2.13 Cell Overhead

As previously noted, the ATM cell consists of a 48-byte payload and five-byte header, resulting in a total cell length of 53 bytes. The overhead associated with transmission of data on an ATM network depends on the average quantity of data being transmitted. Under a best-case scenario, when a user has a multiple of exactly 48 bytes of data to transmit the cell overhead becomes (5/48) * 100, or approximately 9.43 percent. However, very rarely will anyone have a multiple of 48 bytes of information to transmit. Thus, let us examine the effect upon ATM overhead as we transmit different quantities of data.

Suppose you transmit a one-character message transported by an ATM cell. The overhead then becomes 52 bytes in a 53-byte cell, or 98.11 percent. Now suppose your message is 49 bytes in length, requiring two cells to transport the message. In this situation, the overhead decreases to [(5 + 52)/106] * 100, or 53.77 percent. Here, the first cell contains 52 bytes of overhead. Now let us examine a few additional data transfers occurring via ATM cells .

Assume you transmit a 97-byte message that fills the payload area of two cells and requires an additional byte in a third cell to transport the message. The overhead includes five bytes in each of the first two cells and 52 bytes in the third cell, for a total of 62 bytes for the 159 bytes in the three cells. This results in an overhead of (62/159) * 100, or 38.99 percent. Now assume our transmission increased to 193 bytes. With a 48-byte payload five ATM cells are now required, with the fifth cell also transporting one byte of information. Now the overhead is 72 bytes out of a total of 265 bytes, which is (72/265) * 100, or 27.16 percent.

Table 2.11 provides a summary of ATM cell overhead based upon a varying number of data bytes in a transmitted message. In examining the entries in Table 2.11, note that in addition to many common power of 2 message lengths, there are several that may appear curious as to why they were entered in the table. Those entries you may be curious about include message block lengths of 49, 145, 193, 433, 913, and 1393 bytes. Each of those entries represents an increasing message that has one more byte than necessary to fill a cell, resulting in an additional cell being required to transport the message. Note that the cell overhead diminishes, falling from 53.77 percent when a message consists of 49 bytes to 31.60 percent when a message consists of 145 bytes. As the length of the message continues to expand, our worst-case scenario results in a continuing decline in cell overhead. That is, for a 193-byte message, the cell overhead falls to 27.17 percent, while a 433-byte message has a cell overhead of 18.30 percent and a 913-byte message has a cell overhead of 13.87 percent. This indicates that ATM is much more efficient for transporting files than interactive query data. Also note that for messages of 48, 96, and 480 bytes, which exactly fill one or more ATM cells, cell overhead is reduced to 9.43 percent. Thus, we can note from the overhead associated with different message lengths that ATM overhead will vary between a maximum of 98.11 percent and a minimum of 9.43 percent.

Table 2.11: ATM Cell Overhead
Data Bytes per Message	Number of Cells	Overhead to Total Bytes	Cell Overhead
1	1	52/53	98.11
2	1	51/53	96.22
4	1	49/53	92.24
8	1	45/53	84.91
16	1	37/53	69.81
32	1	21/53	39.62
40	1	13/53	24.52
48	1	5/53	9.43
49	2	57/106	53.77
64	2	42/106	39.62
72	2	34/106	32.07
80	2	26/106	24.52
96	2	10/106	9.43
97	3	62/159	38.99
144	3	15/159	9.43
145	4	67/212	28.77
192	4	20/212	9.43
193	5	72/265	27.16
256	6	62/318	19.49
433	10	97/530	18.30
480	10	50/530	9.43
512	11	71/583	12.18
913	20	147/1060	13.87
1024	22	142/1166	12.17
1393	30	197/1590	12.39
1500	32	196/1696	11.56