Definition of Logical Channel in GSM
The physical channel is the medium over which the information is carried.
The logical channel consists of the information carried over the physical channels.
A Physical Channel (a TS, defined by a fixed position (0-7) on a given TDMA frame) may be used to broadcast messages containing different kinds of information:
traffic messages for speech and data,
signaling messages for different procedures and supplementary services,
synchronization messages for temporal and logical synchronization between the mobile stations and the BTS,
measurements messages for uplink report of the downlink measurements,
control messages to manage the access to the network.
All these kinds of messages are classified in Logical Channels. Depending on the quantity of information to transmit and on their consistency, several logical channels may be mapped onto one physical channel, in order to use its successive Time Slots as much as possible (optimization of the resources number by maximizing the occupancy duration of each).
As a conclusion:
Physical Channel = information container
Logical Channel = specification of the information global content