Basic Transmission Time Interval

Description

The Basic Transmission Time Interval (BTTI) is a core timing concept in the GSM/EDGE Radio Access Network (GERAN) architecture, specifically governing the transmission of packet-switched data over the Packet Data Traffic Channel (PDTCH). It represents the shortest time period over which a Transport Block (a unit of data for the physical layer) can be transmitted. In practical terms, one BTTI corresponds to the transmission duration of four consecutive normal bursts on a single timeslot, which equals 20 ms in the standard GSM frame structure (comprising 4.615 ms TDMA frames). This 20 ms interval is the atomic unit for scheduling radio blocks in GPRS and EDGE services.

At the physical layer, BTTI implementation involves precise synchronization between the mobile station (MS) and base station subsystem (BSS). Each BTTI occupies one radio block period within the 52-multiframe structure used for packet data channels. The Radio Link Control (RLC) layer segments higher-layer Protocol Data Units (PDUs) into RLC data blocks that fit within a single BTTI. These blocks are then encoded with channel coding (convolutional coding for GPRS, with more advanced schemes like MCS for EDGE) and mapped to the four bursts of the BTTI. The Medium Access Control (MAC) layer uses BTTI boundaries for scheduling decisions, allocating radio resources to multiple users through temporary block flows (TBFs) on shared timeslots.

The BTTI's fixed duration creates a predictable timing framework for several critical network functions. It enables efficient multiplexing of different users' data on the same physical channel through time-division scheduling. The consistent timing allows for reliable Hybrid ARQ (HARQ) operations in EDGE networks, where retransmissions can be scheduled at known intervals. Furthermore, BTTI boundaries define when channel quality measurements (like RXQUAL) should be taken and reported, and they synchronize power control adjustments. This standardized timing is crucial for interoperability between network equipment from different vendors and for consistent user experience across various GERAN deployments.

Purpose & Motivation

BTTI was created to provide a standardized, efficient timing structure for packet data transmission in GSM networks as they evolved to support GPRS (General Packet Radio Service) and later EDGE (Enhanced Data rates for GSM Evolution). Before packet-switched capabilities, GSM was primarily circuit-switched, with timing optimized for voice calls using different channel structures. The introduction of data services required a new timing model that could handle bursty, variable-rate traffic while efficiently sharing radio resources among multiple users.

The fundamental problem BTTI solves is establishing predictable transmission intervals for packet data that align with GSM's existing TDMA frame structure. Without such a standardized interval, scheduling data transmissions would be chaotic and inefficient. BTTI enables statistical multiplexing of users on shared channels by defining clear boundaries when different users' data can be transmitted. It also provides the timing foundation for essential data services features like link adaptation (switching between different coding schemes based on channel conditions) and incremental redundancy in EDGE.

Historically, BTTI represented a compromise between transmission efficiency and scheduling flexibility. Shorter intervals would allow finer scheduling granularity but increase overhead from more frequent control signaling. Longer intervals would reduce overhead but decrease responsiveness for interactive applications. The 20 ms BTTI was chosen as optimal for the typical latency requirements and channel conditions of 2G/2.5G mobile data services, balancing these competing factors while maintaining backward compatibility with the fundamental GSM frame structure.