The advances include more
efficient single-line transport as well as a novel multi-pair aggregation strategy that
brings a new level of resiliency and bandwidth to the access network.
IEEE 802.3ah developed an encapsulation and loop aggregation technique for
Ethernet, one optimized for the copper access network. As shown in the architecture
diagram in Figure 5.1, the encapsulation and aggregation processes are transparent
to higher layer applications??”they sit below the Ethernet MAC. The switching and
services layer of the device can be consistent across optical, CAT5, and EFM Ethernet
interfaces, giving the provider the ability to offer a consistent service offering over any
type of access media.
2BASE-TL
2BASE-TL offers a nominal symmetric bandwidth of at least 2 Mbps in a typical noise
environment at reasonable distances. 2BASE-TL is based on the same physical layer
Copper 133
as the enhanced SHDSL standards of ITU and ANSI T1 (also known as G.991.2.bis or
E-SHDSL [2]). Whereas symmetric high-speed DSL (SHDSL, G.991.2) [3] has a maximum
symmetric rate of 2.3 Mbps, enhanced SHDSL can run up to 5.7 Mbps on a single
pair. With such high-speed symmetric access, subscribers can be offered a 10 Mbps
Ethernet service on as little as two-pair of copper access lines (the same lines that are
used for telephony services).
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