Connection-oriented communication

Connection-oriented (CO-mode^[1]) communication is a data communication mode in telecommunications whereby the devices at the end points use a protocol to establish an end-to-end logical or physical connection before any data may be sent. In case of digital transmission, in-order delivery of a bit stream or byte stream is provided. Connection-oriented protocol services are often but not always reliable network services, that provide acknowledgment after successful delivery, and automatic repeat request functions in case of missing data or detected bit-errors.

Circuit mode communication, for example the public switched telephone network, ISDN, SONET/SDH and optical mesh networks, are examples of connection-oriented communication. Circuit mode communication provides guarantees that data will arrive with constant bandwidth and at constant delay.

Packet mode communication may also be connection-oriented, which is called virtual circuit mode communication. Due to the packet switching, the communication may suffer from variable bit rate and delay, due to varying traffic load and packet queue lengths.

A connection-oriented transport layer protocol, such as TCP, may be based on a connectionless network layer protocol (such as IP), but still achieve in-order delivery of a byte-stream, by means of segment numbering on the sender side and data packet reordering on the receiver side.

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 denoted 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.

The alternative to connection-oriented transmission is connectionless communication, also known as datagram communication, in which data is sent in form of packets from one end point to another without prior arrangement or signaling. This mode operates without guarantees of delivery according to a best-effort policy. Each data packet must contain complete address information, since packets are routed individually and independently of each other, possibly transmitted along different network paths.

Connectionless protocols are usually described as stateless because the end points have no protocol-defined way to remember where they are in a "conversation" of message exchanges. Because they can keep track of a conversation, connection-oriented protocols are sometimes described as stateful.

Examples of connection-oriented packet mode communication, i.e. virtual circuit mode communication:

• The Transmission Control Protocol (TCP) is a connection-oriented reliable protocol that is based on a datagram protocol (the IP protocol).
• X.25 is a connection-oriented reliable network protocol.
• Frame relay is a connection-oriented unreliable data link layer protocol.
• GPRS
• Asynchronous Transfer Mode
• Multiprotocol Label Switching

References

1. ^ Information Processing Systems - Open Systems Interconnection, "Transport Service Definition", International Organization for Standardization, International Standard 8072, June 1986.

See also

• Connectionless communication
• Packet switching
• Statistical multiplexing
• Packet
• Virtual circuit

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