Operators are struggling to manage their aging SONET/SDH infrastructure, much of which has exceeded its intended lifespan and become a serious operational strain. For years, oper-ators had few architectural options for network modernization. Fortunately, circuit emulation (CEM) has emerged as a means of circuit-to-packet migration, though many operators may not yet be aware of its scalability or its benefits.
This white paper discusses the modernization challenge and presents CEM over MPLS-based networks as the most promising solution for operators and their customers. Underscoring the point that CEM is ready today, we provide a case study of Verizon's circuit-to-packet modernization using CEM with MPLS, which is being deployed today. Verizon's use case has wide applicability to many other operators.
SONET/SDH equipment has had a long and productive lifespan in telecom networks, but the technology and supporting equipment is well past its prime and in need of replacement. Below, we list the current and primary issues with the installed infrastructure:
As shown in Figure 1, annual global SONET/SDH equipment spending dropped from a high of $6 billion down to $1 billion by 2015. But this statistic doesn't account for the massive installed base that remains in operation today. Operators have invested $30 billion on SONET/SDH networks since 2008 alone. Nearly all of this equipment remains in operation today, at least for now.
Figure 1: Global SONET/SDH Equipment Spending, 2007-2016
Source: Heavy Reading, 2017
Operators around the globe face a big challenge: How do they move away from the aging infrastructure in their own networks without losing customers? How do they move from a fixed grid to a more flexible grid? How do they move from 10G to coherent?
In the absence of a good strategy, most operators have simply delayed the decision. How-ever, the "TDM problem" has now become so severe that doing nothing is no longer an op-tion. Verizon, profiled in this paper, is an example of an innovative operator that is leading the market in transformation with a new high-capacity and scalable packet-optical network.
Fortunately, a viable means of network modernization has emerged in the form of circuit emulation (CEM). CEM is designed to closely match the key service characteristics of a circuit switching technology over a packet network. Critical circuit characteristics include operations, administration and management (OA&M) functions; predictable and deterministic transmis-sion; and sub-50 ms resiliency in failures, achievable using packet network protection tech-nology such as MPLS FRR (fast re-route).
CEM itself is not new. The first IETF requests for comment (RFCs) for CEM over packet net-works appeared in 2005, and CEM has evolved and matured since then. Today, there are three major IETF-defined CEM standards addressing different data rates, packet-switched network types (Ethernet, MPLS, IP) and physical media (i.e., copper and fiber).
While CEM has been around for years, scalability and density were required to enable the transformation described in this white paper. Additionally, implementations of CEP defined in RFC 4842 were missing, meaning that providers could emulate T1/E1 and T3/E3 rates (low data rates), but not at the STS or VT path level required for SONET/SDH services. These advances, combined with MPLS pseudowire transport, have made CEM ready for mass acceptance and adoption.
CEM provides a "bookended" architecture for TDM migration, in which conventional SONET/ SDH equipment transmits and receives TDM traffic at the two endpoints, but the entire net-work between those endpoints is replaced. Figure 2 illustrates a TDM-to-packet migration scenario using CEM.
Figure 2: TDM-to-Packet Migration With CEM
Source: Cisco and Heavy Reading, 2018
In the figure, a CEM-capable Layer 2/3 switch/router receives a private line transmission at the endpoint, encapsulates the data using CEM, and transmits the data as packets through an Ethernet, MPLS or IP network. Another CEM-capable Layer 2/3 switch/router at the receiving endpoint removes the CEM encapsulation and hands off the private line data as TDM traffic to the receiving-end multiplexer.
CEM provides several key benefits that make it appealing for operators that need to retire their legacy SONET/SDH and digital cross-connect equipment. Below, we provide an over-view of the main operator benefits:
Since the 2000s, Verizon has been a leading innovator in the advancement and adoption of packet-optical technologies. Its network architectures have been closely followed by all the major operators around the world. This is also the case with CEM adoption, in which Verizon is pioneering the way.
