Why Short Distance MTP-based Connectivity Utilizes OM3 or OM4 Fibers?

As today’s network needs to support more devices and advanced applications than ever before, the amount of data transmitted at the enterprise business level is rapidly climbing. For many data centers, 10G network no long satisfies the need of high speed data transmission. In 2010, the IEEE ratified the 40G ad 100G standard. Then how to realize smooth migration path from 10G to 40G and 100G has become the most concern for data center managers. After some comparison, many data center managers turn to MTP-based connectivity since it can provide fast installation, high density and high performance cabling for data centers. It is not difficult to find that both MTP/MPO trunk cable and MTP/MPO breakout cable (shown as the figure below) used for short distance connectivity utilize OM3 or OM4 fibers. Why short distance MTP-based connectivity utilizes OM3 or OM4 fibers? This article will show you the reason.

MTP MPO trunk cable and MTP MPO breakout cable

Bandwidth

OM3 and OM4 fibers are the only multimode fibers included in the 40/100G standard. Multimode fibers utilize parallel optical transmission instead of serial transmission due to the 850 nm VCSEL (vertical cavity surface emitting laser) modulation limits. And OM3 and OM4 fibers have a minimum 2000 MHz∙km and 4700 MHz∙km effective modal bandwidth (EMB). The minimum EMBc (Effective Modal Bandwidth calculate) method measures the actual fiber bandwidth performance, recognizing the fact that overall system bandwidth is a function of both the bandwidth properties of the fiber and also the particle characteristics of individual laser sources, and this is the most significant factor in determining link performance. In addition, the IEEE model is the industry reference point for calculating the maximum achievable Ethernet link distance and sets out the minimum requirements of components in an optical link. Therefore, knowing the exact minimum bandwidth performance of OM3 and OM4 fibers is a prerequisite to understand the ultimate limitation of MTP-based connectivity, which can ensure the optical infrastructure deployed in the data center will meet the performance criteria set forth by IEEE for bandwidth.

Insertion Loss

No matter what kind of cabling system you are going to deploy, insertion loss is inevitable and it is an essential performance parameter of the network deployment. It is important to note that the total connectivity loss within a cabling system has an effect on the network performance over the maximum link distance at a given data transmission rate. It is not difficult to understand that the higher the total connectivity loss, the shorter the maximum link distance. As a result, the insertion loss specifications of components used for connectivity should be evaluated at first when designing data center cabling infrastructures. The 40G standard specifies that with link distance up to 100 meters, the maximum channel loss of OM3 fiber is 1.9 dB, which includes a 1.5 dB total connectivity loss budget; while for OM4 fiber, it is specified that with link distance up to 150 meters, the maximum channel loss is 1.5 dB, which includes a 1.0 dB total connectivity loss budget. And the maximum attenuation of fiber optic cable at 850 nm is 3.5 dB/km. With low-loss OM3 and OM4 fibers, maximum flexibility can be achieved with the ability to utilize MTP connector in the optical link.

Conclusion

With 850nm EMB of 2000 MHz∙km and 4700 MHz∙km, OM3 and OM4 fibers can provide the bandwidth which is needed in MTP-based connectivity for 40G network. Besides, low-loss within cabling system is another characteristic of OM3 and OM4 fibers, which can ensure the high performance of the network deployment. Therefore, utilizing OM3 and OM4 fibers makes short distance MTP-based connectivity an ideal solution for migration from 10G to 40G in data centers.

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In-depth Understanding of Polarity for MTP System

To meet increasing demands for high-density cabling and wider bandwidth of network applications, many data centers are migrating to the 40G and 100G Ethernet. To prepare for this change, MTP technology is applied to provide an easy migration path. Typically, a fiber optic link needs two fibers for full duplex communications. Thus the equipment on the link should be connected properly at each end. However, high-density connectivity usually requires more than two fibers in a link, which makes it more complex to maintain the correct polarity across a fiber network, especially when using multi-fiber MTP components for high data rate transmission. This article will specifically guide you to understand the polarity for MTP system and three MTP polarity methods.

