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XRT79L71
PRELIMINARY
305
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
REV. P2.0.0
Where for standard (Bellcore GR-499-CORE) applications:
Flag Sequence = 0x7E
SAPI + CR + EA = 0x3C or 0x3E
TEI + EA = 0x01
Control = 0x03
The following text defines each of these bit/byte-fields within the LAPD Message Frame Format.
Flag Sequence Byte
The Flag Sequence byte is of the value 0x7E, and is used to denote the boundaries of the LAPD Message
Frame. Additionally, whenever the Transmit LAPD Controller is not currently transmitting a LAPD Message, it
will instead be transmitting a continuous stream of Flag Sequence bytes via the "N" bit within each outbound
E3 frame.
SAPI - Service Access Point Identifier
Traditionally, for N-ISDN applications, the SAPI field typically indicates the type of data or service being
supported by the LAPD Message.
However, for standard (Bellcore GR-499-CORE) applications, this
parameter has no meaning and is assigned the value "001111b" or 1510 per Bellcore GR-499-CORE
TEI - Terminal Endpoint Identifier
The TEI bit-fields are assigned the value of 0x00. The TEI field is used in N-ISDN systems to identify a
terminal out of multiple possible terminals. However, since DS3 and E3 data is transmitted in a point-to-point
manner, the TEI value is unimportant in this application.
Control
The Control byte-field identifies the type of frame being transmitted. There are three general types of frame
formats: Information, Supervisory, and Unnumbered. For standard (Bellcore GR-499-CORE) applications the
user must use the Control byte the value "0x03". Hence, the XRT79L71 will be transmitting and receiving
Unnumbered LAPD Message frames.
Information Payload
The Information Payload is the 76 bytes, 82 bytes or any number of bytes of data (e.g., the PMDL Message)
that the user has written into the on-chip Transmit LAPD Message buffer (which is located at Address 0x11B0).
A Special Note about the Information Payload when transmitting standard (Bellcore GR-499-CORE)
type LAPD Messages
FIGURE 144. LAPD MESSAGE FRAME FORMAT
FLAG SEQUENCE (8 BITS)
SAPI (6-BITS)
C/R
EA
TEI (7 BITS)
EA
CONTROL (8-BITS)
76 OR 82 OR ANY-SIZE BYTES OF INFORMATION (PAYLOAD)
FCS - MSB
FCS - LSB
FLAG SEQUENCE (8-BITS)