With ever-increasing telecommunications traffic, the need of the hour is to characterize and quantify this growth and select and implement the technology in an appropriate and optimal manner to upgrade the usable bandwidth of the fiber-optic backbone of national and international networks.

The networks to handle such a high bulk of traffic will not only have to be broadband in nature, but will also have to be highly reliable as well as ready for these mission-critical applications. Thus, the network required has to be of large bandwidth, flexible, reliable, highly survivable, simple to manage, and affordable.

Contrary to voice and video traffic, which are local in nature, the Internet generated and the data-based traffic are national and international in nature. However, the high demand for various types of bandwidth hungry services, vis-à-vis availability and emergence of newer and newer technology to meet the bandwidth demand, makes it difficult and confusing to forecast the demand and choose a corresponding network technology for a broadband over a long distance.

Under such uncertainty in the increase in demand of bandwidth, some operators find it extremely difficult to commit large sums upfront to build their infrastructure. In such a situation, the network required is one that is highly reliable, upgradeable, flexible, manageable, and of large bandwidth and affordable.

Rapid advancements in transmission technologies have enabled and encouraged many operators to jump from copper-based networks to fiber optic-based networks. In countries like India, distance along with bandwidth becomes a very important parameter for consideration while evaluating various available technologies.

What is Optical Networking?
The SDH/SONET (Synchronous Digital Hierarchy/Synchronous Optical Network) equipment has the ability to support a network quite successfully. However, this cannot support the capabilities of optical networking such as routing and switching. In the case of SDH/SONET, routing and switching can be performed in digital domain, not in optical domain. The use of WDM (wave division multiplexing) is viewed as the starting platform for optical networking while WDM line system alone can support terms of optical network functionality.

Optical networking initially consisted solely of the dense wave division multiplexing (DWDM) system deployed by long-distance operators for backbone transmission. While the DWDM system is invading other promising transmission environments such as edge, metropolitan, and access segments, multi-channel long-distance backbone applications still lead the optical network transmission market and are the driving force behind the deployment of fiber optic technologies.

Technologies for Large Bandwidth
Single-mode optical fiber has enormous untapped bandwidth. The demand for increased network capacity has pushed the core network service provider to find the best alternative to increase bandwidth beyond the 2.5 gigabits per second rate on optical fibers. There are several other reasons for using optical fiber cables in the national long distance (NLD) backbone network. Single-mode optical fiber can transmit up to 50 gigabits per second over a thousand kilometer, without repeaters. This surpasses all other media-co-axial, microwave-radio, or satellite. Optical fibers are highly reliable and possess very stable characteristics of extremely low loss (attenuation less than 0.1 dB/Km) and insensitivity to Electro Magnetic Interferences (EMI), which make them ideal for low-error transmission media with high reliability and security over a very long distance for large bandwidth.

The way information is physically encoded onto the transport layer involves a number of tradeoffs among utilization of existing infrastructures, investment in new technologies, and scalability of a network. Traditionally, a single optical carrier per fiber is used in fiber optic networks. Originally, optical networks were based on direct detection receiver and regeneration on periodic intervals along with the core over the distance. Later, researches hinted at the use of coherent receivers in order to increase the distance of operation. With the invention of Optical Amplifiers (Erbium-Doped Fiber Amplifiers, or EDFA), the distance of operation has increased enormously. The EDFA provides low noise, polarization-independent optical gain to overcome the losses during the propagation in the fiber. Initial systems employed purely single-carrier information transport based on Time Division Multiplexing (TDM) until it became apparent that the increased capacity could be economically implemented with a wavelength division multiplexing (WDM) overlay on existing TDM network structure.

There are moves to magnify the part played by optical fiber technology transmission with operators installing WDM or DWDM that carries traffic on different wavelength of light in the same window. The operator is laying fibers in bundles, and vendors are developing a system that packs up to 6.4 tn bits per second.

The technology driver for large-scale deployment in optical fiber is quite vivid. Laser diodes, optical amplifiers, dense wavelength division multiplexing (DWDM) tunable lasers, new types of fibers, and optical switches are all for delivering huge bandwidth over a long distance with high degree of reliability at radically lower price point.

The goal of both new entrants and established operators is to use the right combination of optical carriers and data rates to maximize the network performance in terms of bandwidth, reliability, cost, and future growth.

