The first G.703 interface is in Network Timed mode. The second is in Modem Timed mode. See Figure 1 above.
Timing of Application 2: Network Replacement
The first modem uses an internal timing source and supplies the timing for the entire system. The first G.703 recovers the timing and clocks the data into the FIFO. The Tx of the first modem takes its internal clock and sends it (TXCLK1) to the G.703 FIFO for data (TXD1). The Rx of the second modem recovers the clock and pres- ents a “jittery” clock and data to the second G.703’s transmitter. It has to smooth the clock before it uses the clock to transmit.
The Rx of the second G.703 recovers the timing and clocks the data into a FIFO. The Tx of the second modem sends the second modem’s recovered clock (TXCLK1) to the G.703 FIFO for data (TXD1). The Rx of the first modem uses its internal clock (RXCLK1) to send data (RXD1) into the FIFO. The Tx of the first G.703 smooths the first modem’s recovered timing and sends it to the FIFO for data.
Both G.703 interfaces are in Modem Timed mode. See Figure 2 above.
2.4 TIMING MODE SELECTIONS
Based on the timing arrangements and clock sources mentioned above, there are two timing modes in which the interface can work. These two modes select the clock sources mentioned above and determine which clock gets smoothed.
Network Timed - This sets the interface to pass the smoothed recovered timing to the modem as XCLK1, with the Rx data as TXD1, and also to the G.703 Tx side.
Modem Timed - This sets the interface to use the modem’s Tx timing (recovered or internal source) to send the Rx data (as TXD1) to the modem and to smooth the modem’s recovered timing for transmit- ting on the G.703 Tx side.
RX
TX
CSU
DTE
G.703 INTF- | G.703 TX | HOST | |
MODEM | SMOOTHS | MODEM - | |
RX MODEM | RECOVERED | ||
TIMED MODE | |||
CLOCK | TIMING | ||
|
| TX | FIFO | RX |
G.703 | RX | FIFO | TX |
|
|
|
SECOND MODEM
G.703 TX SMOOTHS
RX MODEM CLOCK
In both cases, the transmitter uses the smoothed clock.
2.5 OPERATING MODE SELECTIONS
There are two data modes in which the interface passes data and timing:
Octet Mode - Data is passed at a 128K rate. This mode pre- serves the byte integrity associated with Octet timing. The Octet tim- ing frame is embedded in the data.
Clear Channel Mode - Data is passed at a 64K rate. This does not preserve the byte integrity associated with Octet timing. Instead, an Octet timing alarm (on or off) is passed over the modem similar to
RX
TX
G.703
TX | FIFO |
RX | FIFO |
SMOOTHED
RX
TX
2 or 4 Wire
the way signaling leads are passed.
CSU DTE
G.703 INTF- FIRST MODEM | HOST |
MODEM - | |
MODEM | INTERNAL |
TIMED MODE | TIMING |
Figure 2. Network Replacement Configuration
5 | 6 |
