Considerations for several key factors in Metro wavelength division networking

Abstract: Based on the technical principles and practical experience, this paper discusses the functions and performance requirements of the metro wavelength division Network Design Based on the features of the metro wavelength division technology, the technical requirements for protection methods, osnr indicators, power and dispersion budget are discussed.

0. Introduction

Metro wavelength division technology has been favored since its birth. In recent years, with the rapid increase in demand and the significant increase in device practicability and cost-effectiveness, its large-scale networking applications are accelerating.

In most urban wavelength division applications, ring-based networking is used. The configuration of specific boards such as OTU can be determined based on actual needs, and the consideration of system performance is important, key considerations include selection of protection methods, optical power budget, dispersion and non-linearity, optical signal-to-noise ratio, and other O & M considerations. The general principles of the above aspects are well known, but there are still various key details that must be paid attention to in actual application. We will discuss them one by one.

1. Selection of protection methods

Protection is a major advantage of Metro wavelength division over bare fiber transmission. Whether to use wavelength division-level protection or the protection method needs to be applied according to the actual situation.

For upper-layer signals with existing protection, such as data signals using dual-homing nodes, SDH signals using protection methods such as MS-spring, and RPR signals protected by RPR ring networks, you do not need to provide protection at the wavelength division level. However, it should be noted that for data circuits using dual-homing nodes, the same two starting points must be arranged in two different flows during the track arrangement.

For circuits without upper-layer protection, such as data circuits and San circuits with single-node Mounting Mode, wavelength division protection can be adopted based on the importance of services. In terms of specific protection methods, usrs (one-way Optical Channel Protection Switching) and BSRs (two-way optical channel sharing Protection Switching) are the two most common methods: usrs are the most mature protection methods, simple and reliable, and supported by all devices. The bpsr method saves protection traffic and can overcome the problem of long routing protection after improvement. Generally, it is better than the uhdr method, however, the advantage is not obvious when most of the metro network services are distributed in aggregation mode, and the device must support this function.

The number of nodes in the system is small (2 ~ (4 nodes), but all of them are OTM nodes. You can use the 1 + 1 line protection function of Zhuguang. Compared with upsr and bpsr, it has the advantage of being simple, and the protection cost is not high in this case.

When multiple circuits are enabled for the same flow direction and multiple OTU pairs are configured, you can consider configuring OTU's 1: N protection to improve system reliability at a low cost.

2. optical power budget

In a wavelength division system, the attenuation of the line and insertion loss of the device are often compensated by the optical amplifier, and the use of the optical amplifier will lead to a decrease in the signal-to-noise ratio of the receiving end (RN reference point. Therefore, the optical power budget of the system must be consistent with the optical signal-to-noise ratio. This section only discusses the coordination between the system sending and receiving optical power (including the primary and suboptical channels.

The optical power budget of the sub-optical channel is the power budget between sending OTU and receiving OTU (in many cases, sending OTU and receiving OTU are integrated on a board, it may be only one optical reuse segment or optical regeneration segment, or it may contain multiple optical regeneration segments or optical reuse segments, the attenuation factors mainly include the insertion loss of the combination/splitter, the insertion loss of the optical connector, and the line attenuation. The power increase factor includes the gain of the power amplifier, wire release, and front release. The power budget must meet the following formula (unit: dBm, DB ):

Emitting power of the transmitting OTU-sensitivity of the receiving OTU> gain of each amplifier-line attenuation-various insertion loss-surplus degree.

When the power budget of the main optical channel is met, the gain of the amplifier is greater than the attenuation of the line, while the sensitivity of the light-emitting power-receiving OTU sent by OTU is usually much higher than that of various insertion loss, therefore, the power budget of sub-optical channels is often satisfied. In most cases, you need to add an adjustable attenuation device to eat excess power to avoid overload at the receiver side.

