CTS Mobile Station

Description

The CTS Mobile Station (CTS-MS) is a type of GSM Mobile Station (MS) that includes the additional capability to operate within a Cordless Telephony System (CTS) environment. In essence, a CTS-MS is a dual-mode device that can function both as a standard GSM mobile phone and as a cordless handset when it is within the coverage area of a CTS Fixed Part (CTS-FP). This allows users to make and receive calls through their mobile subscription using a cordless-like experience, often with improved indoor coverage and potentially lower costs. Technically, the CTS-MS is defined as a 'GSM MS with CTS capability.' This means it incorporates the standard GSM protocols and interfaces (such as the radio interface, SIM, and mobility management) but also includes additional functionality to communicate with a CTS-FP over a cordless air interface. The CTS-MS must support the specific radio interface used by CTS, which is typically based on the DECT or a modified GSM air interface, and it must handle the registration, authentication, and call control procedures specific to CTS. When a CTS-MS is within range of a CTS-FP, it can register with that CTS-FP and use it to access the network. In this mode, the CTS-MS communicates with the CTS-FP over the cordless link, and the CTS-FP acts as a gateway to the fixed network or the GSM core network. The CTS-MS may also support seamless handover between the CTS coverage area and the macro GSM network, ensuring that ongoing calls are not dropped when the user moves out of range. The CTS-MS is specified in 3GPP Technical Specifications 42.056, 43.052, and 45.056, which define the requirements and procedures for CTS operation. These specifications cover aspects such as radio performance, security, mobility management, and interworking with the GSM network. The CTS-MS typically includes a user interface that indicates when it is in CTS mode, and it may support features such as a phonebook, call logs, and SMS in both modes. The CTS-MS concept was introduced in 3GPP Release 8 as part of the CTS standard, which aimed to integrate cordless telephony into the cellular network. The CTS-MS is a key enabler for fixed-mobile convergence services, allowing operators to offer a single phone for home and outdoor use. It represents an early attempt to provide personal coverage and seamless service continuity, similar to later technologies like UMA/GAN and VoWiFi.

Purpose & Motivation

The CTS Mobile Station was created to provide users with a single handset that can function as both a cellular phone and a cordless phone, offering the convenience of cordless telephony within the home or office while maintaining cellular connectivity outdoors. Before CTS, users had to carry separate devices for cellular and cordless calls, and cordless calls were typically tied to a fixed line, not the mobile subscription. The CTS-MS solves this by integrating CTS capability into a standard GSM MS, allowing it to connect to a CTS-FP for cordless operation and to the macro GSM network otherwise. This enables operators to offer converged services, potentially reducing churn and improving indoor coverage. The CTS-MS also addresses the need for seamless mobility between indoor and outdoor environments, as it can handover between the CTS-FP and the GSM network.