The present invention relates to a point-to-multipoint communication system in which communication is performed between a single master unit and a plurality of slave units along a transmission line, and more particularly to a point-to-multipoint communication system in which adequate communication can be achieved even when signals containing several megabytes of data are transmitted in bursts.
Point-to-multipoint communication systems such as LANs, CATV networks, satellite communication networks, and optical subscriber access networks are commonly configured such that a master unit and a plurality of slave units communicate by sharing, for example, transmission lines such as those used in coaxial communication, optical-fiber communication, and radio communication.
Here, an access protocol for allocating the use of a shared transmission line among slave units is important for ensuring smooth communication between the master unit and the slave units via this transmission line. There are various types of access protocols.
For example, the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) protocol is one in which xe2x80x9ccarrier sense+multiaccess+collision detectionxe2x80x9d is the basic principle of operation, and the DAMA (Demand Assign Multiple Access) protocol is one in which xe2x80x9ctransmission start demand/end notification+transmission line sharing/releasexe2x80x9d is the basic principle of operation.
In such point-to-multipoint communication systems, burst traffic in which signals containing several megabytes of data are generated in bursts is rapidly becoming more widespread because of the increased popularity of personal computers, a larger number of Internet users, and the development of various multimedia services that blend communication and broadcasting.
Conventional access protocols, however, have the following drawbacks that make these protocols incapable of handling burst traffic containing several megabytes of data, which is expected to grow even more in the future.
When, for example, information signals generated in bursts of several megabytes are transmitted according to the CSMA/CD protocol, the information signals are separated into packets of about 64-1500 kB and repeatedly transmitted. Consequently, increased burst traffic results in a markedly lower throughput because of the higher frequency of retransmission due to packet collisions.
Probability calculations conducted under conditions corresponding to a collision of about 15 packets show a throughput of about 40%, and a period that is 1.5-2.5 times the normal time is needed for data transmission accompanied by collisions. As a result, the delay time increases and is not constant any longer.
On the other hand, transmitting information signals generated in bursts according to a TDMA protocol increases the time needed to complete the transmission when the information signals being transmitted contain several megabytes of data, so an increase in burst traffic sometimes makes a transmission-awaiting slave unit to miss its time slot and to fail to operate within the delay time allowed for data transmission. Thus, conventional access protocols for point-to-multipoint communication systems are disadvantageous in that an increase in burst traffic containing several megabytes of data results in a lower throughput and makes it impossible to comply with time requirements for downstream transmission.
With the foregoing in view, it is an object of the present invention to provide a point-to-multipoint communication system in which adequate communication can be achieved even when signals containing several megabytes of data are transmitted in bursts.
According to the access protocol for the point-to-multipoint communication system of the present invention, the main principle of operation is that xe2x80x9ca report is issued regarding the information amount necessary for signal transmission+an instruction regarding transmission below a specific maximum value is issued on the basis of the information amount reported.xe2x80x9d
First, slave units about to transmit signals issue reports on the information amount needed to transmit the signals in accordance with instructions from the master unit. The master unit, upon receipt of the reports from the slave units, issues instructions for the slave units to transmit the signals at no more than a specific maximum number of kilobits on the basis of the reported information amount.
Because such an arrangement allows slave units to transmit signals in accordance with instructions from the master unit, no signal collisions occur in the transmission line and a throughput reduction such as that observed in the case of CSMA/CD is avoided when signals containing several megabytes of data are transmitted in bursts.
Another feature is that because signal transmission instructions are issued dynamically and efficiently on the basis of the information amount reported by slave units, high throughput can be obtained even under complex conditions created by greater burst traffic or an increased number of slave units.
The aforementioned specific maximum number of kilobits is set such that the following relation is satisfied:
kxe2x89xa6(rxc3x97td)÷(xcex1xc3x97m),
where m is the total number of slave units (m is an integer), xcex1 is the proportion of currently active slave units (0 less than xcex1xe2x89xa61.0), r (b/s) is the data transfer rate, and td (s) is the delay time allowed for data transmission and determined by the system.
