The Introduction to AON

With the rapid development and globalization of the modern society, a large quantity of data needs to be transmitted, thus resulting in the explosive growth of information content. The explosive growth of information content enables people to places a higher demand on bandwidth, which is a symbol of communication content. However, the electronic bottleneck of photoelectric conversion has restricted the high-speed transmission of data, giving rise to the failure of optical communication network to meet the requirements of high-speed, large-capacity and long-haul transmission. In order to make full use of the potential bandwidth of fiber, continuously improve the transmission rate of fiber and accommodate the explosive growth of communication services, all-optical network (AON) is proposed.

2.What is AON?
All-optical network (AON) is emerging as a promising network for very high data rates, flexible switching and broadband application support. In principle, all-optical network is founded on the premise of keeping the transmission and exchange of data signals entirely in the optical domain from source to destination, thus removing the intermediate electronics to eliminate the so-called electronic bottleneck and allow arbitrary signal formats, bit-rates, and protocols to be transported. In an all-optical network, data signals are always maintained in the optical domain except when they enter or exit the network, as shown in Figure 1. It means that there is no electrical signal processing in the entire transmission, so various transmission modes (PDH, SDH, ATM, etc.) can be applied in the AON to significantly improve the utilization of network resources. Being equipped with excellent transparency, survivability, scalability and compatibility, AON can achieve the data transmission of ultra-long haul, ultra-large capacity and ultra-high speed to become the preferred choice of the future high-speed broadband network.

An all-optical network

Figure 1: An all-optical network

3.Properties over the current optical communication network
AONs are able to arm the communication network with better manageability, flexibility and transparency. Compared with the traditional communication networks and the current optical communication networks, AONs are equipped with the following advantages that they don’t possess.
(1) AON provides huge bandwidth. Because the transmission and exchange of signals in AON entirely operate in the optical domain, AON can make the best use of the transmission capacity of fiber.
(2) AON achieves the transparent transmission. Adopting optical circuit switching to choose the routing according to wavelengths, AON is transparent to signal formats, bit-rates and modulation modes. That is to say, AON allows arbitrary signal formats, bit-rates, and protocols to be transported.
(3) AON has nice compatibility. Not only can AON be compatible with the current networks, but also AON is able to support the future broadband integrated services digital network (ISDN) as well as the network upgrade.
(4) AON possesses excellent scalability. Adding new nodes to the network has no effect on the original network architecture and node devices.
(5) AON is equipped with good reconfigurability. According to the requirements of communication capacity, AON can dynamically change the network architecture. AON is capable of recovering, building and removing the wavelength link.
(6) AON adopts lots of passive components to take place of the large photoelectric conversion equipments. Possessing simple configuration, AON is easy to maintain. At the same time, the overall exchange rate of AON can be greatly lifted to improve the reliability of network.

4.Key technologies
The key technologies applied in AONs fall into four categories: all-optical switching technology, optical cross connection (OXC) technology, optical add-drop multiplexing (OADM) technology, all-optical relay technology and optical amplifier technology.

4.1 All-optical switching technology
All-optical switching is the directly switching process which omits the OEO conversion to make full use of optical communication bandwidth. All-optical switching technology contains light-path switching technology and packet switching technology. The light-path switching can be divided into three types: space-division switching, time-division switching, wavelength/frequency-division switching. Asynchronous transfer mode (ATM), belonging to the packet switching technology, has been extensively studied.

4.2 OXC technology
OXCs are the devices applied in the optical network nodes to flexible and effectively manage the fiber transmission network by cross-connecting the optical signals. OXC technology is an important means of achieving the reliable network protection and recovery as well as automatic wiring and monitoring.

4.3 OADM technology
OADM, utilized in the optical network nodes, is able to selectively add or drop some wavelength signals as well as directly pass some wavelength signals without affecting other wavelength channel transmission. That is to say, OADM in the optical domain accomplishes the functions that SDH ADM does in time domain. OADM technology possesses transparency, thus able to deal with the signals of arbitrary formats and rates.

