PRELIMINARY
XRT79L71
550
REV. P2.0.0
1-CHANNEL DS3/E3 CLEAR-CHANNEL FRAMERLIU COMBO - CC/HDLC ARCHITECTURE
The purpose of the Receive LAPD Message Buffer is to store the contents of LAPD/PMDL Messages that have
been received by the Receive LAPD Controller block. The Receive LAPD Message Buffer will serve as a
temporary storage location of these incoming LAPD/PMDL Messages until they can be read out by the
Microprocessor. the Receive LAPD Message Buffer is actually a 90 byte FIFO that is located at Address
Location 0x11C0 within the XRT79L71 address space.
The Receive LAPD Controller
The Receive LAPD Controller permits the user to receive path maintenance data link (PMDL) message from
the remote terminal equipment via the "inbound" DS3 frames. In this case, the PMDL Message is extracted
out of either the "NR" or the "GC" bytes (depending upon user configuration) within each incoming E3 frame.
The on-chip Receive LAPD Controller is capable of receiving 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
Receive LAPD Message buffer), to store the contents of newly received PMDL Messages. The message
format complies with ITU-T Q.921 (LAPD) protocol with different addresses and is presented below in
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 Receive LAPD Controller is not currently receiving a LAPD Message, it will
instead be receiving a continuous stream of Flag Sequence bytes via either the "NR" or the "GC" Byte
(depending upon which byte was configured to function as the LAPD/PMDL Channel) within each "inbound" 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 254. 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)