Transmission Medium Used

Description

The Transmission Medium Used (TMU) is a standardized parameter within the 3GPP IMS architecture, specifically detailed in technical specification 29.163. It functions as an attribute or information element that is communicated between network functions, primarily involving the Policy and Charging Rules Function (PCRF), the Policy and Charging Enforcement Function (PCEF), and online charging systems. The TMU value explicitly identifies the underlying transport technology carrying the user plane data for a specific IP-CAN (IP Connectivity Access Network) bearer or service data flow. This identification is essential for the network to apply the correct charging rates and policy rules that are specific to the cost and characteristics of the transport medium.

In operation, when a bearer is established or modified, the network element responsible for enforcement (e.g., a Gateway GPRS Support Node (GGSN) or a Packet Data Network Gateway (PGW) acting as a PCEF) may report the TMU to the PCRF. The PCRF, in turn, uses this information as part of its decision-making process to select and provision appropriate Policy and Charging Control (PCC) rules. For instance, a rule for a video stream carried over a dedicated, high-quality transport medium like ATM might have different QoS parameters and charging tariffs compared to the same service carried over a best-effort IP transport. The TMU parameter thus provides the granularity needed to map high-level service policies to the specific technological implementation of the bearer.

Architecturally, TMU is a key component in the decoupled charging and policy control framework of modern 3GPP networks. It sits at the intersection of service awareness and transport awareness. While the IMS core handles session-level signaling and service logic, the TMU provides a link to the bearer plane's physical and data-link layer characteristics. This enables operators to implement sophisticated business models, such as offering premium QoS guarantees on specific transport networks (e.g., a leased line) with corresponding premium pricing, while using standard IP transport for best-effort services. Its role, though seemingly technical, is fundamental to enabling accurate, transport-aware monetization of network services.

Purpose & Motivation

TMU was introduced to address the need for transport-layer awareness in IMS charging and policy control. Early mobile data networks often used a single type of transport (e.g., GPRS over IP), but as networks evolved, operators began utilizing multiple transport technologies (like ATM for high-reliability backhaul and IP for general data) within the same network infrastructure. A generic charging or policy rule could not distinguish between these, leading to potential revenue leakage or incorrect application of QoS. For example, an expensive, guaranteed-bit-rate ATM circuit should not be charged the same as a best-effort IP path for the same video service.

Its creation was motivated by the industry's move towards converged, all-IP networks where service logic (IMS) is separated from the underlying connectivity. This separation required a standardized mechanism to correlate the service being delivered with the specific resources being consumed at the transport level. TMU solves this by providing a clear, standardized identifier for the transmission medium, allowing the PCRF to make informed decisions. It ensures that charging records accurately reflect the cost of the underlying network resources used, enabling fair and granular billing. This was particularly important for the commercialization of early IMS-based services like video telephony, where the quality and cost of the bearer directly impacted the user experience and the operator's profitability.

Key Features

• Standardized transport medium identifier for PCC framework
• Enables transport-aware charging and billing differentiation
• Used by PCRF for selecting medium-specific policy rules
• Reported by PCEF/PGW during bearer establishment/modification
• Supports various media types (e.g., IP, ATM, GPRS)
• Integral for accurate service data flow correlation with transport

Evolution Across Releases

Defining Specifications