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Introduction to 40G QSFP+ Cabling Assemblies

Today’s high-performance computing environments featuring by switching and routing, cloud computing and virtualization require higher network speeds, greater scalability, and higher levels of performance and reliability in data centers. Some bandwidth-hungry applications, like video streaming applications, also drive data rates to higher points. These all boost the need for a migration to 40G and 100G interfaces as 1 and 10G can’t meet the bandwidth needs well. 40G interface is QSFP (Quad Small Form-factor Plable) which has several standards requiring different connectors to fit cabling infrastructure, so as to achieve network connectivity. Do you know what cabling infrastructure is needed to support 40G applications? MPO/MTP cable, direct attach cable (DAC), or LC fiber patch cable? Have any ideas? Follow this article and find the answer.
MTP/MPO Cable
MTP is a registered trademark of US Conec used to describe the connector, and MPO stands for multi-fiber push-on or also multi-path push-on. Actually, the former product is 100% compatible with the latter. Thus, only MTP is written for simplicity in the following paragraphs. In 2010, the IEEE 802.3ba standard specifies MTP connectors for standard-length multi-mode fiber (MMF) connectivity. Its small, high-density form factor makes MTP cable ideal for higher-speed 40G networks in data centers.
To support 40G applications, a 12-fiber MPO connector is needed. The typical implementations of MTP plug-and-play systems split a 12-fiber trunk into six channels that run up to 10 Gigabit Ethernet (depending on the length of the cable). 40G system uses 12-fiber trunk to create a Tx/Rx link, dedicating 4 fibers for 10G each of upstream transmit, and 4 fibers for 10G each of downstream receive, leaving the middle 4 fibers unused. The upgrade path for this type of system entails simply replacing the cassette with an MTP-to-MTP adapter module.
Direct Attach Cable
Besides MTP cable, many data centers also like to choose DACs for 40G cabling infrastructure. DAC, a kind of optical transceiver assembly, is a form of high speed cable with “transceivers” on either end used to connect switches to routers or servers. The “transceivers” on both ends of DACs are not real optics and their components are without optical lasers, thus DACs are much cheaper, preferable for 40G data center applications. As such, the fiber connectivity cost is significantly reduced by using either direct attach copper cables or active optical cables (AOCs) instead of costly fiber transceivers and optical cables.
Direct Attach Copper Cable
Direct attach copper cables are designed in either active or passive versions for short-reaches in data center. Compared with active optical cables, these copper cables are less expensive. Nowadays, there are many twinaxial cables available to support 40G (10G x four channels), in QSFP+ to QSFP+ (ie. EX-QSFP-40GE-DAC-50CM) version or in QSFP to 4 SFP+ cable assembly (eg. QSFP-4SFP10G-CU5M).
The issue is that copper cable is stiff and bulky, thus consuming precious rack space and blocking critical airflow. But with the advancing technology, manufactures produce a thinner, uniquely shielded ribbon-style twinaxial cable that can support speeds of 10G per channel while addressing many of the concerns associated with round, bundled cable. And the ribbon-style twinaxial cable is significantly slimmer than its round counterparts. Even better, the cable can be folded multiple times and still maintain signal integrity, allowing for higher density racks and space savings.
Active Optical Cable
Being a form of DAC, AOC integrates single-mode fiber (SMF) or MMF cable terminated with a connector and embedded with transceivers. It uses electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable. AOCs can reach a longer distance copper cables, and use the same interfaces as copper cables, typically used in data center. Similar to direct attach copper cables, AOCs are also available in QSFP+ to QSFP+ (eg. QSFP-4X10G-AOC20M) and QSFP+ to 4 SFP+ cabling (ie. QSFP-4X10G-AOC10M) versions.
Since 40G AOC connectors are factory pre-terminated, 40G AOC is easier for installation and thus less affected by the repeating plug during daily use than MTP cable. In case there was a fault in the interconnection, for AOC, you can just replace it with another AOC.
LC Fiber Cable
Certainly, LC fiber cable can also be the cabling solution for the long-reach 40G QSFP+ modules (40GBASE-LR4). That is, 40GBASE-LR4 QSFP+ uses a duplex LC connector as the optical interface, able to support transmission distance up to 10km over single-mode fiber (SMF).

