This whitepaper takes a deeper look into the various fiber optic cable and connector types used in modern networks, their specifications, benefits and draw-backs. It details typical applications and use in data center settings.
- Understanding Fiber Optic Cables
- Understanding Fiber Optic Connectors
- Understanding Measurements and Classifications
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There are many types of cables used in data centers.
Depending on what you are trying to accomplish, the type of data center build and what equipment you’re deploying, you have many options available.
A change of cable type may also be desired when executing moves, adds or changes in an existing data center, and as technology advances, new options to consider and integrate.
2. Understanding Fiber Optic Cables
2.1 Quick Reference Guide – Fiber Optic Cable Types and Attributes
|AOC – Active optical cable||Conducts light between components, thin cable with a high bend radius, typically used in high data rate applications (up to 40Gbps)|
|APC – Angled polish / Angle physical contact||High-precision fiber optic cable that reflects light back at the source at approx. 8 degrees, typically used in applications sensitive to return loss|
|DAC – Direct attach cables||Carries electrical current between components|
|DUPLEX||Fiber optic cable with two strands, a transmitter and receiver at both ends, can carry information in two directions and the flow of data can be reversed at any time|
|HALF-DUPLEX||Fiber optic cable with a transmitter and receiver at both ends but can only transmit data in one direction at a time|
|MMF – Multimode Fiber||Multimode fiber optic cable with a larger core size than singlemode that allows multiple light signals to be transmitted simultaneously, typically used over shorter distances|
|SMF – Singlemode Fiber||Singlemode fiber optic cable with a small core that allows only one light signal to be transmitted, typically used over longer distances|
|UPC – Ultra polish / Ultra physical contact||Fiber optic cable that reflects light directly back at the source|
2.2 Singlemode (SMF) vs. Multimode (MMF) Fiber Optic Cables
As bandwidth demand increases, a large number of data center managers may feel that singlemode cables are the definitive answer for the future. And to be fair, they do carry a lot more data over longer distances than multimode fiber cables. The real difference between the two is how they transmit light: singlemode fiber cables allow only one ray of light to be transmitted, while multimode fiber cables have several strands in a larger core that allow more “rays” of light to be transmitted simultaneously.
However, the key element in that phrase is “over longer distances.” When it comes to enterprise-level data centers, multimode cables are just as effective for most applications where less distance is involved, and they cost a lot less than their singlemode counterparts.
This cost is felt in the needed transceiver as well. A singlemode optical cable has a small core size, meaning the beam of light it transmits must be much more focused than that needed for a multimode cable, thus leading to the more expensive transceiver.
Multimode cables, at least at the time of this writing, can still handle high-speed data demands at distances less than 500-600 meters. That means that almost any cable internal to an enterprise-level data center can still be multimode and function well.
That being said, there are less bend-sensitive and full-spectrum singlemode cables that offer more bandwidth and are less sensitive to handling of the patch cords. There are also more transceiver options as a result. As these cables get better and more affordable, they may become more common in shorter distance applications.
2.3 Simplex vs. Duplex Fiber Optic Cables
A simplex cable is used when information or data only needs to flow in one direction, not more than one. So for instance, if you had both a transmission and a return path, you would need two simplex cables. This type of cable is also not reversible – it’s like a one-way street. Information can travel from the transmitter to the receiver, but the moment they are reversed, the cable will no longer function properly.
On the other hand, a duplex cable has two strands and can carry information in two directions using a single cable. Both ends have both transmitters and receivers, and the flow of data can be reversed at any time.
There are two types of duplex cables, however. Although they can be reversed, half-duplex cables can only transmit data in one direction at a time. So while there can be transmitters and receivers at both ends, like the center carpool lanes in many large cities, the lane is only open to traffic in one direction at a time.
A full-duplex cable, however, can transmit data in both directions at one time. So what is the difference when it comes to practical applications? Well, that depends almost entirely on the systems you are using.
Some systems require a simplex and even a singlemode simplex cable. Others actually require a full-duplex cable for the data they will transmit. Essentially, you need to determine what the equipment needs are and cable accordingly.
There are SC, ST, LC and other connectors on simplex and duplex cables, and they can be used in a variety of applications as a result. But besides connectors and simplex vs. duplex, other things matter in the area of cable selection as well.
