The architectures presented here are aligned with the detailed descriptions and naming conventions given in [103]. First generation MD-ROADM were implemented using liquid crystal array wavelength blocker devices in a broadcast and select architecture [31], [46], [99], and [100].
The most basic degree-2 ROADM architecture is outlined in Figure 109. All links are bi-directional over a pair of fibres however only one direction is shown for simplicity; wavelength blocker modules include two devices, one per direction.
Figure 109 Degree-2 ROADM node architecture
An incoming WDM signal from direction A passes through a splitter and is broadcasted through a de-multiplexer (typically an AWG) and received at a local drop port, the signal is also broadcasted towards the wavelength blocker. The wavelength blocker can be set to fully attenuate any of the wavelengths in the passband. A blocked wavelength is dropped at the corresponding drop port of the de- multiplexer and received by a tuneable transponder tuned to the correct wavelength where OEO regeneration takes place. A signal at the dropped wavelength can then
B
A
Local Add/Drop Ports
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be added after the wavelength blocker via a multiplexer and coupler to join the aggregate WDM signal thereby allowing wavelength re-use.
Channels which are not dropped locally are said to optically bypass the node without any OEO regeneration thereby significantly saving the number of transponders compared with the OEO architecture. A ROADM does cost significantly more than two back-to-back terminals however since typically more than 50% of traffic entering a node is passing through, the reduction in transponder numbers compensates for the increased DWDM infrastructure cost resulting in an overall cost saving at a network level.
Additional components are required at ROADM nodes (and indeed at terminals) and are omitted from the architecture diagrams for simplicity. EDFAs are present on either side of the wavelength blocker to compensate for the loss of the preceding fibre span and the wavelength blocker, splitters and couplers.
DCMs are included at the mid-stage of DSA EDFAs to optimise the dispersion map on a link by link basis so that all possible connections within the network have near optimal dispersion maps regardless of their origin or destination.
Power monitors and an OSC are required to monitor and control the power levels of each channel in the WDM signal.
Signal pre-emphasis and re-emphasis to a desired profile as detailed in section 2.8 is also possible at ROADM and MD-ROADM nodes by controlling the liquid crystal array to attenuate each channel independently. This is particularly important in transparent networks as different channels are likely to have transmitted over varying distances in the network experiencing different gain and loss profiles resulting in an uneven spectrum. Dynamic gain equalisation functionality is effectively provided for free at ROADM nodes. Prior to ROADMs dedicated channel levelling nodes were required approximately after 8 fibre spans as described in section 2.8, adding significant cost to a network. Levelling nodes may still be required in a ROADM network where certain links between ROADMs are longer than about 8 spans.
Benefits of MD-ROADM: The primary advantage of ROADM and MD-ROADM is
the significant saving of costly OEO regenerators (back to back transponders) that is achieved through optical bypass of transit traffic. A significant saving in CapEx is expected particularly as the network scales to higher capacities.
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OpEx is also expected to reduce as the scalability advantage can lead to reduced power consumption and real estate space requirements.
MD-ROADMs can also improve the flexibility of a network to respond to changing traffic demands. Remote re-configurability is possible via the NMS however tuneable transponders and colourless and directionless and contention-less (CDC) MD-ROADMs are required for full flexibility.
Disadvantages of MD-ROADM: Increased optical transparency comes at a price.
As transmission distances increase, higher specification ULH transmission capabilities are required together with costly ROADM components (WB or WSS) resulting in high upfront costs for DWDM infrastructure. Unless traffic is at a sufficiently high level for the saving in OEO equipment to outweigh the higher DWDM infrastructure cost, the MD-ROADM network may not be economical until late in the network’s lifecycle.
Increased transmission distances and optical bypass leads to an analogue network whereby knowledge of fibre span characteristics and careful management of transmission impairments is required to maintain adequate BER for each connection [85]. Complex wavelength planning is also required to avoid / reduce wavelength contention and optimising network cost.
5.3.1
N
ODES OFD
EGREE>2
USINGROADM
ULH systems and early Multi-Reach DWDM systems were initially limited to degree-2 ROADM functionality. Network nodes of degree greater than 2 were implemented using a combination of degree-2 ROADM and 1 or more terminals. Figure 110 shows an example degree-3 node implementation. The ROADM is oriented along the two directions with the most pass-through traffic (i.e. path A-B) in order to maximise the benefit of optical bypass.
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Figure 110 Degree-3 node using a degree-2 ROADM and a terminal
Traffic routed between A-C and B-C require OEO regeneration between the terminal and the add/drop ports of the ROADM via back to back transponders interconnected by an optical patch panel.
For the general case of a node of degree , OEO regeneration takes place between the add/drop ports of the ROADM and each of the terminal branches within the node and also between each of the terminal branches themselves. Figure 111 and Figure 112 show a degree 4 and degree 5 node respectively.
A significant disadvantage of this architecture is that the optimal direction of pass- through traffic may deviate from that forecasted at the outset of network planning, thus leading to a potentially sub-optimal network as traffic evolves.
B Terminal ROADM A C direction of maximum pass-through traffic O-E-O Regeneration
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Figure 111 Degree-4 node using a degree-2 ROADM and 2 terminals
Figure 112 Degree-5 node using a degree-2 ROADM and 3 terminals
A slightly more optimal node architecture given in [103] and shown here in Figure 113 can be achieved for degrees > 4 by orienting 2 ROADMs along 2 routes of maximum through traffic. Unfortunately this was not implemented at the time of the research however the improved economic impact is likely to be relatively small.
