The Relationship Between GPRS and TBF

TBF (Temporary Block Flow) marks the beginning and end of a user's session. When analyzing GPRS traffic, we often focus on PDCH traffic and the conversion between TCH and PDCH (if the device supports it), using these metrics to assess whether a cell is busy or idle. While this macro-level analysis can reflect the overall load of a cell, it does not accurately represent the actual user experience with GPRS. For example, consider one user accessing the internet via a laptop, occupying four PDCH channels. This user's session corresponds to a single TBF. In this case, although the PDCH channels are heavily loaded, the user experience is excellent because the user exclusively occupies four channels. The RLC-layer bandwidth is roughly 50 Kb/s, and with coding schemes CS3 or CS4, it can be even higher. In contrast, if 100 users share these same four channels—meaning 100 concurrent sessions (i.e., approximately 100 TBFs, though in practice the number is limited by the maximum TBFs per PDCH)—each user would only have about 0.5 Kb/s bandwidth, which is practically unusable. Therefore, judging solely based on PDCH utilization fails to reflect real user experience. We must therefore examine the relationship between TBF and PDCH, analyzing the bandwidth allocated per TBF. However, in practice, many vendors provide minimal TBF-related statistics, making data-driven analysis difficult. This is an area that requires deeper investigation.

Another issue is that there is a limit on the number of concurrent TBFs on a single PDCH. For instance, a single PDCH can support up to four simultaneous uplink or downlink TBFs. This raises the question of how user TBFs are allocated across PDCH channels. Some vendors use a horizontal allocation algorithm—distributing one user's TBF across each of the four channels—so that once four users occupy four PDCHs, no new users can connect until PDCH configuration is adjusted. This adjustment process takes time and involves specific algorithms, which can negatively impact user perception. Another approach is the vertical allocation algorithm, which reserves certain PDCHs for incoming users. However, this may prevent existing users from achieving optimal bandwidth. These are critical considerations in GPRS network optimization.

In the GPRS system, radio resources refer to PDCHs, which are shared and dynamically allocated among all GPRS-attached mobile devices within a cell. These resources are divided and allocated on demand in the form of TBFs (Temporary Block Flows). A TBF is assigned when data (specifically RLC/MAC blocks) needs to be transmitted and is released once transmission completes. TBFs are identified by TFI (Temporary Flow Indicator), with uplink and downlink TBFs managed independently. The TFI uses a 5-bit encoding, allowing up to 32 simultaneous TBFs on a single PDTCH—meaning up to 32 devices can transmit data concurrently (though not all vendors support such a high level of physical channel sharing).

TBF (Temporary Block Flow) refers to a physical connection used between two radio resource entities, designed to support unidirectional LLC PDU transmission over PDCH. The network may assign one or more PDCH channels to a TBF. A TBF consists of multiple RLC/MAC blocks, which carry one or more LLC PDUs. Each TBF is assigned a unique TFI (Temporary Flow Indicator) by the network to identify it.