This thesis is based on the design and development of ADSL access networks. We have completed the hardware and software functions in two main ADSL equipments: ATU C and ATU R. With the system design, we carry out re...This thesis is based on the design and development of ADSL access networks. We have completed the hardware and software functions in two main ADSL equipments: ATU C and ATU R. With the system design, we carry out researches to find out effective methods to improved performance in the following aspects: (a) how to allocate limited bandwith among services with different QoS requirements and (b) what is the effect of asymmetric links on TCP end to end flow control mechanism. We make detailed analyses and propose useful solutions to those problems we find. The main contents of this thesis are as follows: (1) the design and realization of ADSL access system. This part includes the principles of ADSL access technology, ADSL broadband access network architecture, and ATM over ADSL protocol models that we use in our system. We give detailed hardware and software design steps in this part, and list system testing results to show the system performance. In our hardware, the CPU is a RISC microprocessor. Combined with a high speed digital signal processor, the whole system can hold high reliability. Our software design platform is a pSOS multi task real time operation system. We have developed a series of hardware drivers besides other tasks for high level control and interruption processing. Currently, our system has reached the expected performance under test environment. Our next step is the development of DSLAM and in this part we give a feasible design scheme. (2) Packet scheduling policy in ADSL system. In order to optimize ATU R performance, we need a scheduling policy to regulate the order and number of packet output to the uplink. For this purpose, we study the scheduling algorithm design methods in a single network node and their performance. After studying the existing algorithms carefully, we first conclude certain important design principles and criterions for scheduling policy evaluation. And then by using service curve function, we make detailed performance analysis of a typical scheduling policy — the earliest deadline first. The advantage of characterizing service by using a function is that the service curve can provide a wide spectrum of service characterization. With this method, we first develop a generalized algorithm for calculating the deadlines used in the service curve based on the earliest deadline first policy, and then design a simplified computing program for an important curve. After that, holding fair bandwidth allocation and packet deadline guarantee as policy evaluation standards, we analyze this scheduling policy performance and propose solutions to those problems that we find. Finally, we, in theory, prove these improving algorithms. All these solutions and algorithms we design in this part are now under development in real systems and will enhance system performance. (3) Effects of asymmetric links on TCP performance and solutions. TCP is a widely used transmission control protocol. It regulates its forward data sending through reverse ACKs. What are the new problems to its flow control mechanism on asymmetric links and how to solve them are the two main points of this part. We first study the relationship between normalized bandwidth ratio, link buffer capacity and TCP link throughput under asymmetric link models. We find that TCP ack transportation on slow backward link is a key reason that causes the problems in TCP asymmetric links. Serving this as a start point, we present several improving solutions to TCP flow control algorithm and test them through simulation. Besides, TCP has been widely implemented in many networks. To give compatibility with current TCP end users, we use scheduling solutions to regulate data packet and ack packet transmission ratio onto uplinks. In the theoretical analysis of several typical scheduling policies, we find a feasible algorithm to control TCP data efficiency on asymmetric links. Finally, we prove those analyses and propose algorithm through simulation.展开更多
文摘This thesis is based on the design and development of ADSL access networks. We have completed the hardware and software functions in two main ADSL equipments: ATU C and ATU R. With the system design, we carry out researches to find out effective methods to improved performance in the following aspects: (a) how to allocate limited bandwith among services with different QoS requirements and (b) what is the effect of asymmetric links on TCP end to end flow control mechanism. We make detailed analyses and propose useful solutions to those problems we find. The main contents of this thesis are as follows: (1) the design and realization of ADSL access system. This part includes the principles of ADSL access technology, ADSL broadband access network architecture, and ATM over ADSL protocol models that we use in our system. We give detailed hardware and software design steps in this part, and list system testing results to show the system performance. In our hardware, the CPU is a RISC microprocessor. Combined with a high speed digital signal processor, the whole system can hold high reliability. Our software design platform is a pSOS multi task real time operation system. We have developed a series of hardware drivers besides other tasks for high level control and interruption processing. Currently, our system has reached the expected performance under test environment. Our next step is the development of DSLAM and in this part we give a feasible design scheme. (2) Packet scheduling policy in ADSL system. In order to optimize ATU R performance, we need a scheduling policy to regulate the order and number of packet output to the uplink. For this purpose, we study the scheduling algorithm design methods in a single network node and their performance. After studying the existing algorithms carefully, we first conclude certain important design principles and criterions for scheduling policy evaluation. And then by using service curve function, we make detailed performance analysis of a typical scheduling policy — the earliest deadline first. The advantage of characterizing service by using a function is that the service curve can provide a wide spectrum of service characterization. With this method, we first develop a generalized algorithm for calculating the deadlines used in the service curve based on the earliest deadline first policy, and then design a simplified computing program for an important curve. After that, holding fair bandwidth allocation and packet deadline guarantee as policy evaluation standards, we analyze this scheduling policy performance and propose solutions to those problems that we find. Finally, we, in theory, prove these improving algorithms. All these solutions and algorithms we design in this part are now under development in real systems and will enhance system performance. (3) Effects of asymmetric links on TCP performance and solutions. TCP is a widely used transmission control protocol. It regulates its forward data sending through reverse ACKs. What are the new problems to its flow control mechanism on asymmetric links and how to solve them are the two main points of this part. We first study the relationship between normalized bandwidth ratio, link buffer capacity and TCP link throughput under asymmetric link models. We find that TCP ack transportation on slow backward link is a key reason that causes the problems in TCP asymmetric links. Serving this as a start point, we present several improving solutions to TCP flow control algorithm and test them through simulation. Besides, TCP has been widely implemented in many networks. To give compatibility with current TCP end users, we use scheduling solutions to regulate data packet and ack packet transmission ratio onto uplinks. In the theoretical analysis of several typical scheduling policies, we find a feasible algorithm to control TCP data efficiency on asymmetric links. Finally, we prove those analyses and propose algorithm through simulation.
