XRT79L71
PRELIMINARY
495
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
6.2.5.1
AN INTRODUCTION TO SSM (SYNCHRONIZATION STATUS MESSAGES)
If the XRT79L71 is operating in the E3, ITU-T G.832 Frame Format, then Bits 6, 7 and 8 (the three least
significant bit-fields) within the MA byte can be used to transport the Synchronization Status Message, referred
to as SSM from this point on, from one terminal equipment to another. The bit-format of the MA Byte, with
these bit-fields shaded is presented below.
According to ITU-T G.707, the SSM is a four-bit value that is used to identify the quality-level of the
synchronization/timing that the Transmitting E3 Terminal Equipment is currently operating at.
Whenever a given Transmitting E3 Terminal transmits the SSM to the remote terminal, it does so by repeatedly
transmitting this four-bit SSM message, one bit at a time or one bit per E3 frame period via Bit 8 (the least
significant bit-field), within the MA byte. As this Transmitting E3 Terminal repeatedly transmits this byte to the
remote terminal equipment it will use Bits 6 and 7 (within the MA Byte) to indicate which SSM Message bit is
being transported via Bit 8, within the current MA byte. Depending upon which of the four bits (within the SSM)
that is being transported via the MA byte, within a given E3 frame, the Transmitting E3 Terminal will set Bits 6
and 7 to the appropriate value, in order to identify this SSM bit, as depicted below in Table 65.
FIGURE 230. ILLUSTRATION OF THE FUNCTIONAL BLOCK DIAGRAM OF THE TRANSMIT DIRECTION CIRCUITRY, WHEN-
EVER THE
XRT79L71 HAS BEEN CONFIGURED TO OPERATE IN THE E3, ITU-T G.832 CLEAR-CHANNEL FRAMER
MODE (WITH THE TRANSMIT SSM CONTROLLER BLOCK HIGHLIGHTED)
FIGURE 231. THE BIT-FORMAT OF THE MA-BYTE WITHIN THE E3, ITU-T G.832 FRAMING FORMAT
BIT 1
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
BIT 8
FERF/RDI
FEBE/REI
Payload_Type[2:0]
SSM Multi-Frame Indicator[1:0]
SSM Bit
Transmit
Payload Data
Input
Interface
Block
Transmit
Payload Data
Input
Interface
Block
Transmit
DS3/E3
Framer
Block
Transmit
DS3/E3
Framer
Block
Transmit
DS3/E3
LIU Block
Transmit
DS3/E3
LIU Block
TxSer
TxNib[3:0]
TxInClk
TRING
TTIP
Transmit
Overhead Data
Input Interface
Block
Transmit
Overhead Data
Input Interface
Block
TxOHClk
TxOHIns
TxOHInd
TxOH
TxOHEnable
TxOHFrame
TxNibClk
TxFrame
Tx LAPD
Controller
Block
Tx LAPD
Controller
Block
From Microprocessor
Interface Block
Tx LAPD
Buffer
(90 Bytes)
Tx LAPD
Buffer
(90 Bytes)
Tx TTM
Controller
Block
Tx TTM
Controller
Block
Tx SSM
Controller
Block
Tx SSM
Controller
Block