Introduction
It is common knowledge that telecommunication service providers worldwide have transport networks that were originally designed to deliver TDM services. Yet due to the exponential growth in data services we have experienced over the past few years, next-generation transport is facing scalability and cost challenges and therefore embedding packet capabilities.
In order for telecom service providers to preserve their TDM services, know-how and investment, it has become increasingly obvious that some kind of a migration plan is needed. One such approach would be to emulate TDM in packet-based switches and routers. In line, another option would be to embed packet technologies such as Ethernet and MPLS / MPLS-TP into the traditional transport systems.
This document provides an analysis on the current and future TDM plus packet services delivery over networking infrastructure, comparing several alternatives and describing how phased migration can be done with circuit emulation technologies in order to achieve the most scalable and affordable networking environment.
Current status of TDM traffic
A surprising dominancy for Ethernet-over-SDH services
TDM services and SONET/SDH technologies were originally designed to deliver voice. Voice is a mandatory service with specific requirements, but its capacity in the entire network is very small - less than 10 percent in most cases.
Most of TDM services today are leased-lined that enterprises consume to connect their remote offices. TDM is considered as reliable and a secure connection, and therefore it was widely adopted as a VPN enabler. 155Mbps that is achievable with an STM-1 connection is a popular example.
The introduction of carrier-grade switches and routers brought an alternative with a much higher capacity and lower cost. The result was acceptance of Ethernet services for enterprise VPN and residential triple-play service.
A closer look at the actual business that is deriving from the TDM leased-lines reflects this trend - most of the TDM services are actually Ethernet-over-SDH services that are still producing good margins to the telecom service providers.
But the increased competition (a practical alternative for an enterprise to replace its STM-1 service with Gigabit Ethernet service) with the exponential growth of packet services is changing the overall picture. A long term view on the traffic that networks will need to deliver is clear - Ethernet is the growth engine, and Ethernet will capture most of the traffic in the next-generation networks. This is demonstrated in Figure #1:
Figure #1: Ethernet/TDM traffic split in a typical network infrastructure
Thinking about TDM while upgrading networks
Capex and Opex dilemmas when TDM is increasing or decreasing
While the long term vision regarding Ethernet and TDM is clear, the implementation is affected by many constraints, including: current architecture and subscriber base, competition from alternative operators, regulation, required scalability, costs, workforce know-how, existing fiber and other factors.
In some cases, TDM traffic and services are still growing. So a key question is how to enable the new TDM: with new TDM Capex over the existing TDM network, or with new Carrier Ethernet Capex over the new Packet Transport Network - that will anyway carry the majority of the traffic that will arrive from Ethernet? This opens not only a Capex and Opex comparison but a strategic decision regarding one network that will handle all services vs. separated networks that will deliver different types of services.
In other cases, TDM traffic and services are decreasing. This is mainly due to enterprises that are giving up their TDM and move to Ethernet. A key question here is what to do with the existing TDM infrastructure. What is the best way to reuse the exiting subscribers, fibers and network infrastructure? Other than Capex issues to enable the new Packet Transport issues, there are various Opex issues regarding the existing TDM network. Once again, the strategic decision here is about the co-existence of both TDM and packet services in the next-generation transport network.
This perspective is demonstrated in Figure #2:
Figure #2: While Ethernet is rapidly growing, TDM increase/descries over time brings dilemmas regarding the next-generation networks
TDM + packet networks: the alternatives
High level screening, including pros and cons
Service providers face the following alternatives and tradeoffs when designing their next-generation transport networks that should deliver both TDM and Ethernet services:
1. The orthogonal approach: two networks
The orthogonal approach involves the addition of a new and parallel packet-based network to the already-existing TDM network. In this concept, the service provider is maintaining two separate networks: one only for the TDM traffic and another one only for Ethernet traffic. Although it is the most intuitive and simple approach, it is not Capex (network elements, fibers, etc.) or Opex (human resources, customer support, etc.) effective. In addition, this approach is not satisfying the "transport" organization's motivation to embed packet technologies in his existing network.
