Comtech EF Data SDM-650B - Manual (Page 66)

Configuration SDM-650B Satellite Modem
3–24 Rev. 5
3.2.4 G.703, 2048 kbit/s Interface
3.2.4.1 Functional Description
The G.703 2048 kbit/s digital interface (Figure 3-8) provides level translation, buffering,
and termination between the internal modem signals and the G.703 DTE interface on the
rear panel.
G.703 N X 64KBITS SATELLITE MODEM INTERFACE ASSY/0679 REV-4 FSCN 4J515
SWITCH 1
Figure 3-8. G.703 2048 kbit/s Interface Card
The G.703 standard is defined in CCITT Volume III (red book).
Refer to Figure 3-9 for a functional block diagram of the G.703 2048 interface.
This interface meets G.703 specifications at 2.048 Mbit/s, normally referred to as the
“E1” rate. This interface is also unique in that no clock signals are provided.
The clock information is provided in the data signal through the data encoding method
used, AMI. In this method, a mark is transmitted as a positive or negative pulse, one-half
the width of the data-bit time. Each consecutive mark has the opposite sign of its
predecessor. Spaces are transmitted as an all-zero voltage.
Refer to Table 3-4 for line code selection and switch settings for the G.703 2048 kbit/s
interface.
In order to provide a clear channel (i.e., a channel in which an indefinite number of 0s
can be transmitted), HDB3 encoding and decoding is provided. This allows the modem to
interface with DTE utilizing HDB3 coding. HDB3 coding is enabled by an on-board
jumper (SW1-7).