既然NB-Iot的一大亮点是低功耗，那么就必须了解下低功耗的原理和参数。
一、PSM状态
NB进入PSM状态。在此状态下NB的射频被关闭，相当于关机，但是核心网还保留着NB的信息，当NB从PSM唤醒之后进入连接状态（不需要再次进行附着建立）。
在PSM状态下，下行数据不可达，那核心网就是要下发怎么办，此时需要服务网关缓存下行数据并延迟，等NB从PSM唤醒之后，再下发这条数据。那么NB被唤醒有2种方式，第一是NB需要上行发数据的时候，第二是TAU定时器超时，注意TAU和激活定时器是2个不同的定时器。TAU定时器的用途是NB必须多久上报一次，类似心跳。
进入PSM状态，NB不再接收寻呼消息和下行数据，看起来NB和核心网失去连接，但是NB仍然注册在核心网上（核心网还保留着NB的信息）。
PSM的优点是可进行长时间的睡眠，缺点是不能马上收到核心网的下行数据（下行实时性差），主要用在远程抄表等对下行数据实时性要求不高的产品。但是物联网和手机需求不同，手机的话需要随时监听基站的呼叫请求，要不别人找不到你，但是物联网大部分就是要上行数据，比如抄表，只要把每个月的用电量传上去就可以。
在下图中，T3412就是TAU定时器，T3324就是激活定时器。DRX就是周期性寻呼的时间，在空闲状态下，还支持周期性寻呼。
二、参数说明
1、T3412TAU定时器
<Periodic-TAU> string type; one byte in an 8-bit format. Indicates the extended periodic TAU value (T3412) allocated to the UE in E-UTRAN. The extended periodic TAU value is coded as one byte (octet 3) of the GPRS Timer 3 information element coded as bit format (e.g. "01000111" equals 70 hours). For the coding and the value range, see the GPRS Timer 3 IE in 3GPP TS 24.008, Table 10.5.163a/3GPP TS 24.008. See also 3GPP TS 23.682 and 3GPP TS 23.401.
GPRS Timer 3 value (octet 3)
Bits 5 to 1 represent the binary coded timer value.
Bits 6 to 8 defines the timer value unit for the GPRS timer as follows:
Bits
8 7 6
0 0 0 value is incremented in multiples of 10 minutes
0 0 1 value is incremented in multiples of 1 hour
0 1 0 value is incremented in multiples of 10 hours
0 1 1 value is incremented in multiples of 2 seconds
1 0 0 value is incremented in multiples of 30 seconds
1 0 1 value is incremented in multiples of 1 minute
1 1 0 value is incremented in multiples of 320 hours (NOTE 1)
1 1 1 value indicates that the timer is deactivated (NOTE 2).
NOTE 1: This timer value unit is only applicable to the T3312 extended value IE and the T3412 extended value IE (see 3GPP TS 24.301 [120]). If the UE receives the IE in an integrity protected message, then it shall interpret the value as multiples of 320 hours. Otherwise it shall interpret the value as multiples of 1 hour.
If the network receives the IE in S1 mode, the network shall interpret the value as multiples of 320 hours, regardless whether the IE is received in an integrity protected message (e.g., an ATTACH REQUEST message or TRACKING AREA UPDATE REQUEST message (see 3GPP TS 24.301 [120])) or not. Otherwise it shall interpret the value as multiples of 1 hour.
NOTE 2: This timer value unit is not applicable to the T3412 extended value IE. If this timer value is received, the T3412 extended value IE shall be considered as not included in the message (see 3GPP TS 24.301 [120]).
2、T3324PSM状态请求的活动时间
<Active-Time> string type; one byte in an 8-bit format. Indicates the Active Time value (T3324) allocated to the UE in E-UTRAN. The Active Time value is coded as one byte (octet 3) of the GPRS Timer 2 information element coded as bit format (e.g. "00100100" equals 4 minutes). For the coding and the value range, see the GPRS Timer 2 IE in 3GPP TS 24.008 Table 10.5.163/3GPP TS 24.008. See also 3GPP TS 23.682 and 3GPP TS 23.401.
The purpose of the GPRS timer 2 information element is to specify GPRS specific timer values, e.g. for the timer T3302 or timer T3319.
The GPRS timer 2 is a type 4 information element with 3 octets length.
The GPRS timer 2 information element is coded as shown in figure10.5.147/3GPP TS 24.008 and table 10.5.163/3GPP TS 24.008.
8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | ||
GPRS Timer 2 IEI | octet 1 | ||||||||
Length of GPRS Timer 2 contents | octet 2 | ||||||||
GPRS Timer 2 value | octet 3 | ||||||||
Figure 10.5.147/3GPP TS 24.008: GPRS Timer 2 information element
Table 10.5.163/3GPP TS 24.008: GPRS Timer 2 information element
GPRS Timer 2 value is coded as octet 2 of the GPRS timer information element. |
8 7 6
0 0 0 value is incremented in multiples of 2 seconds
0 0 1 value is incremented in multiples of 1 minute
0 1 0 value is incremented in multiples of 6 minutes
1 1 1 value indicates that the timer is deactivated.
三、参数PSM实例
TAU时间：00100011 表示3小时
T3324时间：00100010 表示2分钟
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