Siemens CP 1515 - Manual (Page 79)

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Industrial Ethernet Wireless LAN RLM Manager, CP 1515 Manager
C79000-G8976-C171-01
79
In most situations you will see that the number of lost messages will
increase whenever the level of SNR decreases.
The different fields for messages received at the different transmit rates
(e.g. “11 Mbit/s”
, “5.5 Mbit/s” , “2 Mbit/s” and “1 Mbit/s”) may serve as an
indicator for network throughput efficiency.
It is normal behavior for SIMATIC NET IWL stations to retransmit messages
that were lost (either as a result of a frame-collision, or because the test
partner was “out-of-range”):
• If a message transmission fails, your wireless station will retransmit the
“lost” frame.
• If re-transmitting fails repeatedly, the station will switch to a lower data
speed
1 and try to transmit the message again.
The higher the number of messages received with the highest transmit rate,
the better your throughput efficiency. A relatively high number of messages
received at lower transmit rates may indicate:
• Inadequate radio performance, which can typically be related to the level
of SNR, or
• Network congestion. This may typically be the case when the SNR was
rated “Good”
.
In situations where you see a lot of (re)transmissions at lower data rates, the
lower data speed might be the result of:
• A link test partner that is almost “out-of-range” of your CP 1515 Manager
station. This is easily recognized by a low level of SNR.
1. The range of wireless data is related to the data speed. Radio messages transmitted at lower
data speeds will travel longer distances than messages at maximum data speed. In most
network environments, the “Auto Fall-back” transmit rate will yield the best performance
results.