November 16, 2023

General knowledge of low current systems such as optical fibers, optical modules, optical interfaces and optical jumpers.

ForewordHi everyone, I'm Mr. Lin, well versed in low current systems such as optical fibers, optical modules, optical interfaces and optical jumpers. Let's get a look! TextOptical modules commonly used in Ethernet switches include SFP, GBIC, XFP, XENPAK.

Their full English names:

SFP:Small Form FactorPluggabletransceiver, Compact Pluggable Transceiver

GBIC: GigaBit Interface Converter, Gigabit Ethernet Interface Converter

XFP:10 Gigabit Small Form Factor Pluggable Transceiver

Small size replacement transceiver

XENPAK:10-GigabitEtherNetTransceiverPAcKage 10-Gigabit Ethernet interface transceiver assembly package.

Fiber optic connector

Fiber optic connectors consist of optical fibers and plugs at both ends of optical fibers. Plugs consist of pins and peripheral retaining structures. 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 uses a thread locking mechanism, which is an earlier invention and is most widely used active fiber optic connector.

SC is a rectangular connector designed by NTT. It does not require a threaded connection and can be connected directly. Compared with FC connector, it has a small working space and is easy to use. Entry-level Ethernet products are very common.

LC is a mini type SC connector developed by LUCENT. It is smaller and widely used in system. This is direction for development of active fiber optic connectors in future. Entry-level Ethernet products are very common.

The ST connector was developed by AT&T. It uses a bayonet locking mechanism. In terms of basic parameters, it is comparable to FC and SC connectors, but is not used in companies. Typically used in multimode device connections. often used when connecting equipment from other manufacturers.

KTRJ pins are made of plastic and are positioned with steel pins. As number of connections and disconnections increases, mating surfaces wear out and long-term stability is not as good as ceramic pin headers. .

Fiber Knowledge

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

In single-mode fiber, there is only one primary mode of light transmission, which means that light is transmitted only along inner core of fiber. Because themode dispersion is completely eliminated, transmission bandwidth of single-mode fiber is very wide, so it is suitable for high-speed and long-distance fiber communication.

There are several modes of optical transmission in multimode fiber. Due to dispersion or aberration, transmission performance of this fiber is poor, frequency band is narrow, transmission rate is small, and distance is short.

Fibre characteristics

The optical fiber structure is drawn from a prefabricated silica fiber rod, and outer diameter of multimode optical fiber and single-mode optical fiber used for communication is 125μm.

The slimming body is divided into two areas: core (Core) and shell (Claddinglayer). The core diameter of single-mode fiber is 8~10µm, and core diameter of multimode fiber has two standard specifications: core diameter is 62.5µm (US standard) and 50µm (European standard).

The interface fiber specification has following description: 62.5 µm/125 µm multimode fiber, where 62.5 µm refers to core diameter of fiber and 125 µm refers to outer diameter of fiber.

The wavelength of light used by single-mode fiber is 1310nm or 1550nm.

The wavelength of light used in multimode fiber is mainly 850nm.

Singlemode fiber and multimode fiber can be distinguished by color. The outer shell of single mode fiber is yellow and outer shell of multimode fiber is orange.

Gigabit optical port

Gigabit optical ports can operate in two modes: mandatory and auto-negotiation. In 802.3 specification, Gigabit optical port only supports 1000M speed and supports two duplex modes: full duplex (Full) and half duplex (Half).

The most fundamental difference between auto-negotiation and mandatory is that code streams sent when a physical connection is established are different. The auto-negotiation mode sends /C/ code, i.e. Configuration code stream. , while forced mode sends /I/, which is a stream of wait code.

Gigabit optical port auto-negotiation process

1. Both ends are configured for self-negotiation mode

Parties send codestreams /C/ to each other. If they receive 3 identical /C/ codestreams and received codestreams match operating mode of local end, they return /C with an Ack response to other side./code, after receiving Ack message, peer considers that they can communicate with each other , and sets port to UP.

