XRT72L50 Datasheet, PDF (415/471 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
FIGURE 183. ILLUSTRATION OF THE E3, ITU-T G.832 FRAMING FORMAT
FA1
FA2
EM
TR
MA
NR
GC
60 Columns
530 Octet Payload
9 Rows
1 Byte
59 Bytes
When the Receive E3 Framer block detects the FA1 octet, and determines that this octet is immediately
followed by the FA2 octet, then it will transition to the FA1, FA2 Octet Verification state, per Figure 183.
The FA1, FA2 Octet Verification State
Once the Receive E3 Framer block has detected an 0xF628 pattern (e.g., the concatenation of the FA1 and
FA2 octets), it must verify that this pattern is indeed the FA1 and FA2 octets and not some other set of bytes,
within the E3 frame, mimicking the Frame Alignment bytes. Hence, the purpose of the FA1, FA2 Octet
Verification state.
When the Receive E3 Framer block enters this state, it will then quit performing its bit-by-bit search for the
Frame Alignment bytes. Instead, the Receive E3 Framer block will read in the two octets that occur 537 bytes
(e.g., one E3 frame period later) after the candidate Frame Alignment patterns were first detected. If these two
bytes match the assigned values for the FA1 and FA2 octets, then the Receive E3 Framer block will conclude
that it has found the Frame Alignment bytes and will then transition to the In-Frame state. However, if these two
bytes do not match the assigned values for the FA1 and FA2 octets then the Receive E3 Framer block will
concluded that it has been fooled by data mimicking the Frame Alignment bytes, and will transition back to the
FA1, FA2 Octet Search state.
In Frame State
Once the Receive E3 Framer block enters the In-Frame state, then it will cease performing Frame Acquisition
functions, and will proceed to perform Framing Maintenance functions. Therefore, the operation of the Receive
E3 Framer block, while operating in the In-Frame state, can be found in Section 5.3.2.2 (The Framing
Maintenance Mode).
OOF (Out of Frame) Condition State
If the Receive E3 Framer while operating in the In-Frame state detects four (4) consecutive frames, which do
not have the valid Frame Alignment (FA1 and FA2 octet) patterns, then it will transition into the OOF Condition
State. The Receive E3 Framer block’s operation, while in the OOF condition state is a unique mix of Framing
Maintenance and Framing Acquisition operation. The Receive E3 Framer block will exhibit some Framing
Acquisition characteristics by attempting to locate (once again) the Frame Alignment octets. However, the
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