Understanding Distance Limits with Multimode Fiber

Multimode optical fiber is the most common media choice for both backbone and horizontal distribution within the local area network (LAN) including campuses, buildings, and data centers. Let’s take a closer look at the types of multimode fiber options based on bandwidth and distance needs.

 

1 GB/S NETWORKS

The majority of enterprise fiber networks today still run 1000BASE-SX, delivering up to 1 Gb/s over multimode. OM1 cable will support 1000BASE-SX out to 275 meters, and that distance jumps to 550 meters with OM2 cable. OM3 and OM4 came after the 1000BASE-SX standard was written, so the distances up to 860 meters listed in the chart below are based on the gigabit Fibre Channel values. When IT managers require distances upwards of 860 meters, they will likely want to consider single-mode cable instead of multimode.

 

10 GB/S NETWORKS

Many enterprise networks are moving beyond 1000BASE-SX and transitioning to 10 gigabit networks, such as 10GBASE-SR. This is where distance considerations really come into play. A network using OM1 has a maximum distance of 275 meters for 1000BASE-SX, but it would see a distance limit of only 33 meters for 10GBASE-SR. Similarly, OM2 fiber for 1000BASE-SX has a 550 meter limit, but drops down to 82 meters for 10GBASE-SR. The introduction of OM3 increased that distance to a more usable 300 meters in the enterprise.

 

The distance limit for 10 Gb/s over OM4 is listed at 400 meters in the above chart. This limit is set by TIA and IEEE standards based on worse case assumptions. However, these distances can likely extend out to 500 or 550 meters. The 400-meter limit is based on the transceiver having a spectral width of 0.65 nanometers, but most of these transceivers today are 0.47 nanometers, so you can typically extend farther than 400 meters. That’s a conversation you need to have with the cabling manufacturer.

 

40 AND 100 GB/S NETWORKS

When considering multimode for 40 gigabit Ethernet — namely 40GBASE-SR4 using four transmitters and four receivers — you will need an MPO-style connector, and you can’t use older OM1 or OM2 fiber. Also, the distance limits will drop to 100 meters for OM3 and 150 meters for OM4. The original intent of 40GASE-SR4 was for the data center, with the vast majority of the links in data centers under 100 meters. But enterprise links are typically much longer than 100 meters. These networks will likely deploy 10GBASE-SR throughout the campus, and then 40GBASE-SR4 in server rooms or communications rooms.

 

Moving to 100GBASE-SR4 reduces the supported length further to 70 meters over OM3 and 100 meters over OM4, which is why we are seeing an increase in the deployment of OM4 fiber and the consideration of single-mode, as it is not so distance limited.