4.4 All-optical repeater technology
All-optical repeater technology is to directly amplify the optical signals in the optical path. Replacing the traditional OEO repeaters with the all-optical transmission repeaters, we can settle the problems of the repeater intricacy and electronic bottleneck to achieve the all-actinic signal transmission. The all-optical transmission repeaters include semi-conductor optical amplifier (SOA), Praseodymium-doped fiber amplifier (PDFA) and erbium-doped fiber amplifier (EDFA).

5.Main Components
In all-optical networks, a large quantity of optical components, which include active components and passive components. We will discuss five main components applied in all-optical networks.

5.1 Optical connectors
Optical fiber connectors are used to join optical fibers where a connect/disconnect capability is required. The connectors mechanically couple and align the cores of fibers so light can pass. Fiber Optic Connectors according to connector structure can be divided into: FC,SC, ST, LC, D4, DIN, MU, MTP, MPO and so on in various forms. The optical interface is the physical interface used to connect fiber optic cable.

Optical connectors

5.2 WDM multiplexer/demultiplexer
In a WDM system, multiplexers at the transmitter are used to join the signals together, and demultiplexers at the receiver are utilized to split them apart. According to different wavelength patterns, WDM multiplexer/demultiplexer can be divided into CWDM multiplexer/demultiplexer and DWDM multiplexer/demultiplexer.

Multiplexer Demultiplexer

5.3 OADM
An optical add-drop multiplexer (OADM) is a device used in wavelength-division multiplexing systems for multiplexing and routing different channels of light into or out of asingle mode fiber (SMF). CWDM OADM is designed to optical add/drop one multiple CWDM channels into one or two fibers. DWDM OADM is designed to optical add/drop one multiple DWDM channels into one or two fibers.


5.4 Optical amplifiers
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal.

Optical amplifiers

5.5 Optical switches
An optical switch is a device used to open or close an optical circuit which enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another in telecommunication. In a network system, optical switch plays an important role in protecting the path.

Optical switches

6.Development prospects
All-optical network is the developing goal of the optical communication networks. To achieve the integrated all-optical network, we will experience two phases of development. The first phase is to develop the optical communication network into the all-optical transmission network. During the whole point-to-point fiber transmission process, the photoelectric conversion is not required. The second phase is to achieve the integrated all-optical network. After fulfilling the whole point-to-point transmission, lots of functions, such as signal processing, signal storing, signal exchanging, signal multiplexing/demultiplexing and so on, needs to be completed by the photonic technology. Fulfilling the functions of transmitting, exchanging and processing the end-to-end optical signals is the second developing phase—-the integrated AON.

The WDM System

In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals into a single optical fiber by using different wavelengths of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. A WDM system (Figure 1) uses a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. The concept was first published in 1978, and by 1980 WDM systems were being realized in the laboratory. As a system concept, the ways of WDM includes coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM).


Figure 1: The WDM system

The CWDM System
In simple terms, CWDM equipment performs two functions: segregating the light to ensure only the desired combination of wavelengths are used, multiplexing and demultiplexing the signal across a single fiber link.

Typically CWDM solutions provide 8 wavelengths capability, separated by 20nm, from 1470nm to 1610nm, enabling the transport of 8 client interfaces over the same fiber, as is shown in Figure 2. What’s more, CWDM has the capability to transport up to 16 channels (wavelengths) in the spectrum grid from 1270nm to 1610nm with a 20nm channel spacing. Each channel can operate at either 2.5, 4 or 10Gbit/s. CWDM can not be amplified as most of the channels are outside the operating window of the erbium doped fiber amplifier (EDFA) used in Dense Wavelength Division Multiplexing (DWDM) systems. This results in a shorter overall system reach of approximately 100 kilometers. However, due to the broader channel spacing in CWDM, cheaper un-cooled lasers are used, giving a cost advantage over DWDM systems.


Figure 2:The CWDM system

CWDM proves to be the initial entry point for many organizations due to its lower cost. Each CWDM wavelength typically supports up to 2.5Gbps and can be expanded to 10Gbps support. This transfer rate is sufficient to support GbE, Fast Ethernet or 1/2/4/8/10GFC, STM-1/STM-4/STM-16/OC3/OC12/OC48, as well as other protocols.

