Comtech EF Data RCS10 - Manual (Page 221)

RCS10 Redundant Communication System Principles of Operation
TM058 - Rev. 2.3 Page 5-5
5.5.5.2 Enhanced Multiplexer Mode
Since many of the frame bits in the standard IBS mode are not used, an "Enhanced" multiplexer
mode has been implemented that can be engaged under software control. Since this mode
changes the use of many of the framed non-data bits, this mode is only usable when the DMD10 is
at both ends of a link. In this mode, the overhead signaling bytes 16 and 48 can be used to
implement a significantly higher speed ES to ES Data Channel under software control. When
implemented, this rate is 16 times that of the normal IBS standard, or 1/30
th of the terrestrial data
rate (1/32nd of the aggregate rate). In addition, spare bit 1 of byte 1 and bit 6 of Byte 32 are used
as flags indicating that enhanced asynchronous data is present.
NOTE: The Enhanced Async option requires that CLOSED NET mode must be selected as
the current mode. Since Enhanced Asynchronous operation does not conform to the IESS-
309 standard, it is not an IBS mode function.
The microprocessor on the interface board performs software/hardware assignment of bits to
specific purposes in the enhanced mode and buffers the ES to ES Data Channel to standard
asynchronous data rates.
The processor-controlled primary and secondary ES-ES channels each contain both transmit and
receive data buffers which are 32 bytes in length. These act as FIFOs on the data in each direction.
No data is lost if the actual ES-ES data channel and the buffered user data rates are different
unless the sustained user data rate causes the buffer to overflow. Buffer overflow results in purging
the entire buffer contents. Gaps between characters or messages do not use the buffer and thus
can be used to prevent overflow. If the user data rate is less than the data channel rate, then there
are simply gaps between characters, which is normal in asynchronous communications. The
Enhanced Asynchronous function is available in the following Drop and Insert modes only: PCM31
and PCM30C. All other Drop and Insert modes require signaling information to be carried in bytes
16 and 48 of the IBS frame.
5.6 Additional Clocking Data
SCTE: Serial Clock Transmit External
This clock is the transmit terrestrial clock associated with the interface. With the G.703 interface
selected SCTE is the clock that is recovered from the G.703 data stream. For synchronous
interfaces such as RS-422, SCTE is sometimes referred to as TT (Terminal Timing).
SCT: Serial Clock Transmit
This clock is an internally generated clock that is output from the modem. The clock is generally
used by the terrestrial terminal equipment for clocking the transmit data. The frequency of the clock
is set to be the same as the transmit terrestrial clock rate if internal is selected or is the receive
clock from the demodulator if SCR is selected.
INT CLK: Internal Clock
This clock is set to the Tx data rate and is used inside the modem to produce SCT clock.
SYSTEM EXT EXC: External Clock
This is an independent clock source. This clock is most often used if there is a station master
clock. The EXT EXC can be selected, in the interface/general menu, to be either balanced,
unbalanced, or IDI. IDI is used ONLY for Drop & Insert cases where external framing is selected. In
this case, the EXT EXC must be set to IDI where the receive buffer clock is derived from the
External Receive T1 or E1 trunk. You may select the location from which to retrieve the EXT EXC.
A system clock is available and the frequency is set in the Config. Switch Menu (See Section 3).