Understanding TCX Series Terminology
To understand the TCX Series ring, mesh, and linear multi-hop capabilities, you need to understand certain terms. Throughout this document we use the following reconfigurable optical add drop multiplexer (ROADM) terms:
ROADM Element — A ROADM element is a TCX Series device that provides a designated function as a constituent part of a ROADM node. This function can include switching channels to or from another ROADM degree using a pass-through connection or adding/dropping physically connected channels onto the associated ROADM degree’s fiber span. Examples of ROADM elements are the TCX1000-RDM20 or a combination of the TCX1000-RDM20 and a compatible multiplexer such as the TCX1000-2D8CMD or the BTI7800-FMD96. A ROADM node is built from a number of ROADM elements. The TCX1000-ILA is both a network element and a node.
ROADM Degree — ROADM degrees are groups of elements that are specifically and exclusively assigned to a given network line direction. The term ROADM degree can also be used to simply describe the TCX1000-RDM20 element.
ROADM Node — A ROADM node is a configuration of ROADM elements that together provide a specific role in an optical network. A ROADM node is conceptual only, and exists purely to convey the type of role that the constituent ROADM elements provide. Examples of ROADM nodes are single-degree terminal nodes and multi-degree add/drop nodes.
In addition the following packet optical ROADM node characteristics and terms appear throughout this document:
Colorless — The colorless property of a ROADM node is the ability of the ROADM node to have a multiplex port in which the channel center frequency (or wavelength: they are the same thing) is initially unassigned. This flexibility can be achieved with a broadband multiplexer or a programmable/tunable multiplexer, for example the TCX1000-RDM20. The operational center frequency of the port is determined through provisioning.
The important points are that colorless is a property of the multiplexer (universal) ports, not the line ports, and that the center wavelengths used on the multiplexer port channels must be provisioned. That is, the multiplexer port is colorless until a wavelength (or wavelengths) are assigned to that port.
Directionless — The directionless property is not something that a single ROADM degree, such as a standalone TCX1000-RDM20, can have. There is only one direction (or destination) for the line port on a single ROADM degree. The universal port traffic on the TCX1000-RDM20 can only go in one line direction.
The directionless property is a characteristic of the ROADM node rather than each individual degree.
In multi-degree ROADM node configurations, directionless refers to the ability of the ROADM node to have a multiplexer port that is initially unassigned to a line port direction. Obviously, it would take at least two line ports to offer any choice of line direction, but the choices are not limited to two as long as there are multiple degrees to the ROADM node.
In its simplest form, a directionless configuration consists of two ROADM elements connected to each other and to an add/drop device in common.
In summary, directionless operation is a characteristic of the ROADM node and not the individual element. Directionless operation can be achieved with a direction-switchable or broadcast multiplexer, such as the TCX1000-2D8CMD, where the line port direction used is determined through provisioning.
Contentionless — Contentionless operation refers to the ability to reuse wavelengths within a directionless ROADM node.
In a directionless but non-contentionless architecture, after a wavelength is configured on a multiplexer port (to go in one line direction), the same wavelength cannot be configured on another multiplexer port of the same multiplexer (even if going in a different line direction). This is a blocking architecture.
In a directionless and contentionless architecture, the same wavelength can be reused on different multiplexer ports of the same multiplexer as long as the wavelength on one port uses a different line direction from the wavelength on the other port.
This is a non-blocking architecture.
If the multiplexer element is designed to provide a fully non-blocking solution, then the solution is contentionless (as well as directionless).
Flex Grid — In flex grid operation, not only is the channel center frequency or wavelength provisionable for the multiplexer port, but the channel width is provisionable as well. This capability is important because the channel width can now be optimized to match the spectral width of the channel. For example, a 32 Gbps Baud channel, previously operated in a 50 GHz window aligned with a 50 GHz-spaced grid, can now operate in a 37.5 GHz window, and a 64 Gbps Baud channel can be accommodated with the provisioned 75 GHz channel width.
As with colorless operation, this flexibility can be achieved with a broadband multiplexer or a programmable/tunable multiplexer. The operational width and center frequency of the port is determined through provisioning.
The important point here is that flex grid ports are all colorless, but all colorless ports are not necessarily flex grid ports.
The hardware of the TCX1000-RDM20 is flex grid ready. However the proNX Optical Director does not yet support the ability to configure the channel width.