Upstream rates under discussion fall into
three general ranges:
- 1.6-2.3 Mbps
- 19.2 Mbps
- Equal to downstream
Early versions of VDSL will almost certainly
incorporate the slower asymmetric rate. Higher upstream and
symmetric configurations may be possible only for very short lines.
Like ADSL, VDSL must transmit compressed video, a real-time signal
unsuited to error retransmission schemes used in data
communications. To achieve error rates compatible with those of
compressed video, VDSL will have to incorporate FEC with sufficient
interleaving to correct all errors created by impulsive noise events
of some specified duration. Interleaving introduces delay, on the
order of 40 times the maximum length correctable impulse.
Data in the downstream direction will be
broadcast to every CPE on the premises or be transmitted to a
logically separated hub that distributes data to addressed CPE based
on cell or TDM within the data stream itself. Upstream multiplexing
is more difficult. Systems using a passive network termination (NT)
must insert data onto a shared medium, either by a form of TDM
access (TDMA) or a form of FDM. TDMA may use a species of token
control called cell grants passed in the downstream direction from
the ONU modem, or contention, or both (contention for unrecognized
devices, cell grants for recognized devices). FDM gives each CPE its
own channel, obviating a MAC protocol, but either limiting data
rates available to any one CPE or requiring dynamic allocation of
bandwidth and inverse multiplexing at each CPE. Systems using active
NTs transfer the upstream collection problem to a logically
separated hub that (typically) uses Ethernet or ATM upstream
multiplexing.
Migration and inventory considerations dictate
VDSL units that can operate at various (preferably all) speeds, with
automatic recognition of a newly connected device to a line or to a
change in speed. Passive network interfaces need to have hot
insertion, whereas a new VDSL premises unit can be put on the line
without interfering with the operation of other modems.