B ROADM A C direction of maximum pass-through traffic O-E-O Regeneration D Ter min al Terminal D A B direction of maximum pass-through traffic O-E-O Regeneration E C ROADM Ter min al
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Figure 113 Degree-4 node using 2 degree-2 ROADMs
5.3.2
MD-ROADMB
ASED ONW
AVELENGTHB
LOCKERSA MD-ROADM capable of full optical bypass in up to 4 directions with an architecture based on wavelength blockers (WB MD-ROADM) was developed soon after the launch of the Multi-Reach DWDM system. Figure 114 illustrates the architecture of a degree 3 node.
Figure 114 Degree-3 “broadcast and select” WB MD-ROADM node architecture
Incoming signals from each of the three fibre directions A, B and C are broadcasted to the add/drop de-multiplexer and through a passive splitter into two branches with wavelength blockers along each branch. If a particular channel is to be routed all-
C ROADM D directions of maximum pass-through traffic O-E-O Regeneration ROADM B A directions of maximum pass-through traffic B A C WB Passive Splitter/Combiner
Local Add/Drop Ports Wavelength Blocker A
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optically to path B for example then the wavelength blocker between path A and C would be set to block that particular channel allowing it to only pass through A to B. For the general case of a degree node the through path is split into paths with wavelength blockers on each. The number of wavelength blocker devices per node scales with the square of the node degree and since each wavelength blocker module is bi-directional containing one device per direction the number of wavelength blocker modules NWB scales as
2 1 D D NWB Equation 57 A degree-4 node architecture is shown in Figure 115.Figure 115 Degree-4 “broadcast and select” WB MD-ROADM node architecture
Extending to a degree 5 MD-ROADM node would require 10 wavelength blocker modules as shown in Figure 116, however from an implementation complexity and cost perspective WB MD-ROADM are limited to optically bypass in a maximum of 4 directions. B A C D Passive Splitter/Combiner Local Add/Drop Ports
Wavelength Blocker WB WB A /D A /D A/D A/D
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Figure 116 Degree-5 “broadcast and select” WB MD-ROADM node architecture
Figure 117 Degree-4 “broadcast and select” MD-ROADM upgraded to degree-5 node architecture with an additional terminal
C A
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D
Passive Splitter/Combiner Local Add/Drop Ports
Wavelength Blocker E WB WB A /D A/D A/D C A B D Passive Splitter/Combiner Local Add/Drop Ports Wavelength Blocker WB WB E O-E-O Regeneration A /D A/D A /D A/D
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A degree 5 node is implemented as a degree 4 MD-ROADM with an additional terminal for the 5th degree as shown in Figure 117. OEO regeneration occurs between the add/drop ports of each direction on the MD-ROADM and the terminal where a signal passes through the terminal direction and one of the MD-ROADM branches. To minimise regeneration cost the terminal branch is chosen for the route with the lowest pass-through traffic and highest add/drop traffic.
5.3.3
MD-ROADMB
ASED ONW
AVELENGTHS
ELECTIVES
WITCHWavelength Selective Switches (WSS) are advanced optical switching modules which today have replaced the WB as the technology of choice for implementing broadcast and select ROADM and MD-ROADM node architectures in meshed optical networks [102].
An N×1 WSS is capable of selecting any DWDM wavelength from N different input fibres and combining these wavelengths on a common output fibre. Conversely a 1×N WSS is capable of switching individual wavelengths from a common input port to any of N output fibres.
A key advantage of WSS versus WB is scalability; the number of WSS modules required in a node scales linearly with degree rather than quadratically. WSS result in lower cost and complexity of transparent network nodes and can enable full transparency in more than 4 directions; optical losses are also lower.
In common with WB, WSS are capable of dynamic spectral equalisation of each wavelength to optimise transmission reach performance.
The most common WSS technology is 2D MEMS for high port count WSS (9×1) and liquid crystal arrays for low port counts (2×1 & 4×1) [95]. Liquid crystal on silicon (LCOS) is emerging as a candidate technology for flexible wavelength grid applications i.e. combinations of 50GHz and 100GHz channel spacing and alternatives such as 25GHz closely spaced or 75GHz, 150GHz and higher for high speed rates beyond 100 Gb/s.
Higher port counts modules are appearing on the market with 1×20 WSS recently released [104]63. Despite network nodes being usually no more than degree-8, such high port count WSS are crucially important for implementing colourless,
63
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directionless nodes [105] and also for interconnecting multiple fibre pairs per direction where parallel DWDM line systems are deployed.
Figure 118 Degree-4 MD-OADM node architecture with WSS
An example degree-4 node architecture based on WSS is shown in Figure 118. Each WSS module (shown in green) includes an N×1 WSS on the multiplex side and a 1×N passive splitter on the de-multiplex side. Fixed multiplexers and de-multiplexers (typically AWG) are used for local add/drop. In this configuration add/drop ports are both coloured (wavelength specific) and tied to one particular direction, therefore the architecture is not fully flexible for automated re-configurability in the event of future changes in traffic demand. For add/drop channels the transponder must be manually connected to the ingress/egress mux/demux port of the correct colour and associated with the correct direction.
The ultimate level of add / drop flexibility can be achieved with all-optical switch based on 3D MEMS technology [106]. Such devices can enable a true N:N WSS with 100% add / drop capacity, each with colourless, directionless and contention less add / drop access, however due to the high cost these devices have not been widely adopted in commercial networks.
B A
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Local Add/Drop Ports
Nx1 WSS & 1xN passive splitter A /D A /D A/D A/D W SS W SS WSS WSS
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