CWDM is the technology of choice for cost efficiently transporting large amounts of data traffic in telecoms or enterprise networks. Optical networking and especially the use of CWDM technology has proven to be the most cost efficient way of addressing this requirement.

In CWDM applications, a fiber pair (separate transmit and receive) is typically used to serve multiple users by assigning a specific wavelength to each subscriber. The process begins at the head end (HE) or hub, or central office (CO), where individual signals at discrete wavelengths are multiplexed, or combined, onto one fiber for downstream transmission. The multiplexing function is accomplished by means of a passive CWDM multiplexer (Mux) module employing a sequence of wavelength-specific filters. The filters are connected in series to combine the various specific wavelengths onto a single fiber for transmission to the field. In the outside plant a CWDM demultiplexer (Demux) module, essentially a mirror of the Mux, is employed to pull off each specific wavelength from the feeder fiber for distribution to individual FTTX applications.

CWDM is suitable for use in metropolitan applications, also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example, the downstream signal might be at 1310 nm while the upstream signal is at 1550nm. CWDM can also be used in conjunction with a fiber switch and network interface device to combine multiple fiber lines from the switch over one fiber. CWDM is optimized for a cost conscience budgets in mind, with low-cost, small-powered laser transmitters enabling deployments to closely match guaranteed revenue streams.

The DWDM System
DWDM stands for Dense Wavelength Division Multiplexing. Here “dense” means the wavelength channels are very narrow and close to each other. DWDM uses the same transmission window but with denser channel spacing. Channel plans vary, but a typical system would use 40 channels at 100 GHz spacing or 80 channels with 50 GHz spacing.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber, as is shown in Figure 3. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight OC -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s.


Figure 3: The DWDM system

A basic DWDM system contains five main components: a DWDM terminal multiplexer, an intermediate line repeater, an optical add-drop multiplexer (OADM), a DWDM terminal demultiplexer and an Optical Supervisory Channel (OSC). A DWDM terminal multiplexer contains a wavelength-converting transponder for each data signal, an optical multiplexer and an optical amplifier (EDFA). An intermediate line repeater is placed approximately every 80–100 km to compensate for the loss of optical power as the signal travels along the fiber. An optical add-drop multiplexer is a remote amplification site that amplifies the multi-wavelength signal that may have traversed up to 140 km or more before reaching the remote site. A DWDM terminal demultiplexer consisting of an optical demultiplexer and one or more wavelength-converting transponders separates the multi-wavelength optical signal back into individual data signals and outputs them on separate fibers for client-layer systems (such as SONET/SDH). An Optical Supervisory Channel (OSC) is a data channel which uses an additional wavelength usually outside the EDFA amplification band (at 1,510nm, 1,620nm, 1,310nm or another proprietary wavelength).

DWDM is designed for long-haul transmission where wavelengths are packed tightly together and do not suffer the effects of dispersion and attenuation. When boosted by erbium doped fiber amplifiers (EDFAs)—a sort of performance enhancer for high-speed communications—these systems can work over thousands of kilometers. DWDM is widely used for the 1550nm band so as to leverage the capabilities of EDFA. EDFAs are commonly used for the 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L Band).

A key advantage to DWDM is that it’s protocol and bit rate independence. DWDM-based networks can transmit data in IP, ATM, SONET/SDH, and Ethernet, and handle bit rates between 100Mb/s and 2.5Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel. From a QOS standpoint, DWDM-based networks create a lower cost way to quickly respond to customers’ bandwidth demands and protocol changes.

WDM, as a multiplexing technology in optical field, can form a optic-layer network called “all-optic network”, which will be the most advanced level of optical communications. It will be the future trend of optical communications to build a optical network layer based on WDM and OXC to eliminate the bottleneck of photoelectric conversion with a pure all-optic network. As the first and most important step of all-optic network communications, the application and practice of WDM is very advantageous to developing the all-optic network and pushing forward optical communications!