2.4 DAC and AOC Cables
DAC, or direct attach cables, carry electrical current directly from one component to another. Data rates support some applications and they are better than copper, in the neighborhood of 4Gbps – 10Gbps. Additionally, DAC cables don’t generate heat.
On the other hand, Active Optical Cables, or AOC cables, conduct light rather than electricity, and so are immune to electrical interference. They can handle even higher data rates, up to 40 Gbps. The other advantage to these thinner cables is that they have a higher bend radius for use in high-density data centers, and also due to this, there is more space for airflow.
Either cable has a place in modern data centers, again depending on your specific application and data center build.
2.5 Fiber Optic Cable Jacket Colors
Color codes are used in fiber optics to identify fibers, cables and connectors. When a tech opens a fiber optic cable to prepare it for splicing, they will find a colorful bundle of buffer tubes. Color codes are especially important when making connections by splicing. Each splice tray has 72 splices so the arrangement of the colored buffer tubes and the colored fibers is used to keep all the connections correct.
Perhaps nothing is more complex in fiber optics than maintaining polarity of fibers when using multi-fiber array connectors of the MPO type. In the TIA-568 standard that covers fiber polarity, MPO polarity takes almost 40 pages to explain (so we won’t go into it here).
Patch cords used with patch panels can easily get mixed up. Standards use color codes for fiber and connector types to make it easy to find the right patch cord.
There is a color code standard in TIA, TIA-598 that addresses fiber optic color codes, which most manufacturers adopt and reference, although there are many exceptions based on customer requirements or preferences. Here is what TIA-598 recommends:
Colored outer jackets and/or print may be used on Premises Distribution Cable, Premises Interconnect Cable or Interconnect Cord, or Premises Breakout Cable to identify the classification and fiber sizes of the fiber. (Outdoor cables are generally black for protection against UV light and markings are printed on the cable.)
When colored jackets are used to identify the type of fiber in cable containing only one fiber type, the colors shall be as indicated in Figure 1. Other colors may be used providing that the print on the outer jacket identifies fiber classifications. Such colors should be as agreed upon between manufacturer and user.
Unless otherwise specified, the outer jacket of premises cable containing more than one fiber type shall use a printed legend to identify the quantities and types of fibers within the cable. Figure 1 shows the preferred nomenclature for the various fiber types, for example “12 Fiber, 8 x 50/125, 4 x SM.” Some manufacturers use black as the jacket color for hybrid or composite cables. When the print on the outer jacket of premises cable is used to identify the types and classifications of the fiber, the nomenclature of Figure 1 is preferred for the various fiber types.
|Fiber Type||Non-Military Applications3||Military Applications||Suggested Print Nomenclature|
|Multimode (50/125) (OM2)||Orange||Orange||OM2, 50/125|
|Multimode (50/125) (850 nm Laser-optimized) (OM3, OM4)||Aqua||Undefined||OM3 or OM4, 850 LO 50 /125|
|Multimode (50/125) (850 nm Laser-optimized) (OM5)||Lime Green||Undefined||OM5|
|Multimode (62.5/125) (OM1)||Orange||Slate||OM1, 62.5/125|
|Single-mode (OS1, OS1a, OS2)||Yellow||Yellow||OS1, OS1a, OS2, SM/NZDS, SM|
|Polarization Maintaining Single-mode||Undefined||Undefined2|
Source: Fiber Optic Cable And Connector Color Codes, The Fiber Optic Association Inc. (FOA) Reference Guide
- Natural jackets with colored tracers may be used instead of solid-color jackets
- Because of the limited number of applications for these fibers, print nomenclature are to be agreed upon between manufacturer and end user
- Other colors may be used providing that the print on the outer jacket identifies fiber classifications
- For some Premises Cable functional types (e.g., plenum cables), colored jacket material may not be available
According to the The Fiber Optic Association (FOA) users have been installing hybrid (MMF+SMF) cables in the backbone for years. With the premises fiber optic cabling now including several varieties of 50/125 fiber, 62.5/125 and singlemode fibers, managing the cable plant is more difficult. FOA report having seen instances of users and installers being confused and getting bad test results, as well as having problems with networks operating when connected over the wrong fiber type. Connector color codes may be used to identify fiber type also. If unsure about the fiber, core size can be determined by examining the connector ferrule with a fiber optic inspection microscope while illuminating the fiber with a white light (flashlight). For fuller details, read this document that defines the twelve TIA/EIA colors for fiber conductors.
