An active monitor, in terms of computer networking, is the supervisory role one of the computers on a token ring local area network (LAN) takes on in order to maintain the network. This monitor takes charge of the network by watching all of the other computers attached to the network and making sure that data is able to travel through them to the others on the network. All of the other computers on a token ring network, then, are called standby monitors and will only rally for a new active monitor in the case the current one happens to go off-line.
The birth of the active monitor on a token ring network can occur in a couple different ways. In the case of a token ring starting up as each computer joins the network individually, the first computer to boot up claims the role. In other cases, though, such as if the network is established after all the computers are already running or the existing active monitor leaves the network for some reason, the establishment of the active monitor happens via an election process. Each of the computers on the network will send out what's called a claim token, declaring its intent to take charge of the network. As the claim travels around the network, it is quickly evaluated against all other claims and the computer with the highest media access control (MAC) address will win the election.
An active monitor is absolutely crucial in the operation of a token ring LAN. The first, empty token frame, the package that handles the delivery of data to the other computers on the network, is generated by the active monitor and sent around the ring. The monitor will send out one of these tokens every seven seconds to ensure that all of the computers on the network have learned the address and have subsequently identified themselves to one another. Should any extra tokens be circulating the network, the monitor is responsible for cleaning them up.
As a computer on the network requires the use of the token for transmitting data, it captures the token frame, converts it into a data frame and gives it an address. It then fills it with the data it wants to send, and puts it back out onto the ring. Since only one frame can be in use at a time, the active monitor then watches to make sure that if a data frame is in play, other tokens are withheld. Once the data has reached its destination, the frame is sent back to the originating computer, which is then required to release the empty token frame back onto the ring. If some sort of transmission error happens along the way, the monitor will issue a new token frame back into the ring.
Among the essential auditing of the tokens on the network, the active monitor also sets up a clock that makes sure all of the other computers on the network remain synchronized. They must capture and release the token in time with each other to ensure that the network operates efficiently. To be fair to all of the computers on the network, the active monitor also adds in a delay so that the token can make it around the ring.