A TDMA timeslot allocation algorithm for UWB

Abstract

The present invention provides a TDMA time slot allocation algorithm applied to UWB. The base station uses P frame as a small cycle, and a G frame is composed of multiple P frames. The tag uses the G frame period as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame includes two parts. The first part is the Alloc time slot allocation part, and the second part is the Work working part. The Alloc time slot allocation part consists of a Beacon period, L Join periods and an Idle period. The Work working part consists of M Work time slots. The number of time slots constituting Beacon, Join and Work is determined according to the UWB working rate of 110Kbps, 850Kpbs and 6.8Mbps, the preamble length of PHY and the time required for Work work, which needs to include the UWB wireless air transmission time and the CPU processing time required for the base station and the tag respectively. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, and the Idle period adjusts the duration of the P frame.

Description

Technical Field

[0001] The present invention relates to the field of UWB wireless communications, and in particular to a TDMA time slot allocation algorithm applied to UWB. Background Art

[0002] UWB wireless communication systems face a common challenge: when multiple devices share a channel, they compete for access to the resource and avoid simultaneous signal transmissions that could cause channel conflicts. In practical applications, particularly in high-density environments with a large number of terminals, improper channel allocation can lead to a communication avalanche effect, saturating the UWB channel.

[0003] Therefore, we have made improvements to this and proposed a TDMA time slot allocation algorithm for UWB. TDMA time division multiplexing technology is used to allocate channel time slots, so that each terminal can receive and send data within the specified time slot, effectively avoiding channel conflicts, ensuring communication reliability, reducing terminal power consumption, and extending terminal working time. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems raised by the existing background technology and to achieve the above invention purpose. The present invention provides the following technical solutions: a TDMA time slot allocation algorithm applied to UWB, including a base station and a tag, wherein the base station uses P frame as a small cycle, and a plurality of P frames form a G frame large cycle, and the tag uses the G frame cycle as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame contains two parts, the first part is the Alloc time slot allocation part, and the second part is the Work working part. The Alloc time slot allocation part consists of a Beacon The Beacon, Join, and Work time slots are determined by the UWB working rate of 110Kbps, 850Kpbs, and 6.8Mbps, the preamble length of the PHY, and the time required for the Work operation, which includes the UWB wireless air transmission time and the CPU processing time of the base station and the tag. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, and the Idle period adjusts the time length of the P frame.

[0005] As the preferred technical solution of this application, the base station broadcasts its presence by periodically sending P frames through a timer, and sends a Beacon data frame at the beginning of a P frame. The Beacon frame contains base station information. The base station and the tag apply for and allocate time slots through the following steps: S1, the base station uses T P It periodically broadcasts a Beacon frame at the beginning of the P frame, which carries its own ID and information about whether there is an idle time slot.

[0006] As a preferred technical solution of the present application, the tag also includes S2, monitoring the Beacon frame, checking the available flag therein, and returning to step 1 to continue monitoring if there is no idle time slot, otherwise proceeding to step 3; S3, the tag uses an improved carrier sense multiple access / collision avoidance (CSMA / CA) algorithm to detect whether the UWB channel is idle, and at the same time starts the timer Timerjoin, whose timeout period is L*T join0 –2*T s .

[0007] As the preferred technical solution of this application, it also includes S4, running the CSMA / CA algorithm detection until it detects that the channel is idle and proceeds to the next step. Otherwise, if the timer Timerjoin times out, the monitoring is terminated and the process returns to step S1; S5, the tag sends a Join frame to the base station that receives the Beacon frame and applies for a time slot from the base station. The Join frame contains the tag ID and type information. At the same time, the tag starts a timer Timer ack with a timeout period of T join0 .

[0008] As the preferred technical solution of the present application, it also includes S6, the base station selects a time slot from the S time slots, and sends the allocated time slot ID, base station ID, delayed Work time and T g The time information is sent to the tag via the ACK frame; S7. After receiving the ACK frame, the tag will work at the specified time according to the ACK information.

