Based on the good development trend of FTTH, this paper focuses on the application mode of the access network in FTTH, the design and attention points of ODN optical links, the types of common optical cables and the selection of optical fibers.
1. Application Mode of Access Network in FTTH
The basic composition of FTTH system includes FTTH optical line terminal (OLT), optical distribution network (ODN), and FTTH optical network unit (ONU). In the fiber access network, the location of the ONU has great flexibility. The location of the ONU in the access network is different. The fiber access network can be divided into fiber to the road (FTTC) and fiber to the building ( FTTB), fiber to the office (FTTO), fiber to the home (FTTH) and other modes.
1. Fiber to the roadside
In the FTTC structure, the optical network unit is installed at a roadside cabinet or a branch box (or junction box) on a telephone pole. At this time, the part from the optical network unit to each user is still a twisted pair copper cable. If you transmit data for broadband image services, this part requires coaxial cable or xDSL.
2. Fiber to the building
FTTB puts the ONU directly in the building (usually a residential apartment or an office of a small business institution), and then distributes the business to various users through multiple pairs of twisted pairs or Category 5 lines. FTTB is a point-to-multipoint structure. FTTB's optical fiber is more advanced than FTTC. The optical fiber has been laid in the building, so it is more suitable for high-density user areas, and it is closer to the long-term development goals. It is widely used, especially in new industrial networks and residential buildings.
3. Fiber to the office and fiber to the home
In the original FTTC structure, if the ONU installed on the roadside is moved to the user's home, it is the FTTH structure. If the ONU is placed at the terminal equipment of a large enterprise user building and can provide a certain range of flexible services, it constitutes a so-called fiber-to-office. Consider that FTTO is also a pure fiber connection network, so it can be classified into a structure like FTTH. However, FTTO is mainly used for large enterprises and users, and the business volume is large, and the structure can be applied to point-to-point or ring structure. While FTTH is used for residential residential users, the demand for business volume is small, so the economic structure must be a point-to-multipoint method.
Second, OND optical link design and points of attention
The optical link loss of the ODN in the FTTH system includes the optical loss from the S / R reference point and the R / S reference point, calculated in dB. Including the total attenuation introduced by optical fiber, optical splitter, optical active connector and optical fiber fusion splice. The optical link loss calculation formula is as follows:
ODN optical link loss = optical fiber loss + optical splitter insertion loss + optical active connector loss + optical fiber fusion loss
The relevant parameters during calculation are as follows:
Fiber attenuation
According to the worst-case calculation method for loss, fiber loss is usually calculated according to attenuation at 1310 nm. When designing an ODN optical link, it is usually calculated as 0.40 dB / km.
2. Optical splitter insertion loss
The insertion loss of the optical splitter refers to the number of dB of each output relative to the input optical loss.
3. The insertion loss of the optical active connector is usually selected at 0.5dB.
4. Determination of attenuation of optical fiber fusion splice
² The attenuation of the optical fiber connector of the discrete optical cable is taken as the bidirectional average value: 0.08dB / each connector;
² The attenuation of the optical fiber connector of the ribbon cable takes the bidirectional average value: 0.2dB / each connector;
5. Cold connector
The optical fiber cold splice, also known as the mechanical optical fiber splice, is an abrasive-free field assembly movable connector based on mechanical splicing technology. It is suitable for occasions such as high corridors, narrow spaces, insufficient lighting, and inconvenient on-site power supply. The bidirectional average value is 0.15dB / connector.
When designing passive optical networks, a certain line maintenance margin (Mc) is usually required. The selection of line maintenance margin is generally as follows:
² When the transmission distance is less than or equal to 5 kilometers, the line maintenance margin is not less than 1 dB;
² When the transmission distance is ≤10 kilometers, the line maintenance margin is not less than 2 dB;
² When the transmission distance is> 10 km, the line maintenance margin is not less than 3 dB.
Assuming that the ODN optical link channel of the selected PON system allows a maximum insertion loss of 24dB, as shown in Figure 2, it is assumed that the optical splitter used is 1:16, and the fiber connector losses in the PON system are shown in Table 2. Through the calculation, we can know that the sum of the allowable line loss and the line maintenance margin is 7.55dB, and the line loss calculation is 0.40dB / km, then the maximum transmission distance of the PON system is 11.3km.