Earlier Networks
About fifteen years ago, the bit rate required in the backbone networks was 565 Mbps and 1.2 Gbps, survivable ring topology was not much in use, and maximum distances were limited to 50-60 km. Bandwidth hungry services such as the Internet were limited only to some academic institutions. Data traffic was such a small percentage of the overall network that its contribution to the total growth was minimal. TDM was sufficient to combine information channels.

In TDM, increased data rates are made possible by interleaving more and more pulses while shrinking the pulse width at the same time. With the evolution of standards in various parts of the world, different nomenclatures and rates have been designated. At high data rates, these standards are merged into SONET/SDH. A great deal of multiplexing is required to achieve high data rates. As the data rate increases, so does the cost of the electronic terminal equipment that converts the optical signals to electrical signals. Fiber dispersion places limit on the permissible channel bandwidth to keep the pulse distortion at an acceptable level.

A large number of technologies have been investigated to combat the effects of dispersion. Dispersion compensating fibers with special core design are fabricated to compensate for the installed fiber dispersion characteristics for a typical span. Another method for this compensation is through fiber bragg grating (FBG), which gives comparatively lower loss as compared to dispersion compensating fibers. Researches are going to extend the bandwidth over which the dispersion compensation can be employed. Dispersion compensation has allowed a significant increase in the TDM rate using installed fiber. However, the cost of terminal equipment, including transmitter and receiver, increases significantly at the higher bit rate of transmission.

Today's Network
Today's telecom network supports a number of services by means of time division multiplexing (TDM), asynchronous transfer mode (ATM), and Internet Protocol (IP). In most cases, the transmission bandwidth is managed separately from the services, and the management of the transmission network itself is designed as per the vendor or the operator.

In the near future, along with the growth of IP-based services, TDM and ATM-based services will also grow. IP-based and legacy services co-exist in two layers, namely, data service layer and optical transport layer. The data service layer may consist of IP and multi-services network and the transport layer will have to be fully optical, based on DWDM, which, in turn, will be enhanced by existing SDH-based optical networks.

The Required Network
The backbone network is making use of 10 Gbps TDM mainly on point-to-point topology. The system in use, today, generally has a typical span of 100 km before regeneration is required. Though polarization mode dispersion (PMD), reflective splicing technique, and other non-linear effects in optical fiber at 10 Gbps and above prohibit the use of conventional TDM systems, most of the currently installed single-mode fiber plats may be used.

The TDM-based OC-192/STM-64 technology for 10 Gbps SDH systems is maturing. Simultaneously, a 16-channel DWDM is now being widely deployed worldwide in both 2.5 Gbps and 10 Gbps applications. These systems continue to be used basically for point-to-point applications with high capacity SDH rings. Forty channels, eighty channels, or even hundred channels DWDM systems are appearing in the market.

Currently available DWDM system will increase in capacity by adding more channels and using a higher bit rate per channel by improving spectral efficiency and by enhancing the range of solutions that can be offered to accurately match customers' requirements. Reduced channel spacing, enhanced EDFAs, a new generation of amplifier, called Raman Amplifier, and improved high efficiency modulation format are enabling a technology that will extend the maximum distance that can be achieved without regeneration.

Wavelength Division Multiplexing
The driving force behind the push for WDM is the cost consideration and its ability to compensate for chromatic dispersion.

Many difficulties that limit the performance of the TDM system can be avoided by WDM. While TDM is analogous to packing more cars on a single-lane highway, WDM is adding more lanes to the highway. This means that rather than increasing the data rate in order to handle more information, as the TDM system does, a DWDM system carries several optical signals on different wavelengths.

The WDM system using more than two channels in the 1550 nm spectral range is often referred to as Dense Wavelength Division Multiplexing (DWDM). The most important force behind the success of DWDM has been the EDFA. The EDFA is simply a length of optical fiber whose center is doped with an erbium ion. Pumping of fiber with a laser operating at 980 nm or 1480 nm causes the ions to absorb energy. Later, the ions give up the energy to the incoming 1550 nm wavelength signal, resulting in amplification.

WDM terminals consist of multiple independent TDM transmitters at different wavelengths in the 1550 nm band and an equal number of independent TDM receivers. The output of transmitters is optically multiplexed onto a common output interface to the optical transmission network. Optical input to the WDM (or DWDM) terminal is optically demultiplexed and separated signals are fed to the TDM receiver.

