Virtual circuit

Virtual connections as is known, the logical channels, resulting in a communication network by the allocation of transmission capacity, which provides a physical communication link. About each of these channels has its own logical bitstream can be transmitted in each case. This division into logical channels is realized by the individual bit streams are segmented at the transmission side, and are then transported in the payload of packets or cells through the communication network. On the receiver side, the payload is unpacked and the individual bit streams are restored. The packets or cells obtained but no source or destination address, such as a datagram, but a connection identifier, seen from the to which they belong virtual connection.

The concept of virtual connection has the advantage that when the concentration of the packets or cells, a multiplexing gain occurs: the bandwidth of the line is used effectively. The prerequisite for this is that the individual bit streams do not require constant bandwidth, but have a variable demand. This is for example the dialogue between operating data display terminals and mainframes the case. The multiplexing gain can occur for one on the line connecting the terminal to the packet switch and on the other but also to the trunk lines between the packet switches.

Does the news network packet-switched X.25, so each virtual connection link identifier obtained as a channel group number ≤ 15 and a channel number ≤ 255, so 4096 virtual connections via a communication link can be performed. In each case, 255 virtual channels can thus be grouped into a channel group. From 4095 to 4096 channels are used, channel 0 is used for the signaling protocols between the terminal and exchange.

In this specification, also immediately clear that the connection identifier of a virtual connection is not suitable for the addressing of the participants in a large packet network. At a large packet network must be able to connect millions of devices, and to the address space of the link identifier is not enough. The connection identifier has only a local validity: Each individual connection of the entire address space of 4096 virtual channels is available again. The connection itself, however, has its own number, which follows a numbering plan that allows much higher numbers of participants.

ATM networks have also virtual connections. The connection identifier consists of two phases as in X.25: what does it mean for an X.25 "Channel Group " and "Channel ", ie at ATM " path " and " channel ". A virtual connection, which is denoted by virtual channel, having a virtual channel identifier VCI. A number of virtual channels can be bundled by a virtual path identifier VPI Virtual Path again. Because it is a broadband network to ATM, the address space is very much greater than that of X.25.

A virtual circuit can be dynamically assembled and disassembled are (English: switched virtual circuit SVC ) or be set up (English: permanent virtual circuit, PVC). If you want to leave open whether it is PVC or SVC, it is called a virtual channel (English: virtual circuit).

• Communications Engineering
• Switching technology