The idea of logical and physical used in GSM also used in GPRS. To carry the data and the signaling required to control channels the GPRS uses various Physical and logical channels.
- A new data channel is called the Packet Data Channel (PDCH) is used in GPRS.
- The overall slot structure for this channel is the same as that used within GSM,
- Each burst of information for GPRS is 0.577 mS in length and is the same as that used in GSM. It also carries two blocks of 57 bits of information, giving a total of 114 bits per burst.
- The BSC assigns PDCHs to particular time slots, and there will be times when the PDCH is inactive
Logical channels: There are different channels which are used within GPRS, but all the GPRS actions are carried out within the GPRS logical channels carried within the PDCH.
- Packet Broadcast Central Channel (PBCCH): PBCCH broadcasts general information such as power control parameters, access methods and operational modes, network parameters, etc., required to set up calls.
- This is a downlink only channel that is used to broadcast information to mobiles and informs them of incoming calls etc.
Common control channels:
- Packet Paging Channel (PPCH): used to alert the mobile to an incoming call and to alert it to be ready to receive data. It is used for control signaling prior to the call set up. Once the call is in progress a dedicated channel referred to as the PACCH takes over.
- This is also a downlink channel
- Packet Access Grant Channel (PAGCH): This is also a downlink channel.
- It sends information telling the mobile which traffic channel has been assigned to it. It occurs after the PPCH has informed the mobile that there is an incoming call.
- Packet Notification Channel (PNCH): This is also a downlink channel.
- It used to alert mobiles that there is broadcast traffic intended for a large number of mobiles.
- Packet Random Access Channel (PRACH): This is an uplink channel.
- Enables the mobile to initiate a burst of data in the uplink.
- It has two types, one is an 8 bit standard burst, and a second one using an 11 bit burst has added data to allow for priority setting.
Dedicated control channels:
- Packet Associated Control Channel (PACCH): It is used for control signaling while a call is in progress.
- This channel is present in both uplink and downlink directions.
- It takes over from the PPCH once the call is set up and it carries information such as channel assignments, power control messages and acknowledgements of received data.
- Packet Timing Advance Common Control Channel (PTCCH): This channel is also present in both uplink and downlink directions.
- It is used to adjust the timing advance. It is required to ensure that messages arrive at the correct time at the base station regardless of the distance of the mobile from the base station.
Dedicated traffic channel:
- Packet Data Traffic Channel (PDTCH): It is used to send the traffic and it is present in both the uplink and downlink directions.