GPRS Channels: Physical, Logical and Control Channels

GPRS Physical Channel

GPRS (General Packet Radio Service) extends the basic GSM network structure. It uses the same modulation technique and frame structure as GSM, making it an evolution of the GSM standard. In GPRS, the Base Station Controller (BSC) can dynamically assign time slots to GPRS traffic depending on demand. Any remaining slots may still be used for traditional GSM voice traffic.

GPRS introduces a new data channel called the Packet Data Channel (PDCH). The time slot structure of PDCH is the same as GSM and uses similar power profiles and timing advance techniques to compensate for signal travel
time between the mobile device and the base station. Each GPRS information burst lasts 0.577 ms, the same duration as in GSM. Each burst carries two blocks of 57 bits, which means 114 bits per burst.

A block of 20 ms of data requires four bursts, which together carry 456 bits of encoded data. The BSC assigns PDCHs to particular time slots. During periods when the PDCH is inactive, the mobile device can scan other base stations and measure their signal strength to help the network decide when a handover is required. The base station also determines timing delay using a logical channel called the Packet Timing Advance Control Channel (PTCCH).

GPRS Channel Allocation

Although GPRS mainly uses a single physical channel (PDCH) for transmitting data, it uses several logical channels mapped onto this physical channel to manage packet data communication. Unlike circuit-switched GSM communication, GPRS transmits data in packet form. Data packets are assigned space within the system based on their current transmission needs. The Medium Access Control (MAC) layer plays an important role in managing transmissions. Three different MAC allocation modes are used:

Fixed Allocation

This mode is used when a mobile device needs a constant data rate. A set of PDCH time slots is reserved for a specific period, allowing continuous data transmission without checking slot availability. This method is typically used for applications such as video conferencing.

Dynamic Allocation

In this mode, the network dynamically assigns time slots whenever data needs to be transmitted. The mobile device transmits data only when it detects a matching Uplink Status Flag (USF). This ensures that only one
mobile device uses the slot at a given time, even though multiple devices may share the same slot.

Extended Dynamic Allocation

Extended dynamic allocation allows a mobile device to transmit data in multiple time slots simultaneously, resulting in much higher data rates. When the USF permits this mode, the mobile can transmit across several slots, increasing the speed of data transmission.

GPRS Logical Channels

GPRS uses several logical channels mapped onto the Packet Data Channel (PDCH). These channels are categorized into broadcast channels, common control channels, dedicated control channels, and traffic channels.

Broadcast Channels

Packet Broadcast Control Channel (PBCCH)

• Downlink-only channel
• Broadcasts system information to mobile devices
• Provides parameters such as power control, access methods, and network configuration
• Functions similarly to the GSM BCCH channel

Common Control Channels

Packet Paging Channel (PPCH)

• Downlink-only channel
• Notifies the mobile device of incoming data or calls
• Used for signaling before the call or data session is established

Packet Access Grant Channel (PAGCH)

• Downlink channel
• Informs the mobile device about the assigned traffic channel
• Used after the paging process

Packet Notification Channel (PNCH)

• Downlink-only channel
• Notifies multiple mobile devices about broadcast data
• Commonly used in point-to-multipoint services

Packet Random Access Channel (PRACH)

• Uplink channel
• Allows a mobile device to initiate communication with the network
• Supports both standard 8-bit bursts and extended 11-bit bursts for priority access

Dedicated Control Channels

Packet Associated Control Channel (PACCH)

• Used in both uplink and downlink directions
• Handles signaling during active communication
• Transmits channel assignments, power control commands, and acknowledgements

Packet Timing Advance Control Channel (PTCCH)

• Used for timing advance adjustments
• Ensures signals from different mobiles reach the base station at the correct time
• Essential for accurate synchronization in TDMA systems

Dedicated Traffic Channel

Packet Data Traffic Channel (PDTCH)

• Used to transmit actual user data
• Supports both uplink and downlink transmission
• A mobile device can use up to eight PDTCH channels for higher data rates

Importance of GPRS Protocol Stack

The GPRS protocol stack is an important part of the mobile communication system used in both handsets and base stations. It divides communication functions into different layers so that each layer can handle a specific task efficiently. This layered structure ensures that data transmission is organized, prioritized, and processed in the correct order across the network.