What is a virtual circuit network? Explain ATM as a virtual circuit wide area network.

A virtual circuit network is a packet-switched network that sets up a logical path (virtual circuit) between sender and receiver before data is sent. It works in three phases:

1. Setup: a path is chosen and every switch on it records an entry in its table.
2. Data transfer: every packet carries a short virtual circuit identifier (VCI) instead of a full address, and all packets follow the same path in order.
3. Teardown: the table entries are removed.

It combines features of circuit switching (fixed path, ordered delivery) and packet switching (links are shared). Resources can be reserved, so quality of service can be guaranteed.

ATM as a virtual circuit WAN

Asynchronous Transfer Mode (ATM) is a cell-relay WAN technology built on virtual circuits.

• Cells: all data is carried in fixed-size 53-byte cells (5-byte header and 48-byte payload). Fixed size lets switches work in hardware at very high speed with predictable delay, which suits voice and video.
• Virtual connections: ATM uses two levels of identifier. A virtual path (VP) is a bundle of virtual channels (VC). Each cell header carries a VPI (virtual path identifier) and a VCI (virtual channel identifier), and switches forward cells using these values.
• Connection types: permanent virtual circuits (PVCs), set up by the operator, and switched virtual circuits (SVCs), set up on demand.
• Layers: the ATM Adaptation Layer (AAL) breaks user data into 48-byte pieces and rebuilds it; the ATM layer adds the header and does switching; the physical layer carries the cells (e.g. over SONET).
• QoS: because the path is set up in advance, ATM can guarantee bandwidth and delay for different traffic classes.