ITEXPO begins in:   New Coverage :  Asterisk  |  Fax Software  |  SIP Phones  |  Small Cells

Feature Article
March 2001


TDM-To-IP Evolution In The Metro


[Go right to Offload Where It Counts]

Few will argue the notion that the "network of the future" will be driven by packet-based Internet Protocol (IP) services. It is common knowledge that, somewhere in 1999, packet traffic surpassed that of voice in our nation�s backbone networks. This phenomenon was facilitated primarily by the huge investments in dense wavelength division multiplexing (DWDM) technologies made by long-haul carriers in the past five years. Less clear, however, is the fact that even though packet traffic dominates that of TDM traffic, plain old TDM services still constitute 90 percent of the approximately $200B annual North American service market.

Carriers thus face a quandary: How to leverage and protect the TDM-based networks that currently comprise a majority of their high-margin services, while migrating the network towards a future of IP? Nowhere is this challenge more evident than at the edge of the metropolitan access and transport networks, which act as on-ramps between the user and the network core.

A service provider's access network is necessarily complex, and resists simplification. Generally, efforts to simplify the access network only generate greater complexity. Not only does the equipment chosen for any rehabilitation effort evolve during the process, acquisitions and mergers cause even more problems. For instance a service provider based on standard time-division multiplexing technologies can acquire a competitor that has built a network based on ATM. Over time, most networks become a mix of technologies: Copper, fiber, coaxial, and wireless transmission using analog and digital encoding with time-division, cell-division, and wave-division multiplexing. Despite the diversification of the access plant technologies, TDM technologies account for the vast majority of the existing plant.

One trend that shows no sign of slowing is the movement of intelligent electronics out into the access plant. It is generally cheaper to place remote electronics close to the customer to collect signals and provide service than it is to run individual wires to each customer. This outward movement began with Layer 1 switching and multiplexing elements: Systems that blindly switch bits without regard to content. The advent of ATM-based SONET Ring multiplexors, Digital Subscriber Loop Access Multiplexors (DSLAMs), and ATM Service Access Multiplexors (SAMs) began a move towards Layer 2 switching in the access plant.

As IP becomes more widespread through the backbone network, and carriers provide higher-layer functionality at the edge, the access network is under pressure from both sides to become data aware. Despite this, it must remain compatible with existing infrastructure and services.

Current Access Networks
The particular mix involved in any provider's network will be based on its strategy at initial market entry. An established Local Exchange Carrier will tend to have an access network based on time-division multiplexing technologies. The copper loops connected to each home are multiplexed and switched through the established switching and multiplexing hierarchy: DS0s into DS1s, DS1s into DS3s, STS-1s, STS-3s, etc.

While there are many access topologies, two dominate the metro landscape: Linear, and SONET ring. Linear topologies, which tend to receive less press than ring-based access networks, are less efficient than ring networks, and cost more to deploy. They do, however, dominate the installed base, and will continue to do so for some time to come.

SONET rings, used heavily by the established LECs, are deployed in both Unidirectional Path Switched Ring (UPSR) and Bi-directional Line Switched Ring (BLSR) configurations. UPSR configurations are popular in the access plant, and normally operate at OC-3 (155 Mbps), OC-12 (622 Mbps), and OC-48 (2.488 Gbps) rates.

The important characteristics of UPSR and BLSR rings include the following facts:

  • They use time-division multiplexing. A given circuit uses a fixed amount of bandwidth, even when no active traffic is present. Circuits are available only in fixed sizes, and moving from one to another can be painful.
  • They use occupancy doubling for protection. UPSR and two-fiber BLSR rings both reserve twice the bandwidth actually needed for the signal, while four-fiber BLSR actually doubles the amount of fiber.

These characteristics have caused a search for more efficient ways to build rings. Several vendors are beginning to offer packet-based rings, which offer several advantages over traditional TDM rings, but not without some costs.

Packet Rings
Packet-based rings attack the problem from two

  • Bandwidth is expensive and must be conserved.
  • Customer bandwidth needs are variable, and service must be able to change easily and quickly.

To achieve these goals, the nodes on the ring use packet-based protocols to send traffic around the ring. Each node on the ring is effectively a small router, incorporating a packet routing fabric. Packet rings achieve their stated goals, but the costs are significant:

  • Relatively expensive nodes. The cost of each node on the ring is higher than a standard SONET add-drop multiplexor. This is due to expense of packet processing and switching hardware.
  • Packet latency/reliability. The use of routing protocols requires that each node receive the entire packet prior to forwarding it, which increases the latency.
  • No interoperability. An ATM ring multiplexor cannot share the ring with standard UPSR or BLSR ring multiplexors.
  • Circuit Switching: All circuit-switched data must be converted to packet format (using various Voice over IP protocols). This is much more expensive than traditional TDM technology.

