In-Depth Look at Tx Power and Rx Power of a Fiber Transceiver

ponedjeljak , 27.03.2017.

When choosing fiber transceiver, you may usually hear the description like Tx (transmit) power and Rx (receive) power for a fiber transceiver. Do you know what do these two phrases stand for? If we want to choose a proper kind of fiber transceiver, do we have high requirements on its Tx power and Rx power? In fact, these two phrases are the important parameters of the fiber transceiver which mainly determine the transmission distance of the fiber transceiver. To better know the Tx power and Rx power of a fiber transceiver, let’s take them in the 10GBASE SR SFP and 10GBASE LR SFP transceivers as reference.

Tx Power and Rx Power in 10GBASE SR and 10GBASE LR SFP Transceivers
In general, the optical Tx power is the signal level leaving from the device which should be within the transmitter power range, while the optical Rx power is the incoming signal level being receiving from the far end device that should fall within the receive power range. Both of the two kinds of parameters are very important factors that affect the transmission distance of a fiber transceiver. The following will study the two factors in 10GBASE SR and 10GBASE LR SFP transceivers in details.

As for the 10GBASE SR SFP transceiver, it is a kind of multimode fiber transceiver that can support the distance of 300 m over OM3 multimode fiber patch cable. As for the 10GBASE LR SFP transceiver, it is a single mode type of fiber transceiver which enables the 10G network at lengths up to 10 km over single mode fiber patch cable. Why the former transceiver can be only used for short distance transmission but the latter one is able to support the same signal for a much longer distance? Except the differences of fiber patch cables they always work with, it is also relevant to their Tx power and Rx power parameters.

From the figure below, we can easily learn the different Tx power and Rx power between 10GBASE SR and 10GBASE LR SFP transceivers. As for the 10GBASE SR one, Tx power is between -7.3 dBm and -1 dBm and the maximum Rx power is below -11.1 dBm. With regard to the 10GBASE-LR one, Tx power is from -8.2 to 0.5 dBm and its maximum Rx power is -14.4 dBm. Hence, when choosing 10GBASE fiber transceiver, you are highly suggested to acquire these two information for better knowing the performance of the transceiver.

Optical Power Budget for 10GBASE SR and 10GBASE LR SFP Transceivers
We have noted before that the Tx power and Rx power affect the transmission distance of a fiber transceiver. How does the influence happen? In fact, the Tx power and Rx power determine the optical power budget (maximum allowable loss) of a fiber transceiver, which can have a direct influence on the transmission distance. In short, the bigger the optical power budget is, the longer the fiber transceiver will transmit the signal. How to calculate the optical power budget of a fiber optic transceiver? Here is the formula: Optical power budget = Min Tx power – Max Rx power

Let’s calculate the optical power budget of 10GBASE SR and 10GBASE LR SFP transceivers according to the formula. For the 10GBASE SR SFP transceiver, its optical power budget is 3.8 dBm. And the optical power budget of the 10GBASE LR SFP transceiver is 6.2 dBm, bigger than the former one. Hence, there is no doubt that the 10GBASE LR SFP transceiver can support a much longer transmission than the 10GBASE SR SFP transceiver.

Conclusion
Tx power and Rx power of the fiber transceiver are two main factors which can mainly determine the transmission distance. When choosing the fiber transceiver, you can use the Min Tx power to minus the Max Rx power, and then you would get the optical power budget of the transceiver. The bigger the optical power budget is, the better the transceiver will performs.

Why Not Use QSFP DAC Twinax Cable for 40G Cabling?

petak , 10.03.2017.

With the growing network requirements for higher network speed, greater scalability, higher performance and reliability, 40G network gradually becomes a better choice for most data centers. Under this condition, the 40G QSFP+ copper direct attach cable (DAC) came into market, which is designed for delivering an aggregate data bandwidth of 40Gb/s in a short distance and high density cabling interconnect system. As a highly cost-effective solution for 40G to 40G or 10G to 40G connection, there is no doubt that the QSFP DAC twinax cable plays an important role in 40G cabling for better meeting the increasing requirements.

QSFP DAC Twinax Cable Overview
QSFP DAC twinax cable is also referred to as QSFP+ copper direct attach cable or QSFP copper cable, used for connecting switch to switch or switch to sever. It can be simply divided into three types, QSFP+ to QSFP+ direct attach copper cable, QSFP+ to 4 SFP+ direct attach breakout copper cable and QSFP+ to 4 XFP breakout copper cable. Since the first two types of QSFP DAC twinax cables are more commonly used, the following will mainly talk about these two kinds of QSFP DAC twinax cables.

As for the QSFP+ to QSFP+ direct attach copper cable, it generally consists of a length of copper twinax cable and two QSFP+ connectors, used for 40G to 40G connection, which is capable of distances of up to 10 meters. As for the QSFP+ to 4 SFP+ direct attach breakout copper cable, it is designed for 10G to 40G short reach applications up to 5 meters. Compared to the previous copper cable, it is more complicated, fitted with one QSFP+ connector on one end of the copper twinax cable and four SFP+ connectors on the opposite end of the copper twinax cable.

