Medium Access Control (MAC) and Physical Layer (PHY) - CISE

4-June-07 P1901_PRO_016_r0
In this case, a token loss occurs when the master node transmits a frame and the intended token receiver does not
detect the token or when the master does not detect the token sent by one of its slaves. In case of token loss, the
master node shall detect that the token has been lost.
Using the validity associated to every transmitted token, the master node shall generate a new frame whenever the
validity assigned in a token to one of its slaves expires without receiving back the token. Only HE and TDR
implement this feature.
When a Token Distributor is used to share the channel, the Token Loss Recovery policy becomes more complex,
but equally efficient.
There are two different modes to recover a token lost when using Distributor Tokens:
1. Master node is the only node in charged of recovering a token lost. In this mode, the node that sends the
Distributor token must monitor the channel during all the validity time assigned in the token in order to
detect in each moment if the channel owner transmits.
When Master transmits a Distributor Token checks that the first node on the distribution list starts
transmitting. If Master does not detect the Token Announce from the expected node, it can retransmit the
Distributor Token lost or a new Distributor Token. In that way, the only overhead included in this simple
acknowledgement mechanism is the retransmission of the token, instead of waiting for the maximum time
assigned to the first slave.
If first slave detects the Distributor Token, it starts transmitting and assigns the channel to the second
destination slave. This process will be monitorized by the Master. If Master node detects the first slave
assigns the channel to the next slave, it will check if the second slave starts transmitting. If Master does not
detect the Token Announce from that slave, it transmits a new Distributor Token. This Distributor Token
can include the slave that lost the token or can continue in the next slave on the distribution list.
Master shall repeat all this process till the last destination slave returns the channel to the Master.
Master node shall know in each moment which is the channel owner slave, when it has to start transmitting,
and when it has to release the token to the next slave.
Following diagram shows the monitoring process performed by the Master in order to recover a token lost:
Submission page 128 UPA-OPERA