Packet Optical Transport--Evolution vs. revolution

There is an industry consensus that the long term target network will be a packet-optical converged network built upon Ethernet, IP/MPLS, and DWDM. Today's access and aggregation networks, however, are primarily T1/E1 and SONET/SDH based and heavily reliant on Multi-Service Provisioning Platforms (MSPP). The target network uses Carrier Ethernet Switch Routers (CESR) and Reconfigurable Optical Add Drop Multiplexers (ROADM). There is little consensus, however, on how to make the transition from today's network to the long term target.

The best transition strategy clearly depends on the rate of transition from T1/E1 based subscriber services to IP/Ethernet services. Greenfield deployment of PON, IP DSLAM or Active Ethernet access networks makes rapid transition to CESRs in the aggregation network easy.

Incumbent service providers, however, continue to deploy T1 circuits to support 2G and 3G wireless backhaul, enterprise MPLS VPN, and Internet access services.  Circuit switched voice services, though declining, maintain a large majority of the voice market and many ATM based DSLAMs that use TDM/SONET transport facilities also are still in service. Recognizing that large incumbent service providers must support a mix of legacy and IP/Ethernet services MSPP vendors are repositioning their products as Packet Optical Transport Systems (POTS) by adding ROADM and Ethernet switching to the MSPP chassis. POTS is an obvious choice where the move away from T1/E1 based services is slow.

Most transition strategies fall between these extremes, however, and require a more subtle Total Cost of Ownership analysis. Our studies show that an evolutionary rather than revolutionary move from MSPP to CESR is called for. One reason is that modern MSPPs incorporate a ROADM shelf while CESRs do not. Therefore, even if all of the T1/E1 and SONET/SDH traffic is eliminated the existing MSPP must be kept in place to provide the ROADM function.

Another reason for gradual rather than abrupt transition from MSPP to CESR is that it rarely makes sense to replace an entire aggregation network and all of its subtending access networks at once. This is especially true if the network contains a mix of legacy and Ethernet transport services. For example, MSPP/POTS systems commonly carry Ethernet payloads over DWDM wavelengths and as Packet over SONET (POS). However, though technically feasible it is rare to use a CESR to carry a SONET/SDH payload over Ethernet--the business case is a difficult one.

The most cost effective transition from MSPP/POTS to CESR is one where Ethernet access networks are created to offload all of the Ethernet services from the access MSPPs leaving T1/E1 and SONET/SDH on the existing MSPPs. During the initial transition phase the Ethernet access networks are terminated in aggregation MSPP/POTS systems. In a second phase CESRs are added to the aggregation network to handle the Ethernet payloads. In the long run the MSPPs are retired and the long term packet-optical converged network is realized.

I believe that economic analysis similar to this is what led AT&T to call for the phase out of the PSTN and "Plain Old Telephone Service"--the former meaning of POTS--in its December 21, 2009 filing before the FCC. AT&T's objective is to preemptively eliminate TDM from the network and therefore clear the way for all packet-optical networking. Even such a revolutionary step, however, may not go far enough. Many modern and growing services such as the MPLS VPN, wireless backhaul, and Internet services mentioned above make heavy use of T1 access circuits due to their ubiquity and the relative scarcity of fiber in access networks.

My view is that we will move gradually toward the converged packet-optical networks. Packet Optical Transport Systems will make that transition more affordable than it would otherwise be.

Michael Kennedy is a regular FierceTelecom columnist and is the co-founder and Managing Partner of Network Strategy Partners, LLC (NSP) He can be reached at [email protected]