
XRT84L38
338
OCTAL T1/E1/J1 FRAMER
REV. 1.0.1
In order to guarantee adequate clock recovery from the received PCM data, a minimum "ones density" must be
maintained. In the case of an all zero channel, that is, if all the incoming PCM data of a particular DS0 channel
from the Terminal Equipment is zero, the raw PCM data is replaced by a certain pattern that no more than
fifteen consecutive zeros will occur. It is known as zero code suppression.
In the receive end, the user needs to know what type of zero code suppression scheme is applied on the
receiving data. In this way, a correct decoding method or the reverse of zero code suppression can be done to
extract the payload data from the DS1 LIU Receive Block.
The XRT84L38 framer supports three types of zero code suppression schemes:
AT&T Bit 7 Stuffing - an old coding method that forces Bit 7 (the second LSB of a DS0 channel) to a 1 in an
all zero channel.
GTE Zero Code Suppression - Bit 8 (the LSB of a DS0 channel) is stuffed by 1 in non-signaling frame in an
all zero channel. Otherwise, Bit 7 is stuffed by 1 in signaling frame if the signaling bit is zero.
DDS Zero Code Suppression - an octet with hexadecimal value of 0x98 is used to replace the input data if it
is all zero.
The Receive Zero Code Suppression Select [1:0] bits of the Receive Channel Control Register (RCCR) of a
particular DS0 channel is used to select which type of zero code suppression scheme is applied to the received
DS1 payload data. The table below shows configurations of the Receive Zero Code Suppression Select [1:0]
bits of the Receive Channel Control Register (RCCR).
RECEIVE CHANNEL CONTROL REGISTER (RCCR) (INDIRECT ADDRESS = 0XN2H, 0X60H - 0X7FH)
10.5
How to Configure the XRT84L38 Framer to Extract Robbed-bit Signaling Information
The XRT84L38 T1/J1/E1 Octal Framer supports insertion of Robbed-bit Signaling information into the outgoing
DS1 frame. It also supports extraction and substitution of Robbed-bit Signaling information from the incoming
DS1 frame. The following section describes how does the XRT84L38 framer extract and substitute Robbed-bit
Signaling in DS1 mode.
10.5.1
Configure the framer to receive and extract Robbed-bit Signaling
The XRT84L38 framer supports receiving and extraction of Robbed-bit Signaling in ESF, SF and SLC96
framing formats. The Receive Signaling Extraction Control [1:0] bits of the Receive Signaling Control Register
(RSCR) of each channel select either:
No signaling extraction
Two-code signaling
Four-code signaling or
Sixteen-code signaling
In SF or SLCa96 mode, the Receive Signaling Extraction Control [1:0] bits can select no signaling
(transparent), two-code signaling, or four-code signaling. Two-code signaling decoding is done by stripping the
least significant bit (LSB) of the specific channel in frame 6 and 12 and stores it into the Signaling Bit A position
of RSRA register array. Four-code signaling is done by stripping the LSB of channel data in frame 6 and the
LSB of channel data in frame 12, and store them into Signaling Bit A and Signaling Bit B position of RSRA
B
IT
N
UMBER
B
IT
N
AME
B
IT
T
YPE
B
IT
D
ESCRIPTION
5-4
Receive Zero Code
Suppression
Select
R/W
00 - The received DS1 PAYLOAD data of this DS0 channel is unchanged.
No zero code suppression is used.
01 - AT&T Bit 7 stuffing is used.
10 - GTE zero code suppression is used.
11 - DDS zero code suppression is used.