Abstract:
The invention describes a data transmission method for a multi-user. multipoint-to-multipoint digital data transmission system involving a plurality of users equipment ( 1 ) which are bidirectionally connected via physical medium ( 6 ). The method is intended to communicate one ( 1   a) user to several (   1   b - 1   d ) users equipment at different speeds, sending multiple frames, maintaining the bandwidth and maximum latency values required by each destination user ( 1   b - 1   d ). Further, this method consists in using the percentage reserve of the frame, providing a quality of service based on that required by the user equipment and supplying a criterion for dynamically assigning the packets sent to each user equipment in the frame, grouping or dividing them.

Description:
RELATED APPLICATIONS 
   The present application is a Continuation of co-pending PCT Application No. PCT/ES03/00012, filed Jan. 14, 2003, which in turn, claims priority from Spanish Application Serial No. 200200101, filed Jan. 18, 2002. Applicants claim the benefits of 35 U.S.C. §120 as to the PCT application and priority under 35 U.S.C. §119 as to said Spanish application, and the entire disclosures of both applications are incorporated herein by reference in their entireties. 

   TECHNICAL FIELD OF THE INVENTION 
   The invention applies to various user modems in two-way communication over a physical medium. The process allows multiple transmissions of information at various speeds between a user modem and numerous addressee user modems while maintaining the values for bandwidth and maximum latency required by each of the addressee user modems. 
   The process comprises the sending of various frames of information from a modem to numerous addressee user modems at various speeds and furthermore achieves quality of service per user modem by reserving a percentage of the frame, and provides a criterion to dynamically assign the packets sent to each user modem in the frame, by grouping or dividing the packets to be sent. 
   OBJECT OF THE INVENTION 
   The current invention, as suggested in the title, consists of a process for the transmission of data by a multi-user, point to multi-point digital data transmission system. The process refers to the means of carrying out access control in the transmission medium in the downstream channel, that is to say, the manner by which transmission is carried out from a user modem to numerous other user modems. 
   The principal objective of the process in this invention is to maximize transmission capacity by means of distributing the bandwidth available in the physical medium between the different user modems to whom it is transmitted according to the maximum latency requirements established by each one of the user modems. 
   The technical field of the invention is the telecommunications sector, and more specifically two-way communications between various users using any transmission means. 
   BACKGROUND OF THE INVENTION 
   The use of point to multi-point systems, where various modems send information to each other using the same physical medium, is known in the prior art. The main problem in these systems is that unless the transmission network is symmetrical for each user, something that is unlikely to occur in reality, the user modems receivers will not be able to receive information from the various transmitters at equal levels of quality due to the fact that attenuation and channel noise in the downstream (the channel that runs from a user modem transmitter to the receiver in another user modem) depends on which transmitter and receiver are involved in each case and the behaviour of the channel at each moment. 
   This problem makes it very difficult to maintain both the bandwidth requirements and the maximum latency necessary to be able to transmit different types of traffic in the point to multi-point systems available to date. 
   Furthermore, in cases where the transmission medium response in time is variable rather than fixed, as is the case of low voltage electricity networks or mobile networks, this technical problem becomes more important due to the need to modify the configuration of the communication so as to be able to maintain the requirements for latency and bandwidth at all times. 
   The current invention solves this technical problem, allowing said quality of service requirements (bandwidth and maximum latency) to be maintained independently of the characteristics of the physical medium and the communication. 
   Other systems that allow the inclusion of quality of service (QoS) in a communication exist in the prior art. In this context mention must be made of the Spanish Patent submission No 200003024 concerning a “POINT TO MULTIPOINT SYSTEM AND PROCESS FOR THE TRANSMISSION OVER THE ELECTRICITY NETWORK OF DIGITAL DATA”, as well as Spanish Patent submission No 200100916 concerning a “PROCESS FOR MULTIPLE ACCESS AND MULTIPLE TRANSMISSION OF DATA IN A MULTI-USER SYSTEM FOR THE POINT TO MULTIPOINT DIGITAL TRANSMISSION OF DATA OVER THE ELECTRICITY NETWORK”, both of which present a solution for the inclusion of quality of service (QoS) for communication point to multi-point over the electricity network, and where a single transmitter (the head-end modem) communicates with various users (user modems). The process in the present invention improves this process to allow the inclusion of quality of service (in terms of bandwidth and maximum latency) in point to multi-point communications, so that any user modem may transmit information to any other user and the communication is not limited to a variable time medium such as the low voltage electricity network but can also be employed in any other transmission medium used as a communications channel in point to multi-point transmissions. 
