colorless and directionless multi-degree roadm
TRANSCRIPT
In 1 Out 1
Drops 1 Adds 1
ROADM Degree 1
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Figure 1. Schematic of a conventional N-degree ROADM node
Colorless and Directionless Multi-Degree
Reconfigurable Optical Add/Drop Multiplexers
Philip N. Ji
NEC Laboratories America
4 Independence Way, Princeton, NJ USA
Yoshiaki Aono
Optical Network Division, NEC Corporation
1131 Hinode, Abiko, Chiba 270-1198, Japan
Abstract— The reconfigurable optical add/drop multiplexer
(ROADM) has become one of the most important elements in the DWDM network. The next generation multi-degree ROADM requires two main features: colorless and directionless. Colorless means that add/drop ports are not wavelength specific, and directionless feature enables any transponder to be connected to any degree. In this paper we review and analyze different multi-degree ROADM node architectures that offer full colorless and directionless features. Their key characteristics and properties,
including optical loss, size, cost, modularity and upgradability, are compared and discussed.
Keywords- Reconfigurable optical add/drop multiplexer,
wavelength cross-connect, colorless, directionless, non-blocking,
wavelength-selective switch
I. INTRODUCTION
The reconfigurable optical add/drop multiplexers (ROADMs) have been widely deployed in long haul and metro WDM networks in recent years, enabling the flexible adding and dropping of any or all WDM channels at the wavelength level without manual configuration or the costly and power-consuming O-E-O switches. In mesh or meshed ring optical networks, multi-degree ROADMs (MD-ROADMs) also provide cross-connection functions of WDM signals among different paths, so they are also called the wavelength cross-connects (WXCs).
The current MD-ROADMs are typically constructed by simply interconnecting individual single degree ROADMs together (Fig. 1). However, such node cannot provide full flexibility for add/drop and switching. In this paper, we discuss the requirements for the next generation MD-ROADM nodes, then review and analyze six major node architectures that offer such features.
II. REQUIREMENTS FOR NEXT GENERATION MD-ROADM
The current generation MD-ROADM nodes have two major limitations. Firstly, they are colored. This means that each transponder corresponds to a fixed wavelength, because the wavelength corresponding to each demultiplexer output port is predetermined during the manufacturing process. Even though the photodetector in the transponder can receive any wavelength within the transmission band, and even if the transmitter in the transponder is tunable across the entire band,
the transponder can only operate on a fixed channel determined by the demultiplexer output port it connects to. Therefore all transponders need to be preinstalled if there is possibility for any channel to be dropped. This is costly, especially if the add/drop percentage is small but the add/drop channels change dynamically. Alternatively, manual provisioning by operator is needed to connect individual transponders to the required add/drop ports, leading to higher operation cost and higher possibility of human error. Therefore network owners are demanding colorless feature for next generation MD-ROADM [1-4], where the add/drop ports are not wavelength specific and any transponder can be tuned to any DWDM channel. This feature allows full automation of wavelength assignment. And the network owners do not need to pay high upfront capital expense by installing all transponders, but can have pay-as-you-grow investment strategy. Colorless feature also allows the network owners to use different wavelengths for different sections in the optical path to avoid congestion situations.
The second limitation is that the add/drop operation is directional. In other words, the outbound direction of the add signals is limited to the same degree and cannot reach other ROADM degrees (with optical protection switch, it can reach 2 degrees). This prevents the added signals to be routed to different optical paths and thus limits the flexibility of the node and the network. To overcome this limitation, the next generation MD-ROADM needs to have the directionless feature [1-5]. It is defined as: for any channel dropped at the local node, the corresponding add channel can go to any output port, regardless of which input port it comes from. The directionless feature allows more efficient sharing of
TPND: transponder
978-1-4244-7596-4/10/$26.00 ©2010 IEEE
In 1 Out 1
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Figure 2. Functional schematic of colorless and directionless N-
degree ROADM node
PXC
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(a) (b) (c)
(d) (e) (f)
Figure 3. Schematics of six colorless and directionless MD-ROADM architectures
transponders in a node among different paths, and improves the protection scheme.
