Datum Systems M7 Series - Data Interface Clock Options

Maintenance M7/M7L/M7LT Modular Satellite Modem
3-96 M7/M7L/M7LT - Rev. 0.05
3.8.3. G7 – Dual G.703/E1 Interface
3.8.3.1. E1 Signal Structure
Each port of the Dual G.703/E1 interface operates at a nominal rate of 2.048Mbps and adds 3975 bits
of overhead (<0.2%) to the data rate transmitted via the modulator. This overhead is needed to allow
for independent operation of the two (2) G.703/E1 ports. This overhead is independent of any
additional overhead that is added as a result of the MCC or ESC channels baud rate.
When the G7 is the active interface, selection of the Data Rate Entry Mode in the Modulator and the
Demodulator is automatically set to Interface; <Mod: Data - Rate Entry Mode> = Interface, <Dmd:
Data - Rate Entry Mode> = Interface
3.8.3.2. E1 Line Coding
The basic E1 signal is coded using the Alternate Mark Inversion (AMI) or High-Density Bipolar 3
(HDB3). In AMI coding, “ones” are alternately transmitted as positive and negative pulses, whereas
“zeros” are transmitted as a zero voltage level. AMI is not used in most E1 transmissions because
synchronization loss will occur during long strings of data zeros. In HDB3 coding, a string of four
consecutive zeros is replaced with a substitute string of pulses containing an intentional bipolar
violation. The HDB3 code substitutions provide high pulse density so that the receiving equipment is
able to maintain synchronization with the received signal.
The G7 supports two E1 link line codes:
 AMI coding
 HDB3 coding.
3.8.3.3. Other Interface Specifications
 Balanced: 120 ohm impedance on a RJ48c (Port 1 and Port 2)
 Unbalanced: 75 ohm impedance using J23 and J24 for Port 1 and a connector adapter for Port 2.
 Jitter performance is in according to ITU G.823
3.8.3.4. Alarm Indication Signal (AIS)
The AIS signal is an unframed “all-ones” signal, and is used to maintain line signal synchronization in
case of loss of input signal. The G7 interface will provide an AIS on the receive data line if there is a
fault condition in the demodulator. There will also be an indication on the G7 interface if there is an
AIS condition received on the transmit data line.
3.9. Data Interface Clock Options
The active interface type will determine availability of the bit timing sources. When the Synchronous
Interface is selected as the active interface, the Modulator and the Demodulator each have four (4)
possible sources for bit rate timing. These clock sources may be used in various ways in a system
implementation to provide correct timing at a destination. The network configuration will determine the
requirements for bit rate timing in the modem.
The options for modulator bit timing (Section 0) and demodulator bit timing (section 0) are shown
below. Descriptions of two (2) typical network timing configurations are explained in Section 3.9.3 and
Section 3.9.4.