Verilink PRISM 4051 (34-00253.2) Product Manual - Manual (Page 25)

Front Panel Testing 19
V.54 loops take place at the DTE data port
interface and are activated upon receipt of
in-band V.54 loop codes in the network receive
data stream. V.54 loops are either unidirectional
(Figure 3-2) and bidirectional (Figure 3-3) Unidirectional loops return the DSU
receive data to the DSU transmit data, and then to the DDS transmit data. Receive
data is unaffected and DSR and DCD are optionally forced Off. In the bidirectional
line loop, network receive data is looped back to the network as network transmit
data. DTE transmit data is looped back through the data port as receive data to the
DTE.
When the test switch is returned to the center
position, the 4051 sends five seconds of V.54
loop down code and then returns to its normal
operating mode.
When the Test switch is in the LOC
position, the 4051 performs a network
LLB as shown in Figure 3-4, and the
LOOP LEDs light.
The Local loop is bidirectional or unidirectional and takes place at the DDS
network interface. It returns the DDS receive data to the DDS transmit line and the
DSU transmit data to the DSU receive data output (see Figure 3-4).
For the Line Loop, Data Loop, and V.54 Loop, the user has a choice of what the
receive data sent to the DTE is. This is determined by the setting of the Loop
Mode option. When set to Unidirectional, the remotely activated loops behave as
follows: The network receive data loops back to the network as network transmit
data and continues to pass through the data port to the DTE. Transmit data from
the DTE is terminated.
When set to Bidirectional, remotely activated loops behave as follows: The
network receive data is looped back to the network as network transmit data.
Transmit data from the DTE is looped back through the data port as receive data to
the DTE.
In addition to activating a local loop, the user may also instruct the 4051 to
transmit in-band V.54 loop code to the remote-end unit, causing it to enter a V.54
loop as described above.
The 4051 does not transmit alternating DSU loop code, DSU latch, DSU unlatch, or
cause sealing current reversal to activate a loop on the remote-end unit.
The test switch is also used when upgrading the 4051 software (see Upgrading
Software on page 20).
DTE Network
Figure 3-2 Unidirectional V.54 Loop
DTE Network
Figure 3-3 Bidirectional V.54 Loop
DTE Network
Figure 3-4 Local Loop
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