Section 150-232-100
Revision 01
Page 5
3. FUNCTIONAL
OPERATION
3.01 The HLU-232 is capable of operating in three modes: 1)
normal operation as a standard HDSL line unit, 2)
primary unit operation in a “hot standby” backup system, and 3) protection unit operation in a “hot standby” backup
system.
3.02 In the normal mode of operation, the HLU-232 retains the operational features of the HLU-231. When
configured as the primary line unit of a “hot standby” backup system, the HLU-232 provides DS1 and HDSL
interfaces unless a primary unit failure occurs. When configured as the protection line unit of a “hot standby” backup
system, the HLU-232 monitors the operation of the primary line unit and provides a complete backup system (HLU,
HDU, HRU, and HDSL cable pairs) in case of primary unit failure.
3.03 The HiGain system with the protection switching feature provides automatic switchover to the protection circuit
interface. Switchover occurs in the case of service degradation, alarm condition, and major failure of the primary
circuit.
3.04 HiGain utilizes PairGain’s 2B1Q HDSL transceiver systems to establish two full-duplex 784 kbps data channels
between the HLU-232 and a remotely mounted HRU-412 HiGain Remote Unit. This provides a total capacity of
1.568 Mbps between the two units.
3.05 A block diagram of the HLU-232 is shown in Figure 3. The HiGain HLU-232 receives a 1.544 Mbps DS1 data
stream from the DSX-1 digital cross connect interface. The HLU contains a DS1 frame synchronizer controlled by an
8-bit micro controller that determines the type of framing on the DS1 stream and synchronizes to it. The HLU-232
recognizes SF or ESF framing. When the data is unframed, the HLU-232 arbitrarily defines a frame bit.
3.06 The HLU-232 contains a demultiplexer that generates two parallel 784 kbps data streams. The data streams
contain HDSL frames that are nominally 4704 bits (6 milliseconds) in length. The HDSL frames contain a 14-bit
Frame Sync Word (FSW), 6-bit Cyclic Redundancy Check (CRC), 21-bit operations channel and DS1 payload. The
DS1 stream is separated into two parallel streams that comprise the payloads of the HDSL channels. HDSL channel
1 contains the DS1 time slots 1 thru 12. HDSL channel 2 contains the DS1 time slots 13 thru 24. The 8 kbps frame
bits of the DS1 stream are included on both HDSL channels.
3.07 The two formatted HDSL channels are passed to the HDSL transceivers which convert them to 2B1Q format
on the HDSL lines. The 2B1Q line code is designed to operate in a full-duplex mode on unconditioned pairs. The
transceiver’s echo canceler and adaptive equalizer receive the signal from the remote end in the presence of
impairments and noise on the copper pairs.
3.08 The received HDSL channels are processed by the transceiver and then passed on to the HLU-232 multiplexer
module. The multiplexer provides frame synchronization for each of the two HDSL channels. The multiplexer and
HDSL transceivers work under control of the HLU-232 micro controller and compensate for data inversions caused
by tip-ring reversals and for channel swaps caused by pair reversals. The HiGain system allows for tip-ring or pair
reversals, but does not tolerate split pairs. By synchronizing to the Frame Sync Word (FSW) of each channel, the
multiplexer can reconstruct the original 1.544 Mbps DS1 stream from the payloads of the two HDSL channels. The
CRC fields on the HDSL streams allow the HLU-232 to determine if errors are present on the channel due to
excessive impairments on the HDSL pairs, or due to excessive impulse or crosstalk noise.
3.09 The multiplexer removes data link messages from the HDSL channels and passes them to the micro-
controller. This mechanism allows operations messages and status to be exchanged between the HLU-232 and the
HRU-412 remote unit.
