Building an edge for the future

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By Gady Rosenfeld

The shifting balance between circuit-switched and packet traffic has changed carriers’ requirements for traffic aggregation onto the metropolitan network. As the “core network” becomes more of an “IP core network,” dominated by rising demand for packet and Ethernet traffic, carriers need a converged architecture that provides the service control mechanisms to make sales of both circuit and packet services profitable.

WilTel Communications ( recently announced a move to just such a converged network approach. KDDI Corp. of Japan ( also has deployed a similar resilient packet ring (RPR)-based architecture throughout its entire nationwide infrastructure. Such deployments leverage a new class of optical transport switch. Although the industry refers to it by several labels-transport multiservice equipment, Ethernet transport node, or even lumped in with multiservice provisioning platforms (MSPPs)-this article will call it simply a packet-transport platform. Whatever the system’s label, the fundamental qualities of packet-transport platforms make them appropriate for building converged metro transport networks that support both circuit and packet services on one platform-at a price carriers can afford.

The transformation from circuit-switched to packetized services affects many different types of carriers. Wholesalers and carriers catering to the enterprise market are experiencing tremendous growth in demand for Ethernet connectivity services. The Metro Ethernet Forum (MEF) refers to these services as E-Line (point-to-point private-line connectivity based on Ethernet virtual circuit) and E-LAN (multipoint-to-multipoint Layer 2 VPN service offerings).

The majority of business for these carriers comes from traditional TDM traffic. However, the demand for Ethernet is forcing them to seek an aggregation network that efficiently handles the fast-growing packet traffic yet still accommodates circuit traffic as the largest part of their business.

In addition, many of today’s incumbent carriers continue to reel from the impact of customer demand for advanced triple-play services. At the same time, many carriers face vicious competition to deliver these bundled services rapidly and efficiently.Th 211953

Figure 1. The use of packet-transport platforms-here designated packet add/drop multiplexers, or PADMs-creates a converged architecture that combines the functions of existing network elements into a more streamlined network.

These carriers are realizing very quickly that to roll out triple-play services, they must first upgrade their metro infrastructure and deploy a packet-transport approach. KDDI, for example, is already far along with such an upgrade. The company’s converged packet-transport architecture enables it to use the same infrastructure to not only carry consumer traffic but also business Ethernet services. KDDI is also backhauling 3G wireless traffic handed off on TDM interfaces. The wireless market has strong potential for increasing TDM traffic on the aggregation network, strengthening the case for a packet-transport approach.

As another example of today’s trends, WilTel, a large wholesaler, enjoys a significant presence in most U.S. markets, selling connectivity services to other carriers, ISPs, and large Fortune 500 companies. The company has several points-of-presence (PoPs) in most major U.S. cities. To grow its business, WilTel launched retail offerings to enterprises, focusing on Ethernet services. Although most of the traffic crossing the aggregation network was dominated by circuit TDM, the new offerings created a significant upward trend in packet traffic-on both TDM and Ethernet interfaces.

WilTel concluded that its existing circuit-based platforms would be incapable of scaling to meet the changing demand for packet-based traffic. The company’s network consisted of a combination of WDM equipment, MSPPs for circuit traffic, and Layer 3 or Layer 2 Ethernet switches for Ethernet traffic. The main office network elements included a multiservice switching platform, edge router, 3:1 digital crossconnect system, ATM switch, and the core optical network. The carrier used a combination of routers and add/drop multiplexers to transport and aggregate packet and circuit traffic.

Realizing the need to transport increasing amounts of packet traffic while maintaining its revenue-generating circuit-switched traffic, WilTel decided to deploy a packet-transport infrastructure, particularly at the edge aggregation points, that would support all traffic types.

WilTel’s and KDDI’s requirements are not unique. With the rise in packet services and the increasing demand for Ethernet, service providers from the smallest independent local exchange carrier to the largest RBOC must seek a viable business model that accommodates the growth of packet services. At the same time, providers must continue to deliver TDM services with all the quality and reliability of traditional SONET/SDH.

