Converge IP and DWDM Layers in the Core Network

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Converge IP and DWDM Layers in the Core Network

The Cisco® IP over DWDM (IPoDWDM) solution for IP Next-Generation Networks (NGNs) enables the convergence of service providers’ IP and DWDM core networks, increasing service flexibility and reliability while lowering operating expenses (OpEx) and capital expenditures (CapEx). The Cisco IPoDWDM solution efficiently manages traffic growth through the Cisco CRS-1 Carrier Routing System and the Cisco ONS 15454 Multiservice Transport Platform (MSTP).

Introduction
To meet the demands for converged packet infrastructures, service providers are increasingly moving their networks toward an IP NGN-based architecture. This shift is driven by the need to reduce OpEx and CapEx while offering new revenue-generating services over a single, converged infrastructure. IP NGNs can enable convergence at the application, service, and network layers. This paper focuses on the network layer and specifically on the convergence of two important technologies within service providers’ core networks: IP and DWDM.

Core Network Infrastructure Evolution
One recent core network trend is the consolidation of multiple Layer 2/3 networks into a single IP/Multiprotocol Label Switching (IP/MPLS) infrastructure. In spite of this Layer 2/3 convergence, however, the underlying transport layer (Layer 1) of many service provider core networks has continued to use SONET/SDH, an infrastructure that was introduced in the early 1990s to support traditional time-division multiplexing (TDM)-based data and voice services. This SONET/SDH infrastructure was implemented to support three critical functions: grooming; protection and restoration; and thorough operational support (such as alarming and performance monitoring).
When introduced, SONET/SDH allowed the efficient multiplexing of lower-speed TDM circuits such as T1/E1 and T3/E3 to higher-speed OC-3 and OC-12 trunks for transport across service providers’ core networks. Because of their synchronous nature, the trunk connections could be switched independently when needed without the need for complex demultiplexing as demanded by TDM-based data and voice services. The infrastructure also allowed the growth to faster aggregate links such as OC-48 and higher while also supporting the emerging IP networks of the day. This "grooming" function allowed the bandwidth efficiency and automation that service providers needed to support multiple switched TDM services in the core of their networks by using SONET/SDH add/drop multiplexers (ADMs) and cross-connect equipment. The SONET/SDH infrastructure, which typically uses a ring-based topology within the core, also allowed for protection and fast restoration (50 ms) during a failure on one part of the ring to maximize the availability of the overall network. With the advent of SONET/SDH standardization, a separate, standardized, message-based channel1 was used for alarms, control, monitoring, and administration of the links from a centralized location.
In the latter part of the 1990s, DWDM emerged as a way to significantly increase the efficiency of the installed fiber plant by allowing transmission of multiple wavelengths over a single physical fiber. This function introduced another level of multiplexing and demultiplexing at the optical level to support greatly increased bandwidth at the core of the network, which followed the dramatic rise of IP-based networks fueled by the explosion of the Web. The SONET/SDH layer, which now handled increasing amounts of IP traffic, was mapped into wavelengths at the DWDM transport layer to be carried across the core long-haul2 networks spanning regions and countries in many cases. This has remained largely the case in many service provider networks globally today.

The IP Explosion
The volumes of IP traffic on these core networks have, however, continued to increase steadily to the point where the primary use of these core long-haul networks today is to carry massive amounts of transient IP traffic, significantly outpacing the traffic volumes of traditional voice and data services. Over the next 5 years alone, global monthly IP traffic is expected to rise to 26 exabytes,3 accelerated by the application convergence of all video, voice, and data traffic to IP, resulting in a compound annual growth rate (CAGR) in excess of 56 percent globally. The convergence of traditional applications such as broadcast television, video on demand, and voice to new distribution models over IP as well as the explosion of new applications such as music and video podcasting and peer-to-peer (P2P) file sharing will only continue to fuel this tremendous growth of core IP traffic.