[0009] As a preferred technical solution of the present application, it also includes S8, if the timer Timer ack times out and the tag still does not receive the ack frame, return to step S3; S9, if the timer Timerjoin times out, return to step S1.

[0010] As the preferred technical solution of the present application, the time slot ID range of the base station in S6 is 1 to S. When the time slot is selected, the entire time slot will be divided into two parts, that is, the time slot number ID is odd and even. The odd part is allocated first, and then the even part is allocated. In this way, the access tag is appropriately discretized to avoid interference when adjacent time slot tags send data due to clock drift between the base station and the tag.

[0011] As a preferred technical solution of the present application, when the base station fails to allocate all the cycles for the first time, the time slot ID sequence it allocates is:

[0012] 1, 3, 5, 7...

[0013] 2, 4, 6, 8...

[0014] The base station will map the allocated time slot ID and tag ID and maintain a timer for this time slot with a timeout of n*T. g , the value range of n is [3,6]. When the tag sends data to the base station in the specified time slot, the base station will reset the timer. Once the timer expires, the base station will recycle the allocated time slot. After one allocation, the above sequence will be different. The base station will continue to allocate in the recycled time slot, resulting in unpredictable time slot ID sequence.

[0015] As the preferred technical solution of this application, the tag type can be changed according to the working cycle. The tag is based on the G frame period T g To start working in units, the tag needs to start working at a faster speed. The cycle can be G frame cycle T g A quarter of the time slot allocation algorithm in S6 needs to be fine-tuned, and the time slots corresponding to the first two P frames in the G frame are reserved for the maximum number of such tags, which is 2*M, and the P in the G frame is reserved for the first two P frames. N / 4-1 、P N / 4 、P 2N / 4-1 、P 2N / 4 、P 3N / 4-1 、P 3N / 4 The corresponding time slot ID is also reserved and does not participate in the time slot allocation of ordinary tag types.

[0016] As the preferred technical solution of this application, the tag in S3 will use an improved CSMA / CA algorithm to first determine whether the channel is occupied. If it is occupied, the backoff algorithm will be used to retry access. CSMA / CA is in the Join time slot application stage of the Beacon phase, but there is no need to perform the CSMA / CA backoff algorithm in the Work phase. It detects whether the channel is busy before sending, thereby reducing access conflicts caused by time drift between the base station and the tag. The processing flow of CSMA / CA has the following parameters: NB backoff times, the initial value of NB is 0. When the device has data to send, after a backoff time, if it detects that the channel is busy, it will generate a backoff time again, and the NB value will be increased by 1. In IEEE 802.15.4, the maximum NB value is defined as 4. When the channel is still busy after 4 backoff delay times, wait for the next P frame.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] In the scheme of this application:

[0019] 1. The base station uses P frames as a small cycle, and multiple P frames form a G frame large cycle. The tag uses the G frame period as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame consists of two parts: the first part is the Alloc time slot allocation part, and the second part is the Work work part. The Alloc time slot allocation part consists of a Beacon period, L Join periods, and an Idle period. The size of L is adjusted according to actual conditions.

[0020] 2. The Work section consists of M Work time slots. The Work time slot is the time slot in which the tag works normally after the base station allocates the time slot. The number of time slots that make up Beacon, Join and Work is determined by the UWB working rate of 110Kbps, 850Kpbs and 6.8Mbps and the preamble length of PHY and the time required for Work. It needs to include the UWB wireless air transmission time and the CPU processing time of the base station and the tag respectively. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, which adjusts the duration of the P frame.

[0021] 3. The base station broadcasts its presence by periodically sending P frames through a timer. A Beacon data frame is sent at the beginning of a P frame. The Beacon frame contains base station information. The base station and the tag apply for and allocate time slots through the following steps: S1. The base station sends a P frame with T P A Beacon frame is periodically sent at the beginning of a P frame, carrying its own ID and information about whether there is an idle time slot.

