Fiber: Review of Optics, Cables & Connectors

When I started this blog, I said I wanted to give you useful information, sometimes in the form of lengthy technology overviews, and sometimes in short snippets. I like to dig around the search terms, comments and emails to see what you want to know more about, and I’ve seen a lot of interest in fiber information.

The fiber types (such as multi-mode, single-mode), standards (SX, LX, LH) and  connectors (LC, ST, SC) seem to be topics that need clarification about 80% of the time when we’re working with customers on networking equipment or site surveys.

Here’s a brief review of the various types of fiber, optics, connectors and when to use what. Let’s start with the basic stuff, and move down the line.

Multi-mode vs Single-mode
First of all, we have multi-mode and single-mode fiber. Multimode has a larger diameter ‘core’ or the area in the middle the light travels through. The larger diameter- think of it as a big tunnel- lets the light take different paths, creating multiple rays, or modes (hence multi-mode). The light bounces around more, which means the connectors and splices for multimode are more forgiving than for singlemode, but the bouncing causes dispersion and fidelity loss. On the other hand, singlemode has a much smaller diameter core, giving the light one straight path, or mode, through the cable. Because of this, singlemode offers higher throughput and longer distance, but the light equipment and connectors are much more finely-tuned. Which, of course, means singlemode is much more expensive.

When you’re adding or surveying multimode fiber, you should know what core size you’re working with. The core size affects bandwidth and the maximum distance you can reliably run it. Multimode usually comes in 50- or 62.5-micron, which is the core diameter. The larger the core size, the more bouncing, so the shorter distance you’ can go. To give you a general comparison, most singlemode comes in 9-micron core, which is about 1/6^th the diameter of multimode. The smaller the diameter, the more precision you get.

When to use what. In short, the fiber type you choose will depend on 1) budget and 2) distance. Mostly, you’ll use multimode for short fiber runs, between switches, to servers and possibly between buildings, if they’re adjacent. You should use singlemode when you need higher throughput or a longer distance. Here’s a quick look at the types and maximum distances for each. I’ve also included a proprietary rating, for connectors using 1550nm wavelength over singlemode fiber, to get increased distance. (Standard for singlemode is 1310).

• Multimode – up to 220m with 62.5 micron core
• Multimode – up to 550m with 50 micron core
• Singlemode – up to 5km-10km (standard, using 1310nm optics)
• Singlemode – up to 70+km* (proprietary, using 1550 nm optics)
  

Fiber Optic Standards
You’ll need to know the type of optic to specify for your network equipment. Some vendors have their own proprietary fiber optics, but the standards are 1000Base-SX for multimode, and 1000Base-LX for singlemode. You can use multimode with 1000Base-LX with the addition of a mode-conditioning cable to set the light along the correct path down the cable. LX, which is standard, uses the ~1310nm wavelength. Vendors have created 1000BASE-ZX and 1000BASE-LH, which use the 1550nm optics to obtain longer distances. Note, here we’re talking about 1-Gig fiber, not 10GbE, hence the 1000Base. We usually just refer to these as SX, LX and LH, leaving off the 1000Base– when talking about the optics.

• 1000Base-SX – multimode
• 1000Base-LX – singlemode standard (can be used over MM with mode-conditioning cable)
• 1000Base-LH – singlemode non-standard (proprietary for longer distances at 1550nm)

Connectors
Here’s the fun part, and no one remembers what connectors they have (if they even knew in the first place!). There are several out there, but you’re probably going to only ever run into three – LC, ST and SC.

I’ll start with LC since that’s usually found on switches and other network equipment these days. LC stands for ‘Lucent Connector’ (the creator) and is the connection type on SFPs (Small Factor Pluggable) or Mini-GBICs. They’re small, and were designed to replace the SC connectors.

Since I mentioned SC, let’s go there next. SC, or ‘Standard Connector’ are the predecessor to LC, and are similar in shape, but quite a bit larger. We suggest using the mnemonic ‘Square Connector’ to remember SC.

Last- and possibly least- we have ST, which really means ‘Straight Tip’, but many folks have a better time thinking of ‘Stab and Twist’. You stick it in and lock it in place by turning the outer barrel, sort of like BNC did. And yes, I’m old enough to remember the BNC days ;)

Duplex and Simplex
Most often, you’ll be using duplex fiber, which consists of a pair of fiber for bi-directional communication. Then- of course- you would use simplex fiber cables if you only need to send data a single direction. Those applications are more specific, but they do exist. On duplex cables, you’ll noticed connectors that aren’t fixed-form will be marked or color-coded, one for transmit, one for receive.

Ordering Fiber Cables
If we’re translating all our acronyms and numbers into something we can use, then let’s talk about how you put it all together when you’re looking for the right cables.

For example, let’s say you’re purchasing short fiber jumpers for connecting your patch cable to your switch. Most likely, you’ll want multimode, in a short length (2meters), with LC on the end going to the switch and let’s say SC on your patch panel. In our example, we’re assuming we have 62.5micron mm fiber.

What you’ll ask for is: Fiber jumper, 2 meters, duplex, 62.5-micron multimode, LC to SC.

LC	SC	ST

These are the best images I found to demonstrate the shapes and orientation of the various duplex fiber connectors we talked about. You can find these images and descriptions at Cables To Go.

Wowzers, I said this was going to be a short one. In fact, this post was originally titled “Fiber: A Very Brief Review of Cables & Connectors” but I had to rename it ;) Oh well- now you have all the information in one place for future reference.

# # #