【转】手机制式的解释

Preferred Network Types change the type of frequency the phone searches for.
Android is used on several devices that can gain access to different cell carriers. The 'Testing' menu that contains the 'preferred network types' you are referring you to is used for all of those Android devices as far as I can tell. 

For simplicity, there are two main types of cell carriers, GSM and CDMA.
The following popular data communication technology is associated with each cell carrier type:
GSM = GPRS (2g), EDGE(2g), UMTS(3g), WCDMA(3g), HSDPA(3g) {Cell Carrier examples: AT&T and T-Mobile}
CDMA = CDMA*(2/3g) EV-DO(3g) WiMax(3/4g) {Cell Carrier examples: Verizon and Sprint}

Now that the general stuff is out of the way, here's my take on the Preferred Network types:

WCDMA preferred - The GSM phone is capable of using both 2G and 3G data communication and when signal strength is low 3G is favored more.
GSM only - The GSM phone is capable of using only 2G data communication. When the 2G signal is too low you get nothing at all.
WCDMA only - The GSM phone is capable of using only 3G data communication. When the 3G signal is too low you get nothing at all.
GSM auto (PRL) - The GSM phone is capable of using both 2G and 3G data communication and when signal strength is low 2G is favored more. This one is a bit confusing to me since PRL is associated mostly with CDMA technology and not GSM technology.
CDMA auto (PRL) - The CDMA phone is capable of using both 2G and 3G data communication and when signal strength is low 2G is favored more.
CDMA only - The CDMA phone is capable of using only 2G data communication. When the 2G signal is too low you get nothing at all.
EvDo only - The CDMA phone is capable of using only 3G data communication. When the 3G signal is too low you get nothing at all.
GSM/CDMA auto (PRL) - Some phones are equipped with both GSM and CDMA capabilities. This setting appears to just have the phone attempt to stay connected to the data communication type that works the best. (Maybe the Samsung Galaxy S will take advantage of this???)
Unknown - If none of the above fit or the phone is acting weird as far as connecting to the carrier, you will see your preferred network type is set to this

When using the 'only' options you should still be able to make calls.

To put things into perspective, The Nexus One is set to WCDMA preferred as default and the Droid Incredible is set to CDMA auto (PRL).

Some References:
http://www.google.com/support/forum/p/android/thread?tid=34f1f0c3bc7affe1&hl=en
http://en.wikipedia.org/wiki/3G
http://en.wikipedia.org/wiki/Preferred_Roaming_List
http://ncare.com/2010/03/29/how-to-update-your-phones-with-the-newest-towers/