A slave unit can therefore continuously transmit large signals because it has exclusive use of the upstream transmission line during the transmission of signals whose maximum size is measured in k (bits).
Another feature is that when the maximum value is determined on the basis of the above equation and signal transmission instructions are issued for the slave units, a time of td seconds is necessary for the transmission if all the slave units transmit kilobit signals, making it possible to secure an allowed delay time of td seconds for data transmission by all the slave units.
Fairness is also ensured because all the slave units can invariably transmit their signals within the allowed delay time of td seconds.
Yet another feature is that the master unit can instruct the slave units to issue information amount reports in a dynamic mode in accordance with changes in the transmission line load, the information amount reported, and the like.
In the case of low transmission line load, for example, variations in information amount can be identified by the master unit at a higher speed by reducing the interval between the instructions for information amount reporting.
The master unit can therefore be more flexible in issuing signal transmission instructions under conditions of varying information amount from slave units, making it possible to reduce the latency time elapsed before a slave unit transmits a signal and to lower the capacity of the buffer memory required by the slave unit to temporarily store the signal.
In the converse case of high transmission line load, the band that can be used to transmit signals along a transmission line by increasing the interval between the instructions for information amount reporting, making it possible to increase the throughput even further.
Instructions for a slave unit to report information amount can thus be given in a dynamic mode, making it possible to reduce the latency time elapsed before the slave unit can transmit a signal, to reduce the capacity of the buffer memory required for the slave unit to temporarily store signals, and to increase throughput.
Another feature is that when no instructions have been issued for a slave unit to transmit a signal, the same effect can be obtained by issuing instructions for reporting the information amount of the slave unit and performing a look-ahead procedure for the information amount of each slave unit.
Yet another feature of the present invention is that slave units about to transmit information signals temporarily store these information signals in an information signal storage means, and the storage amounts of the stored information signals are reported to the master unit. Upon receipt of the reports from the slave units, the master unit calculates, based on the storage amounts thus reported, the information amount (information amount allowed for transmission) that can be transmitted by each slave unit at no more than a specific maximum value, and each slave unit is notified of the information amount thus calculated.
Upon receipt of the notification from the master unit, the slave units transmit to the master unit the stored information signals at no more than the information amount specified in the notification.
Because such an arrangement allows slave units to transmit information signals in accordance with the information amount specified in the notification from the master unit, no signal collisions at all occur in the transmission line, and a throughput reduction such as that observed i n the case of CSMA/CD is avoided when signals containing several megabytes of data are transmitted in bursts.
Here, the information amount that allows information signals to be transmitted can be calculated dynamically and efficiently on the basis of reports from slave units such that the data transfer rate is utilized with 100% efficiency, so high throughput can be obtained even under complex conditions created by greater burst traffic or an increased number of slave units.
Another feature of the present invention is that the master unit instructs the slave units to transmit signals successively when the information amount reported by all the slave units still has a transfer rate margin after the signal transmission instructions have been issued.
In such an arrangement, the master unit constantly instructs slave units to transmit signals when the information amount reported by the slave units is of low volume and the communications traffic is slow, making it possible to reduce the latency time elapsed before the slave unit can transmit a signal, to lower the capacity of the buffer memory required to temporarily store the signal, and to further enhance communications efficiency.
Here, instructions concerning signal transmission are issued in accordance with the present invention such that individual procedures are executed independently from each other, making it possible to ensure that access fairness is ensured and delay requirements concerning data transmission are satisfied for all the slave units in the same manner as in the above-described invention.
In addition, the point-to-multipoint communication system of the present invention operates on the following basic principle: the information amount needed for transmission is reported for each service class in accordance with instructions from the master unit, and transmission by service class is enabled by the master unit.
