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XRT79L71
PRELIMINARY
57
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
can be further divided into 8 blocks of 85 bits, with 84 of the 85 bits available for payload information and the
remaining one bit used for frame overhead.
Differences between the M13/M23 and C-bit Parity Frame Formats
The frame formats for M13/M23 and C-bit Parity are very similar. However, the main difference between these
two framing formats is in the use of the C-bits. In the M13/M23 Framing format, the C-bit reflect the status of
stuff-opportunities that either were or were not used while multiplexing the 7 DS2 signals into this DS3 signal.
If at least two of the three stuff bits, within an F-frame are set to "1", then the corresponding Si bit (not shown in
Figure 15) is designated as being a stuff bit. Conversely, if at least two of the three C-bits within an F-frame
are set to "0", then the corresponding Si is not interpreted as being a stuff bit. In the C-bit Parity framing
format, the C-bits take on different roles as is presented below in Table 12.
Definition of the DS3 Frame Overhead Bits
In general, the DS3 Frame Overhead (OH) bits serve the following three purposes.
1.
To support Frame Synchronization between the Near-End and remote DS3 Terminals
2.
To provide parity bits in order to facilitate performance monitoring and error detection within the DS3
data-stream.
3.
To support the transmission of Alarms, Status, and Data Link information between the Near-End and the
remote DS3 Terminals.
The DS3 Overhead bits supporting each of these purposes are further defined below.
Frame Synchronization Bits (Applies to both M13/M23 and C-bit Parity Framing Formats)
Each DS3 Frame or M-frame contains a total of 31 bits that support frame synchronization. Each DS3 M-frame
contains three (3) M-bits. According to Figure 14 and Figure 15, these M-bits reside within the very first bit-
fields within F-frames # 5, 6 and 7. These three bits appear in each M-frame with the repeating pattern of [0, 1,
0]. This fact is also presented in Figure 14 and Figure 15, in which these particular bit-fields are designated
as M0, M1 and M0, where M0 is an M-bit that is set to "0" and M1 is an M-bit that is set to "1".
Each of the seven (7) F-frames, within a DS3 M-frame contains four (4) F-bits, which also aid in
synchronization between the Near-End and remote DS3 terminals. Therefore, each DS3 M-frame consists of
a total of 28 F-bits. These F-bits exhibit a repeating pattern of [1, 0, 0, 1] within each F-frame. This fact is also
presented in Figure 14 and Figure 15 in which these particular bit-fields are designated as F0 and F1, where
F0 is an F-bit that is set to the value "0", and F1 is an F-bit that is set to the value "1".
Each of these bit-fields is used by the Receive DS3/E3 Framer block to perform Frame Acquisition and Frame
Maintenance functions. For more information on how the Receive DS3/E3 Framer block uses these bit-fields,
TABLE 12: C-BIT FUNCTIONS FOR THE DS3, C-BIT PARITY FRAMING FORMAT
C-BIT
FUNCTION OF C-BITS WHILE IN THE C-BIT PARITY FRAMING FORMAT
C11
AIC (C-Bit Parity Mode)
C12
NA (Reserved for Network Application)
C13
FEAC (Far End Alarm & Control)
C21, C22, C23
User Data Link (undefined for DS3 Frame)
C31, C32, C33
CP (Path) Parity Bits
C41, C42, C43
FEBE (Far End Block Error) Indicators
C51, C52, C53
Path Maintenance Data Link
C61, C62, C63, C71, C72, C73
User Data Link (undefined for DS3 Frame)