The optical power budget of the main optical channel refers to the power budget between the first-level optical fiber and the next-level optical fiber in each optical reuse segment or optical regeneration segment (in some cases, if only the first-level optical fiber is used, it refers to the power budget for sending the OTU to the front-end. If there is only a power amplifier, it refers to the power budget between the power amplifier and the receiving OTU ). The main optical channel must meet all the conditions from-wave signal input to full wave signal input. Since the dimmable attenuation can be used to consume excess power, the minimum attenuation of the Light release segment (that is, the overload condition) is not considered ). In general, the attenuation restricted distance is designed using the worst value method. The power budget must meet the following requirements:

PS-pr-C-Cr> A × L

Formula:

PS: minimum output power of a single optical amplifier, in dBm.

PR: Minimum Allowable input power of a single-channel receiver, in dBm.

C: Attenuation of all optical connectors and the attenuation of each optical connector is 0.5db.

Cr: Line surplus, usually 3 dB within 60 km, more than 60 km according to 0.05 dB/Km value.

A: optical fiber loss coefficient (dB/km), including optical fiber attenuation and optical fiber fusion attenuation. The measured value is used as the standard. 0.25db/km is usually used for estimation.

Note: The DWDM System is an osnr restricted system. The above formula only reflects the power limitations of the optical amplifier. The osnr considerations are described below.

In addition, to ensure the long-term stability of the system, the following requirements must be met:

(1) In case of single-wave full-wave input, the main optical channel can satisfy the requirement that the optical power at the end of the line falls within the input power range of the amplifier at the receiving end, so as to ensure smooth system expansion or channel adjustment;

(2) in case of full wave input, the transmit optical power of the main optical channel is less than or equal to 20 dBm, to avoid non-linear and other negative effects caused by excessive optical power;

(2) when the system is initially put into use, various surplus degrees need to be reduced through the electric adjustable attenuation device. If the attenuation increases during operation, the adjustable power-saving and adjustable attenuation device releases a certain degree of surplus, this ensures that the system power budget is normal.

3. color dispersion

Color dispersion is a time-domain broadening effect of the receiver signal due to the different transmission rates of light at different wavelengths (the short-wave length in the optical fiber is faster than the long wavelength. The dispersion problem is not exclusive to wavelength division multiplexing. However, in Wavelength Division Multiplexing Systems, the dispersion slope of light is not zero, leading to the relationship between dispersion characteristics and wavelength. Different wavelength channels have different dispersion sizes, which brings new problems to the dispersion technology. A good dispersion compensation technique should compensate different dispersion sizes of all wavelength channels of Wavelength Division Multiplexing at the same time, that is, the dispersion slope can be compensated.

In order to reduce the effect of dispersion in the DWDM System, OTU adopts out-of-band modulation, pre-cheching, and other technologies to obtain the minimum spectral width of the light source. In addition, the distributed passive Dispersion Compensation Optical Fiber module can be used for compensation.

The total dispersion of an end-to-end link can be obtained by the following formula:

Dispersion total (PS/nm) = D (PS/nm. km) × length (km)

In engineering design, g.652 optical fiber is divided into 16 ~ 22 ps/nm. KM considering the color dispersion, G.655 optical fiber is 4 ~ The color dispersion is considered for 8 PS/nm. km. The measured value is used as the standard. The principle of dispersion budget is that the dispersion value of the receiver is within the dispersion margin of the receiver.

Take the metro wavelength division equipment of a manufacturer as an example: For a 2.5g WDM System, the transmission distance is limited to 640 km due to dispersion, and no dispersion compensation is required. For a 10g system, on a G.655 optical fiber, 200km does not require dispersion compensation. On g.652 optical fiber, 60 ~ No dispersion compensation is required for 80 km.

It is worth noting that, unlike long-distance wavelength division, Metro Wavelength Division usually uses the OADM site, and the business carried by a certain channel may end on the next node, the Influence of dispersion is considered not only between two OADM nodes, but between two upstream and downstream business nodes. Therefore, the dispersion of Metro wavelength division is usually to budget the entire ring network, and ensure that the residual dispersion values of the whole ring traversal and the receiver of a single multiplexing segment do not exceed the dispersion margin range of OTU.

Depending on the length to be compensated, the Dispersion Compensated optical fiber (DCF) can be divided into 40/60/80/100/120 km (equivalent to g.652 optical fiber), usually using distribution compensation or post compensation, A good device can compensate both the fiber dispersion value and the fiber dispersion slope.

Note that the insertion loss introduced by DCF must be considered in the power budget. However, if the same site uses multi-level amplification and DCF is set between two-level amplifiers, you do not need to consider DCF insertion loss in the main optical channel.

4. polarization mode dispersion and nonlinear consideration

In Metro wavelength division, the transmission distance is relatively short, and the influence of polarization mode dispersion (PMD) and nonlinear effect is smaller than that of long-distance wavelength division, but it also needs to be considered to a certain extent.

The polarization mode dispersion is caused by the exclusive refraction and polarization of the single-mode optical fiber. The inherent polarization mode dispersion of the optical fiber is caused by a non-circular fiber core, which forms the phenomenon of refraction, the polarization modulus dispersion caused by refraction is caused by external factors such as mechanical pressure and thermal pressure. Polarization mode dispersion cannot be avoided and can only be minimized.

The following formula is used to obtain the PMD dispersion P total of an end-to-end link:

Pfiber: G.655 optical fiber is calculated based on the value, while g.652 optical fiber is calculated based on the value.

For a 2G system, it is generally not considered because its PMD margin is at least 40 PS/nm. For a 10g system, the theoretical PMD margin is 10 ~ 18 PS/nm. The signal deterioration caused by PMD can be offset by an equivalent increase of osnr surplus, that is, the increase of osnr increases the PMD tolerance. The ing can be simply represented in table 1:

Table 1osnr improvement is equivalent to the corresponding table of the PMD tolerance.

PMD (PS)	Osnr surplus
10	0
11	0.1
12	0.3
13	0.6
14	1
15	1.4
16	1.9

The nonlinear effect of optical fiber is also an important factor limiting the performance of WDM Systems. The nonlinear effects of optical fiber can be divided into two types: Scattering Effect and refraction effect.

The scattering effects mainly include SBS and SRS ). SBS is a narrow band effect. When the laser width after modulation is greater than 50 ~ MHz can effectively suppress SBS. In the actual system, the width of the laser modulated by the 10 Gbit/s reflector is much larger than this value, and the optical output power of each channel is less than 5 MW, therefore, SBS has no significant impact on the WDM System. SRS is a broadband effect with a high threshold power of about 1 W. Therefore, it can be considered that SBS has no significant impact on the WDM System.

Because the scattering effect is related to the size of the incoming optical power, excessive incoming optical power may lead to sudden changes in the scattering effect. Therefore, this is also a reason for limiting the optical power from the sending point into the fiber. According to the national standard, the MPI-S power of the 32/40 wave Metro wavelength division system is not greater than + 17dbm (corresponding to + 2/+ 1 dBm per channel ), the long-distance application time is no greater than + 20dbm (corresponding to + 5/+ 4 dBm per channel ). Therefore, in the light power budget, we must consider not only the incoming Fiber Power of the current track configuration, but also the total power of the Full Wave configuration should not exceed the standard. Otherwise, we need to adjust the power of the main optical path in the future for expansion, this affects activated services. This situation occurs when a wavelength division system has just been introduced to a region and must be avoided.

Refraction effects include self-Phase Modulation Effect (SPM), cross-phase modulation effect (XPM), and four-wave mixing effect (FWM ). SPM can compress the width of the transmitted optical pulse. SPM can be used to compensate for the effect of dispersion widening pulse. Cross-phase modulation (XPM) usually occurs in a DWDM System over 40 channels. The four-wave mixing effect (FWM) is closely related to the fiber dispersion. G.652 optical fiber with a large dispersion coefficient does not have a significant FWM.

