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A connection-oriented service is a technique used to transport data at the session layer. Unlike its opposite, connnectionless service, connection-oriented service requires that a session connection be established between the sender and receiver, analogous to a phone call. This method is normally considered to be more reliable than a connectionless service, although not all connection-oriented protocols are considered reliable.
A connection-oriented service can be a circuit-switched connection or a virtual circuit connection in a packet-switched network. For the latter, traffic flows are identified by a connection identifier, typically a small integer of 10 to 24 bits. This is used instead of listing the destination and source addresses.
A connection-oriented service needs an established connection between peers before data can be sent between the connected terminals. This handles real-time traffic more efficiently than connectionless protocols because data arrives in the same order as it was sent. Connection-oriented protocols are also less error-prone.
Asynchronous transfer mode is a connection-oriented service, and it has yet to be replaced by Ethernet for carrying real-time and isochronous traffic streams. Increasing bandwidth does not always solve service problems. A good connection-oriented service can often deliver more quality than large bandwidth. Even so, some connection-oriented services have been made to accommodate both connectionless and connection-oriented data.
In a connection-oriented, packet-switched data link layer or network layer protocol, all data is sent over the same path during a communication session. The protocol does not have to provide each packet with routing information (complete source and destination address), but only with a channel/data stream number, often called a virtual circuit identifier (VCI). Routing information may be provided to the network nodes during the connection establishment phase, where the VCI is defined in tables in each node. Thus, the actual packet switching and data transfer can be taken care of by fast hardware, as opposed to slow, software-based routing.