In a network cabling, optical fiber cables are usually used for connections between outdoor buildings, while optical fibers or twisted copper ethernet cables are used inside of buildings. So how do you switch the signal between the optical fiber cable outside the building and the Ethernet transmission medium inside the building? What equipment was used in it? What are their functions? What is the relationship between them?
Before clarifying these issues, we first need to understand the following terms:
Pigtail : used in terminal boxes to connect optical fibers in optical cables, and connected to tail fibers or jumpers through terminal box couplers (adapters).
Fiber patch cord cable : Both ends of the cable are movable joints, which serve to connect the pigtail and equipment.
Fiber optic cable terminal box: It is a box used to protect the fiber optic cable and fuse the tail fiber at the end of the fiber optic cable laying.
Fiber optic coupler: It is used for the active connection of two optical fibers or tail fibers, commonly known as a flange.
Fiber optic terminal box: It is the terminal joint of a fiber optic cable, with one end being the fiber optic cable and the other end being the tail fiber, which is equivalent to a device that splits a fiber optic cable into a single fiber optic cable. 
Fiber optic fusion box: It is used to connect two optical cables into a long optical cable. The fiber optic terminal box and fiber optic fusion box cannot be interchanged. The fiber optic cable is connected to the optical transceiver through the fiber optic terminal box, which means that only tail fibers can be inserted on the optical transceiver.
Coupler: It can only connect two tail fibers and has interfaces such as SC/PC FC/PC, while the optical cable and tail fibers are fused using a fusion machine, which is dead.
Terminal box VS fusion box: The former is the fusion of optical cables and tail fibers, while the latter is the fusion between optical cables.
Connection box VS terminal box: The connection box is fully sealed and can be waterproof, but it cannot fix the tail fiber; The terminal box is not waterproof, and if the internal knot is loose, one side can fix the optical cable and the other side can fix the tail fiber.
Pigtail VS fiber patch cord cable : Only one end of the pigtail is a movable joint; Both ends of the jump fiber are movable joints, and there are many types of interfaces. Different interfaces require different couplers, and the jump fiber can be divided into two and used as a tail fiber.
The connection between optical cables, terminal boxes, and pigtails
1: The outdoor optical cable is connected to the terminal box, with the purpose of fusing the optical fiber in the cable with the tail fiber and leading it out through jumpers.
2: Connect patch cord cable to fiber optic transceivers with the aim of converting optical signals into electrical signals.
3: The optical fiber transceiver leads out electrical signals, and the transmission medium used is twisted pair. At this point, the twisted pair can be connected to the RJ-45 port of the network device. So far, the conversion of the photoelectric signal has been completed.
Note: Many network devices now also have optical ports (optical fiber interfaces), but if there is no Optical module (similar to the optical fiber transceiver function), this port cannot be used.
The role of optical fiber cables, terminal boxes, and pigtails
The function of the optical cable terminal box is to terminate the optical cable, connect the fiber core and tail fiber in the optical cable, and the internal structure of the optical cable terminal box. The connected optical cable can have multiple cores,
For example, if a 4-core optical cable (with 4 cores in the cable) passes through the terminal box, a maximum of 4 tail fibers can be fused out, that is, 4 jumper wires can be led out. If only 2 wires are fused, then lead out 2 jumpers.
Pigtails : There is a connector at one end and a broken end of a fiber optic cable core at the other end. Connect to other fiber optic cable cores through fusion. Tail fiber function: mainly used to connect the joints at both ends of the optical fiber. One end of the pigtail is fused with the Optical fiber connector, and the other end is connected through a special connection
The head is connected to a fiber optic transceiver or fiber optic module, forming an optical data transmission path. Generally, we cannot purchase pure tail fibers, but instead the jumper shown in the figure, which is cut open in the middle and becomes a tail fiber. ST, SC and FC Optical fiber connector are standards developed by different enterprises in the early stage, with the same use effect and advantages and disadvantages,
ST and SC connector connectors are commonly used in general networks.
