Understanding Multi-Degree ROADM Nodes
Pass-through enables you to configure multi-degree ROADM nodes and pass express traffic between ROADM degrees. This topic provides an overview of the channel connectivity when using pass-through versus directly connected add/drop channels.
2-Degree ROADM Node Example
Figure 1 shows an example of how devices can be connected together in a 2-degree ROADM node. A 2-degree ROADM node contains two ROADM elements, with each ROADM element connected to a line (or degree). These ROADM elements are labelled A and B in the example.
ROADM Element A consists of a TCX1000-RDM20 and a TCX1000-2D8CMD. The TCX1000-2D8CMD connects to a universal port on the TCX1000-RDM20 in ROADM Element A and to a universal port on the TCX1000-RDM20 in ROADM Element B. By connecting to both TCX1000-RDM20 devices, the 2D8CMD has add/drop access to wavelengths on both degrees, which is a prerequisite to setting up a service with redundant paths.
In contrast, ROADM Element B just consists of a TCX1000-RDM20, so it only has add/drop access to wavelengths on the line that it is attached to. The reason for this is that the TCX1000-RDM20 can only switch wavelengths between its universal ports and its line port. It cannot switch wavelengths from one universal port to another.
The external link shown between the TCX1000-RDM20 in ROADM Element A and ROADM Element B is the pass-through connection.
Understanding Pass-Through Connections
The TCX1000-RDM20 pass-through feature enables you to create multi-degree switching nodes and ring, mesh, and linear multi-span network configurations. Pass-through enables channels to be switched from one degree to another within the same ROADM node. Channels configured to take the pass-through path enter on one degree, are demultiplexed and routed to the designated pass-through universal port on the other ROADM degree, which multiplexes the channels and sends them out the line port of ROADM degree to the next node in the path.
Figure 2 highlights the allowable channel connectivity for this pass-through configuration.
In the configuration shown in Figure 2, channels can be add/dropped to any of the three directions served by the node based on which ROADM element they are physically connected (see A, B, C) to or channels can be passed from one degree to another over the pass-through path between each degree in the node (see green lines between each node).
The multi-degree ROADM node in Figure 2 consists of the following ROADM elements and degrees:
3 ROADM degrees:
Degree 1 is specific to the direction 1: RDM20-A
Degree 2 is specific to the direction 2: RDM20-B
Degree 3 is specific to the direction 3: RDM20-C
3 ROADM elements:
ROADM element: RDM20-A serves ROADM degree 1
ROADM element: RDM20-B serves ROADM degree 2
ROADM element: RDM20-C serves ROADM degree 3
Using pass-through connections, you can create a flexible connectivity map for the ROADM node. By interconnecting all ROADM degrees to all other ROADM degrees within the same node using pass-through, you can establish a fully non-blocking switching matrix.
In Figure 2, a pass-through connection is configured between all three ROADM degrees in the node (see green lines), enabling full intra-ROADM mesh any-to-any connectivity between the ROADM degrees:
Channels to/from direction 1 can connect to/from direction 2 (see red line)
Channels to/from direction 1 can connect to/from direction 3 (see light blue line)
Channels to/from direction 2 can connect to/from direction 3 (see orange line)
It is important to understand that the TCX1000-RDM20 only switches channels between its universal ports and its line port. It does not switch channels from one universal port to another universal on the same ROADM element. Channels connected to the universal ports of the TCX1000-RDM20 are always multiplexed by the ROADM and sent out the ROADM line port.
Connecting and Configuring Pass-Through
To pass-through channels from one ROADM degree to another in a multi-degree node, you connect two universal ports on different ROADM degrees within a multi-degree node with a fiber cable and you create a logical link between the two universal ports. When you create a service, you simply select the two service endpoints and the proNX Optical Director automatically provisions the best path for the service.
You can configure multiple channels to run over a single pass-through connection and you can create multiple pass-through connections between ROADM degrees in a multi-degree node.
Understanding Add/Drop Channel Connectivity in Multi-Degree ROADM Nodes
The ROADM node in Figure 3 is capable of routing any directly connected channels to any degree the node serves as long as you connect your transceiver to the correct ROADM degree. In addition, it is important to take note of which ROADM degree you attach your transceivers to in multi-degree nodes (sites), to ensure the channel traverses the shortest path between source and destination.
Figure 3 shows the same configuration but highlights the allowable add/drop channel connectivity of the ROADM node.
In this configuration, channels (A, B, C) are add/dropped to the Line port of the ROADM element they are physically connected to. The allowable add/drop capabilities in this configuration are:
Channel A can be switched only to/from RDM20-A degree 1 (orange line)
Channel B can be switched only to/from RDM20-B degree 2 (red line)
Channel C can be switched only to/from RDM20-C degree 3 (light blue line)
The following channel connectivity is not allowed on the ROADM node in Figure 3:
Channel A cannot be switched to/from degree 2
Channel A cannot be switched to/from degree 3
Channel B cannot be switched to/from degree 1
Channel B cannot be switched to/from degree 3
Channel C cannot be switched to/from degree 1
Channel C cannot be switched to/from degree 2
The most important thing to understand about directly connecting channels to the universal (add/drop) ports in multi-degree ROADM nodes is that channels connected to the universal ports on the TCX1000-RDM20 are considered to be part of that ROADM degree that they are connected to and are sent or received to/from only that ROADM degree or line port. This capability is referred to as single direction colorless multiplexing. You can configure any wavelength you want on any universal port, that is the colorless operation and it is single direction because directly connected channels are always associated with the ROADM degree to which the channel transceiver is physically connected.
It is important to understand that the ROADM elements in Figure 3 are completely capable of switching each channel in any direction by physically connecting the transceivers to the ROADM element serving the direction you want the channel switched.