The Comparison Of WDM And TDM

The Principles of WDM and TDM System
Wave-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e., colors) of laser light, as is shown in Figure 1.This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity. As an analog process, WDM is based on a well-known concept called frequency division multiplexing (FDM). With this technology, the bandwidth of a channel is divided into multiple channels, and each channel occupies a part of the large frequency spectrum. In WDM networks, each channel is referred to as a wavelength. This name is used because each channel operates at a different frequency and at a different optical wavelength. The wavelengths on the fiber are separated by unused spectrum. This practice makes the wavelengths separate from each other and helps prevent their interfering with each other. This idea is called channel spacing, or simply spacing.


Figure 1: WDM operating principle

Time-division multiplexing (TDM) is a technology of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fraction of time in an alternating pattern, as is shown in Figure 2. TDM is a type of digital (or rarely analog) multiplexing in which two or more bit streams or signals are transferred simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent time slots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during time slot 1, sub-channel 2 during time slot 2, etc. One TDM frame consists of one time slot per sub-channel plus a synchronization channel and sometimes error correction channel before the synchronization. After the last sub-channel, error correction, and synchronization, the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc.


Figure 2: TDM operating principle

The Features of WDM and TDM
WDM, possessing high transmission capacity, can save optic fiber resources. As to the single-wavelength fiber system needs to use a pair of optic fibers to receive and dispatch a signal, while the WDM system, no matter how many signals waited to be transmitted, only needs a pair of optic fibers. Being transparent to various service signals, WDM is able to transmit different kinds of signals, then compounding and decomposing them. As an optimal capacity-expanding method, WDM can introduce various services or expand capacity only by means of changing switch and adding an optical wavelength instead of using lots of fibers or high-speed networking devices. What’s more, using the optical add-drop multiplexer (OADM) and the optical cross connection (OXC), WDM can constitute the all-optic network of high flexibility, high reliability and high survivability.

TDM is designed to accomplish the high-capacity and high-speed transmission. Being able to adopt nonlinear soliton transmission and other useful technologies, TDM can eliminate the effect of chromatic dispersion in the high-speed transmission. At the same time, TDM is able to eliminate the rate effect of electronic devices to accomplish the high-speed transmission on a single wavelength. As an effectively optical multiplexing way, TDM can make full use of spectral resources and greatly improve the utilization of spectral bandwidth. Unlike WDM, TDM is free of the limitations resulted from the nonlinear effect of fibers, thus effectively utilizing optical wavelength and operating in various network of different distances and capacities. Though still immature, TDM is a more long-term technology than WDM.

The Differences between WDM and TDM
TDM and WDM are two methods of multiplexing multiple signals into a single carrier. Multiplexing is the process of combining multiple signals into one, in such a manner that each individual signal can be retrieved at the destination. Since multiple signals are occupying the channel, they need to share the resource in some manner.

The primary difference between WDM and TDM is how they divide the channel. WDM divides the channel into two or more wavelength ranges that do not overlap, while TDM divides and allocates certain time periods to each channel in an alternating manner. Due to this fact, we can say that for TDM, each signal uses all of the bandwidth and some of the time, while for WDM, each signal uses a small portion of the bandwidth and all of the time.

TDM provides greater flexibility and efficiency, by dynamically allocating more time periods to the signals that need more of the bandwidth, while reducing the time periods to those signals that do not need it. WDM lacks this type of flexibility, as it cannot dynamically change the width of the allocated wavelength.

WDM proves much better latency compared to TDM. Latency is the time it takes for the data to reach its destination. As TDM allocates time periods, only one channel can transmit at a given time, and some data would often be delayed, though it’s often only in milliseconds. Since channels in WDM can transmit at any time, their latencies would be much lower compared to TDM. WDM is often used in applications where latency is of utmost priority, such as those that require real-time information.

The Relationship between WDM and TDM
WDM and TDM are all ultrafast transmission technologies. TDM has dispelled the restriction of the speed of the electronic device and is free of the limitation of the nonlinear effect of fibers, realizing high-speed transmission on the single wavelength, but it is still at research and development stage for the present. As to WDM, it is a very mature technology and extensively used in communication networks, but its multiplexing wavelength and transmission diatance are restricted by the nonlinear effect of fibers. In the long term, WDM and TDM can be used in tandem and co-exist in the transmission network. As is shown in Figure 3, we can build a bigger optical transmission network by using TDM high-speed channels to connect the subnets composed of WDM. In the subnets, WDM can significantly improve flexibility and reliability of network. At the same time, TDM is effective to accomplish the high-speed and high-capacity transmission.


