The IEEE 802.4 and 802.5 standards and the IBM Token Ring protocol (with which IEEE 802.5 is compatible) use token passing, an altogether different approach than CSMA/CD, for medium access control. We will consider first the case of token passing in a ring (802.5) and will treat Token Bus (802.4) briefly later in this unit.
This section covers the lower sublayer of the Data Link Layer. This lower sublayer, also referred to as the Media Access Control (MAC) layer, is the framing and access control layer for the Token Ring protocol.
Token Ring Media Access Control
Recall that in ring networks, each nod contains a repeater that receives bits from one of the two links and transmits them on the other. It receives messages simply by copying bits as they go by. The medium access control question with a ring LAN is, "When can the nodes insert bits onto the ring?" The answer lies in the token passing protocol. See the Token Ring Flow diagram below.
Figure 4-16. Token Ring Flow
The idea of token passing is this:
Token Ring Protocol
When a node needs to transmit a message, the following takes place. See the Token Ring Send Algorithm diagram below.
Figure 4-17. Token Ring Send Algorithm
It transmits this information, after the first free token:
The node must stop transmitting after ten milliseconds and wait for the token to come around again if it has more data to send. The token will circulate around the ring, followed by the message, passing through the destination node, until it returns to the source node. As it passes through the destination node, the node copies the data into its own memory. (The A and C bits in the frame status byte are used to signal the sending node that the node is alive and the frame has been copied.) When the source node receives the busy token, it retransmits it as a free token. As it receives the data following the token, it does not retransmit it, thus removing the message from the ring.
The Token Ring Receive Algorithm diagram below illustrates the general process required for a node to receive a frame.
Figure 4-18. Token Ring Receive Algorithm
Token Ring Configurations
This section illustrates two types of Token Ring configurations found in networks today.
The Simple Token Ring diagram below illustrates how several ring segments are connected together via Multistation Access Units (MAUs).
Figure 4-19. Simple Token Ring
The Token Ring and IBM Host Connectivity diagram below illustrates three traditional Token Ring configurations used to access an IBM host. Token Ring nodes can access host resources via front rnd processors, vluster controllers, or through LAN gateway devices such as an IBM 3172.
Figure 4-20. Token Ring and IBM Host Connectivity
Advantages and Disadvantages of Token Ring