Simplex or Duplex: What Is the Difference Between Fiber Optic Patch Cords with 1 and 2 Fibers

When choosing a fiber optic patch cord, it is important to pay attention not only to the SC or LC connector type, UPC or APC polishing, length and fiber type, but also to the cable construction — Simplex or Duplex. This parameter shows how many fibers are used in the patch cord: one or two.

Simplex and Duplex are often confused with the number of fibers in a fiber optic cable. But these are different things. Backbone or distribution fiber optic cables can have 4, 8, 12, 24, 48 or more fibers. In fiber optic patch cords, however, two main options are most common: Simplex — one fiber, Duplex — two fibers.

In this article, we will explain the difference between Simplex and Duplex, when a one-fiber patch cord is needed, when it is better to choose a two-fiber patch cord, and what to choose for PON/GPON/FTTH, an SFP module, media converter, optical box or patch panel.

What Simplex and Duplex Mean in a Fiber Optic Patch Cord

Simplex is a fiber optic patch cord with one fiber. Such a patch cord usually has one optical connector on each end: SC, LC, FC or ST. Simplex is often used in PON, GPON, FTTH, subscriber connections and BiDi connections, where signal transmission and reception can work through one fiber.

Duplex is a fiber optic patch cord with two fibers. Such a patch cord usually has two connectors on each end. Duplex is used where signal transmission goes through one fiber and reception goes through another.

Simplex: Patch Cord with One Fiber

A Simplex patch cord has one optical fiber. It is used when the equipment needs only one fiber to operate or when transmission and reception are performed through one fiber using different wavelengths.

Simplex is often used in:

• PON / GPON / FTTH networks;
• subscriber fiber optic connections;
• optical boxes;
• ONU / ONT connection;
• BiDi SFP modules;
• outdoor SC-SC fiber optic patch cords;
• simple lines where one fiber is required.

For example, SC/APC Simplex patch cords are often used for GPON, because the subscriber line usually works over one singlemode fiber.

Duplex: Patch Cord with Two Fibers

A Duplex patch cord has two optical fibers. One fiber can be used for transmitting the signal, and the second one for receiving it. This option is often required for SFP modules, media converters, fiber optic patch panels and active network equipment.

Duplex is usually chosen if:

• the equipment has two optical TX/RX channels;
• the SFP module works over two fibers;
• an LC-LC Duplex patch cord is needed;
• a connection between a patch panel and active equipment is required;
• transmission and reception go through separate fibers;
• multimode OM3 or OM4 fiber is used for short connections.

The most common example is an LC-LC Duplex patch cord for connecting an SFP module, where two LC connectors are used for signal transmission and reception.

Simplex or Duplex: Main Differences

The main rule is simple: Simplex means one fiber, Duplex means two fibers. But the choice depends not on preference, but on the equipment and connection scheme being used.

Do Not Confuse Patch Cords with Fiber Optic Cables

Simplex and Duplex in patch cords are not the same as the number of fibers in a line fiber optic cable. In a ready-made patch cord, the choice is usually between 1 and 2 fibers. But a fiber optic cable for line installation can have many more fibers: 4, 8, 12, 24, 48 or more.

So if a customer is choosing a ready-made patch cord, the question is usually Simplex or Duplex. If a customer is choosing cable for network installation, then the total number of fibers should be calculated with reserve for future connections.

What to Choose for PON, GPON and FTTH

For PON, GPON and FTTH, Simplex solutions are most often used because the subscriber fiber optic line often works over one fiber. In such systems, signals in different directions can be transmitted over one fiber using different wavelengths.

For GPON/FTTH, the following are often used:

• SC/APC Simplex patch cords;
• SC/APC pigtails;
• SC/APC adapters;
• optical boxes with SC/APC adapters;
• outdoor Simplex fiber optic patch cords;
• subscriber FTTH patch cords.

If the connection goes to an ONU/ONT, optical box or GPON subscriber line, SC/APC Simplex is usually the first thing to check. But before buying, it is still necessary to check the equipment port and installed adapters.

What to Choose for an SFP Module

For an SFP module, the choice between Simplex and Duplex depends on the module type. There are standard SFP modules that work over two fibers, and BiDi/WDM SFP modules that work over one fiber.

