CSMT (Circuit Switched Mobile Terminated)

Circuit Switched Mobile Terminated (CSMT) is a type of mobile network architecture that was widely used in the early days of mobile communication. In this system, voice calls are set up by establishing a dedicated circuit between the calling and called parties, which is reserved for the duration of the call. In contrast to modern cellular networks which use packet-switching, the circuit-switched network is designed to allocate dedicated resources for each call. This approach was used to provide high-quality voice communication between two parties without the need for a buffering mechanism that is required for packet-switching.

In a circuit-switched network, the entire bandwidth of the circuit is reserved for the duration of the call. This approach provides a high level of call quality because there is no contention for bandwidth or network resources. Additionally, circuit-switched networks can provide a guaranteed Quality of Service (QoS) for voice calls, which ensures that the call remains stable throughout its duration.

In the CSMT architecture, when a call is initiated, the mobile station sends a request to the base station. The base station then forwards the request to the Mobile Switching Center (MSC), which is responsible for establishing the circuit between the calling and called parties. Once the circuit is established, the calling party can speak to the called party over the dedicated circuit. When the call ends, the circuit is released, and the resources are returned to the network.

The CSMT architecture is divided into three main components: the Mobile Station (MS), the Base Station System (BSS), and the Mobile Switching Center (MSC).

Mobile Station (MS)

The mobile station is the end-user device that is used to initiate and receive calls. The MS consists of a mobile phone or other mobile device that has a SIM card installed. The SIM card is used to authenticate the user and to provide access to the network. The mobile station communicates with the base station system using a wireless link.

Base Station System (BSS)

The Base Station System (BSS) is the infrastructure that provides wireless communication between the mobile station and the network. The BSS consists of one or more Base Transceiver Stations (BTS) that are responsible for transmitting and receiving radio signals. The BTS communicates with the mobile station using a radio link, and with the Mobile Switching Center (MSC) using a wired link.

Mobile Switching Center (MSC)

The Mobile Switching Center (MSC) is the central component of the CSMT architecture. The MSC is responsible for routing calls between the BSS and the Public Switched Telephone Network (PSTN). The MSC performs call switching, call setup, and call termination functions. The MSC also manages mobility functions, such as handover, which allows a mobile station to maintain a call while moving between different BSSs.

In the CSMT architecture, voice calls are set up using a process called call establishment. The call establishment process consists of several steps, as follows:

Call Request

The call request is initiated by the mobile station. The mobile station sends a request to the base station system to establish a call.

Paging

The base station system broadcasts a paging message to locate the mobile station. The paging message is transmitted on a control channel, which is a dedicated channel used for signaling and control functions.

Call Setup

Once the mobile station is located, the base station system sends a call setup message to the MSC. The MSC then initiates a circuit setup between the calling and called parties. The circuit is reserved for the duration of the call.

Call Connected

When the circuit is established, the calling party can speak to the called party over the dedicated circuit. The call remains connected until one of the parties ends the call.

Call Release

When the call ends, the circuit is released and the resources are returned to the network. The MSC sends a release message to the base station system, which then sends a release message to the mobile station.

CSMT is a reliable technology for voice communication, but it has some limitations. One of the main limitations is that it is designed for voice communication only, and does not support data services such as text messaging or internet access. Additionally, the circuit-switched network requires a large amount of bandwidth, which can limit the number of calls that can be supported on the network. This limitation makes it difficult to scale the network to meet the growing demand for mobile communication.

Another limitation of the CSMT architecture is that it is not well-suited for mobile users who are constantly on the move. When a mobile station moves between different BSSs, a handover process is required to maintain the call. The handover process can be slow, and may result in dropped calls or degraded call quality.

Despite its limitations, the CSMT architecture was widely used in the early days of mobile communication, and it played a significant role in the evolution of mobile networks. Today, most mobile networks have transitioned to packet-switching technology, which provides more flexibility and scalability for data services. However, circuit-switched networks continue to be used for voice communication in some parts of the world, and they remain an important part of the history of mobile communication.

One of the major advantages of the CSMT architecture is its ability to provide a guaranteed Quality of Service (QoS) for voice calls. Since the entire bandwidth of the circuit is reserved for the duration of the call, there is no contention for bandwidth or network resources, which ensures that the call remains stable throughout its duration. Additionally, the circuit-switched network provides a high level of call quality, which is ideal for voice communication.

Another advantage of the CSMT architecture is its simplicity. The architecture is relatively easy to implement, and it requires minimal network infrastructure. This simplicity makes the CSMT architecture an ideal solution for countries or regions with limited network infrastructure or resources.

The CSMT architecture also provides a high level of security. Since the circuit is reserved for the duration of the call, there is no possibility of unauthorized access or eavesdropping on the call. Additionally, the use of SIM cards provides a level of authentication for the user, which helps to prevent fraudulent use of the network.

Despite its advantages, the CSMT architecture has some limitations. One of the main limitations is its inability to support data services. Since the CSMT architecture is designed for voice communication only, it cannot support data services such as text messaging, internet access, or multimedia messaging. This limitation makes it difficult to compete with newer mobile network technologies that offer a wider range of services.

Another limitation of the CSMT architecture is its scalability. Since the circuit-switched network requires a large amount of bandwidth, it can be difficult to scale the network to meet the growing demand for mobile communication. This limitation makes it difficult to support a large number of users, which can result in network congestion and dropped calls.

In conclusion, the CSMT architecture is an important part of the history of mobile communication. While it has some limitations, it played a significant role in the evolution of mobile networks. Today, most mobile networks have transitioned to packet-switching technology, which provides more flexibility and scalability for data services. However, circuit-switched networks continue to be used for voice communication in some parts of the world, and they remain an important part of the mobile network ecosystem.