The FDDI token and frames were patterned on the Token Ring token and frames, but there are some differences. For example, there is no monitor station on an FDDI ring, and no priority or reservation bits are used, so no access control field is present. Another difference is the use of special characters to delimit frames.
Figure 19.21 shows the format of an ordinary (unrestricted) token. A token must be preceded by four or more idle symbols. The starting and ending delimiters are made up of pairs of special symbols. These are represented by 5-bit code-groups that never are used to represent data bytes.
The starting delimiter code-groups are called J and K.
The 10 bits ...