID", it still can't/won't connect. ifdown and then ifup doesn't help, disabling and then reenabling the wireless radio through the kill switch doesn't help, nor does biting one's nails. It *seems* to be set up correctly in every way imaginable, but it doesn't do anything.
Only rebooting or manually doing:
zorael@azrael:~$ sudo modprobe -r ipw3945 && sudo modprobe ipw3945
...gives me another golden attempt. Which doesn't spell well for when the machine loses connection when unattended. (For instance, if the router randomly restarts when I have class, or if the chipset itself decides to disconnect for no apparent reason. Which happens, all too often.)
NOTE that this only grants me another *try* at connecting; there's no guarantee I'll actually be able to establish one until after five more attempts. It's iffy, sketchy, unreliable.
Sometimes, if I connect with DHCP, it just freezes at Obtaining IP address.
Other times, both with DHCP and with a static ip, it connects and reports a 0% signal.
Further other times, it connects and reports a valid signal, but only one ping request goes through before the connection dies. (I usually run a 'ping 192.168.0.1' in another terminal so I know if I succeeded.)
As mentioned in a comment below: When I successfully get through but with only a 0% signal, there's no successful traffic, but it reports being connected. Workaround ritual: disconnecting (ifconfig down or through wicd), stopping/starting the module, connecting with DHCP but cancelling when obtaining IP, disconnecting, stopping/starting module, then connecting with static IP again fixes it in 90% of all cases.
I forgot to mention, I experience the same thing in Ubuntu/Kubuntu 7.04 as in Ubuntu 7.10 tribe 4. I only tested briefly in 7.10 tribe 4 with Gnome's network manager, since it was on a Live CD session, but if it helps the process along I can download tribe 5 and install it properly.