Verizon's network has hundreds of thousands of circuits, ranging from DS-0 rates to OC-192. This includes 74 different platforms and more than 150,000 TDM-based elements in its central offices. Operating one of the world's largest networks, Verizon had been struggling to determine exactly how to modernize its network. Figure 3 illustrates Verizon's legacy SONET-based transport network.
Figure 3: Verizon's Legacy Transport Network
Source: Verizon, 2018
Verizon began to address the problem when it launched its metro network RFP in 2014. CEM was high on the solution list for that RFP, but a lack of OC-192 support created a problem. Emulating some of the TDM network while maintaining TDM infrastructure for its 10 Gbit/s circuits meant that two separate networks would still be required.
Further, Verizon's network has a lot of channelized OC-192 interfaces, so 10 Gbit/s was a critical piece of the TDM migration strategy. "The standardization of OC-192 rate CEM, com-bined with the availability of products supporting OC-192, removed the 10 Gbit/s barrier and was key to making CEM a viable option," says Verizon's Director of Optical Transport Network Architecture, Design and Planning, Glenn Wellbrock.
Additionally, two other CEM advances were critical for the operator:
Using CEM, Verizon has built a graceful migration architecture that retains the customer's endpoint as long as required, but eliminates the intermediate digital cross-connects between those two endpoints. As illustrated in Figure 2, once TDM traffic is encapsulated at the CEM packet switch, it becomes part of the packet network. Verizon selected Cisco's NCS 4200 system as one of its CEM packet switches. The packet network is Verizon's MPLS core.
Each time Verizon rolls a TDM customer to the packet network, intermediate SONET multi-plexers and digital cross-connect ports are idled throughout the network. When a multi-plexer or digital cross-connect no longer carries customer traffic, it can be removed from the network with no customer impact – freeing up the associated space, power and maintenance activities.
According to Wellbrock, digital cross-connect and SONET removal yields the biggest immedi-ate benefits. Digital cross-connects are among the oldest systems in the network (sometimes 20 years old or more), and this older technology occupies massive central office footprints that can immediately be put to better use. For example, digital cross-connect replacement at a single site can remove many rows of equipment that can be substituted with just a few racks, with space savings of up to 90 percent.
Verizon's CEM over MPLS architecture allows the operator to maintain the same SLAs as the legacy circuit-switched network that is being replaced. When Verizon converts a customer to the packet network, it makes no changes to the contract and no changes to customer pricing. This yields two significant benefits:
Verizon is "seeding" the network with MPLS packet switches in advance of moving circuits. Specifically, Verizon is using the Cisco NCS 4200 today in edge offices as well as core cen-tral offices. The NCS 2000 and NCS 4000 platforms are also deployed.
Verizon is beginning the rollovers with a targeted plan that identifies high-impact sites through the use of visualization tools and a set of machine learning algorithms. The operator recognizes that the circuit-to-CEM rollover process will take several years.
Figure 4 shows Verizon's next-generation transport network using CEM and an IP/MPLS core.
Figure 4: Verizon's Next-Generation iEN Transport Network
Source: Verizon, 2018
The aging infrastructure problem has reached such a critical point that operators can no longer ignore it. Today they face a big challenge: how to migrate away from legacy multi-plexers and digital cross-connects without losing customers that require the private line functionality. CEM has matured to provide the most efficient solution to the problem, ena-bling a measured migration for operators and their customers while retaining reliability (including 50 ms resiliency) and SONET-like OA&M. With CEM, operators can reap the oper-ational benefits of a modern packet-switched network while retaining existing private line service revenue.
A leading innovator in packet-optical transport, Verizon is a first mover in TDM-to-IP/MPLS migration with CEM and has already begun seeding its metro network with CEM equipment, with a multi-year plan to shut down the legacy network. Significantly, the TDM-to-IP approach championed by Verizon has wide applicability to many operators around the world struggling with similar problems. Verizon is committed to raising the visibility of CEM and sharing its learnings with other operators along the way.
Topic: NCS 2000 | NCS 4200 | NCS 4000 | Optical | MPLS | FlexLSP | EPNM | NSO | WAE