Learning Five Ways to Test Fiber Optic Cables

In this technological world filled by fiber optic systems everywhere, one won’t fail to enjoy the benefits brought by fiber optics in daily life. In a whole fiber optic system, the most essential part should be the fiber optic cable. This cable is made up of incredibly thin strands of glass or plastic capped with the same (eg. ST ST fiber cable) or different connector types (LC ST patch cable) on the ends, used as the medium to carry information from one point to another with light-based technology. Just like electricity that can power many types of machines, beams of light can carry many types of information, so fiber optics do great to people in many ways, like broadcasting, transportation, medicine, etc..Along with the heavy use of fiber optic cables, testing the installed cables also gains importance in practical use. Since there are many standards available for testing, some people may get confused. But don’t worry. This text is written with an attempt to clear off this confusion.
Testing Principles
Generally speaking, five ways are listed in various international standards from the EIA/TIA and ISO/IEC to test installed cable plants. First three of them use test sources and power meters to make the measurement, while the last two use an optical time domain reflectometer (OTDR). Let’s first see the different results from these methods, and then delve into each one.
The use of source and power meter method, also known as “insertion loss”, simulates the way the actual network uses the cable plant. The test source mimics the transmitter, and the power meter the receiver. But insertion loss testing requires reference cables attached to the source and meter to connect to the cable under test. This insertion loss test can use 1, 2 or 3 reference cables to set the “zero dB loss” reference for testing. Each way of setting the reference gives a different loss. While OTDR is an indirect method, using backscattered light to imply the loss in the cable plant, which can have large deviations from insertion loss tests. OTDRs are more often used to verify splice loss or find damage to cables.
Source/Power Meter Method
In source and power meter method, all the three tests share the same setup (shown below), but the reference power can be set with one, two or three cables as explained next. In general, the 1 reference cable loss method is preferred, but it requires that the test equipment uses the same fiber optic connector types as the cables under test. If the cable (ST ST fiber cable) has different connectors from the test equipment (SC-SC on the tester), it may be necessary to use a 2 or 3 cable reference, which will give a lower loss since connector loss is included in the reference and will be subtracted from the total loss measurement.
Reference per TIA OFSTP-14 (1 Cable Reference)
This method, formerly called method B, uses only one reference cable. The meter, which has a large area detector that measures all the light coming out of the fiber, effectively has no loss, and therefore measures the total light coming out of the launch reference cable. When the cable is tested as below, the measured loss will include the loss of the reference cable connection to the cable plant under test, the loss of the fiber and all the connections and splices in the cable plant and the loss of the connection to the reference cable attached to the meter.
Reference per TIA OFSTP-14 (2 Cable Reference)
This one, formerly called method A, uses two reference cables with one launch cable attached to the source, and the other receive one attached to the meter. (The two cables are mated to set the reference.) Setting the reference this way includes one connection loss (the mating of the two reference cables) in the reference value. When one separates the reference cables and attaches them to the cable under test, the dB loss measured will be less by the connection loss included in the reference setting step. This method gives a loss that’s less than the 1 cable reference.
Reference per TIA OFSTP-14 (3 Cable Reference)
Reference cables are often patch cords with plugs, while the cable under test has jacks on either end. The only way to get a valid reference is to use a short and good cable as a “stand-in” for the cable to be tested to set the reference. To test a cable, replace the reference cable with the cable to test and make a relative measurement. Obviously this method includes two connection losses in setting the reference, so the measured loss will be less by the two connection losses and have greater uncertainty. Finally, here goes the picture showing the testing case with one, two, three reference cables.
OTDR Testing
With only one lunch cable, the OTDR can measure the length of the cable under test and the loss of the connection to the cable under test plus the loss of the fiber in the cable under test, and any other connections or splices in the cable under test. However, this method doesn’t test the connector on the far end of the cable under test, because it isn’t connected to another connector, and connection to a reference connector is necessary to make a connection loss measurement.
If a receive cable is used on the far end of the cable under test, the OTDR can measure the loss of both connectors on the cable under test as well as the fiber in the cable, and any other connections or splices in the cable under test. The placement of the B marker after the connection to the receive cable means some of the fiber in the receive cable will be included in the loss measured.