[0022] 4. Run the CSMA / CA algorithm until the channel is idle and proceed to the next step. Otherwise, the timer Timerjoin times out and the monitoring is terminated, returning to step S1. S5. The tag sends a Join frame to the base station that received the Beacon frame and requests a time slot from the base station. The Join frame contains the tag ID and type information. The tag starts a timer Timer ack with a timeout of T. join0 . BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the G frame structure provided for this application;

[0024] Figure 2 Schematic diagram of the P frame structure provided for this application;

[0025] Figure 3 Periodic formula diagram provided for this application;

[0026] Figure 4 The tag time slot application flow chart provided for this application;

[0027] Figure 5 CSMA / CA algorithm diagram provided for this application. DETAILED DESCRIPTION

[0028] To make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiment is a specific implementation of the present invention and is not limited to all embodiments.

[0029] Therefore, the following detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely represents some embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort shall fall within the scope of protection of the present invention.

[0030] like Figure 1-Figure 5 , a TDMA time slot allocation algorithm applied to UWB, the base station uses P frame as a small cycle, and a G frame is composed of multiple P frames. The tag uses the G frame period as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame contains two parts, the first part is the Alloc time slot allocation part, and the second part is the Work working part. The Alloc time slot allocation part consists of a Beacon period, L Join periods and an Idle period. The size of L is adjusted according to the actual situation. The Work working part consists of M Work time slots. The Work time slot is the time slot in which the tag works normally after the base station allocates the time slot. The number of time slots that make up Beacon, Join and Work is determined according to the UWB working rate of 110Kbps, 850Kpbs and 6.8Mbps, the preamble length of PHY and the time required for Work work, which needs to include the UWB wireless air transmission time and the time required for the base station and tag CPU processing. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, and the Idle period adjusts the duration of the P frame.

[0031] As a preferred embodiment, based on the above method, the base station further transmits P frames periodically through a timer to broadcast its presence, and sends a Beacon data frame at the beginning of a P frame. The Beacon frame contains base station information. The base station and the tag apply for and allocate time slots through the following steps: S1, the base station uses T P A Beacon frame is periodically sent at the beginning of a P frame, carrying its own ID and information about whether there are idle time slots.

[0032] As a preferred embodiment, based on the above method, it further includes S2, the tag monitors the Beacon frame and checks the available flag therein. If there is no idle time slot, it returns to step 1 to continue monitoring, otherwise it goes to step 3; S3, the tag uses an improved carrier sense multiple access / collision avoidance (CSMA / CA) algorithm to detect whether the UWB channel is idle, and at the same time starts the timer Timerjoin, whose timeout period is L*T join0 –2*T s .

[0033] As a preferred implementation, based on the above method, it further includes S4, running the CSMA / CA algorithm detection until it detects that the channel is idle and proceeds to the next step. Otherwise, the timer Timer join times out and terminates the monitoring and returns to step S1; S5, the tag sends a Join frame to the base station that receives the Beacon frame, and applies for a time slot from the base station. The Join frame contains the tag ID and type information, and the tag starts a timer Timer ack with a timeout period of T join0 .

[0034] As a preferred embodiment, based on the above method, further comprising S6, the base station selects a time slot from the S time slots, and sends the allocated time slot ID, base station ID, delayed Work time and T g The time information is sent to the tag via the ACK frame; S7. After receiving the ACK frame, the tag will work at the specified time according to the ACK information.

[0035] As a preferred implementation, based on the above method, further, it includes S8, if the timer Timerack times out and the tag still does not receive the ack frame, returning to step S3; S9, if the timer Timerjoin times out, returning to step S1.

[0036] As a preferred implementation method, based on the above method, further, the time slot ID range of the base station in S6 is 1 to S. When the time slot is selected, the entire time slot will be divided into two parts, that is, the time slot number ID is odd and even. The odd part is allocated first, and then the even part is allocated. In this way, the access label is appropriately discretized to avoid interference when adjacent time slot labels send data due to clock drift between the base station and the label.

[0037] As a preferred embodiment, based on the above method, further, when the base station does not allocate all the cycles for the first time, the timeslot ID sequence it allocates is:

[0038] 1, 3, 5, 7...

