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

4-June-07 P1901_PRO_016_r0
The channel access is done through the use of a special MAC element called a token. Master nodes decide the
type of frame that their slave nodes are going to transmit next, based on the type of token included in the current
frame, or the kind of channel access the slave nodes can perform. Tokens have several intended uses, depending
on the type of token, and the actions to be undertaken upon its reception also depend on the type of token.
The master node manages the type of channel access, contention free access to the channel with a
predetermined decision about which of its slaves will be allowed to transmit data, in what order, and for how
long, or a contention based access of the slaves to the channel.These decisions will be published by the use of
different types of tokens. In both schemes the final result is a node holding a token. The holder of the token has
the right to access the medium for a specific time and after that time it has to assign the token to the next
destination node. By the control of the frequency of the token occupancy and the length of the time the node is
allowed to hold the token, it is possible to guarantee the requested QoS.
4.1.1 Contention free medium access examples
One of the simplest ways of distribute the channel among several users is the use of Data Frames. Let’s the
following scenario:
CPE
N3
Submission page 83 UPA-OPERA
HE
N1
TDRep
N2
CPE
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Figure 21 Simple Access Topology example
The Data Token has a specific destination node, which, upon its reception, must enter transmission mode. This
destination node might then return the token to its master, or transmit it to its own slaves, in the case of a Time
Division Repeater. A tipical transmission using data tokens can be seen in the following figure: