Optical Transport Network (OTN) for High Speed Service

Nowadays, the SONET/SDH network is an universal network that combines with WDM (wavelength division multiplexing) technique to transmit multiple optical signals over a single fiber. In future networking, high speed transmission is no doubt the migration trend. Inspired by the SONET/SDH network, ITU-T (ITU Telecommunication Standardization Sector) has defined the optical transport network (OTN) to achieve a more cost-effective high speed network with the help of WDM technology.

Generally speaking, OTN is a network interface protocol put forward in ITU G.709. OTN adds OAM (operations, administration and maintenance) functionality to optical carriers. It allows network operators to converge networks through seamless transport of the numerous types of legacy protocols, while providing the flexibility required to support future client protocols. Unlike the previous SONET/SDH, OTN is a fully transparent network that provides support for optical networking on a WDM basis. Since multiple data frames have been wrapped together into a single entity in OTN, it is also known as the “digital wrapper”.

Working Principle of OTN

You may wonder how OTN works in practice. Actually, its working structure and format very resemble the SONET/SDH network. Six layers are included in the OTN network: OPU (optical payload unit), ODU (optical data unit), OTU (optical transport unit), OCh (optical channel), OMS (optical multiplex section) and OTS (optical transport section).

OPU, ODU and OTU are the three overhead areas of OTN frame. OPU is similar to the “path” layer of SONET/SDH, which provides information on the type of signal mapped into the payload and the mapping structure. ODU resembles the “line overhead” layer of SONET/SDH, which adds the optical path-level monitoring, alarm indication signals, automatic protection switching bytes and embedded data communications channels. OTU is like the “section overhead” in SONET/SDH, and it represents a physical optical port that adds performance monitoring and FEC (forward error correction). OCh is for the conversion of electrical signal to optical signal and modulates the DWDM wavelength carrier. OMS multiplexes several wavelengths in the section between OADMs (optical add drop multiplexer). OTS manages the fixed DWDM wavelengths between each of the in-line optical amplifier units.

OTN-structure

Advantages of OTN

There are many advantages of OTN. Firstly, it separates the network against uncertain service by providing transparent native transport of signals encapsulating all client-management information. Secondly, it performs multiplexing for optimum capacity utilization which enhances network efficiency. Thirdly, it improves maintenance capability for signals transmitting through multi-operator networks by providing multi-layer performance monitoring.

Migration to High Speed OTN

With the fast evolution of networking, OTN standard is able to reach a higher speed service. Its multiplexing hierarchy allows any OTN switch and any WDM platform to electronically groom and switch lower-rate services within 10 Gbps, 40 Gbps, or even 100 Gbps wavelengths. This eliminates the need for external wavelength demultiplexing and manual interconnects. OTN network is definitely the best solution for future high speed networking over long distance. The picture below shows the OTN mapping diagram for high speed transmission.

OTN-high-speed-mapping-diagram

Conclusion

Over the years, OTN has never stopped improving itself. Driven by the needs for high speed transmission, OTN combined with WDM is obviously a better choice in networking. It is a cost-effective way to build an optical transport network accommodating high throughput broadband services. I believe more and more people will employ this standard in their own network in the near future.

Comparison Between CWDM & DWDM Technology

For a better signal transmission in fiber-optic communication, different kinds of technologies are applied to the industry. Wavelength-division multiplexing (WDM) is one of the commonly used technologies which multiplexes a number of optical carrier signals onto a single optic fiber by using different wavelengths of laser light. That is to say, WDM enables two or more than two wavelength signals to transmit through different optical channels in the same optical fiber at the same time.

WDM

In the WDM system, there are two types of divisions – CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing). They are both using multiple wavelengths of laser light for signal transmission on a single fiber. However, from the aspects of channel spacing, transmission reach, modulation laser and cost, CWDM and DWDM still have a lot of differences. This article will focus on these distinctions and hope you can have a general understanding about CWDM and DWDM technology.

Channel Spacing

As their names suggest, the words “coarse” and “dense” reveal the difference in channel spacing. CWDM has a wider spacing than DWDM. It is able to transport up to 16 wavelengths with a channel spacing of 20 nm in the spectrum grid from 1270 nm to 1610 nm. But DWDM can carry 40, 80 or up to 160 wavelengths with a narrower spacing of 0.8 nm, 0.4 nm or 0.2 nm from the wavelengths of 1525 nm to 1565 nm (C band) or 1570 nm to 1610 nm (L band). It is no doubt that DWDM has a higher performance for transmitting a greater number of multiple wavelengths on a single fiber.

