2010

Demystifying Optical Ethernet Networks


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).

altScalability 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 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.