Common knowledge of weak electrical systems such as optical fibers, optical modules, optical interfaces, and optical jumpers

Optical switches commonly used in Ethernet switches include SFP, GBIC, XFP, and XENPAK.

Their full English names:

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 EtherNetTransceiverPAcKage 10 Gigabit Ethernet interface transceiver set package.

The optical fiber connector

The optical fiber connector is composed of an optical fiber and a plug at both ends of the optical fiber, and the plug is composed 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.

FC connector adopts thread locking mechanism, it is an optical fiber movable connector that was invented earlier and used most.

SC is a rectangular joint developed by NTT. It can be directly plugged and unplugged without screw connection. Compared with FC connector, it has a small operating space and is 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 a direction for the development of fiber optic active connectors in the future. Low-end Ethernet products are very common.

The ST connector is developed by AT & T and uses a bayonet-type locking mechanism. The main parameters are equivalent to FC and SC connectors, but it is not commonly used in companies. It is usually used for multimode devices to connect with other manufacturers Used more when docking.

KTRJ’s pins are plastic. They are positioned by steel pins. As the number of mating times increases, the mating surfaces will wear out, and their long-term stability is not as good as that of ceramic pin connectors.

Fiber knowledge

Optical fiber is a conductor that transmits light waves. Optical fiber can be divided into single-mode fiber and multi-mode fiber from the mode of optical transmission.

In single-mode fiber, there is only one fundamental mode of optical transmission, that is, light is transmitted only along the inner core of the fiber. Because the mode dispersion is completely avoided and the transmission band of the single-mode fiber is wide, it is suitable for high-speed and long-distance fiber communication.

There are multiple modes of optical transmission in a multimode fiber. Due to dispersion or aberrations, this fiber has poor transmission performance, a narrow frequency band, a small transmission rate, and a short distance.

Optical fiber characteristic parameters

The structure of the optical fiber is drawn by prefabricated quartz fiber rods. The outer diameter of the multimode fiber and single mode fiber used for communication is 125 μm.

Slim body is divided into two areas: core and cladding layer. The core diameter of single-mode fiber is 8 ~ 10μm, and the core diameter of multimode fiber has two standard specifications. The core diameters are 62.5μm (American standard) and 50μm (European standard).

The interface fiber specifications are described as follows: 62.5μm / 125μm multimode fiber, where 62.5μm refers to the core diameter of the fiber and 125μm refers to the outer diameter of the fiber.

Single-mode fiber uses a wavelength of 1310nm or 1550 nm.

Multimode fibers use mostly 850 nm light.

Color can be distinguished from single-mode fiber and multi-mode fiber. The single-mode fiber outer body is yellow, and the multi-mode fiber outer body is orange-red.

Gigabit optical port

Gigabit optical ports can work in both forced and self-negotiated modes. In the 802.3 specification, the Gigabit optical port only supports a 1000M rate, and supports two full-duplex (Full) and half-duplex (Half) duplex modes.

The most fundamental difference between auto-negotiation and forcing is that the code streams sent when the two establish a physical link are different. The auto-negotiation mode sends the / C / code, which is the Configuration code stream, while the forcing mode sends / I / code, which is the idle code stream.

Gigabit optical port auto-negotiation process

First, both ends are set to auto-negotiation mode

The two parties send / C / code streams to each other. If 3 consecutive / C / codes are received and the received code streams match the local working mode, they will return to the other party with a / C / code with an Ack response. After receiving the Ack message, the peer considers that the two can communicate with each other and sets the port to the UP state.

Second, Set one end to auto-negotiation and one end to mandatory

The self-negotiating end sends / C / stream, and the forcing end sends / I / stream. The forcing end cannot provide the local end with the negotiation information of the local end, nor can it return an Ack response to the remote end, so the self-negotiation end is DOWN. However, the forcing end itself can identify the / C / code, and considers that the peer end is a port that matches itself, so the local end port is directly set to the UP state.

Third, both ends are set to force mode

Both parties send / I / stream to each other. After receiving the / I / stream, one end considers the peer to be a port that matches itself, and directly sets the local port to the UP state.

How does fiber work?

Optical fibers for communications consist of hair-like glass filaments covered with a protective plastic layer. The glass filament is essentially composed of two parts: a core diameter of 9 to 62.5 μm, and a low refractive index glass material with a diameter of 125 μm. Although there are some other types of optical fiber according to the materials used and the different sizes, the most common ones are mentioned here. Light is transmitted in the core layer of the fiber in a “total internal reflection” mode, that is, after the light enters one end of the fiber, it is reflected back and forth between the core and cladding interfaces, 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 single mode fiber?

Multimode:

Fibers that can propagate 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 graded 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. Multimode optical transceivers can transmit up to 5 kilometers through multimode fiber. Use light emitting diode or laser as light source.

Single mode:

Fibers that can only propagate one mode are called single-mode fibers. The refractive index profile of standard single-mode (SM) fibers is similar to that of step-type fibers, except that the core diameter is much smaller than that of multimode fibers.

The size of the single-mode fiber is 9-10 / 125 μm, and it has the characteristics of infinite bandwidth and lower loss than the multi-mode fiber. Single-mode optical transceivers are mostly used for long-distance transmission, sometimes reaching 150 to 200 kilometers. Use LD or LED with narrow spectral line as light source.

Difference and connection:

Single-mode equipment can usually run on single-mode fiber or multi-mode fiber, while multi-mode equipment is limited to operate on multi-mode fiber.