After inserting the ST head, it rotates half a circle and has a bayonet for fixation, but the disadvantage is that it is easy to break;
The SC connector is directly inserted and unplugged, which is very convenient to use, but the disadvantage is that it is easy to fall out;
FC connectors are generally used in telecommunications networks, with a nut screwed onto the adapter. The advantages are reliability and dust resistance, while the disadvantage is that the installation time is slightly longer.
The MTRJ type fiber optic jumper consists of two high-precision plastic shaped connectors and optical cables. The external components of the connector are precision plastic components, including a push pull insertion and locking mechanism. Suitable for indoor applications in telecommunications and data network systems.
Types of fiber optic interface connectors
TF-FC, TF-ST, TF-FC/APC, TF-SC/APC, TF-SC
There are many kinds of optical fiber connectors, that is, Optical fiber connector connected to Optical module, and they cannot be used mutually. People who do not frequently touch optical fibers may mistakenly think that the fiber optic connectors of GBIC and SFP modules are the same, but they are not. SFP modules are connected to LC fiber optic connectors, while GBIC is connected to SC fiber optic connectors.
Below is a detailed explanation of several commonly used fiber optic connectors in network engineering:
① FC type fiber optic connector: The external reinforcement method is to use a metal sleeve, and the fastening method is to use a screw buckle. Usually used on the ODF side (most commonly used on distribution frames)
② SC type optical fiber connector: It is a connector connecting GBIC Optical module. Its shell is rectangular, and its fastening method is plug and pull pin door type, without rotation. (Most used on router switches)
③ ST type fiber optic connector: commonly used in fiber optic distribution frames, with a circular shell and a screw buckle fastening method. (For 10Base-F connections, connectors are usually ST type. Commonly used for fiber optic distribution frames)
④ LC type fiber optic connector: A connector for connecting SFP modules, made using a modular jack (RJ) latch mechanism for easy operation. (commonly used in routers)
⑤ MTRJ transceiver integrated square fiber optic connector, one end dual fiber transceiver integrated
Introduction to various types of fiber optic interfaces
FC round threaded (most commonly used on distribution frames)
ST card type circular shape;
SC card style square (most commonly used on router switches)
PC microsphere surface grinding and polishing;
APC is at an 8 degree angle and subjected to microsphere surface grinding and polishing
MTRJ type, one end dual fiber transceiver integrated (useful on Huawei 8850)
Fiber optic module: Generally supports hot swapping,
GBIC Giga Bitrate Interface Converter, mostly using SC or ST type fiber optic interfaces
SFP small package GBIC, using LC type fiber optic
Fiber optic used:
Single mode: wavelength 1310nm, 1550nm
Multi mode: wavelength 850nm
In the labeling of pigtail joints, we often see “FC/PC”, “SC/PC”, etc., which have the following meanings:
The “SC” joint is a standard square joint made of engineering plastic, which has the advantages of high temperature resistance and low oxidation resistance.
The “LC” joint is similar in shape to the SC joint, but smaller than the SC joint.
The “FC” connector is a metal connector, usually used on the ODF side, and the metal connector can be plugged and unplugged more times than plastic.
There are many more types of connectors with various signals, including MTRJ, ST, MU, etc.
PC is most widely used in the equipment of telecom operators, with a flat joint cross-section.
The attenuation of “UPC” is smaller than that of “PC” and is generally used for equipment with special needs. Some manufacturers use FC/UPC for internal fiber skipping in ODF racks, mainly to improve the indicators of ODF equipment.
Fiber optic connectors
Fiber optic connectors are devices that allow for detachable (movable) connections between optical fibers. They precisely connect the two end faces of the fiber to maximize the coupling of the optical energy output from the transmitting fiber to the receiving fiber, and minimize the impact on the system due to its involvement in the optical link. This is the basic requirement of fiber optic connectors. To a certain extent, fiber optic connectors also affect the reliability and performance of optical transmission systems.
