A convolutional code can be defined by three variables: n, k, and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block, and K to the memory of the encoder. Convolutional Coding Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and impor- tance. The most important class is the class Ia containing 50 bits. The class Ib is next in importance, which contains 132 bits. The class II is the least important, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class la bits are block coded. Three parity bits, used for error detection, are added to the 50 class la bits. The resultant Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction to GSM Chapter 1 53 bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50 + 3 + 132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. The class II bits are added, without any protection, to the output block of the convolutional coder. An output block of 456 bits is finally obtained. This description is meant to set the stage for understanding the under- lying network that supports the GPRS systems. Much more detail and descriptive materials can be found in other publications, SO that the reader can gain a better description if needed. In the next chapter, the focus will be on the motivators for the operators to move from a GSM-only network to one that overlays GPRS on the GSM network. In each of the following chap- ters, we will look at the interfaces between each of the components in a GSM/GPRS network. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004