Optical fiber module and optical interface common knowledge

The commonly used optical modules of Ethernet switches are SFP, GBIC, XFP, and XENPAK.
Their full English name:
SFP: Small Form-factorPluggabletransceiver, Small Form Factor Pluggable Transceiver
GBIC: GigaBit InterfaceConverter, Gigabit Ethernet Interface Converter
XFP: 10-Gigabit smallForm-factorPluggable transceiver 10 Gigabit Ethernet Interface
Small package pluggable transceiver
XENPAK: 10-Gigabit EtherNet TransceiverPAcKage 10 Gigabit Ethernet Interface Transceiver Package.
The optical fiber connector
The fiber optic connector consists of a fiber and a plug at both ends of the fiber. The plug consists of a pin and a peripheral locking structure. According to different locking mechanisms, fiber optic connectors can be divided into FC type, SC type, LC type, ST type, and KTRJ type.
The FC connector adopts a thread locking mechanism, which is an earlier and most used optical fiber movable connector.
SC is a rectangular connector, developed by NTT, without thread connection, can be directly inserted and removed, compared with the FC connector has a small operating space, easy to use. Low-end Ethernet products are very common.
LC is a Mini-type SC connector developed by LUCENT. It has a smaller size and has been widely used in the system. It is the future direction of the development of optical fiber connectors. Low-end Ethernet products are very common.
The ST connector was developed by AT&T and uses a bayonet type locking mechanism. The main parameters are the same as those of the FC and SC connectors. However, they are not widely used in the company. They are usually used in the connection of multi-mode devices and devices from other manufacturers. Use more when docking.
KTRJ's pin is plastic. Through the positioning of steel needles, with the increase of the number of plugs, each mating surface will wear, and the long-term stability is inferior to the ceramic pin connector.
Optical fiber knowledge
An optical fiber is a conductor that transmits light waves. Optical fibers can be divided into single-mode fibers and multi-mode fibers from the mode of optical transmission.
In the single-mode fiber, there is only one mode of light transmission, that is, light is transmitted only along the inner core of the fiber. Due to the complete avoidance of mode dispersion makes the transmission bandwidth of single-mode fiber very wide, it is suitable for high-speed, long-distance fiber communication.
There are multiple modes of optical transmission in a multimode optical fiber. Due to dispersion or aberration, the transmission performance of this optical fiber is poor, the frequency band is narrow, the transmission rate is small, and the distance is short.
Optical fiber characteristic parameters
The structure of the optical fiber is prepared by prefabricated quartz fiber rods, and the outer diameters of the multimode fiber and the single mode fiber for communication are both 125 μm.
The slimming body is divided into two areas: the core and the Cladding layer. Single-mode fiber core diameter of 8 ~ 10μm, multimode fiber core diameter has two standard specifications, the core diameter of 62.5μm (US standards) and 50μm (European standard).
The interface fiber specification has this description: 62.5 μm/125 μm multimode fiber, where 62.5 μm refers to the core diameter of the fiber and 125 μm refers to the outer diameter of the fiber.
The wavelength of light used by a single-mode fiber is 1310 nm or 1550 nm.
The optical wavelength used for multimode fiber is mostly 850 nm.
Single mode fiber and multimode fiber can be distinguished from each other in color. The single-mode fiber outer body is yellow, and the outer body of the multi-mode fiber is orange-red.
Gigabit optical port
Gigabit optical ports can work in both mandatory and self-negotiating modes. In the 802.3 specification, the Gigabit optical ports only support the 1000M rate and support full-duplex and half-duplex modes.
The most fundamental difference between auto-negotiation and enforcement is that the code streams sent when the two physical links are established are different. The self-negotiation mode sends the /C/ code, which is the Configuration stream, and the mandatory mode sends the I/code, ie idle stream.
Gigabit optical auto-negotiation process
First, both ends are set to auto-negotiation mode
Both parties send /C/code streams to each other. If three consecutive /C/ codes are continuously received and the received code stream matches the working mode of the local end, a /C/ code with an Ack reply is returned to the other party. After receiving the Ack information, the peer considers that the two can communicate with each other and set the port to the UP state.
