The MIB module defining objects for the management of a pair of
VDSL modems at each end of the VDSL line.  Each VDSL line has
an entry in an ifTable. 
        
An agent may reside at either end of the VDSL line however the
MIB is designed to require no management communication between
them beyond that inherent in the low-level VDSL line protocol.
The agent may monitor and control this protocol for its needs.
        
VDSL lines may support optional Fast or Interleaved channels.
If these are supported, additional entries corresponding to the
supported channels must be created in the ifTable. Thus a VDSL
line that supports both channels will have three entries in the
ifTable, one for each physical, fast, and interleaved, whose
ifType values are equal to vdsl(97), fast(125), and
interleaved(124), respectively. The ifStackTable is used to
represent the relationship between the entries.
        
Naming Conventions:
    Vtuc -- (VTUC) modem at near (Central) end of line
    Vtur -- (VTUR) modem at Remote end of line
    Vtu  -- One of either Vtuc or Vtur
    Curr -- Current
    Prev -- Previous
    Atn  -- Attenuation
    ES   -- Errored Second.
    LCS  -- Line Code Specific
    Lof  -- Loss of Frame
    Lol  -- Loss of Link
    Los  -- Loss of Signal
    Lpr  -- Loss of Power
    xyzs -- interval of Seconds in which xyz occurs
            (e.g., xyz=Lof, Los, Lpr)
    Max  -- Maximum
    Mgn  -- Margin
    Min  -- Minimum
    Psd  -- Power Spectral Density
    Snr  -- Signal to Noise Ratio
    Tx   -- Transmit
    Blks -- Blocks, a data unit, see cvdslVtuXChanCrcBlockLength