 

Four Aspects About Multi-mode Fibers

Data centers are never ceased their steps to bring greater speed and efficiency to telecommunication and datacoms industries. An enormous amount of data is transmitted, gathered and analyzed everyday, all which requires a vast number of high-bandwidth interconnections between data centers, and people. During these interconnections, fiber optic cables see their heaviest use.
Fiber optic cables can deliver more bandwidth for voice, video and data applications, and carry thousands of times more information than copper wire. With fiber optic cables, reliable and secure data transmission is ensured. Fiber optic cables are available in single-mode and multi-mode versions based on transmission mode standard. This article puts its focus on the latter version: multi-mode fiber (MMF), discussing MMF from its core size attenuation, bandwidth and manufacturing ways.
MMF: Larger Core Size
It’s known that MMF has a much larger core size and cladding diameter, whose different types are distinguished by jacket color: for 62.5/125 µm (OM1) and 50/125 µm (OM2), orange jackets are recommended, while aqua is recommended for 50/125 µm “laser optimized” OM3 and OM4. MMF’s larger core endows it greater light gathering capacity, allowing multiple modes of light to propagate through the fiber simultaneously. Thus, MMF is more suitable for relatively shorter-reach application, usually less than 600m. When it’s deployed in GbE applications, the maximum reach is 550m in combination of 1000BASE-SX SFP (ie. 1783-SFP1GSX).
MMF: Attenuation/Signal Loss
Attenuation refers to the reduction of signal loss when light travels through the fiber optic cable, which is measured in decibels per kilometer (db/km). Insertion loss is the total attenuation from all sources plus any reflection losses over a specific fiber length. Such attenuation is often caused by absorption of optical energy by tiny impurities in the fiber such as iron, copper, or cobalt. Sometimes, the scattering of the light beam as it hits microscopic imperfections, called Rayleigh scattering can also lead to signal loss phenomenon. Attenuation problem is a commonplace in MMFs.
MMF: More Bandwidth
Bandwidth quantifies the complicated data-carrying capacity of MMF, given in units of megahertz-kilometer (MHz·km). Bandwidth behavior of MMF arises from multi-modal dispersion (multi-path signal spreading) which happens as the result of light traveling along different modes in the core of fibers. The bandwidth specification of performance of a MMF is verified through optical measurements during fiber manufacture. Actual system performance and data-rate handling rely heavily on bandwidth, affected by transceiver technology and device characteristics.
MMF: Manufacturing Ways
MMF can be manufactured in two ways: step-index or graded index.
Step-index fiber has an abrupt change or step between the index of refraction of the core and the index of refraction of the cladding. Multi-mode step-index fibers have lower bandwidth than other fiber designs.
Graded index fiber is designed to reduce modal dispersion inherent in step index fiber. This design maximizes bandwidth while maintaining a larger core diameter for simplified system assembly, connectivity and lower network costs. Graded index fiber is made up of multiple layers with the highest index of refraction at the core. Each succeeding layer has a gradually decreasing index of refraction as the layers move away from the center. High order modes enter the outer layers of the cladding and are reflected back towards the core. Multi-mode graded index fibers have less attenuation (loss) of the output pulse and have higher bandwidth than multi-mode step-index fibers.
MMF related transceivers: Multi-mode Transceivers
A fiber optic transceiver is a package, usually a plable module, comprising of a receiver on one end of the fiber and a transmitter on the other end. Over the years, multi-mode bandwidth specifications and measurement methods have evolved along with the transceiver technology, so as to keep up with delivery of higher transmission speeds. The combination of transceiver and fiber optic cable plays an important role in fiber’s practical link length. As for multi-mode transceivers which have larger core, they are often used in short-reach applications with 850mn wavelength. Listed below are several commonly-used multi-mode transceiver ports: 1000BASE-SX, 10GBASE-SR, 10GBASE-LRM, among which 10GBASE-SR port type enjoys widely deployment in 10GbE applications when the required distance is not so long. Take F5-UPG-SFP+-R for example, this F5 compatible 10GBASE-SR SFP+ transceiver listed in Fiberstore takes OM3 MMF as its transmission medium for 300m reach.
Besides what have been discussed above, there is also another MMF feature that comes into your mind: that is the affordability. MMF is less expensive than its counterpart single-mode fiber (SMF). Because of this, more people prefer MMF to SMF when the required distance is not so long. Thus, this big saving can be re-invented in other projects.

