![](http://datasheet.mmic.net.cn/Exar-Corporation/XRT79L71IB-F_datasheet_100145/XRT79L71IB-F_274.png)
XRT79L71
PRELIMINARY
259
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
5.0
ARCHITECTURAL/FUNCTIONAL DESCRIPTION OF THE XRT 79L71 -
E3, ITU-T G.751 MODE
OPERATION
This particular section discusses Clear-Channel Framer over E3, ITU-T G.751 operation of the XRT79L71.
Prior to discussing the architecture and the role of the DS3/E3 Framer blocks, whenever the XRT79L71 has
been configured to operate in the E3, ITU-T G.751 Mode, it is imperative to discuss the E3, ITU-T G.751 Frame
structure.
5.1
DESCRIPTION OF THE E3, ITU-T G.751 FRAME STRUCTURE AND THE OVERHEAD BITS
The E3, ITU-T G.751 frame contains 1536 bits, of which 12 bits are overhead bits and the remaining 1524 bits
are payload bits.
Each E3 frame consists of the following twelve (12) overhead bits.
A 10-bit FAS (Framing Alignment Signal) pattern. This pattern is assigned the constant pattern of [1, 1, 1, 1,
0, 1, 0, 0, 0, 0], and is used by the Receive E3 Framer block to acquire and maintain Frame Synchronization
with the incoming E3 frame.
The A (or Alarm) Bit
The N (or National) Bit
The BIP-4 Bits (if configured)
The frame repetition rate for this type of E3 frame is 22,375 frames per second, thereby resulting in the
standard E3 bit rate of 34.368Mbps. Figure 113 presents an illustration of the E3, ITU-T G.751 Frame Format.
Configuring the XRT79L71 to operate in the E3, ITU-T G.751 Clear-Channel Framer Mode
The user can configure the XRT79L71 to operate in the E3, ITU-T G.751 Clear-Channel Framer Mode by
executing the following three steps.
STEP 1- Configure the XRT79L71 to operate in the Clear-Channel Framer/HDLC Controller Mode
This can be accomplished by setting Bit 0 (Configuration Control) within the Operation Control Register - Byte
3 to "1" as depicted below.
STEP 2 - Configure the XRT79L71 to operate in the Clear-Channel Framer Mode
FIGURE 113. ILLUSTRATION OF THE E3, ITU-T G.751 FRAMING FORMAT
Operation Control Register - Byte 3 (Address = 0x0100)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
Configura-
tion Control
R/O
R/W
0
1
Frame
Alignment
Signal
A
N
Data
BIP-4
if Selected
1
10
11
12
384 385
768
769
1152 1153
1532
1536
Framing Alignment Signal Pattern = 1111010000