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A token bus network is a local area network (LAN) in which stations share a physical bus but control who may transmit by passing a control mechanism called a token. Only the station that currently holds the token may send data. The access method is associated with the IEEE 802.4 standard. In short, it pairs a bus as the physical medium with token passing as the medium access control method.
How token passing controls access
The token is a permission mechanism, not a data carrier. Because a station must hold the token before it can transmit, two stations never compete to send at the same moment. IEEE Technology Navigator’s overview of token networks describes this as deterministic access, meaning the order of turns is defined in advance. That description refers to the access method in general; it is not a promise of a specific waiting time in every token bus installation.
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The token cycle, step by step
- The station holding the token checks whether it has data queued for transmission.
- If it has data, it transmits. No other station may transmit while it holds the token.
- The station then passes the token to the next station in the logical sequence.
- A station with nothing to send passes the token onward immediately. Idle stations still take part in the rotation.
Because every station gets a turn in a fixed sequence, the design avoids the collisions that occur when stations on a shared channel transmit at the same time without coordination.
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Bus layout versus logical order
The word “bus” describes the physical layout: all stations attach to one shared cable. The token, however, travels along a logical order that the network defines, and that order does not have to match the physical positions of the stations along the cable. A token bus therefore behaves like a ring in the sequence of turns, even though the cable itself is not closed into a physical loop.
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This is the main point that separates a token bus from a token ring. In a token ring, the physical wiring forms the ring and the token moves around that physical path. A token bus keeps the bus wiring and imposes the ring-like order in software-level logic. The sources reviewed explain this concept at a conceptual level. They do not give installation rules, cabling details or wiring diagrams, so this article does not make those claims.
How token bus compares with token ring and Ethernet
The comparison below uses the axes that matter most for a definition: physical layout, who may transmit, how contention is handled, and predictability. Where a cited source does not cover a cell, the table says so.
| Attribute | Token bus (IEEE 802.4) | Token ring | Contention-based Ethernet |
|---|---|---|---|
| Physical layout | Shared bus cable | Physical ring | Not stated in the cited sources |
| Logical transmission order | Defined token sequence, which can differ from physical station order | Follows the physical ring | Not applicable; stations compete for the channel |
| Who may transmit | Only the station holding the token | Only the station holding the token | Any station that senses the channel is idle |
| Contention handling | Avoided by design, because access is granted by the token | Avoided by design, because access is granted by the token | Stations contend for access, so simultaneous transmissions can collide |
| Predictability under load | Described as more predictable than a contention-based shared channel; no universal latency guarantee | Not compared in the cited sources | Less predictable under contention; no universal ranking is claimed here |
The token ring entries in this table describe the general token-passing principle, not a specific token ring standard. Avoid reading the table as a performance ranking. The sources support the conceptual differences, not a verdict that one method is always faster.
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Token passing was chosen where orderly access and predictable waiting times mattered more than the simplicity of contention. A National Institute of Standards and Technology workshop report on analytic and simulation modeling of IEEE 802.4 token bus local area networks treats the network in the context of industrial automation and performance assessment. That report shows that engineers studied token bus behavior through modeling and simulation, not only through live deployments.
Current status and historical context
The IEEE Technology Navigator states that Ethernet displaced token-based LANs in most enterprise settings by the mid-1990s. That is a broad historical characterization. It does not mean every legacy token network was removed, and it does not mean token bus has no remaining specialist or archival use.
An older IETF document also shows the technology’s history. RFC 1230, “IEEE 802.4 Token Bus MIB,” published in May 1991, defines a management information base for IEEE 802.4 token bus interfaces. It is evidence that these interfaces were managed as a network technology in 1991. It is not evidence of present-day deployment.
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What the sources do not establish
- The current formal revision status of IEEE 802.4. Check the IEEE standards catalogue before citing a particular revision.
- Exact physical-layer options, the full frame format, or the detailed token-handling rules.
- Whether any current manufacturer still sells token bus equipment, parts or accessories.
- Any latency figure for a specific token bus configuration.
Reader questions this definition answers
If you encountered the term in a textbook, a course or an industrial system document, the working definition is this: a bus-cabled LAN that uses token passing to grant transmission turns in a defined logical order, standardized under IEEE 802.4. For a background reference on the wider category, the ScienceDirect overview of token bus places the topic inside computer networking reference material.
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