[0039] 2, 4, 6, 8...

[0040] The base station will map the allocated time slot ID and tag ID and maintain a timer for this time slot with a timeout of n*T. g , the value range of n is [3,6]. When the tag sends data to the base station in the specified time slot, the base station will reset the timer. Once the timer expires, the base station will recycle the allocated time slot. After one allocation, the above sequence will be different. The base station will continue to allocate in the recycled time slot, resulting in unpredictable time slot ID sequence.

[0041] As a preferred embodiment, based on the above method, further, the tag type can be changed according to the working cycle, and the tag is based on the G frame period T g To work in units, the tag needs to start working at a faster speed. The cycle can be G frame cycle T g A quarter of the time slot allocation algorithm in S6 needs to be fine-tuned, and the time slots corresponding to the first two P frames in the G frame are reserved for the maximum number of such tags, which is 2*M. N / 4-1 、P N / 4 、P 2N / 4-1 、P 2N / 4 、P 3N / 4-1 、P 3N / 4 The corresponding time slot ID is also reserved and does not participate in the time slot allocation of ordinary tag types.

[0042] As a preferred implementation, based on the above method, further, the tag in S3 will use an improved CSMA / CA algorithm to first determine whether the channel is occupied. If it is occupied, the backoff algorithm will be used to retry access. CSMA / CA is used in the Join time slot application stage of the Beacon phase, but no CSMA / CA backoff algorithm is required in the Work phase. Before sending, it detects whether the channel is busy, thereby reducing access conflicts caused by time drift between the base station and the tag. The CSMA / CA processing flow has the following parameters: NB backoff times, the initial value of NB is 0. When the device has data to send, after a backoff time, if it detects that the channel is busy, it will generate a backoff time again, and the NB value will be increased by 1. In IEEE802.15.4, the maximum NB value is defined as 4. When the channel is still busy after 4 backoff delay times, it waits for the next P frame.

[0043] Working principle: During the use of the present invention, a TDMA time slot allocation algorithm applied to UWB is used. The base station uses P frame as a small cycle, and a G frame is composed of multiple P frames. The tag uses the G frame period as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame contains two parts. The first part is the Alloc time slot allocation part, and the second part is the Work working part. The Alloc time slot allocation part consists of a Beacon period, L Join periods and an Idle period. The size of L is adjusted according to the actual situation. The Work working part consists of M Work time slots. The Work time slot is the time slot in which the tag works normally after the base station allocates the time slot. The number of time slots that make up Beacon, Join and Work is determined according to the UWB working rate of 110Kbps, 850Kpbs and 6.8Mbps and the preamble code length of PHY and the time required for Work work. It needs to include the UWB wireless air transmission time and the time required for the base station and tag's respective CPU processing. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, and the Idle period adjusts the duration of the P frame.

[0044] The base station broadcasts its presence by sending P frames periodically through a timer. At the beginning of a P frame, a Beacon data frame is sent. The Beacon frame contains base station information. The base station and the tag apply for and allocate time slots through the following steps: S1. The base station sends a P frame with T PThe tag periodically sends a Beacon frame at the beginning of the P frame, carrying its own ID and whether there is an idle time slot. It also includes S2, the tag monitors the Beacon frame and checks the available flag in it. If there is no idle time slot, it returns to step 1 to continue monitoring, otherwise it goes to step 3; S3, the tag uses the improved carrier sense multiple access / collision avoidance (CSMA / CA) algorithm to detect whether the UWB channel is idle, and at the same time starts the timer Timerjoin, whose timeout period is L*T join0 –2*T s , S4, run CSMA / CA algorithm detection until the channel is detected to be idle and go to the next step, otherwise the timer Timerjoin times out and terminates the monitoring and returns to step S1; S5, the tag sends a Join frame to the base station that receives the Beacon frame and requests a time slot from the base station. The Join frame contains the tag ID and type information, and the tag starts a timer Timer ack with a timeout of T join0 , S6, the base station selects a time slot from the S time slots and sends the allocated time slot ID, base station ID, delayed Work time and T g The time information is sent to the tag via the ack frame. S7: After receiving the ack frame, the tag will work at the specified time according to the ack information. S8: If the timer Timer ack times out and the tag still has not received the ack frame, the process returns to step S3. S9: If the timer Timerjoin times out, the process returns to step S1. The time slot ID range of the base station in S6 is 1 to S. When selecting the time slot, the entire time slot is divided into two parts, that is, the time slot ID is odd and even. The odd part is allocated first, and then the even part is allocated. This appropriately discretizes the access tag to avoid interference when adjacent time slot tags send data due to clock drift between the base station and the tag. If the base station does not complete the cycle in the first allocation, the time slot ID sequence it allocates is:

