Which band will be scanned when a mobile turned on?

[dmg123]

Can some experts tell me which frequency band will be scanned when a mobile turned on? 900 or 1800 or 2100... and it will scan ascendingly or descendingly?

[mahdi]

When you first power on the device or your device got into out of coverage and try to detect/search a new cell, UE does not have any idea on which frequency it has to try camp on. so it may have to do some kind of blind search.
For example, let's assume that your device support WCDMA Band I.
The NodeB around your UE may use any frequency channel from 10562 to 10838.
There can be 276 possibilities of frequencies that eNB would use. Then how UE can detect/find the cell(NodeB) it would camp on ?
There can be many different algorithm to try.
These algorithms are not defined in 3GPP. so it is all up to the implementation on UE side or chipset implementation.

[patek1968]

Can some experts tell me which frequency band will be scanned when a mobile turned on? 900 or 1800 or 2100... and it will scan ascendingly or descendingly?

Depends what's the last Network settings saved on your phone. Like GSM preferred / WCDMA only/ LTE recommended. So base on this you have roughly an idea that's depends how it camp on specific band due to UE capabilities itself.