2. The overlay approach: Ethernet over TDM
The overlay approach involves backhauling of the upcoming Ethernet services over the existing TDM network. This is a starting point in networks that are heavily based on TDM infrastructure and are starting to offer Ethernet services. This is a costly alternative that is not reasonable for a large amount of Ethernet services, since it cannot take the advantage of statistical multiplexing. Therefore a lot of unutilized ports will pass through the entire network to the expensive TDM and router core interfaces.
3. The convergence approach: MSPP/P-OTS
The convergence approach is an all-inclusive approach, creating a converged networking layer including native support for TDM services and native support for packet services. This is being deployed by using MSPP/P-OTS products that implement a hybrid, complex, and expensive fabric. A typical MSPP/P-OTS product embeds a wide variety of technologies, including DWDM, SONET/SDH and IP/Ethernet/MPLS. The next chapter will describe in detail the pros and cons of this approach.
4. The migration approach: PTN with circuit emulation
The migration approach uses the standard and interoperable Packet Transport Network switches that were originally built for the delivery of large capacity packet services and add circuit emulation technology to enable TDM services. The packetized TDM services are equivalent in their quality to those being provided by the traditional SDH/SONET networks. This approach provides the opportunity for smooth migration from a TDM network to a packet network, since the TDM linecards can be added and removed at any time. The next chapter will describe in details the pro's and con's of this approach.
Figure #3 presents the four alternatives:
Figure #3: TDM + packet networks: the alternatives
Migration versus convergence
Circuit emulation advantages over hybrid platforms
Migration and convergence are the two main alternatives for implementing next-generation networks. The migration concept is to move From "A" (TDM) to "B" (Packet), while the convergence concept is to move from "A" (TDM) to "A" + "B" (TDM + packet). Typical migration products are PTN switches that enable TDM with circuit emulation technologies, while typical convergence products are MSPP/P-OTS that have native TDM and optical capabilities.
The migration approach: PTN with circuit emulation
The migration approach is based on MPLS and MPLS-TP PTN products. These products are based on a packet fabric and therefore can only perform packet processing. This means that for Ethernet services the solution is highly scalable, and cost optimized but TDM services need to be packetized and handled specially by traffic management modules.
The TDM traffic is transported on top of synchronization technologies (such as Synchronous Ethernet and IEEE 1588v2) and TDM emulation protocols (such as CEP, SAToP and CESoP). This package enables any rate of TDM services - from PDH E1/E3 till SDH STM-1/4/16 services.
Other than scalability, performance and cost, the migration approach embeds TDM in optional linecards. This means that the service provider can add and remove the TDM linecards in order to cope with its current TDM traffic, without impacting the Ethernet performance. This pay-as-you-grow approach enables packet transport networks that will evolve to carry mainly packet services.
TDM services being offered using circuit emulation technologies are equivalent in their behavior and quality to those being provided on the traditional SDH/SONET networks.
Figure #4: The migration approach - a PTN port - services hierarchy
The convergence approach: MSPP/P-OTS
The convergence approach is based on MSPP/P-OTS products. These hybrid products pack the functionality of Ethernet and TDM on a single fabric and transport them in their native form. Therefore the products integrate smoothly into existing transport networks and provide satisfying solutions for networks that need to start introducing Ethernet services on their large capacity TDM infrastructure.
MSPP/P-OTS product complexity is higher than a PTN switch or a router since two different domains coexist - classic transport with DWDM and SDH, and classic packet with Ethernet/MPLS. Operating these platforms and debugging problems is a challenging task.
In addition, since the long term plans for the next-generation network is to support a massive amount of Ethernet services, and TDM is not optional on the products, this approached will turn to become not competitive in its price point and performance to the pure switches and routers alternative.
Figure #5: The convergence approach - a P-OTS port - packaging services
The table below summarizes the main differences between PTN with circuit emulation solution (migration approach) versus MSPP/P-OTS solution (convergence approach):
Figure #6: Carrier Ethernet versus P-OTS
Smooth migration from TDM to a unified MPLS / MPLS-TP Packet Transport Network
How to smoothly migrate: step-by-step guidelines
While building the next-generation infrastructure, a gradual transition of TDM services enables a smooth network migration towards a unified packet network. The transition involves the following steps:
Step 1: Building MPLS / MPLS-TP aggregation network and offloading high capacity TDM traffic from the TDM backbone
A typical starting point is a TDM network, based on PDH/SDH technologies, that is enabling both TDM services such as E1 and STM-1/4/16, as well as 10M/100M Ethernet services.