[0045] 1, 3, 5, 7...

[0046] 2, 4, 6, 8...

[0047] The base station will map the allocated time slot ID and tag ID and maintain a timer for this time slot with a timeout of n*T. g , the value range of n is [3,6]. When the tag sends data to the base station in the specified time slot, the base station will reset the timer. Once the timer expires, the base station will recycle the allocated time slot. After one allocation, the above sequence will be different. The base station will continue to allocate in the recycled time slot, resulting in unpredictable time slot ID sequence.

[0048] The tag type can be different and the working cycle can also change accordingly. The tag follows the G frame period Tg To work in units, the tag needs to start working at a faster speed. The cycle can be G frame cycle T g A quarter of the time slot allocation algorithm in S6 needs to be fine-tuned, and the time slots corresponding to the first two P frames in the G frame are reserved for the maximum number of such tags, which is 2*M. N / 4-1 、P N / 4 、P 2N / 4-1 、P 2N / 4 、P 3N / 4-1 、P 3N / 4 The corresponding time slot ID is also reserved and does not participate in the time slot allocation of ordinary tag types. The tag in S3 will use the improved CSMA / CA algorithm to first determine whether the channel is occupied. If it is occupied, the backoff algorithm will be used to retry access. CSMA / CA is used in the Join time slot application stage of the Beacon phase, but no CSMA / CA backoff algorithm is required in the Work phase. Before sending, it detects whether the channel is busy, thereby reducing access conflicts caused by time drift between the base station and the tag. The CSMA / CA processing flow has the following parameters: NB backoff times, the initial value of NB is 0. When the device has data to send, after a backoff time, if it detects that the channel is busy, it will generate a backoff time again, and the NB value will increase by 1. In IEEE802.15.4, the maximum NB value is defined as 4. When the channel is still busy after 4 backoff delay times, it waits for the next P frame.

[0049] The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Although this specification has described the present invention in detail with reference to the above embodiments, the present invention is not limited to the above specific implementation methods. Therefore, any modification or replacement of the present invention; and all technical solutions and improvements thereof that do not depart from the spirit and scope of the invention are included in the scope of the claims of the present invention.