3. Understanding Fiber Optic Connectors
3.1 Reference Guide – Fiber Optic Connector Types and Attributes
|BACK REFLECTION||How much light bounces back as light travels from one fiber to the next|
|LC CONNECTOR||Also known as a Lucent connector or Small form factor connector. Fiber optic cable connector (1.25mm housing) with a latch mechanism that connects to data center devices|
|POLISH||Refers to the back reflection level of a connector, measured in db|
|REFLECTANCE||Typically a negative value of a connection (two mated connectors)|
|RETURN LOSS||Also called insertion loss. The amount of light reflected from a single discontinuity in an optical fiber link, such as a connector pair, typically of a positive value|
|SC CONNECTOR||Also known as square connector or standard connector. Fiber optic cable connector (2.5mm housing) with a locking tab|
|ST CONNECTOR||Also known as straight tip connector. Fiber optic cable connector with a round ferrule, typically used in limited space applications|
3.2 LC to LC Connections
The LC refers to a specific type of connector developed by Lucent Technologies and later standardized in EIA/TIA-604–10. The connector is now made by other manufacturers, although still referred to as a Lucent Connector, or sometimes a “small form factor connector.” The purpose was to replace the SC connectors (more on that in a moment) with something smaller that offered a secure connection and as little insertion loss as possible.
The LC is half the size of the SC, at 1.25 mm, and offers a latch mechanism that enables the technician to ensure a “pull-proof” connection in system rack mounts. The push and latch design is the real key.
Of course, an LC to LC patch cord can come in several types, including:
- Simplex, duplex, or multi-fiber assemblies
- Polish / physical contact type of connectors
- Custom fiber optic cable lengths and jacket colors
- 0.9mm, 1.2mm, 1.6mm, 1.8mm, 2.0mm, and 3.0mm outer diameter cables
- Different jacket types, such as riser, PVC, plenum-rated, or LSZH – all RoHS compliant
- Various fiber types and wavelengths, typical 9/125 single-mode fiber (OS2) (SMF), 50/125 (OM2, OM3, OM4, OM5) and 62.5/125 (OM1) multi-mode fiber (MMF)
Polish or physical contact type refers to how much light bounces back as light travels from one fiber to the next, called back reflection (measured in db). Anything over 60db is ineffective in modern networks:
- PC – 40db or better
- UPC – 50db or better
- APC – 60db or better
Polarity defines direction of flow, such as the direction of a magnetic field or an electrical current. In fiber optics, polarity is directional; light signals travel through a fiber optic cable from one end to the other. A fiber optic link’s transmit signal (Tx) at end of the cable must match the corresponding receiver (Rx) at the other end. Here’s a good resource that goes through the basics of fiber polarity.
In addition, there are other even more specialized applications. There are armored patch cables for applications where more durability is desired or even needed, and there are also multi-core LC to LC cables for various applications, including video and other applications.
LC to LC cables are used in LAN, WAN, CATV, and other applications. LC cables can be made with other types of connectors as well, for flexibility in a variety of applications.
3.3 Common Connector Types
The Lucent Connector or LC is just one of several types of connectors. They can be combined in a number of ways and may be suitable for different applications or adaptations.
3.3.1 The SC Connector
SC and LC connectors are perhaps the most common of all connectors. Most connectors have been developed for a specific hardware or application. While the formal name of the SC connector is the subscriber connector, it has also been called the square connector or standard connector. No matter what you call it, the application is the same.
The connector comes with a locking tab and is very good at aligning optical cables in connections. Also called “stick and click” it does just that. Insert the male connector into the female end, hear a click, and you know your connection is secure. They are especially handy for applications where there is a risk of return loss, such as video delivery over RF.
The big con of SC connectors is that, well, they are big. Twice as large as LC connectors, they have a 2.5 mm housing, one that makes it less suitable for densely populated racks or server cabinets.
3.3.2 The ST Connector
This brings us the ST, or straight tip connector. Rather than a square housing, it has a similar round ferrule that still aligns and latches securely. Larger than the LC, the round design makes it easier to deploy on racks and in tight spaces. In fact, for very limited space applications, a 90-degree option is available, often solving some simple logistical challenges.
