22. Hold it! What is a modulo system and how is it used to number frames?

A modulo system is a mathematical system that cyclically repeats itself. For example, the set {0, 1, 2, 3} is a finite set that contains only the four given elements, namely, 0, 1, 2, and 3. This means that if we were to work with these set elements in sequence, after using the element 3, we must cycle back and start with 0. In general, a mathematical system that consists of the set of elements {0, 1, 2, 3,..., m - 1} is called a modulo or mod m system. Most sliding window protocols support either a three-bit or seven-bit window field. A three-bit field specifies a mod 8 system because 2³ = 8, and a seven-bit field specifies a mod 128 system because 2⁷ = 128.