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VDSL is still in the
definition stage; some preliminary products exist, but not enough is
known yet about telephone line characteristics, radio frequency
interface emissions and susceptibility, upstream multiplexing
protocols, and information requirements to frame a set of
definitive, standardizable properties. One large unknown is the
maximum distance that VDSL can reliably realize for a given data
rate. This is unknown because real line characteristics at the
frequencies required for VDSL are speculative, and items such as
short bridged taps or unterminated extension lines in homes, which
have no effect on telephony, ISDN, or ADSL, may have very
detrimental affects on VDSL in certain configurations. Furthermore,
VDSL invades the frequency ranges of amateur radio, and every
above-ground telephone wire is an antenna that both radiates and
attracts energy in amateur radio bands. Balancing low signal levels
to prevent emissions that interfere with amateur radio with higher
signals needed to combat interference by amateur radio could be the
dominant factor in determining line reach.
A second dimension of VDSL that is
far from clear is the services environment. It can be assumed that
VDSL will carry information in ATM cell format for video and
asymmetric data communications, although optimum downstream and
upstream data rates have not been ascertained. What is more
difficult to assess is the need for VDSL to carry information in
non-ATM formats (such as conventional Plesiochronous Digital
Hierarchy [PDH] structures) and the need for symmetric channels at
broadband rates (above T1/E1). VDSL will not be completely
independent of upper-layer protocols, particularly in the upstream
direction, where multiplexing data from more than one CPE may
require knowledge of link-layer formats (that is, ATM or not).
A third difficult subject is premises
distribution and the interface between the telephone network and CPE.
Cost considerations favor a passive network interface with premises
VDSL installed in CPE and upstream multiplexing handled similarly to
LAN buses. System management, reliability, regulatory constraints,
and migration favor an active network termination, just like ADSL
and ISDN, that can operate like a hub, with point-to-point or
shared-media distribution to multiple CPE on-premises wiring that is
independent and physically isolated from network wiring.
However, costs cannot be ignored. Small ONUs
must spread common equipment costs, such as fiber links, interfaces,
and equipment cabinets, over a small number of subscribers compared
to HFC. VDSL, therefore, has a much lower cost target than ADSL
because VDSL may connect directly from a wiring center or cable
modems, which also have much lower common equipment costs per user.
Furthermore, VDSL for passive NTs may (only may) be more expensive
than VDSL for active NTs, but the elimination of any other premises
network electronics may make it the most cost-effective solution,
and highly desired, despite the obvious benefits of an active NT.
Stay tuned.
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