CWDM-VS-DWDM

Transmission Reach

Since the wavelengths are highly integrated in the fiber during light transmission, DWDM is able to reach a longer distance than CWDM. The amplified wavelengths provide DWDM with the ability of suffering less interference over long-haul cables. Unlike DWDM system, CWDM is unable to travel unlimited distance. The maximum reach of CWDM is about 160 kilometers but an amplified DWDM system can go much further as the signal strength is boosted periodically throughout the run.

Modulation Laser

CWDM system uses the uncooled laser while DWDM system uses the cooling laser. Laser cooling refers to a number of techniques in which atomic and molecular samples are cooled down to near absolute zero through the interaction with one or more laser fields. Cooling laser adopts temperature tuning which ensures better performance, higher safety and longer life span of DWDM system. But it also consumes more power than the electronic tuning uncooled laser used by CWDM system.

Cost

Because the range of temperature distribution is nonuniform in a very wide wavelength, so the temperature tuning is very difficult to realize, thus using the cooling laser technique increases the cost of DWDM system. Typically, DWDM equipment is four or five times more expensive than CWDM equipment.

Conclusion

CWDM and DWDM are both coming from the WDM technology that is capable of conveying multiple wavelengths in a single fiber. But with different characteristics, people should think twice before choosing the CWDM or DWDM system. CWDM usually costs less but its performance is far behind DWDM. Both your requirements and budget need to be taken into consideration. Moreover, the WDM products including CWDM mux/demux module, DWDM mux/demux module and optical splitter are highly welcome in the market.

Overview of Single-mode Fiber Types

According to the light transmission mode, optic fibers can be classified into single-mode and multimode. It’s easy to categorize multimode fiber into four types of OM1, OM2, OM3 and OM4. However, when it comes to single-mode, it may not be as simple as you think. The classification of single-mode fiber is much more complicated than multimode fiber. ITU-T G.65x series and IEC 60793-2-50 (published as BS EN 60793-2-50) are two primary sources for single-mode fiber specification. This article will mainly focus on the ITU-T G.65x series.

The following table introduces 19 ITU-T specifications of single-mode fiber:

Name Type
ITU-T G.652 ITU-T G.652.A, ITU-T G.652.B, ITU-T G.652.C, ITU-T G.652.D
ITU-T G.653 ITU-T G.653.A, ITU-T G.653.B
ITU-T G.654 ITU-T G.654.A, ITU-T G.654.B, ITU-T G.654.C
ITU-T G.655 TU-T G.655.A, ITU-T G.655.B, ITU-T G.655.C, ITU-T G.655.D, ITU-T G.655.E
ITU-T G.656 ITU-T G.656
ITU-T G.657 ITU-T G.657.A, ITU-T G.657.B, ITU-T G.657.C, ITU-T G.657.D

Each type has its own area of application and the evolution of these optical fiber specifications reflects the evolution of transmission system technology from the earliest installation of single-mode optical fiber to the present day. Choosing the right one for your project can be vital in terms of performance, cost, reliability and safety. Now, let’s have a look at the differences of G.65x series specifications for single-mode fiber respectively.

G.652

The ITU-T G.652 fiber is known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. The G.652.A and G.652.B fibers are designed to have a zero-dispersion wavelength near 1310 nm, therefore they are optimized for operation in the 1310nm band. They can also operate in the 1550nm band, but it is not optimized for this region due to the high dispersion. These optical fibers are usually used within LAN, MAN and access network systems. The more recent variants (G.652.C and G.652.D) feature a reduced water peak that allows them to be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.

G.652

G.653

G.653 fiber was developed to address this conflict between best bandwidth at one wavelength and lowest loss at another. It uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest losses in the fiber. Therefore, G.653 fiber is also called dispersion-shifted fiber (DSF). G.653 has a reduced core size, which is optimized for long-haul single-mode transmission systems using erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. One of the most troublesome, four-wave mixing (FWM), occurs in a Dense Wavelength Division Multiplexed (CWDM) system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.

G.653

G.654

The G.654 specifications entitled “characteristics of a cut-off shifted single-mode optical fiber and cable”. It uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550nm band. It usually also has high chromatic dispersion at 1550 nm, but is not designed to operate at 1310 nm at all. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.

G.655

G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber overcomes problems associated with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. They have respectively a negative and positive slope versus wavelength. Following picture depicts the dispersion properties of the four main single-mode fiber types. The typical chromatic dispersion of a G.652 compliant fiber is 17ps/nm/km. G.655 fibers were mainly used to support long-haul systems that use DWDM transmission.

G.655

G.656

As well as fibers that work well across a range of wavelengths, some are designed to work best at specific wavelengths. This is the G.656, which is also called Medium Dispersion Fiber (MDF). It is designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind of fiber was developed to support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allows the easier deployment of CWDM in metropolitan areas, and increases the capacity of fiber in DWDM systems.

