Which Fiber Loopback Should I Use for My Transceiver?

In telecommunication, fiber loopback is a hardware designed to provide a media of return patch for a fiber optic signal, which is typically used for fiber optic testing or network restorations. When we need to know whether our fiber optic transceiver is working perfectly, we can use a fiber loopback cable as an economic way to check and ensure it. Basically, the loopback aids in debugging the physical connection problem of the transceiver by directly routing the laser signal from the transmitter port back to the receiver port. Since fiber optic transceivers have different interface types and connect different types of cables, it is not that simple to choose a right loopback for our transceiver. This post will be a guide on how to choose a right loopback cable for specific transceiver module.

Fiber Loopback Types and Configurations

Before deciding which loopback cable to use, we should firstly know the structure and classification of fiber loopback cable. Generally, a fiber loopback is a simplex fiber optic cable terminated with two connectors on each end, forming a loop. Some vendors provide improved structure with a black enclosure to protect the optical cable. This designing is more compact in size and stronger in use. Based on the fiber type used, there is single-mode loopback and multimode loopback, available for different polishing types. According to the optical connector type of the loopback, fiber loopback cables can be divided to LC, SC, FC, ST, MTP/MPO, E2000, etc. In testing fiber optic transceiver modules, the most commonly used are LC, SC and MTP/MPO loopback cables.

lc&sc loopback cable
Figure 1: LC & SC Loopback Cable

The LC and SC loopbacks are made with simplex fiber cable and common connectors; it’s not difficult to understand their configurations. As for the MTP/MPO loopback, it is mainly used for testing parallel optics, such as 40G and 100G transceivers. Its configuration varies since the fiber count is not always the same in different applications.

8 Fibers MTP/MPO Loopback Cable Configuration

In a 8 fibers MTP/MPO loopback, eight fibers are aligned on two sides of the connector, leaving the central four channels empty. And the fibers adopt a straight configuration of 1-12, 2-11, 5-8, 6-7. The polarity channel alignment is illustrated in the following figure.

8 Fibers Loopback Polarity Channel Alignment
Figure 2: 8 Fibers Loopback Polarity Channel Alignment
12 Fibers MTP/MPO Loopback Cable Configuration

The only difference between the 12-fiber MTP loopback and the 8-fiber loopback is that the central four channels are not empty. Its alignment is 1-12, 2-11, 3-10, 4-9, 5-8, 6-7.

12 Fibers Loopback Polarity Channel Alignment
Figure 3: 12 Fibers Loopback Polarity Channel Alignment
24 Fibers MTP/MPO Loopback Cable Configuration

The 24 fibers MTP loopback also adopts type 1 polarity. Its alignment design is shown below.

24 Fibers Loopback Polarity Channel Alignment
Figure 4: 24 Fibers Loopback Polarity Channel Alignment
Which to Choose for a Specific Transceiver?

Considering the common features of the transceiver and the loopback, we should think about the connector type, polish type, and cable type when selecting a loopback for the transceiver. The selection guide for some mostly used transceiver modules is summarized in the following tables.

Table 1: Loopback choices for 10G SFP+ transceivers

Model Interface type Cable Type Suited Loopback

LC/UPC Duplex Multimode Fiber Loopback


LC/UPC Duplex Single-mode Fiber Loopback


Table 2: Loopback choices for 40G QSFP+ transceivers

Model Interface type Cable Type Suited Loopback

8/12 Fibers MTP/UPC Multimode Fiber Loopback


8/12 Fibers MTP/APC Single-mode Fiber Loopback

40GBASE-LR4 LC Duplex (PC) SMF

LC/UPC Duplex Single-mode Fiber Loopback


LC/UPC Duplex Multimode/Single-mode Fiber Loopback

Table 3: Loopback choices for 100G QSFP28 transceivers

Model Interface type Cable Type Suited Loopback

8/12 Fibers MTP/UPC Multimode Fiber Loopback


8/12 Fibers MTP/APC Single-mode Fiber Loopback

100GBASE-LR4 LC Duplex (UPC) SMF

LC/UPC Duplex Single-mode Fiber Loopback

Table 4: Loopback choices for CFP transceivers

Model Interface type Cable Type Suited Loopback

8/12 Fibers MTP/UPC Multimode Fiber Loopback


SC/UPC Duplex Single-mode Fiber Loopback


24 Fibers MTP/UPC Multimode Fiber Loopback


This post discusses specific fiber loopback choices for some most commonly used fiber optic transceivers. For other transceiver modules that are not mentioned in this post, we can also know how to choose a suitable loopback for it by getting details about its interface type, physical contact and cable type.

