Description
Unnumbered Acknowledgement (UAU) is a specific type of control frame defined within data link layer protocols, such as the one specified in 3GPP TS 27.010 for serial data communication. It operates as part of a protocol's frame structure, which typically includes information frames (I-frames) carrying user data and supervisory frames (S-frames) like UAU for control purposes. The UAU frame is sent by the receiver to positively acknowledge the correct receipt of one or more previously transmitted I-frames, but it does not contain a sequence number (N(R)) like a standard Receive Ready (RR) supervisory frame might.
In operation, the UAU provides a simple acknowledgement mechanism. When a receiver sends a UAU, it indicates that all frames up to a certain point have been received correctly, often implicitly acknowledging the last frame received. This is particularly useful in scenarios where the protocol uses a connection-oriented, reliable transfer mode but may employ a simplified acknowledgement scheme for efficiency. The transmission of a UAU frame resets any timer related to awaiting an acknowledgement for the sender.
Its role in the network, as per TS 27.010, is within the context of circuit-switched data services and terminal adaptation, ensuring reliable data transfer over the serial interface (e.g., between a Terminal Equipment (TE) and a Mobile Termination (MT)). The UAU works in conjunction with other frame types like Unnumbered Information (UI) and Set Asynchronous Balanced Mode (SABM) to establish, maintain, and tear down data links, providing flow control and error recovery at the data link layer for auxiliary services.
Purpose & Motivation
The UAU mechanism exists to provide a lightweight, efficient method for positive acknowledgement in data link protocols, particularly in environments where minimizing overhead is beneficial. It addresses the need for reliable data transfer without the constant use of sequence-numbered acknowledgements, which can add header overhead. In the context of 3GPP TS 27.010, which defines the serial data protocol for use with mobile stations, such efficiency is valuable for auxiliary data services.
Historically, as mobile systems evolved to support data services beyond voice, protocols like the one in TS 27.010 were adapted from established standards (e.g., based on HDLC principles) to manage communication over serial links. The UAU offers an alternative to numbered acknowledgements (like RR with N(R)), simplifying the receiver's logic in certain operational modes or states. It solves the problem of providing reliable delivery confirmation while keeping the protocol adaptable for different service requirements, supporting both acknowledged and unacknowledged operation modes defined in the specification.
Key Features
- Control frame type for positive acknowledgement without a sequence number
- Used in data link layer protocols per 3GPP TS 27.010
- Indicates successful receipt of information frames
- Helps in flow control and error recovery procedures
- Can be used in specific protocol modes to simplify acknowledgement
- Works in conjunction with other unnumbered and supervisory frames
Evolution Across Releases
Defining Specifications
| Specification | Title |
|---|---|
| TS 27.010 | 3GPP TS 27.010 |