We used to choose fiber optic transceiver and patch cable to deploy the fiber link, which requires us to firstly insert the transceiver to the switch and then connect the patch cable to the transceiver. However, for the QSFP DAC twinax cable, it can be directly plugged into the switch to finish the 40G connection, without any transceiver, as shown in the following figure. In short, the QSFP DAC twinax cable is really a cost-effective solution for interconnecting high speed 40G switches to 40G switches or 40G switches to existing 10G equipment.

QSFP+ to QSFP+ Direct Attach Copper Cable for 40G to 40G Connection
If you want to deploy a 40G to 40G connection, then you are suggested to choose the QSFP+ to QSFP+ direct attach copper cable that provides a highly economical way to set up a 40G short distance link between QSFP+ ports of QSFP+ switches within racks and across adjacent racks. In principle, this kind of QSFP DAC twinax cable enables four-lane high speed interconnects, and each of the lane operates at 10Gb/s transmission speeds like the lane of SFP+ cable. Obviously, one QSFP+ to QSFP+ direct attach copper cable link is equivalent to 4 SFP+ cable links, that provides greater density and reduced system cost.

QSFP+ to 4 SFP+ Direct Attach Breakout Copper Cable for 10G to 40G Connection
As we know, most data centers can deployed switches with 40G Ethernet ports, but their servers are still fitted with 10G Ethernet ports. Taking this case into consideration, the QSFP+ to 4 SFP+ direct attach breakout copper cable should be chosen for the 10G to 40G connection. When putting it into use, the QSFP+ connector will be connected to the 40G QSFP port of a switch on one end, while the four SFP+ connectors will be connected to four 10G SFP+ ports of a switch on the other end. That’s to say, it allows a 40G Ethernet port to be used as four independent 10G ports thus providing increased density, while permitting backward compatibility and a phased upgrade of equipment.

Conclusion
QSFP DAC twinax cable is an ideal solution for 40G to 40G or 10G to 40G connection, which is highly cost effective to meet the increasing requirements of 40G network. At present, there are two main types of QSFP DAC twinax cables. One is the QSFP+ to QSFP+ direct attach copper cable, which is very suitable for 40G to 40G connection with the distance up to 10 meters. And the other is the QSFP+ to 4 SFP+ direct attach breakout copper cable that can be used for 10G to 40G short reach applications up to 5 meters.

SFP+ Twinax DAC Cables for 1G to 10G Migration

petak , 03.03.2017.

As we know, the 10G SFP+ interface offers the highest density, lowest cost and lowest power 10 Gigabit Ethernet solution, while the SFP+ transceiver is published and available for achieving the migration from 1G to 10G network. However, if the mass 10G migration is required, the total cost for SFP+ fiber optical solution is still high. In order to reduce the cost for 10G migration, experts come up with the SFP+ twinax DAC cable solution for 10G short distance transmission as a ideal cost effective alternative to the fiber optical solution. In this paper, it will mainly talk about the SFP+ twinax DAC cable, which may be useful for you to deploy a 10G short distance transmission in a cost effective manner.

What’s the SFP+ twinax DAC cable?
SFP+ twinax DAC cable, also referred to as SFP+ DAC or 10G twinax cable, is a copper interconnect using a twinax cable assembly that connects directly into an SFP+ housing, designed for 10G short distance transmission. Compared to the fiber optical solution that basically requires two SFP+ transceivers and a optical fiber patch cable (SMF or MMF), the SFP+ twinax DAC cable replaces them with a 10G twinaxial copper cable assembly that is very easy to install. In simple words, you only need to incorporate the SFP+ twinax DAC cable into the physical infrastructure directly, then the 10G short distance connection can be accomplished without any extra signal processing or conversion.

As for its advantage, the SFP+ twinax DAC cable not only features low cost, low power and low latency, but also has the smallest 10G form factor that facilitates the cabling. At present, it can be simply classified into two types, SFP+ passive copper cable and SFP+ active copper cable, which are designed for different applications.

SFP+ Passive Copper Cable vs. SFP+ Active Copper Cable
As for the SFP+ passive copper cable, it costs much less and has fewer components, only including capacitors, resistors, EEPROM and cable. Although it has no active Tx/Rx components, it is more reliable than the active one for no LOS, no TX disable and no interrupts. Meanwhile, there are limited management interfaces, which requires the host to do the work of driving it properly. In general, the SFP+ passive copper cable has a fixed length, typically from 1 m to 7 m.

The SFP+ active copper cable can be longer than the passive one, up to 15 m, which has a higher cost for the added benefits of being "optical-module" like. The most significant benefit is that there is no need to worry the host Tx/Rx for driving copper cables. Meanwhile, it enables pre-emphasis transmission and enhanced signal integrity that equalizes the active and adaptive receiving. In comparison with the passive one, the SFP+ active copper cable has LOS, TX disable and interrupts, and more management interface, which can function as an optical transceiver.

Conclusion
Undoubtedly, the SFP+ twinax DAC cable is an ideal solution for making the migration from 1G to 10G with lower cost, lower power and lower latency. Since there are two kinds of SFP+ twinax DAC cables available on the market, it can be concluded from this paper that the SFP+ passive copper cable is more reliable for the 10G short distance transmission but needs the host to drive the copper cable, which can be got at a fixed length from 1 m to 7 m, and it can be also concluded that the SFP+ active copper cable is more expensive but has no the requirement for the host, which can be ordered with longer length, up to 15 m.