Token Ring Networks

In 1984, IBM introduced the 4 Mbit/s Token Ring network. Instead of the normal plug and socket arrangement of male and female gendered connectors, the IBM data connector (IDC) was a sort of hermaphrodite, designed to mate with itself. Although the IBM Cabling System is to this day regarded as a very high quality and robust data communication media, its large size and cost – coupled with the fact that with only 4 cores it was less versatile than 8-core UTP – saw Token Ring continue fall behind Ethernet in the popularity stakes. It remains IBM’s primary LAN technology however and the compatible and almost identical IEEE 802.5 specification continues to shadow IBM’s Token Ring development. The difference between Token Ring and IEEE 802.5 networks are minor. IBM’s Token Ring network specifies a star, with all end stations attached to a device called a multistation access unit (MSAU). In contrast, IEEE 802.5 does not specify a topology, although virtually all IEEE 802.5 implementations are based on a star. When a Token Ring network starts up, the nodes all take part in a negotiation to decide who will control the ring, or become the Active Monitor – responsible for making sure that none of the members are causing problems on the network, and for re-establishing the ring after a break or if an error has occurred. To do this it performs Ring Polling every few seconds and ring purges whenever it detects a problem. The former allows all nodes on the network to find out who is participating in the ring and to learn the address of their Nearest Active...

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