The TDM-to-packet network transformation has been underway in transport/ telecommunications networks for some years now, fueled primarily by two trends: (a) the advent of triple-play (voice, video, data) for enterprise and residential customers and, lately, the explosion in video and mobile data services, and (b) the evolution in both packet- and transport-network equipment.

Figure 1. Three key components of Optical Ethernet: service, transport, PHY, together with the technologies and standards organizations involved in specifying/developing each component.

In this regard, there have been rapid advancements to make packet technologies, such as IP and Ethernet, more “circuit-like”, and to make transport technologies and equipment more dynamic and, thus, “packet friendly.” These developments have led, over the last few years, to the emergence of a mélange of terms — “optical Ethernet”, “metro optical Ethernet”, “packet-optical transport”, “Carrier Ethernet”, “metro Ethernet” — which are often used interchangeably, blurring the distinction between them, and leading to confusion in industry circles. Our objective here is to define the terms optical Ethernet, Carrier Ethernet, and packet-optical transport, explain their relationships, and show how they all fit together in emerging optical Ethernet networks.

Versatile Packet Networking

Before defining the term “optical Ethernet,” it is useful to point out that the term “Ethernet” itself can apply to any one of the three roles of Ethernet technology: as a service, as a transport technology, and as a PHY layer (Figure 1).

Figure 2. Relationships of the different layers: service layer, transport layer, and PHY layer, and their corresponding entities.

An Ethernet service is offered to the end-customer (the enterprise or residential customer), runs end-to-end (customer premise-to-customer premise), and is one in which the traffic flow into/out of the system at the customer consists of Ethernet frames. An Ethernet service is thus the Ethernet connectivity between customer equipment. A carrier-grade Ethernet service is one that is scalable (to many MAC addresses and end points), offers QoS (traffic management), reliability (protection), and manageability (OAM and monitoring), and can span long distances (of MAN/WAN scope; typically tens to thousands of kilometers).

Ethernet transport refers to the ability to switch/route Ethernet frames (belonging to an Ethernet service) between network nodes, by setting up/using connection-oriented, traffic engineered paths in the network with deterministic performance (QoS, delay, jitter, loss, reliability). In other words, Ethernet transport refers to the setting up of the “pipe” through which the Ethernet frames travel, and to determining its routing within the cloud.

Ethernet transport makes it possible to realize connection-oriented Ethernet (COE). COE, in essence, refers to the collection of control-plane protocols and data-plane settings that create a connection- oriented capability for transferring the frames of an Ethernet service. We mention that Ethernet transport could be provided either by enhancing Ethernet technology (e.g. as is done in Provider Backbone Bridging with Traffic Engineering, PBB-TE, in the IEEE 802.1Qay standard) or by a different technology (e.g. using MPLS-TP technology being developed jointly by the IETF & the ITU-T). Both of these forms of transport involve switching/routing data frames and are, therefore, referred to as Layer 2 (or L2) transport.

It is also possible to embed Ethernet frames in a different transport networking layer, such as the one provided by the ITU-T’s G.709 OTN (Optical Transport Network) standard. This form of transport involves switching/routing traffic at the optical channel data unit (ODU) level and is, therefore, referred to as Layer 1 (or L1) transport.

Ethernet PHY refers to the framing and timing of the actual bits of the Ethernet frame, and their transmission over a physical medium — copper wire, coaxial cable, or optical fiber — to connect switches at the physical layer. Some common Ethernet PHYs are the 1 GE (IEEE 802.3z), 10 GE (IEEE 802.1ae), and 100 GE (IEEE 802.3ba) Ethernet PHYs. Note that Ethernet frames can also be embedded in other PHY framing standards, such as those in the ITU-T’s G.709 OTN (Optical Transport Network) standard.

Optical Ethernet Network

With this background, we may now define an Optical Ethernet Network as a network spanning a MAN/WAN that offers a carrier-grade Ethernet service, running over a connection-oriented Ethernet (COE) transport infrastructure over an optical PHY (Figure 2). The optical PHY could be provided either by the OTN’s optical channel (OCh), or by an Ethernet PHY running over optics, and may be multiplexed onto a given fiber using CWDM/DWDM technology.

A key characteristic of optical Ethernet is that its scope is beyond the enterprise LAN, and spans a metropolitan- area or wide-area network.

“Carrier Ethernet” vs Optical Ethernet

The term “Carrier Ethernet” was formalized by the work of the MEF (Metro Ethernet Forum) in the 2004-2005 time frame, which defines Carrier Ethernet as “a ubiquitous carrier-grade Ethernet service that has the following five attributes: standardized services, scalability, reliability/ protection, hard QoS, and service management.” The technical work of the MEF (as described in its specifications) together with the technical work of associated standards bodies (ITU-T, IEEE, IETF) enable the functionality and attributes of Carrier Ethernet.