What is the transmission loss when using optical cables?

This depends on the wavelength of the transmitted light and the type of fiber used.

850nm wavelength for multimode fiber: 3.0 dB / km

1310nm wavelength for multimode fiber: 1.0 dB / km

1310nm wavelength for single-mode fiber: 0.4 dB / km

1550nm wavelength for single-mode fiber: 0.2 dB / km

What is GBIC?

GBIC is the abbreviation of Giga Bitrate Interface Converter, which is an interface device that converts gigabit electrical signals into optical signals. GBIC is designed for hot plugging. GBIC is an interchangeable product that complies with international standards. Gigabit switches designed with GBIC interface occupy a large market share in the market due to their flexible interchange.

What is SFP?

SFP is the abbreviation of SMALL FORM PLUGGABLE, which can be simply understood as an upgraded version of GBIC. The size of the SFP module is reduced by half compared to the GBIC module, and the number of ports can be more than doubled on the same panel. The other functions of the SFP module are basically the same as those of the GBIC. Some switch manufacturers call the SFP module a mini-GBIC (MINI-GBIC).

Future optical modules must support hot plugging, that is, the module can be connected or disconnected from the device without cutting off the power supply. Because the optical module is hot pluggable, network managers can upgrade and expand the system without closing the network. The user does not make any difference. Hot swappability also simplifies overall maintenance and enables end users to better manage their transceiver modules. At the same time, due to this hot-swap performance, this module enables network managers to make overall plans for transceiver costs, link distances, and all network topologies based on network upgrade requirements, without having to completely replace system boards.

The optical modules that support this hot-swap are currently available in GBIC and SFP. Because SFP and SFF are approximately the same size, they can be directly plugged into the circuit board, saving space and time on the package, and have a wide range of applications. Therefore, Its future development is worth looking forward to, and may even threaten the SFF market.

SFF (Small Form Factor) small package optical module uses advanced precision optics and circuit integration technology, the size is only half that of ordinary duplex SC (1X9) fiber optic transceiver module, which can double the number of optical ports in the same space. Increase line port density and reduce system cost per port. And because the SFF small package module uses a KT-RJ interface similar to the copper network, the size is the same as the common computer network copper interface, which is conducive to the transition of existing copper-based network equipment to higher-speed fiber optic networks. To meet the dramatic increase in network bandwidth requirements.

Network connection device interface type

BNC interface

BNC interface refers to the coaxial cable interface. The BNC interface is used for 75 ohm coaxial cable connection. It provides two channels of receiving (RX) and transmitting (TX). It is used for the connection of unbalanced signals.

Fiber interface

A fiber interface is a physical interface used to connect fiber optic cables. There are usually several types such as SC, ST, LC, FC. For the 10Base-F connection, the connector is usually ST type, and the other end FC is connected to the fiber optic patch panel. FC is the abbreviation of FerruleConnector. The external reinforcement method is a metal sleeve and the fastening method is a screw button. ST interface is usually used for 10Base-F, SC interface is usually used for 100Base-FX and GBIC, 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 commonly used name, which refers to the standardization by IEC (60) 603-7, using 8 positions (8 pins) defined by the international connector standard. 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 is a standard for serial communication jointly developed by the American Electronics Industry Association (EIA) in 1970 in conjunction with Bell systems, modem manufacturers, and computer terminal manufacturers. Its full name is “serial binary data exchange interface technology standard between data terminal equipment (DTE) and data communication equipment (DCE)”. The standard stipulates that a 25-pin DB25 connector is used to specify the signal content of each pin of the connector, as well as the level of various signals.

RJ-11 interface

The RJ-11 interface is what we usually call a telephone line interface. RJ-11 is a generic name for a connector developed by Western Electric. Its outline is defined as a 6-pin connection device. Originally called WExW, where x means “active”, contact or threading needle. For example, WE6W has all 6 contacts, numbered 1 to 6, WE4W interface uses only 4 pins, the two outermost contacts (1 and 6) are not used, WE2W uses only the middle two pins (that is, for telephone line interface).

CWDM and DWDM

With the rapid growth of IP data services on the Internet, the demand for transmission line bandwidth has increased. Although DWDM (Dense Wavelength Division Multiplexing) technology is the most effective method to solve the problem of line bandwidth expansion, CWDM (Coarse Wavelength Division Multiplexing) technology has advantages over DWDM in terms of system cost and maintainability.

Both CWDM and DWDM belong to the wavelength division multiplexing technology, and they can couple different wavelengths of light into a single-core fiber and transmit them together.

CWDM’s latest ITU standard is G.695, which specifies 18 wavelength channels with a 20nm interval from 1271nm to 1611nm. Considering the water peak effect of ordinary G.652 optical fibers, 16 channels are generally used. Because of the large channel spacing, multiplexing and demultiplexing devices and lasers are cheaper than DWDM devices.

The channel interval of DWDM has different intervals such as 0.4nm, 0.8nm, 1.6nm, etc. The interval is small and additional wavelength control devices are needed. Therefore, the equipment based on DWDM technology is more expensive than the equipment based on CWDM technology.

A PIN photodiode is a layer of lightly doped N-type material between a P-type and N-type semiconductor with a high doping concentration, which is called an I (Intrinsic) layer. Because 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 photodiodes have not only optical / electrical conversion but also internal amplification. The amplification is accomplished by the avalanche multiplication effect inside the tube. APD is a photodiode with gain. When the sensitivity of the optical receiver is high, APD is helpful to extend the transmission distance of the system.