We may sometimes take mobile networks for granted but we must appreciate that they do a lot of work in the background to make the communication happen for all of us. Even services as simple as voice calls involve a lot of signalling, handovers, circuits, and even packets of data depending on the network technology. Did you know that when we make voice calls on 2G, 3G and sometimes 4G, the mobile network uses a very specific set of resources that are different from the resources used for data services e.g. a WhatsApp message? This is where the concept of circuit-switched and packet-switched techniques becomes relevant in mobile communications. Generally, the circuit-switched components in the mobile network mainly enable voice calls, and the packet-switched network resources make mobile data services possible.
What is circuit-switched?
The term circuit-switched in mobile communications refers to the switching technique used traditionally for real-time services such as voice. With Circuit-Switched or CS, a dedicated circuit is engaged for the entire duration of a conversation between two users. The circuit-switched technology is a highly reliable way of ensuring Quality of Service (QoS) for voice calls. However, this technique is not the most efficient as it puts certain limitations on the ability of the network to accommodate simultaneous voice calls at any given time. The voice calling in GSM (2G) and UMTS (3G) networks employs the circuit-switched technique. With LTE, any Voice over LTE (VoLTE) calls can work without the circuit-switched technique but they do have a circuit-switched fall back system to accommodate voice calls on 2G/3G when VoLTE calls are not possible. Traditionally, the GSM networks were designed for voice calls which could be offered in best quality through the use of dedicated circuits (circuit-switching). The circuit-switched technique can also enable limited data services, a capability that was used in GSM networks before GPRS.
What is packet-switched?
When GPRS (General Packet Radio Service) was introduced in the GSM networks for efficient mobile data services, the mobile network architecture saw some changes. A packet-switched extension was added to the existing GSM networks. New network nodes SGSN and GGSN were also introduced in the core network to support packet-switching. That way, all data services were offered through the packet-switched part of the network while the voice and SMS part continued to use the circuit-switched capability. You can learn more about SGSN and GGSN in this post.
Later, the 3G UMTS networks followed the same approach, i.e. CS for voice and PS for data. As briefly mentioned earlier, 4G LTE networks have the capability to accommodate voice calls over IP using Voice over LTE (VoLTE) protocol. However, they also have a circuit-switched Fall Back option which uses CS to facilitate voice calls when VoLTE support isn’t available.
The packet-switched technique provides much higher levels of efficiency as compared to the circuit-switched technique. It sends packets of data bursts at different time intervals while sharing the available capacity among multiple users. These packets have a ‘header’ with the destination information of each packet and a payload which contains the actual data or information that is being transmitted. These headers are used by the switching nodes to determine the source and destination of the packets so that the data packets can be directed to the desired subscribers (devices) using the best possible route.
So to summarise…
Circuit-switching and packet-switching are two key techniques used in mobile communications. With Circuit-Switched (CS), a dedicated circuit is engaged for the entire duration of a call or session. The packet-switched (PS) technique is more efficient and shares the available capacity with multiple users by sending packets of data bursts at different time intervals.