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Second-Generation Optical Networks – A Brief Overview

>> The Advantages of Optical Networks

1. Flexible Bandwidth Delivery

In addition to providing enormous capacities in the network, an optical network provides a common infrastructure over which a variety of services can be delivered. These optical networks are also increasingly becoming capable of delivering bandwidth in a flexible manner where and when needed.

2. High Capacity and Immune to EMI and RFI

Optical fiber offers much higher bandwidth than copper cables and is less susceptible to various kinds of electromagnetic interferences and other undesirable effects.

As a result, it is the preferred medium for transmission of data at anything more than a few tens of megabits per second over any distance more than a kilometer. It is also the preferred means of realizing short-distance (a few meters to hundreds of meters), high-speed (gigabits per second and more) interconnections inside large systems.


>> Current Optical Networks (as of 2011)

Optical fibers are widely deployed in all kinds of telecommunications networks. The amount of deployment of fibers is often measured in sheath miles.Sheath miles is the total length of fiber cables, where each route in a network comprises many fiber cables.

For example, a 10-mile-long route using three fiber cables is said to have 10 route miles and 30 sheath (cable) miles. Each cable contains many fibers. If each cable has 20 fibers, the same route is said to have 600 fiber miles.

A city or telecommunication company may present its fiber deployment in sheath miles; for example, a metropolitan region may have 10,000 fiber sheath miles. This is one way to promote a location as suitable for businesses that develop or use information technology.


>> Two Generations of Optical Networks

When we talk about optical networks, we are really talking about two generations of optical networks.

In the first generation, optics are essentially used for transmission and simply to provide capacity. Optical fiber provided lower bit error rates and higher capacities than copper cables. All the switching and other intelligent network functions were handled by electronics. Examples of first-generation optical networks are SONET (synchronous optical network) and the essentially similar SDH (synchronous digital hierarchy) network, which form the core of the telecommunications infrastructure in North America and in Europe and Asia, respectively, as well as a variety of enterprise networks such as Fibre Channel.

Second-generation optical networks have routing, switching, and intelligence in the optical layer. We will discuss second-generation optical network in the below section.


>> Second-Generation Optical Networks

Optics is clearly the preferred means of transmission, and WDM (Wavelength Division Multiplexing) transmission is widely used in networks. Optical networks are capable of providing more functions than just point-to-point transmission. Major advantages are to be gained by incorporating some of the switching and routing functions that were performed by electronics into the optical part of the network.

For example, as data rates get higher and higher, it becomes more difficult for electronics to process data. Suppose the electronics must process data in blocks of 70 byes each (e.g., a small Ethernet package). In a 100Mb/s data stream, we have 5.6us to process a block, whereas at 10 Gb/s, the same block must be processed within 56ns.

In first-generation networks, the electronics at a node must handle not only all the data intended for that node but also all the data that is being passed through that node on to other nodes in the network. If the latter data could be routed through in the optical domain, the burden on the underlying electronics at the node would be significantly reduced. This is one of the key drivers for second-generation optical networks.

Optical networks based on this paradigm are now being deployed. The architecture of such a network is shown in the following figure.

We call this network a wavelength-routing network. The network provides lightpaths to its users, such as SONET terminals or IP routers. Lightpaths are optical connections carried end to end from a source node to a destination node over a wavelength on each intermediate link.

At intermediate nodes in the network, the lightpaths are routed and switched from one link to another link. In some cases, lightpaths may be converted from one wavelength to another wavelength as well along their route.

Different lightpaths in a wavelength-routing network can use the same wavelength as long as they do not share any common links. This allows the same wavelength to be reused spatially in different parts of the network. For example, the above figure shows six lightpaths. The lightpath between B and C, the lightpath between D and E, and one of the lightpaths between E and F do not share any links in the network and can therefore be set up using the same wavelength λ1. At the same time, the lightpath between A and F shares a link with the lightpath between B and C and must therefore use a different wavelength. The two lightpaths between E and F must also be assigned different wavelengths. Note that these lightpaths all use the same wavelength on every link in their path. We must deal with this constraint if we do not have wavelength conversion capabilities within the network.

Suppose we had only two wavelengths available in the network and wanted to set up a new lightpath between nodes E and F. Without wavelength conversion, we would not be able to set up this lightpath. On the other hand, if the intermediate node X can perform wavelength conversion, then we can set up this lightpath using wavelength λ2 on link EX and wavelength λ1 on link XF.

Key Elements of an Optical Network

The key network elements that enable optical networking are optical line terminals (OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs), as shown in the above figure.

An OLT multiplexes multiple wavelengths into a single fiber and demultiplexes a set of wavelengths on a single fiber into separate fibers. OLTs are used at the ends of a point-to-point WDM link.

An OADM takes in signals at multiple wavelengths and selectively drops some of these wavelengths locally while letting others pass through. It also selectively adds wavelengths to the composite outbound signal. An OADM has two line ports where the composite WDM signals are present, and a number of local ports where individual wavelengths are dropped and added.

An OXC essentially performs a similar function but at much larger sizes. OXCs have a large number of ports (ranging from a few tens to thousands) and are able to switch wavelengths from one input port to another. Both OADMs and OXCs may incorporate wavelength conversion capabilities.

Optical networks based on the architecture described above are already being deployed. OLTs have been widely deployed for point-to-point applications. OADMs are now used in long-haul and metro-networks. OXCs are beginning to be deployed first in long-haul networks because of the higher capacities in those networks.

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