Migrating to an Ethernet-centric Infrastructure

Good question. Many providers of 2G/3G mobile wireless backhaul are interested in converging their TDM and packet traffic over a single converged Ethernet network. In addition, many providers of TDM private lines are looking at ways to support incremental sales of private lines over their new Ethernet networks so they don't have to continue investing in legacy SONET/SDH equipment. To make this a reality, a few key standards have emerged to transport TDM over packet or Ethernet, commonly referred to as TDMoP. The IETF has defined two standards--Structure Agnostic TDM over Packet (SAToP) and Circuit Emulation Services over Packet Switched Networks (CESoPSN). Both are critical to achieve TDMoP especially if both channelized and unchannelized services are required. Essentially, they take TDM traffic and encapsulate it into a either Dry Martini or MPLS pseudowires for transport over Ethernet. By using pseudowires, the provider can leverage the OAM and network resiliency mechanisms. Another emerging standard is the MEF's CESoE which provides TDMoP without pseudowires. This approach is less mature. In terms of synchronization, this does present a challenge for TDMoP once native TDM connectivity is completely eliminated from the site requiring the TDM service (i.e. no physical T1/E1 to derive timing and synchronization from). So again, a few standards are emerging to overcome this challenge. One is from the IEEE called 1588 and another is from the ITU called G.8261. G.8261 specifies the timing requirements of Ethernet networks at the boundaries of Ethernet and TDM networks. It outlines the minimum requirements for network element synchronization using two methods for clocking distribution: Synchronous Ethernet and packet based methods (i.e. 1588). Synchronous Ethernet, as defined by G.8261, uses the physical layer of Ethernet and can only distribute frequency and not time of the day. It is not affected by impairments introduced by the higher levels of the network. IEEE 1588 is independent of the physical Ethernet layer and can distribute time of the day and frequency. However, it can be affected by impairments of the network such as packet delay variation.