User guide

Canopy T1 / E1 Application Note
Issue 1, June 2005 13
There is a means of setting the converter cable’s “balun” to optionally ground (earth) the
RJ45 unused pins and shield. Motorola recommends that these unused pins and shield
be “grounded”, which is usually the factory default setting.
This leaves the difficulty of determining the “length” of a converter cable in the sense of
the TMUX configuration described in Section 5.4 above. Motorola has no explicit
recommendation in this regard, but suggests setting the length such that E1 performance
is maximized.
5.6 “Outside” T1/E1 Termination Device Clock Source Settings
In addition to knowing the “outside” T1/E1 device port modulation and framing type being
used it is important to know about that device’s “clock source”. The highest quality clocks
are called Stratum clocks. This clocking reference is attached to various circuit devices
via what is termed a “BITS” clock (Building Integrated Timing Systems). Such devices
are “precise enough” that other circuit devices, those without stratum clock input, can use
their T1/E1 circuit connection as a clock source.
Circuit devices on customer premises, because they do not have BITS availability, often
use such a central-office clocked T1/E1 as a reference clock source.
Consequently, it is important for the Canopy TMUX operator to know which “outside”
T1/E1 terminations can provide “clock” and which must derive “clock” from the TMUX
end.
The Canopy T1/E1 Multiplexer User Guide defines these two modes of operation:
• Loopback mode: the T1/E1 Transmit clock (transmit from the Canopy T1/E1 Multiplexer)
is derived from the incoming Receive clock on the T1/E1.
• Recovered mode: the T1/E1 Transmit clock (transmit from the Canopy T1/E1
Multiplexer) is derived from the far-end device transmit clock (the far end Canopy T1/E1
Multiplexer Receive clock). In this mode, a buffer is used to accommodate bursts of
Ethernet traffic. The buffer is, on average, maintained at a constant level. As the buffer
fills, the Transmit clock increases in frequency to avoid overflow. As the buffer empties,
the Transmit clock decreases in frequency in order to maintain a constant level.
Figure 7 above illustrates the TMUX GUI screen for setting these parameters and
assigning them to particular T1/E1 TMUX ports.
• Set the TMUX port facing a T1/E1 device having Stratum clock synchronization to
“loop timing”; this could be one or both TMUXs; these connections typically face
Central Office-based devices.
• Set the TMUX port facing a T1/E1 device expecting to receive synchronization to
“derived timing”; this is typically a CPE device; the other TMUX unit should be set to
“loop timing”.
Note the circumstance. A TMUX in “loop” mode receives clocking on a T1/E1; this could
be one or both TMUXs. A TMUX in “recovered” mode receives timing from across the
Canopy radio link, and employs that timing in the T1/E1 framing sense toward whatever
CPE T1/E1 devices on the far end.
In particular note that if the TMUXs are both configured to “recovered” clock then neither
is obtaining clock from the “outside” T1/E1 devices. This means that the TMUXs are
forced to operate independently of a precise clock source, meaning they operate on their
built-in TMUX clock that is not stratum compliant. Motorola recommends avoiding this
operation scenario by configuring the “Central Office end” TMUX to “loop” and the other
TMUX to “recovered”.