Optical networks - ROADM
Older optical networks deploy SDH / SONET technologies for the transport of data over the optical network. These networks are relatively easy to plan and design. New network elements can be easily added to the network. WDM static networks may require less investment in equipment, especially in metro networks. However, planning and maintaining these networks can be a nightmare as the engineering rules and scalability are often quite complex.
Bandwidth and wavelengths must be pre-allocated. Because wavelengths are grouped into groups and not all groups end at every node, accessing specific wavelengths may not be possible at some sites. Network extensions may require new amplifiers and optical-electrical regeneration -optics or at least power adjustments in existing sites. Operating a static WDM network is very labor intensive.
Network and bandwidth planning should be as easy as in SDH / SONET networks in the past. In the given bandwidth of the ring, for example STM-16 or OC-48, each node could provide as much bandwidth as needed.
Full bandwidth access was possible at each ADM. Network expansion, for example, inserting a new node into an existing ring, was relatively easy and did not require any on-site visits to existing nodes. The network diagram on the left illustrates this: Digital interconnect systems link with multiple SDH / SONET optical rings.
Reconfigurable optical networks act differently: bandwidth can be planned on demand and range is optimized as optical power is now managed byr WDM channel. Scalability increases dramatically.
The key element in enabling such a reconfigurable optical network is the Reconfigurable Optical Multiplexer (ROADM) . It allows optical wavelengths to be redirected to client interfaces with one click in the software. The other traffics remain unchanged. All this is achieved without the need to drive to the respective sites to install filters or other equipment.
Reconfigurable WDM Network with ROADMs
Static WDM engineering rules and scalability can be quite complex (OADM in each node).
- Bandwidth and wavelength pre-allocation
- Margin allocation for a fixed filter structure
- Insufficient power management
- Network expansion requires optical-electrical-optical (OEO) regeneration
SDH / SONET networks are easy to plan.
- Full bandwidth access on each ADM
- Simple engineering rules (single hop only)
- Easy addition new network elements
A reconfigurable optical layer enables the following.
- On-demand bandwidth planning
- Transparent reach extended through power management by WDM channel
- Safe Scalability
Static photonic layers are made up of separate optical rings. Consider a number of DWDM systems located on each of these rings. Often times the information or data just stays on the same ring, so there is no problem. However, what happens in cases where the data needs to be transferred to a different optical ring?
In static systems, a large number of transponders are needed wherever a transition between the rings is required.e. Indeed, each wavelength which passes from one ring to another needs two transponders: one on each side of the network. This approach involves high costs and a lot of upfront planning, taking into account the allocation of bandwidth and channels.
Now imagine a dynamic reconfigurable photonic layer. Here there is only one DWDM system forming the interface between two optical rings. Therefore, transponder-based regeneration disappears and the number of DWDM systems decreases. The whole network design is simplified and wavelengt hs can now travel from one ring to another without further obstruction.
Any wavelength can propagate to any ring and to any port. The key to such a fully flexible and scalable network design, with optical passage from the core to the access area, is the ROADM and the GMPLS control plane.
Simplifications via ROADM
ROADMs simplify the service provider or carrier network and processes. This interaction sums up some of these simplifications. After all, we have to keep in mind that all of these benefits translate into reduced time and costs. But what is more important is that they also lead to increased customer satisfaction and, therefore, customer loyalty.
Network planning is greatly simplified with ROADMs. Just consider the greatly reduced number of transponders, which must be stored in the warehouse.
Installation and commissioning - for example, when setting up a new wavelength on the network - requires much less effort and is much less complex . Service technicians only need to visit the respective end sites to install transponders and ROADMs. Fixed optical multiplexers(FOADM) required a visit to each staging site so that installation work and fixes could be carried out.
Operations and maintenance are greatly simplified when a dynamic optical network is deployed. Optical diagnosis can be performed in minutes rather than hours as it used to be. Deficiencies can be dynamically detected and cleared instead of triggering truck runs to external locations.
With the deployment of tunable lasers and colorless ROADMs, maintenance of the fiber plant is made easier. With these features, providing services is now easier than ever. As with installation and commissioning work, it is also much easier to carry out network maintenance and possible upgrades.
Many advantages The ages that the ROADMscontributing to the design and operation of the network have been dealt with in the previous sections. Here are a few more -
- Monitoring and power leveling per channel to equalize all DWDM signal
- Complete traffic control from network remote operations center
One question, however, has remained unanswered so far: how does a ROADM work? Let's take a look at some basics.
A ROADM usually consists of two major functional elements: a wavelength splitter and a selective wavelength switch (WSS). Look at the block diagram above : A pair of optical fibers at network interface # 1 is connected to the ROADM module.
The fiber carrying incoming data (from the network) is routed to the wavelength splitter. Now all wavelengths are available on all splitter output ports, in this case 8. The traLocal add / subfile (wavelengths) can be multiplexed / demultiplexed with a matrix waveguide filter (AWG). Using an AWG implies a fixed wavelength allocation and direction.
The Selective Wavelength Switch (WSS) selectively joins the different wavelengths and feeds them to the output of network interface # 1. The remaining splitter ports are connected to other network directions, for example, three other directions at a 4 degree junction node.
Note - One of the modules shown (completely gray boxed) is required per network direction at this node. Or to be more precise: in a trunk node serving four directions (4 degrees), four of these modules are needed.
