Most existing 1G short reach interfaces were installed using 62.5 micron core multimode fiber with a modal bandwidth of 200 MHz*Km (called OM1 fiber). This fiber was sufficient for 1G transmission over 200 meters (656 ft). ‘Laser optimized’ OM2 fiber with 500MHz*Km modal bandwidth, allowed transmission distances of up to 500 meters (1,640ft) at 1Gbps. However, network operators upgrading their 1G links to 10G who assumed they could use a short reach 10G transceiver on that existing OM1 fiber were in for a rude awakening.
At 10Gbps, the individual optical pulses composing the transmission are 10 times closer together than at 1Gbps. Therefore, the pulse-spreading effect of modal dispersion becomes a problem proportionally sooner (distance-wise). As shown in the table below, a standard 10G short reach (850nm) transceiver will only reliably link over about 33 meters (about 108 feet) of OM1 fiber.
|MMF Type||Modal BW||1000Base-SX||10GBase-SR|
In a perfect world, an upgrade from 1G to 10G would simply involve installing the new 10G equipment as close as possible to the older 1G gear, then unplugging the fiber from the 1G interfaces and plugging it into to the new 10G transceivers. As shown in the table above, that migration technique becomes problematic even on relatively short links.
To solve this problem a new type of 10G interface has been developed, referred to as “LRM”, short for Long Range Multimode. These interfaces are designed specifically for backward compatibility with OM1 fiber. LRM interfaces transmit in the 1300nm window and are capable of the same transmission distance over OM1 or OM2 fiber. The following table shows the capability of 10G LRM interfaces compared with 1G short reach.
|MMF Type||Modal BW||1000Base-SX||10GBase-LRM|
As shown above, the new 10G LRM are actually capable of greater reach than 1G transceivers over OM1 fiber! Over OM2 and OM3 fibers, the 10G-LRM transceivers do not quite match 1G performance but certainly expand the number of links for which fiber upgrade is avoided.
The key to the new LRM interfaces is the use of sophisticated signal processing technology in the receiver portion of these devices. The specialized signal processing is referred to as Electronic Dispersion Compensation or EDC. EDC is accomplished in the electronic rather than the optical domain. An EDC chip is located immediately behind the optical detection system and implements a continuously adaptive filtering technique known as Continuous Time Filtering.
It should be noted that in some cases, to achieve the maximum distances shown above, mode conditioning patch (MCP) cables may be required. Mode conditioning fiber precisely controls the launch angle/location of the singlemode fiber (SMF) LRM source into the multimode fiber (MMF).
As shown in the diagram above, the mode-conditioning fiber is a special assembly that precisely offsets the SMF centerline with the MMF centerline. MCP, if needed, are only used when connecting a 1300nm source to MMF.
It is generally not known beforehand when an MCP cable will be required. If a substantial number of links must be upgraded from 1G to 10G and these links are 150 meters or more in length, it is good practice to have several MCP cables on hand. If, when moving a link from a 1G interface to a 10G, the new span fails to link up, an MCP may be inserted on the transmit side of one or both ends of the link.
FLUXLIGHT LRM SOLUTIONS
Fluxlight, the price performance leader in optical transceivers, offers 10G LRM interfaces compatible with a wide variety of OEM switch manufacturers equipment, the following are links to some of these:
MANUFACTURER PART NUMBER
For Mode-Conditioning Patch cables, contact FluxLight at email@example.com or 888-874-7574 for special order.
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