Optical networking

One of the major issues in the networking industry today is tremendous demand for more and more bandwidth. Before the introduction of optical networks, the reduced availability of fibers became a big problem for the network providers. With the development of optical networks and the use of Dense Wavelength Division Multiplexing (DWDM) technology, a new and probably, a very crucial milestone is being reached in network evolution.

Optical networks transmit data by sending laser light through filaments of glass fiber; this distinguishes them from traditional networks, which transmit electrical signals along copper wires. Although optical hardware and transmission media are more expensive, they can handle far higher capacity loads at significantly faster speeds.

Optical networks are high-capacity telecommunications networks based on optical technologies and components that provide routing, grooming, and restoration at the wavelength level as well as wavelength-based services.

As networks face increasing bandwidth demand and diminishing fiber availability, network providers are moving towards a crucial milestone in network evolution: the optical network. Optical networks, based on the emergence of the optical layer in transport networks, provide higher capacity and reduced costs for new applications such as the Internet, video and multimedia interaction, and advanced digital services.



History

Telecommunication networks have evolved during a century-long history of technological advances and social changes. The networks that once provided basic telephone service through a friendly local operator are now transmitting the equivalent of thousands of encyclopedias per second. Throughout this history, the digital network has evolved in three fundamental stages: asynchronous, synchronous, and optical.

Asynchronous

The first digital networks were asynchronous networks. In asynchronous networks, each network element's internal clock source timed its transmitted signal. Because each clock had a certain amount of variation, signals arriving and transmitting could have a large variation in timing, which often resulted in bit errors. More importantly, as optical-fiber deployment increased, no standards existed to mandate how network elements should format the optical signal. A myriad of proprietary methods appeared, making it difficult for network providers to interconnect equipment from different vendors.

Synchronous

The need for optical standards led to the creation of the synchronous optical network (SONET). SONET standardized line rates, coding schemes, bit-rate hierarchies, and operations and maintenance functionality. SONET also defined the types of network elements required, network architectures that vendors could implement, and the functionality that each node must perform. Network providers could now use different vendor's optical equipment with the confidence of at least basic interoperability.

Optical

The one phpect of SONET that has allowed it to survive during a time of tremendous changes in network capacity needs is its scalability. Based on its open-ended growth plan for higher bit rates, theoretically no upper limit exists for SONET bit rates. However, as higher bit rates are used, physical limitations in the laser sources and optical fiber begin to make the practice of endlessly increasing the bit rate on each signal an impractical solution. Additionally, connection to the networks through access rings has also had increased requirements. Customers are demanding more services and options and are carrying more and different types of data traffic. To provide full end-to-end connectivity, a new paradigm was needed to meet all the high-capacity and varied needs. Optical networks provide the required bandwidth and flexibility to enable end-to-end wavelength services.

Optical networks began with wavelength division multiplexing (WDM), which arose to provide additional capacity on existing fibers. Like SONET, defined network elements and architectures provide the basis of the optical network. However, unlike SONET, rather than using a defined bit-rate and frame structure as its basic building block, the optical network will be based on wavelengths. The components of the optical network will be defined according to how the wavelengths are transmitted, groomed, or implemented in the network. Viewing the network from a layered approach, the optical network requires the addition of an optical layer. To help define network functionality, networks are divided into several different physical or virtual layers. The first layer, the services layer, is where the services-such as data traffic-enter the telecommunications network. The next layer, SONET, provides restoration, performance monitoring, and provisioning that is transparent to the first layer.

Emerging with the optical network is a third layer, the optical layer. Standards bodies are still defining the optical layer, but it will eventually provide the same functionality as the SONET layer, while operating entirely in the optical domain. The optical network also has the additional requirement of carrying varied types of high bit-rate non SONET optical signals that bypass the SONET layer altogether. Just as the SONET layer is transparent to the services layer, the optical layer will ideally be transparent to the SONET layer, providing restoration, performance monitoring, and provisioning of individual wavelengths instead of electrical SONET signals.

Dense Wavelength Division Multiplexing (DWDM)

Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission technique. It involves the process of multiplexing many different wavelength signals onto a single fiber. So each fiber have a set of parallel optical channels each using slightly different light wavelengths. It employs light wavelengths to transmit data parallel-by-bit or serial-by-character. DWDM is a very crucial component of optical networks that will allow the transmission of data: voice, video-IP, ATM and SONET/SDH respectively, over the optical layer.

Hence with the development of WDM technology, optical layer provides the only means for carriers to integrate the diverse technologies of their existing networks into one physical infrastructure. For example, though a carrier might be operating both ATM and SONET networks, with the use of DWDM it is not necessary for the ATM signal to be multiplexed up to the SONET rate to be carried on the DWDM network. Hence carriers can quickly introduce ATM or IP without having to deploy an overlay network for multiplexing.

DWDM SYSTEM

Optical networks use Dense Wavelength Multiplexing as the underlying carrier. The most important components of any DWDM system are transmitters, receivers, Erbium-doped fiber Amplifiers, DWDM multiplexors and DWDM demultiplexors. Fig gives the structure of a typical DWDM system.

The concepts of optical fiber transmission, amplifiers, loss control, all optical header replacement, network topology, synchronization and physical layer security play a major role in deciding the throughput of the network.

SONET

The Synchronous Optical NETwork (SONET) standard for fiber optic networks was developed in the mid-1980s. It remains in widespread use today. In a nutshell, SONET allows multiple technologies and vendor products to interoperate by defining standard physical network interfaces.

Technology

SONET commonly transmits data at speeds between 155 megabits per second (Mbps) and 2.5 gigabits per second (Gbps). To build these high-bandwidth data streams, SONET multiplexes together channels having bandwidth as low as 64 kilobits per second (Kpbs) into data frames sent at fixed intervals.

Compared to Ethernet cabling that spans distances up to100 meters (328 feet), SONET fiber typically runs much further. Even short reach links span up to 2 kilometers (1.2 miles); intermediate and long reach links cover dozens of kilometers.

The Future of SONET

Because SONET can carry very large amounts of traffic, it would seem on the surface to be an ideal technology for future voice and data broadband networks. SONET competes with several other viable technologies including ATM and Gigabit Ethernet for this role.




Resources:

http://www.iec.org
http://compnetworking.about.com