Fiber Transmissions at Higher-speed Ethernet


When moving to 40/100GbE, the most important difference in backbone and horizontal multimode applications is the number of fiber strands. 40GBASE-SR4 uses 4 strands to transmit and receive for a total of 8 strands. 100GBASE-SR10 uses 10 lanes to transmit and receive for a total of 20 strands. SMF remains a 2-strand application and although the fiber is less expensive, SMF optics and electronics can be 10x more expensive. In data centers and backbones, it may be possible to have 8 or 20 individual strands of fiber. However, those strands may take disparate paths from one end to the other and this can cause delay skew (known as bit skew) resulting in bit errors. For this reason, the 40/100GbE standards are written around fiber optic trunk assemblies that utilize a MPO or MTP multi-fiber array connector. In these assemblies, all strands are the same length. Also referred to as “parallel optics,” this construction minimizes bit/delay skew, allowing the receive modules to receive each fibers information at virtually the same time.

MPO (Multi-fiber push-on) and MTP (Mechanical Transfer Push-on) are available in both 12 and 24 strand termination configurations used at the end of a trunk assembly. The MTP design is an improved version of the MPO. The patented MTP connector is a ruggedized version with elliptical shaped, stainless steel alignment pin tips to improve insertion guidance and reduce guide hole wear. The MTP connector also provides a ferrule float to improve mechanical performance by maintaining physical contact while under an applied load. MPO MTP trunks also support for the 10GBASE-SR/SX applications although only two fiber strands are used. In this case trunks are connected to cassettes and/or hydra assemblies, which break out the multiple fibers into two-strand connections (typically LC or SC).

MPO Cabling

The second difference in high-speed fiber configurations is polarity. For 2-strand applications such as 10GbE transmission, managing polarity is as simple as reversing the strands somewhere over the channel. This is true if the channel is constructed of individual strands or is part of a trunk assembly. In trunk assemblies, which have historically been 12-strand, there are three suggested polarity methods in the standards (as shown in the following table).

As shown above, 2-strand application polarity managing is relatively easy. When migrating from 2-strand to multi-strand parallel optics, it is important to note which polarity method was selected to assure that the correct assemblies are purchased for higher speeds. All polarity methods can be converted from 2-strand to 12-strand applications.


It is important to note that these polarity methods are suggested in the standards, not mandated. However, the mandate does state that a polarity method should be established and maintained throughout all fiber channels, mapping the transmit strand from one end to the receive strand at the other. This does not change for higher fiber count transmissions, with the exception that more strands are involved. To better visualize the transmission for multistrand applications, consider the following diagrams:

40GBASE-SR uses 8 strands of a 12-strand MPO/MTP trunk cable, (4 to transmit and 4 to receive). The middle 4 strands in the MPO/MTP connector remain dark. The interface on equipment will accept an MPO/MTP array connector rather than a traditional LC.

100GbE has three approved methods for transmission including one 24-strand (shown left) or two 12-strand trunks either “over and under” or “side-by-side” (shown right- Side-by-side configuration is not shown). The transmission uses 10 strands to transmit and 10 to receive leaving the outer unused strands dark. It is also possible to connect two 1- strand trunks via a “Y” assembly that converts two 12-strand trunk assemblies to one 24-strand assembly. Polarity must also be considered regardless of the method chosen and supported by the electronics.

The new MTP cables can bridge legacy 1Gbps/10Gbps networks over to 40Gbps/100Gbps networks, and can act as the trunk line on a network backbone. Since a single fiber cable can connect up to 24 devices, fewer cables are needed, cutting down on installation labor. The high-density, small form factor also saves space and improves air flow. With its push-pull release mechanism, the MTP connector is easy to engage and disengage.

FiberStore provide a wide range of MTP/MPO products including single mode or multimode MPO and MTP fiber cable. High density MTP/MPO trunk cables with up to 144 fibers in a single cable. Fiberstore also offer wide range of MTP MPO cassette. The standard cassettes can accommodate 12 and 24 port configurations. Different sizes of cassettes are available. Available in all fibermodes and connector options. Custom Options available including MPO MTP taps and MTP/MPO Silitter combinations.