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PRELIMINARY
XRT79L71
472
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The Transmit LAPD Message Buffer
The purpose of the Transmit LAPD Message Buffer is to permit the user to write and temporarily store the
contents of the very next outbound LAPD Message that is to be transmitted. The Transmit LAPD Message
Buffer is actually a 90 byte FIFO that is located at Address location 0x11B0 within the XRT79L71 address
space.
The Transmit LAPD Controller
The Transmit LAPD Controller permits the user to transmit path maintenance data link (PMDL) messages to
the remote terminal equipment via the outbound E3 Frames. In this case the message bits are inserted into
and carried by either the NR or the GC byte within each outbound E3 frame. The on-chip Transmit LAPD
Controller permits the user to transmit both standard and non-standard PMDL Messages of any length up to 82
bytes. The XRT79L71 allocates a block of 90 bytes of on-chip RAM (e.g., the Transmit LAPD Message buffer),
to store the contents of the outbound PMDL message to be transmitted. The message format complies with
ITU-T Q.921 (LAPD) protocol with different addresses and is presented below in Figure 226.
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 either the NR or GC byte,
depending upon user selection, 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
FIGURE 226. 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)