Logical channels are easy to understand once you try to put yourself in their shoes 🙂
Paging for GPRS is always the routing area (RA), defined by the couple Location Area Code (LAC) + Routing Area Code (RAC).
Most operators define RAC=0, so that there is only one RA per LA. This is sufficient in networks with low PS paging. Nowadays though, there might be a need to split 1 LA in 3 or 4 RA because there is heavy PS Paging generated by smartphones apps.
For example :