Technique of packet switching allows the connection information from the beginning of the end to end through the process of sharing resources, using the procedures and links have different speeds to transfer the packet and can transfer the package on many different paths. There are two basic packages: switch type switch datagram (datagram) and virtual channel switches (Virtual Circuit).Datagram switches: datagram switches provide the service do not require real time. The delivery depends on the high layer protocol or data link. Datagram type switches do not need the connection setup phase and are suitable for low-traffic data form and short existence. Datagram switches are switches type maximum effort (best effort), the information about the delays will not be guaranteed as well as the repeated packet loss, packet phenomenon also easily happen with this switch type. The datagram must contain all the information about the destination and address the requirements of the service layer above is shown in the title, so the title of the datagram is quite large. However, datagram switches allow selection of the path to a destination the fastest meet the quickly changing network. Virtual channel switch: switches the virtual channel (Virtual Channel) ask the stage set route between the sending and the receiving device device information, a virtual channel is formed between the devices in the path switches; virtual channel is the only channel to be determined when the data are transmitted and not dependent on the time logic. Virtual channel switches require an online immediately in the process of routing and virtual channels are identified through the school virtual channel identifier VCI (Virtual Channel Identifier) is set in the packet header. In the process of setting up the virtual channel, virtual channel identifier VCI was created by the switching node to specify the resources of the packet will move through (for example, the buffer capacity of the link). Once the virtual channel is set up along the route from the source to destination via the links and nodes, then the channel is used to transmit the packet. The VCI in the title can be used as pointers to access to the information stored in the switch button. The virtual channel identifier field need not only to distinguish the information that people use and reuse. If using the VCI for the entire network, then quantity of VCI huge and constantly increases with the size of the network. Therefore, people using the virtual channel identifier according to the local region, even on each link. This way, when a VC to initialize each one button switches along the route will have to establish the identity of the virtual channels on the link input and output links of the button switches. The buttons must deal with each other on single virtual channel identifier on the link between the two nodes for a virtual channel. Virtual channel identifier on the input and output links don't need to be the same, the information transfer based on the packet headers contain VCI will perform the conversion information in the input to the output VCI VCI. All packages are sent on the same virtual channel VC will follow the same path, so the order and time delay spread is control. This is very useful for real time traffic and time exists. If the virtual channel errors or corrupted, the routing system will find a different route instead. The method of search path and set the path will be presented in the routing section. When it wants to liberate the virtual channel, interrupt control connection packets are transmitted to all the devices virtual channel goes through to release resources and virtual channels will be reused for another connection.
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