What is Demand Priority?
Just like most other networks, a VG node can receive a packet at any time but can only transmit at certain times. The set of rules governing when a node can transmit one or more packets on the network are often called media access rules. The media access rules for VG are called Demand Priority. Other networking technologies, including Ethernet, Fast Ethernet, Token Ring, ARCnet, and FDDI, implement their media access rules in the nodes themselves (there is no central access control). In contrast, all the media access rules for VG are implemented in the hub. The hub, or hubs, decide when each node can transmit. Nodes do not cooperate with each other to decide when they are able to transmit. VG nodes don’t pass tokens (even when using Token Ring frame types), or detect and resolve collisions (except when plugged into a 10Base-T hub). It works like this: When a node has one or more packets to transmit, it sends a signal (a demand) to the hub that says “hey! let me transmit a packet!” When