In the short term, the lack of interoperability, and the high expense of voice traffic handling, makes relying on packet access rings alone unfeasible.

Adding Intelligence to the Network
Another approach in migrating TDM-based networks to IP is to add more intelligence to SONET ring nodes. This concept that was first pioneered by Cerent Corporation (now a part of Cisco), and has now been expanded upon by several metro equipment vendors.

Instead of creating a completely incompatible ring, bandwidth on a standard TDM ring can be reserved for packet or cell traffic. The traffic offered to the reserved bandwidth is statistically multiplexed, while the remaining bandwidth is dedicated to existing circuit services. Such rings can be made to work with the reserved bandwidth being allocated for ATM cells, similar to an ATM ring -- or for packet traffic, using PPP or Frame Relay.

The advantage of such a hybrid ring is that it is backward compatible with an existing UPSR or BLSR ring. For a carrier that has a large installed base, the attractiveness of such a solution is obvious: It allows the ring to migrate towards data services while maintaining compatibility with the installed base of TDM services. The advantages of such rings over standard rings are many, including the following characteristics:

  • Compatibility with existing SONET rings.
  • Processing can vary according to data type. Packet connections can be made with packets, and circuit connections can be made with circuits.
  • Protection can be variable, based on data type. IP traffic can take advantage of the inherent protection of the routing protocols, avoiding the bandwidth doubling inherent in conventional UPSR and BLSR rings. Circuit traffic can use standard SONET protection if desired.

Effects on Linear Access Structures
As noted earlier, linear access structures predominate. So what does the future hold for the linear side of the equation? Nearly the same thing as it does for rings.

An M13 multiplexor has been one of the workhorses of the access network for years. Like the standard SONET multiplexor, it is a layer 1 device; every bit that comes in goes out. Frame Relay switches have included direct M13 interfaces for some time. A Frame Relay switch that allows for multiple DS1s in and a single M13 out is not much different than an M13 multiplexor with integrated layer 2 switching capability. All it lacks is the ability to perform direct circuit connections for some of the DS1s. A similar argument can be made for ATM multiplexors.

The same pressures that exist in the ring access structures apply in the linear segment of the market as well. Just as a ring multiplexor can dedicate sections of the bandwidth for packets and cells, a linear multiplexor can devote sections of a DS3 or OC-3 bandwidth for the same purpose. Groups of DS1s or DS2s can be allocated as Inverse Multiplexing over ATM (IMA), Multi-link PPP, or Multi-link FR to provide variable bandwidth pipes while still interworking with conventional DS3 systems.

By allocating specific sections of bandwidth for specific traffic, significant improvements in bandwidth utilization can be provided while maintaining interoperability with existing DS1 and DS3 cross-connect systems.

Finally, the power of statistical multiplexing and multilayer switching can convert that linear multiplexor into an intelligent element that can make more efficient use of bandwidth. When used to feed ring elements, highly flexible and fault tolerant networks can be built on the same infrastructure as today's access network.

The outward movement of intelligence into the access plant can transform the behavior of the network without requiring major upheaval of its physical structure. The key is to use nodes that provide true multilayer and multiprotocol capability. Using IP within a TDM-based access plant can provide for efficient protection by making use of the inherent self-healing capabilities of standardized routing protocols. Existing equipment can interoperate with intelligent nodes, allowing new services to be deployed as needed without disrupting the existing network. The result is a graceful migration from the dominant TDM services of today to the up-and-coming IP services of tomorrow.

Kevin Wayne Williams is chief technical officer and Scott Lukes is marketing director at at MAYAN Networks. MAYAN Networks was founded in 1998, to simplify the delivery of services, and to reduce cost and complexity at the edge of the optical network. Please visit their Web site at

Return To The March 2001 Table Of Contents

Offload Where It Counts


Not too long ago, the public switched telephone network provided reliable service that was always available. These days, the once rare �All circuits are busy now� message increasingly interrupts us when we try to place a call. The explosion of Internet usage is largely responsible. With substantially increased hold-times, dial-up Internet access calls create a challenging traffic environment, making the busy-out message a more common occurrence as the PSTN is strained and the carriers seek answers.

Attempting to address both planned network traffic expansion and the impact of dial-up Internet hold-times, carriers search for answers to economically scale their networks to meet these demands. To date, the few carriers that have done anything have implemented a variety of stopgap measures. These have typically been �trunk-side� or �post-ingress switch� products that have met with limited success, as they merely shift the problem, not eliminate it.

