Unnumbered Information with Header Check

Description

Unnumbered Information with Header Check (UIH) is a specific command/response frame defined in the 3GPP TS 27.010 protocol, titled "Multiplexing protocol for mobile equipment interfaces." This protocol operates over a serial interface, historically the RS-232 or USB serial emulation, between Terminal Equipment (TE) like a laptop or PDA and Mobile Termination (MT) like a mobile phone or data card. The UIH frame is one of several frame types used to establish and manage multiple logical data channels (virtual circuits) over a single physical serial connection. The UIH frame is primarily used for the transfer of user data. Its structure includes an Address field (identifying the data link connection endpoint), a Control field (specifying the frame type as UIH), an optional length indicator, the user data payload, and a Header Check Sequence (HCS). The HCS is a CRC calculated over the header fields (Address and Control) to protect against errors in these critical parts of the frame, ensuring the data is routed to the correct logical channel. Unlike other frame types like SABM (Set Asynchronous Balanced Mode) used for link establishment, UIH frames do not require acknowledgment at the data link layer; they are sent in an unnumbered format, meaning they do not have sequence numbers. This makes them efficient for streaming data. The protocol uses these frames to multiplex different higher-layer protocols (e.g., AT command channel, PPP for IP data, voice call control) simultaneously over one serial port. The receiving entity checks the HCS; if it fails, the frame is discarded.

Purpose & Motivation

The UIH frame and the TS 27.010 multiplexing protocol were created to solve the problem of supporting multiple concurrent services over the single, limited serial interface available on early mobile devices. Before multiplexing, a device could typically only handle one logical connection at a time—for example, either an AT command session or a PPP data session, but not both simultaneously. This was a major limitation for applications requiring simultaneous voice and data or multiple data channels. The 27.010 protocol, inspired by the ITU-T V.42bis / LAPM standards, introduced a data link layer multiplexer. The UIH frame specifically provides an efficient mechanism for transferring user data for established logical channels. Its purpose is to offer a lightweight, acknowledged (via HCS for header integrity) but not sequence-controlled data transfer service, minimizing overhead while ensuring basic frame addressing integrity. This addressed the need for reliable channel separation and data transfer for auxiliary services like fax, circuit-switched data, and supplementary service control alongside the primary data connection, enabling more advanced mobile station functionality in the GSM and UMTS eras.

Key Features

• Data link layer frame for user data transfer in TS 27.010 multiplexing
• Includes a Header Check Sequence (HCS) for error detection in the frame header
• Operates in unnumbered mode (no sequence numbers for flow control)
• Used for data on established logical channels between TE and MT
• Supports multiplexing of multiple protocols over a single serial interface
• Enables concurrent services like voice, data, and AT commands on a single port

Evolution Across Releases

Defining Specifications