 

Three Useful Fiber Patch Cords and Their Use

With the rapid advancement of fiber optic technology and trend towards optical communications, fiber optic patch cord has realized its great use in high speed data transmission networks, found in routers, fiber patch panels, media converters and even in hubs and switches. Compared to its previous counterpart, fiber optic jumper causes lower signal loss, delivers more bandwidth and carries more information, becoming more and more popular in cabling installation or upgrading between or inside buildings. Just like the transceiver modules which fall in many types based on different standards, fiber optic patch cables are also available in several kinds, including single-mode/multi-mode, simplex/duplex, MPO/MTP cable, armored patch cord, and so on. This article aims to introduce the last three useful fiber patch cords and their use.
Simplex/Duplex Patch Cables
Simplex cable, also known as single strand cables, has one fiber, tight-buffered (coated with a 900micron buffer over the primary buffer coating) with Kevlar (aramid fiber) strength members and jacketed for indoor use. The jacket is usually 3mm (1/8 in.) diameter, but some 2mm cable is sometimes used with small form factor connectors. Duplex (zipcord) cable has two fibers joined with a thin web.
Since simplex patch cord consists of only one fiber link, it’s used in such applications that only require one-way data transfer. But when the equipment can transmit and receive on two different wavelengths, simplex cable can also be considered. For example, transmit could be at 1310nm and receive could be at 1550nm. This application is found more with single-mode simplex patch cable.
Duplex patch cable is suitable for applications that require simultaneous, bidirectional data transfer. Typical applications include workstations, fiber switches and servers, Ethernet switches, backbone ports, and similar hardware.
MPO/MTP Cable
MPO/MTP cable uses multi-fiber MPO/MTP connectors for setting up high-performance data networks in data centers, so as to achieve greater bandwidth and handle network traffic requirements. Specifically, in MPO/MTP cable component, each one of the connector are used with ribbon type fiber optic cables which contain multi-fiber in one single jacket, so that MPO/MTP patch cord greatly saves space, very convenient to use. Based on single ferrule MT technology, the MPO/MTP cable assemblies are able to provide up to 72 fiber connections in a single point, reducing the physical space and labor requirement, while providing the same bandwidth capacity of a multi-fiber cable with individual fiber connector terminations per cable. MTP cables can be divided into trunk and harness versions (image below).
MPO/MTP patch cables have great use in Gigabit applications, especially in 40GbE. Often, MPO/MTP connectors terminate OM3 or MO4 to form structured cabling, serving as the transmission medium for 40GBASE optics (ie. QFX-QSFP-40G-SR4).
Armored Patch Cord
Armored patch cord enjoys all the features of standard fiber patch cord, available in single-mode and multi-mode version (shown below), except its much stronger characteristic. It won’t get damaged even it is stepped by an adult. What’s more, this kind of patch cord is anti-rodents, and when it’s utilized, people do not need to worry that the rodent animals like the rats may bite the cables and make them broken. Although armored fiber cables are strong, they are actually as flexible as standard fiber jumper cords, and they can be bent randomly without being broken.
Armored patch cable can be made with the similar outer diameter to the standard patch cable, which makes it a space-saving design. In addition, armored fiber cables can be with different jacket colors and jacket types, like OFNR. Light in weight, armored fiber patch cords can be with SC, ST, FC, LC, MU, SC/APC, ST/APC, FC/APC, LC/APC types of terminations.
The armored fiber optic patch cords are more robust designed, suitable to be deployed in FTTH projects inside the buildings. They use stainless steel armor inside the jacket to be resistant of high tension and pressure, able to resist the weight of an adult person.

