Token ring – IEEE 802.5

A token ring is an outdated form of network technology, and today Ethernet is the most prevalent type of wired LAN connection. But that doesn't make the old technology any less interesting. Older methods, now confined to the realms of history, can also help us understand modern networks and why technology has developed as it has.

However, a token ring is not really a ring network, also referred to as a ring topology, even if the name suggests this. In fact, the only ring created is a logical one, not a physical one. We will explain the differences below.

Even before the introduction of the token ring, there were, of course, other computer networks. These were not available to the public, like ARPANET, but were mostly developed and used by individual institutions for their own purposes and were therefore not standardised. In the early 1980s, the companies Procom, Apollo Computer, and Proteon were the first to offer token ring networks on a larger scale. IBM then took the matter up and launched its own product in the middle of the decade.

Due to how widespread IBM PCs became, the token ring also began to quickly take over the tech market – and was directly slowed down again. Ethernet technology, which was already developed in the 1970s, also gained more and more popularity in the 1980s, partly thanks to the support of Intel and Xerox. Ethernet beat the competition between the two technologies by introducing low-cost, thin co-axial cables. IBM had charged such high fees for the license of the token ring process that no inexpensive products could come onto the market.

While IBM initially tried to convince the public of the advantages of a token ring, they gave up at the end of the 1990s: Although the IEEE had approved models with 100 Mbit/s and 1,000 Mbit/s, the former was marketed only slightly, and with the latter not a single product made it onto the market.

The question of what a token ring network is easier to answer if you know what it is not: a ring topology. A ring topology in the physical sense consists of a circular arrangement of computers. Each net participant is connected to its neighbours to the left and right, so that the network is a closed ring. As soon as a computer within the LAN fails or a connection is otherwise disconnected, the whole network breaks down, and the connection is lost. If another computer wants to participate in the network, the LAN must be interrupted for at least a short moment.

A token ring works somewhat differently to other ring topologies, which is why it’s said that this technology is based only logically on a ring topology. The token ring topology uses Multistation Access Units (MAUs), which allow a star-shaped connection of the connections involved. The distributor is a node that is connected to all computers on the network. There is no direct connection between the individual computers.

Nevertheless there is still a logical ring involved, which is due to the physical star structure, because the data transmission takes the form of a ring – on an abstract level. Although the data is repeatedly transported to the MAU, it is not sent from there to a specified subscriber, but simply to the next computer in the fixed sequence.

Issues can occur in any network, and in most there is the risk that some would cause the network to break down. In a token ring, all computers are standby monitors (SM) by default, but you usually check on the network in the role of an active monitor (AM). Each station can take on this role. The decision as to which station will become AM – and so also which ones remain as SM – is made by a fixed procedure: monitor contention, also called token claiming. The procedure is initiated when a station has determined that the current AM no longer transmits the mandatory active monitor present frame via the ring.

When this happens, the protocol requires a choice for a new active monitor: The first station that has noticed the malfunction of the AM begins to send a so-called claim token frame. The next station in the ring compares the sender's MAC address with its own. If the address of this second network subscriber has a higher value, it replaces the frame with its own. At the end only one station remains: If its frame has circled the ring three times without any other station intervening, a new active monitor is determined.

The active monitor is responsible for the smooth running of the token ring, but standby monitors can also intervene in some cases. In the following scenarios, the token ring network can heal itself:

As soon as a token passes the active monitor, it sets a timer. If the time span (10 milliseconds) has elapsed before the token reaches it again, the AM detects a problem in the network and generates a new, free token.

If one station sends a packet and the recipient station would fail at this very moment, the packet would circle endlessly on the ring, since no station identifies itself as the recipient. When the packet passes the AM for the first time, it converts a specific bit. If the packet passes the AM again, he recognises from the bit that the packet has not reached his recipient. Only the AM can set the bit to 1 and determines that it must already have come into contact with the packet. The active monitor destroys the package and creates a new, free token.

In a normal ring topology, the failure of a station or connection means the complete failure of the network. With a token ring topology, however, the MAU can simply bridge the interface. Faulty stations are detected by a direct neighbour: To do this, the participant located directly behind the damaged computer sends test frames to the neighbour. All other stations enter a waiting mode: no station sends data. If the faulty node detects that its neighbour has submitted a complaint, it starts an error analysis and takes itself off the network.

However, the error can also be at the original station: The fact that the computer can no longer access data may also be due to a faulty network card. So, the participant sets a timer that would give the neighbour enough time to perform a self-test. If no frame arrives after the expiration of the timer, the station assumes an error and starts a test.

The faulty station – if it was not the AM – is simply bypassed during transmission until a solution has been found. If it is the AM, the token claiming starts. When everything works again, the AM creates a new token and mains operation can continue normally.

The advantages and disadvantages of a token ring can be seen in a comparison with Ethernet systems. The fact that collisions cannot occur makes the token ring an interesting concept: In a (half-duplex) Ethernet network, collisions caused by several stations transmitting at once are likely. With the help of CSMA/CD you can control how these collisions are handled, but they reduce the speed anyway. In a token ring, however, collisions cannot occur. The token-passing procedure prevents several stations from transmitting simultaneously.

This compensates for the token ring’s lack of speed. While token ring technology was stopped in its development at 16 Mbit/s, Ethernet networks with 100 Mbit/s already existed at that time. If you compare a simple ring topology with the token ring via MAUs, the latter has the advantage of flexible nodes. Adding or removing individual computers from the network is no problem. Even if individual stations fail unexpectedly, the network is not endangered. This in combination with effective troubleshooting makes a token ring a very stable system.