Trunk-Side Offload: The Shell Game
Dial-up Internet hold-times have created PSTN congestion �hotspots� which have in turn led carriers to seek relief from a variety of offerings. For example, both carriers and Internet service providers (ISPs) initially benefited from remote access servers (RAS), and then from central office-based remote access servers (COBRAS) as a way to provide greater control over modem traffic engineering. Meanwhile, in each case the egress Class 5 switch remains a network hotspot.

Egress switch congestion led carriers to look at offload architectures that focused on the core of the network, including direct RAS connection to the tandem switch. Later, as the tandem switch became congested, connection was made directly to the trunk side of the ingress switch. Eventually, NGN tandems were employed to improve the situation. Unfortunately, the result of these approaches varies little. Regardless of where trunk-side offload is employed standalone, it�s a shell game serving only to move the problem somewhere else in the network.

Pre-Switch Offload (PsO): Offload Where It Counts
Contrary to these other approaches, pre-switch offload delivers Internet PSTN relief at the earliest possible point in the network -- before traffic hits the first switch. The key component is the access gateway (AGW), which separates Internet and voice traffic before the ingress switch and routes each accordingly. Narrowband voice calls go to the Class 5 switch as always, while dial-up Internet calls are routed around the Class 5 switch to the ISP.

The benefits of pre-switch offload are manifold. It offloads the PSTN in ways other alternatives plainly cannot. PsO simplifies the network by consolidating offload into one box. Furthermore, it uniquely maximizes the carriers� substantial investment in current circuit-switched infrastructure. By offloading line-side, the Class 5 switch can again simply carry the voice traffic for which it was designed, enabling carriers to cap their ongoing expenditure on legacy PSTN infrastructure.

Pre-Switch Packetization (PsP): The Road To Convergence
Taking pre-switch solutions to a higher level, the huge leap forward in PSTN offloading is pre-switch packetization. By providing even greater improvements in both offload and economics, it also positions the infrastructure for next-generation voice and data.

Pre-switch packetization enables the carrier to exploit the public data network (PDN) rather than tandem trunks to transport data directly to the ISP as packets instead of circuits. PsP offloads the entire network through redirection of Internet traffic around the circuit-switched PSTN and interoffice facilities. This approach provides carriers the added benefit of reduction in interoffice facilities requirements by a ratio of more than six to one.

Moreover, pre-switch packetization provides for strategic investment in voice/data convergence. Advancing circuit-to-packet conversion into the line side, new convergence platforms will emerge to packetize all voice and data traffic to form the foundation for next-generation network infrastructure.

Economics Case Study
To further examine the impact of different offload approaches, a study was conducted with an actual carrier LATA with over 1M subscribers and 23 central offices. Network expansion costs were analyzed for a five-year period using their present method of operation (adding Class 5 and TDM infrastructure), application of standalone trunk-side offload, pre-switch offload, and pre-switch packetization solutions. The study indicated savings estimates for pre-switch offload and packetization techniques of up to $10M, not including operational savings from network simplification and other effects.

NGN Evolution: Offload, Augment -- Then Converge
Economics provide strong incentive to pursue next-generation networks. But only the emergence of truly deployable solutions, that help realistically evolve carriers� networks, will induce them to readily join the circuit-to-packet revolution.

Uniquely coexisting with the existing infrastructure, the carrier-grade next-generation convergence platform will initially offload and extend the life of PSTN investments, while augmenting the network and its equipment with increased capability over time. Finally, the converged network will evolve from the very platform used to offload and extend the life of PSTN gear. By enabling a graceful migration to NGN -- without a forklift changeover -- the next-generation convergence platform will ultimately replace legacy infrastructure. c

Fred Ellefson is vice president of marketing for Rapid5 Networks. Rapid5 is dedicated to the advancement of the circuit-to-packet revolution by delivery of a three-dimensional, carrier-grade platform to perform Internet PSTN offload, broadband access and multiservice gateway functionality. Mr. Ellefson may be reached at For more information about Rapid5, visit

Return To The March 2001 Table Of Contents

Today @ TMC
Upcoming Events
ITEXPO West 2012
October 2- 5, 2012
The Austin Convention Center
Austin, Texas
The World's Premier Managed Services and Cloud Computing Event
Click for Dates and Locations
Mobility Tech Conference & Expo
October 3- 5, 2012
The Austin Convention Center
Austin, Texas
Cloud Communications Summit
October 3- 5, 2012
The Austin Convention Center
Austin, Texas