 

SMF&MMF 40G QSFP+ Transceiver Overview

The demand for better network throughput and performance has never ceased. Instead, it has become more and more vigorous. The server consolidation, virtualization, as well as networking-service performance improvements, all these have pushed the necessity for dense 40GbE switch connections in data centers.
But when migrating to 40GbE from10GbE, some companies or organizations are challenged by two main factors in re-configuring the physical layer of the network: firstly, the possible reduced reach of the OM3/OM4 multi-mode optics from 10GBASE-SR (300/400 m) to 40GBASE-SR4 (100/150m), and secondly, the need to upgrade the existing fiber optic cabling plant so as to support the IEEE-defined 40GBASE-SR4 parallel optics. In order to avoid these questions, SMF&MMF 40G QSFP+ transceiver is brought to the market.
SMF&MMF 40G QSFP+ Transceiver Definition
It’s know that a fiber optic transceiver may either operate on single-mode fiber (SMF) or multi-mode fiber (MMF). However, this SMF&MMF 40G QSFP+ transceiver is able to communicate with both SMF and MMF, without the need for any software/hardware changes to the transceiver module or any additional hardware in the network. It has 4 channels (1270, 1290,1310, and 1330nm) of 10G multiplexed inside the module to transmit and receive an aggregate 40G signal over 2 strands of fiber with a duplex LC connector.
Based on IEEE defined 40GBASE-LR4 specifications, this supports distances up to 150m over OM3 or OM4 MMF and up to 500m over SMF. Certainly, different fiber optic equipment vendors may have different specifications.
SMF&MMF 40G QSFP+ Transceiver Advantages
SMF&MMF 40G QSFP+ transceiver is designed for seamless migrations from existing 10GbE to 40GbE networking without modification or expansion of the fiber network. It addresses several challenges faced by today’s data centers and the passages highlight the advantages of this transceiver.
No Redesign or Expansion of Fiber Network
Other short-reach 40G QSFP+ transceiver types, such as MMF 40GBASE-SR4 transceivers (100m over OM3 MMF), utilize four independent 10G transmitters and receivers for an aggregate 40G link. These 40GBASE-SR4 transceivers (eg. JG325B) use a MPO-12 connector and require 8-fiber parallel OM3 or OM4. As a result, customers installing MTP/MPO fiber systems may need to deploy new fiber while upgrading from 10G to 40G. However, SMF&MMF 40G QSFP+ transceiver uses duplex LC connector, which is consistent with the existing 10G connections. It works on existing OM3 and OM4 MMF infrastructure which is widely installed and used for 10GbE networks, thus free from redesign or expansion of the fiber network.
Increase in the Number of 40G Links
The existing MMF 40GbE solutions use of 8 fibers for a 40G link, and customers have to add additional fiber if they want to increase the number of 40G links. But if you deploy SMF&MMF 40G QSFP+ transceiver, the number of 40G links is 4our times of that existing MMF 40GbE solutions without any changes to their fiber infrastructure. During this link increase, the network scale and performance are also expanded.
A Cost-effective Solution for SMF Infrastructure
Limited in the distance reach that multi-mode transceivers can support, the migration from 10G to 40G, to 100G, or even 400G would become simpler with SMF. But single-mode transceivers typically cost up to 4 times more compared to multi-mode transceivers. Since SMF&MMF QSFP+ transceiver interoperates with QSFP-LR4 and QSFP-LR4L optics, it’s a cost effective solution for SM fiber infrastructure for distances up to 500m. And customers can deploy mixed connections without fiber concerns.
Simplification in Infrastructure Deployment
SMF&MMF QSFP+ transceiver boasts of the unique characteristic of working through both SMF and MMF without any requirement for additional fiber. Customers can consolidate their optics and use SMF&MMF QSFP+ transceiver in their network without concern about the fiber type, which makes the full use of existing cabling infrastructure, leading to the reduced equipment cost and simplification of deployment.

 

