This was posted over on Hofo, credit goes to that poster:
HowardForums: Your Mobile Phone Community & Resource - 8320 Stable Wi-Fi UMA Connection Setting

== snip ==
This may help:

Per LINKSYS tech support and RIM use the following values in your router:

DTIM: 2 (1-255)
Fragment Threshold : 2304 (256-2346)
RTS Threshold : 2304 (0-2347)
Beacon Interval : 50 (20-1024 ms)
Channel = 3 (or least used channel in your area)

This has solved many issues with failed UMA connections.
== snip ==

I am not a big UMA user and/or have tried, just passing on some info.

Thanks Rivvie--I'll post it over on the FAQ as well.

Thanks. It is very interesting that I've been having a lot of issues with the phone not being able to start UMA or have it drop the UMA, but I changed these settings around (in a WRT54G) and my phone locked immediately into UMA mode. I don't know if it'll still drop randomly, but it seems to have made an improvement right off the bat. Thanks again!

Thanks Rivvie.

Mine was set to:

Fragment Threshold : 2346 (256-2346)
RTS Threshold : 2347 (0-2347)
Beacon Interval : 100 (20-1024 ms)
Channel = 2

and I haven't had much trouble. However, I changed to "your" settings and will try it out.

**Can anyone tell me (a layman) what that stuff does?

I have no idea what that does or even how to input that into my Linksys WRT54GS router, can anybody help me out?

These are the default settings on a T-Mo Linksys router:
There are some differences from those recommended above.
I will try them out.

(Wireless>Advanced Wireless)
Authentication Type: Default (grayed out)
Basic Rate: Default
Transmission Rate : Auto
CTS Protection Mode : Disabled
Frame Burst : Enable
Beacon Interval: 100
DTIM Interval: 1
Fragmentation Threshold : 2346
RTS Threshold : 2347
AP Isolation : Off

One day later and it didn't drop out of UMA once at home.

These appear to be the default settings for ANY Linksys Router. That's what mine was set to.

It's hard to tell if mine is better since, so far, I've had pretty good luck overall. It's definitely not worse, though.

It's a highly technical subject, it's better if I link you to some long pages instead of trying to describe it all in one sentence.

http://www.wi-fiplanet.com/tutorials...le.php/1468331 (Fragmentation)
http://www.wi-fiplanet.com/tutorials...le.php/1445641 (RTS/CTS)
http://www.wi-fiplanet.com/tutorials...le.php/1492071 (Beacons)

Home now and I'm at my DD-WRT config screen, here's the quick help blurb they include that boils these settings down into a paragraph:

Fragmentation Threshold
This value should remain at its default setting of 2346. The range is 256-2346 bytes. It specifies the maximum size for a packet before data is fragmented into multiple packets. If you experience a high packet error rate, you may slightly increase the Fragmentation Threshold. Setting the Fragmentation Threshold too low may result in poor network performance. Only minor modifications of this value are recommended.

RTS Threshold
This value should remain at its default setting of 2347. The range is 0-2347 bytes. Should you encounter inconsistent data flow, only minor modifications are recommended. If a network packet is smaller than the preset RTS threshold size, the RTS/CTS mechanism will not be enabled. The router sends Request to Send (RTS) frames to a particular receiving station and negotiates the sending of a data frame. After receiving an RTS, the wireless station responds with a Clear to Send (CTS) frame to acknowledge the right to begin transmission.

Beacon Interval
The default value is 100. Enter a value between 1 and 65,535 milliseconds. The Beacon Interval value indicates the frequency interval of the beacon. A beacon is a packet broadcast by the router to synchronize the wireless network. 50 is recommended in poor reception.

I'm about to try changing these on mine just to see if it helps with my Wifi performance without UMA, I'm getting a bit tired of having to bounce the connection because it's gone dead. :(

Changed mine, guess we'll see how it goes.

How's yours doing? On my end it feels like I acquire the connection a lot faster (no more death pauses turning on Wifi when I get home, connects almost instantly now) which is good. I don't really have a scientific way to test it though, could just be luck of the draw that it's playing nice right now.

Mine seems to be working great with this. I still get UMA dropping to EDGE when I get more than about 20 feet from my router. I'd like to get a nice commercial Cisco router or something. Our Cisco router at work has an awesome range on it.

I never drop calls at home, though and connections to UMA seem to be pretty quick now.

Buy yourself a WRT54GL (L model) then throw DD-WRT on it, it allows you to adjust the mW output power. The default is 28mw, max is 200mW, "safe" is considered no higher than 70mW (at that point it starts to get too noisy on the RF). I run mine at 50mW, and at the office where I have a bunch in a WDS I run them at 50mW as well for better range.

I've considered the DD-WRT before but wasn't quite ready to dive in. Is that power setting not available on the WRT54G with DD-WRT?

It is - I was trying to avoid the Linksys version fiasco for you. I'm actually running a WRT54G v4 at home (and a backup v3 on the shelf with stock firmware), which is identical to the newer L models. I've never tried to run DD-WRT on the v5+ models at all due to their crippled status - not worth my time.

Mine's a v1.1. Should I go for it? Any link to the best instructions for loading it? I've seen a few just from Google searches but not sure about them.

I thinks this may be it... I've been using an older WAP54G from Linksys (Access Point, not Router) and while UMA has worked for the most part it seemed to drop for no real reason. I checked the settings from OP and changed to them with the exception of the RTS Threshold. In my WAP, the value is from 256 to 2346 and the default was 2346. Changing that to one lower than recommended above, 2303 (my range is one lower so I took a stab and logic) and blam - UMA snaps right in; definitely locks in way faster. Changing that value to 2304 kills it until I change back to 2303.

Other interesting things are that the default for me for CTS protection mode is Auto and the DTIM Interval default is at 3 (range is 1-255)

This would be a cool tweak if it improves things!

Loaded DD-WRT and cranking up the power helped quite a bit. I can walk through my entire house without losing the UMA now. I added the settings above and have a very stable UMA connection. The dd-wrt allows much more QoS customization than the stock...I was able to give my BB's mac full priority over everything else and, for a nice bonus, even increased priority for my Slingbox based on the port it's plugged into.

I used info from here:
Hack Attack: Turn your $60 router into a $600 router

Open source for the win! Welcome to the beauty of DD-WRT camp, it's fantastic eh?