Multi-Service Access Nodes (MSANs) are also being deployed by some carriers. These
are DSLAMs with POTS voice capabilities??”able to connect directly to the customer
line, rather than in parallel with a class 5 voice switch and with a passive splitter directing
POTS voice to the switch and broadband DSL to the DSLAM.
Although, in some cases, carriers have opted to connect the DSLAM or MSAN directly
to the BRAS over a point-to-point GigE link, this is typically much more expensive than
deploying a layer of Ethernet aggregation between the DSLAM and BRAS??”especially
if the carrier is able to bypass the BRAS for voice and video traffic (known as a multiple
edge architecture??”see Figure 14.9). In the longer term, many carriers plan to replace
dedicated BRAS devices with MPLS-enabled service edge routers that incorporate a
subset of BRAS functionality, for example, using DHCP and 802.1x for session management
rather than PPPoE.
EoMPLS proves to be an ideal solution for Ethernet aggregation in the ???multiple
edge??? case. Layer 2 U-PE devices are typically deployed in rings to aggregate DSLAMs,
whereas Layer 3 capable N-PE devices are located at carrier head-end locations with
each ring terminated on two N-PEs (often in different locations).
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