170 Communication Networks
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F2 0000 0101
Digital Section Frame alignment
Section error monitoringreporting F3
0000 1001 Transmission Path
Network status monitoring Header error monitoring
Path error monitoringreporting Cell delineationrate decoupling
The main advantage of the cell-based interface is its inherent simplicity. The main advantage of the SDH-based interface is that it provides a means for combining
multiple ATM channels to achieve a higher bit rate.
12.3.3. Cell Delineation
Because the cell-based interface involves no framing, some other means of synchronization is needed. Cell delineation provides this by identifying cell
boundaries. The HEC field which applies to the remaining fields in the header is used for this purpose.
Cell boundary is identified by comparing potential header bits against their corresponding HEC bits using the state diagram shown in Figure 12.145.
Figure 12.145 Cell delineation state diagram.
Hunt Presynch
Synch
Correct HEC Incorrect HEC
Bit-by-bit check Cell-by-cell check
Cell-by-cell check delta consecutive
alpha consecutive incorrect HEC
correct HEC
A receiver is initially in the hunt state. Here a bit-by-bit checking of the cell stream is made with the objective of finding a header with matching HEC field. When
a correct HEC is detected, the presynch state entered. In this state HEC fields are checked on a cell-by-cell basis. When delta usually set to 6 consecutive correct
HEC fields are detected the synch state is entered. While in presynch state, an incorrect HEC field results in a return to the hunt state. In the synch state HEC fields
are still checked on a cell-by-cell basis. Detection of alpha usually set to 7 consecutive incorrect HEC fields causes the receiver to revert back to the hunt state.
The information contents of cells is scrambled as a means of protection against imitated HEC field correlations which may have been generated accidently or
maliciously. With even a relatively high error rate, the receiver will spend very long periods in the synch state and extremely short periods in the other two states.
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12.3.4. HEC Generation and Verification
The HEC field is generated by the transmitter using the first four octets of the cell header. The outcome is a single octet which becomes the final octet in the cell
header. A HEC field is calculated as follows:
1. Multiply the polynomial denoted by the first 32 bits of the header to which HEC applies by x
8
. 2. Divide the outcome by the generator polynomial x
8
+ x
2
+ x + 1 using modulo 2 division.
3. The resulting 8 bits are added to 01010101 using modulo 2 addition to produce the final HEC value.
Because the HEC field is fairly large compared to the data to which it applies, it also provides some scope for error correction. This includes all single-bit errors and
some multiple-bit errors. Figure 12.146 illustrates the receiver behavior in response to HEC values.
Figure 12.146 HEC state diagram for a receiver.
Correction Detection
No error detected no action
Error detected No error detected
no action discard cell
Single-bit error detected correct it
Multiple-bit error detected discard cell
The receiver is initially in the correction state. A single-bit error in this state is corrected and results in a transition to the detection state. A multi-bit error causes the
cell being discarded and also results in a transition to the correction state. While in correction state, detection of any error causes the cell to be discarded. A correct
cell, however, results in a return to the correction state. The receiver remains in the correction state as long as no errors are detected.
The above algorithm is designed to take into account the error behavior of optical fiber transmission media i.e., a combination of single-bit errors and error
bursts.
12.3.5. Cell Rate Decoupling
ATM provides an asynchronous means of cell transfer. However, actual physical transmission remain synchronous fixed rate. Hence cells need to be transmitted
even when the transmitter has no data to send. To adapt the cell rate to the
