DVB Recept Considerations at Delivery System Boundaries

A bug was reported from [C] that something wrong with our DVB receipt device. Some duplicated channels appeared, and some EPG missed. Thanks to [C], he helped to point out the possible missed considerations by explain the specifics in his environment. That occurs when the receipt device is in somewhere signal coverage from different DVB transmit stations overlapped. And the stations are not configured as SFN.
Here is an example.
Freeview on the Hannington transmitter
Freeview on the Rowridge transmitter
Freeview on the Salisbury transmitter
The area of Salisbury east is covered by the three transmitters.

As a result, BBC may appear twice or more in a channel list. All of them carry the same program, but from different stations or different frequencies. That is absolutely not what an user would like to see.

Here I have a plan to fix it.
1. select a network
* with a specific network_id
* consider to compare the signal strength as the mechanism to select
** check the get_signal_strength function work
2. scan those frequencies in NIT frequency_list_descriptor
* record service_id in each entry
* consider what to do when frequency_list_descriptor is not available
3. scan PAT, PMT, SDT, EIT on those frequencies found in 2.
* discard data if the network_id, service_id is not match

Reference:
DVB, ETR 211