XRT72L50 Datasheet, PDF (353/471 Pages) Exar Corporation – SINGLE CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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XRT72L50
SINGLE CHANNEL DS3/E3 FRAMER WITH HDLC CONTROLLER
REV. 1.2.1
Framer Operating Mode Register (Address = 0x00)
BIT 7
BIT 6
Local Loop-back DS3/E3*
R/W
R/W
x
1
BIT 5
Internal LOS
Enable
R/W
x
BIT 4
RESET
R/W
0
BIT 3
BIT2
Interrupt Frame Format
Enable Reset
R/W
R/W
x
x
BIT 1
BIT 0
TimRefSel[1:0]
R/W
R/W
x
x
6.1.1.1 Frame Alignment (FA1 and FA2) Bytes
FA1 and FA2 are known as the frame alignment bytes. The Receive E3 Framer, while trying to acquire or
maintain framing synchronization with its incoming E3 frames, will attempt to locate these two bytes. FA1 is
assigned the value, 0xF6, and FA2 is assigned the value, 0x28.
6.1.1.2 Error Monitor (EM) Byte
The EM byte contains the results of BIP-8 (Bit-Interleaved Parity) calculations over an entire E3 frame. The Bit
Interleaved Parity (BIP-8) byte field supports error detection, during the transmission of E3 frames, between
the Local Terminal Equipment and the Remote Terminal Equipment.
The Transmit E3 Framer will compute the BIP-8 value over the 537 octet structure, within each E3 frame. The
resulting BIP-8 value is then inserted into the EM byte-field within the very next E3 frame. BIP-8 is an eight bit
code in which the nth bit of the BIP-8 code reflects the even-parity bit calculated with the nth bit of each of the
537 octets within the E3 frame. Thus, the BIP-8 value presents the results for 8 separate even-bit parity
calculations.
The Receive E3 Framer will compute its own version of the EM bytes for each E3 frame that it receives.
Afterwards, it will compare the value of its locally computed EM byte with the EM byte that it receives in the
very next E3 frame. If the two EM byte values are equal, then the Receive E3 Framer will conclude that this E3
frame was received in an error-free manner. Further, the Receive E3 Framer block will inform the Remote
Terminal Equipment of this fact by having the Local Terminal Equipment set the FEBE (Far-End-Block Error)
bit, within the MA Byte of an Outbound E3 frame (to the Remote Terminal Equipment) to “0”. Please see
Section 6.1.1.4 for a discussion of the MA Byte.
However, if the Receive E3 Framer block detects an error in the incoming EM byte, then it will conclude that the
corresponding E3 frame is errored. Further, the Receive E3 Framer block will inform the Remote Terminal
(e.g., the source of this erred E3 frame) of this fact by having the Local Terminal Equipment (e.g., the Transmit
E3 Framer block) set the FEBE bit, within an Outbound E3 frame (destined to the Remote Terminal) to “1”.
NOTE: A detailed discussion on the practical use of the EM byte is presented in Section 6.3.2.7.
6.1.1.3 The Trail-Trace Buffer (TTB) Byte
This byte-field is used to repetitively transmit a Trail-access point identifier so that a trail receiving terminal can
verify its continued connection to the intended transmitter. The trail access point identifier uses the 16-byte
numbering format as tabulated in Table 65.
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