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Chapter 10
Synchronous Ethernet Analysis
xGenius may optionally support the ITU-T defined Synchronous Ethernet standard
which enables these equipments qualify Synchronous Ethernet network equipment or
to generate Ethernet signals synchronized to various timing sources, including GPS
and TDM.
10.1.Introduction to Synchronous Ethernet
Synchronous Ethernet is an ITU-T standard that provides mechanisms to transfer
frequency over the Ethernet physical layer, which can then be made traceable to an
external source such as a network clock. As such, the Ethernet link may be used and
considered part of the synchronization network.
The proposal to specify the transport of a reference clock over Ethernet links was
brought by operators to ITU-T Study Group 15 in September 2004. The aim of
Synchronous Ethernet is to avoid changes to the existing IEEE Ethernet, but to extend
it working within its protocol definitions.
Despite being an IEEE standard, Ethernet architecture has been described in ITU-T
G.8010 as a network made up of an ETH layer and a ETY layer. Put in simple terms,
the ETY layer corresponds the physical layer as defined in IEEE 802.3, while the ETH
layer represents the pure packet layer. Ethernet MAC frames at the ETH layer are
carried as a client of the ETY layer. In OSI terminology, ETY is layer 1, ETH layer 2.
Synchronous Ethernet is based on the ITU-T G.8010 description of the Ethernet
architecture.
A key topic in Synchronous Ethernet is the definition of the mechanisms necessary to
achieve interworking between SDH / SONET and Synchronous Ethernet equipment.
These mechanisms and procedures are found fundamentally in three different
recommendations: ITU-T G.8261, G.8262 and G.8264. The aspects covered there
include the following:
• Extension of the synchronization network to include Ethernet as a building block
(ITU-T G.8261). This enables Synchronous Ethernet network equipment to be
connected to the same synchronization network that SDH / SONET. Synchroniza-
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