Specifications

Watson TDM
Operating Manual
Watson-TDM-Manual-W.doc
Version 2.0-01
3-10
Revision: 2008-02-07
3.3 ISDN PRA Interface
In PRA mode, the DSL modem offers the functions of an ISDN PRA NT1, an LT
or a combination of the functions of NT1 and LT. This allows for the following
configurations:
• The modem at the customer premises is configured as NT1, the modem at
the central office is configured as LT.
• The modem at the customer premises is configured as NT1 & LT. Direct ac-
cess to the exchange is established by a bit-transparent DSL-Link. All data,
including timeslot 0, from the exchange must be transmitted transparently to
the customer modem, therefore the DSL equipment providing the line termi-
nation in the exchange is working in transparent mode (see Figure 3-9).
NT2/TE NT1 LT
DSL
Trans-
ceiver
DSL
Trans-
ceiver
ET
T V3 V3'
DSL Link
Figure 3-9: Reference Points of the PRA
The E1 interface of the DSL modem is the user/network-interface for primary rate
access (T reference point in ISDN terminology). The equipment at the user side
of the T reference point, which may be TE1, TA or NT2, is termed NT2/TE. The
interface towards the exchange (ET function) is the V3 reference point.
3.3.1 CRC4 Processing Options
In addition to the usual PRA operation with CRC4 processing in both directions,
the PRA interface also offers other modes of operation. Following ITU-T Rec.
I.604 Annex A, three subscriber access options for a digital link are supported.
The PRA interface can be configured to work either as a digital link without CRC
processing (option 1), a digital link with CRC processing in the NT1 (option 2) or
a digital link with CRC monitoring only in the NT1 (option 4). A digital link with
CRC processing in the LT and NT1 is possible by using a DSL link consisting of
an NT1 and an LT, both configured with CRC processing on.
Digital Link without CRC Processing (Option 1)
In this mode, transparent transmission between the ET and the NT2/TE is possi-
ble. There is no CRC4 processing in the DSL modems; the CRC processing is
only done in the ET and the NT2/TE.
When loss of incoming signal is detected on either side, AIS is transmitted to the
opposite side.
The detection of events and the state information are still valid as in normal PRA
operation mode (option 2).