SONET and DWDM:
Competing Yet Complementary Technologies
for The Metro Network
Introduction
The next generation of SONET equipment has been well accepted by service providers and is being
usefully deployed into networks today to carry continually increasing network traffic. Compared to
traditional SONET equipment, NGADM is characterized by simple architecture, scalability, high capacity
add/drop, multiple ring terminations, multiservices, and multiple fabrics, including STS and VT switching
capabilities. At the same time, DWDM equipment is increasingly showing its viability in today’s metro
networks with tremendous available bandwidth over the existing fiber plant, along with a variety of service
interfaces and superior optical transmission capabilities. A new generation of equipment is also being
rolled out that provides features of both SONET and DWDM. Some are DWDM systems with integrated
SONET units that provide grooming, add/drop and protection of a wavelength. Others are NGADMs with
built in DWDM capability that provide optical multiplexing, amplification and transmission.
Although a higher capacity technology such as DWDM brings about many benefits, this technology can
also incur higher initial cost. In quite a few situations, we need to evaluate the benefits gained from DWDM
and SONET against the economical impact that each inherits. Important questions that need to be
reviewed for each type of system include: why is DWDM needed; when is it needed; under what
circumstances is DWDM more economical than a SONET architecture; what are the factors that need to be
taken into account for the deployment of DWDM technology; and when is SONET grooming beneficial?
The Scenarios of the Study
This paper investigates an example metro network from some real life IOF applications to answer some of
the questions previously mentioned. In this example network, three network architectures are discussed
(see Figure 1):
• PMO – a SONET ring architecture with cross-connects and local rings
• SONET Overlay – a SONET ring architecture that uses optimized overlaid rings to avoid ports at the
pass-through nodes in the network
• DWDM – an architecture in which DWDM equipment is deployed for transport with SONET aggregation
as needed for each wavelength
The main objective of this study is to investigate the crossover points at which the economical advantages
of the SONET approaches and the DWDM approach exchange positions. The factors that directly influence
the crossover point in question are the network traffic volume, distances between sites in the network,
circuit types (OC-3, OC-12, OC-48 or Gigabit Ethernet), density of interface cards and shelf itself, diversity of
traffic and the equipment cost. We will also discuss other impacts on selecting a core SONET versus a core
DWDM architecture in a carrier’s metro network.
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For your convenience, a list of acronyms can be found at the end of this document.
1
60 k m
R
60 km
Ring
ing
SONET
SONET
SONET
SONET
SONET
Present Mode of Operation
SONET
SONET
15 k m
Route
SONET
Route
Ring
105 km
15 k m
15 k m
SONET
15 k m
SONET
oints
2 Endp
oints
oints
3 Endp
Route
oints
4 Endp
Route
15 k m
SONET
15 k m
1 Endp
SONET
SONET Overlay
15 k m
15 k m
SONET
SONET
105 km
Ring
DWDM
DWDM
DWDM
DWDM
DWDM
DWDM
15 k m
DWDM
DWDM
Figure 1: The Three Main Scenarios
Analysis of Crossover Points
Four types of traffic demands have been tested in the models: OC-3, OC-12, OC-48 and GigE—one type of
demand at a time. Each end-to-end demand is called a circuit. For example, two end-to-end OC-48
demands in route one are defined as two (OC-48) circuits. In the modeling, each type of traffic demand has
been routed evenly into the four routes so that each route gets the same amount of demands or number
of circuits. The physical span distances are set to be uniformly 15 km in the initial comparisons. Then, some
sensitivities have been conducted to evaluate the impact of distance change on the SONET solutions.
Protection is provided at both the tributary and network levels. Tributary interface cards are 1+1 protected
for the SONET solutions as well as the DWDM solution, except in the GigE case where no protection is
provided on the interface card. SONET provides BLSR protection and DWDM provides a BLSR-like shared
path protection. SONET designs are all OC-192 rings and the DWDM mux/demux has a capacity of 80
unprotected or 40 protected wavelengths.
For SONET designs, an abundant supply of fibers is assumed to be available and fiber spans between sites
can be connected together to allow an optical bypass of a site by a SONET ring. For DWDM design, limited
fiber is assumed and DWDM equipment is required at each site to enable an optical bypass or termination
as might be needed. This makes the model more generic.
