Metrodata FM4900, Руководство пользователя

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TECHNICAL OVERVIEW
7 76-02-011F
3 TECHNICAL OVERVIEW
The FM4900 is used on unframed G.703 digital services, and can be used in either unframed
G.703 or framed E3 G.751 modes. Technical overviews of G.703 and G.751 are provided.
3. 1 G.703 Signal Transmission
The signal is transmitted on 75 Ohm RG59 unbalanced coax. 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. 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 34
.
368 Mbps.
3. 2 G.751 Framing
Groups of 1524 bits are grouped into frames together with a 12-bit overhead at the start of
the frame. The frame length is therefore 1536 bits, and the frame repetition rate is 22
⋅
375
KHz. Data from the payload is presented to the HSSI DTE in bursted fashion. Bursted data
comprises a sequence of bits with a bit rate equal to the line rate, followed by a gap in the
data equivalent in length to the synchronisation pattern.
3. 3 Path Overhead
The 12 bits are used for path overhead and provide framing, alarm information, error
detection and management. They are used as follows:
Figure 3. 1 Path overhead
3. 4 Payload
The FM4900 operates with a DTE port data rate of 34
.
368 Mbps in unframed mode, or
34.099 Mbps in framed mode.
Bit No Function
1 to 10 Frame alignment signal (FAS) 1111010000
11 Remote alarm indication (RAI)
12 National bit