Channel Switch Announcement element

IEEE 802.11-2012

The Channel Switch Announcement element is used by an AP in a BSS, a STA in an IBSS, or a mesh STA in an MBSS to advertise when it is changing to a new channel and the channel number of the new channel. The format of the Channel Switch Announcement element is shown in Figure 8-102.The Length field is set to 3.

Element ID

Length

Channel Switch Mode

New Channel Number

Channel Switch Count

Octets:    1                                1                                 1                              1                                  1

Figure 8-102—Channel Switch Announcement element format

    The Channel Switch Mode field indicates any restrictions on transmission until a channel switch.

An AP in a BSS or a STA in an IBSS sets the Channel Switch Mode field to either 0 or 1 on transmission.

In an MBSS, the Channel Switch Mode Field is reserved. See 10.9.9. The New Channel Number field is set to the number of the channel to which the STA is moving (as definedin 18.3.8.4.3).For nonmesh STAs, the Channel Switch Count field either is set to the number of TBTTs until the STA sending the Channel Switch Announcement element switches to the new channel or is set to 0.

A value of 1 indicates that the switch occurs immediately before the next TBTT.

A value of 0 indicates that the switch occurs at any time after the frame containing the element is transmitted.For mesh STAs, the Channel Switch Count field is encoded as an octet with bits 6 to 0 set to the time, in units of 2 TU when the MSB (bit 7) is 0, or in units of 100 TU when the MSB (bit 7) is 1, until the mesh STA sending the Channel Switch Announcement element switches to the new channel. A value of 0 for bits 6 to 0 indicates that the switch occurs at any time after the frame containing the element is transmitted. For example, a 200 TU channel switch time is encoded as X’82’ and a 10 TU channel switch time is encoded as X’05’.

The Channel Switch Announcement element is included in Channel Switch Announcement frames, as described in 8.5.2.6, and may be included in Beacon frames, as described in 8.3.3.2, and Probe Response frames, as described in 8.3.3.10. The use of Channel Switch Announcement elements and frames is described in 10.9.8.

Length : set to 3 bytes
Channel Switch Mode :
Indicates any restrictions on transmission until a channel switch. An AP in a
BSS or a STA in an IBSS sets the Channel Switch Mode field to either 0 or 1 on transmission. In an MBSS,the Channel Switch Mode Field is reserved
New Channel Number: set to the number of the channel to which the STA is moving.
Channel Switch Count: for nonmesh STAs, this  field either is set to the number of TBTTs until the STA sending the Channel Switch Announcement element switches to the new channel or is set to 0. A value of 1 indicates that the switch occurs immediately before the next TBTT. A value of 0 indicates that the switch occurs at any time after the frame containing the element is transmitted.

This Channel Switch Announcement element present in beacons & probe responses. Channel Switch Announcement element also associated with an action frame (spectrum management type or category type =0) that can be sent by the AP between beacons to announce the channel switch.

Here is the frame format of Action Frame that contain the Channel Switch Announcement field.The Channel Switch Announcement frame uses the Action frame body format and is transmitted by an AP in a BSS, a STA in an IBSS, or a mesh STA in an MBSS to advertise a channel switch. The format of the Channel Switch Announcement Action field is shown in Figure 8-436

Here are some important points about Channel Switch Announcement in a BSS.

• An AP shall inform associated STAs that the AP is moving to a new channel and maintain the association by advertising the switch using Channel Switch Announcement elements in Beacon frames, Probe Response frames, and Channel Switch Announcement frames until the intended channel switch time.
• The AP may force STAs in the BSS to stop transmissions until the channel switch takes place by setting the Channel Switch Mode field in the Channel Switch Announcement element to 1.
• The channel switch should be scheduled so that all STAs in the BSS, including STAs in power save mode, have the opportunity to receive at least one Channel Switch Announcement element before the switch.
• The AP may send the Channel Switch Announcement frame in a BSS without performing a backoff, after determining the WM is idle for one PIFS period.
• A STA that receives a Channel Switch Announcement element may choose not to perform the specified switch, but to take alternative action. For example, it may choose to move to a different BSS.
• A STA in a BSS that is not the AP shall not transmit the Channel Switch Announcement element

 

Selecting and advertising a new channel

1.General

An attempt may be made to move a BSS to a new operating channel. It is an objective that disruption to the BSS is minimized in this process, although it should be recognized that a channel switch might not successfully move all STAs. It should also be stressed that the channel switch process is distinct from the regulatory requirement to cease transmission on a particular channel in the presence of radar.