Figure 3: The optical transmission network of WDM and TDM

WDM, as a mature and high-capacity optical transmission technology, has already been extensively adopted in the network now. It is equipped with the advantages of transparency, reconfigurability and excellent network survivability. The future WDM optical network will develop towards the flexible networking direction based on the optical wavelength routing and exchanging, which possesses the ability of fast network recovery and reconfiguration and will play a main role in the future optical transmission network. As a very effective multiplexing technology, TDM can make full use of spectral resources and dispel some restrictions of WDM system caused by nonlinear effect. In recent years TDM has made great progress in the research field, but not ripe enough. In a nutshell, WDM and TDM have their own advantages and disadvantages as the optical multiplexing technology. With the relevant study lucubrating constantly, WDM and TDM can be combined together to be extensively applied to the ultra fast transmission network.

CWDM DWDM Networking Solutions

Wavelength division multiplexing is a cost effective and efficient way for expanding the fiber optic transmission capacity, because it allows using current electronics and current fibers and simply shares fibers by transmitting different channels at different color (wavelength) of light.

Wavelength Division Multiplexing, WDM is a technique that multiplexing several signals over a single fiber optic cables by optical carriers of different wavelength, using light from a laser or a LED. According to the number of wavelengths it supports, there are Coarse Wavelength Division Multiplexing (CWDM) and Dense Wavelength Division Multiplexing (DWDM).

CWDM was introduced as a low-cost approach to increasing bandwidth utilization of the fiber infrastructure. By using several wavelengths/colors of the light, 18 channels are viable and defined in the ITU-T standard G.694.2. CWDM systems typically provide 8 wavelengths, separated by 20nm, from 1470nm to 1610nm.

Benefits of CWDM
Passive equipment that uses no electrical power
Extended Temperature Range (0-70C)
Much lower cost per channel than DWDM
Scalability to grow fiber capacity with little or no increased cost
Protocol Transparent
Simple to install and use

Drawbacks of CWDM
16 channels may not be enough
Passive equipment offers no management capacities

DWDM packing WDM channels denser than in CWDM systems, 100 GHz spacing (approx. 0.8nm), more channels and higher capacity can be achieved using DWDM. IUT-T recommendation G.694.1 defines the DWDM channels spectrum. DWDM comes in two different versions: an active solution and a passive solution. An active solution is going to require wavelength management and it a good fit for applications involving more than 32 lines over the same fiber. In most cases, passive DWDM is looked at as a more realistic alternative to active DWDM.

Benefits of DWDM
Up to 32 channels can be done passively
Up to 160 channels with an active solution
Active solutions typically involve optical amplifiers to achieve longer distances

Drawbacks of DWDM
DWDM is very expensive
Active solutions require a lot of set-up and maintenance expense
“Passive” DWDM solution still requires power

Optical Add/Drop Multiplexing (OADM)
By optical add/drop multiplexing techniques, wavelength channels may be added and dropped at intermediate nodes using passive optical components only. Optical Add/Drop Multiplexers are used in WDM Systems for multiplexing and routing fiber optic signals. They can multiplex several low-bandwidth streams of data into a single light beam, and simultaneously, it can drop or remove other low-bandwidth signals from the stream of data and direct them to other network routers. There are CWDM OADM and DWDM OADM.

FiberStore offer a wide range of WDM optical networking products that allow transport of any mix of service from 2Mbps up to 200Gbps. Our highly reliable WDM/CWDM/DWDM products include CWDM multiplexers and demultiplexer, DWDM Multiplexers and demultiplexers, CWDM & DWDM Optical Add-drop Multiplexer, Filter WDM modules, CATV amplifier, OEO converters as well as many other most demanding CWDM DWDM networking infrastructure equipment.