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Specifying ATM1 Shaping Values

For ATM1 interfaces, you can specify the rates in bits per second or cells per second. For OC3c interfaces, the highest rate is 135,631,698 bps (353,207.55 cps), which corresponds to 100 percent of the available line rate. For OC12c interfaces, the highest rate is 271,263,396 bps (706,415.09 cps), which corresponds to 50 percent of the available line rate. Table 30 lists some of the other rates you can specify. If you specify a rate that is not listed, it is rounded to the nearest rate.

The exact number of values differs between OC12c and OC3c interfaces. OC12c interfaces have about four times as many value increments as OC3c interfaces.

For OC12c rates between 1/2 of the line rate and 1/128 of the line rate, there are 128 steps between each 1/n value. This means that there is 128 steps between the 1/2 and 1/3 line rate values, and another 128 steps between 1/3 and 1/4 and so on. For rates smaller than 1/127, there are (16,384 minus 127) or 16, 257 values. The reason for this is that fractional shaping is ignored at rates below 1/127. This results in a total of about 32,384 distinct rates for OC12c. When n is larger than or equal to 127, the steps are 1/n.

For OC3c, the starting point is full line rate, the fraction/integer breakpoint is about 1/31, and there is a maximum of 4096 scheduler slots for use after 1/31 of line rate, producing about 8032 total distinct rates. When n is larger than or equal to 31, the steps are 1/n.

For ATM1 interfaces, the following formula can be used to predict the actual shaping rate:

OC3 shaping settings between 135,631,698 bps (OC3 ATM cell line rate) and 4,375,216 bps (1/31 of OC3 ATM cell line rate).
OC12 shaping settings between 271,263,396 bps (half OC12 ATM cell line rate – the highest rate supported) and 4,271,864 bps (1/127 of OC12 ATM cell line rate).actual-rate = (128 * line-rate) / (trunc ((128 * line-rate) / desired-rate))

line-rate is the maximum available rate on the interface (in bits per second) after factoring out the overhead for SONET/SDH and ATM (per-cell) overheads. For OC3c interfaces, the line rate is calculated as follows:




line-rate = 155,520,000 bps x (26/27)
x (48/53) = 135,631,698.1 bps

For OC12c interfaces, the line rate is calculated as follows:




line-rate = 622,080,000 bps x (26/27)
x (48/53) = 542,526,792.45 bps

desired-rate is the rate you enter in the vbr statement, in bits per second.

The trunc operator indicates that all digits to the right of the decimal point should be dropped.

For shaping settings smaller than 1/31 of OC3 ATM cell line rate (4,375,216 bps) and 1/127 of OC12 ATM cell line rate (4,271,864 bps), you can predict the actual shaping rate using the following formula:



actual-rate = ( 1 / ( trunc ( line-rate / desired-rate
) +1 ) ) * line-rate

For example, for OC12 interfaces, the actual rates for shaping below 4,271,864 bps are calculated as follows:



1 / 127 * 542,526,792.45 bps = 4,271,864 bps (11124 cells/second)
1 / 128 * 542,526,792.45 bps = 4,238,490 bps (11038 cells/second)
1 / 129 * 542,526,792.45 bps = 4,205,634 bps (10952 cells/second)
...

Buffers are shared among all VCs, and by default, there is no limit to the buffer size for a VC. If a VC is particularly slow, it might use all the buffer resources.

Table 30 shows ATM1 traffic-shaping rates.

Table 30: ATM1 Traffic-Shaping Rates

Interface Type	Line Rate (bps)	Line Rate (cps)	Percentage of Total Line Rate
OC3
	135,600,000	353,125	100.00
	134,542,320	350,370.66	99.22
	133,511,760	347,686.88	98.46
	132,494,760	345,038.44	97.71
	131,491,320	342,425.31	96.97
	130,501,440	339,847.5	96.24
	129,525,120	337,305	95.52
	128,562,360	334,797.81	94.81
	127,626,720	332,361.25	94.12
	126,691,080	329,924.69	93.43
OC12
	271,263,396	706,415.09	50.00
	270,207,897	703,666.40	49.81
	269,160,579	700,939.01	49.61
	268,121,349	698,232.68	49.42
	267,090,113	695,547.17	49.23
	266,066,779	692,882.24	49.04
	265,051,257	690,237.65	48.85
	264,043,458	687,613.17	48.67
	263,043,293	685,008.58	48.48
	262,050,677	682,423.64	48.30

Example: Specifying ATM1 Shaping Values

Determine the actual rate in ATM1 interfaces when the desired rate is 80 percent of the maximum rate:

OC3c:135,600,000 bps * 0.8 = 108,480,000 bps
Because 108,480,000 bps is greater than 1/31 of OC3 ATM cell line rate:
actual-rate = (128 * 135,600,000.1) / (trunc ((128 * 135,600,000.1) / 108,480,000)) actual-rate = 17,356,800,013 / (trunc (17,356,800,013 / 108,480,000)) actual-rate = 17,356,800,013 / 160 actual-rate = 108,480,000 bps
OC12c:271,263,396 bps * 0.8 = 217,010,716.8 bps
Because 217,010,716.8 bps is greater than 1/127 of OC12 ATM cell line rate:
actual-rate = (128 * 542,526,792.45) / (trunc ((128 * 542,526,792.45)/217,010,716.8)) actual-rate = 69,443,429,434 / (trunc (69,443,429,434 / 217,010,716.8)) actual-rate = 69,443,429,434 / 320 actual-rate = 217,010,717 bps

Determine the actual rate in ATM1 interfaces when the desired rate is 3,000,000 bps:

OC3c:
Because 3,000,000 bps is smaller than 1/31 of OC3 ATM cell line rate:
actual-rate= ( 1 / ( trunc ( line-rate / desired-rate ) + 1 ) ) * line-rate actual-rate = ( 1 / ( trunc ( 135,631,698 / 3,000,000 ) + 1 ) ) * 135,631,698 actual-rate = ( 1 / ( 45 + 1 ) ) * 135,631,698 actual-rate = ( 1 / 46 ) * 135,631,698 actual-rate = 2,948,515 bps
OC12c:
Because 3,000,000 bps is smaller than 1/127 of OC12 ATM cell line rate:
actual-rate = ( 1 / ( trunc ( line-rate / desired-rate ) + 1 ) ) * line-rate actual-rate = ( 1 / ( trunc (542,526,792 / 3,000,000 ) + 1 ) ) * 542,526,792 actual-rate = ( 1 / ( 180 + 1 ) ) * 542,526,792 actual-rate = ( 1 / 181 ) * 542,526,792 actual-rate = 2,997,386 bps

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