DWDM Network Elements
The DWDM networking requires performing optical amplifications, wavelength multiplexing, and routing. Wavelengths can be separated from the data stream using optical add/drop multiplexer (OADM). OADMs and Optical Cross Connects Switches (OCX) are also envisioned for future optical wavelength routed networks.

The use of specific technologies varies from application to application. An all-optical network can provide operation independent of data rate, allowing an easy upgradation from a lower data rate to a higher data rate or from a relatively smaller bandwidth to a larger bandwidth. In such a situation, wavelength conversion would have to be performed with an all-optical method.

DWDM techniques offer great flexibility over pure TDM methods in EDFA-equipped networks. Optical channels can be added or dropped as per the requirements at intermediate points to match the demand. DWDM is cost effective since the most exciting OC-48 (2.5 Gb/x) network facility can be readily incorporated into them by treating each OC-48 path as an individual wavelength channel.

Challenges before DWDM
The challenges faced by DWDM are mainly the evolution of optical frequency standards, non-linear behavior of optical fiber network architecture and its manageability, setting wavelength, the stability of laser beam of the transmitter, and its narrow channel spacing in a multi-beam transmitter. It is further complicated by a phenomenon, called Four-wave Mixing. Currently, ITU-T (International Telecommunication Union under the UN Charter) has allocated channel frequency grid referenced at 193.1 THz (1552.52 nm) with the channel spacing at the integral multiple of 100 GHz channel from 196.1 THz (1528.77 nm) to 192.1 THz (1530.61nm).

The complexity of network management for DWDM system can vary tremendously depending on the architecture. For point-to-point DWDM system, the management is not like a network that simply added more fibers along a span using DWDM. In transparent networks, signals pass without bandwidth limitation, thereby not requiring opto-electronic regeneration; but it requires a wavelength interchange and switching capability allowing wavelength reuse.

Several non-linear optical phenomena occurring in fiber itself such as Stimulated Brillouin Scattering (SBS) and Stimulated Raman Scattering (SRS) affect high power narrow bandwidth channels in DWDM transmission. This can be controlled by a proper combination of standard dispersion un-shifted fiber, dispersion shifted fiber, and non-zero dispersion shifted fiber. Though this may solve the problems up to 2.5 Gbps or a maximum of 10 Gbps, the technologies of improved fibers don't solve all the problems at a bit rate beyond 10 Gbps. These limitations will, probably, prevent the commercial system with bit rates of 40 Gbps and channel counts of more than 100.

In the near future, emerging amplifier technologies in 1300 nm region and above 1550 nm region may provide some relief from the need to space channel more closely, but, again, the installed fiber base poses key limitations since attenuation is higher at 1300 nm region and dispersion (pulse spreading phenomenon in optical fibre) was neither controlled nor measured above 1565 nm.

Challenges Ahead
Flexible and scalable DWDM-based optical network solutions have to be foreseen for the use in the transition period, depending on the technology available to match the needs and requirements of the operator. Meeting the needs of both new and legacy operators, a considerable challenges are faced in the following areas:

• Increasing network bandwidth vis-a-vis cutting the implementation cost
• Designing flexible and future proof solution
• High reliability
• End-to-end manageability and high quality of service (QoS)
• Short provisioning time

DWDM for backbone applications are increasing the number of wavelength that can be carried on the fiber while increasing the transmission distance. Therefore, consideration is needed for packaging the wavelength in DWDM so closely together to have a trade-off with the distance of operation More wavelength means more transmitters, which, in turn, means more cost selection of the optical cross connect to handle the desired traffic Comparison between costs for different technologies is required to determine the most cost effective way of some total DWDM bandwidth

The initial phase of DWDM introduction may follow the following path:

• Mostly up to 8 channels with further upgradation to 16-channel DWDM system
• Multiplexing of STM-16 tributary
• Adding/dropping wavelength at a very few number of desired stations

A proper integration of TDM and DWDM will make it possible to provide the required network bandwidth at the maximum possible distance with the desired error performance survivability and manageability.

Anuj Kumar Srivastava
vadmail@cybermedia.co.in
The author is general manager, (East Area), MTNL