Claims

1. A TDMA time slot allocation algorithm for UWB, including a base station and a tag, characterized in that: The base station uses P frame as a small cycle, and a G frame large cycle is composed of multiple P frames. The tag uses the G frame period as the working cycle. The duration of the P frame determines the number of time slots in TDMA and the period value of the G frame. The P frame includes two parts, the first part is the Alloc time slot allocation part, and the second part is the Work working part. The Alloc time slot allocation part consists of a Beacon period, L Join periods and an Idle period. The size of L is adjusted according to the actual situation. The Work working part consists of M Work time slots. The Work time slot is the time slot in which the tag works normally after the base station allocates the time slot. The number of time slots that make up Beacon, Join and Work is determined according to the UWB working rate of 110Kbps, 850Kpbs and 6.8Mbps and the preamble code length of PHY and the time required for Work work. It needs to include the UWB wireless air transmission time and the time required for the base station and tag's respective CPU processing. The Idle period occupies at least 2 T s The time unit is 2ms. The Alloc and Work parts each have an Idle period, which adjusts the duration of the P frame. The base station broadcasts its presence by periodically sending P frames through a timer. At the beginning of a P frame, a Beacon data frame is sent. The Beacon frame contains base station information. The base station and the tag apply for and allocate time slots through the following steps: S1. The base station sends a P frame with T P A Beacon frame is periodically sent at the beginning of a P frame, carrying its own ID and information about whether there is an idle time slot. The process also includes S2, where the tag monitors the Beacon frame and checks the available flag. If there is no idle time slot, it returns to step 1 to continue monitoring. Otherwise, it goes to step 3. S3, where the tag uses an improved Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA) algorithm to detect whether the UWB channel is idle and starts a timer called Timer join, whose timeout period is L*T. join0 –2*T s ; The process also includes S4, running the CSMA / CA algorithm until the channel is detected to be idle and proceeding to the next step. Otherwise, if the timer Timerjoin times out, the monitoring is terminated and the process returns to step S1. S5, the tag sends a Join frame to the base station that receives the Beacon frame and requests a time slot from the base station. The Join frame contains the tag ID and type information. The tag starts a timer Timer ack with a timeout of T. join0 ; The base station also includes S6, selecting a time slot from the S time slots, and sending the allocated time slot ID, base station ID, delayed Work time, and T g The time information is sent to the tag via the ack frame; S7. After receiving the ACK frame, the tag will work at the specified time according to the ACK information; The process also includes S8: if the timer "Timer ack" times out and the tag still does not receive the ack frame, returning to step S3; S9: if the timer "Timer join" times out, returning to step S1.

2. A TDMA time slot allocation algorithm for UWB according to claim 1, characterized in that: The time slot ID range of the base station in S6 is 1 to S. When selecting the time slot, the entire time slot will be divided into two parts, that is, the time slot number ID is odd and even. The odd part is allocated first, and then the even part is allocated. In this way, the access tag is appropriately discretized to avoid interference when adjacent time slot tags send data due to clock drift between the base station and the tag.

3. A TDMA time slot allocation algorithm for UWB according to claim 1, characterized in that: When the base station fails to allocate all the cycles for the first time, the time slot ID sequence allocated by the base station is: 1、3、5、7…… 2、4、6、8…… The base station will map the allocated time slot ID and tag ID and maintain a timer for this time slot with a timeout of n*T g , the value range of n is [3,6]. When the tag sends data to the base station in the specified time slot, the base station will reset the timer. Once the timer expires, the base station will recycle the allocated time slot. After one allocation, the above sequence will be different. The base station will continue to allocate in the recycled time slot, resulting in unpredictable time slot ID sequence.

4. A TDMA time slot allocation algorithm for UWB according to claim 2, characterized in that: The tag type can be different and the working cycle can also change accordingly. The tag is based on the G frame period T g To start working in units, the tag needs to start working at a faster speed. The cycle can be G frame cycle T g The time slot allocation algorithm in S6 needs to be fine-tuned. The time slots corresponding to the first two P frames in the G frame are reserved for a maximum of 2*M for such tags. The time slot IDs corresponding to PN / 4-1, PN / 4, P2N / 4-1, P2N / 4, P3N / 4-1, and P3N / 4 in the G frame are also reserved and do not participate in the time slot allocation of ordinary tag types.

5. The TDMA time slot allocation algorithm for UWB according to claim 1, characterized in that: The tag in S3 will use the improved CSMA / CA algorithm, first determine whether the channel is occupied, and if it is occupied, it will perform a backoff algorithm to retry access. CSMA / CA is used in the Join time slot application phase of the Beacon phase, but does not require a CSMA / CA backoff algorithm in the Work phase. It detects whether the channel is busy before sending, thereby reducing access conflicts caused by time drift between the base station and the tag. The CSMA / CA processing flow has the following parameters: NB backoff times, the initial value of NB is 0. When the device has data to send, after a backoff time, if it detects that the channel is busy, it will generate a backoff time again, and the NB value will increase by 1. In IEEE802.15.4, the maximum NB value is defined as 4. When the channel is still busy after 4 backoff delay times, it waits for the next P frame.

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