3.3.3 SC-ACP and LC-UPC Cables
The simplified explanation of the difference between these two cable types is the manufacturing process and return loss. Think of it this way: an APC cable reflects light back at the source at an angle, approximately 8 degrees. This is ideal for applications that are much more sensitive to return loss, and require a high precision signal, such as FTTX (Fiber To The X), video delivery through RF signal, WDM (wavelength division multiplexing) applications and analog equipment like CCTV.
A UPC connector reflects light back directly at the source and is better for applications less sensitive to return loss with no other particular demands. In these cases, the LC-UPC connector can work as effectively as an SC-APC connector and at a lower cost. It is also worth noting that like other connector types, there is a significant size difference, so for servers or racks with a lot of connections, where space is a premium, the lower profile LC-UPC is often a better choice.
3.4 The Difference Between MTP and MPO Connectors
While these terms are often thrown about interchangeably, the primary thing to understand is that MTP is a registered trademark. It refers only to MPO cables made by US Connec. All MTPs are MPOs, but the reverse is not true. MTP stands for Multi-Terminal Push-On connectors.
- MTP connectors are built with metal pin clamps that help center the push spring
- The MPO connector has chamfered guided pins that can chip the ferrule and cause the material to drop into the guided pin holes or on the ferrule end face
MPO is a type of cable connection referred to as Multi-Fiber Push-On. It is used to terminate multi-fiber cables in an indoor environment only. There are male and female MPO connectors. While non-MTP MPO cables may look identical to MTP, there are some small differences, and they are not always compatible.
3.5 Fiber Optic Connector Colors
Since the earliest days of fiber optics, orange, black or gray was multimode and yellow singlemode. However, the advent of metallic connectors like the FC and ST made connector color coding difficult, so colored strain relief boots were often used.
|Fiber Type||Connector Body||Strain Relief/|
|50/125 laser optimized (OM3, OM4)||Aqua||Aqua|
|OM5 wideband fiber||Lime||Lime|
Source: Fiber Optic Cable And Connector Color Codes, The Fiber Optic Association Inc. (FOA) Reference Guide
4. Understanding Measurements and Classifications
4.1 Reference Guide – Fiber Optic Measurements and Classifications
|GAUGE||Refers to wire diameter, current the wire can carry, weight and resistance (measured in Ohms)|
|OM||Standard used to classify multimode cable|
|OS||Standard used to classify singlemode cable|
4.2 Fiber Optic Cable Sizes
In this case, the very term size can be deceptive. We’re going to look at three different cable “sizes”: 9/125, 50/125, 62.5/125. The 125 on all of the following cable types indicates the size of the cladding of the cable (125 microns). However, the real differences lay on the inside.
First things first: 9/25 refers to singlemode cables, while 50/125 and 62.5/125 refer to multimode cables. As discussed previously, those differences will depend on the application you are using the cable for. Because of that, we will look more in-depth at the differences between the legacy 62.5/125 cable and the newer 50/125 multimode cable.
What’s changed since 62.5/125 was the standard? Well, the major difference is the adoption of the Gigabit Ethernet standard. However, that does not mean you should replace every one of your legacy 62.5 cables right away. You can still achieve the Gigabit standards without taking that drastic step.
First, the real difference in the cables is the diameter of the core, or light carrying region of the cable, 62.5 and 50 microns respectively. What does this mean to you? Well, there are three factors to consider:
- Power consumption
Example: the 50/125 cable has nearly triple the bandwidth of a 62.5 cable. (500 MHz-km vs. 160 MHz-km at a distance of 850m) This means the 50/125 cable can transmit more data over a longer distance. What’s the downside? The 50/125 can cause a power budget reduction in LED-based applications, reducing the number of links allowed in a link.
Transmitter and receiver characteristics also play a role. Using more expensive 1300-nm lasers rather than the newer and less expensive 850-nm lasers such as vertical-cavity surface-emitting lasers (VCSELs) can enable you to achieve the same results using 62.5/125 cables.
The cost savings on the lasers needs to be weighed with the potential cost of replacing most or all of an existing cable structure. Of course, if you are starting from scratch rather than working with an existing data center, the decision is much simpler.