As carriers upgrade the network edge, they must maintain or improve the existing levels of availability on the network: not just SONET/SDH-class resiliency and protection switching, but also manageability, performance monitoring, and operations, administration, and maintenance (OAM) testing ability. The bottom line is to develop the ability to handle Ethernet, circuit, and packet traffic over TDM interfaces-scalable to whatever proportions are required.

Carriers also need to be capable of supporting their “on-net” customers (those who connect directly to network PoPs) as well as “off-net” customers (those enterprise customers who require the rollout of Ethernet services). It all boils down to an ability to support efficient aggregation of packet traffic, even when it enters the network through TDM interfaces.

Transport equipment for these converged networks must provide carriers with the best of both worlds. They must be optimized for packet aggregation and multiplexing yet accommodate the full range of TDM services: T-1, T-3, OC-3, OC-12, OC-48, and beyond.

T-3, for example, remains an important service offering for carriers today that are extending their networks into the enterprise business market. For WilTel, much of the long-distance transport traffic enters the network through T-3 interfaces. These interfaces are still important to the carrier reaching enterprise customers where fiber is not present to provide Ethernet services. Carriers simply cannot afford to replace these circuits with new equipment to meet the increased demand for packet transport.Th 211954

Figure 2. Packet-transport platforms support both packet and circuit-switched traffic via the same system. They create an intelligent transport network that can accommodate a variety of traffic types simultaneously.

However, carriers that are intent on keeping their TDM infrastructure also want to see increased functionality. For instance, the ability to achieve new applications for Layer 2 interworking promises substantial cost savings. Much of the current business stems from customers who only require access to Layer 3 VPN services. This is typically provided across T-1 or N×T-1 lines coming to the customer router that simply maps IP traffic onto the circuits.

A better approach is to terminate those T1 lines wherever they hit the network, extract the IP traffic, and map it into an Ethernet connection. This allows carriers to efficiently multiplex packet traffic on those TDM interfaces at the aggregation point and hand off the traffic from a simple, inexpensive Ethernet interface onto an edge router at the network hub.

In short, traffic is packetized the moment it hits the network. Therefore, carriers can benefit from both efficient traffic aggregation and multiplexing as well as the ability to hand off traffic to a standard Ethernet interface. This benefit is only possible using a packet-transport platform that can terminate packet traffic even when it enters the network through a TDM interface.

Carriers can also use the same basic capability with interconnect services sold to ISPs. Most ISPs buy or lease OC-3/STS-3 circuits to interconnect their routers, typically running packet-over-SONET/SDH traffic that can be very bursty. The average port utilization is very low-in many cases, consisting of only Internet traffic. However, with a packet-transport platform, the data from those OC-3 circuits is terminated the moment it hits the network and transported like any other packet traffic: efficiently aggregated and handed off to an edge router at the hub.

The packet-transport approach also simplifies the typical hub site by eliminating expensive 3:1 crossconnects and enabling a distributed crossconnect function. The hub becomes more scalable and achieves higher levels of availability and reliability by consolidating all traffic into just two main network elements.

First, the edge router handles all types of packet traffic, Ethernet services, access to Layer 3 VPN services, and interconnection between routers via a universal, inexpensive Ethernet-based interface. Second, the long-distance optical network continues to transport all private-line or native circuit traffic across the network.

Both initial and operational cost savings are achieved by better bandwidth usage and the reduction of network elements, combined with the elimination of expensive channelized TDM ports at the label edge router. The services are also handed off through universal interfaces, which reduce operational costs through the ability to deliver any service, on any port, and at any rate.

There are alternatives to packet-transport platforms. Many carriers will opt to continue on their current course, using circuit-based platforms to handle more and more packet traffic. However, circuit platforms are simply not built to aggregate large amounts of packet traffic.

Another alternative is to build a completely new overlay network. However, this option presents significant issues: quality of service, restoration, circuit emulation, fiber availability, and management.

The best approach is a converged network that can handle all types of traffic: TDM, Ethernet, and packet traffic entering via TDM interfaces. A packet-transport platform enables all services and requires less equipment, fiber, management, provisioning time, and expense in terms of both initial and operating costs.

Gady Rosenfeld is vice president of marketing at Corrigent Systems (, headquartered in San Jose, CA. He can be contacted via e-mail at

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