Using circuit emulation to sell TDM over GPONs

In fact, more than $25 billion worth of TDM services are sold every year worldwide to businesses of all sizes for leased-line or network-access applications. T1/E1 services still account for a substantial amount of operator revenue in 2005, and they will continue to do so for many years to come.

Therefore, it is important that new access technologies such as gigabit passive optical networking (GPON) be capable of delivering TDM services. GPON is a low-cost, high-performance broadband fiber optic technology that delivers the processing power needed for applications such as HDTV, VoIP and IPTV (see Fig. 1). GPON delivers unprecedented high bit rate support of up to 2.488 Gbit/s while enabling the transport of multiple services, specifically data and TDM, in native formats and with extremely high efficiency. In January 2003, GPON standards were ratified by the ITU-T and are known as ITU-T Recommendations G.984.1, G.984.2 and G.984.3.

An upswing of GPON deployment is rapidly approaching. Current forcasts predict that approximately 12.5 million GPON links will be deployed by 2008. Experts estimate that approximately 1 million of those links will be used to deliver TDM services.

Delivering the goods
To manage this key requirement for GPON's support of TDM, the Full Service Access Network (FSAN) Group, a forum of operators and vendors that worked to standardize GPON in ITU-T G.984, made TDM service transport a central element of the FSAN standard.

There are currently two alternatives for delivering TDM services over GPON. The first appears in ITU-T G.984, which defines a solution for transporting TDM over GPON using GPON Encapsulation Method (GEM). This mode is commonly referred to as native TDM over GPON Encapsulation Method, or as TDM over GEM (see Fig. 2).

Native TDM over GEM provides transport of unstructured TDM in the GPON network from the optical line termination (OLT) to the optical network unit (ONT). The TDM network is terminated at the OLT, and TDM circuits are recreated at the ONU.

Native TDM over GEM supports only the GPON part of the path traversed by the TDM service. A TDM service needs to be delivered to the OLT, and encapsulated using GEM for transport over the GPON network before it is finally delivered to the ONU. TDM services arrive through a TDM cross-connect into the OLT. Within the OLT, a TDM backplane carries the traffic onto dedicated TDM circuitry off of the OLT line card, where it is encapsulated into GEM.

With the adoption of IP and packet-based infrastructure as a ubiquitous unifying network, a second approach to delivering TDM services over packet networks is now feasible. This approach makes fewer assumptions about the underlying properties of the physical transport and is known as circuit emulation service.

What is CES?
CES is a straightforward tunneling technology derived from the pseudo-wire approach developed in the Internet Engineering Task Force (IETF) Pseudo-Wire Edge-to-Edge Emulation (PWE3) working group. Pseudo-wire was developed to support transport of non-Ethernet/IP traffic such as T1/E1, ATM and frame relay across packet networks. CES, specifically, is the technology used for transporting TDM and synchronization over Ethernet, IP and MPLS networks.

First made available to the market in 1998, CES was not standardized until the IETF made it part of the standardization efforts of the PWE3 working group. The work done in PWE3 formed the basis for parallel efforts in ITU-T, the Metro Ethernet Forum (MEF) and the MPLS Forum – all of which ratified standards and specifications for CES in 2004.

Currently, CES comprises two modes – the first is structure-agnostic and is referred to as Structure Agnostic TDM over Packet (SAToP). The second mode supports structured and fractional T1/E1 services and is referred to as CES over Packet Switched Networks (CESoPSN). The IETF is finalizing a third mode, referred to as circuit emulation over packet (CEP), to be used for transporting high rate TDM traffic including STM-1/OC-3.

CES already has been implemented in network solutions in metro Ethernet, DOCSIS hybrid fiber coax (HFC) and fixed wireless applications worldwide. It has gained industry-wide acceptance as a key element in the evolution of Ethernet, IP and MPLS networking infrastructure and the convergence of voice and data.

CES works by taking a TDM stream at the ingress to the packet network link, cutting it up into segments referred to as payloads. The relevant packet header (Ethernet, IP and/or MPLS) is added to each payload before it is transmitted over the packet network. At the far end of the packet link (the egress), the CES interworking function receives the incoming packets with the TDM payload, removes the headers, reassembles them in the correct sequence and releases the reconstructed TDM stream to the TDM user equipment connected to the CES-based device. CES also provides the ability to deliver the TDM clock over the packet network along with the TDM circuit to ensure synchronization of TDM equipment at both ends of the packet link according to G.823/G.824 jitter/wander standards.

The basic element of CES implementations, the pseudo-wire, functions as a point-to-point connection. CES implementations supporting multiple simultaneous pseudo-wires can then operate in multipoint (star) and multipoint-to-multipoint (mesh) topologies. CES sessions are available all the time and in the event of a link failure, are recreated extremely quickly.

CES over GPON
CES over GPON (CESoGPON) refers to the overlay of TDM services over a GPON. To understand how CES is used in conjunction with GPON, it is useful to consider a trivial example of using CES.

For example, Fig. 3 shows a T1/E1 running across a standard Ethernet switch connecting a PBX via standard T1/E1 interfaces to a CES gateway. In turn, the gateway is connected to the switch over standard CAT5 Ethernet cabling on both sides of the switch. In this implementation, CES can be used to transport standard telephony across an Ethernet switch. No special configuration of the network is required, nor is there a need for any additional gateways. There will also be no degradation in the quality of the voice experienced by the PBX users.

To create CESoGPON, all an operator needs to do is simply replace the switch with a GPON access network (see Fig.4).

The next step is examining how CESoGPON is implemented in a real GPON application.

On the ONU subscriber premises side, CES interworking functionality is integrated inside the ONU. The CES function directly interfaces with the G.984 MAC in the ONU via a standard Fast Ethernet or GigE interface.