Fig. 2 shows the functional schematic of a MD-ROADM with both colorless and directionless features. Both features aim to enable pre-fibering and software-based service provisioning, thus eliminating the needs for manual provisioning and reducing the operation expense of the network.
Besides colorless and directionless, the next generation MD-ROADM node must be non-blocking. This is also called contentionless and includes two aspects: at the drop path, if total p channels from all N input degrees need to be dropped and p does not exceed the total number of transponders in the shared transponder bank, then all these p channels can be dropped regardless of whether they contain channels with the
same wavelength from different input ports. At the add path, contentionless means: the signals from different sources (cross-connect or add) that need to go to the particular output port do not have duplicated wavelengths. (Or if some wavelengths are duplicated, only one of them contains useful signal while the rest can be discarded.)
Also, the node needs to have reasonable optical power budget and does not require high level of optical amplification. Similarly, the hardware footprint also needs to be as compact as possible, and the power consumption needs to be low.
Other requirements include modularity, which refers to the ability to separate the node into functional elements for easy reconfiguration, upgradeability to add more degrees or add/drop ports, and not having a large single point-of-failure.
Finally, the deciding factor to the adaptation and deployment of new technology is the cost. Therefore the overall cost (capital expense for the equipment and the on-going operation expense) of the MD-ROADM node needs to be low.
III. COLORLESS AND DIRECTIONLESS MD-ROADM NODE
ARCHITECTURES
MD-ROADM node architectures with different configurations and components can be used to achieve the colorless and directionless features. Here we review six major colorless and directionless MD-ROADM architectures. All of them are contentionless too.
A. Architecture 1: AWG+PXC
The first architecture uses AWG optical multiplexers,
demultiplexers and photonics cross-connect (PXC, which is a multi-port fiber switch) to achieve colorless and directionless switching ([6, 7], Fig. 3(a)). Here all the DWDM channels
from the N input ports are fully demultiplexed into N×K individual channels (where K is the total number of DWDM channels from each input), these signals are combined with the
added channels (up to L×N, where L is the maximum number
of local add/drop channels per degree, and L ≤ K) and sent to a
large scale PXC with the dimension of [(L+K)×N] ×
[(L+K)×N]. At the output end, the signals for each output port are combined through respective multiplexer, and the drop channels are sent to the transponders in the shared transponder
bank containing L×N colorless transponders with tunable wavelength. Since the PXC can switch any input fiber to any output fiber, any channel can be switched to any output port or dropped to any transponder, realizing the fully colorless and the fully directionless features. The problem of this architecture is that it requires large scale PXC. This is not only costly but also presenting a single point-of-failure.
B. Architecture 2: WSS + switch + tunable filter array
The second architecture does not require PXC switch. This architecture and the subsequent architectures have similar structure as the general schematic shown on Fig. 2: at each input port, the DWDM signal is split into N groups, among which N-1 groups are sent to the other N-1 degree output ports (assuming no loop-back path is reserved), and the last group contains the drop signals. Each dropped channel is switched to a destination colorless transponder within the shared transponder bank through a transponder aggregator and achieves the colorless function, and this aggregator also switches the added signals to respective target output port to achieve directionless feature. At each output port, a wavelength-selective switch (WSS) is used to select the appropriate output signals and avoid wavelength contention. The main issue for these architectures is how to design the transponder aggregator efficiently.
In this architecture (Fig. 3(b)), the drop signals from each
input port is sent to a 1:(L×N) splitter. These signals are then
sent to an array of L×N units of selector, which contains an
N×1 optical switch followed by a tunable filter. In such case,
each of the N×1 switch can receive all input channels from all input ports, and it then selects the input port from which the respective drop channel comes. The tunable filter subsequently selects the particular channel from the selected DWDM signals to be received at the connected colorless transponder. The add part has the same configuration, but does not require the tunable filters [6].
The disadvantages of this architecture include the large
optical loss due to the 1:(L×N) splitters and the high components cost due to the large number of optical switches and tunable filters required. Another approach is to use “WSS
+ splitter” instead of “splitter + 1×N switch” before the tunable filters [2]. This approach has smaller optical loss, but has wavelength contention issue among the drop signals from different input ports.
