Circuit Switching
The traditional method of establishing a dedicated communication path (e.g., telephone calls).
The Concept of Circuit Switching
Circuit switching is one of the foundational methods for building communication networks. Its core principle is the establishment of a dedicated, end-to-end communication path (a "circuit") between two parties before any data is sent. This path is reserved for the entire duration of the communication session, ensuring that the resources are exclusively available to the connected parties.
The most classic and intuitive example of circuit switching is the traditional analog telephone network. When you make a phone call, a physical electrical circuit is created and maintained between your phone and the person you're talking to, lasting from the moment they pick up until one of you hangs up.
The Three Phases of Communication
Every communication session in a circuit-switched network proceeds through three distinct and sequential phases:
- Phase 1: Connection Setup
Before any data can be transferred, a complete, physical (or logical) path must be established from the source to the destination. This involves sending signaling messages through the network to reserve channels on each link along the chosen route. If User A wants to connect to User B, a request travels through network nodes (switches), each node reserving an outbound channel to the next. This setup phase introduces a delay, known as the connection setup time.
- Phase 2: Data Transfer
Once the circuit is established and confirmed, data can be transferred between the two parties. Because the path is dedicated, the data flows at a constant rate with a consistent, minimal delay. There is no need for individual data chunks to carry address information, as the path is already determined.
- Phase 3: Connection Teardown
When the communication is complete, one of the parties initiates a termination signal. This signal propagates back through the network, releasing the reserved channels on each link. These resources (channels) are then made available for other users to establish new connections.
Characteristics: Advantages and Disadvantages
While simple and reliable, the circuit-switching model has a distinct set of trade-offs that make it suitable for some applications but inefficient for others.
Advantages
- Guaranteed Bandwidth: Once a circuit is established, its full bandwidth is available exclusively to the connected parties. It's like having a private, dedicated lane on a highway.
- Constant Low Delay (Latency): Since the data follows a fixed path and no routing decisions are needed during transfer, the delay is constant and minimal. This is ideal for real-time services like voice and video.
- High Reliability: After the initial setup, the data transfer is simple and reliable. Data arrives in the same order it was sent, without the need for complex reordering mechanisms at the receiver.
Disadvantages
- Inefficient Resource Utilization: The dedicated channel is reserved for the entire session, even when no data is being sent (e.g., during moments of silence in a phone conversation). This is a significant waste of valuable network resources.
- Long Connection Setup Time: The delay before data transfer can begin is significant due to the time required for signaling and resource reservation across the network.
- Blocking: If all channels on any link along the path to the destination are already in use, the connection cannot be established, and the call is "blocked." The caller receives a busy signal, even if the destination user is free.
Applications of Circuit Switching
Circuit switching is the foundational technology for networks that prioritize constant delay and guaranteed quality for real-time communication.
- : The traditional global telephone system is the primary example of a circuit-switched network.
- : A set of digital communication standards that provided circuit-switched access for voice and data over copper telephone lines, essentially a digital upgrade to the analog PSTN.
- : The voice communication part of many 2G and 3G mobile networks (like GSM) operates on a circuit-switched basis to ensure consistent call quality.