Over the past several years, telecommunication service providers have been experiencing a dramatic shift from legacy TDM circuits to next generation Ethernet traffic. This shift has been driven by both residential triple-play services and business Ethernet services, and it has also been marked by the introduction of Ethernet-based 3G Node B and 4G/LTE mobile networks.
Nevertheless, the demand for SDH circuits is expected to remain solid for a long time. While small offices and home offices (SOHO) have already made the shift towards Ethernet services (mostly based on broadband xDSL and CATV services), two significant market segments present an ongoing demand for legacy TDM services.
The first is medium and large enterprise customers. Although such companies are moving to Ethernet based services, many of them maintain their legacy systems. Carriers, therefore, will continue to provide them with TDM services. The other segment that demands legacy services is the mobile market. In order to prevent major forklift upgrades, mobile operators will continue to support 2G and 3G infrastructure. These technologies contribute to the SDH demand in metro networks.
Therefore, it is clear that for the foreseen future, TDM and Ethernet will have to coexist in the metro. Yet curiously, initial deployments of carrier Ethernet solutions have not offered any support for SONET/SDH. Standards bodies such as the MEF decided to focus on the transport of PDH rates with circuit emulation services but did not define a circuit emulation technique or demand for real, high-rate SDH services.
As a result, most of the carriers today use two separate transport systems in their metro area networks: The first is a legacy SDH network (or MSPP based Next Generation SDH). At the same time, they have also deployed a packet network composed of Layer2 aggregation (in some cases, Carrier Ethernet-based aggregation) and a Layer3 IP-MPLS core.
"Packet Transport Networks" (PTN) equipment and "Packet Optical Transport Systems" (POTS) are innovative solutions that address the challenge of an integrated, single-layer approach for a next generation metro network. Both solutions include advanced technologies to address Ethernet and SDH in their native forms. This white paper will provide a comprehensive comparison between these two solutions.
The POTS solution
POTS products provide an integrated solution for several technologies in a single box using a multipurpose centralized switching fabric. This fabric is typically based on cell switching technology and is capable of performing native packet switching and native SDH switching simultaneously. POTS products can be designed so that the packet switching technology of choice is pure Ethernet (Provider Bridge), MPLS, MPLS-TP, Provider Backbone Bridging or Traffic Engineering (PBB, PBB-TE). For TDM switching, POTS can be configured with High Order (HO) or Low Order (LO) switching granularity for SONET or SDH. TDM tributaries may include low rate services such as E1/T1 and up to high rate STM-64/OC-192 10G interfaces.
As illustrated in Figure 1, POTS use different line-cards for TDM interfaces and for packet interfaces. Every line card processes the relevant technology and converts the processed traffic into a pre-defined cell format, which is then forwarded to the egress card regardless of the originating frame format. The egress card converts the cells into standard frames in the required format. The resulting architecture provides a multi-purpose switching platform, capable of splitting its switching capacity between packet switching and SDH circuits
Figure 1: Typical POTS Architecture
POTS Services and network architecture
The separation of the two switching entities within the universal switching fabric suggests that the POTS platform is composed of two separate systems: packet switch and TDM switch. Simply put, the POTS solution is a combination of a Carrier Ethernet Switch and MSPP in a single box.
Today's carriers' environment generates a large amount of SDH traffic and packet traffic that needs to be transported between sites in the metro. The deployment of POTS would require two fiber pairs for every link. One pair would be used to interconnect the SDH part of the network and the other pair would be used to carry the Packet traffic. The complete separation between the two technologies implies that with POTS, the operator actually builds and maintains two networks on the same physical node.
This significant waste of CAPEX (or OPEX, in the case of leased fibers) is overcome in one of two ways: Ethernet over SDH (EoS) or WDM/ROADM technologies with OTN capabilities.
With EoS, POTS implements Virtual Concatenation (VCAT), Generic Framing Procedure (GFP) and Link Capacity Adjustment Scheme (LCAS) on dedicated hardware to map Ethernet into a standard SDH payload. This approach is very inefficient for high capacities of packet traffic and services.
By using integrated WDM/ROADM with OTN capabilities in POTS platforms, POTS can carry both SONET/SDH and Ethernet over a single fiber pair over 2 separate wavelengths or 2 sub-wavelengths. Although WDM/OTN technologies significantly increase bandwidth, they also add a significant amount of complexity and require special care by trained professionals. Moreover, as operators often already use a WDM network in the metro, the integrated WDM and ROADM capabilities of POTS adds little value. In this case, deployment of POTS will require two wavelengths on an existing WDM infrastructure.
