A Study about SFP Modules

A small form-factor pluggable (SFP) transceiver is a compact, hot-swappable, input/output transceiver used in data communication and telecommunications networks. SFP interfaces are generally used in almost all communication devices like switches, routers, and servers. SFP transceivers support communications standards including synchronous optical networking (SONET)(Standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs).), Ethernet and fiber channel. Hence the availability of SFP modules can be either for fiber optic cables or copper cables.

SFP Transceivers have a wide range of detachable interfaces to multimode/single-mode fiber optics, which allows users to select the appropriate transceiver according to the required optical range for the network. SFP transceivers are also available with copper cable interfaces, which allows a host device designed primarily for optical fiber communications to also communicate over unshielded twisted pair networking cables. 

Functionality of SFP Modules

The simplest way to describe the functionality of SFP modules is that these are transceivers(a device that transmits and receives)which can also convert signals from one form to another form. As you know the machines can process the data only in the form of digital signals(1’s & 0’s), In the case of data transmission through fiber optic cables, data has to be converted into light signals. Here SFP modules will convert digital signals to light signals on the sender side and light signals to digital signals on the receiver side. When it comes to copper cable communications, digital signals are converted as electric signals (analog signals). The  SFPs used in this scenario can act as Digital to Analogue Converter (DAC) and Analogue to Digital Converter (ADC).

Modern optical SFP transceivers support digital diagnostics monitoring (DDM) functions, also known as digital optical monitoring (DOM). This feature gives users the ability to monitor the real-time parameters of SFP, such as optical output power, optical input power, temperature, laser-bias current and transceiver supply voltage.

Evolution of SFP Modules

With the increasing demand for higher bandwidth, multiple types of transceiver modules are designed for data transmission, including GBIC, SFP, SFP+, XFP, QSFP, QSFP+ and so on. Well, what do GBIC, SFP, SFP+, and QSFP stand for? Among them, SFP has been used in popularity for over 18 years.

The first generation of a transceiver is GBIC (Gigabit interface converter), which is a hot-swappable transceiver developed in 2000. However, with the demand for higher bandwidth and faster transmission speed, The Small Form Factor Committee, a group consisting of major equipment vendors, created the Mini GBIC(SFP) specification and first published it in 2001. SFP modules built to the initial specifications support data rates of up to 1 Gb. The specification has since been updated to support increased data rates.

The most common name used for Mini GBIC is nothing but the SFP which has replaced GBIC in most applications for the small size. Just as its name, Mini GBIC is similar to the former GBIC in function but with a smaller form factor. It provides flexibility for the transmission in both data and telecommunication applications. Instead of being standardized by official standards, SFP is specified by the multi-source agreement (MSA). MSA is an agreement between many different transceivers manufactures to make standardized products. So the SFP is compatible across a range of telecoms vendors’ hardware. In addition, SFP module can be categorized into various types under Gigabit Ethernet standards and industry-accepted standards, including 1000BASE-T, 1000BASE-TX, 1000BASE-SX, 1000BAS-LX, 1000BASE-LX10, 1000BASE-BX10, 1000BASE-LX/LH, 1000BASE-EX, 1000BASE-ZX, 1000BASE-SR and so on.

SFP Transceivers Over Copper Cabling: 1000BASE-T vs. 1000BASE-TX

1000BASE-T (IEEE 802.3ab) transceiver is 1Gb copper SFP that transmits GE over twisted pairs. They make GE a desktop technology since users can use their existing copper cabling infrastructure. 1000BASE-TX (TIA/EIA-854) is similar copper SFP version but utilizes only two pairs of wires. Below table will give you a better idea of each of these SFPs specifications.

Though 1000BASE-TX needs fewer wires for transmission, 1000BASE-T is still more widely. Possible reasons are that Cat6 cable is more expensive and 1000BASE-T has become cheaper. 1000BASE-T and 1000BASE-TX are generally not inter-operable in switches without a dual physical layer (PHY) since they use different coding approaches.

SFP Transceivers Over Optical Fiber Cabling

SFP transceivers that work over optical fiber consist of standard 1000BASE-SX, 1000BASE-LX and 1000BASE-LX10, and non-standard industry accepted 1000BASE-LX/LH, 1000BASE-LH, 1000BASE-EX ,1000BASE-ZX and 1000BASE-SR. Have a look at the below table which will give you a better idea of each of these SFPs specifications.

SFP modules designed for use with fiber support a variety of wavelengths and maximum distances up to 2 km with multimode and 100 km with a single-mode. 
If you ever wonder why SFP Transceivers over copper cabling is limited than SFP transceiver over fiber cabling, the answer is simple, it’s due to the distance limitation of UTP/STP cables(maximum up to 100m) and its noise disruption characteristics.