Second, one end is set to auto-negotiation and one end is set to mandatory
The negotiating side sends the /C/ bitstream and forces the peer to send the /I/ bitstream. The forced end cannot provide the local end negotiation information to the peer end and cannot return the Ack reply to the peer end, so the self-negotiation peer goes DOWN. However, the forcible end itself can identify the /C/ code, and thinks that the peer end is a port that matches itself, so directly set the local port to the UP state.
Third, both ends are set to mandatory mode
Both parties send /I/streams to each other, and when one end receives the /I/stream, the peer is considered to be a port that matches itself, and the local port is set to the UP state.
How does fiber work?
Communication optical fibers consist of thin, hair-like glass filaments coated with a protective layer of plastic. The glass filament consists essentially of two parts: a low refractive index glass material having a core diameter of 9 to 62.5 μm and an outer covering diameter of 125 μm. Although there are some other kinds of optical fibers according to the materials used and different sizes, the most common ones are mentioned here. Light is transmitted in the "core internal reflection" mode at the core part of the fiber, that is, after the light enters one end of the fiber, it is reflected back and forth between the core and the clad interface, and then transmitted to the other end of the fiber. An optical fiber with a core diameter of 62.5 μm and a cladding outer diameter of 125 μm is called 62.5/125 μm light.
What is the difference between multimode and singlemode fiber?
Fibers that can transmit hundreds to thousands of modes are called multimode (MM) fibers. According to the radial distribution of the refractive index in the core and cladding, it can be divided into step multimode fiber and gradient multimode fiber. Almost all multimode fiber sizes are 50/125 μm or 62.5/125 μm, and the bandwidth (the amount of information transmitted by the fiber) is usually 200 MHz to 2 GHz. Multi-mode optical transceivers can transmit up to 5 kilometers over multimode fiber. Use a light emitting diode or laser as the light source.
Single mode:
Fibers that can only propagate one mode are called single-mode fibers. Standard single-mode (SM) fiber refractive index profiles are similar to step-type fibers, except that the core diameter is much smaller than the multimode fiber.
Single-mode fibers are 9-10/125 μm in size and have an infinite amount of bandwidth and lower loss characteristics than multimode fibers. Single-mode optical transceivers are often used for long-distance transmission, sometimes reaching 150 to 200 kilometers. Use LD or narrow-spectrum LED as the light source.
Differences and links:
Single-mode devices typically operate on single-mode fibers and on multimode fibers, while multimode devices are limited to multimode fibers.
How is the transmission loss when using fiber optic cable?
This depends on the wavelength of the transmitted light and the type of fiber used.
850nm wavelength for multimode fiber: 3.0dB/km
1310nm wavelength for multimode fiber: 1.0dB/km
1310nm wavelength for single-mode fiber: 0.4dB/km
1550nm wavelength for single-mode fiber: 0.2dB/km
What is GBIC?
GBIC is the abbreviation of Giga Bitrate Interface Converter and is an interface device that converts Gigabit electrical signals into optical signals. GBIC is designed to be hot swappable. GBIC is an interchangeable product that meets international standards. The Gigabit switch with GBIC interface design has a large market share due to its flexibility in exchange.
What is SFP?
SFP is the abbreviation of SMALL FORM PLUGGABLE and can be simply understood as an upgraded version of GBIC. The SFP module is half the size of the GBIC module and can be configured with more than double the number of ports on the same panel. The other functions of the SFP module are basically the same as those of the GBIC. Some switch vendors call the SFP module a miniaturized GBIC (MINI-GBIC).
Future optical modules must be hot-swappable. That is, the module can be connected to or disconnected from the device. The optical module is hot-swappable. Network administrators can upgrade and expand the system without shutting down the network. Users will not have any effect. Hot pluggability also simplifies overall maintenance work and allows end users to better manage their transceiver modules. At the same time, thanks to this type of heat exchange performance, the module enables network managers to perform overall planning of send and receive costs, link distances, and all network topologies based on network upgrade requirements without having to replace all system boards.