In engineering design, the impact of the above factors can be taken into account at the cost of optical channels. The cost of optical channels includes the effects of PMD dispersion, ASE noise, channel crosstalk, and inter-code interference. Generally, the value range is 1 ~ 2 dB. The optical traffic cost is generally included in the osnr value surplus.

5. Considerations for Optical Signal-to-Noise Ratio

The optical signal-to-noise ratio (osnr) is defined as osnr = the signal optical power of each channel/the noise optical power of each channel.

The optical amplifier generates the so-called amplified spontaneous radiation ase in the spectral area of dozens of nanometers. This ASE is a noise for signal light. In a transmission system with several cascade EDFA, the ASE noise of the optical amplifier repeats a periodic attenuation and amplification like the signal light.

The ASE noise spectrum distribution is also expanded along the system length. When the ASE noise from the first optical amplifier is sent to the second optical amplifier, the gain distribution of the second optical amplifier changes due to gain saturation. This effect will be passed downstream to the next optical amplifier. Even if a narrowband filter is used at each optical amplifier, ASE noise will accumulate, this is because the noise exists within the Signal Band.

The accumulation of ASE noise has an impact on the system's SNR, because the SNR deterioration of the received signal is mainly related to the ASE-related differential beat noise. The difference noise increases linearly with the increase in the number of optical amplifiers. The error rate degrades with the increase of the number of optical amplifiers, and the noise accumulates exponentially with the gain of the amplifier. As a result of the gain of the optical amplifier, the ASE noise spectrum after many optical amplifiers has a wavelength spike caused by the self-emission effect. In particular, if we consider using a closed all-optical loop network system, ASE noise will accumulate infinitely if there are infinite cascade optical amplifiers, the SNR degrades with the increase of the optical amplifier. More than one OTM or parallel OADM node must be set in the Metro wavelength division ring to avoid this problem.

It can be seen that the optical amplifier not only scales up the optical signal, but also adds noise (additional parasitic power) around and below the signal, which affects the deterioration of the signal-to-noise ratio, in addition, osnr is further reduced with the increase of the number of optical dropped connections, and the error rate is increased at the receiver end. After theoretical derivation and practical test, the approximate relationship between osnr and bit error rate at the receiver end is obtained. Figure 1 is the BER-OSNR curve of 2.5g system.

 

Figure 12.5g system's ber ~ Osnr Curve

The optical power of the DWDM System Noise of multiple cascaded optical amplifiers is mainly dominated by the amplified spontaneous radiation noise. The approximate calculation formula of osnr for optical amplifier cascade is as follows:

Osnr = POUT-L-NF-10LogN + 58

Pout is the output power per channel (unit: dBm), L is the loss of Optical Fiber segments (unit: DB) between optical amplifiers, NF is the noise index of EDFA (unit: DB ), n is the number of Optical Fiber segments in the link. It is assumed that the loss of all optical fiber segments in the link is equal.

The optical signal-to-noise ratio (SNR) is one of the most important factors that affect the performance of the DWDM System. Due to its complexity, it is also the performance parameter most considered in the design of the wavelength division system. In the metro wavelength division system, FEC technology can be used to increase the osnr tolerance of the receiving end, which is generally increased by 4 ~ 7db. From the BER-OSNR curve in Figure 2, we can see the improvement effect of FEC on the bit error rate (taking the 10g system as an example ).

