CN102118211A - Timing and synchronization method for media-sharing STDM system - Google Patents

Abstract

The invention discloses a timing and synchronization method for a statistical time-division multiplexing system of a shared medium, which mainly solves the problems of large timing synchronization overhead and complicated round-trip delay measurement method in the prior art of point-to-multipoint network. The implementation steps are: the central station periodically sends timing and synchronization frame D without time stamp; the slave station sends the round-trip delay measurement frame U at the agreed offset time based on the time when the D frame is received; the central station receives After the U frame, the round-trip delay is obtained by the difference between the offset value of the arrival time of the U frame and the agreed offset value; the central station takes the time when the D frame is sent locally as the reference, and uses the offset value relative to the reference to represent the channel planning information, and Perform delay compensation processing; after receiving the channel planning information, the slave station accesses the channel according to the offset time specified by the central station. The invention has the advantages of small timing and synchronization overhead and simple round-trip delay measurement, and can be used in the field of point-to-multipoint access network and local area network sharing media.

Description

Timing and Synchronization Method for Shared Media Statistical Time Division Multiplexing System

technical field

The invention belongs to the technical field of communication, and relates to a timing and synchronization method, in particular to a timing and synchronization method for a shared medium statistical time division multiplexing system, which can be used for a point-to-multipoint local area network based on a shared medium statistical time division multiplexing technology and access networks.

Background technique

Statistical time-division multiplexing (STDM) is a commonly used channel multiplexing technology. In this technology, channels are divided in a time-division manner and dynamically shared among multiple users. This technology dynamically allocates channel resources according to the actual needs of users. When a user has data to transmit, channel resources are allocated to the user; when the user suspends data transmission, channel resources are not allocated to the user. The transmission capacity can be used by other users, thus having a high channel utilization rate.

This technology is widely used in local area networks and access networks based on shared media, and its common form is a point-to-multipoint network with a center. In this kind of network, there is a central station that allocates channel resources uniformly, and other stations except the central station are called subordinate stations. When the subordinate stations have data to transmit, the central station must first allocate channel resources for them. Typical The method is that the subordinate station sends a request to the central station, and then the central station grants the allocation, or the central station plans a period of time for multiple stations to share this channel resource in a competitive manner, and the subordinate station must strictly follow the time specified by the central station. into the channel.

The above method of allocating channel resources by the central station based on the statistical time division multiplexing technology actually divides the channel into continuous time periods of different sizes, and each subordinate station uses the channel resources in an orderly manner according to the time period specified by the central station. In order to make the above-mentioned shared media system based on statistical time division multiplexing technology work normally, two problems need to be solved at first. One is the timing problem, that is, how the central site indicates the start and end times of each time segment it divides on the channel time axis; It is a problem of synchronization, that is, the clock of the slave station must establish synchronization with the clock of the central station, so that the central station and the subordinate station can access the channel based on the same time reference. In addition, in an access network with a large coverage area, the distance between each subordinate site and the central site is greatly different, resulting in a large difference in the signal propagation delay between each subordinate site and the central site, which will lead to different The degree of synchronization between the stations is different, which may cause the signals of different subordinate stations to collide. Therefore, it is also necessary to solve the problem of synchronization correction, that is, to eliminate the influence of signal propagation delay on synchronization.

In actual use of the above shared medium statistical time division multiplexing technology, the channel is generally divided into continuous data transmission periods, and each data transmission period DTC is composed of several time periods, and each time period is a data transmission opportunity. The timing and synchronization of the system is issued by the central site. The typical way is to periodically issue beacon frames carrying system time stamps. The slave sites reset the local timers according to the system time issued by the central site, thereby establishing the system time of each slave. Synchronization between sites and central site. The start and end times of each data transmission cycle are represented by the system clock value, and each time period in each data transmission cycle is represented by an offset value relative to the starting point of the data transmission cycle. The schematic diagram of the data transmission cycle of the above-mentioned existing shared media statistical time division multiplexing system using the existing timing and synchronization method is shown in Figure 1, wherein the B frame is the Beacon frame issued by the central station at a fixed period, that is, the beacon frame, carrying the system Timestamp value. When the slave station receives the B frame, it resets its own local clock value, thereby establishing clock synchronization with the central station. The channel allocation of DTC in each data transmission cycle is issued by the central site through a special channel planning frame, and the system time value is used in the channel planning frame to indicate the start and end moments of DTC and the start and end moments of each transmission period in DTC, each time The start and end times of a segment are represented by offset values relative to the start time of the DTC.