Solutions 40G Parallel & Bidirectional Optical Transceiver Introduction

Speeds in data centers have maintained their growth in the past years, and will continue to do so in the predictable future. High-data-rate systems have become increasingly popular among some enterprises for high-performance computing networks, such as 40 Gigabit Ethernet (GbE) infrastructure, in which 40G fiber optic transceivers and cables are needed to ensure the high-performance and great-bandwidth of the 40GbE system. This article mainly introduces 40G fiber optic transceivers: the plable optical Enhanced Quad Small Form-Factor Plable (QSFP+), with focus on the bidirectional optical transceivers and parallel optical transceivers.
The transceiver is an electronic device that receives an electrical signal, converts it into a light signal, and launches the signal into a fiber. It also receives the light signal, from another transceiver, and converts it into an electrical signal. With the 40G QSFP being the dominant transceiver form factor used for 40GbE applications, the IEEE standard 802.3ba released several 40-Gbps based solutions in 2010, including a 40GBASE-SR4 parallel optics solution for multi-mode fiber (MMF). Another solution is a bidirectional 40-Gbps transceiver that uses a two-fiber LC optical interface.
40G Parallel Optical Transceiver
40G parallel optical transceiver enables high-bandwidth 40G optical links over 12-fiber parallel fiber terminated with MPO/MTP connectors. Four fibers on one side are used to transmit, while another four on the other side are utilized to receive, leaving the middle four fibers unused. In total, eight of the twelve fiber are used. That is to say, when used for 40GBASE-SR4 and 40GBASE-CSR4, parallel optical transceiver has 10-Gbps electrical lanes that are mirrored in the optical outputs, causing the requirement of eight fibers with a MTP connector interface. Each fiber either transmits (Tx) or receives (Rx) 10-Gbps traffic at a single wavelength.
Just as mentioned above, 40GBASE-SR4 QSFP+ transceiver belongs to 40G parallel optical transceivers, which uses multi-mode MPO trunks to establish 40G links. This port type 40G QSFP+ module can support link lengths of 100 meters and 150 meters over laser-optimized OM3 and OM4 MMFs respectively. It can also be used to connect with four 10GBASE-SR optical interfaces using an 8-fiber MTP to 4 duplex LC cable. Fiberstore listed 40GBASE-SR4 optical transceivers are fully compatible with such famous brands, as Cisco, Intel, Juniper (QFX-QSFP-40G-SR4), and so on. All are quality-and compatiblity-assured, offering the high performance to customers.
40G Bidirectional Optical Transceiver
By contrast, 40G bidirectional optical transceiver consists of two 20-Gbps transmit and receive channels, enabling an aggregated 40-Gbps link over a two-strand MMF connection. That is, the bidirectional optical transceiver used for 40GBASE-SR-BD uses the same 10-Gbps electrical lanes, which are then combined in the optical outputs, thus requiring two fibers with an LC connector interface. Each fiber simultaneously transmits and receives 20-Gbps traffic at two different wavelengths.
Cabling Options for 40G Parallel & Bidirectional Optical Transceiver
Cabling Options for 40G Parallel Optical Transceiver
As previously mentioned, in 2010 IEEE 802.3ba approved the 40GBASE-SR4 physical-medium-dependent (PMD) multi-mode parallel optic solution, which uses eight fibers to transmit four duplex channels each at 10-Gbps. This is an economical path to 40GbE data rates, while using many of components of 10GbE solutions. The main advantage of the parallel optical transceiver over the bidirectional transceiver at 40GbE is the reach. For example, if you cable your data center with OM3 at 10GbE, you can support distances up to 300m. Then if you move to 40GbE, you can support the same 300m distance with the same OM3 fiber and a 40GBASE-CSR4 transceiver. However, if your cabling distances do not justify the extra distance capability, then the bidirectional solution would be used.
There exists a problem in this parallel optical cabling solution—MTP cable assemblies which built on 12-fiber position connectors, leaving four unused fibers in each link. There are several basic cabling options for parallel optics connectivity. One approach is to ignore the unused fibers and continue to deploy 12 fibers. Another approach is to use a conversion device to convert two 12-fiber links into three 8-fiber links.
Cabling Options for 40G Bidirectional Optical Transceiver
This two-fiber 40G bidirectional multi-mode solution tackles the challenge—polarity correction that occurs in a 12-fiber MTP connector., using two different transmission windows (850 and 900nm) that are transmitted bidirectionally over the same fiber. This approach allows the use of the same cabling infrastructure for 40GbE as was used for 1 and 10 Gigabit Ethernet. The plable bidirectional transceiver has the same QSFP+ format as the existing 40GBASE-SR4 transceivers. Therefore, the same switch line card with QSFP+ ports can support either parallel optics 40GBASE-SR4 or bidirectional optics 40GBASE-SR-BD solutions.
As such, while connecting a 40GbE bidirectional transceiver to another bidirectional transceiver, a Type A-to-B standard LC duplex patch cord can be considered, with one fiber in connector position A on one end and in connector position B on the other end. Such reverse fiber positioning allows a signal to be directed from the transmit position on one end of the network to the receive position on the other end of the network. However, this direct connectivity is recommended only within a given row of cabinets.