Gigabit transceivers datasheet with errors?


http://learn.extremenetworks.com/rs/extreme/images/Pluggable-Transceivers-DS.pdf contains on page 3 the information that the MGBIC-LC03 supports 2 km with OM1 cable while at the same time the device supports ONLY 1 km with a better fiber cable of type OM4.
Can you elaborate this scenario "better cable but less distance"?

7 replies

Userlevel 7
Good question...
We've forwarded your question to one of our product directors to review. Hope to have an answer for you soon.
Short answer: The data sheet is correct in specifying the 2km reach for OM1 62.5μ fiber with the gigabit MGBIC-LC03 SFP transceiver versus 1km for the "better" OM2 fiber type. This is a design attribute of the transceiver which has been optimized for OM1 62.5μ fiber.

Long answer: Just to level set - OM1 62.5μ and OM2 50μ multi-mode fibers are graded index fibers. A consequence of the OM1 and OM2 graded index fiber manufacturing, however, is that there is a discontinuity in the index of refraction at the centerpoint of the OM1/OM2 fiber. This is not a big problem for the performance of an 850nm LED or VCSEL laser source which have relatively large diameters for the power (Encircled Flux) launched into the fiber. Since only a relatively small % of the launch power is at the fiber centerpoint, the discontinuity there is not a major impairment for the overall performance.

A 1310nm laser source, however, is a highly focused light source generally intended for use with single mode fiber. A 1310nm laser, like a 1000BASE-LX gigabit transceiver, can be used with OM1 and OM2 multi-mode fiber but to avoid launching the bulk of the signal power directly into the center defect of the OM1/OM2 fiber, a mode conditioning patch cord (MCP) is required. An MCP offsets the launch centerpoint of a focused laser source away from the center of the fiber.

The MGBIC-LC03 is a version of a 1000BASE-LX transceiver which has been optimized for use with OM1 and OM2 fiber. The MGBIC-LC03 has an integrated centerpoint launch offset which eliminates the need for an added external mode conditioning patch cord - saving cost and reducing connectors and loss. However, the optimal launch offset to use on OM1 62.5μ fiber is different than the optimal launch offset for OM2 50μ fiber. This particular transceiver is optimized for 62.5μ fiber, hence the better reach versus OM2 50μ MMF. In both cases, this transceiver provides a longer reach than a standard SX transceiver over both MMF types or comparable 1000BASE-LX solutions used with an external MCP.

edit: LX, not LR.
Userlevel 7
Ottman, Randy wrote:

Short answer: The data sheet is correct in specifying the 2km reach for OM1 62.5μ fiber with the gigabit MGBIC-LC03 SFP transceiver versus 1km for the "better" OM2 fiber type. This is a design attribute of the transceiver which has been optimized for OM1 62.5μ fiber.

Long answer: Just to level set - OM1 62.5μ and OM2 50μ multi-mode fibers are graded index fibers. A consequence of the OM1 and OM2 graded index fiber manufacturing, however, is that there is a discontinuity in the index of refraction at the centerpoint of the OM1/OM2 fiber. This is not a big problem for the performance of an 850nm LED or VCSEL laser source which have relatively large diameters for the power (Encircled Flux) launched into the fiber. Since only a relatively small % of the launch power is at the fiber centerpoint, the discontinuity there is not a major impairment for the overall performance.

A 1310nm laser source, however, is a highly focused light source generally intended for use with single mode fiber. A 1310nm laser, like a 1000BASE-LX gigabit transceiver, can be used with OM1 and OM2 multi-mode fiber but to avoid launching the bulk of the signal power directly into the center defect of the OM1/OM2 fiber, a mode conditioning patch cord (MCP) is required. An MCP offsets the launch centerpoint of a focused laser source away from the center of the fiber.

The MGBIC-LC03 is a version of a 1000BASE-LX transceiver which has been optimized for use with OM1 and OM2 fiber. The MGBIC-LC03 has an integrated centerpoint launch offset which eliminates the need for an added external mode conditioning patch cord - saving cost and reducing connectors and loss. However, the optimal launch offset to use on OM1 62.5μ fiber is different than the optimal launch offset for OM2 50μ fiber. This particular transceiver is optimized for 62.5μ fiber, hence the better reach versus OM2 50μ MMF. In both cases, this transceiver provides a longer reach than a standard SX transceiver over both MMF types or comparable 1000BASE-LX solutions used with an external MCP.

edit: LX, not LR.

Wow. Excellent answer to an excellent question! Thanks Randy!
Correction of an error in my response above - all references to "1000BASE-LR" should of course be "1000BASE-LX" - apologies for any confusion.
Userlevel 7
Ottman, Randy wrote:

Correction of an error in my response above - all references to "1000BASE-LR" should of course be "1000BASE-LX" - apologies for any confusion.

Thanks. I went ahead and edited your post to show this correction.
Ottman, Randy wrote:

Correction of an error in my response above - all references to "1000BASE-LR" should of course be "1000BASE-LX" - apologies for any confusion.

Randy,
I'm really impressed by the speed and quality of your answer!
Thanks a lot!!!
My question has been answered.
Userlevel 7
I packed up Randy's answer and put it into GTAC Knowledge for reference in the future:
Why does the MGBIC-LC03 datasheet state 2 km distance with OM1 fiber and only 1 km with "better" OM4 fiber?

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