Re: Multiple WiFi adapters

On Sun, 18 Jan 2009 08:18:51 -0700, Dave Rudisill <denali@alaska.net>
wrote:

>A while back there was a discussion of how Windows chooses which
>network connection to use when there is both a wired and wireless
>connection:
>(http://groups.google.com/group/alt.i...de116db990c9be)
>
>Does that same discussion apply to multiple wireless connections?

Yes. The major issue is that each "interface" (that's an ethernet
port, wireless card, or other network connection) must have a unique
MAC address associated with it. You can't have the same IP address
assigned to multiple MAC addresses. Therefore, you can't have
multiple connections with the same source IP address. If you
simultaneously run multiple ethernet or wireless cards, the output of:

ipconfig /all | find "Address"

Physical Address. . . . . . . . . : 00-02-B3-1E-43-17
IP Address. . . . . . . . . . . . : 10.0.0.11
Physical Address. . . . . . . . . : 00-02-B3-1E-39-ED
IP Address. . . . . . . . . . . . : 192.168.1.11

The two IP addresses are the result of having two ethernet adapters in
my desktop.

If I connected to a remote VPN server, there would be a third IP.
Might as well try... yep, it works:

ipconfig /all | find "Address"
Physical Address. . . . . . . . . : 00-02-B3-1E-43-17
IP Address. . . . . . . . . . . . : 10.0.0.11
Physical Address. . . . . . . . . : 00-02-B3-1E-39-ED
IP Address. . . . . . . . . . . . : 192.168.1.11
Physical Address. . . . . . . . . : 00-53-45-00-00-00
IP Address. . . . . . . . . . . . : 192.168.111.141

Anyway, if I try to assign the same IP address to any two of these
interfaces, Windoze will complain. This may not seem relevent, but
please read on.

>I.e., if my laptop's internal WiFi adapter and my USB-connected
>WiFi adapter are both connected to the same NAP, is the "least
>expensive" connection still used?

Yes. The problem is the same as previously mentioned. Each interface
on your laptop has to be unique. Different MAC and by implication,
different IP. However, your IP route to the router will be different
for each wireless card. You can connect, associate, login, and
connect, but the traffic will only move through one of the wireless
connections. Unless you're running some load balancing router
software, that distributes the traffic from multiple IP's to a single
destination IP (i.e. your router's IP address), it's going to be very
unbalanced traffic. Worse, you can't switch destinations on the fly,
because that will initiate a disconnect.

All is not lost. You can connect two seperate wireless cards to two
seperate access points (each with a unique IP address) and
simultaneously move traffic to both. That's done with a static route
to one of the routers and the network behind it. The other route is
the default route, which takes care of everything else.

You might want to read the research articles under:
<http://research.microsoft.com/en-us/projects/VirtualWiFi/>
It's not directly related to what you're doing, but the last two
articles do expand on my previous ranting.

>Does each connection automatically get a different Metric value?

I dunno. The above example with 3 connections yielded a metric of 1
for all connections. There's actually a 3rd ethernet card installed
that I forgot to enable. When I turned it on, it didn't get a DHCP
assigned IP address and received a metric of 30. However, in all
other respects, the other 3 interfaces have equal route "costs".
Incidentally, this is on Windoze 2000 SP3. I'm too lazy to get my XP
laptop out of the truck and see what it does. I suspect it's
different, as Vista might be because I vaguely recall that there were
some changes in various service packs.

>Can those values be manually assigned?

Yes.

Configuring Multiple Adapters on the Same Physical Network
<http://support.microsoft.com/kb/258487/EN-US/>
The bottom of the page shows how to manually tweak the route metric.

An explanation of the Automatic Metric feature for Internet Protocol
routes
<http://support.microsoft.com/kb/299540>

>Is there any down side to having both connections active?

Self interference. Even if the two wireless adapters are on
different 2.4GHz channels (which is impossible when connecting to a
single access point), you're gong to have mutual interference. You
might get away with connecting to two different access points, on
different channels, or preferably, different bands.

Also, stuffing in a 2nd interface to the same destination does NOT
double your thruput. You get whatever speed is supplied by the single
wireless interface, with the other wireless interface doing nothing.

If you have any questions, BEFORE you ask, please supply:
1. What are you trying to accomplish?
2. What do you have to work with?
3. What have you done so far and what happened?


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558