| OTN - Ethernet Frames over Transport |
Ethernet is today the dominant LAN technology in the private and enterprise sector.
It is defined by a set of IEEE 802 standards. Emerging multi-protocol/multi-service
Ethernet services are also offered over public transport networks. Public Ethernet services
and Ethernet frames over transport standards and implementation agreements are being debated
in the ITU-T and other organizations. Specifically, the ITU-T SG15 is focused on developing
Recommendations related to the support and definition of Ethernet services over traditional
telecommunications transport, such as PDH, SDH, and OTN. Ethernet can be described in the
context of three major components: services aspects, network layer, and physical layer.
This description is meant to provide a brief overview of Public Ethernet considering each
of the above aspects.
The Public Ethernet services aspects (for service providers) include the different service
markets, topology options, and ownership models. Public Ethernet services are defined to
a large extent by the type(s) of topologies used and ownership models employed. The topology
options can be categorized by the three types of services they support: Line services,
LAN services and Access services. Line services are point-to-point in nature and include
services like Ethernet private and virtual lines. LAN services are multi-point-to-multi-point
(such as virtual LAN services).
Access services are of hub-and-spoke nature and enable single ISP/ASP to serve multiple, distinct, customers. (Due to the similar aspects from a public network perspective, Line and Access services may be essentially the same.)
The services can be provided with different service qualities. A circuit switched technology
like SDH provides always a guaranteed bit rate service while a packet switched technology
like MPLS can provide various service qualities from best effort traffic to a guaranteed
bit rate service. Ethernet services can be provided for the Ethernet MAC layer or Ethernet
physical layer.
The Ethernet network layer is the Ethernet MAC layer that provides end-to-end transmission of Ethernet MAC
frames between Ethernet end-points of individual services, identified by their MAC addresses. Ethernet MAC
layer services can be provided as Line, LAN and Access services over circuit switched technologies like SDH
VCs and OTN ODUs or over packet switched technologies like MPLS and RPR. For the Ethernet LAN service
Ethernet MAC bridging might be performed within the public transport network in order to forward the MAC
frames to the correct destination. Ethernet MAC services can be provided at any bit rate. They are not
bound to the physical data rates (i.e. 10 Mbit/s, 100 Mbit/s, 1 Gbit/s) defined by IEEE.
IEEE has defined a distinct set of physical layer data rates for Ethernet with a set of interface options (electrical or optical). An Ethernet physical layer service transports such signals transparently over a public transport network. Examples are the transport of a 10 Gbit/s Ethernet WAN signal over an OTN or the transport of a 1 Gbit/s Ethernet signal over SDH using transparent GFP mapping. Ethernet physical layer services are point-to-point only and are always at the standardized data rates. They are less flexible compared to Ethernet MAC layer services, but offer lower latencies.
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