WAN Switching

There are three basic types of WAN transmission technologies:

• Circuit-switched WANs (public switched telephone networks [PSTNs]) create a dedicated circuit, or channel, which is used for the duration of the transmission.

• Packet-switched WANs (Frame Relay, Internet, IP Virtual Private Networking [IP VPN]) separate messages into variable-length segments and send these segments across dynamic connections.

• Cell-switched WANs (ATM) separate messages into fixed-length cells and send these cells across dynamic or permanent connections.

Each of these circuit types is discussed in the following sections.

Circuit-Switched WANs

Circuit-switched WANs, the most prevalent example of which is the PSTN, are designed for analog voice transmission and have the following characteristics:

• Circuit-switched networks are connection-oriented, requiring call setup before information can be exchanged. This temporary connection is known either as a circuit or channel.

• The circuit bandwidth is constant and dedicated throughout the duration of the call.

• There is no error recovery because circuit-based switches maintain small buffers.

• Circuit-switched network providers charge customers for the duration of the connection in a cost-per-minute (CPM) fashion.

Packet-Switched WANs

Packet-switching, which divides messages into smaller packets for transmission, was developed for the purpose of sending data over unreliable analog circuits. The Internet is a packet-switched network in which web pages, e-mail, and file transfers are broken down into smaller packets that are reassembled at the destination.

Packet-switched networks have the following characteristics:

• Packet-switched networks are connection-oriented, meaning that the receiving host must acknowledge the sending host before communication can begin.

• Packet-switched connections do not require call setup because each packet contains a destination address that is used to route the packet through the network. This dynamically routed connection is known as either a virtual circuit or virtual channel (VC).

• Dynamic routing results in flexible use of bandwidth and network resources, enabling the network service providers to share backbone bandwidth across several subscribed customers.

• Packet-switched network service providers charge based on subscribed bandwidth and usage instead of the CPM model of a circuit-switched network.

Cell-Switched WANs

Cell-switching divides messages into small, fixed-length cells for transmission. ATM WANs are the most prevalent use of cell-switching, using cells 53 bytes in length (48 bytes for user payload, and 5 bytes for the cell header). Cell switching combines the guaranteed bandwidth of a circuit-switched network and the bandwidth-sharing and prioritization capabilities of a packet-switched network.

Cell-switched WANs have the following characteristics:

• Like packet-switched networks, cell-switched networks are connection-oriented, meaning that the receiving host must acknowledge the sending host before communication can begin.

• Cell-switched (ATM) WANs do not require call setup because each 53-byte cell contains the destination address used to forward the cell through the network.

• Cell-switched networks enable both dynamic (on-demand) and permanent connections. Dynamic connections are known as switched virtual circuits (SVC) and permanent connections are known as permanent virtual circuits (PVCs).

• The logical (virtual) circuits of a cell-switched network enable network service providers to guarantee Quality of Service (QoS) connections to their customers.

• Like packet-switched services, cell-switched network service providers charge based on subscribed bandwidth and usage instead of the CPM model of a circuit-switched network.