There are a number of options for modernizing your transport network, including native TDM switching over the optical transport network (OTN), a complete overlay network with routers and DWDM, and finally migrating to packet optical using circuit emulation (CEM). Each option has value, but the value depends on your end goal.

In a previous post,A Requirements Checklist for the Next Generation Transport Network, we discuss how a new transport network modernization solution could affect an organization's business model, future growth opportunities, OpEx and CapEx targets, and integration into existing operational models, which includes staffing, skillsets, and OSSs and business support systems (BSSs). In this post, we'll explain the different technology paths to transport network modernization and how they each modernization path aligns with business goals.

The Best Fit for Your Organization

If you don't have extensive TDM connections and related infrastructure in your network and central office, retooling to provide an all IP next-generation transport network is simpler and more cost-effective. For those with extensive TDM services to support, a more gradual migration using a hybrid transport network that supports TDM, Ethernet, and IP/packet services can be the more prudent way to move forward.

Option 1: Native TDM Switching over OTN

With native TDM switching over the OTN, the OTN switch becomes a way to backhaul TDM traffic. T1, DS3, OC3, OC12, 1 Gigabit, and 10 Gigabit connections are multiplexed for transport over an optical data unit (ODU) connection. This solution lets you retain TDM interfaces while using OTN and low-speed interfaces for transport.

Drawbacks to this approach:If you want the solution to support Ethernet, there can be problems. The low-density ASICs in a SONET platform typically allow for only 40 to 60 gigabytes of low-order/high-order (LO/HO) switching. Capacity utilization on each system is limited to a maximum of approximately 40 percent. It's not really a viable solution for packet traffic, which requires a more expansive environment for traffic diversity and overall traffic and bandwidth growth. The SONET cross-connect can only be connected to a maximum of 16 destinations, limiting scale. And OpEx is high given the complexity of the SONET/SDH over OTN grooming solution. OTN is not optimized for LO/HO grooming of containers within the SONET payload because of hardware capacity limitations. As a whole, this solution is very inefficient because it doesn't use the available capacity in your network and it also limits scale. You can use it to support your TDM needs, but it's at the expense of adding Ethernet traffic.

Option 2: Overlay with Routers and DWDM

This Ethernet-only network requires that you change out your optical carrier technology (for example, OC3 or OC12) to Ethernet. You'll get rid of TDM interfaces and a lot of SONET/SDH gear and replacing it with Ethernet technology. It is a comprehensive changeover instead of a gradual migration to a hybrid network.

Drawbacks to this approach:It would force your customers to buy new CPE devices and change their service contracts. The greatest risk is for customer churn; will your customers agree to incur the expense of new equipment? Are they willing adjust their operations environments to support the new interface? Or will they put their contract out to bid and possible find another provider? Your TDM services would be converted to VoIP. Management will be different. You'll want to consider how it would support a 99.999 percent SLA for uptime and 50ms protection for voice.

This option does not support TDM, Ethernet, and IP traffic. It will require expertise both within your organization and among your customers. It could be expensive and take time to implement. It's a risky move as it directly affects your customers and may result in increased customer churn rates.

Option 3: Packet Optical with Circuit Emulation (CEM)

Cisco's high-density circuit emulation technology allows TDM services to be migrated across an asynchronous IP/Multiprotocol Label Switching (MPLS) network with no errors and a constant delay. It provides the ability to terminate TDM traffic over SONET/SDH as an interface, continuing fault propagation between SONET/SDH and the IP/MPLS network using pseudowires running over dynamic label-switched paths (LSPs). LSPs are paths through MPLS networks set up by a signaling protocol.

The CEM solution is supported by multiple industry standards and deployed through control planes for both SONET/SDH and IP/MPLS. The CEM solution provides protection; operations, administration, and management (OAM); simplicity; and manageability on a par with those provided by SONET/SDH. It is predictable and deterministic, with sub-50ms resiliency. IP/MPLS packet services run natively over the network.

TDM traffic is restricted to the edges, and IP/MPLS is added as the major transport network. The scalable MPLS infrastructure is overlaid on top of the 100GB ROADM and reconfigurable OTN DWDM infrastructure. With a SONET/SDH look and feel, one-to-one path protection, and an A to Z provisioning system, the CEM solution lets you trim down your legacy transport network as part of a next-generation migration. DCSs can be eliminated right away, dramatically reducing costs. SONET/SDH ADMs may be eliminated now or later. With Gigabit Ethernet over SONET.SDH, Gigabit Ethernet routers may also be taken away or reduced. You'll be able to use as little as 1/10 the space you're using now in your central office and get rid of many electrical cables running between floors. No need to cannibalize other systems to keep other legacy gear running or buy expensive replacement parts, if they are even available.

Advantages of this approach:This solution is ideal for organizations that want their TDM services to be integrated along with IP services across SONET/SDH transport networks. You get SONET/SDH-like features, such as OAM and manageability, and IP/MPLS and other technologies help eliminate errors and delay. Your next-generation transport network becomes predictable and deterministic, with sub-50ms resiliency. The solution is supported by industry standards. And it's invisible to end customers, with no new CPE required. Packet optical with CEM is a great way to modernize, scale, trim OpEx and CapEx, and future-proof your transport network. It lets you evolve towards Metro Ethernet, Layer 3 VPN, and full IP transport whenever you're ready.

A Roadmap for Transport Network Modernization

In our recently published E-Book,A Roadmap for Transport Network Modernization, we cover the next-generation transport network solution options in more detail. In addition, we look at both the business and technology requirements of transport networks today and into the next decade. We conclude the E-Book with a look at the Cisco Transport Modernization Solution. We'll show you how our next-generation approach, using Cisco's high-density circuit-emulation (CEM) technology, provides you with a cost-effective network modernization path, while still supporting ongoing TDM services requirements.

If you are ready to modernize your network, find out how with A Roadmap for Transport Network Modernization.Download your copy today.

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