Multiple Applications of 1U Detachable Horizontal Cable Management Panel

Good cable management is essential to keep the fiber cables in acceptable condition and ensuring transmission performance with high-density cabling. A 1U detachable horizontal panel is an amazing part in cable management due to its multiple choices for application. This article will introduce the structure of a 1U detachable cable management panel and its diverse applications in network installation.

Structure of the 1U Detachable Cable Management Panel

This 1U detachable horizontal cable management panel consists of four parts that are held together by screws and can be removed by tools. They are illustrated in the following figure, including horizontal laser bar, horizontal laser panel, D-rings and 1U patch panel.

Structure of the 1U Detachable Cable Management Panel

Multiple Application Choices of the Detachable Cable Management Panel

This 1U detachable horizontal cable management panel can be used to manage cables for different active or passive network devices because they can be assembled randomly to meet your needs. Here are four application examples of this detachable cable management panel.

It is a good choice for holding different types of adapter panels. As a whole, this detachable cable management panel can be used with LC adapter panel. Here it is used with four fiber adapter panels with 12 LC duplex single-mode adapters. Besides, taking off part 4, it can also be used with a 12/24-port adapter in 1U patch panel form.

1U adapter panel is used with LC adapter panel

Combining part 2, 3 and 4 mentioned above, it can be used with MTP/MPO adapter panel. Here it is used for four 8 MTP adapter panel. It should be noted that for each MTP/MPO adapter, 8-fiber MTP/MPO connector is also suited to be linked with 12-fiber MTP/MPO connector when only 8 fibers are used in the 12-fiber MTP/MPO (such as in 40G network).

1U detachable panel is used with MTP adapter panel

By using only part 2 and part 3, cable management for a 1U rack mount fiber enclosure is perfectly achieved.

1U detachable panel is used with enclosure

The combination in the last example can also be used to manage cables coming from 1U WDM Mux/Demux or switch. In the following figure, on the top this detachable panel is used with a 18 channels CWDM Mux/Demux and at the bottom it is used with a switch.

1U detachable panel is used with CWDM Mux/Demux and switch


From the application examples presented above, we can see that this 1U detachable horizontal cable management panel is very capable in different cabling situations. You may notice that cable ties are also used to help organize the cables placing inside the D-rings. If you interested in this multifunctional assembly, you can find it on our site (FS.COM). You can also find many other helpful cable management assemblies for managing efficiently high-density structured cabling in your data centers and telecommunication rooms.

The following table displays some details of several cable management devices provided by us They are all in stock at Seattle warehouse and are available for same-day shipping.

Picture Description
cable management-1 1U Detachable Horizontal Panel with D-ring & Lacing Bar
cable management-2 1U Plastic Single-side Horizontal Cable Management Panel with Finger Duct
cable management-3 24 Ports UTP Cat5e 110 Punch Down 1U 19” Fast Ethernet Patch Panel
cable management-4 1U Metal Horizontal Lacer Panel with D-rings

MTP Specifications and Deployment for 40GBASE-PLRL4 QSFP+

Commonly, QSFP+ transceiver designed with LC interface works with single-mode fiber for long distance application, while QSFP+ transceiver with MTP/MPO interface is used over multimode fiber for short distance transmission. For instance, 40GBASE-ER4 QSFP+ is designed with LC duplex interface, and it supports maximum transmission length of 40 km over single-mode LC duplex fiber; 40GBASE-SR4 QSFP+ with MTP/MPO interface supports a transmission distance no more than 150m over multimode fiber. However, in order to meet user’s diverse needs in real applications, some 40G transceivers are designed not following this rule, like 40GBASE-PLRL4 (parallel LR4 Lite). This transceiver is with MTP/MPO interface design but is used over single-mode fiber for long distance transmission. This article will introduce the MTP/MPO specifications for this transceiver and its deployment cases.

40GBASE-PLRL4 transceiver

MTP Specifications for 40GBASE-PLRL4 QSFP+

QSFP-40G-PLRL4 transceiver uses MTP-12 interface to achieve parallel transmission, supporting maximum data links up to 1.4 km. The cable type required for 40GBASE-PLRL4 is an APC (angle polished connector) single-mode MTP-12 cable. The cable is similar to the 40G-SR MTP or MPO, with the only change being the use of single-mode fiber. UPC (ultra polished connector) is another type of connector for MTP-12 cables, but it is not suited for single-mode fiber in market. APC is the only available type for single-mode MTP-12 fiber. The MTP-12 connector plugged into the QSFP-40G-PLRL4 transceiver carries the 40G signal over only 8 of the 12 fibers, remaining four fibers unused, and these four can optionally be not presented in the cable for economic reason. The used 8 fibers are mapped as 4x10G Tx and Rx pairs. In addition, the MTP cables connected to QSFP-40G-PLRL4 transceiver can be either MTP trunk cables or MTP splitter cables.

