AT&T 2500 series, 7500 series, 7100 series, 7200 series, 7400 series, 7300 series manual D4 Framing

1-24MULTIPLEXED COMMUNICATION

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1 SUPERFRAME = 12 FRAMES

FT 1

FS

COMBINED F 1

FRAME #

F1

0

0

F2

0

0

F3

0

0

F4

	1.5 MS, 2316 BITS
1			0
	1			1
1			0		1
		1
F5		F6	F7		F8

1

1

F9

0

1

10

F10 F11

0

0

F12

1

1

1 FRAME

125 μSEC. 193 BITS

					TIME
		1		2	SLOT #		24
F	1 2 3 4 5 6 7 8		1 2 3 4 5 6 7 8		BIT #	1 2 3 4 5 6 7 8

FRAME		F	d1	d2	d3	d4	d5	d6	d7	d8
#							X	X		X
1		1	X	X	X	X			X
2		0	X	X	X	X	X	X	X	X
3		0	X	X	X	X	X	X	X	X
4		0	X	X	X	X	X	X	X	X
5		1	X	X	X	X	X	X	X	X
	6	1	X	X	X	X	X	X	X	A	A SIGNALING FRAME (OPTIONAL WITH ROBBED-BIT SIGNALING)
7		0	X	X	X	X	X	X	X	X
8		1	X	X	X	X	X	X	X	X
9		1	X	X	X	X	X	X	X	X
10		1	X	X	X	X	X	X	X	X
11		0	X	X	X	X	X	X	X	X
12		0	X	X	X	X	X	X	X	B	B SIGNALING FRAME (OPTIONAL WITH ROBBED-BIT SIGNALING)

Figure 1-7. D4 Framing

Framing

A frame is a set of 24 8-bit time slots grouped as a single transmission unit. Each DS1 frame has 192 bits (24 × 8), plus 1 bit, known as the framing bit, that is inserted at the beginning of each frame. Since each frame repeats time slots in the same sequence as previous frames, the time slots representing a single conversation or data stream form a channel.

Two framing formats are currently in use: the D4 frame and the extended super frame (ESF). The ESF format is preferred because of its superior reliability. However, either format may be chosen; the decision must depend on the network equipment and application.