POTS Operation Administration and Maintenance (OAM)
SONET/SDH technology uses an advanced OAM suite. With a multilayer OAM approach, SONET/SDH offers the ability to continuously monitor every single circuit, path, multiplexer section and regenerator section, and initiate consequent actions (such as switch to protection) upon failures in these layers. It also includes advanced Alarm Indication Signals (AIS) and Remote Defect Indications (RDI), loopbacks and alarm correlation tools.
Carrier-grade packet switching technologies are enhanced to provide a similar level of OAM by providing standard tools for OAM at different levels. At the Ethernet level, IEEE 802.1ag - Connectivity Fault Management (CFM) offers Continuity Check, Loopback and Link trace (trace route) while ITU-T Y.1731 extends this standard and offers service-level OAM enhancements such as AIS for alarm indication and suppression and SLA verification - delay, delay variation and frame loss rate.
Although these standards are sufficient for Ethernet-based packet forwarding (PB and PBB), additional OAM tools are required for MPLS-based networks. This includes IETF RFCs such as LSP Ping (RFC 4379) for data and control plane connectivity check, performance monitoring (delay, jitter and packet loss) and LSP Trace for fault isolation.
The use of POTS in a metro network requires full suite SDH OAM and Packet OAM. As a result, operators should actually manage two separate networks that happen to share the same boxes. The operational complexity is high and lack of any interworking functionality between the Packet and TDM layers prevents the operational simplicity promised by the POTS technology.
The PTN approach
Carrier Ethernet (CE) technology is ideally designed to cater to Ethernet services in carrier network environments. With Carrier Ethernet, carriers benefit from a complete OAM solution as well as high availability and manageability features. The underlying transport technology varies from system to system and can be based on Ethernet technologies, such as Provider Bridge or PBB/PBB-TE, or on MPLS technologies, such as IP-MPLS (VPLS/VPWS) or the newly defined MPLS-TP.
PTN was specially designed by Orckit-Corrigent and integrated into its CM-4000 family of products offering an enhanced Carrier Ethernet solution with unique transport capabilities of SDH services. As an example, while regular CE platforms implement SAToP and CESoPSN circuit emulation technologies, they are limited to E1/T1 circuits over packet and are far from providing a real replacement for legacy TDM.
PTN solution is capable of providing circuit emulation for any SONET/SDH payload. It also provides HO and LO cross connection and grooming capabilities, fueling true network convergence over a single platform and significantly reducing OPEX.
Figure 2 below depicts the architecture of a typical PTN platform. The system is built around a packet switch, which handles all traffic flowing through the product. A second switch can be added and configured in standby mode for 1:1 protection of the switching fabric.
Figure 2: Typical PTN System Architecture
In order to keep costs low, the switching fabric shares a blade with Ethernet UNI and NNI ports. High port fan-out is achieved by enabling the ports next to the standby switch to be fully functional regardless of the status of their collocated switch. With an extremely low entry cost, additional services and interfaces are provided by inserting extension modules. With a pure packet-based switching fabric, the packet extension modules are also very cost-efficient.
PTN is Enabling TDM and Synchronization
PTN solution can provide a full set of SONET/SDH services over packet networks using Circuit Emulation over Packets (CEP) encapsulation. These services are transported with the same delay, jitter and wander tolerances as in traditional SONET/SDH systems and are compliant with ITU-T and Telcordia specifications. The CEP implementation is based on IETF standards and provides service protection in under 50msec for fiber cut or node failure.
This effectively enables circuit switching of STM-1/OC-3, STM-4/OC-12 and STM-16/OC-48 signals together with HO/LO cross-connections and grooming of multiple channelized OC-n/STM-n signals, as illustrated in the figure below.
Figure 3: PTN high rate TDM Services
With the integration of Synchronous Ethernet and IEEE 1588v2 Technology, the PTN circuit emulation solution is based on a synchronized packet network enabling it to provide the same quality as traditional SONET/SDH networks.
The circuit switching (HO and LO cross connection) and circuit emulation (converting TDM payload into packets) is performed on the TDM interface cards. This maintains a low cost for the packet services and adds the TDM cost burden to the TDM interfaces alone.
PTN Services and network architecture
A PTN is a pure packet network. With Layer 2 MPLS as the underlying packet technology, PTN provides network-wide Traffic Engineering, advanced QoS and Connection Admission Control (CAC) for SLA assurance.