为将一个基础服务集（BSS）移动到一个新的可用频道上去可能会进行一个尝试。主管上我们在这个过程中要将BSS的终端最小化，然而客观上我们必须认识到信道切换并非能将所有的工作站都切换成功。还有一点要强调的是：信道切换程序应该符合法规，不能在有雷达存在的那些特殊信道上进行数据传输。

2.Selecting and advertising a new channel in an infrastructure BSS

The decision to switch to a new operating channel in an infrastructure BSS shall be made only by the AP. AnAP may make use of the information in Supported Channel elements and the results of measurements undertaken by the AP and other STAs in the BSS to assist the selection of the new channel. The algorithm to choose a new channel is beyond the scope of this standard, but shall satisfy applicable regulatory requirements, including uniform spreading rules and channel testing rules. The AP shall attempt to select a new channel that is supported by all associated STAs, although it should be noted that this might not always be possible.

在一个基础设施基本服务集上应该只有由AP来决定切换到一个新的工作频道上。一个AP可能会利用“支持的信道元素”中的信息以及由该AP和在该基础服务集上的其他工作站做的一些测量结果来协助做出新的信道选择。选择新的信道机制不在本标准的讨论范围中，但是应该符合相应的监管要求，包含同一的传播规则和信道测试规则。AP应该尽可能的选择一个所有关联着的工作站都支持的信道，虽然这种情况（一个信道被所有的设备都支持）并不是总是可能的。

An AP shall inform associated STAs that the AP is moving to a new channel and maintain the association byadvertising the switch using Channel Switch Announcement elements in Beacon frames, Probe Responseframes, and Channel Switch Announcement frames until the intended channel switch time. The AP mayforce STAs in the BSS to stop transmissions until the channel switch takes place by setting the ChannelSwitch Mode field in the Channel Switch Announcement element to 1. The channel switch should bescheduled so that all STAs in the BSS, including STAs in power save mode, have the opportunity to receiveat least one Channel Switch Announcement element before the switch. The AP may send the ChannelSwitch Announcement frame in a BSS without performing a backoff, after determining the WM is idle forone PIFS period.

A STA that receives a Channel Switch Announcement element may choose not to perform the specified switch, but to take alternative action. For example, it may choose to move to a different BSS.

A STA in a BSS that is not the AP shall not transmit the Channel Switch Announcement element.

3.Selecting and advertising a new channel in an IBSS

DFS in an IBSS is complicated by the following:

— There is no central AP function for collating measurements or coordinating a channel switch. If STAs make independent decisions to switch channel in the presence of radar, there is a danger that all STAs announce switches to different channels if several of them detect the radar.

——没有中心AP功能来整理测量或者协调信道切换。如果工作站在雷达工作的信道上独自切换信道，那么将有个危机就是当他们中的几个或者大多数都检测到了雷达信号的时候，那么所有这些工作站都会通知信道切换，但是这些通知中的目标信道可能都不一样。

— There is no association protocol that can be used to

——没有可以使用的关联协议

— Exchange supported channel information and

— Determine membership of the IBSS at a given instant for requesting measurements.

 

— Beaconing is a shared process; therefore, there is no guarantee that a STA that has something to send

(e.g., a channel switch message) will be the next STA to transmit a Beacon frame.

— A 20/40 MHz IBSS cannot be changed to a 20 MHz IBSS, and a 20 MHz IBSS cannot be changed to a 20/40 MHz IBSS.

The DFS owner service, IBSS DFS element, and Channel Switch Announcement frame address these complications.

— The DFS owner service provides a central point of coordination for a channel switch. It attempts to minimize the probability that multiple STAs concurrently decide to switch to different channels. The DFS Owner field and DFS Recovery Interval field within the IBSS DFS element support the DFS owner service.
— Each STA shall include a Channel Map field within the IBSS DFS elements that it transmits. The channel map communicates the STA-supported channel set and basic measurement reports for that STA.

