802.15.4 信道接入

翻译自802.15.4 2006版的7.5.1，和上一篇CSMA-CA算法是姊妹篇，本篇主要是信道接入、超帧结构的介绍。

7.5.1 Channel access 信道接入

This subclause describes the mechanisms for accessing the physical radio channel.

 

7.5.1.1 Superframe structure 超帧结构

A coordinator on a PAN can optionally bound its channel time using a superframe structure. A superframe is bounded by the transmission of a beacon frame and can have an active portion and an inactive portion. The coordinator may enter a low-power (sleep) mode during the inactive portion.

 

PAN网络下的协调器可以选择用超帧结构来确定它的信道时间。一个超帧是和一个信标帧的发射绑定的，分为活动和非活动部分。在非活动部分协调器可以进入低功耗（睡眠）模式。

 

The structure of this superframe is described by the values of macBeaconOrder and macSuperframeOrder. The MAC PIB attribute macBeaconOrder, describes the interval at which the coordinator shall transmit its beacon frames. The value of macBeaconOrder, BO, and the beacon interval, BI, are related as follows: for 0 ≤ BO ≤ 14, BI = aBaseSuperframeDuration * 2BO symbols. If BO = 15, the coordinator shall not transmit beacon frames except when requested to do so, such as on receipt of a beacon request command. The value of macSuperframeOrder shall be ignored if BO = 15.

 

超帧结构由macBeaconOrder和macSuperframeOrder来描述。MAC PIB属性macBeaconOrder描述了协调器信标帧的发射间隔。macBeaconOrder的值，BO，以及信标间隔BI的关系如下：0 ≤ BO ≤ 14,

 

The MAC PIB attribute macSuperframeOrder describes the length of the active portion of the superframe, which includes the beacon frame. The value of macSuperframeOrder, SO, and the superframe duration, SD, are related as follows: for 0 ≤ SO ≤ BO ≤ 14, SD = aBaseSuperframeDuration * 2SO symbols. IfSO = 15, the superframe shall not remain active after the beacon. If BO = 15, the superframe shall not exist (the value of macSuperframeOrder shall be ignored), and macRxOnWhenIdle shall define whether the receiver is enabled during periods of transceiver inactivity.

 

MAC PIB属性macSuperframeOrder描述了超帧活动部分的长度，包括信标帧。macSuperframeOrder的值，SO，以及超帧持续时间SD的关系如下：0 ≤ SO ≤ BO ≤ 14,个符号。如果SO=15，在信标之后超帧将没有活动部分。如果BO=15，超帧将不存在，同时macRxOnWhenIdle将定义在收发机不活动的时间内接收是否被允许。

 

The active portion of each superframe shall be divided into aNumSuperframeSlots equally spaced slots of duration 2SO * aBaseSlotDuration and is composed of three parts: a beacon, a CAP and a CFP. The beacon shall be transmitted, without the use of CSMA, at the start of slot 0, and the CAP shall commence immediately following the beacon. The start of slot 0 is defined as the point at which the first symbol of the beacon PPDU is transmitted. The CFP, if present, follows immediately after the CAP and extends to the end of the active portion of the superframe. Any allocated GTSs shall be located within the CFP.

 

超帧的活动部分将会被平均分成aNumSuperframeSlots份，每份时长为。活动部分由三部分构成：信标、CAP和CFP。信标在时隙0传输，不采用CSMA机制。CAP紧跟着信标发射。时隙0的开始被定义信标PPDU的第一个符号开始发送。CFP跟随CAP扩展了超帧的活动部分。每一个分配的GTS位于CFP之内。

 

The MAC sublayer shall ensure that the integrity of the superframe timing is maintained, e.g., compensating for clock drift error.

 

MAC子层将保证超帧时序正确性的维护，如补偿时钟偏移的误差。

 

PANs that wish to use the superframe structure (referred to as a beacon-enabled PAN) shall set macBeaconOrder to a value between 0 and 14, both inclusive, and macSuperframeOrder to a value between 0 and the value of macBeaconOrder, both inclusive.

 

PAN网络如果使用超帧结构（信标网络）将设置macBeaconOrder的值在0到14之间。同时macSuperframeOrder的值在0到macBeaconOrder之间。

 

PANs that do not wish to use the superframe structure (referred to as a nonbeacon-enabled PAN) shall set both macBeaconOrder and macSuperframeOrder to 15. In this case, a coordinator shall not transmit beacons, except upon receipt of a beacon request command; all transmissions, with the exception of acknowledgment frames and any data frame that quickly follows the acknowledgment of a data request command (see 7.5.6.3), shall use an unslotted CSMA-CA mechanism to access the channel. In addition, GTSs shall not be permitted.

 

PAN网络不希望使用超帧结构（非信标网络）将设置macBeaconOrder和macSuperframeOrder为15。对于这种情况，协调器将不发送信标，除非在收到信标请求命令之后。所有的发送，除了确认帧和紧随数据请求命令确认帧的数据帧，将使用unslotted CSMA-CA机制来实现信道接入。另外，GTS将不被允许。