Second, one end is set to self-negotiate and other end is required

Self-negotiating side sends streamcode /C/, and forced negotiation side sends code stream /I/. end, so auto-negotiate end DOWN. But force end itself can recognize /C/ code and think that opposite end is a port that matches itself, so directly set local end port to UP state.

Third, both ends are set to mandatory

Both sides send code stream /I/ to each other. After receiving /I/ code stream, one side considers other side to be a port that matches itself and directly sets local port to UP state.

How Optical Fiber Works Optical fibers for communication are made from thin strands of glass coated with a protective plastic layer. The glass filament is mainly composed of two parts: a core with a diameter of 9 to 62.5 µm and a low refractive index glass material with a diameter of 125 µm. Although there are other types of optical fiber depending on material used and different sizes, most common ones are mentioned here. Light is transmitted at center of fiber by "total internal reflection", which means that after light hits one end of fiber, it is reflected back and forth between core layer and cladding interface, and then transmitted to other end of fiber. An optical fiber with a core diameter of 62.5 µm and an outer cladding diameter of 125 µm is called a 62.5/125 µm optical fiber.

What is difference between multimode and singlemode fiber?


Fibers that can transmit hundreds to thousands of modes are called multimode (MM) fibers. According to radial distribution of refractive index in core and cladding, it can be divided into stepped multimode fiber and smooth transition multimode fiber. Almost all multimode fibers are 50/125 µm or 62.5/125 µm, and bandwidth (the amount of information carried over fiber) is typically between 200 MHz and 2 GHz. The multimode optical transceiver can transmit up to 5 kilometers over multimode optical fiber. LEDs or lasers are used as light sources.

Single mode:

A fiber that can only propagate one mode is called a single-mode fiber. Standard single-mode (SM) fiber has a refractive index profile similar to that of stepped fiber, except that core diameter is much smaller than that of multimode fiber.

Singlemode fiber is 9-10/125um in size and has characteristics of unlimited bandwidth and lower loss than multimode fiber. The single mode optical transceiver is mainly used for long distance transmission, sometimes up to 150-200 kilometers. Use an LD or LED with a narrow spectral line as light source.

The difference between and sligature:

Singlemode equipment typically works with both singlemode and multimode fiber, while multimode equipment is limited to working on modal fiber.

What is transmission loss when using fiber optic cable?

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

Using 850 nm multimode fiber: 3.0 dB/km

Using 1310 nm multimode fiber: 1.0 dB/km

Using 1310 nm single mode fiber: 0.4 dB/km

Using 1550 nm single-mode fiber: 0.2 dB/km

What is a GBIC?

GBIC is short for Giga Bitrate Interface Converter, which is an interface device that converts Gigabit electrical signals to optical signals. The GBIC is designed to be hot swappable. GBIC is an interchangeable product that complies with international standards. Gigabit switches designed with GBIC interface have a large market share due to their flexible interchangeability.

What is SFP?

SFP is short for SMALL FORM PLUGGABLE, which can simply be understood as an updated version of GBIC. The size of an SFP module is half that of a GBIC module, and more than twice as many ports can be configured on a single panel. The other functions of SFP module are basically same as those of GBIC. Some switch manufacturers refer to SFP module as Miniaturized GBIC (MINI-GBIC).

Future optical modules should be hot-swappable, meaning module can be plugged in or out of device without powering down. Because optical module is hot-swappable, network administrators can upgrade and expand system without network outages affecting online users. The hot-swap capability also simplifies overall maintenance and allows end users to better manage their transceiver modules. At same time, with hot-swap performance, module allows network administrators to make overall plans for sending and receiving costs, communication distances, and all network topologies in accordance with network upgrade requirements without replacing all motherboards.

Currently, there are GBIC and SFP optical modules that are hot swappable. Since SFPs and SFFs are same size, they can be inserted directly onto PCB, which saves space and time in packaging and has a wide range of applications. , so its future development is worth waiting for and may even threaten small form factor market.

