The Concept of Circuit Switching
Circuit switching is one of the basic methods for building communication networks. Its core principle is the establishment of a dedicated communication path with reserved resources(a "circuit") between two parties before data is sent. The path may be physical or logical and remains reserved for the session, giving the connected parties exclusive access to the allocated resources.
A classic example of circuit switching is the traditional analog telephone network. Lifting the handset seizes the subscriber loop; after dialing, the switches connect a circuit that remains assigned until the call is released. In digital networks, an allocated channel or time slot provides the equivalent path.
The Three Phases of Communication
Every communication session in a circuit-switched network proceeds through three distinct, sequential phases:
- Phase 1: Connection Setup
Before data can be transferred, a complete physical or logical path must be established from the source to the destination. The process sends signaling messages through the network and reserves a channel on each link along the selected route. If User A wants to connect to User B, a request travels through network nodes such as switches, and each node reserves an outbound channel to the next node. A successful confirmation must return before the data phase begins; if a link cannot reserve a channel, signalling clears any partial reservations. This phase introduces a delay called connection setup time.
- Phase 2: Data Transfer
Once the circuit is established and confirmed, data can be transferred between the two parties. The allocated resources provide a fixed rate and predictable delay without contention from other connections for the same channel. Individual data units do not need to carry address information because the route has already been determined. Because the reservation belongs to the session, silence does not make the channel available to another call until teardown.
- Phase 3: Connection Teardown
When the communication is complete, one of the parties initiates a termination signal. The signal propagates through the network, and each switch releases its reserved channel state. These resources then become available for other users establishing new connections.
Characteristics: Advantages and Disadvantages
The circuit-switching model has defined advantages and limitations. It suits some applications and uses resources less efficiently in others.
Advantages
- Guaranteed Bandwidth: Once a circuit is established, the allocated channel capacity is available exclusively to the connected parties.
- Constant Low Delay (Latency): Data follows a fixed path, and the network does not need to select a route for each unit during transfer. The delay is therefore predictable and usually low, which suits real-time services such as voice and video.
- High Reliability: After the initial setup, data follows an established path. It arrives in transmission order without reordering mechanisms at the receiver, although circuit switching itself does not remove transmission errors.
Disadvantages
- Inefficient Resource Utilization: The dedicated channel is reserved for the entire session, including periods when no data is sent, such as silence during a phone conversation. Unused capacity cannot serve another connection.
- Connection Setup Delay: Data transfer starts only after signaling and resource reservation across the network have completed. The procedure adds an initial delay.
- Blocking: If no channel is free on any link along the selected path, setup fails. Signalling reports an unavailable circuit or channel, and a telephone user may hear a busy tone even when the destination user is free.
Applications of Circuit Switching
Circuit switching remains suitable for networks that require allocated capacity and predictable delay for real-time communication.
- : Traditional fixed-line voice service is the canonical example of circuit switching.
- : ISDN defined circuit-switched B-channel bearer services, commonly at 64 kbit/s, for voice and data.
- : Voice calls in GSM networks and in some 3G systems use circuit switching, which reserves resources for the duration of the call.