— The ability to send a Channel Switch Announcement element within a management frame other than a Beacon frame or Probe Response frame is essential.The potential for hidden nodes within an IBSS means that the IBSS channel switch protocol is best effort.All members of an IBSS shall have an individual responsibility to cease transmission on a particular channel in the presence of radar.

A STA at which an IBSS is started shall be a DFS owner in that IBSS. That STA shall include its MAC address in the DFS Owner field of the IBSS DFS element and DFS Recovery Interval field from the MLME.START.request primitive parameter. The purpose of the DFS owner is to coordinate a channel switch when required. All STAs within a spectrum-managed IBSS shall have the ability to become DFS owner.

Each STA in an IBSS shall adopt the DFS owner and the DFS owner recovery interval from any valid IBSS DFS element when the frame contained a matching SSID and the value of the timestamp is later than the STA’s TSF timer. The STA shall include the adopted values within the IBSS DFS elements it transmits. Because all STAs in an IBSS participate in sending Beacon frames, this process always, over a number of beacon intervals, results in a unified view of one DFS owner throughout the IBSS.In order to attempt a channel switch using the DFS owner, a STA that detects radar shall broadcast one or more Measurement Report frames indicating the presence of the radar.

A DFS owner receiving a Measurement Report frame indicating the presence of radar in the current channel shall select and advertise a new operating channel (including the possibility of no change). The DFS owner may make use of information received in Channel Map fields and from measurements undertaken by other members of the IBSS to assist the selection of the new channel. The algorithm to choose a new channel is beyond the scope of this standard, but shall satisfy any regulatory requirements, including uniform spreading rules and channel testing rules. The DFS owner shall attempt to select a new channel that is supported by all members of the IBSS. It should be noted that this process might be imperfect in that the DFS owner may have incomplete knowledge and there may be no suitable channel.

The DFS owner shall attempt to make the members of the IBSS aware of the new operating channel bybroadcasting at least one Channel Switch Announcement frame. The DFS owner shall also include theChannel Switch Announcement element in all Beacon frames, Probe Response frames, or Channel SwitchAnnouncement frames until the intended channel switch time. A STA that receives a valid Channel SwitchAnnouncement element shall repeat this element in all Beacon frames and Probe Response frames that ittransmits. The DFS owner may attempt to silence STAs in the IBSS until the channel switch takes placeusing the Channel Switch Mode field in the Channel Switch Announcement element. If possible, the channelswitch should be scheduled so that all STAs in the IBSS, including STAs in power save mode, have theopportunity to receive at least one Channel Switch Announcement element before the switch.

If a STA does not receive a valid Channel Switch Announcement element from the DFS owner within DFSrecovery time measured from the end of the frame within which radar notification was first transmitted bythe STA or received from another STA, then it shall enter a DFS owner recovery mode. In DFS ownerrecovery mode, the STA shall assume the role of DFS owner, shall select a new operating channel, and shalladvertise the new channel by transmitting a Channel Switch Announcement frame using the contentionresolution algorithm defined for beacon transmission at TBTT in 10.1.3.3. The STA shall also include theChannel Switch Announcement element in all Beacon frames and Probe Response frames until the intendedchannel switch time. A STA that is not the DFS owner shall not initiate a channel switch.

If the STA receives a valid Channel Switch Announcement element from another member of the IBSS, the STA shall leave DFS owner recovery mode prior to the channel switch and adopt the received channel switch information. If the Channel Switch Announcement element was within a Beacon frame or Probe Response frame, the STA shall also adopt the DFS owner address from the IBSS DFS element. If the Channel Switch Announcement element was within a Channel Switch Announcement frame, the STA shall adopt the DFS owner from the transmitter address of the received frame.

There are several circumstances when DFS owner recovery is required (e.g., if the original DFS owner hasleft the network or if the original measurement report was not received by the initial DFS owner). It shouldbe noted that DFS owner recovery might temporarily give rise to more than one DFS owner within the IBSS.

This risk is mitigated by the random nature of the IBSS DFS recovery mechanism. However, because all STAs in an IBSS participate in sending Beacon frames, over a number of beacon periods, there will be convergence from multiple DFS owners to one DFS owner.