ver, only one bit of information is carried for can be constrained SO that it never produces certain symbol every two bits transmitted and SO the efficiency is only 50%. In this sequences. Thus, the occurrence of these sequences at the case, this results in a channel bandwidth requirement of twice the receiver provides a means of estimating the link error perfor- uncoded bandwidth. However, its simplicity makes it attractive for mance. applications where transmission bandwidth considerations are less 5. Efficiency. In order to provide the above features it will usually crucial; it is used in the magnetic recording of digital signals and in be necessary to add extra information (redundancy as far as the the Ethernet local area network system (ANSI/IEEE 802-3) data is concerned) into the digit stream. This will lower the efficiency of the line code which can be defined as the ratio of the average information carried per transmitted symbol to the 28.3.3.2 Code mark inversion (CMI) maximum possible information per symbol (when assuming no added redundancy), expressed as a percentage. It is shown in Figure 28.11(b) and is similar to the Manchester code except that it achieves a polarity balance over a longer term by 28.3.3 Classification of line codes alternating between a positive symbol and a negative symbol when a binary one (mark) is to be transmitted. It is specified by CCITT Line codes can be classified in a variety of ways, one method is to (CCITT, 1988c) as an interface code for short distance connections identify the following categories: between certain types of transmission equipments. 1. Bit-by-bit codes. 2. Block codes: bit insertion and block substitution. 28.3.3.3 Alternate Mark Inversion (AMI). 3. Partial response codes. This differs from the above line codes in that the transmitted symbol Some codes will fall into more than one of these classes. The rate is the same as the binary information rate. It provides the following are representative examples from each category. desired features by using a red