Appreciating the problem:
Like you, wireless networking worked for me in version 10.04 LTS. This was also true for questions 31483, 67077, and others that are all closed as "too localized". This is an IAQ (Infrequently Answered Question).
My wireless network is slower on Ubuntu than on Windows boxes, or even the same machine booted into Windows.
I get a peculiar saw-toothed graph of ping times from my laptop to a wireless router (e.g., 10.0.0.1 or 192.0.0.1 at home). Here are my ping milliseconds from my laptop to my coffee shop's AP, adding parenthesis to illustrate the saw-tooth cycle.
(36, 59, 81, 104, 127) (47, 59, 92, 116) (36, 59, 81, 104) (24, 150; 73,92,114)
(38,48,81,103) (23,45,67,91) ...
I can connect to some coffee shop APs and not others. The results are almost, but not always the same. Once in a blue moon, I do connect to an AP I had written off as not connectable.
Much of the syslog is verbose, about 50 lines for an attempt to connect. Much is concerned with the unused IPv6. In comparing a good connection versus a bad connection, a good connection does (DHCPDISCOVER then DHCPREQUEST then receives DHCPOFFER) while a failing connection does DHCPDISCOVER and has no DHCPREQUEST or DHCPOFFER lines.
Gathering the Information
Many questions like this have no great information. Here's a bit of help in gathering information:
The usual most relevant log is /var/log/syslog which collects all sorts of random information. A wireless connection generally generates at least 100 lines.
# tail -20 /var/log/syslog ;# see the last 20 lines of the log
# tailf /var/log/syslog ;# watch the log expand with usage.
# logger ====== Trying again ;# add a message to the syslog
Another log is dmesg, the kernel ring buffer. It might, but usually wont say anything useful. The usual networking message is "No IPv6 Routers Present"
# dmesg | tail -3 ;# look at last three lines
[ 422.328274] eth1: no IPv6 routers present
[ 474.578004] eth1: no IPv6 routers present
[ 3327.234898] eth1: no IPv6 routers present
The ifconfig command relates to networking interfaces, both wired and unwired. It shows, and can configure, hardware addresses and packet counts.
# ifconfig -a ;# show all interfaces, configured or not
# ifconfig eth1 ;# just show eth1
eth1 Link encap:Ethernet HWaddr c4:17:fe:71:59:f1
inet addr:192.168.19.79 Bcast:192.168.19.255 Mask:255.255.255.0
inet6 addr: fe80::c617:feff:fe71:59f1/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:7549 errors:0 dropped:0 overruns:0 frame:18706
TX packets:7230 errors:142 dropped:0 overruns:0 carrier:0
RX bytes:6541083 (6.5 MB) TX bytes:960689 (960.6 KB)
The iwconfig command is about wireless interfaces. Note that you want to run this as root because it sometimes gives abbreviated results from the user account.
# iwconfig eth1 ;# show eth1 wireless information
eth1 IEEE 802.11abgn ESSID:"Quickly-WiFi"
Mode:Managed Frequency:2.412 GHz Access Point: 06:16:16:03:6A:10
Bit Rate=11 Mb/s Tx-Power:24 dBm
Retry min limit:7 RTS thr:off Fragment thr:off
Power Managementmode:All packets received
Link Quality=1/5 Signal level=-85 dBm Noise level=-90 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:145 Invalid misc:0 Missed beacon:0
You might also use these commands:
# cat /etc/lsb-release ;# See your current Ubuntu version
# lshw -c network ;# See your networking hardware, at least what's recognized.
# ls /etc/modprode.d ;# config probing hardware, blacklists, and add exceptions
# lsmod ;# which modules are used by what
# lspci ;# more hardware information
# rfkill list all ;# see status of hardware and software on/off switches
# nc ;# a hard to use swiss army knife of networking.
Troubleshooting and Solutions
There seems to be a sequence of hints.
Rule out the careless. Make sure your wireless in enabled, make sure you have the right password, etc. Boot in Windows to check. Look for the DHCPDISCOVER but not request in /var/log/syslog.
Start seeing where networking breaks. Ping loopback (127.0.0.1), try with wired to the access point, try pinging out (126.96.36.199 is Google), try pinging google.com (using DNS). Try a wget on http://www.google.com.
Try blacklisting your wireless card or suspending the hardware probe using files in /etc/modprobe.d.
Try going into the network manager and giving yourself a fixed, reasonable IP address and see the router then answers you.
Try rebooting. There are occasional problems with network cards coming back from power saving modes.
There are some known issues detailed in Hardware Support as well.
Good luck, and I'll try to update this answer from comments.