Fiber optic connectors can be divided into common single-mode and multimode connectors for silicon based fibers based on different transmission media, as well as other fiber optic connectors using plastic as the transmission medium; According to the structural form of the connector, it can be divided into various forms such as FC, SC, ST, LC, D4, DIN, MU, MT, etc. The following are some commonly used fiber optic connectors:
(1) FC fiber optic connector
This type of connector was first developed by NTT in Japan. FC is the abbreviation for Ferrule Connector, indicating that its external reinforcement method is
Use a metal sleeve and tighten it with a screw buckle. The earliest FC type connectors used ceramic pin docking ports. This type of connector has a simple structure, convenient operation, and easy production, but the fiber end is more sensitive to dust and is prone to Fresnel reflection. Improving the return loss performance is relatively difficult. Later, improvements were made to this type of connector, using a spherical mating pin (PC), while the external structure remained unchanged, resulting in a significant improvement in insertion loss and return loss performance.
(2) SC type fiber optic connector
This is a fiber optic connector developed by NTT Corporation in Japan. Its shell is rectangular, and the structural dimensions of the pin and coupling sleeve used are identical to those of the FC type,. The end face of the pin is mostly ground using PC or APC type grinding methods: the fastening method is to use a plug-in pin door type, without the need for rotation. This type of connector has low price, convenient insertion and removal operation, small fluctuation in intervention loss, high compressive strength, and high installation density.
ST and SC interfaces are two types of fiber optic connectors. For 10Base-F connections, the connector is usually ST type, while for 100Base-FX, the connector is mostly SC type. The core of the ST connector is exposed, while the core of the SC connector is inside the connector.
(3) Biconic Connector
The most representative product of this type of optical fiber connector is developed by Bell Labs in the United States. It consists of two cylindrical plugs with truncated conical ends formed by precision molding and a coupling component with a double conical plastic sleeve inside.
(4) DIN47256 fiber optic connector
This is a connector developed by Germany. The structural dimensions of the pin and coupling sleeve used in this connector are the same as those of the FC type, and the end face treatment adopts PC grinding method. Compared to the FC type connector, its structure is more complex, and there are springs in the internal metal structure that control pressure, which can avoid damaging the end face due to excessive insertion pressure. In addition, this type of connector has a high mechanical accuracy, resulting in a smaller intervention loss value.
(5) MT-RJ type connector
MT-RJ originated from the MT connector developed by NTT, with the same latch mechanism as the RJ-45 LAN electrical connector. It is aligned with the optical fiber through guide pins installed on both sides of the small sleeve. To facilitate connection with the optical transceiver, the connector end face optical fiber is designed with a dual core (0.75mm interval) arrangement, and is the next generation of high-density optical fiber connectors mainly used for data transmission.
(6) LC type connector
The LC type connector was developed by the renowned Bell Institute and is made using a modular jack (RJ) latch mechanism that is easy to operate. The size of the pins and sleeves used is half of the size used for ordinary SC, FC, etc., which is 1.25 mm. This can increase the density of fiber optic connectors in the fiber optic distribution frame. At present, in terms of single mode SFF, LC type connectors have actually occupied a dominant position, and their applications in multimode have also grown rapidly.
(7) MU connector
The MU (Minimum Unit Coupling) connector is the world’s smallest single core fiber optic connector developed by NTT, based on the most commonly used SC type connector,. The connector adopts a 1.25mm diameter sleeve and a self retaining mechanism, which has the advantage of achieving high-density installation. NTT has developed the MU connector series using the 1.25mm diameter sleeve of MU. They have socket type connectors (MU-A series) for fiber optic cable connections, backplane connectors with self retaining mechanisms (MU-B series), and simplified sockets (MU-SR series) for connecting LD/PD modules to plugs. With the rapid development of fiber optic networks towards larger bandwidth and capacity, and the widespread application of DWDM technology, the demand for MU connectors will also rapidly increase.
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