The ROADM Heart - the WSS module
Let's start with the WDM signal coming from the left. It passes through the optical fiber from above and is directed towards a diffraction grating in menough. This mass diffraction grating acts like a kind of prism. It separates the different wavelengths in different directions, you gh the angle variation is quite small. The separated wavelengths hit a spherical mirror, which reflects the rays onto a set of micro-electromechanical systems (MEMS). Each microswitch is struck by a different wavelength, which is then reflected back to the spherical mirror.
From there the rays are sent back to the bulk diffraction grating and sent to the optical fiber. But now it is a different fiber than the one we started with. The single wavelength output signal indicates that this has occurred. This signal can then be combined with other single wavelength signals to fill another transmission fiber.
There are different versions available - the keywords here are colorless, without direction, etc.
ROADM - Degrees, colorless, withoutdirection, and more
| Term || Explanation |
| Degree || The term Degree describes the number of supported DWDM line interfaces. A 2 degree ROADM node supports two DWDM line interfaces. It also allows two add / remove branches from all line interfaces. |
| Multi Degree || Multi-degree ROADMs support more than two interfaces of DWDM line. The number of possible add / remove branches is determined by the number of WSS ports. |
| Colorless || Colorless ROADM allows flexible assignment of any what wavelength or color at any port. Filter modules must be connected to implement this function. |
| Directionless || |
A directionless ROADM does not require physical reconnection of the transmission fibers. Restrictions on directions are removed.
Directionless ROADMs are deployed for restoration purposes or for temporary rerouting of services (for example, to cause network maintenance or on-demand bandwidth requirements).
| Contentionless || Contentionless ROADMs eliminate the potential problem of the collision of two identical wavelengths in the ROADM. |
| Gridless || Non-network ROADMs support |
To understand this leveled ROADM approach, here are some key terms often used in relation to ROADMs.
Simple ROADMs include a WSS for each direction, also called “a degree”. Wavelengths are always assigned and added / removed fixed transceivers are used. Colorless ROADMs remove this limitation: with such ROADMs, any wavelength or color can be assigned to any port. No truck roll is required as the entire setup is software controlled. Filter modules must be implemented to achieve the colorless function.
This often appears in conjunction with the term "colorless". A directionless design removes another ROADM limitation. The need to physically reconnect transmission fibers is eliminated by using directionless ROADMs as there are no restrictions on the direction, for example, to the south or to the north.
Although iColorless and directionless, ROADMs already offer great flexibility, two wavelengths using the same frequency could still collide in a ROADM. Contention-free ROADMs provide a dedicated internal structure to avoid such blockage.
Gridless ROADMs support a very dense wavelength channel grid and can be adapted to future transmission speed requirements. This functionality is required for signal rates over 100 Gbit / s and different modulation formats within the same network.
When without direction
Directionless ROADMs are the most popular ROADM design as they allow the addition / removal of wavelength from ITU grid supported on any line interface. In the case of a directionless variant only, the add / remove ports are specific to a length of onde defined. By using the colorless option, ports can also be wavelength non-specific.
Directionless technology is primarily deployed for wavelength rerouting to other ports as needed for restoration purposes. Other applications are also possible, for example, in bandwidth on demand situations. ROADMs that do not support the directionless function are subject to certain limitations in terms of flexibility.
When they are colorless
Colorless ROADMs allow the wavelengths of a specific optical channel to be changed without any physical rewiring. A colorless ROADM can be reconfigured to add / remove any supported ITU grid wavelength on any add / remove port. The added / deleted wavelength can change (adjustable DWDM interface). This allows -
Improved flexibility for wavelength provisioning and wavelength restoration
Restore switching, directional switching, and color switching
The main advantage of colorless add / remove ports in combination with adjustable DWDM line interfaces is increased flexibility for wavelength provisioning and wavelength restoration. Automatic adjustment of the next free wavelength on a requested optical path.
One of the last bits of full optical network automation is the deployment of colorless ROADMs. Using such ROADMs allows the add / remove of any supported ITU grid wavelength on any add / remove port. The wavelength on the port may change as tunable transceivers are useds as optical fronts.
Supplying and restoring wavelength is even easier than before. When a wavelength is occupied, the system can automatically tune the transceiver to the next available free wavelength. ROADMs provide the ability to use fixed and colorless add / remove functionality in the same ROADM node.
When C ontentionless
ROADMs without contention can add / remove any wavelength to any add / remove port without no contention grid on any add / remove port. A dedicated wavelength color can be added / removed multiple times (from different DWDM line interfaces) on the same add / remove branch. If only 8 add / remove ports are equipped, it must be possible to drop the same wavelength from8 different line directions on the 8 add / drop ports. As long as free add / remove ports are available, the ROADM node must be able to add / remove any wavelength from / to any line interface.
The combination of colorless, directionless and contention-free (CDC) functionality provides the ultimate level of flexibility.
When nodes without a network
ROADM nodes without a network support different ITU-T channel grids in the same DWDM signal. Network bandwidth can be provided per channel.
The gridless function is required for networks operating at data rates greater than 100 Gbit / s or for network operating with different modulation schemes. It is intended for next generation networks with consistent line interfaces. Different data rates require different wavelength requirements depending on theu modulation scheme and data rate.
Transmission speeds are increasing and modulation schemes are becoming more and more complex. Several modulation technologies can now be mixed on a single optical fiber. All of this links back to ROADM technology and generates the requirements for networkless ROADMs. These ROADMs operate on a dense frequency grid and allow channel-based bandwidth provisioning. Data channels now require different wavelengths depending on their modulation scheme and data rate.
Typical applications are networks operating at data rates above 100 Gbit / s or performing different modulation schemes in parallel. The latter situation can, for example, easily exist when deploying coherent transmission technologies.