The Exabyte Era is upon us. Cisco research indicates that the total volume of worldwide IP traffic is doubling every two years. By 2011, the world's IP networks will transport 29 exabytes of traffic - equivalent to 144 times the total amount of printed data that exists today - each month. This growth will be driven not just by an expanding user base, but by the emergence of a new generation of complex, highly personalized, bandwidth-intensive media applications.
The only available technology to support an adequate amount of bandwidth is DWDM. Traditional 10-Gbps transmission systems cannot handle these traffic volumes and require bit-rate upgrades. The migration to 40 Gbps and 100 Gbps has become inevitable. This paper describes how network operators can smoothly upgrade to 40-Gbps DWDM transport systems with highly flexible and robust solutions based on the Cisco® ONS 15454 Multiservice Transport Platform (MSTP).
The traffic carried on core and metro DWDM networks is growing exponentially, while operators' revenues are failing to keep pace. Cisco ONS 15454 40-Gbps solutions can dramatically lower the cost to carry this traffic, helping operators to maintain and even increase profitability.
Internet usage is still skyrocketing, primarily due to demand for next-generation services such as quadruple play, video distribution, IPTV, and an array of high-bandwidth services. These services are creating bottlenecks in DWDM networks that require a technology leap to support the surge in traffic. Upgrading from 10 Gbps to 40 Gbps solves this problem, quadrupling the bandwidth that can be transported over existing fiber plants.
In addition, Cisco ONS 15454 50-GHz "touchless" reconfigurable optical add-drop multiplexer (ROADM) solutions provide a way to cope with unpredictable network growth and reconfiguration, helping reduce operational expenses. Fully reconfigurable 80-channel, 50-GHz touchless networks that are capable of supporting 40-Gbps and 100-Gbps wavelengths are the cornerstone of next-generation optical networks.
Network operators are considering migrating their DWDM system to 40-Gbps technology for several reasons:
• 40-Gbps router interconnect for business services and research networks
The exponential growth in Internet services along with research network application development are forcing network operators to scale their high-end router interfaces up to 40 Gbps and soon to 100 Gbps. These interfaces were initially deployed for intra-POP connections, but operators now need them to help transport high-bandwidth services across a transport domain.
• 40-Gbps links in national carrier backbone networks
The enormous traffic increase at the edge creates an avalanche effect in DWDM backbone networks, where 800 Gbps of maximum capacity (80 channels x 10 Gbps) is not enough. A flexible and high-performance 80-channel ROADM network, based on 40-Gbps technology, is now a critical requirement for any new greenfield deployed backbone network.
• 40 Gbps as a way to help relieve fiber exhaust in high-traffic regional and long-haul networks
Many network operators have a wide installation of 10-Gbps systems that are not at full capacity so they cannot justify a new overbuild of their network based on bandwidth forecasts. 40-Gbps transmissions can solve this problem, allowing operators to increase the capacity of existing 10-Gbps systems by adding 40-Gbps wavelengths on the free channels.
Network operators are facing the dilemma of how to support a bandwidth increase in their transmission network. 40 Gbps seems a logical path but there are several deployment issues to consider:
• What is the impact of the 40-Gbps wavelengths over my existing infrastructure?
There are different 40-Gbps transmission technologies that may have an impact on existing 10-Gbps wavelengths. Some modulation techniques in fact would require the creation of a "guard band" between 40-Gbps channels and 10-Gbps channels. This needs to be evaluated in the planning phase of the network and may have route-constraint impact in optical mesh networks.
• Do I need to change the installed hardware?
This is a key criterion to defining the 40-Gbps upgrade path. Operators need to understand if the 40-Gbps units would require new hardware chassis, separate controller units, or an upgrade to the power system of the installed bay. Moreover they need to evaluate if there are any limitations on where the new cards can be placed in existing chassis, as this could create operational nightmares. In addition, the fundamental transmission aspects need to be carefully evaluated to determine the impact of 40-Gbps wavelengths onto existing DWDM systems: Do DCU maps need to be changed? Do you need to add more optical amplifiers?
• Or do I need to deploy a complete, 40-Gbps dedicated network?
If most of the answers above are positive and the guard band cannot be tolerated in order to achieve acceptable network efficiency, you can consider a complete network overbuild, which requires significant investment and longer ROI.
• What are the limitations of 40-Gbps transmission? Do I need to care about my fiber plant?
40-Gbps transmission is intrinsically more sensitive to fiber characteristics. One of the main parameters that need to be evaluated is Polarization Mode Dispersion (PMD), which can limit the application distance of a 40-Gbps signal if not properly counter-measured. High PMD is generally associated with old fiber plant, typically deployed before 1999, a time of lower-quality fiber manufacturing processes. Or it can be associated with fibers subjected to temperature fluctuations, such as aerial fibers.