C. Architecture 3: WSS + AWG + PXC
In this architecture (Fig. 3(c)), the transponder aggregator is constructed using N units of 1:K AWG demultiplexers and a
(N×K) × (N×L) PXC [7]. Since the number of input fibers in
the PXC (N×K) is greater than the number of the output fibers
(N×L), some signals are discarded by the PXC, these are the channels to be cross-connected to output ports directly and do not need to be dropped locally.
D. Architecture 4: Cascaded standard WSS + PXC
Fig. 3(d) shows the fourth architecture. It uses
commercially available WSS (with port count up to 1×9) in the transponder aggregator [7]. For cases with L>9 per degree,
1:L/9 splitter is used to separate the signal into multiple standard WSS units. The WSS’s select the channels to be dropped among the DWDM signals from each input and sends
to an (N×L) × (N×L) PXC that is shared among all input ports. The output fibers of the PXC are connected to the shared transponders. Another PXC with the same dimension is used to separate Add signals for each output port, these signals are
combined by N units of L:1 couplers. The 1:L/9 splitter can
be replaced by a 1×L/9 WSS, this will reduce the optical loss if L is large, but is also more costly.
E. Architecture 5: Standard WSS + HPC WSS + PXC
The fifth architecture is similar to Architecture 4, but instead of having cascaded coupler with WSS at the
transponder aggregator, high port count (HPC) 1×L WSS’s are used (Fig. 3(e)). This will further reduce the size and cost of the
node due to integration, but WSS with port count beyond 1×9 is not commercially available yet.
F. Architecture 6: Super HPC WSS + PXC
Architecture 6 further combines the WSS in each ROADM unit and the WSS in the transponder aggregator (Fig. 3(f)). It
requires even larger size WSS with port count of 1×(N+L-1).
For Architectures 2-5, the path separation and combining devices at the input and output ends of each degree can be (1) both WSS’s, a solution that is more costly but has lower insertion loss and can further avoid wavelength contention, or (2) splitter at the input and WSS at the output, as shown on Fig. 3, or (3) WSS at the input and coupler at the output end. These options do not affect the colorless and directionless add/drop property. For Architecture 6, Option (2) is not available.
IV. ANALYSIS AND COMPARISON OF COLORLESS AND
DIRECTIONLESS MD-ROADM ARCHITECTURES
All these six architectures described above achieve the colorless, directionless and contentionless features required for the next generation MD-ROADM node, we now compare and analyze other aspects of their design, including optical loss, hardware size, cost, upgradability, and modularity.
TABLE I. COMPONENT COMPARISON AMONG SIX MD-ROADM ARCHITECTURES
Arch. Mux/Demux
1:K PXC ports
WSS Coupler/splitter Switch
1××××N
Tunable
filter 1××××N 1××××L 1××××(N+L-1) 1:L/9 1:N 1:L 1:LN
# 1 2N
2N (L+K)
(1 of [N(L+K)]
×[N(L+K)] )
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# 2 0 0 N 0 0 0 N 0 2N 2NL NL
# 3 2N 2N(K+L)
(2 of NK×NL ) N 0 0 0 N 0 0 0 0
# 4 0 4NL
(2 of NL×NL ) N+NL/9 0 0 N N N 0 0 0
# 5 0 4NL
(2 of NL×NL ) N N 0 0 N N 0 0 0
# 6 0 4NL
(2 of NL×NL ) 0 0 N 0 N N 0 0 0
N: Number of node degrees (N×N node); K: Channels per port/degree; L: Local add/drop channels per degree (L<=K); INT: Upper bound integer
TABLE II. MD-ROADM INSERTION LOSS AND SIZE COMPARISON
Arch. Path Insertion loss (dB) Size (relative)
#1
Through/Cross 13.4
1.5 Drop 8.4
Add 8.4
#2
Through/Cross 16.2
6 Drop 41.8
Add 35.6
#3
Through/Cross 16.2
2 Drop 15.6
Add 11.6
#4
Through/Cross 16.2
2 Drop 21.8
Add 21.6
#5
Through/Cross 16.2
1.5 Drop 17.6
Add 21.6
#6
Through/Cross 18.2
1 Drop 8.4
Add 25.6
Table I lists the components required for each architecture. These figures are used for the analysis below. For general
analysis, we assume that it is an 8×8 node with 80 DWDM channels from each input port, and the maximum number of add/drop channels per degree is 20 channels (25%).