To solve the synchronization correction problem described above, it is necessary to measure the round-trip delay between the central site and the slave sites. The commonly used method of measuring the round-trip delay is shown in Figure 2. When the slave station receives the measurement frame with a time stamp from the central station, it resets the local clock to the received time stamp value and waits for a period of time to proceed. After processing, a response frame with the local current timestamp is returned. When the central station receives the response frame from the slave station, it will use the timestamp value carried in the frame to calculate the round-trip delay between it and the slave station. The round-trip delay is equal to the sum of the downlink delay and the uplink delay. From Figure 2, it can be seen that the round-trip delay RTT=T _down +T _up =T _response -T _wait =(t ₂ -t ₀ )-(t ₁ -t ₀ ) =t ₂ -t ₁ , that is, the round-trip delay is equal to the difference between the current local clock value of the central site and the time stamp value in the received response frame. After the round-trip time delay is obtained by the above method, the central station adjusts the sending time of the subordinate stations to ensure that the signal arrives at the correct time expected by the central station, thereby avoiding collisions.

The above timing and synchronization methods rely on the timestamp issued by the central site, and the timing of the data transmission cycle needs to be represented by the system time, which is expensive; the measurement of the round-trip delay requires the design of a special process, which is relatively complicated.

Contents of the invention

The purpose of the present invention is to avoid the deficiencies in the above-mentioned prior art, and propose a timing and synchronization method for a shared medium statistical time division multiplexing system based on a point-to-multipoint network, so as to reduce the timing and synchronization overhead of the system and simplify measurement A method of round-trip delay between the central site and the slave sites.

The technical solution for realizing the object of the present invention comprises the following steps:

1) The central station sends a timing and synchronization frame D without a time stamp with a fixed cycle length or a variable cycle length;

2) The slave station uses the time when the timing and synchronization frame D without a time stamp is received locally as a time reference, and sends a round-trip delay measurement frame U without a time stamp at the time corresponding to the offset value specified by the system;

3) After the central station receives the round-trip delay measurement frame U without a timestamp, it uses the timing and synchronization frame D sent locally without a timestamp as the time reference, and subtracts the corresponding count value from the current moment when the U frame is received Go step 2) in the offset value that sends the system regulation of U frame, draw the round-trip time delay between central station and above-mentioned subordinate station;

4) The central station uses the timing and the time when the synchronization frame D is sent locally without a time stamp as the time reference for channel planning, and uses the offset value relative to the reference to represent the channel planning information, and uses the round-trip delay obtained in step 3) to The offset value is processed for delay compensation, and then the channel planning information is released by sending the channel planning frame E;

5) When the slave station receives the channel planning frame E, it uses the time when it receives the timing and synchronization frame D locally without a time stamp as the time reference, and transmits and receives data at the time specified by the offset value in the channel planning frame E.

The timing and synchronization frame D without a time stamp and the round-trip delay measurement frame U without a time stamp are further used to carry any useful information as required.

The above-mentioned "representing channel planning information with an offset value relative to the reference" is based on the timing of the local transmission without a time stamp and the time of the synchronization frame D, and uses an offset value relative to the reference to indicate the start and end of the data transmission cycle Time: Taking the start time of the data transmission cycle as a reference, the start and end time of each time period in the data transmission cycle is represented by an offset value relative to the reference.

The "delay compensation processing" refers to that the central site subtracts the round-trip time delay value between the central site and the subordinate site from the planning value of data sent and received by the subordinate site.

The present invention has the following advantages:

1) The present invention uses the timing based on no time stamp to establish system synchronization with the synchronization frame D, and uses the starting moment of D as the time reference, and uses the offset value relative to the reference to represent the channel planning information, thereby reducing the system's Timing and synchronization overhead.

2) In the present invention, since the subordinate station sends the round-trip delay measurement frame U without a time stamp at a fixed offset moment relative to the timing without a time stamp and the synchronization frame D, the central station only needs to compare the time of arrival of the U frame with the specified The round-trip delay between the central site and the subordinate sites can be obtained from the difference of the arrival time, which simplifies the measurement process of the round-trip delay.

