Token Ring Topology: Working, Token Passing, Frame Format, Errors & Advantages Explained

Introduction

Token Ring is a networking technology used to create local area networks (LANs). It was introduced by IBM in 1984 and standardized as IEEE 802.5 in 1989.

It uses a three-byte frame called a token that circulates around a logical ring of workstations or servers. This channel access method ensures equal access for all stations and eliminates collisions that occur in contention-based networks. Token Ring was popular in corporate environments but was eventually replaced by advanced Ethernet technologies.

What Is a Token Ring?

A token ring network should not be confused with a physical ring topology. In a physical ring topology, computers are arranged in a continuous circle, where each device is connected to its left and right neighbors. If one computer or link fails, the entire network can collapse.

Operating Modes

• Listen Mode: The node listens for data and forwards it to the next node, with a one-bit transmission delay.
• Transmit Mode: The node sends its own data to the network and discards incoming data.
• By-pass Mode: Activated when the node is offline. Incoming data is ignored, and no one-bit delay occurs.

What Is Token Passing?

Token passing is the method used by Token Ring networks to control access. A station must capture the token before transmitting data. The token contains three main fields that circulate until captured and removed by a station.

Token Fields

• Starting Delimiter: Identifies the beginning of the token frame.
• Access Control: Determines station priority (0–7). Used by the active monitor to detect continuously circulating frames.
• Ending Delimiter: Marks the end of the token frame.

Active and Standby Monitors

Every station is either an active monitor (AM) or a standby monitor (SM). Only one active monitor exists at a time. It performs key functions such as:

• Synchronizing the ring signal
• Adding a 24-bit delay for buffering
• Ensuring a token circulates when no data is transmitted
• Detecting and removing broken or stuck frames

Token Format

The token is the shortest possible frame (24 bits). The MSB (Most Significant Bit) is always transmitted first.

• SD – Starting Delimiter (1 octet)
• AC – Access Control (1 octet)
• ED – Ending Delimiter (1 octet)

Frame Format

Like the token, the MSB is always transmitted first.

• SD – Starting Delimiter (1 octet)
• AC – Access Control (1 octet)
• FC – Frame Control (1 octet)
• DA – Destination Address (2 or 6 octets)
• SA – Source Address (2 or 6 octets)
• DATA – Up to 4027 octets
• CRC – Checksum (4 octets)
• ED – Ending Delimiter (1 octet)
• FS – Frame Status

Possible Errors in Token Ring Networks

1. Token Lost

The active monitor sets a timer when a token passes. If the token doesn’t return within 10 ms, the AM generates a new token.

2. Packet Circulates Without Recipient

If a destination fails, the packet loops endlessly. The AM marks the packet on first pass and deletes it if it returns unchanged, then generates a new token.

3. Faulty Interfaces

The MAU (Multistation Access Unit) can bypass faulty nodes. Neighboring stations send test frames, and faulty nodes isolate themselves. If the AM fails, token claiming occurs to assign a new AM.

Who Removes the Packet From the Ring?

There are three possibilities:

• The source removes the packet after one full round.
• The destination removes it (problematic for broadcast/multicast).
• A special node removes packets (inefficient and slow).

The preferred method is the first— the source removes its packet after one complete ring cycle.

Cycle Time

The time taken by a token to complete one revolution of the ring is the cycle time.

Cycle Time = Tp + (THT × N)

• THT – Token Holding Time
• Tp – Propagation delay (distance/velocity)

Token Holding Time (THT)

THT is the maximum time a station can hold the token (default: 10 ms). No station may exceed this time.

Advantages of Token Ring

• Low chance of data collisions
• Better performance under heavy traffic
• No server required for node connectivity management

Disadvantages of Token Ring

• Each packet passes through all nodes, slowing routing
• Complex network management and fault handling
• Considered obsolete due to Ethernet dominance