Supporting this hot swappable optical module is currently GBIC and SFP. Because SFP and SFF are similar in size, it can be directly inserted on the circuit board, which saves space and time on the package, and has a wide application area. Therefore, Its future development is worth looking forward to, and may even threaten the SFF market.
What is SFF?
The Small Form Factor (SFF) small-package optical module uses advanced precision optics and circuit integration technology. It is only half the size of an ordinary duplex SC (1X9) optical fiber transceiver module, and can double the number of optical ports in the same space. Increase line port density and reduce system cost per port. Because the small SFF module adopts a KT-RJ interface similar to a copper network, the size is the same as that of a common computer network copper interface, which facilitates the transition from a copper-based network device to a higher-speed fiber network. To meet the dramatic increase in network bandwidth requirements.
Network connection device interface type
BNC interface
The BNC interface refers to the coaxial cable interface. The BNC interface is used to connect the 75-ohm coaxial cable. It provides two channels (RX and TX) for unbalanced signal connection.
Fiber Interface
Fiber interfaces are physical interfaces used to connect fiber cables. Usually there are SC, ST, LC, FC and other types. For a 10Base-F connection, the connector is usually of ST type and the FC at the other end is connected to an optical fiber cradle. FC is the abbreviation of FerruleConnector. The external reinforcement method is the use of metal sleeves. The fastening method is a turnbuckle. ST interface is usually used for 10Base-F, SC interface is usually used for 100Base-FX and GBIC, and LC is usually used for SFP.
RJ-45 interface
The RJ-45 interface is the most commonly used interface for Ethernet. RJ-45 is a common name, referring to the standardization of IEC(60)603-7 and the use of 8 positions defined by international connector standards (8 pins). Modular jack or plug.
RS-232 interface
RS-232-C interface (also known as EIA RS-232-C) is the most commonly used serial communication interface. It was established in 1970 by the United States Electronic Industries Association (EIA) in conjunction with the Bell system, modem manufacturers and computer terminal manufacturers for the serial communication standard. Its full name is "a serial binary data exchange interface technology standard between data terminal equipment (DTE) and data communication equipment (DCE)." The standard specifies the use of a 25-pin DB25 connector to specify the signal content of each pin of the connector and to specify the level of various signals.
RJ-11 interface
The RJ-11 interface is what we usually call a telephone line interface. The RJ-11 is a generic name for a connector developed by Western Electric. Its shape is defined as a 6-pin connection device. Formerly known as WExW, x stands for "active", contacts or needles. For example, WE6W has all 6 contacts, numbers 1 to 6, WE4W interface uses only 4 pins, and the outermost two contacts (1 and 6) are not used. WE2W only uses the middle two pins (ie, the telephone line interface).
With the rapid growth of the Internet's IP data services, the demand for transmission line bandwidth has increased. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective method to solve line bandwidth expansion, CWDM (Coarse Wavelength Division Multiplexing) technology has advantages over DWDM in terms of system cost and maintainability.
Both CWDM and DWDM are wavelength division multiplexing technologies. Both can couple different wavelengths of light to a single core fiber for transmission.
The ITU standard for CWDM is G.695, which specifies 18 wavelength channels with a spacing of 20 nm from 1271 nm to 1611 nm. In consideration of the water peak effect of an ordinary G.652 fiber, 16 channels are generally used. Because of the large channel spacing, the multiplexing and demultiplexing devices and lasers are cheaper than DWDM devices.
The DWDM channel spacing needs 0.4nm, 0.8nm, 1.6nm and other different intervals according to the needs, the interval is small, need extra wavelength control device, so the equipment based on DWDM technology is higher than the equipment based on CWDM technology.
The PIN photodiode is a lightly doped N-type material, known as an I (Intrinsic) layer, between P-type and N-type semiconductors with high doping concentrations. Since it is lightly doped, the electron concentration is very low and a wide depletion layer is formed after diffusion, which can improve its response speed and conversion efficiency.
APD avalanche photodiode, which not only has optical/electrical conversion, but also has internal amplification. Its amplification is accomplished by the avalanche multiplication effect inside the tube. APD is a photodiode with gain. In applications where the optical receiver sensitivity is high, the use of APD is beneficial to extend the system's transmission distance.

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