Before buying a patch cord for an SFP module, check the module itself. If it has two optical openings, Duplex is most often required. If it has one optical port and the module is BiDi, Simplex is most often required.

What to Choose for a Media Converter

Media converters can work with either one or two fibers. It depends on the exact model and optical port type.

Before choosing a patch cord for a media converter, check:

• whether the media converter works over one fiber or two fibers;
• which connector is installed — SC or LC;
• whether UPC or APC polishing is required;
• whether singlemode or multimode fiber is used;
• what patch cord length is required;
• which connector and connection type are used on the other end of the line.

If the media converter has one SC port, a Simplex patch cord may be needed. If it has two optical TX/RX ports or uses an SFP module with two ports, Duplex is required.

SC Simplex, LC Duplex, SC-LC: How Not to Get Confused

Simplex or Duplex means the number of fibers, while SC or LC means the connector type. These parameters should be considered separately.

More details about the difference between SC and LC are available in the article SC or LC: Which Fiber Optic Connector to Choose.

UPC/APC: Does Polishing Affect Simplex or Duplex?

UPC and APC do not determine whether a patch cord is Simplex or Duplex. This is a separate parameter that describes the polishing type of the fiber optic connector.

For example, the following options are possible:

• SC/APC Simplex;
• SC/UPC Simplex;
• LC/UPC Duplex;
• LC/APC Duplex;
• SC-LC UPC Duplex;
• SC-LC APC Simplex.

If APC is used in the equipment or adapter, an APC patch cord should be selected. If UPC is used, a UPC patch cord is needed. More details are available in the article UPC or APC: What Is the Difference Between Blue and Green Fiber Optic Connectors.

Singlemode and Multimode: Does It Depend on Simplex or Duplex?

Simplex and Duplex do not determine the fiber type. A patch cord can be Simplex or Duplex with both singlemode and multimode fiber.

Possible options include:

• Singlemode Simplex;
• Singlemode Duplex;
• Multimode Simplex;
• Multimode Duplex;
• LC-LC Duplex OM3;
• SC-SC Simplex G.657.A1;
• SC/APC Simplex G.652D.

For PON/GPON/FTTH, singlemode fiber is usually used. For short connections in server rooms and data centers, multimode OM3 or OM4 patch cords may be used if required by the equipment.

Simplex or Duplex for an Outdoor Fiber Optic Patch Cord

For outdoor fiber optic patch cords, the choice between Simplex and Duplex depends on what equipment needs to be connected and how many fibers are required for the line to operate.

Outdoor Simplex patch cords are often used for:

• FTTH / PON / GPON;
• subscriber connections;
• optical boxes;
• one-fiber connections;
• BiDi solutions;
• outdoor lines to a house, cabinet or technical room.

An outdoor Duplex patch cord is needed where equipment works over two fibers: one for transmission and one for reception. This option can be used for connections between equipment, patch panels, media converters or SFP modules.

How to Choose Simplex or Duplex Correctly

To avoid mistakes, before buying, check not only the cable length but also the equipment type on both ends of the line.

Before choosing, check:

• how many optical ports the equipment has — one or two;
• whether transmission is required over one fiber or two fibers;
• which connector type is used — SC or LC;
• which polishing type is required — UPC or APC;
• whether singlemode or multimode fiber is needed;
• whether an indoor or outdoor patch cord is required;
• what cable length is needed;
• which port or adapter is installed on the other end of the line.

The simplest rule is: if the equipment has one optical port and works over one fiber, Simplex is needed. If the equipment has two optical TX/RX channels, Duplex is needed.

Common Mistakes When Choosing Simplex and Duplex

• buying Simplex when the equipment requires Duplex;
• buying Duplex for GPON where one SC/APC Simplex is needed;
• confusing the number of fibers in a patch cord with the number of fibers in a fiber optic cable;
• choosing the correct number of fibers but the wrong SC or LC connector;
• confusing UPC and APC;
• not checking whether the SFP module is standard or BiDi;
• ordering LC-LC Duplex when SC is required on one side;
• not considering Singlemode or Multimode;
• choosing a patch cord only by length;
• not checking equipment ports before buying.

Most mistakes happen when a buyer looks only at the patch cord length but does not check the Simplex/Duplex construction, connector type, polishing and equipment compatibility.