Optical SFP Transceivers of Different Fiber Types

We all know that there are two types of fiber optics cables are available. Hence the use of SFP transceivers that work over optical fiber can also be categorized into different types according to their fiber types. There are single-mode SFP and multimode SFP, as well as simplex SFP and duplex SFP.

The primary function to meet the communication using SFP’s are, both the end to end must have similar components. Let’s say for example if you want to interconnect two switches using SFP ports, If this is a single-mode fiber optics cable communication then both the SFP modules used in both the end should be single mode. If this is a multi-mode fiber communication then both the SFP modules which are used in communication should support multimode communication. So let’s say that we can’t make the communication channel by using different mode components. 

Single-mode SFP vs. Multimode SFP

Based on the types of optical fibers SFP transceivers work with, SFP transceivers are divided into single mode SFP that works with single-mode fiber and multimode SFP that works with multimode fiber. Let’s see what’s the major differences between them.

Simplex SFP & Duplex SFP

Apart from the above mentioned SFP transceivers that use dual fibers for duplex transmission (one transmit and one receive separately), there are SFP transceivers that use only a single fiber for transmission. The latter are called simplex SFPs, or more commonly known as bidirectional (BiDi) SFPs, which are equipped with WDM couplers/diplexers. BiDi SFPs support bidirectional communication over a single fiber. While other SFP types require two fibers -- one to send and one to receive -- BiDi SFPs use different wavelengths with one wavelength for sending data and the other for receiving. The most frequently used wavelength pairs of BiDi SFPs are 1310nm/1550nm, 1310nm/1490nm, and 1510nm/1590nm. And the transmission distances vary from 10 km to 160 km when using different wavelengths. It is very easy to distinguish simplex SFP and duplex SFP from the receptacle.

All SFP transceivers should be used in pairs. For duplex SFPs at the two sides, we should connect two SFPs of the same wavelengths. For example, two 850nm SFPs or two 1310nm SFPs. However, for simplex/BiDi SFPs, we should use two SFPs that have opposite wavelengths for transmitter and receiver.

Long-Reach WDM SFP Transceivers of Multiple Wavelengths

As an advanced technology that allows to transmit multiple signals simultaneously on a single fiber, wavelength-division multiplexing (WDM) is utilized by telecom systems in long distance transmission, either DWDM (dense WDM) or CWDM (coarse WDM). In these systems, the lasers of SFP transceivers are chosen with precise wavelengths closely spaced but not so close they interfere with each other.

CWDM SFPs and DWDM SFPs both connect with LC duplex single-mode fiber cables, but they have more differences than similarities. Generally, when the customer needs a number of channels up to 16, it is more economical to deploy CWDM SFP vs. DWDM SFP. For information about how to choose CWDM SFP+ and DWDM SFP+, you can read the article

Video SFP Transceivers for High-Quality Video Services

3G-SDI (serial digital interface) video SFP transceivers are designed to meet the high standard video transmission needs in the High Definition (HD) environment. The key difference between video SFPs and normal SFPs is that the video transmission is uni-directional. Thus, the video SFP can have either two optical transmitters, two optical receivers, a single transmitter, a single receiver, or one optical transmitter and one receiver. Similar to normal SFP transceivers, 3G-SDI video SFPs can be multimode, single-mode, BiDi or CWDM transceivers of different wavelengths, and can have different reaches of distances. More details about video SFP transceivers, please read Understanding Video SFP Transceivers.

PON SFP Transceivers in FTTx Networks

Passive Optical Networking is the key technology used in fiber-based (FTTx) access networks. PON SFP transceivers are used in the Optical Line Terminal (OLT) at the Central Office and the Optical Network Terminal/Unit (ONT/ONU) at the subscriber’s premises. PON SFP differs from other SFP transceivers in that the OLT SFP and ONT SFP are not used in pairs, instead, an OLT SFP should communicate with up to 32 or 64 ONT SFPs. Generally, GPON SFP consists of two types: Class B+ and Class C+. Here are their differences.

Advancing to Higher Speed: From SFP to SFP+ and SFP28

The trend towards higher speed and higher bandwidth is always unstoppable, from Fast Ethernet to Gigabit Ethernet, and then to 10 Gigabit Ethernet and 25 Gigabit Ethernet. At the same time, new devices for transmitting data are published, SFP+ for 10 Gigabit and SFP28 for 25 Gigabit Ethernet. The most obvious difference between them is the data rate, while they all use the same form-factor packaging. Let see what’s the difference SFP vs. SFP+ vs. SFP28.

For these transceivers with the same packaging, a lower speed optical transceiver can be plugged into the slot of higher speed and work at the lower data rate, such as an SFP can be used in an SFP+ slot for 1G transmission, but the contrary is not feasible.

Knowledge & Picture credits: www.fs.com

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