In each service class, slave units about to transmit information signals issue reports for the master unit regarding the information amount needed to transmit the information signals in accordance with the instructions from the master unit. Upon receipt of the reports from the slave units, the master unit issues transmission instructions for the slave units in each service class on the basis of the information amount reported for each service class. At this time, the master unit issues instructions to transmit data in sequence from higher-priority service classes.
Upon receipt of transmission instructions from the master unit regarding a certain service class, the slave units transmit the data for the corresponding service class to the master unit.
Such an arrangement involves reporting information amount by service class and issuing transmission instructions in sequence from higher-priority service classes, so the stringent delay requirements imposed on higher-priority service classes can be satisfied because the transmission instructions are issued independently for the higher-priority service classes even when several megabytes or more are transmitted in bursts for a lower-priority service class.
Another feature of the point-to-multipoint communication system of the present invention is that it operates on the basic principle that xe2x80x9ca report is issued regarding the information amount necessary for signal transmission in all service classes+an instruction regarding signal transmission below a specific maximum value is issued on the basis of the information amount reported.xe2x80x9d
First, the master unit establishes a plurality of service classes on the basis of differences in the delay time allowed for data transmission, and signals to be transmitted to the master unit are classified by service class and stored in the buffer memory. Before transmitting the signals stored in the buffer memory, slave units inform the master unit about the information amount needed to transmit the signals for all the service classes in accordance with the instructions from the master unit.
Upon receipt of the reports from the slave units, the master unit instructs the slave units to transmit signals at a specific maximum kilobit value on the basis of the information amount reported.
In addition, the slave units transmit signals in accordance with the instructions from the master unit, starting from signals that are classified as belonging to a higher-priority service class and requiring shorter allowed delay times for data transmission.
With such an arrangement, the master unit issues signal transmission instructions in a dynamic mode in accordance with the information amount reported by the slave units, and the slave units transmit signals in accordance with the instructions from the master unit, making it possible to prevent signal collisions in the transmission line and to maintain high throughput even under conditions of increased traffic. Limitations are imposed on the time during which a signal can be continuously transmitted by a slave unit, so the traffic of one slave unit has no effect on another slave unit. Because the slave units transmit signals in accordance with instructions from the master unit, starting from signals that are classified as belonging to a higher-priority service class and having shorter allowed delay times, it is possible to provide a real-time service such as that employed in telephony or video transmission, a non-real time service such as that employed in file transfer, or other type of communication involving a plurality of classes having different traffic characteristics or requiring different allowed delay times for data transmission, performed simultaneously and with high efficiency.
Another feature of the present invention is that the time slots for transmitting information signals by slave units are calculated by the master unit dynamically and efficiently on the basis of reports from slave units to ensure efficient utilization of data transfer rate, making it possible to obtain high throughput even under complex conditions created by greater burst traffic or an increased number of slave units.
Yet another feature is that because the maximum number of time slots that can secure a circuit in a single session is defined in advance, any slave unit can transmit data within a period based on the product of the maximum number of time slots and the number of the slave units even when different amounts of data are stored in the slave units. Specifically, the maximum data transfer delay time can be specified, and access fairness ensured.
In addition, telephone voice transmission and other types of guaranteed service can be performed simultaneously.
Another feature of the present invention is that the master unit divides terminal-addressed input packets and attaches headers to create fixed-length cells. Each header contains slave unit addresses for use in the point-to-multipoint communication system and retrieved based on the terminal addresses contained in the input packets. Because the slave units operate such that data is retrieved only if the addresses contained in the aforementioned headers match the addresses of the local stations, the transfer rate in the downstream direction for each of the slave units can be varied in a simple manner using the master unit alone. In addition, short bit strings are used and the transfer rate can be utilized efficiently because the addresses of the slave units used in each header are those that have significance solely inside a point-to-multipoint communication system.
Furthermore, the address architecture used in the point-to-multipoint communication system is hierarchized, and the addresses are set such that a subnet is used and the slave units constitute a single network within the entire point-to-multipoint communication system. The structure of the master unit can thus be simplified because the portion of a terminal address for identifying the subnet can be used directly as a slave unit address.