One key design decision for ATM was whether to use fixed- or variable-length cells.

Let us consider this decision from the point of view of efficiency. We can define transmission efficiency as N =

Number of information octets Number of information octets + Number of overhead octets

a. Consider the use of fixed-length packets. In this case the overhead consists of the header octets. Define L = Data field size of the cell in octets H = Header size of the cell in octets X = Number of information octets to be transmitted as a single message Derive an expression for N. Hint: The expression will need to use the operator < ~ =, where

b. If cells have variable length, then overhead is determined by the header, plus the flags to delimit the cells or an additional length field in the header. Let Hv = additional overhead octets required to enable the use of variable-length cells. Derive an expression for N in terms of X, H, and Hv.

c. Let L = 48, H = 5, and Hv = 2. Plot N versus message size for fixed- and variable-
length cells. Comment on the results.