Network Sales and Services Handbook (Cisco Press Networking Technology)
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.