What is SFF?

The SFF (Small Form Factor) small optical module adopts advanced precision optics and circuit integration technology, its size is only half size of a conventional SC (1X9) duplex fiber optic transceiver module, and optical port can bebe doubled in same case. space This number can increase line port density and reduce system cost per port. And since SFF small body module uses KT-RJ interface, similar to copper wire network, size is same as copper wire interface of general computer network, which is conducive to transition of existing copper cable network equipment to high-speed fiber optic network To meet dramatic increase demand for network bandwidth.

Network connection device interface type

BNC interface

The BNC interface refers to coaxial cable interface. The BNC interface is used to connect a 75 ohm coaxial cable. It provides two receive (RX) and send (TX) channels. It is used to connect unbalanced signals.

Fiber interface

A fiber optic interface is a physical interface used to connect a fiber optic cable. There are usually several types such as SC, ST, LC and FC. For a 10Base-F connection, connector is usually ST type, and other end is FC connected to optical cable rack. FC is an abbreviation for FerruleConnector, external reinforcement method is a metal sleeve, attachment method is a lanyard. The ST interface is typically used for 10Base-F, SC interface is typically used for 100Base-FX and GBICs, and LC interface is typically used for SFP.

RJ-45 interface

The RJ-45 interface is most commonly used interface for Ethernet. RJ-45 is a common name that refers to IEC(60) 603-7 standardization using 8 positions defined by international connector standard (8-pin) modular jack or plug.

RS-232 interface

The RS-232-C interface (also known as EIA RS-232-C) is currently most commonly used serial communication interface. It is a serial communication standard developed by Electronic Industries Association (EIA) in conjunction with Bell System, modem and computer terminal manufacturers in 1970. Its full title is "Technical Standard for a Serial Binary Data Exchange Interface between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE)". The standard specifies that a 25-pin DB25 connector is used, signal content of each pin of connector, and levels of various signals.

RJ-11 interface

The RJ-11 interface is what we commonly refer to as a telephone line interface. RJ-11 is generic name used for connectors designed by Western Electric. Its shape is defined as a device with a 6-pin connector. Formerly known as WExW, x here stands for "active", a pin or wire. For example, WE6W has all 6 pins numbered 1 to 6, WE4W interface usesThere are only 4 pins, two outermost pins (1 and 6) are not used, WE2W uses only two middle pins (i.e. for telephone line interface).


With rapid growth of Internet IP data services, demand for transmission line bandwidth continues to grow. Although DWDM (Dense Wave Division Multiplexing) is most efficient solution for extending line bandwidth, CWDM (Coarse Wave Division Multiplexing) technology has advantages over DWDM in terms of system cost and serviceability.

Both CWDM and DWDM refer to a wavelength division multiplexing technology that can combine light of different wavelengths into a single-core optical fiber for co-transmission.

The latest ITU standard for CWDM is G.695, which provides 18 wavelength channels spaced at 20nm from 1271nm to 1611nm. Considering water peak effect of conventional G.652 optical fiber, 16 channels are commonly used. Due to large channel spacing, multiplexer/demultiplexer devices and lasers are cheaper than DWDM devices.

The DWDM channel spacing has different spacings such as 0.4nm, 0.8nm and 1.6nm depending on needs. The spacing is small and additional wavelength control devices are required, so equipment based on DWDM technology, more expensive than equipment based on CWDM technology.

A PIN photodiode is a layer of lightly doped N-type material between P-type and highly doped N-type semiconductors, called I (intrinsic) layer. Because it is lightly doped, electron concentration is very low, and a wide depletion layer is formed after diffusion, which can improve its response speed and conversion efficiency.

The APD avalanche photodiode not only has a photo/electrical conversion function, but also has an internal amplification function, and its amplification function is complemented by an avalanche multiplication effect inside tube. The APD is an amplifying photodiode. In case where sensitivity of optical receiver is high, use of APD is beneficial to increase transmission distance of system.