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Service routing in the all-optical network

Given these limitations it's extremely difficult to implement ESI effectively in an all-optical network because a large part of what ESI does is concerned with involves the routing and signalling of connections. In an all-optical network the network elements are often unable to flexibly route connections, or are prevented from routing connections for reasons such as wavelength blocking.

This diagram shows the way that a conventional optical network tends to implement ESI. There are broadly two kinds of device in this network:

  • All-optical reconfigurable optical add-drop multiplexers (ROADM) devices that cannot take a direct part in most ESI operations because they have no visibility of the digital character of services.

  • Opto-Electronic-Opto (OEO) cross-connect devices (OXC) that do have digital visibility.

Although both types of device (OXC and ROADM) may have ESI capability, it's simply not possible to bring up services dynamically on an all-optical ROADM because the appropriate transponders may not be installed, and even if they were pre-positioned, there's no guarantee that a particular wavelength could support the required service. In the past, the all-digital world of SONET/SDH could have coped with the demands of ESI, but with the advent of DWDM the costs of converting many wavelengths of SONET/SDH from optical to electrical and back to optical at each hop (known as OEO conversions) was perceived to be unacceptable.


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