Simultaneous Use of Services

Description

Simultaneous use of services refers to the capability of a mobile station (MS) to engage in both circuit-switched (CS) and packet-switched (PS) services at the same time. In the context of 3GPP, this typically means using a circuit-mode service, such as a voice call or circuit-switched data, while simultaneously using packet-mode services, such as GPRS for internet access or multimedia messaging. This capability was introduced in Release 4 and has been maintained through subsequent releases, although its relevance has evolved with the advent of all-IP networks like LTE and 5G.

The architecture that enables simultaneous use of services involves the coexistence of the circuit-switched domain and the packet-switched domain within the same network. In GSM/GPRS and UMTS networks, the Mobile Switching Center (MSC) handles circuit-switched services, while the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) handle packet-switched services. The mobile station must support dual-mode operation, meaning it can maintain a connection to both domains simultaneously. This is achieved through mechanisms such as dual transfer mode (DTM) in GSM and multi-radio access bearer (multi-RAB) in UMTS.

In GSM, DTM allows a mobile station to have a circuit-switched connection and a packet-switched connection at the same time on the same radio channel. This is accomplished by allocating dedicated timeslots for CS and PS traffic, and by coordinating the scheduling between the two domains. In UMTS, the concept is realized through the support of multiple radio access bearers (RABs), where one RAB carries CS voice and another carries PS data. The Radio Network Controller (RNC) manages the allocation of resources and ensures that the quality of service (QoS) requirements for each bearer are met.

The key components enabling simultaneous use include the mobile station's capability to handle multiple connections, the radio access network's ability to multiplex CS and PS traffic, and the core network's ability to route and manage both types of traffic. The Home Location Register (HLR) and Visitor Location Register (VLR) store subscription information that indicates whether the user is allowed to use simultaneous services. The GPRS support nodes manage the packet data sessions, while the MSC manages the circuit-switched sessions.

In terms of operation, when a user initiates a voice call while a packet data session is active, the mobile station requests the establishment of a CS connection. The network checks the subscription and available resources, and if successful, allocates the necessary radio resources. The packet data session continues in parallel, with the network ensuring that the QoS of both services is maintained. If resources are scarce, the network may prioritize the CS service or may reject the request for simultaneous use. The mobile station may also indicate its capability for simultaneous use during attach and routing area update procedures.

The simultaneous use of services was particularly important in the early 2000s when GPRS was being deployed and users wanted to browse the internet while on a voice call. It was also a differentiator for 3GPP networks compared to other technologies. However, with the migration to all-IP networks like LTE, where voice is carried over IP (VoLTE) and there is no separate circuit-switched domain, the concept has become less relevant. In 5G, all services are packet-based, and simultaneous use is inherent. Nevertheless, the term remains in the 3GPP vocabulary to describe the capability in legacy networks and for interworking scenarios.

Overall, simultaneous use of services represents a significant milestone in mobile communications, enabling the concurrent operation of CS and PS services and paving the way for the all-IP paradigm.

Purpose & Motivation

The simultaneous use of services capability was created to address the need for users to access packet data services without interrupting circuit-switched voice calls. In the early days of GSM, voice was the primary service, and data services were limited to circuit-switched data at low speeds. With the introduction of GPRS, packet-switched data became available, but initially, a mobile station could not use both CS and PS services at the same time. This meant that a user had to choose between making a voice call and using data services. The simultaneous use of services solved this problem by allowing both to operate concurrently.

The primary motivation was to enhance user experience and network efficiency. Users wanted to be able to browse the web, send emails, or use instant messaging while on a voice call. This was especially important for business users who needed to stay connected. From a network perspective, simultaneous use allowed for better utilization of radio resources, as CS and PS traffic could be multiplexed, and it enabled new applications that combined voice and data, such as push-to-talk over cellular (PoC) and video telephony.

Historically, the concept was introduced in 3GPP Release 4, which was the first release to fully specify GPRS and its interworking with GSM. Prior to that, in Release 97 and 98, GPRS was defined but simultaneous use was not standardized. Release 4 added the necessary mechanisms, such as DTM in GSM and multi-RAB in UMTS, to support concurrent CS and PS services. The specifications 21.905 (Vocabulary for 3GPP Specifications) and 22.060 (General Packet Radio Service (GPRS); Service description; Stage 1) provide the definitions and requirements.

Over time, the importance of simultaneous use has diminished with the advent of all-IP networks. In LTE, voice is carried as VoIP over the packet-switched domain, so there is no separate CS domain. In 5G, all services are packet-based, and simultaneous use is inherent. However, the term is still used in the context of interworking between legacy networks and new networks, and for describing the capabilities of multi-mode devices. The concept also laid the foundation for the quality of service frameworks that ensure different services can coexist with appropriate prioritization.