What Products Are Bought Together with Simplex and Duplex Patch Cords

For correct optical line connection, the following products are often ordered together with Simplex or Duplex patch cords:

• fiber optic patch cords — for connecting equipment, boxes, patch panels and adapters;
• outdoor fiber optic patch cords — for outdoor or protected installation;
• fiber optic pigtails — for splicing with fiber optic cable;
• fiber optic adapters — for connecting SC, LC, FC or ST connectors;
• fiber optic boxes — for protecting and organizing optical connections;
• fiber optic patch panels — for installing fiber optics in server cabinets;
• SFP modules — for connecting optical lines to equipment;
• media converters — for converting fiber optics to Ethernet;
• fiber optic cables — for building fiber optic lines.

The main rule is that all components of one line must be compatible with each other: number of fibers, connector type, polishing type, fiber type and cable length.

FAQ

What Is Simplex in a Fiber Optic Patch Cord?

Simplex is a fiber optic patch cord with one fiber. It usually has one connector on each end and is used for connections where one fiber is enough.

What Is Duplex in a Fiber Optic Patch Cord?

Duplex is a fiber optic patch cord with two fibers. It is used where signal transmission and reception go through separate fibers.

Which Is Better: Simplex or Duplex?

One option is not always better than the other. Simplex is needed for one-fiber connections such as GPON or BiDi. Duplex is needed for equipment that works over two TX/RX fibers.

What Should I Choose for GPON — Simplex or Duplex?

For GPON, Simplex is most often used, usually SC/APC Simplex on singlemode fiber. But before buying, check the equipment port and the adapter in the optical box.

What Should I Choose for an SFP Module?

If the SFP module is standard and has two optical TX/RX channels, Duplex is usually required. If it is a BiDi/WDM SFP module that works over one fiber, Simplex is needed.

Can an LC Patch Cord Be Simplex?

Yes. An LC patch cord can be either Simplex or Duplex. It depends on how many fibers are required for the specific equipment.

Can an SC Patch Cord Be Duplex?

Yes. An SC patch cord can also be Duplex if it has two fibers and two SC connectors on each end.

Does UPC or APC Depend on Simplex/Duplex?

No. UPC and APC are connector polishing types, while Simplex and Duplex describe the number of fibers in the patch cord. These are different parameters.

Does Singlemode or Multimode Depend on Simplex/Duplex?

No. A patch cord can be Singlemode Simplex, Singlemode Duplex, Multimode Simplex or Multimode Duplex. The fiber type should be selected according to the equipment and distance.

How Is Simplex/Duplex Different from the Number of Fibers in a Fiber Optic Cable?

Simplex and Duplex refer to ready-made patch cords with 1 or 2 fibers. A fiber optic cable for line installation can have 4, 8, 12, 24, 48 or more fibers.

Useful Articles

• SC or LC: Which Fiber Optic Connector to Choose
• UPC or APC: What Is the Difference Between Blue and Green Fiber Optic Connectors
• How to Choose an Outdoor Fiber Optic Patch Cord
• What Is the Difference Between OM2, OM3, OM4 and Singlemode
• What Is an SFP Module and How to Choose It

Conclusion

Simplex and Duplex describe the construction of a fiber optic patch cord by the number of fibers. Simplex has one fiber, while Duplex has two fibers. Simplex is more often used in PON, GPON, FTTH, subscriber lines and BiDi solutions. Duplex is more often required for SFP modules, media converters and equipment where signal transmission and reception use separate fibers.

To choose the right patch cord, check the ports on both ends of the line, equipment type, required number of fibers, SC or LC connector, UPC/APC polishing, Singlemode/Multimode fiber type and cable length.

Do not confuse Simplex and Duplex with the number of fibers in a fiber optic cable. For ready-made patch cords, the choice is usually 1 or 2 fibers, while fiber optic cables for line installation can have 4, 8, 12, 24, 48 or more fibers.

Need help choosing a Simplex or Duplex patch cord? Net-Server specialists will help you select an SC-SC, LC-LC, SC-LC, Simplex or Duplex fiber optic patch cord, as well as adapters, pigtails, SFP modules, media converters and other components for your fiber optic network.