Perle Systems Technical Notes
Maintaining a Transparent and Flexible Fiber to Fiber Solution
Fiber to Fiber Media Converters provide a great solution for connecting multimode fiber connections across single mode or multimode fiber. Various products are offered by different vendors. Rudimentary implementations offer a simple transceiver pair. However, while this is a lower cost option, there is a serious drawback. The end to end connection will always appear as if it is up and running even though the fiber connection may be broken or disconnected. The end devices, such as switches, continue to see the connection as healthy and never report a problem to an enterprise network management system.
For mission-critical links, it is important that these connections behave like a real fiber to fiber connection. This is accomplished through an onboard processor in the media converter that continuously monitors the status of the links connected to its transceiver ports. If there is a break or failure in the link, both end devices become aware and react accordingly. An application on a server or a switch can then take the appropriate action should a link fail. A number of Link Pass-Through features are available to provide this link transparency troubleshooting capability.
Perle’s Fast Ethernet Media Converters combine an existing 802.3 standard along with its own Link Pass-Through feature in order to accomplish a transparent link.
With Link Pass-Through, the state of one fiber receiver is passed to the other fiber transmitter. A typical scenario may be that the multimode fiber cable to an important remote fiber switch port is accidently disconnected. The media converter detects that the link is now lost and then drops the fiber connection. In conjunction with the Far-End Fault feature, the remote media converter connected to a fiber switch can now pass the failed state of the connection along the switch which can then send an SNMP alert to its enterprise NMS system so that corrective action can take place.
Far-End Fault ( FEF ) is an IEEE 802.3u standard that is designed to assist fiber gear in detecting link faults on the fiber link. Should the link on the fiber receiver connection fail, the media converter will transmit a Far-End Fault signal pattern over its fiber transmit connection. The media converter continuously monitors the fiber connection for a valid signal. Once the fiber problem has been corrected, the media converter ceases to transmit the FEF.
The action the media converter takes on receiving a Far-End Fault Indication is dependent on the Link Pass-Through switch setting. With Link Pass-Through enabled, the FEF signal received on its fiber receiver will cause the media converter to drop the other fiber link so that the end device gear knows that the fiber link is not completely functional.
Therefore when considering a Fast Ethernet Multimode to Single Mode Media Converter for important network links, it is very important that the media converter be able to present conditions on the entire fiber connection transparently.
Media converters with onboard processors such as Perle’s Gigabit media converters can provide the necessary signaling with the remote peer ensuring that there is always end-to-end transparency.
A Link Mode switch provided on the media converters, defines the behavior of the fiber ports under various conditions. In the default “Normal” position, the media converter will operate as per the natural auto-negotiation behavior of the Gigabit transceiver ( with Fiber Auto-negotiation enabled ). If the user desires to have the fiber ports reflect the state of the peer fiber port, the Link Mode switch can be placed into Smart Link Pass-Through mode -- an advanced feature that provides a true reflection on what is happening on the other port. Smart Link Pass-Through will perform this whether Fiber Auto-Negotiation is enabled or disabled.
Perle SFP to SFP media converters are protocol transparent solutions that enable multiple fiber types and wavelengths to be incorporated in a network. Supporting speeds up to 4.25 Gbps, numerous fiber to fiber mode conversion applications are possible. This results in significant cost saving when compared to replacing an optical blade on network equipment.
When enabled, Smart Link Pass-Through ensures that the link state on a fiber connection is directly reflected through the media converter to the other connection. If the link is lost on one of the connections, the other link will be brought down by the media converter. This feature applies when both SFP slots are occupied.
When disabled, the media converter transmits a 25Mhz keep-alive signal to artificially keep the link up.
If your fiber media conversion project involves links that are mission critical to your operation, it is important that the media converters used are intelligent enough to properly reflect conditions on all parts of the link and provides complete link transparency to network equipment end points.