   Another relevant background art can be found in document EP 0 973 290 A2 which relates to multiplexing data packets in a data service channel with data in one or more digital video signals channels to form a multiplexed output signal. The data in the data service channel may include control data, conditional access data, electronic program guides, paged data services, service information, broadcast internet information, and business information such as financial share information. The data packets each comprise a time stamp indicating a requested delivery time and the data packets are sorted into a queue in time stamp order. The urgency of the data service channel is calculated as a function of the queue length and requested delivery times. The share of the bit rate of the multiplexed output signal allocated to the data service channel is varied according to its urgency. An error value is calculated for each data packet to represent the error between the expected delivery time and the requested delivery time to the head of the queue and the urgency of the data channel is derived as an average of the error values. The average may be a weighted average. 
   Therefore, the present invention presents a process that maintains said requirements including when attenuation, noise and channel response vary dynamically not only in frequency but also in time for each user modem. 
   DESCRIPTION OF THE INVENTION 
   To achieve the objectives and avoid the inconveniences indicated in the previous paragraphs, this invention as claimed has developed a new process for the transmission of data in the downstream of a multi-user, point to multi-point, digital data transmission system where various user modems are in two-way communication over a physical medium. The downstream channel is that which is used by a user modem to transmit information to a series of user modem that receives the information and the upstream channel is by definition, the opposite. 
   The process described in this invention is characterized because it comprises transmission of information at different speeds to different user modems using the same physical medium; because time division in the downstream channel adaptable to the different transmission speeds and the different users is carried out, because packet size is optimised to the means of communication, and this size may be different to that used in the upper layers of the communication by the transmitter; and because packets with different priorities for user modems or groups of user modems are transmitted by means of reservation of channel time based on the quality required by these. 
   All these characteristics allow to guarantee a determined bandwidth and a determined maximum latency for the various users in the downstream channel, delivering quality of service (QoS) for the communication traffic based on different bandwidth requirements and latency for different user modems. 
   To carry out time division in the downstream channel that is adaptable to the transmission speeds of the various users, all the packets sent by the physical medium must occupy the same channel time but at different lengths, since the information is transmitted at different speeds and these transmission speeds depend of the number of bits per symbol to be used by each user modem. The length of the packets sent may be less than or equal to a fixed maximum (in number of bits). 
   In order that the packets occupy the same channel time, the number of symbols for each packet is fixed in what is conventionally called the Maximum Tranfer Unit (MTU) in the transmitter. This number of symbols is adjusted to a value that is adequate to reach a compromise between efficiency in the transmission of bits per second and quality of service in terms of latency. The process allows different types of traffic from the unit used for transmitting packets (MTU). Among the various types of traffic allowed is traffic without minimum latency and bandwidth requirements, traffic with constant bit rates (CBR), traffic with variable bit rates (VBR), and traffic with reserved bandwidth and fixed maximum latency. 
   Due to the fact that the number of symbols sent is a fixed number per packet, the packets that must be sent to the various-users are grouped and/or divided in the transmitter. In this way, packet size is fixed to the number of symbols in the unit used for transmission, the Maximum Transfer Unit (MTU). 
   Due to the fact that the duration of the symbols is fixed and always the same, the number of symbols for each packet sent is also fixed. 
   The downstream channel is divided into a series of frames with a fixed number of packets. In these frames different percentages of packets to be sent to the addressee user groups are reserved. Each user group contains users who share the same quality of service requirements for their traffic, this quality being reserved bandwidth and/or maximum latency. 
