Request unit size

The session parameters specify the maximum length of request units that can be sent by the logical units. This information is divided into two parts: the secondary logical unit send request unit size (byte 9) and the primary logical unit send request unit size (byte 10). Both bytes have the same format; however, they can be set to different values depending upon the requirements of the logical units. The bits that can be set (using the RUSIZES operand of the MODEENT macroinstruction) follow:

Bit setting 0123 4567	(Bytes 9 and 10) Meaning
`0... ....`	(byte 9) 6K bytes is the default maximum length of the request unit that can be sent by the SLU. VTAM® will terminate a session on behalf of the PLU if a larger RU is received.
`0... ....`	(byte 10) There is no limit specified for the size of the request unit that can be sent by the PLU.
`1000 ....` through `1111 ....`	This is the mantissa (m) of the formula m X 2ⁿ used to determine the maximum length of request units that can be sent by the primary or the secondary logical unit.
`.... 0000` through `.... 1111`	This is the exponent (n) of the formula m X 2ⁿ used to determine the maximum length of request units that can be sent by the primary or secondary logical unit.

For example, if byte 9 or 10 is set to hex 85, the mantissa is 8 and the exponent is 5; the associated RU size is 256 bytes (8 x 2⁵). For a discussion of maximum RU size when using SEND OPTCD=LMPEO, refer to LMPEO operating considerations.

The maximum size of request units is shown in Table 1.

Table 1. Maximum size of request unit (in decimal)
Decimal value of bits 4–7	Decimal value of bits 0–3
Decimal value of bits 4–7	8	9	10	11	12	13	14	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	1024	1152	1280	1408	1536	1664	1792	1920
8	2048	2304	2560	2816	3072	3328	3584	3840
9	4096	4608	5120	5632	6144	6656	7168	7680
10	8192	9216	10240	11264	12288	13312	14336	15360
11	16384	18432	20480	22528	24576	26624	28672	30720
12	32768	36864	40960	45056	49152	53248	57344	61440
13	65536	73728	81920	90112	98304	106496	114688	122880
14	131072	147456	163840	180224	196608	212992	229376	245760
15	262144	294912	327680	360488	393216	425984	458752	491520
Note: The bits this table refers to are contained in the RU size bytes (bytes 9 and 10).