Token Ring ... I thought I was out, but they pulled me back in ......
Token Ring, as the name suggests, uses a logical ring topology (topology = "What shape is it?") that is usually a physical star (i.e., central hub, with spokes to the hosts).
One of the hosts on the ring is considered a "Master;" he generates a bit pattern that is recognized as a "token."
The Token is send to the Master's downstream neighbor. The rule of Token Ring is that a station is not allowed to put data on the ring unless it controls the Token.
If the station that has the Token doesn't have any data to send, it regenerates the Token and sends it to his downstream neighbor. This continues ("passing the Token") until one of the hosts has data to send.
If the host has data, it sets a flag in the Token, affixes the data, and send the Token+Data downstream.
As each host receives the Token+Data, it looks at the destination address. If it is addressed to him, he copies the data (and send it up the stack), sets another flag in the Token to indicate the the data was "copied." When the transmitting station gets the Token(+Data) back, he sees the "copied" flag, and flushes the data.
The Master watches the Token+Data circulate on the ring; if he sees the same Token+Data more than once, he decides that the transmitter was lost and removes the Data, regenerates the Token, and sends it on.
That's it in a very basic nutshell.
You cannot do back-to-back connections with Token Ring ... you must use a Multi-Station Access Unit (MAU) or a device that emulates one (Token Ring Switch). This is because the original Token Ring was meant to be able to have MAUs in the ceiling (for example) where there is no power. Each station provides (indivisually and collectively) the power to operate the MAU.
The 802.5 spec also provides a sequence for how a host enters the ring ... that cannot really be emulated in a host-to-host connection.
Again, that's it in a very brief nutshell. There is not enough space in a single post to provide a detailed description.
Token Ring is still being used in some shops, most are using "Big Iron" (IBM Mainframes). Token Ring is very dependable and provides a self-healing physical architecture.
It tends to be a little on the expensive side.
Let us know if you have any additional questions.
Good Luck
Scott
