Dual Transfer Mode

Description

Dual Transfer Mode (DTM) is a feature introduced in 3GPP Release 8 for GSM/EDGE networks that allows a mobile station to have both a circuit-switched (CS) connection and one or more packet-switched (PS) Temporary Block Flows (TBFs) simultaneously. This means a user can engage in a voice call while maintaining a data session, such as downloading a file or browsing the web. DTM is optional for both the mobile station and the network, and it requires the Base Station Subsystem (BSS) to coordinate the allocation of radio resources to support both CS and PS traffic on the same timeslot or on different timeslots.

In a DTM operation, the mobile station is considered a class A mobile, which means it can support simultaneous CS and PS services. The mobile station must be capable of handling both CS and PS signaling and data transfer. The network must also support DTM and coordinate the radio resource allocation between the CS and PS domains. This coordination is typically performed by the Base Station Controller (BSC), which manages the allocation of timeslots and the scheduling of CS and PS traffic. The BSS ensures that the CS connection (e.g., a voice call) is given priority and that the PS TBFs are allocated resources in a way that does not degrade the CS service.

DTM was developed to address the limitation of earlier GPRS implementations, where a mobile station could either have a CS connection or a PS connection, but not both at the same time. This meant that a user could not receive a voice call while downloading data, or could not browse the web during a voice call. With DTM, the mobile station can maintain both connections, providing a more seamless user experience. DTM is particularly useful for applications that require simultaneous voice and data, such as push-to-talk, voice over IP, or interactive gaming.

The implementation of DTM involves several enhancements to the GSM/EDGE radio interface. The mobile station must be able to monitor both the CS and PS channels, and the network must be able to schedule both types of traffic. The BSS uses a coordinated scheduling algorithm to allocate timeslots for CS and PS traffic, ensuring that the CS connection is not interrupted. The mobile station must also support the necessary signaling procedures for DTM, such as the DTM Request and DTM Assignment messages. These procedures allow the mobile station to request DTM operation and the network to assign the necessary resources.

DTM is specified in 3GPP TS 43.055 (Dual Transfer Mode; Stage 2) and TS 44.060 (General Packet Radio Service (GPRS); Mobile Station (MS) - Base Station System (BSS) interface; Radio Link Control/Medium Access Control (RLC/MAC) protocol). These specifications define the procedures and requirements for DTM operation, including the signaling messages, the radio resource allocation, and the performance requirements. DTM has been maintained in subsequent releases, although its relevance has diminished with the advent of LTE and 5G, which natively support simultaneous voice and data through VoLTE and other mechanisms. However, DTM remains an important feature for legacy GSM/EDGE networks and for ensuring backward compatibility.

Purpose & Motivation

Dual Transfer Mode was created to overcome a fundamental limitation of early GPRS networks: the inability to support simultaneous circuit-switched and packet-switched connections. Before DTM, a mobile station could either be in a dedicated mode for a voice call or in a packet transfer mode for data, but not both. This meant that a user could not receive a voice call while actively using a data service, and vice versa. This limitation was a significant drawback for users who wanted to multitask, such as talking on the phone while looking up information on the web.

The motivation for DTM was to provide a more seamless and efficient user experience by allowing both CS and PS services to coexist. This was particularly important for business users and for applications that require real-time interaction, such as push-to-talk and voice over IP. DTM also improved the efficiency of radio resource usage by allowing the network to dynamically allocate resources between CS and PS traffic based on demand.

DTM was standardized in 3GPP Release 8, although the work began earlier. It was designed to be backward compatible with existing GPRS and GSM networks, meaning that DTM-capable mobile stations could still operate in networks that did not support DTM, albeit without the simultaneous services. The introduction of DTM required changes to both the mobile station and the network infrastructure, including the BSS and the core network. Despite its benefits, DTM adoption was limited because it required significant upgrades to the radio access network and because the emergence of 3G and LTE provided alternative solutions for simultaneous voice and data. Nevertheless, DTM remains a part of the 3GPP specifications and is supported in many GSM/EDGE networks.