XRT72L50 Datasheet, PDF (144/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
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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 and C-Bit Parity Frame Formats
The frame formats for M13 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 Format, the C-bits reflect the status of stuff-opportunities
that either were or were not used while multiplexing the 7 DS2 signals into this DS3 signal. If two of the three
stuff bits within a F-frame are "1", then the associated stuff bit, Si (not shown in Figure 29), is interpreted as
being a stuff bit. In the C-bit Parity framing format, the C bits take on different roles, as presented in Table 14.
TABLE 14: C-BIT FUNCTIONS FOR THE C-BIT PARITY DS3 FRAME FORMAT
C - BIT
C11
C12
C13
C21, C22, C23
C31, C32, C33
C41, C42, C43
C51, C52, C53
C61, C62, C63, C71, C72, C73
C-BIT FUNCTION IN THE C-BIT PARITY FRAMING FORMAT
AIC (C-Bit Parity Mode)
NA (Reserved for Network Application)
FEAC (Far End Alarm & Control)
(UDL) User Data Link (undefined for DS3 Frame)
CP (Path) Parity Bits
FEBE (Far End Block Error) Indicators
(DL) Path Maintenance Data Link
(UDL) User Data Link (undefined for DS3 Frame)
Definition of the DS3 Frame Overhead Bits
In general, the DS3 Frame Overhead Bits serve the following three purposes:
1. Support Frame Synchronization between the Local and Remote DS3 Terminals
2. Provide parity bits to facilitate performance monitoring and error detection.
3. Support the transmission of Alarms, Status and Data Link information to the Remote DS3 Terminal.
4.1.1 Frame Synchronization Bits (Applies to both M13 and C-bit Parity Framing Formats)
Each DS3 Frame (M-frame) contains a total of 31 bits that support frame synchronization. Each DS3 M-frame
contains three M-bits. According to Figure 28 and Figure 29, these M-bits are the first bits in F-frames 5, 6 and
7. These three bits appear in each M-frame with the repeating pattern of "010". This fact is also presented in
Figure 28 and Figure 29, which contains bit-fields that are designated as: M0, M1, and M0 (where M0 = "0",
and M1 = "1").
Each F-frame contains four F-bits, which also aid in synchronization between the Local and the remote DS3
terminals. Therefore, each DS3 M-frame consists of a total of 28 F-bits. These F-bits exhibit a repeating
pattern of "1001" within each F-frame. This fact is also presented in Figure 28 and Figure 29, which contains
bit-fields that are designated as: F1, F0, F0, and F1 (where F0 = "0", and F1 = "1").
Each of these bit-fields are used by the Receive DS3 Framer block within the remote terminal equipment to
perform Frame Acquisition and Frame Maintenance functions.
NOTE: For more information on how the Receive DS3 Framer uses these bit-fields, please see Section 4.3.2
4.1.2 Performance Monitoring/Error Detection Bits (Parity)
The DS3 Frame uses numerous bit fields to support performance monitoring of the transmission link between
the Local Transmitting Terminal and the Remote Receiving Terminal. The DS3 frame can contain two types of
parity bits, depending upon the framing format chosen. P-bits are available in both the M13 and C-bit Parity
Formats. However, the C-bit Parity format also includes additional CP-Parity bits.
P-Bits (Applies to both M13 and C-Bit Parity Frame Formats)
131