Unit 4

Token Ring

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:

on an inactive LAN, a three-byte token circulates endlessly
the token is like a message, except that bit 4 in the second byte indicates it's a token
three priority bits indicate whether the token can be grabbed by the station. If the priority of the token is higher than the frame to be transmitted, the token is passed on.
a node transmits its message by inserting it after the token when the token is free
each node retransmits the message that follows a busy token
only one token and at most one message can circulate on the ring at one time. (This is true for 802.4 or 802.5, but not for FDDI, as you will see later.)

   Token Ring Protocol

When a node needs to transmit a message, the following takes place. See the Token Ring Send Algorithm diagram below.

Fig. 58

Figure 4-17. Token Ring Send Algorithm

It transmits this information, after the first free token:

the address of the destination node
its own address — the source address
the data
a checksum
ending control information

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.

Fig. 59

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

Simple engineering because it is point-to-point digital — no analog.
Standard twisted pair medium is cheap and easy to install.
Easy detection and correction of cable failures.
Deterministic and traffic can be prioritized.
No padding of data required in frame, so frames are short.
Excellent performance under conditions of heavy load.
Since rings can be bridged by their wiring concentrators into what is effectively one ring, ring size has no practical limit.
Necessity of having a monitor function.
Under conditions of low load, substantial delay waiting for token to come around, even though network is idle.
Can require significantly more wire to be run than a bus architecture.

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