Advanced Configuration Features
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Horizon Compact Release 1.01.00 Wireless Ethernet Product User Manual – Volume 2
3.9 Rapid Link Shutdown (RLS)
Networks containing alternate or redundant routing paths will typically rely on protocols such as Rapid
Spanning Tree Protocol (RSTP) to invoke a reroute when communications to the far end unit fail or
experience high data error rates. The protocols are implemented on the Ethernet switch and rely on
polling or messaging to determine that communications to the far end has been disrupted. Although very
effective, these protocols can result in slow reaction times to determine link issues. It is often desirable to
inform a switch or router of a network issue in the quickest manner possible. The Horizon Compact
system provides the Rapid Link Shutdown feature for this purpose. Notification to the switch or router is
provided by shutting down the Ethernet ports connecting the Horizon Compact systems to the network.
The Ethernet ports at both ends of the link are shut down. The Ethernet switch immediately recognizes
the loss of Ethernet connection and invokes its alternate path algorithm.
The default setting for RLS is OFF, meaning that the Ethernet port will remain connected during failure
conditions.
Notes:
1. RLS cannot be invoked if Automatic Adaptive Modulation (AAM) is enabled, or if 1+1
Redundancy is enabled.
2. RLS should be disabled during link alignment to prevent the condition where a link goes in and
out of synchronization resulting in the RLS being invoked and released causing loss of Ethernet
management to the systems.
3. A disconnected Ethernet cable at one end of the link will trigger an RLS event and will result in
the Ethernet port on the other end being shut down if RLS is enabled.
Determination of Error Rates on Horizon Compact Systems
Horizon Compact systems employ forwards and backwards error correction to compensate for and
correct errors occurring over the RF link. Once these errors are corrected, the user data can flow error-
free. The post-correction data error rate is reported as "Modem Block Receive Errors" within the Horizon
Compact, accessible through traffic statistics in CLI or through the Performance web page. Modem Block
Error Rate is also available and is reported in scientific notation.
The Horizon Compact systems communicate with each other via “modem blocks” at all times even if there
is no Ethernet traffic. The modem blocks are transmitted at a fixed rate and therefore a modem block
error rate is also available. The incoming Ethernet data stream is treated as a bit stream and sub-divided
into modem blocks for transmission over the RF link. Modem blocks are continuously transmitted to the
far end modem regardless of whether or not Ethernet traffic exists. Modem blocks are less than 256
Bytes in length including overhead. The modems transmit approximately 59,000 blocks per second on a
100 Mbps link. In order to determine a modem block error rate of 1x10e-6 there would have to be a
minimum of 1 million modem blocks transmitted. At 59,000 blocks per second, it would take 16.8
seconds to transmit 1 million blocks. A facility exists in the Horizon Compact that allows the sample time
to be shortened (see “soft” Error mode below).
Horizon introduces enhancements to the DragonWave RLS functionality consisting of:
• The fade monitor (“soft failure” mode) has been simplified to analyze the modem block error rate
over time increments. RLS will trigger when the user-configurable error rate threshold is
exceeded
o The enhancements include the addition of a “make” sample time parameter (“make”
relates to the re-establishment of the link), whereas previously the same sample time was
used for both “make” and “break” (“break” relates to the shutting down of the link).
• A “hard failure” mode has been introduced that triggers on a percentage of errored packets on a
link. This mode is applicable to link outages or severely degraded data transfer.
o The default value of error threshold is a loss of 50% of modem blocks (packets) over a
default sample time of 50 msec. The sample time and error threshold are user-
configurable.
• Enhancement to RLS Advanced Mode providing support for a combined “hard”/”soft” failure mode
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