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In the SONET PMO scenario, as illustrated in Figure 1, two four-node topological rings exist in this eightnode network, one on the left and one on the right. The traffic demands or circuits carried in routes one
and four could only be routed through the two SONET NEs in the upper middle of the network. The optimal
routing for the other two routes is just direct connection between the two nodes on the far left and/or the
far right. See Figure 2 for an example of the traffic routes.
oints
1 Endp
Route
oints
2 Endp
Route
oints
3 Endp
Route
oints
4 Endp
Route
g
Rin
60 km
R
60 km
ing
SONET
SONET
SONET
SONET
SONET
SONET
15 k m
SONET
SONET
Figure 2: Traffic Routing in PMO
Let’s first assume four OC-48s are terminated at each of the four terminating sites (two OC-48 circuits for
each route). A simple design shows two OC-192 rings, one on each topological ring, and 16 OC-192 ports
on the line and cross-connect sides of the SONET NEs in the network. If eight OC-48s are terminated at each
terminating site (four OC-48 circuits for each route), then four OC-192 rings, two on each topological ring,
and 32 OC-192 ports are required.
Another solution for the same problem is to overlay the SONET rings to bypass the cross-connect point in
the network. In this solution, only one topological ring is formed.
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3
Route
Route
Route
Route
1 Endp
oints
2 Endp
oints
3 Endp
oints
4 Endp
oints
SONET
15 k m
SONET
105 km
Ring
SONET
SONET
SONET
SONET
Figure 3: Traffic Routing in the SONET Overlay Scenario
Figure 3 shows the optimal routing of the demands for this SONET overlay solution. Given four OC-48
demands terminated at each end site, two OC-192 rings still exist (with longer links than PMO, though) but
only 12 OC-192 line side ports in a six-node network. If eight OC-48s are terminated at each terminating
site (four OC-48 circuits for each route), then four overlaid OC-192 rings and 24 OC-192 ports are required.
The third approach is a DWDM deployment in which DWDM core equipment is deployed in seven locations
and transponders (4:1 muxponder, OC-48 to 10 Gbps in this example in Figure 4) are installed in the four
terminating nodes. DWDM equipment is deployed at all nodes, including the three nodes without circuit
terminations in the model, to enable reconfigurable optical bypass as needed.
DWDM
DWDM
DWDM
DWDM
DWDM
4:1
nders
Muxpo
DWDM
DWDM
4:1
nders
Muxpo
Figure 4: Wavelength Traffic in the DWDM Core Scenario
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In the case of eight OC-48 terminations per node (four circuits per route), the ring has four total
wavelengths in any particular span. The thin lines represent protection wavelengths and the thicker lines
represent working wavelengths. Assuming the DWDM system can handle 40 protected wavelengths in a
ring, a single such system can accommodate up to 20 protected 4:1 transponders or 80 protected OC-48
terminations at each terminating node.
Traffic volume, in Gbps, is a function of the number of circuits in all four routes. For example, traffic volume
is said to be 16 x 2.5 Gbps = 40 Gbps when 16 OC-48 circuits exist (four in each route). Each OC-48 circuit
carries 2.5 Gbps of traffic in the network.
For OC-3 circuits, Figure 5 shows the economic trend of each solution as the number of circuits in the
network increases. As indicated in the chart, the DWDM solution has a higher initial cost than the two
SONET solutions. However, when the number of OC-3 circuits in the network reaches 52, or the traffic
volume reaches 108 x 0.15552 Gbps = 16.8 Gbps (each OC-3 carries 155.52 Mbps or 0.15552 Gbps of
information), DWDM becomes less expensive than the PMO. And when the traffic volume reaches 132 x
0.15552 Gbps = 20.5 Gbps, it becomes less expensive than the SONET overlay solution.
100.0
90.0
80.0
Normalized Cost
70.0
60.0
50.0
40.0
30.0
20.0
PMO
DWDM
SONET Overlay
10.0
0.0
4
20 36 52
68 84 100 116 132 148 164 180 196 212 228 244 260 276 292 308
Number of Circuits
Figure 5: SONET Versus DWDM for OC-3 Circuits
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100.0
90.0
80.0
Normalized Cost
70.0
60.0
50.0
40.0
30.0
20.0
PMO
DWDM
SONET Overlay
10.0
0.0
4
20 36 52
68 84 100 116 132 148 164 180 196 212 228 244 260 276 292 308
Number of Circuits
Figure 6: SONET Versus DWDM for OC-12 Circuits
100.0
90.0
80.0
Normalized Cost
70.0
60.0
50.0
40.0
30.0
20.0
PMO
DWDM
SONET Overlay
10.0
0.0
4
20
36
52
Number of Circuits
68
84
100
Figure 7: SONET Versus DWDM for OC-48 Circuits
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For OC-12 demands, the crossover point is at traffic volume of 68 x 0.622 Gbps = 42.3 Gbps for PMO and
132 x 0.622 = 82.1 Gbps for SONET overlay (Each OC-12 delivers 0.622 Gbps of information). In the OC-48
case (see Figure 7), DWDM at 24 x 2.5 Gbps = 60 Gbps becomes more competitive than the SONET PMO
and at 28 x 2.5 Gbps = 70 Gbps overtakes the SONET Overlay.
