Rainbow Electronics DS2180A - Manual (Page 25)

DS2180A
041995 25/36
193S FRAMING FORMAT Table 8
FRAME
NUMBER
F-BIT USE BIT USE IN EACH CHANNEL
SIGNALING-BIT USE
NUMBER
F
T
1 F
S
2 DATA SIGNALING 4
1
2
3
4
5
6
7
8
9
10
11
12
1
–
0
–
1
–
0
–
1
–
0
–
–
0
–
0
–
1
–
1
–
1
–
0 3
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–7
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–8
BITS 1–7
BIT 8
BIT 8
A
B
NOTES:
1. F
T
(terminal framing) bits provide frame alignment information.
2. F
S
(signaling frame) bits provide multiframe alignment information.
3. The S-bit in frame 12 may be used for yellow alarm transmission and detection in some applications.
4. The user may program any individual channels clear, in which case bit 8 will be used for data, not signaling.
Line Coding
T1 line data is transmitted in a bipolar alternative mark
inversion line format; 1’s are transmitted as alternating
negative and positive pulses and 0’s are simply the ab-
sence of pulses. This technique minimizes DC voltage
on the T1 span and allows clock to be extracted from
data. The network currently has a 1’s density constraint
to keep clock extraction circuitry functioning which is
usually met by forcing bit 7 of any channel consisting of
all 0’s to 1. The use of Bipolar Eight Zero Substitution
(B8ZS) satisfies all the 1’s density requirement while al-
lowing data traffic to be transmitted without corruption.
This feature is known as clear channel and is explained
more completely in ATT C.B. #144. When the B8ZS fea-
ture is enabled, any outgoing stream of eight consecu-
tive 0’s is replaced with a B8ZS code word. If the last 1
transmitted was positive, the inserted code is 000+-0-+;
if negative, the code word inserted is 000-+0+-. Bipolar
violations occur in the fourth and seventh bit positions
which are ignored by the DS2180A error monitoring log-
ic when B8ZS is enabled. Any received B8ZS code word
is replaced with all 0’s if B8ZS is enabled. Also, the re-
ceive status register will report any occurrence of B8ZS
code words to the host controller. This allows the user to
monitor the link for upgrade to clear channel capability
and respond to it. The B8ZS monitoring feature works at
all times and is independent of the state of CCR.2.