Fig. 1. AWG architecture

Fig. 2. Implemented AWG card

The AWG is a passive WDM device which can be used as a multiplexer, demultiplexer, a drop-and-insert element, or a wavelength router. In general AWG consists of M input and M output slab waveguides and two identical focusing planar star couplers connected by M dispersive waveguide array. An important property of the AWG is the free spectral range (FSR), also known as the demultiplexer periodicity. The free spectral range denotes the wavelength and frequency spacing between the maxima of the the interference pattern because of the periodicity characteristic of the AWG. For the three 18X18 AWGs specifically designed for the ADRENALINE testbed, since this kind of equipment is not available commercially, the FSR comprises 18 wavelengths, from ITU frequency 192.10THz to 193.8THz spaced 100GHz. This property allows that wavelengths can be reused if the input ports are different, that is, we use a single AWG for multiplexing and demultiplexing the same wavelengths but using different ports, as shown in figure 1. Note that the ADRENALINE testbed employs 7 wavelengths, from 193.0THz (1553.33nm) to 193,6THz (1548.51nm). Figure 2 shows the implemented AWG card.

Fig. 3. Implemented array of 2x2 optical switches card

The implemented add/drop stage makes use of two 2x2 optical switches card. Each of these cards holds nine 2x2, add/drop optical fiber switches, whose configuration is handled by a controller card. The optical switches are provided with some electric contacts by which their state (cross/bar) can be driven, and some other contacts allow the readout of the present switch position. The switches are latched-type, which means that the device remains in its latest position following an undesired power loss. A Complex Programmable Logic Devices (CPLD) placed on each of these boards acts as the bridge interpreter between the controller card and the array of switches, setting them according to the commands received from the former. It reduces the microcontroller load and offers switches state information constantly update. The CPLD is reprogrammable via a Joint Test Action Group JTAG header that is sited on the board.

A customized serial port physically supports communications between these boards. The two 2x2 optical switches cards share such serial port, and thus a careful arbitration must be laid in order to avoid any potential conflict. In regards of this fact and the possibility of future additional switches cards, each of them is identified with a particular address that is set manually with their respective address switch. Figure 3 shows the implemented array of 2x2 optical switches card.

Fig. 4. Implemented controller card

For each add/drop stage, its arrays of 2x2 optical switches are managed by the controller card, which integrates a micro controller 32-bit ARM7TDMI. Such micro controller is uClinux based, and is connected to the rest of the system through an Ethernet plug, although other channels are available on the card: standard serial port and current latch (RS-422). The micro controller is reprogrammable through any of these mentioned channels. Another customized serial port communicates with the CPLD on each of the two switches boards, trafficking commands to set or read the switches states. Both the arrays of switches and the controller card can be reset either via hardware (with a switch provided with the controller card) or software, thus allowing synchronization at the start-up of all the cards of the system.

The ARM7TDMI 32-bit processor, named Optical Switching Module Controller (OSMC), is a 55 MHz CPU and its functions are to configure, to supervise and to control the Add-Drop Stage, that is, to verify and to modify the status of the 2x2 optical switches (cross/bar). The OSMC uses 2 different communication protocols. The communication protocol with the arrays of 2x2 optical switches is a RS-232 protocol, while the communication with the control plane of the ADRENALINE testbed is property, implemented via Ethernet. When the OSMC starts up, it opens the communication channels and then it tries to configure all optical switches of the Add-Drop Stage. If the operations are completed successfully, the OSMC informs to the control plane (R-OADM controller) that it is ready to receive its requests. There are two possible requests coming from the control plane. The first one is to monitor the status of the optical switches. The second one is configuration of the optical switches. If the OSMC receives a status monitor request, it verifies the last status of the 2 optical arrays, and then it returns the physical status of each 2x2 optical switch. So the control plane check the status of all optical switches at the same time. However, the performance of the status configuration request is a little bit different, since the control plane can change the status of any status 2x2 optical switch separately, although the OSMC will return the status of the 2 arrays of 2x2 optical switches. The time spent to perform every request is 15 ms. approximately, therefore it is a fast optical commutation system for wavelength-routed networks. When the OSMC does not receive requests from the control plane, it monitors periodically (every 20 ms.) the status of the 2 optical arrays in order to supervise the system, since in this way it can report to the control plane of any irregular event happened in the Add-Drop Stage. The OSMC has also a user interface (UI) via Ethernet, which indicates the current status of all the system. Figure 4 shows the implemented controller card.

Fig. 5. Example of Pass-through of channel 30 from port 1 to port 2

Fig. 6. Example of Pass-through in protection of channel 30 from port 1

The add/drop stage has been implemented in such a way that can work as a unidirectional ring or bidirectional ring via software configuration. An incoming signal (in figure 5 port 1) composed by control (1310nm) and WDM data (1550nm) signal is demultiplexed using a second/third window multiplexor. Then the WDM data signal enters to the 2x2 OMS protection switch (PS1 in the figure) of the first array of 2x2 optical switches. If this switch is in pass-through, the WDM signal is demultiplexed by the AWG into separate wavelengths. Then each wavelength enters again to the array of 2x2 optical switches (from S1 to S7). The same process apply for the incoming signal from port 2, but in this case using the second array of 2x2 optical switches (S9 to 14). These switches are capable of dropping the wavelength, adding a new one, or passing-through the wavelength. Each one of the seven wavelength has its own switch, so all the wavelengths that cross the node can be added or dropped. Added or dropped wavelengths are connected to the distribution stage.

In the general case, all the 2x2 optical switches from both arrays can be used for adding or dropping purposes, except when the OMS protection is required. If the node is configured as OMS unidirectional, all the switches located in the second 2x2 array are disabled for adding or drooping purposes, and are configured as pass-through. For a OMS bidirectional configuration, each array of 2x2 optical switches must use a different set of wavelengths in order to avoid wavelength contention when the traffic of one fiber is switched over the other fiber. Therefore only the half of the 2x2 switches in each array can be used for adding or dropping purposes, and the rest must be configured as pass-through.

Finally, each output wavelength from the 2x2 array is wavelength-multiplexed by the AWG before the signal is sent to the output OMS protection switch (PS2). If this protection switch is in pass-through, the WDM signal is multiplexed with the control signal coming from the OCC and transmitted to the output fiber (port 2). If the OMS protection is activated (figure 6), the protection switches (PS2 and PS3) are in cross, and the WDM data signal is inserted again in the AWG, wavelength-demultiplexed, passing-through the 2x2 optical swicthes and wavelength-multiplexed. Last, the WDM signal arrives to the protection switch (PS4) in pass-through and transmit the signal to the output fiber (Port 1).