4.3 OS and OM Classifications
First, as with our discussion above, we first need to be sure we are comparing like items. OS always refers to a singlemode cable, and OM always refers to a multimode cable. Therefore, of this group, the OS2 is the only singlemode option.
There is an OS1 cable option, but it is antiquated and has been phased out of most existing builds, and simply is not used in new ones. It is designed only for distances less than 2km and with a transmission speed of just 10Gbps. These limitations in distance and bandwidth don’t meet the needs of today’s modern data centers.
When it comes to OM cables, there are 5 classifications OM1-5. OM1 is the older of these players, but despite its age, it is still in use. The reason is the cost is much more affordable for short runs. It’s a 62.5/125 core (see above), The others all work with a 50/125 core.
The second difference is the transmission method. OM1 and OM2 use LED transmitters, which results in a limitation on bandwidth. The max you can get from these cables is around 1Gbps. The others use laser transmission which is more expensive but offers a much wider bandwidth.
- OM3 up to 10 Gbps
- OM4 up to 40 Gbps
- OM5 up to 100 Gbps
There are of course other distances, but the primary thing to remember is that OM1 and OM2 are usually used in shorter distances with lower bandwidth requirements. OM3 – OM5 are used in higher bandwidth applications, but at least the initial cable cost is higher.
4.4 Wire Gauge
Gauge simply refers to the diameter of the wire and determines the amount of current the wire can carry, the weight of the wire, and the resistance (measured in Ohms). The larger the wire, the smaller number the gauge carries. For instance, a 24 AWG wire is nearly twice as thick as a 28 AWG.
In general, the larger the cable, the less resistance it has. This also means the cable creates less heat. These cables are best for longer distances, whereas shorter, thinner cables offer some significant cooling and space-saving benefits. There are some cables out there designated “thin” such as the 28 AWG skinny. These cables offer the same characteristics as their full-sized counterparts but can be 25% thinner.
Thin cables also provide more visibility at the connection point to the server and are often used in those applications for that reason coupled with the airflow and cooling advantages mentioned above. Future thoughts include cables with different insulating materials and a move toward more “skinny” or “thin” cables for short cable runs and better options for longer ethernet cable runs and applications.
The key takeaway of this guide is that when doing moves and changes or even developing a data center plan from the start, you need to know your cables. Understand your application and needs on a deep level, and have a cable management plan. If you are making moves and changes, be sure you have a similar cabling plan, including an overall cable labeling system and nomenclature.
Additionally, the space for cables is constantly changing, and what may be standard practice and true today may not be tomorrow. Keep up with the industry, and when you need to upgrade, be sure to plan accordingly and have the supplies on hand you need to execute those changes.
AOC “Active optical cable”
APC “Angled Polish / Angle Physical Contact”
AWG “American Wire Gauge”, a logarithmic stepped standardized wire gauge system for the diameters of round, solid, nonferrous, electrically conducting wire
BEND RADIUS Measured on the inside curvature, the minimum radius one can bend a cable without kinking it, damaging it, or shortening its life
CATV “Cable television”
CONNECTORS Electromechanical device used to join electrical conductors and create an electrical circuit
DAC “Direct attach cables”
db Unit of measurement for level of ‘Back reflection’
FIBER OPTIC CABLE – High-speed, high bandwidth, short and long distance communication cables
FTTX “Fiber to the X”
LAN “Local area network”
MPO “Multi fiber push-on” cable connector
MTO Trademarked multi-terminal push-on connector
MTP “Multi terminal push- on” connector
Ohms Unit of measurement for level of cable resistance and wire gauge
PC “Simple Polish / Physical Contact”
RECEIVER Component of a fiber optic cable connector that receives data from the transmitter
RF “Radio frequency”
TRANSCEIVER Component of a fiber optic cable connector that send and receives data by converting electrical signals into optical (light) signals and optical signals into electrical signals
TRANSMITTER Component of a fiber optic cable connector that sends data to the receiver
UPC “Ultra Polish / Ultra Physical Contact”. Fiber optic cable that reflects light directly back at the source
VCSELs “Vertical cavity surface-emitting lasers”
WAN “Wide area network” WDM “Wavelength division multiplexing”