1. Demystifying Optical Ethernet Networks
Monday, February 01 2010
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
technologiesand 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.

2. 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.

3. 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 PHYcomponents 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

4. 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 PHYcomponents 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).

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.

6. 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 vsharma@metanoia-
inc.com, Mr. Davari at davari@broadcom.com, or visit http://info.hotims.com/28050-201.