Contents

1 Circuit switching

A circuit-switched network is one that establishes a dedicated circuit (or channel) between nodes before they may communicate (example: a subscriber that makes a telephone call).

Usually, the internal path taken by the circuit between exchanges is shared by a number of conversations by means of time-division multiplexing (TDM).

PSTN and ISDN are two types of circuit-switching technology that may be used to implement a WAN.

2 Packet switching

Packet switching splits traffic data into packets that are routed over a shared network. No circuits are established and allow many pairs of nodes to communcated over the same channel using TDM.

There are two approached to the link determination:

1. Connectionless systems, such as the Internet, carry full addressing information in each packet and each switch evaluate the address fo determine here to send the packet.
2. Connection-oriented systems predetermine the route for a packet, and each packet only has to carry and identifier.

Because the internal links between the switches are shared between many users, the costs of packet switching are lower than those of circuit switching. Delays (latency) and variability of delay (jitter) are greater in packet-switched than in circuit-switched networks. This is because the links are shared, and packets must be entirely received at one switch before moving to the next. Despite the latency and jitter inherent in shared networks, modern technology allows satisfactory transport of voice and even video communications on these networks

The route that packets of the same conversation follow is called a Virtual Circuit (VC). Therefore, a VC is a logical circuit created within a shared network between two network devices. Two types of VCs exist:

1. Permanent Virtual Circuit (PVC): A permanently established virtual circuit that consists of one mode: data transfer. PVCs are used in situations in which data transfer between devices is constant. PVCs decrease the bandwidth use associated with establishing and terminating VCs, but they increase costs because of constant virtual circuit availability. PVCs are generally configured by the service provider when an order is placed for service.
2. Switched Virtual Circuit (SVC): A VC that is dynamically established on demand and terminated when transmission is complete. Communication over an SVC consists of three phases:
   1. Circuit establishment: involves creating the VC between the source and destination devices.
   2. Data transfer: involves transmitting data between the devices over the VC.
   3. Circuit termination: involves tearing down the VC between the source and destination devices.

   SVCs are used in situations in which data transmission between devices is intermittent, largely to save costs. SVCs release the circuit when transmission is complete, which results in less expensive connection charges than those incurred by PVCs, which maintain constant virtual circuit availability.

Examples: X.25, Frame Relay, Metro Ethernet and ATM.