SDH-Based Interface Physical Layer

www.pragsoft.com Chapter 13: Broadband ISDN and ATM 171

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 172 Communication Networks Copyright © 2005 PragSoft transmission rate, the transmitter inserts idle cells in the stream when it has no cells to send. The receiver identifies and discards idle cells upon their arrival. Idle cells are similar to OAM cells in that they have their VPI set to 0 and their VCI set to a fixed value 1. The cell header conatins a valid HEC and the cells information octets are set to the bit pattern 01101010.

12.4. ATM Layer

The ATM layer uses a cell as its basic unit of communication. Unlike the physical layer, at the ATM layer only the logical structure of a cell is of interest. This was described in Section 12.2.2. Cells which are for the use of the physical layer only are distinguished by having the VPI and VCI set to 0, and the least significant bit of the fourth octet in the cell header set to 1. Except for the HEC field which is not used by the ATM layer, the remaining ATM cell fields are separately discussed below.

12.4.1. Generic Flow Control

The GFC field is 4 bits wide and defaults to 0. The GFC field provides a means of exercising UNI-based control over the flow of traffic to counter overload situations. It only applies to traffic generated by the CPE as opposed to the network. The exact GFC details remain largely undefined. The ATM network itself has no provision for flow control comparable to those provided by packet networks. This essentially refelects its streamlined nature.

12.4.2. Virtual Path Identifier

The VPI field is 8 bits wide for the UNI and 12 bits wide for the Network-Network Interface NNI. It is used to distinguish between VP links multiplexed into the same physical connection. All cells belonging to the same VP are assigned the same VPI. As explained earlier, certain cells may be assigned predetermined VPIs. A VPI is mapped at a VP link termination, such as a switch. A Virtual Path Connection VPC is a concatenation of VP links, along which the VCIs may be mapped link-by-link. A VPC preserves the cell sequence for each of the channels it contains.

12.4.3. Virtual Channel Identifier

The VCI field is 16 bits wide and is used to distinguish between VCs sharing the same VP connection. Combined with the VPI, it provides a complete routing identifier. All cells belonging to the same virtual channel are assigned the same VCI.