The demand for GigE transport in metro networks is growing at a very fast pace. We have modeled GigE in
our study with an eight-port GigE card that multiplexes eight GigE signals into a 10 Gbps wavelength on
the DWDM system. Figure 8 is a pictorial description of the comparison among the three scenarios for GigE
circuits.
100.0
90.0
80.0
Normalized Cost
70.0
60.0
50.0
40.0
30.0
20.0
PMO
DWDM
SONET Overlay
10.0
0.0
4
20
36
52
68
84
100
116
132
148
Number of Circuits
Figure 8: SONET Versus DWDM for GigE Circuits
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The crossover points for the GigE model are 32 x 1.25 Gbps = 40 Gbps for PMO and 52 x 1.25 Gbps = 65 Gbps
for SONET overlay.
Sufficient numbers of circuits are tested in each of the models so that the general trends of the economics
can be clearly seen in the displays. The crossover points are sensitive to price and unit interface density.
Improvements in one of these factors can shift the crossover point towards SONET or towards DWDM.
Although the density and relative pricing of the equipment may influence when the periodical hikes are
happening in the graphs, the overall crossing and separation of the curves is eventually inevitable as the
traffic volume escalates. However, a decision still needs to be made for a particular network with a forecast
of growth: SONET or DWDM? Table 1 is a summary of the crossover points for all four demand types we
have modeled in the study. Note that at any traffic volume that is larger than or equal to a crossover point,
the DWDM approach becomes more economic.
Circuit Type
OC-3
OC-12
OC-48
GigE
SONET PMO (Gbps) SONET Overlay (Gbps)
16.8
20.5
42.3
82.1
60.0
70.0
40.0
65.0
Table 1: Summary of Crossover Points in Gbps
In all cases, the SONET overlay tends to delay the time when the curves switch positions. At the crossover
points, the total numbers of OC-192 rings in the SONET overlay network are 4, 10, 8 and 8 for the OC-3,
OC-12, OC-48 and GigE models, respectively—suggesting that, when 4 to 10 OC-192 rings exist in this
example IOF SONET overlay network design, a DWDM solution should be considered. Please note that this
situation is ideal where only high speed (OC-3 and above) TDM and GigE service interfaces are involved,
and the traffic pattern is perfect to carry end-to-end wavelengths in the most efficient way. In situations
where low speed services, such as DS1 and DS3 are included, the crossover points will be delayed because
of the efficiency of SONET solutions in handling such services. While in other situations in which such data
services as FICON/ESCON are mandatory, DWDM would be more favorable than SONET. For more
complicated network and traffic distribution, Fujitsu is conducting research using more sophisticated
modeling tools and design skills, and additional papers will be published in the future.
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Sensitivity to Span Distance and Fiber Cost
SONET solutions in the model use overlaid OC-192 rings as the demands grow. Additional fibers are used
by each overlaid ring, which requires a lot of fibers (one pair per ring). We assume that the wideband
OC-192 optics have an 80 km budget without dispersion compensation or additional amplification. We
then examined the impact of the physical span distance, from 15 km, 20 km, 25 km to 30 km, on the
crossover points of the DWDM and the SONET overlay scenarios. The physical spans are between two
physically adjacent nodes regardless of whether they are terminating or pass-through (without terminating
any demands).
From Figure 1 and Figure 3 and previous discussions, we learned that SONET overlay has a longer ring
distance than the SONET PMO, although SONET reduces the number of network ports and therefore the
overall cost. In the four OC-48 termination example, two rings are required in both the PMO and the SONET
overlay scenarios. However, the total distance of a ring is only 60 km (4 x 15) in PMO while it is 105 km in
SONET overlay. The maximum link (between two terminating nodes) distance is 30 km for PMO and 45 km
for SONET overlay. We then applied a simple cost weight of $300 per single fiber mile in the models and
necessary cost weights to the dispersion and amplification efforts. When the physical span distance
increases, the fiber cost increases more drastically in SONET than DWDM because SONET uses a pair of
fibers for every ring carrying up to 10 Gbps traffic, while DWDM uses one ring to carry up to 40
wavelengths of 10 Gbps traffic per wavelength. See Figure 9 for the crossover points relative to span
distance.
90.0
OC-3
80.0
OC-12
Crossover Point in Gbps
70.0
OC-48
GigE
60.0
50.0
40.0
30.0
20.0
10.0
0.0
15 km
20 km
Span Distance
25 km
30 km
Figure 9: Crossover Point Sensitivity to Span Distance
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A major drop occurs at the 30 km span distance because that is where the total SONET link distance is over
80 km in SONET overlay and the cost of amplification and dispersion compensation kicks in. At this point
the number of OC-192 rings in SONET overlay is 2, 4, 4 and 4 for OC-3, OC-12, OC-48 and GigE, respectively.
