Telecommunication service providers hold large SONET/SDH infrastructure and provide large amount of TDM services mainly to enterprise customers and mobile backhauling purposes. The adoption of packet based technologies brings the challenges of network convergence and services migration. Competitive infrastructure needs to enable both TDM and packet delivery and deal with wide range of traditional transport requirements as well as scalable bandwidth rates in next-generation networks.

Packet based networks are originally asynchronous. The need to enable TDM services makes synchronization an important extension to the classic Ethernet technology. In order to meet delay, jitter and wander requirements as define by the SONET/SDH standards there is a need for high quality end-to-end network synchronization. "Synchronous Ethernet" (ITU-T G.8261) is the basic mechanism to provide synchronization to the TDM services.

There are several technologies for clock distribution over packet networks: Adaptive buffering, IEEE1588v2, or Synchronous Ethernet. Synchronous Ethernet uses only the physical layer (thus, it is not affected by impairments introduced by the higher levels of the network) and does not depend on the network rates and quality. Therefore, Synchronous Ethernet is the most reliable method for clock distribution, and it offers high timing accuracy under any network condition and network topology. 

Circuit Emulation over Packets (CEP) technology enables TDM services delivery

With Circuit Emulation over Packets (CEP) standard technology, low rates and high rates of TDM services are achieved, and therefore smooth migration to packet based networks and truly alternative to MSPP platforms is practical.

CEP packet: with CEP implementation, TDM frames are encapsulated into data packets (according to RFC 4842) to deliver TDM services in a packet based network.
The simplest form of CEP encapsulation is with an STS-1 (VC-3) payload. This basic structure is mapped into a CEP packet that includes CEP header and Synchronous Payload Envelope (SPE) to carry the TDM data.
An STS-3c (VC-4) payload is mapped into three SPEs. Fractional STS-n/VC-n payloads are mapped into shorter SPEs whose length is associated with the actual amount of provisioned bandwidth within the high-order SONET/SDH payload.

CEP bandwidth efficiency: CEP technology is very efficient for high order (VC-3 and above) TDM services. Headers overhead consume approximately 5% of overall traffic, compared to SONET/SDH which increases bandwidth by 3.5%.
And for low order TDM services (VC-11, VC-12, VT-1.5, VT-2) - since CEP enables transport of multiple LO payloads in a single CEP packet, efficiency increases when large number of payloads go to the same destination.

Other alternatives: while CESoPSN and SAToP alternative technologies specialized in the delivery of E1 and DS3 rates, CEP is the only available technology with inherent support for STM-1, STM-4 and STM-16 rates. Therefore, in access networks and mobile backhauling applications there are few alternatives for circuit emulation, but for metro aggregation networks and MSPP alternatives CEP has clear advantages.

MPLS as infrastructure layer for TDM over packet services delivery

TDM circuits are carried over point to point MPLS connections (VPWS). When CEP packets are encapsulated into standard MPLS packets, forming a PW connection, MPLS PW becomes an efficient technique to transport TDM services over packet-based network. This enables TDM circuit provisioning, digital cross-connection and grooming between various SONET/SDH ports for both HO and LO granularity.