   If the percentage reserved for a user or group of addressee users is not used, this percentage may be distributed between the remaining modems or groups of user modems that will transmit information. This is carried out according to a determined configuration of traffic to be transmitted. 
   In the percentage reserved for packets in the frame by each user group, each user in the group has a certain percentage according to its own quality of service depending on whether it has reserved bandwidth and/or maximum latency requirements. 
   The process of the invention includes storing of packets to be transmitted to different users in a series of queues in the transmitter. These transmission queues are queues per user where the packets are ordered according to priority, said priority being fixed by an upper protocol. Having the packets stored in such queues allows the extraction of packets ordered according to priority. Therefore, the transmitter groups packets and decides on not only which is the next packet to be transmitted, but to whom, and the size of that packet. 
   The following drawings are provided to facilitate a better understanding of the present invention and while forming an integral part of the detailed description and the claims, they offer an illustrative but not limited representation of the principles of this invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 . Shows a typical multi-user, point to multi-point communication scenario where a modem communicates with various user modems distributed through different buildings. 
       FIG. 2 . Shows an example of communication between a user modem and three other modems showing the transmission frame and the different speeds for each one of the users. 
       FIG. 3 . Shows a possible example of the reservation of percentages of the packets in the frame for two addressee users with different quality requirements. 
   

   DESCRIPTION OF AN EMBODIMENT OF THE INVENTION 
   Hereafter follows a description of a preferred embodiment of the invention, making reference to the numeration used in the Figures. 
   This example concerns a digital transmission system for the two-way transmission of data point to multi-point, and where therefore there are numerous user modems  1 . 
   The example in  FIG. 1  shows a series of user modems  1  in a building  2  and a user group  1  in the building  3 . 
   Since the invention is applicable to the downstream channel,  FIG. 2  shows a user modem  1   a  transmitting data to three user modems  1   b - 1   d  with different speeds v i , v i+1 , and v i+2 . 
   The speed v i , is the number of bits per second that can be sent from the transmitter  1   a  to the user modem  1   b - 1   d.    
   The transmitter takes charge of assigning communication bandwidth over the physical medium  6  by means of an arbiter unit as will be mentioned later. 
   In a point to multi-point system with different transmission speeds the need to define quality of service that ensures differing efficient latencies for different users is considered. 
   The transmission speed for each user modem may be very different. For example in an “OFDM” (orthogonal frequency division multiplexing) communication system, the speed depends on the number of bits per carrier, and this last value depends on the quality of certain channel parameters in this carrier. 
   In this communication system, for example, if a user modem  1   b  is much closer to the transmitter  1   a  than another modem  1   d , giving a more optimistic bit configuration per carrier to the modem  1   b  than to the modem  1   d  in each OFDM symbol, more bits can be sent to the nearest rather than the furthest away user, without loss of quality, due to the fact that attenuation is less. 
   In a traditional system, where the speed that unites the origin with all the addressees is the same, a maximum number of bytes (MTU) for all packets that will be transmitted is fixed. Therefore, each packet occupies the same channel time and it can be guaranteed that the reception latencies for all the remote users will not surpass a certain value. On the contrary in a system where each addressee has a different speed, if there is the same MTU for all addressees the channel time that the packets will use depends on the speed, so latencies cannot be guaranteed. 
   The process of this invention sets the channel time to be equal for all the addressee modems, independently of the speed in each channel. Therefore transmission to each addressee modem  1   b - 1   d  has a maximum number of bytes (MTU), that will be different for each modem. 
   Consider the example in  FIG. 2 , where two user modems  1   b  and  1   c  are selected, and where  1   b  is further away from the modem  1   a  than the modem  1   c . Consider that there are speeds v i  and v i+1  associated to communications  1   a  to  1   b  and  1   a  to  1   c  respectively (where v i  is greater than v i+1 ). Taking into account that in an OFDM communication system the speeds v i  and V i+1  are translated into number of bits per carrier, then in each OFDM symbol one can introduce and send N 1  bits for the modem  1   b  and N 2  bits for the modem  1   c , since the time that each symbol lasts is known and it is the same for each addressee modem. 