Core Network Infrastructure Challenges
Despite the trend toward IP convergence, multiple equipment layers to support core long-haul networks continue to exist, creating OpEx and CapEx concerns for service providers as well as the challenges of profitability and return on investment. Furthermore, as customers demand increasingly stringent service-level agreements (SLAs), service providers must maintain higher levels of reliability while still having the flexibility or "speed to service" to accommodate change based on service demands or traffic growth characteristics within the network core. To meet these requirements, service providers must consolidate their core networks and move toward more efficient ways to handle the increased IP traffic loads - yet at the same time they are confronted with problems at multiple levels to achieve this objective.

Multiple Transport Layer Elements
Some network inefficiencies result from the way core transport networks are built out today to support the IP layer over the SONET/SDH layer, supported by an underlying DWDM infrastructure. Consider the paths of two types of traffic entering and exiting a typical service provider point of presence (POP). The first scenario is IP traffic that needs a Layer 3 lookup at the POP and therefore is riding a wavelength that will terminate on a router. The second is called "pass-through" (or transient) traffic, which stays in the transport domain and bypasses the router to travel on to an adjacent POP in the service provider’s core network.

Router-Terminated Traffic
The IP traffic comes into the POP today typically through 10-Gbps SONET/SDH OC-192/STM-64 circuits, which are composed of colored wavelengths multiplexed through DWDM on to a physical fiber. This fiber is fed into a DWDM demultiplexer, which splits out the individual colored wavelengths. These individual wavelengths that are to be terminated on the router are then fed into transponders, which convert them from optical (colored) to electrical and then to a standard short-reach wavelength ("grey light"). This optical-to-electrical-to-optical (OEO) conversion is used because historically short-reach optics have been used for connectivity inside the POP environment. The grey light is then typically fed into a short-reach interface on a SONET/SDH cross-connect,4 which recovers the SONET/SDH clocking, performs any grooming necessary, checks for errors, and monitors for loss of signal (LOS) so that it can perform SONET/SDH-level restoration if needed. However, in most cases today, no grooming is actually needed because the full 10 Gbps is being connected to the router (rather than 2.5 Gbps or lower speed links in the past). Therefore, from a connectivity perspective, the cross-connect is serving essentially as a patch panel. The SONET/SDH cross-connect then feeds the 10 Gbps to the router, which performs performance monitoring at Layer 1 through Layer 3, monitors for LOS so it can perform MPLS Fast Reroute (FRR) restoration, and performs a Layer 3 and above lookup to route the packet to its destination. On the aggregation side the core router is typically aggregating multiple lower-speed links and grooming the IP traffic into well-used 10-Gbps links to present back into the core transport network.

Pass-Through Traffic
As traffic patterns in the core have become more distributed, the amount of traffic passing through a given POP purely at the transport layer (as opposed to terminating on a IP router) has tended to increase, and can sometimes be as high as 70 to 80 percent of the overall traffic that the POP handles. In this case the incoming DWDM link goes through a similar method of interconnections through the DWDM demultiplexer and transponders to the SONET/SDH cross-connect through short-reach optics. It checks for errors and monitors for LOS so that it can perform SONET/SDH restoration. Again the grooming function that would have occurred here previously is no longer required because typically full 10-Gbps links are being passed through the POP. Hence the cross-connect is again serving as a patch panel from a connectivity perspective. A similar process of interconnections occurs for outgoing traffic from the POP.
These OEO conversions and the associated electrical processing result in an additional cost in terms of space, because many racks of shelves may be required in a service provider POP, as well as additional power and cooling that is necessary because of the active electronics components that they contain. Furthermore, in this core network scenario the SONET/SDH functions are redundant because of the capabilities that have been integrated into the router.

• Grooming: Because most traffic has moved to IP, the router now performs the grooming function by aggregating IP traffic and presenting it to the core transport layer within well-used 10-Gbps links.

• Operational support: The router and its associated interfaces can measure errors at Layers 1 through 3, collect performance statistics, generate appropriate alarms, etc.

• Protection and restoration: Using MPLS FRR, the router can provide 50-ms protection or better and do so much more efficiently than the traditional SONET/SDH protection schemes (such as BLSR5), which waste up to 50 percent of the bandwidth for protection purposes.
For these reasons, service providers have already started using manual patching in place of the cross-connect to save costs.