
xr
XRT94L33
3-CHANNEL DS3/E3/STS-1 TO STS-3/STM-1 MAPPER IC DATA SHEET
Rev.1.2.0.
407
2.3.4.1.2
HEC Byte VerificationOnce the HEC Byte Verification block is properly delineating cells, then
the HEC Byte Verification block will (as its name implies) perform “HEC Byte Verification” of incoming cells
data from the Receive STS-3c POH Processor block in order to protect against misrouted or mis-inserted
cells. In performing HEC Byte Verification the HEC Byte Verification block will take the first four byte of each
cells (e.g., the header bytes) and will independently compute its own value for the HEC byte. Afterwards, the
HEC Byte Verification block will compare its value of the HEC byte with the fifth octet that it has received from
the Receive STS-3c POH Processor block. If the two “HEC byte” values match then the “Receive ATM Cell
Processor” block will retain this cell for further processing.
However, if the HEC Byte Verification block
detects errors in the header bytes of a cell, then the HEC Byte Verification block will call up and employ the
“HEC Byte Error Correction/Detection” Algorithm (see below).
The HEC Byte Verification block will compute its version of the HEC byte via the generating polynomial x
8 + x2
+ x + 1. The user should be aware that the HEC bytes of the incoming cell might have been modulo-2 added
with the Coset polynomial x
6 + x4 + x2 + 1. If this is the case then the Receive ATM Cell Processor block must
be configured to account for this by writing a “1” to Bit 1 (COSET Polynomial Addition) within the “Receive
ATM Cell Processor Block – Receive ATM Control Register – Byte 1.
Receive ATM Cell Processor Block – Receive ATM Control Register – Byte 1 (Address = 0xN702)
BIT 7
BIT 6
BIT 5
BIT 4
BIT 3
BIT 2
BIT 1
BIT 0
Unused
GFC
Extraction
Enable
HEC Byte
Correction
Enable
Uncorrectable
HEC Byte
Error Discard
COSET
Polynomial
Addition
Regenerate
HEC Byte
Enable
R/O
R/W
0
1
0
1
0
The “HEC Byte Error Correction/Detection” Algorithm
If the HEC Byte Verification block detects one or more errors in the header bytes of a given incoming ATM
cell, then the “HEC Byte Error Correction/Detection” algorithm will be employed.
The “HEC Byte Error
Correction/Detection” Algorithm has two states: “Detection” Mode and “Correction” Mode.
Figure 99 presents a State Machine Diagram of the “HEC Byte Error Correction/Detection” Algorithm. Each
of these states is discussed below.