168 Communication Networks
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The CLP field prioritizes the order in which cells may be discarded. The HEC
field is an 8-bit CRC calculated over the remaining 32 bits in the header. It is used for error detection and single-bit error correction.
12.3. Physical Layer
This section describes the transmission convergence functions of the physical layer. The physical layer may be either cell-based or SDH-based SDH stands for
Synchronous Digital Hierarchy and will be explained in Section 12.7. The generation, recovery, and adaptation of transmission frames are done differently for
these two interfaces, and are therefore described separately below. The remaining functions are identical in both cases.
12.3.1. SDH-Based Interface
Figure 12.142 shows the transmission frame format of the 155.52 Mbps SDH- based interface. It consists of a table of 9 rows by 270 columns of octets total of
2430 octets. The frame is transmitted row-by-row at a frequency of 8 kHz, thus yielding a data rate of 2430 × 8kHz × 8 bits = 155.52 Mbps. The technical term for
the frame format is Synchronous Transport Module 1 STM-1.
Figure 12.142 Frame format for 155.52 Mbps SDH-based interface STM-1.
9 Octets 261 Octets
Section AU-4 Pointer
Administrative Unit 4 AU-4 Overhead
The first 9 columns of octets in the frame consist of transmission overheads called the Section Overhead SOH. This includes framing bytes, parity bytes,
additional narrowband channels for other uses, and the AU-4 pointer which points to a Virtual Container 4 VC-4 block. The VC-4 block starts somewhere in the AU-4
which comprises the remaining 261 columns of the frame.
The format of a VC-4 block is shown in figure 12.143. It consists of 9 rows by 261 columns of octets. Of these, the first column is called the Path Overhead POH
and the remaining 260 columns consist of ATM cells. Because the total size of the latter is not an integral multiple of 53 octets, ATM cells may cross VC-4 boundaries.
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169
Figure 12.143 VC-4 block format for 155.52 Mbps SDH-based interface.
1 Octet 260 Octets
J1 B3
C2 Path
G1 Overhead
F2 Container 4 C-4
H4 Z3
Z4 Z5
The POH octets are as follows. J1 provides a narrowband channel which repeatedly transmits a fixed-length, user-specified string so that path integrity can be
verified by the receiver. B3 provides path-level parity. C2 specifies the path signal label. G1 contains status information. F2 provides a narrowband channel for the path
user. H4 is the cell offset indicator It may assume a value in the range 0-52 which acts as an offset from the H4 octet to the first cell boundary in the same row. Z3, Z4,
and Z5 are reserved for future use.
The effective payload capacity of STM-1 is 260 × 9 × 8kHz × 8 bits = 149.76 Mbps. Physical layer Operation, Administration, and Maintenance OAM functions
are allocated to the frame overheads SOH and POH. The frame format for the 622.08 Mbps SDH-based interface is STM-4 and is
four times the size of the STM-1. It consists of an overhead of 9 × 4 columns and a payload part of 261 × 4 columns. The payload contains only one POH of one
column, thus providing a payload capacity of 600.768 Mbps.
12.3.2. Cell-Based Interface
The cell-based interface is considerably simpler than the SDH-based interface because there is no framing involved. Transmission consists of a continuous stream of
ATM cells.
To cater for OAM functions, ATM cells with unique header patterns are used. This pattern consists of setting the first three octets in the cell header to 0 and setting
the fourth octet to the values shown in Figure 12.144. The latter determines the cell type which in turn classifies the OAM functions it caters for. OAM cells are inserted
in the transmission stream on a per demand basis.
Figure 12.144 Physical layer OAM cells. OAM Cell Type
Fourth Octet Applies to Level
OAM Functions
F1 0000 0011
Regenerator Section Frame alignment
Section error monitoring
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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.
