146 Software Networks
managed locally by the relay. In the standard, the aim is not to create a
mesh network, as the number of hops is limited to two.
Energy savings are also of crucial importance in the current
context, where consumption is increasing exponentially, and the IEEE
802.11ah standard does take steps in this direction. For this purpose,
the connected machines are divided into two classes: TIM (Traffic
Indication Map) terminals and non-TIM terminals. TIM terminals
periodically receive information about the traffic on the access point.
Non-TIM terminals use the new mechanism TWT (Target Wake
Time) to reduce the overhead of the signaling procedure.
TWT is a function which allows an access point to define a time, or
a set of times, for certain terminals to have access to
the communication bearer. The terminal and the access point
exchange information including the allotted duration of the activity, to
enable the access point to control any competition and clashes
between competing terminals. The access point can reserve time
periods for a terminal to transmit thanks to various protective
mechanisms. The use of TWT is negotiated in advance between an
access point and a terminal. The TWT mechanism is also used to
reduce energy consumption, because stations using it can enter a semisleep state until their TWT comes around.
Another important mechanism in IEEE 802.11ah is the RAW
(Restricted Access Window), which limits the number of terminals
which have the right to use the access point. This limitation
is achieved by putting the terminals into groups. Channel access is
restricted solely to those terminals belonging to a given group, over
the course of a set period of time. This mechanism helps reduce
competition and avoid simultaneous transmissions from too high a
number of machines which are unaware of one another’s presence.
The IEEE also defines a TXOP (Transmission Opportunity),
wherein an access point and a client can carry out a set of exchanges
during a reserved period. The purpose of this mode of operation is to
reduce the number of competing channels, improve the efficacy of the
channels by minimizing the number of exchanges and help extend the
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managed locally by the relay. In the standard, the aim is not to create a
mesh network, as the number of hops is limited to two.
Energy savings are also of crucial importance in the current
context, where consumption is increasing exponentially, and the IEEE
802.11ah standard does take steps in this direction. For this purpose,
the connected machines are divided into two classes: TIM (Traffic
Indication Map) terminals and non-TIM terminals. TIM terminals
periodically receive information about the traffic on the access point.
Non-TIM terminals use the new mechanism TWT (Target Wake
Time) to reduce the overhead of the signaling procedure.
TWT is a function which allows an access point to define a time, or
a set of times, for certain terminals to have access to
the communication bearer. The terminal and the access point
exchange information including the allotted duration of the activity, to
enable the access point to control any competition and clashes
between competing terminals. The access point can reserve time
periods for a terminal to transmit thanks to various protective
mechanisms. The use of TWT is negotiated in advance between an
access point and a terminal. The TWT mechanism is also used to
reduce energy consumption, because stations using it can enter a semisleep state until their TWT comes around.
Another important mechanism in IEEE 802.11ah is the RAW
(Restricted Access Window), which limits the number of terminals
which have the right to use the access point. This limitation
is achieved by putting the terminals into groups. Channel access is
restricted solely to those terminals belonging to a given group, over
the course of a set period of time. This mechanism helps reduce
competition and avoid simultaneous transmissions from too high a
number of machines which are unaware of one another’s presence.
The IEEE also defines a TXOP (Transmission Opportunity),
wherein an access point and a client can carry out a set of exchanges
during a reserved period. The purpose of this mode of operation is to
reduce the number of competing channels, improve the efficacy of the
channels by minimizing the number of exchanges and help extend the
www.it-ebooks.info