XRT86L30
REV. 1.0.1
SINGLE T1/E1/J1 FRAMER/LIU COMBO
11.3
Brief Discussion of Robbed-bit Signaling in DS1 Framing Format
Signaling is required when dealing with voice and dial-up data services in DS1 applications. Traditionally,
signaling is provided on a dial-up telephone line, across the talk-path. Bit robbing, or stealing the least
significant bit (8th bit) in each of the twenty-four voice channels in the signaling frames allows enough bits to
signal between the transmitting and receiving end. That is where the name Robbed-bit signaling comes from.
These ends can be CPE to central office (CO) for switched services, or CPE to CPE for PBX-to-PBX
connections.
Signaling is used to tell the receiver where the call or route is destined. The signal is sent through switches
along the route to a distant end. Common types of signals are:
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•
•
•
•
•
On hook
Off hook
Dial tone
Dialed digits
Ringing cycle
Busy tone
Robbed-bit Signaling is supported in three DS1 framing formats.
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•
•
Super-Frame (SF)
SLC®96
Extended Super-Frame (ESF)
In Super-Frame or SLC®96 framing mode, frame number 6 and frame number 12 are signaling frames. In
channelized DS1 applications, these frames are used to contain the signaling information. In frame number 6
and 12, the least significant bit of all twenty-four timeslots is 'robbed' to carry call state information. The bit in
frame 6 is called the A bit and the bit in frame 12 is called the B bit. The combination of A and B defines the
state of the call for the particular timeslot that these two bits are located in.
F
RAME
N
UMBER
S
IGNALING BIT
6
A
B
12
In Extended Super-Frame framing mode, frame number 6, 12, 18 and 24 are signaling frames. In these
frames, the least significant bit of all twenty-four timeslots is 'robbed' to carry call state information. The bit in
frame 6 is called the A bit, the bit in frame 12 is called the B bit, the bit in frame 18 is called the C bit and the bit
in frame 24 is called the D bit. The combination of A, B, C and D defines the state of the call for the particular
timeslot that these signaling bits are located in.
F
RAME
N
UMBER
S
IGNALING BIT
6
A
B
C
D
12
18
24
11.3.1
Configure the framer to transmit Robbed-bit Signaling
The XRT86L30 framer supports transmission of Robbed-bit Signaling in ESF, SF and SLC®96 framing
formats. Signaling bits can be inserted into the outgoing DS1 frame through the following:
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•
Signaling data is inserted from Transmit Signaling Control Registers (TSCR) of each timeslot
Signaling data is inserted from TxSig_n pin
220
EXAR [ EXAR CORPORATION ]