Description of drawings

FIG. 1 is a schematic diagram of a data transmission cycle using an existing timing and synchronization method;

FIG. 2 is a schematic diagram of an existing round-trip delay measurement method;

Fig. 3 is a schematic diagram of a data transmission cycle using the timing and synchronization method of the present invention;

Fig. 4 is a schematic diagram of the timing and synchronization method of the present invention.

Detailed ways

The content of the present invention will be further elaborated below in conjunction with the accompanying drawings.

In this embodiment, assume that the network environment is a coaxial cable access network, and when the timing and synchronization frame D without a time stamp is sent with a fixed cycle length, the cycle is 64ms. When it is sent with a variable cycle length, the sending cycle interval is the smallest 60ms to a maximum of 64ms, the transmission offset time of the round-trip delay measurement frame U without a time stamp is 32ms, the system clock counter accuracy is 20ns, and the access technology of time division duplex and time division multiple access is adopted, and the central site is each slave The station allocates channel resources, and uniformly schedules the time for each subordinate station to send and receive data.

Referring to Fig. 4, the implementation steps of this embodiment are as follows:

Step 1, the central station sends a timing and synchronization frame D without a time stamp, and clears the local clock counter at the moment when the D frame is sent.

The data transmission period of the timing and synchronization frame D sent by the central site without a time stamp is shown in Figure 3, where the D frame has no time stamp, and there are multiple data transmission periods DTC between the D frames, and each DTC contains multiple transmission data time period. There are two ways for the central site to send timing and synchronization frames D without time stamps. One is to send at a fixed cycle interval of 64ms, and the other is to send at a cycle with a minimum interval of 60ms and a maximum interval of 64ms. Each transmission method At the moment when the timing and synchronization frame D is sent, the local clock counter must be cleared.

Step 2, the slave station sends a round-trip delay measurement frame U without a time stamp at the agreed offset time.

When the slave station receives the timing and synchronization frame D without a timestamp, it clears the local clock counter. According to this embodiment, it is assumed that the offset time of sending the round-trip delay measurement frame U without a timestamp is 32ms, which is converted into a counter value It is 32ms/20ns=1600000, therefore, when the slave station waits until the value of the local clock counter is 1600000, it sends a round-trip delay measurement frame U without a time stamp.

In step 3, the central station calculates the round-trip delay value when receiving the U frame.

The central site subtracts the agreed counter value at the time of sending the U frame from the local counter value at the time of receiving the U frame to obtain the round-trip delay with the slave site. Taking the local clock counter value of 1600500 when the central site receives the U frame as an example, it can be seen from step 2 that the counter value corresponding to the offset time of sending the U frame agreed by the system is 1600000, thus the distance between the central site and the subordinate sites can be obtained The round-trip time delay RTT=(1600500-1600000)*20ns=10us, the count value of the counter corresponding to the round-trip time delay is: 1600500-1600000=500.

Step 4, the central site plans the sending and receiving time of each data of the subordinate sites, and performs delay compensation processing.

The central station allocates the channel time according to the resource requirements of the subordinate stations, that is, arranges the time for the subordinate stations to access the channel. Firstly, the start and end times of the data transmission period are represented by the offset value relative to the start time of the local D frame, and then expressed by the data The offset value at the beginning of the transmission cycle is the reference, and the start and end of each time period in which the slave station accesses the channel in the data transmission cycle is represented by the offset value relative to the reference; after the above offset value information is obtained, the time delay is performed Compensation processing is to subtract the counter value corresponding to the round-trip delay from the offset value. Take the planning of a certain data transmission period T between two D frames by the central site as an example, and take the start time of sending D frames as the benchmark, assuming that the offset values of the start and end times of the data transmission period T are 8ms and 10ms respectively , it is planned that the start offset time of the slave station sending data P in the data transmission period T is 20us, and the end offset time of sending data P is 220us, that is, the length of the sending time period corresponding to data P is 200us.

