Demystifying Optical Ethernet Networks

Tuesday, January 26 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.

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).

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.