Questions:- What is LinkSentryTM how does it work?
- Why will some 10/100 unmanaged hubs and switches not work properly with my MiLAN media converter?
- What does the Auto-negotiation jumper do? Does it make the converter run in 10 or 100 mode?
Answers:What is LinkSentryTM how does it work?- LinkSentryTM is the mechanism that monitors the receive links and disables the transmit on the opposite media when the link is down, and enables transmit when link is up. This allows end devices to know when either a cable or device has gone down. If LinkSentryTM was not available, then the converter would always generate a signal and the device connected to it would not know if the fiber had broken, or if the far end converter or device had failed. With LinkSentryTM this information is passed through allowing for graceful failover to a backup port if needed. The Classic Converters have LinkSentryTM hard wired in their design. There are some situations, see the next question, where it may be necessary to generate the link signal on the UTP port even if the fiber port is not up yet. The MIL-C2000 series of converters have dip switch selections to allow for disabling the LinkSentryTM feature.
Why will some 10/100 unmanaged hubs and switches not work properly with my MiLAN media converter?- There are three main ways unmanaged 10/100 devices configure their ports for speed, switch selection per port, auto-sensing and auto-negotiation. The LinkSentryTM and the auto-negotiation features of the converter affect these methods. The media converter automatically senses if there is no link on the fiber receive and disables output on the UTP port, and sense if there is no link on the UTP port and disables transmit on the fiber port.
If a switch selectable 10/100 device is set to 10 Mbps, then the converter will not see a link and will turn off the fiber transmit. The lack of a link LED on the converter for the TX indicates that either the hub/switch is set for 10 Mbps. If, in the case where two converters are connected together using fiber cables, and the UTP cables are each connected to malfunctioning hubs or switches, then no link will appear on any of the devices. Set the hubs at both ends to 100 Mbps mode to solve the problem.
The next level of sophistication after switch selection is auto-sensing. The device monitors incoming traffic and if it sees 100 Mbps pulses on a port, then the port is put into 100 Mbps mode. If it sense 10 Mbps pulses on a port, then the port is put into 10 Mbps mode. If no signal is seen, the port may either default to 10 Mbps or 100 Mbps. The IEEE 802.3 standard says that the port should default to the highest possible value. Some devices assume that they are going to connect low end 10 Mbps product, so they set to 10 Mbps. Other, correctly designed devices, default to generating 100 Mbps fast link pulses.
This becomes critical when two devices using auto-sensing are interconnected via two of the C100 series media converters. If a device defaults to 10 Mbps, then the converter attached to it will see no valid 100 Mbps signal, and will disable the output on the fiber
side. This will cause the far end converter to turn off transmit on the UTP side connected to the far end device. That device, not seeing a 100 Mbps signal, will generate a 10 Mbps signal, which the converter will ignore causing the LinkSentryTM feature to disable the fiber transmit. This stalemate will continue indefinitely. Either one of the two devices needs to be swapped with a more sophisticated networking device that allows for forcing the port to 100 Mbps, or utilize the higher end C2000 series converter which allows for disabling LinkSentryTM allowing for generating the 100 Mbps signal the devices need.
If the converter is connected to a device that auto-negotiates speed and duplex mode, then the converter will generate the signal indicating it supports 100 Mbps, full duplex mode of operation. If there is no active signal coming in on the fiber port, then the UTP port transmit is disabled, and the converter will not send the auto-negotiation signal. For the converter to operate properly, then all connections need to be made and the devices at both end need to generate 100 Mbps signals. One concern is that one of the devices support 100 Mbps full duplex mode with auto-negotiation, and the other supports 100 Mbps half duplex only. In that case, the auto-negotiation jumper needs to be changed to disable generating the auto-negotiation signal. This will cause the auto-negotiation device to operate in 100 Mbps half duplex mode.
The rack mount versions of these converters have dipswitch selections to enable or disable the LinkSentryTM feature. The MIL-C211x series converters or MIL-S311x two port switches are alternative products with dipswitch selections to enable or disable the LinkSentryTM feature, eliminating the issue with the malcontent hubs and switches.
What does the Auto-negotiation jumper do? Does it make the converter run in 10 or 100 mode?- The Auto-negotiation jumper allows for sending or not sending the auto-negotiation signal on the 100Base-TX port. The converter is designed to generate a fixed signal indicating that 100Base-TX full duplex mode is support. This will help devices that support auto-negotiation to communicate with our converter and select the highest level of performance. If a device does not understand the auto-negotiation handshake, this feature has no effect. The converter operates in 100 Mbps mode; it does not support 10 Mbps mode of operation. 10 Mbps traffic is dropped.
Half or full duplex mode does not affect the media converter, it always operates in full duplex, with the two end points determining what happens to the traffic. If either end of the link, or both ends support only half duplex mode, it is highly recommended that the auto-negotiation feature be disabled on the converter. This will eliminate the potential for one end negotiating to full duplex and the other running in half duplex, causing unnecessary collisions.
For those who do not wish to worry about whether or not a device is 10 or 100 Mbps or half or full duplex, then the MIL-S3000 series of two port switches with LinkSentryTM is the recommended product.
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