As virtual connections are called the logical channels, in a communication network by the allocation of transmission capacity arise that a physical communication link provides. A separate bit stream can be sent via each of these logical channels .
The division into logical channels is implemented by segmenting the individual bit streams on the transmission side and then transporting them in the payload of packets or cells via the communications network. On the receiving end, the payload is unpacked again and the individual bit streams are restored. However, the packets or cells do not receive a source or destination address such as B. a datagram , but a connection identifier, which shows which virtual connection they belong to.
A virtual connection can
- dynamically established and dismantled ( switched virtual circuit, SVC ) or
- permanently set up (English: permanent virtual circuit, PVC ).
- If you want to leave open whether it is PVC or SVC, one speaks of a virtual channel ( virtual circuit, VC ).
The concept of the virtual connection has the advantage that bundling the packets or cells results in a multiplex gain: the bandwidth of the line is used more effectively. The prerequisite for this is that the individual bit streams do not require a constant bandwidth, but rather have variable requirements. This is the case , for example, with dialog operation between data display terminals and mainframes . The multiplex gain can arise on the one hand on the line that connects the terminal with the packet switch , but on the other hand also on the long-distance routes between the packet switches.
ID and address space
If the communications network is packet-switched according to X.25 , each virtual connection receives a channel group number ≤ 15 and a channel number ≤ 255 as a connection identifier, so that 4096 virtual connections can be routed via a communications link. 255 virtual channels can be bundled into a channel group. 4095 of the 4096 channels can be used, channel 0 is used for the signaling protocols between the terminal and the exchange .
This definition also makes it clear that the connection identifier of a virtual connection is not suitable for addressing the participants in a large packet network. Millions of end devices must be able to be connected to a large packet network, and the address space of the connection identifier is insufficient for this. The connection identifier only has local validity: the entire address space of 4096 virtual channels is available at each individual connection. The connection itself, however, has its own phone number that follows a numbering plan that allows significantly higher numbers of participants.
ATM networks also have virtual connections. As with X.25, the connection identifier has two levels: what is called “channel group” and “channel” with X.25 is called “path” and “channel” with ATM. A virtual connection, which is referred to as a virtual channel , has a Virtual Channel Identifier VCI. A number of virtual channels can in turn be bundled into a virtual path using the Virtual Path Identifier VPI . Since ATM is a broadband network, the address space is much larger than that of X.25.
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