What Is Polarity?

To form a fiber optic link, the optical transmitter at one end is connected to the optical receiver at the other end. This matching of the transmit signal (Tx) to the receive equipment (Rx) at both ends of the fiber optic link is referred to as polarity. In other words, polarity is the term used in the TIA-568 standard to explain how to make sure that proper connection is made between the transmitter at one end and the receiver at the other end. Once the component is connected to the wrong polarity, the transmission process will be unable to go on.

Structure of MTP Connector

As shown in the following picture, MTP connector is pin and socket connector, which requires a male side and a female side. And each MTP connector has a key on one side of the connector body. When the key sits on top, this is referred to as the key up position, and when the key sits on bottom, we call it key down position. Moreover, each of fiber holes in the connector is numbered in sequence from left to right. We will refer to these connector holes as positions, or P1, P2, etc. Besides, each connector is additionally marked with a white dot on the connector body to designate the position 1 side of the connector when it is plugged in.

Structure of MTP Connector

MTP Adapter Keying Options

MTP adapter contains an asymmetrical housing including an inverted key to achieve the appropriate fiber polarity. On type A adapters, the keys are inverted to ensure that the fiber at position 1 is connected to position 1 in the MTP fiber cable connector at the opposing end.

MTP key up to key down adapter

On type B adapters, both keys are oriented facing up in order that both MTP fiber cable connectors are mated “key up”. The fiber at position 1 is connected to position 12 in the MTP connector at the opposing end.

MTP key up to key up adapter

Three Polarity Methods for MTP System

The TIA standard defines two types of duplex fiber patch cables terminated with LC or SC connectors to complete an end-to-end fiber duplex connection: A-to-A type patch cable is a cross version and A-to-B type patch cable is a straight-through version. Based on this, there are three polarity connecting methods for MTP system. The following part will introduce them in details.

A-to-A type patch cable and A-to-B type patch cable

Method A

Method A utilizes “key up to key down” adapters to connect the MTP connectors. As the following figure shows, this method maintains registration of Fiber 1 throughout the optical circuit. Fiber 1 in the near end cassette mates to Fiber 1 in the trunk cable assembly, which mates to Fiber 1 in the remote cassette. The fiber circuit is completed by utilizing one flipped patch cord, either at the beginning or end of the permanent link, to insure proper transceiver orientation. Method A provides the simplest deployment, and works for single-mode and multimode channels, as well as can easily support network extensions.

Method A

Method B

Different from method A, method B uses “key up to key up” adapters. The fiber circuit is completed by utilizing straight patch cords at the beginning and end of the link, and all of the array connectors are mated key up to key up. This type of array mating results in an inversion, meaning that Fiber 1 is mated with Fiber 12, while Fiber 2 is mated with Fiber 11, etc. To ensure proper transceiver operation with this configuration, one of the cassettes needs to be physically inverted internally so Fiber 12 is mated with Fiber 1 at the end of the link. This method requires a more in-depth planning stage in order to properly manage the polarity of the links, and to identify where the actual inversions need to occur. Moreover, it only supports multimode fiber.

Method B

Method C

With the use of “key up to key down” adapters, method C looks like method A. However, the difference between method C and method A is that the flip does not happen in the end patch cords, but in the array cable itself. This method requires a more in-depth planning stage in order to properly manage the polarity of the links, and to identify where the actual flipped array cord is placed in the link. An additional drawback to this method is that if this link was to be extended, a straight array cord as used in Method A would need to be used to revert the polarity back to straight array polarity condition. In other words, unflip the array cable.

Method C

Conclusion

Knowing the polarity of MTP system helps you better upgrade the 40G and 100G networks. According to different polarity methods, choosing the right MTP patch cables, MTP connectors and MTP cassettes will provide greater flexibility and reliability for your high-density network.