The Cisco ONS 15454 MSTP supports scalable, fully reconfigurable, high-capacity networks. The platform is flexible, scaling from optical access networks to metro-regional and up to ultra-long-haul (ULH) backbones.
The Cisco ONS 15454 MSTP scales both in terms of ROADM flexibility and 40-Gbps solutions. Cisco's ROADM portfolio provides multiple solutions and architectures that are optimized for various network applications: from low-channel-count, 100-GHz ROADMs up to high-end 50-GHz touchless networks.
Similarly, Cisco provides three different 40-Gbps options, targeting different network applications: upgrade of traditional 10-Gbps to 40-Gbps networks, access and metro-regional 40-Gbps networks, and high-end 40-Gbps ULH backbone networks. The following high-performance plug-in modules for the Cisco ONS 15454 MSTP support all three options:
• 4 x 10-Gbps full C-band tunable muxponder card, capable of multiplexing any mix of 10-Gbps and
• 40-Gbps transponder card supporting OC-768, STM-256, OTU3, and 40 Gigabit Ethernet constant bit rate (CBR) services
Cisco ONS 15454 MSTP 40-Gbps solutions support existing DWDM systems without any need for additional equipment (such as a DCU). In addition, the MSTP's full-band tunable 40-Gbps cards have been designed to meet the following critical requirements:
• 40-Gbps signals will work on already deployed 10-Gbps systems.
– 40-Gbps technology will support at least comparable Chromatic Dispersion (CD) robustness, Polarization Mode Dispersion (PMD) robustness, and optical signal-to-noise ratio (OSNR) with existing 10-Gbps transponders.
Cisco solutions support the same CD robustness as industry-standard 10-Gbps systems with comparable PMD and OSNR specifications, helping ensure the smoothest possible upgrade of existing 10-Gbps DWDM systems. The high-end option actually outperforms any available 10-Gbps technology.
– 40-Gbps technology will work on 50-GHz systems as well as current 10-Gbps wavelengths (with comparable filtering penalty).
Cisco solutions, thanks to the efficient spectral density of the modulation technology, offer the capability to cope with 50-GHz network without any impairment caused by the filter cascading effect.
• 40-Gbps units will fit mechanically and thermally with existing installed shelf with no impact on existing units, allowing full compatibility and no restriction on where the units can be placed.
Cisco solutions offer the possibility to install 40-Gbps units in every free space in an already installed Cisco ONS 15454 shelf, offering strong investment protection and avoiding the installation of new expensive and space-consuming chassis.
• 40-Gbps units will support the widest variety of signals without any impairment and with full compliancy to any international standard.
Cisco solutions offers the widest possible flexibility in terms of supported services:
– STM-64/OC-192/10GE LAN PHY/10GE WAN PHY/OTU-2/10-Gbps Fibre Channel and 8-Gbps Fibre Channel over the 40-Gbps muxponder card
– STM-256/OC-768/40GE CBR over the 40-Gbps transponder card
Cisco solutions represent the smart way to add 40 Gbps to network operators' DWDM systems.
They are also fully integrated in the Cisco ONS 15454 MSTP management system (Cisco Transport Manager and Cisco Transport Controller) as well as in the Network Design Planning Tool (Cisco Transport Planner).
40-Gbps Transmission Technology
Physics tells us that 40-Gbps signals require four times better OSNR than an equivalent modulated 10-Gbps signal. This is one of the main impairments that technology had to solve before offering an efficient 40-Gbps solution.
In addition, assuming use of the NRZ modulation format, Chromatic Dispersion (CD) robustness decreases by a factor of square the bit rate increase (4^2 = 16).
On top of this, additional impairments hinder 40-Gbps transmission. An example is Polarization Mode Dispersion (PMD), which is a consequence of the fiber core not being perfectly symmetrical. As a result, the light traveling along the fiber has a different speed on the two components of the electrical field aligned along the X and Y axis. This leads to a pulse broadening effect that is proportional to the distance traveled. The core non-uniformity is not constant along the fiber, which requires a statistical approach to PMD evaluation: It has to be accounted for mean value rather than instantaneous value. PMD can therefore be expressed by a coefficient multiplied by the square root of the total fiber length. It is measured in
Figure 1. Polarization Mode Dispersion Along an Optical Fiber
PMD is function of bit rate; the greater the bit rate, the greater the dependence on PMD.