Deployment of 40GBASE-PLRL4 QSFP+

The QSFP-40G-PLRL4 is optimized to guarantee interoperability with any IEEE 40GBASE-LR4 and 10GBASE-LR. So when the link for 40G network and 10G to 40G migration is less than 1.4 km, it will be very appropriate to use 40GBASE-PLRL4 QSFP+ transceiver with single-mode MTP cables.

In the first case, you can choose a MTP trunk cable together with the 40GBASE-PLRL4 QSFP+ module for direct 40G connection. The following picture shows two 40GBASE-PLRL4 QSFP+ transceivers connected by a single-mode 12-fiber MTP trunk cable.

40GBASE-PLRL4 QSFP+ for 40G connection

In the second case, you can simply use an 8-fiber MTP to 4xLC duplex harness cable with one 40GBASE-PLRL4 QSFP+ and four 10GBASE-LR SFP+ to achieve 10G to 40G.

40GBASE-PLRL4 QSFP+ for 10G to 40G migration

You can see in the above two cases, MTP cable plays an important role and due to the special requirements of 40GBASE-PLRL4 for single-mode MTP APC fiber, it is necessary to choose the right MTP products connected to this 40G QSFP+.


40GBASE-PLRL4 QSFP+ module has special interface design which can be only compatible with single-mode MTP connector. During the deployment of 40GBASE-PLRL4 QSFP+ module, selecting proper MTP assemblies are essential to successfully accomplish the link. FS.COM is a professional fiber optic transceiver vendor and MTP product manufacturer, supplying compatible 40GBASE-PLRL4 QSFP+ transceiver of different brands, such as Cisco, Arista, Brocade, Huawei, etc. Also other customized compatible brands are available for your requirements. MTP cables and assemblies are available for same-day shipping at low prices, including customized 8 fibers MTP/MPO trunk cable. You will be surprised to see how many kinds of network devices FS.COM can offer and you will get more than cost-effective products but also impressive service.

MTP HD Cassette and TAP Cassette Over Standard LGX Cassette

Pre-terminated fiber cabling has become a favorable choice for today’s high speed networks in data centers as this technology enables high bandwidth, high port density, easy management, future data rates migration and security monitoring. And modular system allows for rapid deployment of high density data center infrastructure as well as improved troubleshooting and reconfiguration during moves, adds and changes. MTP cassette is such a modular module. Usually it employs configuration of 12 fibers or 24 fibers, containing MTP adapter, LC/SC adapter, MTP-LC or MTP-SC patch cable etc, of which MTP-LC cassette is more widely used. As an assembly of the high density MTP/MPO pre-terminated fiber devices, it is dominant in high-density data centers for its reliable interface, optimized performance and minimized rack space. There are commonly three types of MTP cassettes available in market, including MTP LGX cassette, MTP HD cassette and MTP TAP cassette. This article will introduce the advantages of MTP HD cassette and MTP TAP cassette over the standard MTP LGX cassette.

FS.COM MTP cassette

MTP HD Cassette Over MTP LGX Cassette

Though the MTP cassette is preferred for its high density, there is still difference in used rack space between MTP LGX cassette and HD cassette. For standard LGX cassette, usually 3pcs of LGX cassette are put inside a 1U19’’ rack, or 12pcs inside a 4U 19’’ rack, as shown in the figure below. However, HD cassette is more optimized for high-density applications than LGX cassette for being more compact in package. 5pcs of HD cassette can be put inside a 1U 19’’ rack. So if the space for a data center is urgent to be saved, MTP HD cassette will be the best selection to help minimize the rack space for the most fiber count.

standard LGX in 1U and 4U fiber enclosures

MTP TAP Cassette Over MTP LGX Cassette

TAP (traffic access point) is usually added in the network for network monitoring. MTP TAP cassette is an effective device for real-time monitoring in high performance network and high density cabling. TAP cassette integrates TAP functionality into cable patching system. A TAP uses a passive fiber optic splitter to create an exact copy of the light signal passing through it. The fiber carrying the signal from a device’s transmit port is connected to the splitter input; the splitter’s live output is connected to the receive port of the downstream device, while a second output carries the copy of the live signal for out-of-band access. A TAP uses two of these splitters, installed on the two fibers supporting both channels of a duplex fiber channel link, to create a complete copy of all traffic between the two devices. And the transmission for the network data will not be affected since there are ports for monitoring and ports for transmission. The MTP TAP cassette can adopt both the package of HD and LGX cassette. The MTP TAP cassette can be easily deployed in network by connecting to the monitoring device and the user device with MTP trunk/breakout cables or LC/SC patch cables. For this hardware tool, TAP cassette is more expensive than the other two cassette types.