On the central office side, CES interworking functions can be integrated either within the OLT, or located externally to the OLT in a separate device. When collocated outside the OLT, the CES gateway can be connected directly to the OLT if there is a TDM backhaul link immediately available to the OLT. Alternatively, the CES gateway can be located remotely from the OLT on the far side of a packet backhaul link such as Metro Ethernet.

If the CES interworking is integrated inside the OLT, it can be added as a pluggable card that plugs into an Ethernet backplane (see Fig. 5).

This integrated CES function in the OLT provides two services in GPON – aggregation of all the TDM services from the remote ONUs onto an OC-3/STM-1 link, and cross connect functionality to reduce the number of TDM uplinks to the backhaul.

Technology Challenges
CES performance is affected directly or indirectly by five main characteristics of the packet network:

• packet loss
• packet mis-order
• packet delay variation (PDV)
• packet network latency and PDV modulations.

GPON provides an ideal transport medium for CES in that packet loss and packet mis-order are either non-existent, or so negligible as to be completely unnoticeable in the context of CES.

PDV, also known as packet jitter, is a function of the scheduling performance of GPON, which is under the complete control of the GPON operator. CES is capable of complying with jitter/wander requirements in ITU-T G.823/G.824 at PDV levels as high as 20 ms. As a result, even PDVs of a few milliseconds in a GPON is not considered to be problematic for CES compliance with very strict jitter/wander requirements.

PDV modulations resulting from variations in traffic patterns in the GPON network, for example due to IP video streaming sessions being created and torn down, may have a potential effect on the performance of the clock recovery in CES. However, CES supports two mechanisms for clock recovery. The first is adaptive clock recovery, in which all the synchronization is based on timing information derived only from the CES packet stream. The second mechanism, differential timing, uses an out-of-stream clock source as a reference for clock recovery. In the case of GPON, a very accurate 8kHz clock source is provided over the GPON, and this can be used by CES to overcome any potential issues of synchronization in the event of significant PDV modulations.

CESoGPON introduces packet headers that consume bandwidth. Depending on the mode used, and the packet payload size, the bandwidth overhead is at least 4%. CES also requires a finite time to packetize and reassemble the TDM stream at the ingress and egress of the packet link. The end-to-end delay (not including the network latency introduced by GPON) can be reduced below 1 millisecond depending on the configuration of the CES and the GPON.

Benefits of CESoGPON
CESoGPON has the following key benefits:

• already standardized in IETF, ITU-T, MEF and MPLS Forum
• mature and deployed over other access infrastructures
• support for fractional T1/E1 as well as unstructured TDM services
• support of multi-operator leased line deployment
• reduced complexity and bill of materials in the OLT
• removes dependency on availability of TDM backhaul link for OLT
• scalable aggregation and cross-connect capacity
• enabler of end-to-end network convergence.

As mentioned above, CES was first tested in 1998 and it has been adopted in many market segments including metro Ethernet, DOCSIS access (HFC) and fixed wireless access, resulting in a broad based, tried and tested technology that is available now for GPON operators.

An additional benefit afforded by CESoGPON is its full support of fractional T1/E1 services up to single DS0 granularity. This is in addition to support for unstructured (clear channel) transport of Nx T1/E1 services where required. Fractional T1/E1 services comprise the majority of TDM services offered today and form an essential part of a service offering to commercial users.

Perhaps the most significant benefit of CESoGPON is its cost effectiveness. Eliminating costly TDM infrastructure simplifies the OLT architecture. Removing complexity drives down costs and enables fast time-to-market service delivery of TDM services to business users and non-TDM services to residential and commercial users, the vast majority of the target market for GPON today. Eliminating the need for a direct TDM link next to the OLT also has the advantage of broadening the GPON footprint beyond locations where availability of TDM capacity is available today.

CES also supports delivery of full, and fractional, leased line services from multiple operators over a single GPON due to the fact that is supports multiple clock domains and recovery. This provides GPON operators with the option of selling First Mile leased line infrastructure service to other operators.

Finally, CES is highly scalable and supports aggregation of many TDM bundles ( > 2000 for STM-1) at up to DS0 granularity for rates up to STM-1/OC-3. This enables operators to easily match the OLT TDM service capacity to the specific requirements of areas in which either businesses or residences predominate.

Summary
The telecommunications industry has invested more than forty years in building high performance TDM-based infrastructure for the transport of both voice and data applications. As result, more than $25 billion worth of TDM services are transported each year over that infrastructure. The gradual replacement of TDM infrastructure with packet-based infrastructure such as GPON, must support the continued huge demand for TDM services in a cost-effective, flexible way that blends in easily with the hybrid TDM-Ethernet telecommunication networks that will exist for many years to come.

CESoGPON provides a smooth and fast migration path for operators that are deploying GPONs to deliver both TDM-based commercial services and IP-based services over high performance fiber access networks and simultaneously reduce their cost of deploying the technology.

About the authors
David Brief is director of product architecture for Resolute Networks. David has been active in technology development throughout his career at the chip, module and system level in the fields of Ethernet, FDDI, Fast Ethernet, IEEE 1394, USB, wirespeed network processing and now Circuit Emulation. David is the holder of 18 US patents, as well as a BSEE from the University of Michigan - Ann Arbor and a CMA from Harvard University. David can be reached at davidb@resolutenetworks.com.

Gal Sitton is the Director of GPON development at BroadLight. Gal has been active in technology development throughout his career at the algorithm, architecture, chip and system level design in the fields of signal processing, network processing, DSL and Passive Optical Networks development. Gal is an active participant in the FSAN committee and he is the editor of the GPON Implementers Guide. Gal holds a BSc from the Technion University and an MSc from the Tel-Aviv University in Electrical Engineering.Gal can be reached at gal@broadlight.com.