   In this example, each packet at a maximum occupies N number of OFDM symbols. Therefore, the channel time for each packet is controlled and it can be calculated that the MTU for modem  1   b  is N1*N and the MTU for modem  1   c  is N2*N, guaranteeing the latencies for all the user modems to whom they transmit information. 
   Another problem that is found in a multipoint configuration where each addressee has a different speed is the need to guarantee reception bandwidth in the different addressee user modems  1   b - 1   d . To achieve this, the channel is divided in time. For example, in an OFDM communication system the number of symbols are grouped in frames  4 . The definition of each frame is carried out taking into account the bandwidth that it is wished to reserve for each user modem  1   b - 1   d . 
   For example, taking the configuration presented previously in  FIG. 2 , it is possible to reserve 80% of the bandwidth for the modem  1   b  and the remaining 20% for the modem  1   c . This scenario is demonstrated in  FIG. 3 . The transmitter uses an arbiter unit to determine to which addressee modem the current packet should be transmitted. 
   Taking into account that the channel is divided into frames  4  and that each packet occupies the same channel time, the channel can be divided into packets  5 , thus facilitating the functioning of the arbiter. 
   The arbiter must decide, at each moment, which packet must be transmitted. In the previous example the modem  1   b  has 80% reserved and the modem  1   c  20%, this reserve implies that the transmitter had to send P 1  packets to the user modem  1   b  where P 1  is 80% of the frame  4  and P 2  packets for the user  1   c  here P 2  is 20% of the packets in the frame  4 . 
   In this situation, the arbiter decides at each moment which packet to transmit taking into account that of the P packets in the frame, the percentage P 1  must be the percentage of packets for the user of modem  1   b  and P 2  the percentage of packets for the user of modem  1   c.  7     
   The packets that will be sent to each user modem are stored in the transmitter  1   a . In the management of these stored packets, one must take into account that for each addressee user there are a number of stored packets and the transmission order of these packets for each user is carried out according to packet priority and if there are two packets with the same priority then these are sent by order of arrival (from the oldest to the most recent). For example, a possible implementation of this storing could occur in a FIIFO (first in, first out) memory for each user, address and priority. When the arbiter decides which user to transmit to, it selects the packet to transmit taking into account the priorities. 
   The concepts of reserved bandwidth and maximum latency unite to offer different qualities of service. For this, every time that a packet can be sent, the user to receive the packet is identified and furthermore, the maximum length that this packet can have (MTU) is calculated. Taking the MTU and the addressee into account, the packets are extracted from the queues according to the priorities of the packets, and then they are aggregated. To construct the packet with the maximum length, not only are packets joined, but they are also divided to adjust to the maximum length (MTU) calculated for this addressee user modem. 
   The users are grouped depending on the quality of service required. A group of users without quality of service requirement also exists, that is, they do not require bandwidth or maximum latency. The packets for users without quality of service requirements enter in a frame in a moment when the arbiter has no packets to send to the users that require quality of service. 
   For example, in a situation with three user modems  1   b - 1   d,  where  1   b  has 80% reserved and the modem  1   c  has 20% reserved, then  1   d  has 0% reserved. The arbiter always sends packets to the modems  1   b  and  1   c  and in the case that it does not have packets for these user modems, it sends data to the modem  1   d.  The user modem  1   d  has a channel speed of v i+2  and therefore it has a MTU that does not affect the latencies of the users profile (in this example  1   b  and  1   c ). 
   In particular, the users are divided into two principal groups, one group with a percentage of reserved bandwidth (type A) and the other without any reserved percentage (type B). The arbiter gives priority to type A. The possibility of considering a percentage of reserved bandwidth for all type B exists, thereby guaranteeing that all the type B users will have very low bandwidth but will receive packets. A third option consists of considering all the type B users globally as one extra type A user. 
   On the other hand, data packets on the line include a header that occupies certain channel time. The process proposed optimises the time applied to headers with respect to the time used in the channel for the transmission of data, arriving at an efficient compromise between the time needed for headers and the time used for the transmission of data.