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SFP Transceiver: To Be or Not To Be?

Gigabit Ethernet has supplanted its predecessor—Fast Ethernet in wired local networks and becomes ubiquitous throughout the world, serving as one of the most prevalent enterprise communication standards. The Gigabit Ethernet standard supports a theoretical maximum data rate of 1 gigabit per second (Gbps) (1000 Mbps), that is 10 times faster than Fast Ethernet, yet is compatible with existing Ethernet. To link your switches and routers to a Gigabit Ethernet network, you need a Gigabit Ethernet transceiver as a transmission medium. This tutorial intends to introduce the most commonly used one—SFP transceivers.

What Is SFP Transceiver?

SFP, short for small form-factor pluggable, is a compact, hot-pluggable transceiver used for both telecommunication and data communications applications. SFP transceiver can be regarded as the upgrade version of GBIC module. Unlike GBIC with SC fiber optic interface, SFP is with LC interface and the main body size of SFP is only about half of GBIC so that it can save more space. SFP interfaces a network device mother board (for a router, switch, media converter or similar devices) to a fiber optic or copper networking cable. Meanwhile, SFP is a popular industry format supported by many network component vendors. It is designed to support SONET, Gigabit Ethernet, Fibre Channel, and other communications standards.

Evolution of SFP Transceiver

GBIC (Gigabit Interface Converter) and SFF (Small Form Factor) have had a very extraordinary history in the development of optical transceivers, but with the rapid development of network, they are gradually replaced by SFP optical modules.

The SFP optical module not only inherits the hot plug features of GBIC, but also draws on the advantages of miniaturization of SFF. SFP is only half size of GBIC with LC interface, greatly increasing port density of network devices. Consequently, SFP modules have seen widespread application by equipment manufacturers instead of GBIC transceivers. Due to a uniform standard, SFP products from various manufacturers can be mutually compatible, and SFP products can be purchased as a separate network device.

Although there comes out more higher technology along with newer optical modules, the SFP optical transceivers will continue to exist for a long time. After SFP, optical modules are turning towards higher rate, and now, 10G, 40G, and 100G optical transceivers are already available on the market. Besides, SFP+ optical module is believed to be a widely used product after SFP.

Types & Applications of SFP transceivers

SFP transceivers are available with various transmitter and receiver types, which facilitates users to select the appropriate transceiver module for different optical reach and optical fiber type (single-mode fiber or multimode fiber) required by every different link. SFP transceiver modules can be divided into several different categories:

Types Transmission Medium Wavelength Distance
1000BASE-T SFP Twisted-pair cabling - 100 m
1000BASE-SX SFP Multimode fiber 770-860 nm OM1-275 m/OM2-550 m
1000BASE-LX/LH SFP SMF & MMF 1270-1355 nm MMF-550 m/SMF-5 km
1000BASE-ZX SFP Single mode fiber 1550 nm 70 km
1000BASE-EX SFP Single mode fiber 1310 nm 40 km
1000BASE-BX10 SFP Single mode fiber 1480-1500 nm downstream, 1260-1,360 nm upstream 10 km
CWDM and DWDM SFP Single optical fiber various wavelengths various maximum distances

SFP modules are found in Ethernet switches, routers, firewalls and network interface cards. Storage interface cards, also called HBAs or Fibre Channel storage switches, also make use of these modules, supporting different speeds such as 2Gb, 4Gb, and 8Gb. Because of their low cost, low profile, and ability to provide a connection to different types of optical fiber, SFP provides such equipment with enhanced flexibility.

A Great Contributor to SFP Transceiver–MSA

At the mention of optical transceiver module, an important contributor can’t be ignored, that is multisource agreement. A multi-source agreement (MSA) is an agreement between multiple manufacturers to make products which are compatible across vendors, acting as de facto standards, establishing a competitive market for interoperable products. Both SFP and SFP+ optical transceivers are standardized by multi-source agreements (MSAs). These documents strictly define sufficient characteristics of an optical transceiver so that system (e.g. Ethernet switch, router, and media converter) vendors may implement ports on their devices, and so MSA compliant pluggable optical transceivers from any vendor will function properly.

What Are Defined In the SFP MSAs?

The major elements or characteristics defined in the SFP MSAs are:

 Mechanical Interface

1. Mechanical dimensions (H: 8.5 mm, W: 13.4 mm, D: 56.5 mm)

2. Transceiver edge connector to host PCB-mounted electrical connector mating

3. Host board layout (location/size of solder pads, etc.)

4. Insertion, extraction and retention forces

5. Labeling

6. Bezel design considerations for host systems

7. Electrical connector mechanical specifications

8. Cage assembly dimensions

 Electrical Interface

1. Pin definitions

2. Timing requirements of control and status I/O

3. Module definition interface and data field description

Why Are MSAs Important?

As we talk above, MSAs define the characteristics of fiber optic transceiver system. To users, MSAs are important because they give consumers piece of mind that transceiver devices will have a basic level of operability. With MSAs, consumers will have more freedom in the choices. Of course, some products may be superior than other. This will allow people to compete and gain a share of the market without creating a design that’s completely different from all other devices. To the industry, before MSAs launched, a couple of companies get together, work behind closed doors, and now these standards force suppliers to be efficient and creative to find ways to drive costs down and offer customers the widest array of options.

A Fiber Tester Inside SFPs–DDM/DOM

If you take a look at the description of a SFP transceiver module, you will see the “DDM” or “DOM” labeled in the product description. What does it mean? In fact, DDM or Digital Diagnostics Monitoring, as the name implies, is used for monitoring some parameters of the transceiver, which can help to identify the location of the fiber link failure, simplify maintenance, and improve system reliability. This feature is also known as digital optical monitoring (DOM). Measurement of these parameters can help network administrators to check and ensure that the module is functioning correctly. These real-time operating parameters include:

 Optical Tx power  Optcal Rx power  Laser bias current  Temperature  Transceiver supply voltage

A DDM interface allows end users to display diagnostics data and alarms for fiber optical transceivers and can be used to diagnose why a transceiver optics is not working, increasing popularity of transceiver optics with DDM. Generally, the transceiver vendor sets the thresholds that trigger a high alarm, low alarm, high warning, or low warning before shipment. In order to be able to take advantage of DDM/DOM capability, most of the modern pluggable transceiver optics support DDM/DOM interfaces.

New Upgrade, Coming Soon–SFP-DD

In July, 2017, the SFP-DD MSA Group has committed to develop brand new specification for small form-factor pluggable (SFP-DD) interface which features in high-speed and double-density. New SFP-DD based networking equipment will support legacy SFP modules and cables, such as DAC and AOC cables, as well as new double density products. Originally, the SFP interface’s single electrical lane operates up to 25 Gbps NRZ or 56 Gbps PAM4. while the new SFP-DD is designed to use electrical lanes that run at up to 25 Gbps via NRZ or 56 Gbps via PAM4, providing solutions up to 50 Gbps or 112 Gbps PAM4 aggregate. Systems designed for SFP-DD modules will be backward compatible with SFP modules to provide maximum flexibility for end users, network platform designers and integrators.

sfp dd transciever

Summary

SFP transceiver modules were widely used due to its small size and pluggability, gradually taking the place of GBIC and SFF. But with the increasing requirement for high throughput and bandwidth, the data center is now shifting into optical transceivers with higher port density and high bitrate such as SFP+, QSFP and SFP28, etc. SFP transceiver is, to some extent, gradually fading away. However, it is undeniable that there are still some vendors or customers using SFP transceiver to link switches or routers. So SFP: to be or not to be? Who knows.