4a) Calculate the counter value corresponding to the start and end moments of the data transmission cycle T, the count value corresponding to the start moment of T in this embodiment is 8ms/20ns=400000, and the count value corresponding to the end moment is 10ms/20ns=500000; The starting time of the transmission cycle T is used as a reference, and the count value corresponding to the start and end time of each time period sent by the slave station within a data transmission period T is calculated. In this embodiment, the starting time of the time period for sending data P by the slave station corresponds to The count value of the time period is 20us/20ns=1000, and the corresponding count value at the end of this time period is 220us/20ns=11000;

4b) Subtract the count value corresponding to the round-trip delay from the offset value corresponding to the start and end moments of the slave station sending time period to obtain the time period offset value after delay compensation. In this embodiment, the time when the slave station sends data P The value of the offset value at the beginning of the segment after delay compensation is 1000-500=500, and the value of the offset value at the end of the time segment after delay compensation is 11000-500=10500;

4c) Encapsulate the planned offset value information in steps 4a) and 4b) into the channel planning frame E, which contains the start and end offset values of the data transmission cycle based on the timing and synchronization frame D sent locally without a time stamp time, and with the start time of the data transmission cycle as the reference, the start and end offset value time after delay compensation of each time period in the data transmission cycle; since the interval between the two D frames in this embodiment is at most 64ms , and its corresponding count value is 3,200,000, so in this embodiment, at most 22 binary bits are needed in the channel planning frame E to represent the planning offset information.

Step 5, after receiving the channel planning frame E, the slave station accesses the channel according to the specified time.

5a) According to the offset value information in the E frame, calculate the counter value corresponding to the start and end time of each time period in the data transmission cycle, and the count value at the start time of the transmission is the offset value of the start time of the data transmission cycle and the data transmission cycle The sum of the start time offset values of each sending time period, the count value of the sending end time is the sum of the offset value of the start time of the data transmission cycle and the end time offset value of each sending time period in the data transmission cycle; this implementation In the example, the count value corresponding to the start time of the data P sent by the slave station is 400000+500=400500, and the count value corresponding to the end time of sending is 400000+10500=410500;

5b) The slave station accesses the channel according to the counter value obtained in step 5a). In this embodiment, the slave station starts sending data P when the counter value is 400500, and ends sending data P when the counter value is 410500.

In this embodiment, in addition to being used for timing and synchronization, the above-mentioned D frames and U frames may further carry any useful information according to system requirements, for example, the D frames and U frames are used to carry channel training information.

The above is only a preferred example of the present invention, and does not constitute any limitation to the present invention. Obviously, according to the concept of the present invention, those skilled in the art can make various modifications and replacements, but these are all included in the protection of the present invention.

Claims (4)

1. a timing and a method for synchronous that is used for sharing medium STDM system comprises the steps:

1) central site sends the timing and synchronization frame D of no-timestamps with fixed cycle length or variable cycle length;

2) the subordinate website with the moment of local timing of receiving no-timestamps and synchronization frame D as time reference, the round-trip delay that sends no-timestamps in the deviant moment corresponding of system's regulation is measured frame U;

3) central site is after the round-trip delay of receiving no-timestamps is measured frame U, the timing that sends no-timestamps with this locality and the moment of synchronization frame D are as time reference, count value with the current time correspondence of receiving the U frame deducts step 2) the middle deviant that sends system's regulation of U frame, draw the round-trip delay between central site and the above-mentioned subordinate website;

4) central site carries out channel plan with the moment of synchronization frame D as time reference with the timing that this locality sends no-timestamps, use with respect to the deviant of benchmark and represent channel plan information, and the round-trip delay that obtains in the use step 3) carries out delay compensation to deviant and handles, again by transmitting channel planning frame E issue channel plan information;

5) the subordinate website is when receiving channel plan frame E, with moment of local timing of receiving no-timestamps and synchronization frame D as time reference, the moment of deviant appointment is carried out data transmit-receive in channel plan frame E.

2. timing described in claim 1 and method for synchronous, wherein the timing of no-timestamps is measured frame U with the round-trip delay of synchronization frame D and no-timestamps, is further used for carrying any useful information as required.

3. timing described in claim 1 and method for synchronous, step 4) described " use with respect to the deviant of benchmark and represent channel plan information " wherein, be the timing that sends no-timestamps with this locality with the moment of synchronization frame D as benchmark, use represent data transfer cycle with respect to the deviant of benchmark start-stop constantly; With the initial moment of data transfer cycle as benchmark, use represent each time period in this data transfer cycle with respect to the deviant of benchmark start-stop constantly.

4. timing described in claim 1 and method for synchronous, step 4) described " delay compensation processing " wherein is meant that central site deducts round-trip delay value between central site and this subordinate website with the planning value of slave station point transmitting-receiving data.

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