The first step is to build in parallel to the TDM metro network, a new Packet Transport Network that will be used for metro aggregation. This network is based on MPLS / MPLS-TP plus circuit emulation technologies. At this step, the new network is used to carry high capacity TDM traffic that is coming from the access rings, and is doing so, offloading the TDM backbone. During this phase, the service provider also learns how to operate the new network and gains confidence with the new technologies and capabilities.
Subscribers are not affected at this point, as well as in any other point of the migration.
Figure #6: Step 1 - Building MPLS / MPLS-TP aggregation network and offloading high capacity TDM traffic from the TDM backbone
Step 2: Adding a multi-service access network to carry any service from the access network
When the metro network is stable and robust, a new multi-service network is built in the access network. The multi-service network elements combine Ethernet and TDM interfaces and are used both for new and existing subscribers.
This process is completing a new networking layer that is capable of carrying any type of service over packet. This network is scalable and cost optimized for the expected Ethernet traffic as well as capable of upgrading the existing subscribers to a higher capacity network with the same service level agreement.
The new multi-service switches is can be added as fast as the service provider needs.
Figure #7: Step 2 - New multi-service access network migrates existing services and enables any new services
Step 3: Phasing out the TDM backbone while preserving the investment, subscribers and services in the TDM access
At a certain point, most of the TDM traffic in the backbone was already migrated to the packet network, and therefore the TDM backbone is consuming Opex that is higher than its revenues. In addition, the level of confidence and stability of the Packet Transport Network is high. At this point, everything is ready to remove the TDM backbone.
This significant step is to release the fiber that is used by the next-generation PTN, while preserving the very large number of network elements in the access network. Preserving the TDM installed base in the access is one of the most important achievements of the smooth migration, both economically and technically.
Figure #8: Step 3 - Offloading the TDM backbone while preserving the TDM installed based in the access network
Step 4: Completing the phased migration with a standard and interoperable Carrier Ethernet network
The long-term vision for a unified Packet Transport Network is achievable in a standard and interoperable way. The new PTN is scalable and has a high performance. The result is a network based on MPLS / MPLS-TP, and therefore the service provider is leveraging all of the technology's connection-oriented, traffic engineering, QoS, OAM and protection mechanisms. The circuit emulation technologies are also standard and available on today's networking platforms. Since circuit emulation was used, its linecards on the switches can be replaced with Ethernet linecards at any time without impacting the switching capacity.
Figure #9: Step 4 - Phasing out TDM infrastructure and achieving a unified MPLS / MPLS-TP PTN
Orckit-Corrigent PTN solution for TDM migration
Delivering any TDM service (from E1/E3 till STM-1/4/6) over a standard and interoperable MPLS / MPLS-TP network
Orckit-Corrigent's CM-4000 family of Packet Transport Network (PTN) switches for metro aggregation networks offers service providers a carrier grade platform based on standard and interoperable Layer 2 MPLS and MPLS-TP technologies. The CM-4000 enables any type of Ethernet services as well as any rate of TDM services over packet using a dense mixture of 10GE, GE and PDH/SDH interfaces.
The CM-4000 significantly reduces Capex and Opex by providing smooth migration from legacy TDM to Ethernet networks with its seamless support of PDH services and SONET/SDH grooming, cross-connect and transport services. By cost effectively enabling a mixture of Ethernet and TDM traffic, the CM-4140 provides an agile platform enabling advanced residential multi-play, enterprise VPN and mobile backhauling services over a unified MPLS / MPLS-TP networking infrastructure.
Summary
Most telecommunication service providers have a large TDM networking infrastructure. However, as the demand for bandwidth intensive applications continues to grow, the Ethernet traffic is expected to capture a significant portion in next-generation networks - therefore driving service providers to migrate their networks beyond legacy TDM technologies.
The migration from TDM networks towards a Packet Transport Networks can be performed, and should be performed today. Preserving the existing TDM access network, subscribers and services is achievable with circuit emulation technologies that are implemented on metro aggregation MPLS / MPLS-TP PTN switches. This migration process is being done using standard protocols and has Capex and Opex advantages over any other alternative that is combining TDM and Ethernet.
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