Circuit emulation packets are assigned with the highest possible priority and strict priority queues. Synchronization is also applied on the packet interfaces using Synchronous Ethernet or IEEE 1588v2. QoS and network synchronization provide "SONET/SDH-like" quality across the network and are used to meet the required jitter, delay and wander performance levels of TDM circuits.
PTN Operation Administration and Maintenance
The PTN OAM approach is significantly simple, compared to the POTS OAM. With a single transport technology for both packet and TDM, the only OAM tools that are used are the Ethernet and MPLS OAMs. SONET/SDH OAM is terminated by the TDM line cards, thereby significantly simplifying the OAM processes.
Case Study - Cost comparison
POTS and PTN are optimized for converged solutions that offer a mixture of SDH and Ethernet services. A typical node configuration would include the following characteristics:
- Fully redundant node configuration for switching fabric, control and power
- 33% of the bandwidth allocated for NNI interfaces and 66% of the bandwidth allocated for UNI interfaces
- Traffic mixture of 60% packet and 40% TDM
Figure 4 compares the cost breakdown of a typical POTS system versus a PTN platform. The PTN solution provides a 40% lower cost, mainly due to the very low cost of the NNI solution. In fact, almost 40% of the POTS system cost is associated with NNI interfaces. The PTN solution, with its on-board UNI/NNI interfaces and the use of one type of NNI interface (namely Ethernet NNI), maintains a much more competitive price point.
Figure 4: Typical POTS and PTN cost breakdown
The migration from legacy SDH to packet-based systems in the metro calls for a hybrid solution that can cost-effectively address this challenge with a single platform. The two leading candidates for this purpose are POTS with its multipurpose switching capabilities and PTN which integrate advanced circuit emulation technology into a state-of-the-art carrier Ethernet platform. The following table compares the key characteristics of the two technologies.
|Unified switching entity for packet and TDM - PTN packet switching architecture forwards packets and TDM in the same way, forming a unified switching architecture.
||Two logically separated switching entities (packet plus TDM) - the multipurpose switching fabric forms two separate switching entities in a single box. |
|Networking||A unified packet network, capable of transporting any mix of packet and TDM traffic over 1GE and 10GE interfaces.
||Two separate networks, one for TDM and the other for Packets. Both can use the same fiber pairs by adding WDM components at extra cost, instead of service cards.|
|OAM||Carrier Ethernet OAM tools including Ethernet OAM (IEEE-802.1ag CFM and ITU-T Y.1731). TDM OAM at TDM termination points only.
||Separate OAM approach for Packet traffic and TDM traffic. Carriers need to master both technologies to manage their network.|
| Network Management
||PTN uses MPLS-based dynamic control plane with full routing and signaling capabilities. This significantly simplifies the establishment and management of all services. Use of NMS is optional and is mostly used for GUI-based service management with full FCAPS support.
||NMS in POTS is compulsory. All provisioning is static and is based on centralized path computation for SDH services and on Path Computation Element for packet services. Management of services is complex and differs significantly from TDM to Packet.|
|Cost||The PTN architecture enables low cost solutions. With a lower entry cost, PTN technology is extremely cost effective. This cost advantage grows as the node capacity scales.
||The POTS architecture with its multipurpose switching fabric imposes high system cost due to high cost of networking interfaces.|
POTS is the technology of choice for multiservice national backbones which require integration between high capacity ROADM technology, packet transport and TDM transport in a single box. When attempting to use the same technology for metro applications, however, its major disadvantages become clear. The multiservice switching technology forces the POTS devices to act as two separate products in one cage with TDM traffic handled completely separate from the packet transport. Converging the two technologies by means of Ethernet over SONET/SDH simply turns the POTS into a regular MSPP.
Furthermore, the POTS technology is very expensive both in terms of CAPEX and OPEX. POTS' high entry cost is mostly driven by the need for expensive NNI interfaces. The cost of expansion cards is also high due to the complex switching architecture and the need to convert any traffic to cells. In terms of OPEX, operators are actually running two networks in one box. This means that network operators are running a TDM network in parallel with a packet network and, in some cases, even with a WDM layer for fiber relief.
PTN solution, on the other hand, is designed and optimized for metro applications. Its unified packet switching keeps the system cost at a very low price point and the OPEX is kept low with a single transport approach. The selection of MPLS and MPLS-TP as the underlying transport technologies, introduces state-of-the-art control plane into the packet transport t world, providing simple operation with assured SLA.
Therefore, it is clear that the optimal technology for next generation transport networks, which are capable of converging TDM with packet technologies, is PTN.