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IDT82P2281
SINGLE T1/E1/J1 LONG HAUL / SHORT HAUL TRANSCEIVER
Functional Description
45
October 7, 2003
3.11.2.2
SS7 Mode
In SS7 mode, there are three kinds of signaling units - MSU, LSSU
and FISU (refer to Figure 14). Their opening flag and closing flag are
both 7E (Hex). The closing flag may also serve as the opening flag of
the next HDLC packet.
Figure 14. Standard SS7 Packet
After the stuffed zero (the zero following five consecutive ’One’s) is
discarded, the extracted SS7 data stream is compared with the standard
SS7 packet. If the value of the 6-bit length indication is equal to ‘0’, the
SS7 packet is FISU; if it is equal to ‘1’ or ‘2’, the SS7 packet is LSSU; if it
is more than ‘2’, the SS7 packet is MSU.
The data stream between the opening flag and the FCS are divided
into blocks. Each block (except the last block) has 32 bytes. The block
will be pushed into a FIFO with one-byte overhead until any of the fol-
lowing invalid packet conditions occurs:
- A packet with error FCS;
- The data between the opening flag and the closing flag is less
than 5 bytes (including the FCS, excluding the flags);
- The extracted SS7 packet does not consist of an integral number
of octets;
- A 7F (Hex) abort sequence is received;
- If the SS7 packet is FISU, the data between the opening flag and
the closing flag is not 5 bytes (including the FCS, excluding the flags);
- If the SS7 packet is LSSU, the data between the opening flag and
the closing flag is not 6 or 7 bytes (including the FCS, excluding the
flags);
- If the SS7 packet is MSU, the data between the opening flag and
the closing flag is less than 8 bytes or more than 271 bytes (including
the FCS, excluding the flags).
If any of the above conditions is detected, the current block will be
discarded, but the one-byte overhead will still be written into the FIFO.
The overhead consists of the M[2:0] bits and the length indication bits as
shown in Figure 13. In FISU/LSSU, if the FISU/LSSU filter is set by the
FISUFIL/LSSUFIL bit respectively, the current FISU/LSSU will be dis-
carded if it is the same with the previous FISU/LSSU. In this condition,
no data and overhead of the current FISU/LSSU will be written into the
FIFO.
The FIFO depth is 128 bytes. The FIFO is accessed by the
DAT[7:0] bits. When the overhead is read from the FIFO, it will be indi-
cated by the PACK bit. When all valid SS7 blocks are pushed into the
FIFO or all the blocks are read from the FIFO, it will be indicated by the
EMP bit.
The interrupt sources in this block are summarized in the Table 30.
When there are conditions meeting the interrupt sources, the corre-
sponding Interrupt Indication bit will be set to ‘1’ and the Interrupt Indica-
tion bit will be cleared by writing a ‘1’. A ‘1’ in the Interrupt Indication bit
Flag
one byte
'01111110'
FCS
two bytes
Signaling Field
n bytes (n>1)
Flag
one byte
'01111110'
Service
Information
Octet
one byte
two
bits
Length
Indication
six bits
( > 2 )
Forward
Indication
Bit
one bit
Forward
Sequence
Number
seven
bits
seven
bits
Backward
Indication
Bit
one bit
Backward
Sequence
Number
Flag
one byte
'01111110'
two
bits
Length
Indication
six bits
( = 1 or 2 )
Forward
Indication
Bit
one bit
Forward
Sequence
Number
seven
bits
seven
bits
Backward
Indication
Bit
one bit
Backward
Sequence
Number
Flag
one byte
'01111110'
FCS
two bytes
Status
one or two bytes
Flag
one byte
'01111110'
two
bits
Length
Indication
six bits
( = 0 )
Forward
Indication
Bit
one bit
Forward
Sequence
Number
seven
bits
seven
bits
Backward
Indication
Bit
one bit
Backward
Sequence
Number
Flag
one byte
'01111110'
FCS
two bytes
Message Signaling Unit (MSU)
Link Status Signaling Unit (LSSU)
Fill In Signaling Unit (FISU)