G.657

G.657 optical fibers are intended to be compatible with the G.652 optical fibers but have differing bend sensitivity performance. It is designed to allow fibers to bend, without affecting performance. This is achieved through an optical trench that reflects stray light back into the core, rather than it being lost in the cladding, enabling greater bending of the fiber. As we all know, in cable TV and FTTH industries, it is hard to control bend radius in the field. G.657 is the latest standard for FTTH applications, and, along with G.652 is the most commonly used in last drop fiber networks.

Conclusion

There are different types of single-mode fiber used for different application. G.657 and G.652 are typically favored by planners and installers, and G.657 is particularly deployed for FTTH applications because of a larger bend radius. And G.655 has been taken the place of G.643 used for WDM system. In addition, G.654 is usually applied to the subsea area. To know more information about single-mode fiber, you are welcome to visit the website at FS.COM.

What Should You Know Before Choosing the Single-mode Fiber?

Fiber optical cable has single-mode and multimode type. Multimode fiber includes types of OM1, OM2, OM3, 0M4. How many kinds of single-mode fiber? There are two primary specifications of single-mode fiber. One is the ITU-T G.65x series, and the other is IEC 60793-2-50 (published as BS EN 60793-2-50). This article will introduce ITU-T G.65x series.

single-mode fiber

There are 19 types of single-mode fiber specifications defined by ITU-T (shown in the following table). Different type has different application area. From the change of single-mode fiber specifications, we can see the evolution of transmission system technology. As so many kinds of single-mode fiber, which one should you choose to get perfect performance with the fewest cost? Following will tell about each specifications in details.

ITU-T Specifications Type
ITU-T G.652 ITU-T G.652.A, ITU-T G.652.B, ITU-T G.652.C, ITU-T G.652.D
ITU-T G.653 ITU-T G.653.A, ITU-T G.653.B
ITU-T G.654 ITU-T G.654.A, ITU-T G.654.B, ITU-T G.654.C
ITU-T G.655 ITU-T G.655.A, ITU-T G.655.B, ITU-T G.655.C, ITU-T G.655.D, ITU-T G.655.E
ITU-T G.656 ITU-T G.656
ITU-T G.657 ITU-T G.657.A, ITU-T G.657.B, ITU-T G.657.C, ITU-T G.657.D

ITU-T G.652

ITU-T G.652 fiber is also known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. G.652.A and G.652.B fibers are designed with a zero-dispersion wavelength near 1310 nm, which can be optimized for the operation in 1310nm band. They can also operate in 1550nm band, but it is not optimized for this region due to the high dispersion. The two fibers are usually used within LAN, MAN and access network systems. While G.652.C and G.652.D reduce water peak and can be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.

ITU-T G.653

ITU-T G.653 fiber uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest loss in the fiber. It can address this conflict between best bandwidth at one wavelength and lowest loss at another. So G.653 fiber is also called dispersion-shifted fiber (DSF). It has a smaller core size, which is optimized for long-haul transmission system combined with erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. What’s more, four-wave mixing (FWM) occurs in a Dense Wavelength Division Multiplexed (CWDM) system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.

ITU-T G.654

G.654 is called “characteristics of a cut-off shifted single-mode optical fiber and cable”. It uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550nm band. It has high chromatic dispersion at 1550 nm but can’t operate at high chromatic dispersion of 1310 nm. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.

ITU-T G.655

G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber can deal with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550-nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. Each one has a negative and positive slope versus wavelength. G.655 fibers are mainly used to support long-haul transmission in DWDM system.

ITU-T G.656

G.656 fiber is called Medium Dispersion Fiber (MDF). It’s designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind for fiber can support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allows the easier deployment of CWDM in metropolitan areas, and increase the capacity of fiber in DWDM systems.

ITU-T G.657
G.657 fiber was originally designed to be compatible with the G.652 fibers but have different bend sensitivity performance. It allows fibers to bend without affecting performance. This is achieved through an optical trench that reflects stray light back into the core and avoids the light lost in the cladding. In reality, it’s hard to control bend radius in the field, such as FTTH applications. G.657 is the latest standard for FTTH applications, and, along with G.652 is the most commonly used in last drop fiber networks.

Conclusion

From the above, different kinds of single-mode fibers have different applications. G.643 is not often used in WDM system because of some problems and is replaced by G.655. G654 is mainly for submarine use. G656 is designed for specific wavelengths. G.657 is compatible with the G.652 but has a larger bend radius than G.652, which is especially suitable for FTTH applications. Now a better understanding of these single-mode fibers will help you to choose the most suitable single-mode fiber.