Image Processing Reference
In-Depth Information
DIFS
DIFS
Contention window
SIFS
Busy medium
Backoff window
Next frame
Slot time
Defer access
Decrement backoff as long
as medium is idle
FIGURE .
Timing diagram for wireless Ethernet (IEEE .).
prevent collisions. Thus, after a packet has been successfully received by its destination node, the
receiver sends a short ACK packet back to the original sender. If the sender does not receive an ACK
packet, it assumes that the transmission was unsuccessful and retransmits.
he CA mechanism in . works as follows. If a network node wants to send while the network is
busy, it sets its backoff counter to a randomly chosen value. Once the network is idle, the node waits
first for an interframe space (DIFS) and then for the backoff time before attempting to send (see
Figure .). If another node accesses the network during that time, it must wait again for another
idle interval. In this way, the node with the lowest backoff time sends first. Certain messages
(e.g., ACK) may start transmitting after a shorter interframe space (SIFS), thus they have a higher
priority. Collisions may still occur because of the random nature of the backoff time; it is possible for
two nodes to have the same backoff time.
Several refinements to the protocol also exist. Nodes may reserve the network either by sending a
request to send message or by breaking a large message into many smaller messages (fragmentation);
each successive message can be sent after the smallest interframe time. If there is a single master
node on the network, the master can poll all the nodes and effectively create a TDM contention-free
network.
In addition to time delays, the difference between the theoretical data rate and the practical
throughput of a control network should be considered. For example, raw data rates for . wireless
networks range from to Mbits/s. he actual throughput of the network, however, is lower due to
both the overhead associated with the interframe spaces, ACK, and other protocol support transmis-
sions, and to the actual implementation of the network adapter. Although .a and .g have
the same raw data rate, the throughput is lower for .g because its backward compatibility with
.b requires that the interframe spaces be as long as they would be on the .b network. Com-
puted and measured throughputs are shown in Table . []. [].The experiments were conducted by
continually sending more traffic on the network until a further setpoint increase in traffic resulted in
no additional throughput.
Experiments conducted to measure the time delays on wireless networks are summarized in
Table . and Figure . []. Data packets were sent from the client to the server and back again,
with varying amounts of cross-traffic on the network. he send and receive times on both machines
weretime-stamped.hepacketlettheclientattime
t
a
and arrived at the server at time
t
b
;thenlet
TABLE .
Maximum hroughputs for
Different . Wireless Ethernet Networks
Network Type
.a
.g
.b
Nominal data rate
Theoretical throughput
.
.
.
Measured throughput
.
.
.
Note:
All data rates and throughputs are in Mb/s.