Joint point ABCDE
Number of movable joints 11101
Number of welding joints 22100
Number of cold junctions 00010
Loss 0.660.660.580.150.5
The above is the calculation of ODN optical link loss in a more ideal environment. In actual system applications, there are different types of optical fiber connection loss and bending loss, which will make the PON system transmission distance shorter than the above value. In addition, we can see from the numerical values ​​of optical fiber loss, optical splitter insertion loss, optical active connector loss, and optical fiber splice loss, the optical splitter insertion loss and optical active connector loss are large, in order to ensure sufficient transmission distance of the system , We also need to pay attention to the following two points in the system design:
1. Flexible use of optical splitters with different splitting ratios. Large splitting ratio optical splitters can reduce the use of optical fibers and reduce the cost of optical cable lines. However, the application of large splitting ratio optical splitters will greatly shorten the transmission distance. The increase in the number of equipment installation equipment rooms leads to a significant increase in the overall cost. Therefore, the selection of the splitting ratio should take into account the flexible application of the local environment. Usually, the optical splitter with a large splitting ratio is used in the nearer area.
2. Minimize the number of active connectors and use one less active connector, which is equivalent to an increase in transmission distance of more than 1km. When the optical cable line is designed to be handed over, it is advisable to use a first-level handover. The junction box should not be arranged layer by layer, because the optical cable is fully used The function of the branch joint box.
3. Commonly used optical cable types in FTTH construction
According to the different positions of the optical cable in the FTTH network system, it can be divided into feeder optical cable, distribution optical cable and home optical cable, as shown in Figure 3, considering its application, it is summarized as follows:
² Feeder cable-outdoor cable
² Distribution optical cable-outdoor optical cable, indoor and outdoor optical cable and indoor optical cable are possible
² Home optical cable-indoor optical cable
1. Outdoor optical cable
The outdoor optical cables currently on the market, in addition to the traditional ordinary layer stranded and center tube discrete optical fiber cables or optical fiber ribbon cables, can also use special optical cables according to the application, such as: road micro-groove cable, drainage pipe optical cable, air Send micro cable and small 8-shaped optical cable.
Pavement micro-groove optical cable drainage pipe optical cable air delivery micro cable small 8-shaped optical cable
2. Indoor and outdoor optical cables
Indoor and outdoor optical cables are specially used for direct conversion of optical fibers from outdoor to indoor. They will be used more and more in FTTH distribution optical cables. These types of optical cables mainly include loose sets of optical cables, tight sets of optical cables and butterfly-type optical cables. Because its application occasions include indoor and outdoor, it must meet the requirements of outdoor high and low temperature changes and waterproof, and also meet the requirements of indoor flame retardancy.
Loose sleeve optical cable tight sleeve fiber optic cable pipeline butterfly-introduced optical cable self-supporting butterfly-introduced optical cable
3. Indoor optical cable
There are many kinds of indoor optical cables. Currently, the most widely used ones are single-core, dual-core, multi-core distribution optical cables, branch optical cables and butterfly optical cables.
Double-core distribution optical cable, multi-core distribution optical cable branch, micro cable, butterfly optical cable
4. Selection of optical fiber types in FTTH construction
According to YD / T1636-2007 "Fiber to the Home (FTTH) Architecture and General Requirements", the wavelength range for upstream signals is 1260 to 1360 nm, and the wavelength range for downstream signals is 1480 to 1500 nm. G.652 and G are usually used for this window channel fiber .657 fiber. In FTTH construction, some optical cables are laid inside the building, especially indoors. During construction, it is necessary to consider not only the optical characteristics of the optical cable, but also the convenience of construction and beautiful decoration. However, G652 fiber has good attenuation characteristics when the bending radius is more than 30mm. Therefore, the current indoor cable in FTTH construction usually uses bending. The insensitive G.657 fiber makes the bending radius even less than 15mm.
Because the bending-insensitive G657 fiber is used, its mode field diameter is much smaller than that of the conventional G.652 fiber. Therefore, when the fiber is connected, the fusion splicing method must be used, and the cold connection cannot be used. At the same time, it should be noted that the fusion between these two optical fibers is very different from the fusion between the same type of optical fibers. We must carefully explore the fusion process in order to obtain better fusion results.
V. Summary
There are many problems involved in the construction of FTTH, and it is only slowly summed up in practice, comprehensively considering the types of users, line conditions, etc., so that FTTH construction has better economic and technical benefits, and then promote the great development of FTTH in China.
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