# Understanding RU Sizing

The following table shows how to find the appropriate value to specify for the RUSIZES parameter. For the formula RUSIZES=X‘abab’, the first ab pair applies to the secondary logical unit (SLU). The second ab pair applies to the primary logical unit (PLU). The table shows the values for a and b. The number of bytes is found where the a column and the b column intersect in the table.

For example, the bold a and b numbers in the table indicate how it is used to determine the RUSIZES=87F8 for SNA connections. Because the primary LU is able to receive 3,840 bytes, the secondary LU can send 3,840 bytes at a time (the intersection of F and 8). The primary LU can send 1,024 bytes at a time because the secondary LU is able to receive 1,024 bytes (the intersection of 8 and 7).

a
b 8 9 A(10) B(11) C(12) D(13) E(14) F(15)
0 8 9 10 11 12 13 14 15
1 16 18 20 22 24 26 28 30
2 32 36 40 44 48 52 56 60
3 64 72 80 88 96 104 112 120
4 128 144 160 176 192 208 224 240
5 256 288 320 352 384 416 448 480
6 512 576 640 704 768 832 896 960
7 1,024 1,152 1,280 1,408 1,536 1,664 1,792 1,920
8 2,048 2,304 2,560 2,816 3,072 3,328 3,584 3,840
9 4,096 4,608 5,120 5,632 6,144 6,656 7,168 7,680
A(10) 8,192 9,216 10,240 11,264 12,288 13,312 14,336 15,360
B(11) 16,384 18,432 20,480 22,528 24,576 26,624 28,672 30,720
C(12) 32,768 36,864 40,960 45,056 49,152 53,248 57,344 61,440
D(13) 65,536 73,728 81,920 90,112 98,304 106,496 114,688 122,880
E(14) 131,072 147,456 163,840 180,224 196,608 212,992 229,376 245,760
F(15) 262,144 294,912 327,680 360,448 393,216 425,984 458,752 491,520

The table is derived from the following calculations:

The RUSIZE of ab means RUSIZE equals a x (2 **b). The b of each ab pair is used as an exponent of base two. The resulting value is multiplied by a to get the RUSIZE for that logical unit.

Using the same example, RUSIZES=87F8, the SLU and PLU values are determined as follows.

 if RUSIZES=X‘87F8’ then the SLU RUSIZE is 87 which indicates 8  x  2**7  = 8 x 128 = 1024 (8 times 2 to the 7th power) the PLU RUSIZE is F8 which indicates F  x  2**8 = 15 x 2**8 = 15 x 256 = 3840 (15 times 2 to the 8th power)