Provisioning LTPH-UM-1049-02, Issue 2
26 January 9, 2002 H2TU-C-319 List 4E
Fractional T1 (FT1) Option. Fractional T1 circuits can be used in feeder networks to provide frame relay
service. If such circuits are maintained by a DDS test group, then these circuits must respond to DDS/DS0 latching
loopback commands, the only tool test groups have at their disposal. A latching loopback, once it has been
initiated by the correct sequence, remains locked or “latched” until the correct loopdown sequence has been
detected.
The FT1 option, when enabled, allows the H2TU-C to respond to DS0 latching loopback commands and therefore,
supports fractional T1 frame-relay applications. This is in addition to the standard full bandwidth T1 2-in-5 loopup
and 3-in-5 loopdown SmartJack commands. FT1 supports both the full T1 enable commands and the new DDS
latching loopback commands, which must also be enabled. (For more information about latching loopback
commands, refer to BellCore TA-TSY-000077, Issue 3, April 1986.)
The FT1 option supports both the DDS NI and DDS DS0 Data Port (DP) latching loopback sequences listed in
Table11.
The sequences in Table11 are sent in time slot 1. The S in the Network Code column is a “don’t care” bit. The
loopback is activated after the detection of Sequence 6. Upon completion of the enable sequence, the Test Center
continues to transmit FEV bytes in multiples of 20 until FEV confirmation bytes are returned or until about 2
seconds have elapsed. If the confirmation bytes are not received, a failed attempt is reported. Anticipate the
minimum number of bytes when the loopback code is transmitted by a preprogrammed machine test pattern
generator. This minimum number of bytes will most likely be exceeded when the codes are sent manually. Also,
manual testing may inject random data signals between valid control sequences. The detection algorithm ignores
these occurrences and only responds to valid control codes.
Table12 lis ts the relationship between the latching loopback sequences and the specific HiGain HDSL2 loopback
they initiate.
The NLOC loopback is called tandem DS0 DP loopback. It is used to select one of many loopback points when
there are several identical data ports in tandem. The NI loopback is assigned to the H2TU-R (NREM) since it is
usually located at the Network Interface (NI). The DS0 DP tandem loopback is assigned to the rest of the HiGain
HDSL2 loopbacks because most DDS test sets support this tandem command set.
The Test Center transmits a group of 40 TIP bytes to loop down the loopback. It continues to transmit TIP bytes
in multiples of 20 until the TIP bytes are not returned or until about 2 seconds have elapsed. If the bytes are absent,
Table 11. DDS NI and DS0 DP Latching Loopback Sequence
Enable Sequence Minimum
Number of Bytes Byte Name Network Code
1 35 Transition in Progress (TIP) S0111010
2A 35 NI Loop Select Code (LSC) S1000001
2B 35 DS0 DP Loop Select Code (LSC) S0000101
3 100 Loopback Enable Code (LBE) S1010110
4 35 All Ones S1111111
5 100 LBE S1010110
6 32 Far End Voice (FEV) S1011010
Disable/Loopdown 35 TIP S0111010
Table 12. HiGain HDSL2 Loopback vs. Latching Sequence
HiGain HDSL2 Loopback Latching Sequence
NREM 1, 2A, 3, 6
NLOC 1, 2B, 3, 6