What is the Optical Layer in an Optical Network?
>> The Protocol Stacks
The classic 7 layers OSI model view of networks needs some embellishment to handle the variety of networks and protocols that are proliferating today. A more realistic layered model for today’s networks would employ multiple protocol stacks residing one on top of the other. Each stack incorporates several sublayers, which may provide functions resembling traditional physical, data link, and network layers. Let’s consider the IP over Ethernet network shown in the following figure.
In this case, the IP network treats the SONET network as providing it with point-to-point links between IP routers. The SONET layer itself, however, internally routes and switches connections, and in a sense, incorporates its own physical, data link, and network layers.
>> The Optical Layer
The introduction of second-generation optical networks adds yet another layer to the protocol hierarchy – the so-called optical layer. The optical layer is a server layer that provides services to other client layers. This optical layer provides lightpaths to a variety of client layers, as shown in the following figure.
Examples of client layers residing above a second-generation optical network layer include IP, Ethernet, and SONET/SDH, as well as other possible protocols such as Fibre Channel.
As second-generation optical networks evolve, they may provide other services besides lightpaths, such as packet-switched virtual circuit or datagram services. These services may directly interface with user applications, as shown in the above figure. Several other layer combinations are possible and not shown in the above figure, such as IP over SONET over optical.
The client layers make use of the lightpaths provided by the optical layer. To a SONET, Ethernet, or IP network operating over the optical layer, the lightpaths are simply replacements for hardwired fiber connections between SONET terminals or IP routers. A lightpath is a connection between two nodes in the network, and it is set up by assigning a dedicated wavelength to it on each link in its path. Note that individual wavelengths are likely to carry data at fairly high bit rates (in the range of a few to 10 gigabits per second), and this entire bandwidth is provided to the higher layer by a lightpath.
Depending on the capabilities of the network, this lightpath could be set up or taken down in response to a request from the higher layer. This can be thought of as circuit-switched service, akin to the service provided by today’s telephone network: the network sets up or takes down calls in response to a request from the caller. Alternatively, the network may provide only permanent lightpaths, which are set up at the time the network is deployed. This lightpath service can be used to support high-speed connections for a variety of overlying networks.
The functions performed by the optical layer are in many ways analogous to those performed by the SONET layer. The optical layer multiplexes multiple lightpaths into a single fiber and allows individual lightpaths to be extracted efficiently from the composite multiplex signal at network nodes. It incorporates sophisticated service restoration techniques and management techniques as well.
The following figure shows a typical layered network hierarchy, highlighting the optical layer. The optical layer provides lightpaths that are used by SONET and IP network elements. The SONET layer multiplexes low-speed circuit-switched streams into higher-speed streams, which are then carried over lightpaths. The IP layer performs statistical multiplexing of packet switched streams into higher-speed streams, which are also carried over lightpaths. Inside the optical layer itself is a multiplexing hierarchy. Multiple wavelengths or lightpaths are combined into wavelength bands. Bands are combined to produce a composite WDM signal on a fiber. The network itself may include multiple fibers and multiple-fiber bundles, each of which carries a number of fibers.
>> Why Do We Need So Many Layers?
Hence, why have multiple layers in the network that perform similar functions? The answer is that this form of layering significantly reduces network equipment costs.
Different layers are more efficient at performing functions at different bit rates. For example, the SONET layer can efficiently (that is, cost-effectively) switch and process traffic streams up to 10 Gb/s today. However, it is very expensive to have this layer process a hundred 10 Gb/s streams coming in on a WDM link.
The optical layer, on the other hand, is particularly efficient at processing traffic on a wavelength-by-wavelength basis, but is not particularly good at processing traffic streams at lower granularities, for example, 155 Mb/s. Therefore, it makes sense to use the optical layer to process large amounts of bandwidth at a relatively coarse level and the SONET layer to process smaller amounts of bandwidth at a relatively finer level. This fundamental observation is the key driver to providing such functions in multiple layers.
A similar observation also holds for the service restoration function of these networks. Certain failures are better handled by the optical layer and certain others by the SONET layer or the IP layer.