The economics of DWDM is very much improved at this critical point. Note that DWDM does not reach a
point where additional equipment is required until the spans are well above 100 km. The additional
equipment at nodes for optical bypass enables even longer spans in the DWDM model.
A Point-to-Point SONET Scenario
An interesting alternative for the SONET scenarios establishes point-to-point connectivity with diversely
routed protection as shown in Figure 10. Working routes are shown with thicker lines.
15 k m
15 k m
15 k m
15 k m
15 k m
15 k m
15 k m
15 k m
Figure 10: A SONET Point-to-Point Scenario
A simple count of OC-192 line ports shows a total of 16 when each terminating site has eight OC-48s (four
OC-48s per route). This count is eight ports less than the SONET overlay. Table 2 gives the crossover points,
including this new SONET point-to-point case.
Circuit Type
OC-3
OC-12
OC-48
GigE
SONET PMO (Gbps)
16.8
42.3
60.0
40.0
SONET Overlay (Gbps) Point-to-Point (Gbps)
20.5
40.4
82.1
No
70.0
130.0
65.0
85.0
Table 2: Summary of Crossover Points in Gbps, Including SONET Point-to-Point
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The point-to-point solution seems to delay the crossover points significantly in all cases. Especially in the
OC-12 case, we could not find a crossover point after inputting 324 total OC-12 circuits (equivalent to 48
point-to-point OC-192 pipes), into the model.
While performing well in the short span settings depicted in Figure 10, the OC-192 pipes really traverse a
long distance from end-point-to-end-point, either for the working paths or for the protection paths. In the
distance sensitivity test, when each physical span increases to 21 km, the protection path of circuit Route 1
(refer to the route indices in Figure 1) and the working path of Route 3 are 84 km, exceeding the dispersion
and dB loss budgets (80 km). When the span distance increases to 27 km or higher, both the working and
protection paths of Routes 1 and 3 exhaust their reach. And, the protection paths of Routes 2 and 4 are
reaching 81 km. The following are the sensitivities of distance on the crossover points between the SONET
point-to-point and DWDM technologies. We observed a sharper drop of almost all crossover points than
the case of SONET overlay. Note that the OC-12 model does not have a crossover point in the number of
circuits we were able to input in the model. We assume it is at a very large number or infinity in the
sensitivity test in Figure 11.
200.0
OC-3
180.0
OC-12
Crossover Point in Gbps
160.0
OC-48
140.0
GigE
120.0
100.0
80.0
60.0
40.0
20.0
0.0
15 km
20 km
Span Distance
25 km
30 km
Figure 11: Crossover Point Sensitivity to Span Distance: DWDM versus SONET Point-to-Point
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Conclusions
As expected, the SONET scenarios have a low initial cost. When the traffic volume is low, a SONET
architecture is more economical than the DWDM architecture. Our modeling indicates that when designing
a SONET overlay network with OC-3, OC-12, OC-48 and Gigabit Ethernet demands and when the design
requires less than four to ten OC-192 rings, a SONET network is a better choice. As the traffic volume grows,
DWDM will eventually prevail and become the choice of the network technology. Timing of this crossover is
sensitive to such things as the span distances, pricing and interface density. The differences between
demand types are mainly caused by the design efficiency of the two technologies’ interface cards in terms
of density and cost. Our study also shows span distances usually trigger the extra requirement for
regenerators, amplifiers and DCMs in the routes. Long span distance tends to favor a DWDM architecture
because of the efficient use of fibers and better optical bypass capabilities at intermediate nodes.
Additionally, higher fiber cost (more than $300/fiber mile modeled in this study) and situations in which
fiber constraints are enforced will lead to more consideration for DWDM than SONET because DWDM saves
a tremendous amount of fiber in the network.
Different alternatives and their economic impact in designing the same network is an interesting study. We
investigated three different approaches in designing a SONET network. SONET overlay seems to always be
better than PMO. SONET point-to-point performs even better. These results may not apply in all situations;
however, the implication is that, in large network designs, the most optimized network may not necessarily
be one single architecture. One part of the network may adopt rings while another part implements pointto-point. Most often, the core part of the network will justify a DWDM architecture.
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12
Acronym
BLSR
DCM
DWDM
GigE
IOF
NE
NGADM
PMO
SONET
STS
TDM
VT
Descriptor
Bidirectional Line Switched Ring
Dispersion Compensation Module
Dense Wavelength Division Multiplexing
Gigabit Ethernet
Interoffice Facility
Network Element
Next-Generation Add/Drop Multiplexer
Present Mode of Operation
Synchronous Optical Network
Synchronous Transport Signal
Time Division Multiplexing
Virtual Tributary
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13