Maximum possible AIUR

Description

Maximum possible AIUR (Air Interface User Rate) is a service-level parameter defined in 3GPP specifications TS 21.905 and TS 22.034. It represents the highest possible user data rate that a set of multiple full-rate traffic channels (TCH/F) can deliver over the air interface. The term is specific to GSM and GPRS systems, where user data is carried over circuit-switched or packet-switched traffic channels with defined data rates. For example, two TCH/F channels using the TCH/F9.6 coding scheme provide a maximum possible AIUR of 19.2 kbit/s, which is twice the 9.6 kbit/s rate of a single channel.

The concept of AIUR is rooted in the GSM architecture, where the air interface between the mobile station and the base transceiver station is the bottleneck for user data rates. A single TCH/F channel supports a limited set of user rates depending on the channel coding scheme: 9.6 kbit/s, 4.8 kbit/s, or 2.4 kbit/s for circuit-switched data, and various rates for GPRS packet data. To achieve higher throughput, multiple TCH/F channels can be aggregated using multi-slot operation or channel bundling. The maximum possible AIUR is the sum of the individual channel rates, assuming ideal radio conditions and no protocol overhead.

In practice, the achievable AIUR is lower than the maximum possible AIUR due to protocol overhead, retransmissions, and radio link quality. The AIUR parameter is used in service specifications to define the capabilities of bearer services and teleservices. It allows operators and equipment vendors to specify the performance of data services in a technology-neutral way, independent of the underlying channel configuration. For example, a service may require an AIUR of 19.2 kbit/s, which can be provided by two TCH/F9.6 channels or by other combinations of channels and coding schemes.

The parameter is particularly relevant to High-Speed Circuit-Switched Data (HSCSD), a feature introduced in GSM Phase 2+ that allows multiple timeslots to be allocated to a single user for circuit-switched data. HSCSD uses multiple TCH/F channels to increase the user data rate, and the maximum possible AIUR defines the theoretical peak rate for a given number of timeslots. Similarly, in GPRS, multiple packet data channels (PDCH) can be allocated to a user, and the maximum possible AIUR represents the aggregate throughput.

Maximum possible AIUR is a static, configuration-dependent quantity. It does not account for dynamic factors such as channel availability, interference, or scheduling. Instead, it provides a reference value that can be used for service definition, network dimensioning, and performance benchmarking. It is closely related to other parameters such as AIUR (the actual achieved rate), mean bit rate, and guaranteed bit rate.

The term has been carried forward unchanged from Rel-4 through Rel-19 in the 3GPP glossary, indicating that the concept remains valid for legacy GSM/GPRS systems even as the industry has moved to LTE and NR. It serves as a reminder of the evolutionary path of mobile data services, from circuit-switched multi-slot operation to packet-switched broadband.

Purpose & Motivation

Maximum possible AIUR exists to define the upper bound of user data throughput that can be achieved over the GSM air interface using multiple traffic channels. It provides a standardized way to specify the capabilities of bearer services and to compare different channel configurations. Without this parameter, service descriptions would need to enumerate every possible combination of channel type, coding scheme, and number of timeslots, which would be impractical.

The parameter was introduced in Rel-4 as part of the 3GPP glossary to support the definition of data services in GSM and GPRS. At that time, mobile data rates were limited by the air interface, and techniques such as HSCSD and GPRS multi-slot operation were being standardized to increase throughput. Maximum possible AIUR provided a clear, technology-neutral metric for expressing the peak rate of these services.

It also supports network planning and dimensioning. By knowing the maximum possible AIUR for a given channel configuration, operators can estimate the resources required to deliver a service with a certain data rate. It helps in capacity planning, admission control, and quality-of-service management. Although the term is less prominent in modern LTE and NR specifications, it remains a useful concept for understanding the evolution of mobile data services.