 

Figure 210g system BER-OSNR curve (out-of-band FEC compared with non-FEC)

In system design, the common principle is that the osnr analog computing value of the RN reference point must be greater than the nominal value of the device (generally the nominal value corresponding to the BER = 10-15, factors such as optical channel cost and receiver aging have been taken into consideration ). For example, according to the national standard, the osnr requirements for 2.5g and 10g Wavelength Division systems are shown in table 2 (initial system values ):

System Type	2.5 GB	10g
Minimum signal-to-noise ratio of each path at rn points	20 dB (no FEC)	25 dB (no FEC) or 20 dB (out-of-band FEC)

Table 2 Requirements for osnr of wavelength division system rn points

System type: 2.5 GB, 10 GB

Minimum optical signal-to-noise ratio of Rn points per path: 20 dB (no FEC) 25 dB (no FEC) or 20 dB (out-of-band FEC)

System type: 2.5 GB, 10 GB

Minimum optical signal-to-noise ratio of Rn points per path: 20 dB (no FEC) 25 dB (no FEC) or 20 dB (out-of-band FEC)

Based on experience, it is recommended that the osnr be 1 ~ The additional surplus of 3 dB facilitates the long-term stability of the system. The additional surplus of 3 dB is of little reference significance.

In the system design, osnr is also an important consideration for the setting of the online site. In general, the use of FEC and other technologies can minimize the impact on the system performance. However, unlike long-distance wavelength division, Metro Wavelength Division usually uses OADM sites. Services carried by a certain channel may end on the next node, the impact on osnr is not the distance between two OADM nodes, but the distance between the two upstream and downstream business nodes. Therefore, whether to save the online storage must consider the business situation, not just the distance between sites.

6. Other considerations

In the design of networking, you also need to consider multiple aspects for the operation and maintenance after the system is put into use to reduce the subsequent operation costs.

Network management is an important part of the wavelength division system. It requires comprehensive functions, user-friendly interfaces, and convenient operations, and fast and reliable communication with network elements through reasonable network management routing planning (including OSC and external routing design.

The network management capability is the maximum number of network elements that can be managed by a set of network administrators to ensure the specified performance indicators. Reasonable hardware and software configurations are required to ensure excellent operation and control performance of the network management.

System jitter is also considered. Only the OTU part of the entire wavelength division system has optical/electrical conversion, and the rest are all optical processing and transmission. Therefore, OTU is the only source for introducing jitter, and requires 2 ~ 5-level Cascade. However, the 3R OTU has a good jitter suppression feature, which usually enables multi-level Cascade without error codes. For devices of mainstream manufacturers, the number of 2.5 Gbit/s OTU cascade connections can reach 128 or more, and the number of 10 Gbit/s OTU cascade connections can reach 64 or more. This indicator is much higher than the number of practical OTU cascade applications for long-distance wavelength division, and is more suitable for Metro wavelength division applications. Therefore, the impact of jitter is usually unnecessary in the design of Metro wavelength division.

The wavelength division system is a simulated system to a large extent. Therefore, regular online monitoring of analog data is an essential part of maintenance. During system design, you can configure a built-in spectral analyzer to monitor parameters such as optical power, osnr, bias current, and wavelength per wave through the system's uninterrupted service monitoring port, it is also reflected in the real-time interface and history of the network management, so as to promptly detect and eliminate hidden dangers and ensure system performance.

Changes in the main optical path have a significant impact on the performance of the wavelength division system, such as optical path deterioration, optical cable cutover, and interruption of the expansion of the main optical path. Changes in the optical path parameters will change the system performance. Therefore, in the system design, an electric dimmable attenuation device should be configured in the main optical channel, which can be used to conveniently and quickly adjust the optical power, re-optimize the system, and restore the optimal performance.

In addition, the interconnection between OTU customer interface and business signal, the division of specific channels, whether to configure the adjustable wavelength OTU for General spare parts and other issues should also be considered during system design.

7. Conclusion

Large-scale applications of Metro wavelength division are accelerating. wavelength division multiplexing is a combination of analog and digital technologies, and system design must be emphasized. Details determine success or failure. If you copy the initial solution of the manufacturer and do not carefully check the key factors, the long-term stability of the metro wavelength division system may be affected. The negative examples are not uncommon. The more rigorous and comprehensive the design considerations, the more stable and reliable the system will be, thus improving the entire network.