Single Mode SFPs connected to Multimode ?

Answered Question
Jul 28th, 2008

Alright, I'm hoping that someone can confirm or deny this. In our campus, we have a mix of single mode & multimode fiber cabling between floors & buildings. I have always been an advocate of keeping things simple and the same, but unfortunately the cabling plant pre-dates my time and I can't do anything to fix the situation.

Now this is where the question comes in. Another engineer here ordered several 3750's for a standard lifecycle upgrade. However, when he ordered the SFP's for these switches he ordered all Single Mode, long range fiber. I questioned him about this because we are going to have mismatches between the fiber and the transceivers. This individual is very well respected in our organization and so I have to know my stuff whenever I challenge him on technical issues. He is telling me that Single Mode SFP's will operate just fine when connected to Multimode fiber. This just doesn't seem right to me. I belive the transceivers have major architecture differences and that this simply won't work.

Can anyone confirm for me whether you can use Single Mode SFP's on Multimode fiber?

Thank you,

Jim

I have this problem too.
0 votes
Correct Answer by Jerry Ye about 8 years 4 months ago

Hi Jim,

Single mode and multimode are not interchangable. The main reason is the wave length of the laser and core size of the fiber.

However, there is 1 type of the Cisco GBIC which can support both single mode and multimode fiber. Here is the link for more info, please refer to part number "SFP-GE-L or GLC-LH-SM":

http://www.cisco.com/en/US/docs/routers/7200/install_and_upgrade/gbic_sfp_modules_install/5067g.html#wp24482

HTH,

jerry

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (2 ratings)
Loading.
Correct Answer
Jerry Ye Mon, 07/28/2008 - 12:31

Hi Jim,

Single mode and multimode are not interchangable. The main reason is the wave length of the laser and core size of the fiber.

However, there is 1 type of the Cisco GBIC which can support both single mode and multimode fiber. Here is the link for more info, please refer to part number "SFP-GE-L or GLC-LH-SM":

http://www.cisco.com/en/US/docs/routers/7200/install_and_upgrade/gbic_sfp_modules_install/5067g.html#wp24482

HTH,

jerry

Rik Guyler Wed, 07/30/2008 - 04:36

Well, by the book you are correct. However an LC GBIC/SFP will light up MMF for short distances.

If you compare the two types of fiber (MMF vs SMF) you will find the key difference is the number of "modes". Simply put, a mode is a light path. Since LC transceivers are based on a laser transmitter, they have a very tightly concentrated light path so SMF is well suited to handle that type of transmission. SC transceivers use an LED transmitter, which is not so tight and readily disperses. Since LED light disperses, it is able to light up several modes and so uses MMF. This is a much simplified view of things but gets the idea across.

As a result of what I described above, you can use MMF with LC and SMF with SC but your distances will be very much shorter in the end since you're not using the optimal solution. One option that may be useful is the addition of mode conditioning cables. The special cables allow you to use the higher powered LC GBIC/SFPs on MMF. The initial 12 inches or so is SMF but the transmit fiber is connected to a light diffuser and then coupled onto MMF. In the end this gives you a means to gain additional length on the light signal but still use MMF, saving the expense of running SMF.

In this particular case where your co-worker is advocating mixing the two fiber types in the same run, that's a bad idea. If you only have one switch over and the last leg to the GBIC/SFP is very short then it might actually go but certainly not a supported solution. If it does light up, test it for some time and check the interface counters for CRC errors and runts.

Rik

rmalone011 Mon, 01/04/2016 - 08:12

I have some extensive experience in this very situation in a running singlemode lasers over old multimode cable in manhole system of a campus and had limited success with it.  The problem is worsened by longer lengths from pulse spreading on the multimode cable to the point where it becomes unintelligble.

Actions

This Discussion