The services defined by the MEF are in terms of an Ethernet Virtual Connection (EVC), which is defined as an association of two or more User Network Interfaces (UNIs) at the edge of a metro Ethernet network (MEN) cloud (i.e. subscriber sites), where the exchange of Ethernet service frames is limited to the UNI’s in the EVC. The MEF defines three standardized services: E-Line (a point-topoint EVC), E-LAN (a multipoint-to-multipoint EVC), and E-Tree (a point-to-multipoint “rooted” EVC, where the root(s) can communicate with any of the leaves, but the leaves must communicate with each other only via the root).

Figure 3. Optical Ethernet Network with the service, transport and PHY components in operation.

Scalability refers to a service that scales to millions of UNIs (end-points) and MAC addresses, spanning access, local, national, and global networks, with the ability to support a wide bandwidth granularity and versatile QoS options. Reliability refers to the ability to detect and recover from errors/faults without impacting customers, typically with rapid recovery times, as low as 50ms. Hard QoS implies providing end-to-end performance based on rates, frame loss, delay, and delay variation, and the ability to deliver SLAs that guarantee performance that matches the requirements of voice, video, and data traffic over heterogeneous converged networks. Service management implies having carrier-class OAM, and standards-based, vendor-independent implementations to monitor, diagnose, and manage networks offering Carrier Ethernet service.

Thus, we see that Carrier Ethernet comprises the service component of optical Ethernet networks (Figure 1, Figure 2, and Figure 5).

Packet-Optical Transport

Packet-optical transport systems (P-OTS or P-OTP) are a new class of networking platforms that combine the functions and features of SONET/SDH/OTN ADMs or cross-connects, Ethernet switching and aggregation systems, and WDM/ROADM transport systems into a single network element, thus providing “data-aware optical networking.”

A P-OTS network element typically will have ITU-T G.709 OTN support, a COE component, and support for WDM. These elements also offer transport of a wide range of client signals — Ethernet (dominant), legacy SONET/ SDH, SAN traffic, IP/ATM, video traffic, and can switch at the wavelength level Figure 3. Optical Ethernet Network with the service, transport and PHY components in operation. (WDM), sub-wavelength (or ODU) level, TDM level (SONET/SDH), and packet level (Ethernet, MPLS). A P-OTS network element enables a carrier, especially in the MAN/WAN, to quickly and cost-effectively change connectivity and bandwidth in the network, without knowing about the actual services.

Key architectural features of P-OTS elements are:

  • Universal switching architecture/fabric for switching traffic at different layers (OTN, TDM and packet)
  • Ability to switch, groom, and manage traffic in its native format (i.e. SONET/SDH traffic as TDM traffic, and IP or Ethernet traffic as packet traffic), thus, allowing for the percentage of each traffic type to vary dynamically (all Ethernet to all SONET/SDH and anything in between, for instance)
  • Software-selectable ports that can switch between switching SONET/ SDH to switching Ethernet, depending on the traffic

Even as this definition is gaining industry consensus, according to research firm Heavy Reading, there are three architectures that are currently deemed to fall under the packet optical transport umbrella, shown in Figure 4.

Thus, P-OTS platforms provide the transport and PHY components of optical Ethernet networks (Figure 5).

Optical Ethernet Applications

So which applications/services are optical Ethernet being used for (or envisaged for) today?

Figure 4. Packet-Optical Transport Systems (P-OTS): Architectures in use today.

As expected, it is the business or residential services with triple-play applications (voice, video, and data to the desktop), mobile backhaul applications (where the Ethernet PHY is used between the base-station and the first switching node, and regular optical Ethernet networks are used in the backhaul and backbone networks), and utility infrastructure networks (where oil, gas, water, and electric utilities are transforming their aged communication systems into “data-aware” systems that allow for automation of functions such as billing, monitoring, meter reading).

Applications such as software-as-a-service, VoIP, VoD, and hosted unified communications are driving demand, as are ICT trends such as virtualization, data center outsourcing, data replication, disaster recovery, remote backup, and IT outsourcing.

How It All Fits Together

Figure 5. Optical Ethernet: How it all fits.

Thus, we see that in the trio that are the components of optical Ethernet — service, transport and PHY — Carrier Ethernet provides the service component, packet-optical transport gear provides the transport and PHY component, and the various IETF, IEEE, and ITU-T standards provide the specifications for the PHY layers, as well as connection-oriented Ethernet (Figure 5).

As optical Ethernet evolves over the next few years, there will be further reduction in the layers leading to a fused Ethernet-WDM packet transport layer with circuit-like capabilities, and to packet- optical systems optimized for it. This allows the providers to handle increasing volume of data traffic, while reducing the number of network elements by using Ethernet as the common packet technology in access, aggregation, and core networks.

This article was written by Vishal Sharma, Ph.D., Principal Consultant & Technologist, Metanoia, Inc. (Mountain View, CA) and Shahram Davari, MASc, Associate Technical Director, Network Switching, Broadcom Corporation (San Jose, CA). For more information, contact Dr. Sharma at This email address is being protected from spambots. You need JavaScript enabled to view it., Mr. Davari at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/28050-201.


Photonics Tech Briefs Magazine

This article first appeared in the February, 2010 issue of Photonics Tech Briefs Magazine.

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