wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Wireless Local Area Networks 48-5
After experiencing a maximum number of collisions, a packet is dropped. In the case of a successful
transmission, the CW value is reset to CWmin before the random backoff interval is selected.
According to the IEEE 802.11 MAC protocol, an explicit acknowledgment (ACK) frame has to be sent
by the receiver to notify the transmitter of the successful reception of a data frame. The time interval
between the reception of a data frame and the transmission of the relevant ACK is defined as the Short
Interframe Space (SIFS). This small gap between transmissions gives priority to the ongoing frameexchange
sequence, by preventing other nodes that have to wait for the medium to be idle for a longer
time interval (e.g., at least for the DIFS time), from accessing the channel.
Physical and virtual carrier-sense functions are used to determine the state of the medium. When
either function indicates a busy medium, the medium is considered busy, otherwise it is considered idle.
The physical carrier-sense mechanism is provided by the 802.11 PHY, while the virtual carrier-sense
mechanism is provided by the MAC and is referred to as the Network Allocation Vector (NAV). The
NAV maintains a prediction of future traffic on the medium based on the duration information that is
either announced in Request to Send/Clear to Send (RTS/CTS) frames prior to the actual data exchange
or can be used by the PCF and the HCF.
The RTS/CTS mechanism is very effective in reducing the length of the frames involved in the contention
process. In fact, assuming perfect channel-sensing by every station, collisions may only occur
when two or more stations start RTS transmission within the same time slot. If both sources employ the
RTS/CTS mechanism (the decision to use RTS/CTS depends on the packet length, as will be explained
subsequently), collisions would only occur while transmitting the RTS frames and would promptly be
detected by the source lacking the CTS responses. The RTS/CTS therefore, significantly lowers the temporal
overhead of a collision (i.e., collisions are much shorter in time). This feature is beneficial for timeconstrained
traffic.
However, the RTS/CTS mechanism has some drawbacks as the additional RTS and CTS frames add a
protocol overhead and thus reduce protocol efficiency especially for short data frames. For this reason,
each station can be configured to use the RTS/CTS mechanism either always, or never, or only on frames
longer than a specified length.
As the typical control traffic exchanged on the factory floor consists of short frames, the use of the
RTS/CTS mechanism to transmit this kind of traffic is not advisable. In addition, the RTS/CTS mechanism
cannot be used for broadcast and multicast transmissions, as there are multiple recipients for the
RTS, and thus potentially, multiple concurrent senders of the CTS in response.
The RTS/CTS mechanism can be successfully exploited to cope with the hidden node problem, which
arises when there are nodes that are out of range of some other nodes belonging to the same wireless
network, thus creating a potential for collisions with transmissions from those nodes. Before sending a
packet, the transmitter sends an RTS frame and waits for a CTS frame from the NAV.
The reception of a CTS frame notifies the transmitter that the channel is clear in the receiver area, so
the receiver is able to receive the packet. Any other node in the receiver area will hear the CTS even if it
cannot hear the RTS and will avoid accessing the channel after hearing the CTS, even if its carrier sense
mechanism indicates that the channel is idle. The source is only allowed to transmit the data packet if
the CTS frame is correctly received within a duration called the CTS Timeout. More details on the DCF
function can be found in [IEE97].
48.4.2 Point Coordination Function
The PCF access method uses a point coordinator (PC), acting as the polling master, which determines
which station currently has the right to transmit. The operation of the PCF may require additional
coordination that is not specified in the standard [IEE97], to permit efficient operation in cases where
multiple point-coordinated networks are operating on the same channel, and to share an overlapping
physical space.
© 2011 by Taylor and Francis Group, LLC