All these impairments have been overcome with the introduction of sophisticated modulation schemes, as shown in Table 1.
Table 1. Modulation Format Comparison
Required OSNR B2B (dB)
50 GHz Compatible
Chromatic Dispersion Robustness with ½ dB of OSNR margin (+/- ps/nm)
PMD Robustness with ½ dB of OSNR margin (ps)
Only the last two modulation formats in Table 1 show a behavior either comparable with or exceeding 10-Gbps NRZ. These are the two formats chosen by Cisco for its 40-Gbps units' trunk interfaces. They are optimized for specific network applications.
Differential Quadrature Phase Shift Keying (DQPSK) provides a tradeoff between performance and complexity and it is the absolutely best modulation format to cope with existing 10-Gbps systems. In fact having comparable OSNR, CD, and PMD robustness allows the smoothest possible upgrade of the existing system.
Polarization Multiplexed Quadrature Phase Shift Keying, "PolMux" for short (PM-QPSK), provides the best absolute performance, but it is sensitive to two factors:
• PolMux signals suffer from the presence of other 10-Gbps signals, because the NRZ signals cause a cross-phase effect over the phase-sensitive PM-QPSK signal. This forces the creation of a so-called "guard band" between 10-Gbps and 40-Gbps signals in hybrid 10-Gbps and 40-Gbps links.
• PolMux signals work better if no Dispersion Compensation Unit (DCU) is installed along the optical line. As a vast majority of existing 10-Gbps systems have a DCU inline, this adds additional penalty to PM-QPSK signals.
Consider the following when selecting a Cisco 40-Gbps solution:
• RZ-DQPSK units (Solution 1): This does not suffer from the previously mentioned impairments; it is the ideal solution for upgrading existing 10-Gbps channels. Suitable for the upgrade of 10-Gbps installed networks with inline DCU. No guard band is required between older 10-Gbps channels and new 40-Gbps channels. Compatible with 50-GHz, 80-channel ROADM networks.
• High-end PM-QPSK units (Solution 2) are suitable for brand-new, high-speed, 50-GHz, 80-channel ROADM networks, optimized for 40 Gbps (and future 100 Gbps) only channels to maximize network performance and scalability. This is the best option to transport high-capacity traffic on bad fiber, with very high PMD.
• Low-end PM-QPSK units (Solution 3): This is a lower-performance, low-cost version of Solution 2. The tradeoff in performance makes it an interesting 40-Gbps option suitable for cost-sensitive metro regional applications. Compatible with 50-GHz, 80-channel ROADM networks.
Table 2 summarizes the performance of the three Cisco ONS 15454 MSTP 40-Gbps solutions.
Table 2. Table 2: Cisco ONS 15454 MSTP 40-Gbps Performances
> 1000 km
+/- 750 ps/nm
> 2000 km
+/- 34,000 ps/nm
~ 1000 km
+/- 3000 ps/nm
How does QPSK Modulation Work?
QPSK modulation is a phase modulation: the information is transported by the phase of the signal, not by the intensity like 10-Gbps NRZ signals.
Four signal "points" are used and, in this way, each "phase" or "signal point" is associated with two bits as shown in Figure 2:
Figure 2. Four-Level Phase Modulation
The advantages of QPSK modulation are the following:
• As the baud rate is 20 Gbps, the OSNR and PMD robustness is greatly enhanced.
• Differential mode (DQPSK) is extremely helpful to heavily reduce cross-phase modulation noise generated by 10-Gbps NRZ channels, allowing adjacency among 40-Gbps and 10-Gbps signals without impairments.
PM-(D)QPSK can carry two DQPSK modulated signals on two orthogonal polarizations, doubling the baud rate of DQPSK and multiplying by four the baud rate of NRZ Amplitude Modulation. This makes the transmitter and receiver design more complex than the DQPSK trunk, but enormously increases its performance.
All three 40-Gbps solutions are 50-GHz stable and can be managed by any MSTP ROADM node.
Traffic growth is now forcing the vast majority of network operators to enhance their optical transport systems with higher-bit-rate transmission and fully flexible ROADM nodes.
Cisco 40-Gbps solutions fully meet the requirement to migrate DWDM transmission from traditional 10-Gbps systems to next-generation, 40-Gbps DWDM transport systems, multiplying by a factor of four the capacity of optical networks. The complete compatibility and integration with fully reconfigurable 50-GHz ROADM nodes, supporting touchless reconfiguration, allows building DWDM infrastructures that are hardware-ready to support WDM switched optical network solutions that will interoperate with the MPLS IP and OTN cores. Cisco is offering the smart way to build fully flexible, 40-Gbps networks.