INTRODUCTION
3
reasonable protection against harmful interference when the equipment is operated in a
commercial environment. This equipment generates, uses, and can radiate radio frequency
energy and, if not installed and used in accordance with the instruction manual, may cause
harmful interference to radiocommunications. Operation of this equipment in a residential
area is likely to cause harmful interference in which case the user will be required to correct
the interference at its own expense.
1. 8 Technical Overview
The DC3X00 is used on unframed E1/G.703, framed E1/G.704 (CRC4) or
E1/G.704 (no CRC4
)
digital services. Technical overviews of G.703 and G.704 are provided.
1 . 8. 1 G.703 Signal Transmission
The signal is transmitted on 75 ohm unbalanced coax or 120 ohm balanced twisted pair. The
signal has alternate mark inversion (AMI) characteristics in accordance with G.703. A mark
is transmitted as a 0.5 unit interval (UI) wide pulse of amplitude 2.37V on 75 ohm coax, or
3.0V on 120 ohm twisted pair. Alternate marks have opposing polarity so that '111' is
transmitted as a positive pulse, a negative one and then another positive one. The pulses
have a duration of 50% so that strings of '1s' can be identified as a series of pulses. This is
because clocking information is derived from the transmitted signal. In addition, strings of
zeros are replaced with high-density binary 3 (HDB3) code words to ensure pulse density
(and therefore clocking information) and an average DC potential of 0V.
The transmission rate is 2.048 Mbps. The worst case delay through the DC3X00 is 2
milliseconds, and the worst case round trip delay is 8 milliseconds.
1 . 8. 2 G.704 Framing
Groups of 248 bits are grouped into frames together with an 8-bit overhead at the start of the
frame called
TIME SLOT 0
(TS0). The frame length is therefore 256 bits, and the frame
repetition rate is 8KHz. The 248 bits of payload are divided into 31 timeslots of 8 bits each
(TS1 - TS31). With the data in each timeslot regarded as an individual channel, 31 channels
may be multiplexed together into one E1 trunk. As well as dividing the trunk between payload
and overhead, groups of frames are associated into multi-frames. A synchronisation pattern
is spread across the multiframe. Frames are alternately
FAS
(Frame Alignment Signal
)
and
NFAS
(Non Frame Alignment Signal) frames
.