This article compares two MTP cassettes with the MTP standard LGX cassette and states the advantages of them. So if space is the primary consideration in high density cabling, MTP HD cassette is a better design choice; if a secure network with high performance is the priority, MTP TAP cassette is recommended to be deployed in the network. For special applications where high density and monitoring are both required, MTP TAP cassette with compact design is the best choice!

MTP-8: Simplest Way to Get 40G Connection

As data centers networks are shifting from 10G to 40G and beyond, it is necessary to seek ideal ways to connect 40G high speed switches populated with higher rate transceivers QSFP+, and to connect 40G switch to existing 10G elements populated with SFP+ modules. There are different approaches to connect 40G switches, or to connect 40G switch to 10G switch. However, by using MTP-8 solution, the simplest way to achieve direct 40G connectivity has been proved feasible and favorable in real applications. This article will introduce the deployment of MTP trunk cable in 40G to 40G connection, and MTP harness cable in 10G to 40G connection.

Basis of MTP Trunk and Harness Cable

MTP trunk cable has MTP connectors terminated on both ends of the fiber optic cable. It is often used to connect MTP port modules for high density backbone cabling in data centers and other high dense degree environments. Currently, most of the MTP trunk cables for high data rate like 40G and 100G are still 12-fiber or 24-fiber. MTP harness cable, also called MTP breakout or fan-out cable, has MTP connectors on one end and discrete connectors (duplex LC, SC, etc.) on the other end. The most common configurations of MTP-LC harness cables are 8-fiber MTP to 4 LC duplex, 12-fiber MTP to 6 LC duplex and 24-fiber MTP to 12 LC duplex. A single MTP connector is able to terminate the combination of 4, 8, 12, 24, 48 fiber ribbon cables. The multi-fiber design provides quick deployment and scalable solution that improves reliability and reduces installation or reconfiguration time and cost.

10G to 40G Connection via MTP Harness Cable

In order easily and quickly finish the migration from 10G network to 40G network, you can use 8-fiber MTP to 4 LC duplex harness cable, 40GBASE-SR4 QSFP+ and 10GBASE-SR SFP+ modules. The configuration of such a link is illustrated by figure 1. On the left the 8-fiber MTP connector is plugged into the MTP port of the 40GBASE-SR4 QSFP+ transceiver installed on the 40G switch; on the right side four duplex LC connectors are plugged into the ports of four 10GBASE-SR SFP+ transceivers installed on the 10G switch. In 10G to 40G migration, using 8-fiber MTP to LC harness cable can ensure every strand of fiber be used, and no one wasted.

10G to 40G via MTP-8 harness

Figure 1: 10G to 40G Migration via MTP-LC Harness Cable

40G to 40G Connection via MTP Trunk Cable

To support your 40G networking needs, you can simply use 12-fiber MTP trunk cable and 40GBASE-SR4 QSFP+ transceiver to accomplish a quick connection for two 40G switches in your network. The following figure shows a concrete example which uses one 12-fiber MTP trunk cable and two 40GBASE-SR4 QSFP+ transceivers to connect two 40G switches. Though the MTP trunk cable in this case is base-12, the fiber count actually in use is eight, leaving four strands unused. That is to say delivering 40G over 4 lanes multimode fiber at 10 Gb/s per lane. Totally only eight fibers (4 transmit, 4 receive) are required for the 4x10G solution. It is the same as the 4x25G solution for 100G.

40G connection via MTP-8 trunk

Figure 2: 40G to 40G Connection via MTP Trunk Cable

The above two examples are both applications of MTP-8 solution in 40G connectivity. You will find that only a few components are needed in the whole installation, and the link will be very easy and flexible, as well as cost-effective.


Current 40G connectivity can be obtained by MTP-8 solution. Though present market is still popular with 12-fiber or 24-fiber MTP, 8-fiber MTP solutions that are starting to hit the market are considered the most efficient option since they support current and future duplex fiber applications (such as 200G and 400G) and using modules that break out 8-fiber MTPs to duplex LCs, as well as current and future 8-fiber applications without the need for conversion cords or modules.