Using typical loss figures from commercially available devices, the optical insertion loss figures for different paths in each architecture are calculated and listed in Table II. These figures show that Architecture 1 has the lowest loss, particularly for the add and the drop paths, since it uses a large integrated PXC element. Architecture 2 has the highest loss for the add and the drop paths where the loss is about 40 dB. This
is mainly due to the 1:(L×N) splitters/couplers used. Such figure is too large even with the optical amplifiers. Architectures 3-6 have moderate loss, mostly within 10 to 20
dB range. Optical amplifier(s) can be placed in the node to compensate for the loss.
Another important factor to select the suitable architecture is the hardware size. As the node degree and the add/drop channel count increase, the MD-ROADM node is also scaled up significantly and may require multiple racks to accommodate all the equipments. We use the actual and expected size of the typical devices to estimate the node hardware size. The comparison results are also shown on Table II. They show that Architecture 2 needs to occupy largest amount of space. Node based on Architecture 6 occupies the least amount of space due to high level of device integration. Architectures 1, 3, 4 and 5 require moderately more space than Architecture 6, but still significantly more compact than Architecture 2.
We then analyze the hardware cost. Among the components, the HPC WSS and super HPC WSS are not available now, so the unit prices are estimated based on the market projection. Other components are currently commercially available, we use the today’s ballpark market prices for the analysis. Fig. 4(a) shows the change of hardware cost from 2 degree to 20 degree whereas the add/drop ratio per degree is kept at 25%. As shown, the hardware cost increases approximately proportional with the number of degree for all architectures, except for Architecture 2 where the increase has exponential-like behavior. This is mainly due to the
requirement of high port count splitters and N×1 switches in this architecture. Fig. 4(b) analyses the hardware cost variation for different add/drop percentages. The result shows that Architectures 1 and 3 have higher upfront cost at low add/drop percentage, whereas the other architectures has lower initial cost but the cost increases more dramatically as the number of add/drop channels increases. The crossing point for the break equal case is about 35% for Architecture 2 and 50% for Architecture 4. The cost of Architectures 5 and 6 at high add/drop percentage is not studied since they require WSS with port count beyond 40, which will not likely to be available within the next few years. Such high port count WSS can be constructed using cascaded lower port count WSS’s, which becomes Architecture 4.
Among the six architectures, only Architecture 1 does not have the “ROADM units + transponder aggregator” structure, and instead has a large centralized/shared PXC. This reduces the flexibility to modularize, and presents a large single point-of-failure. The node in Architecture 2 to 6 can be partitioned into N individual ROADM units (one per degree) and a shared transponder aggregator. Among them, Architecture 2 allows further dividing of selector sub-modules since it does not require shared PXC. However this level of modularity will require large amount of additional fiber connection in between.
These sub-modules can be combined if integrated components are available. Architectures 3-6 have similar modularity level.
In terms of upgradability, Architecture 2 is relatively better since it does not require large scale PXC, and its transponder aggregator can be further divided into sub-units.
V. CONCLUSIONS
The next generation MD-ROADM node requires the colorless feature where add/drop ports are not wavelength specific and the directionless feature that enables any transponder to be connected to any degree. In this paper, we reviewed six major node architectures that offer fully colorless and fully directionless features yet without wavelength contention problem. Their optical loss performance, equipment size, hardware cost, modularity, point-of-failure issue, and upgradability are analyzed. MD-ROADM node with these features will improve switching flexibility, allow better restoration scheme, and reduce hardware and operation cost in DWDM optical network.
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[6] V. Kaman, R. J. Helkey, and J. E. Bowers, “Multi-Degree ROADM’s
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Figure 4. MD-ROADM node hardware cost comparison: (a) at
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