ransfer commands and responses 3. Recovery commands and responses 4. Miscellaneous commands and responses. The information field follows the control field (Figure 10.25) and precedes the frame check sequence (FCS) field. The I field is present only in information (I) frames and in some unnumbered (U) frames. The I field may contain any number of bits in any code, related to character structure or not. Its length is not specified in the standard (ISO 3309, Ref. 25). Specific system implementations, however, usually place an upper limit on I field size. Some versions require that the I field contain an integral number of octets. Frame check sequence (FCS). Each frame includes an FCS field. This field immedi- ately follows the I field, or the C field if there is no I field, and precedes the closing flag (F). The FCS field detects errors due to transmission. The FCS field contains 16 bits, which are the result of a mathematical computation on the digital value of all bits exclud- ing the inserted zeros (zero insertion) in the frame and including the address, control, and information fields. REVIEW EXERCISES 1. What is the basic element of information in a binary system? How much information does it contain? DATA COMMUNICATIONS 2. In reference to question 1 above, how does one extend the information content of that basic information element, a bit? For example, how would we develop a binary code that represents our alphabet? 3. There are many ways we can express the binary 1 and binary 0. Give at least four. How do we remove the ambiguity about meaning of a bit (reversing sense)? 4. How many distinct characters and/or symbols can be represented in a 4-bit binary code? a 7-bit binary code? an 8-bit binary code? How many information elements are in an ASCII character? 6. Give two causes of burst errors. 7. Give the two generic methods of correcting errors on a datalink. 8. Name the three different types of ARQ and define each. 9. Describe the difference between neutral and polar transmission. 10. On a start-stop circuit