Dynamic band-allocating subscriber-line-terminating apparatus

Details Dynamic band-allocating subscriber-line-terminating apparatus Dynamic band-allocating subscriber-line-terminating apparatus

2000-12-26

2004-09-14

Nguyen, Chau (Department: 2663)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S443000, C370S449000

Reexamination Certificate

active

06791984

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
In general, the present invention relates to a subscriber-line-terminating apparatus. More particularly, the present invention relates to dynamic allocation of a band required for transmission of information to optical network units.
2. Description of the Related Art
ATM-PON is an access communication system making use of characteristics of both an ATM (Asynchronous Transfer Mode) technology and a PON (Passive Optical Network) technology. The ATM technology makes integrated processing possible by dividing a variety of multimedia services of various kinds such as sounds, pictures and data into cells each having a fixed length. On the other hand, the PON technology makes a system cost low by letting a plurality of users share an optical transmission facility. An ATM-PON system comprises, among others, a subscriber-line-terminating apparatus (abbreviated hereafter to an SLT) installed typically at a telephone office, a plurality of optical network units (abbreviated hereafter to ONUS) installed at buildings and homes and an ATM exchange. The SLT is connected to the ONUs by a splitter called a star coupler, which is a passive device, through optical fibers. A direction from the SLT to an ONU is referred to as a descending direction. On the other hand, a direction from an ONU to the SLT is known as an ascending direction. Information is included in an ATM cell. Information is transmitted in cell units with each cell having additional bytes of data required for communication through a PON on a transmission line. Information is transmitted from the SLT to a plurality of ONUs in the descending direction as a signal comprising successive ATM cells. An ONU receives the transmitted information only after verifying that the information includes data indicating that the information is destined for the ONU. Each ONU sets ATM cells to a time slot obtained information transmission permission by the SLT for avoiding collision so as to transfer the same so that an ascending signal from an ONU to an SLT is commiunicated. The SLT allocates a time slot for an ascending signal to a specific ONU by transmission of a polling signal appending a transmission right for the specific ONU to a descending signal.
Polling in the ATM-PON system is implemented by providing a polling cell called a PLOAM cell at a specific location in a descending frame comprising typically 56 cells. An identifier for identifying an ONU serving as a recipient of a transmission right is set in the PLOAM cell. In this way, a descending signal can be transmitted to a plurality of ONUs. In the conventional system, polling is carried out for allocation of fixed-band portions. In this description, a band portion is a partial band allocated to an ONU. That is to say, a band portion allocated to each ONU is fixed. However, this conventional technique is not capable of well keeping up with a case in which traffic occurs irregularly as a burst as is the case with data traffic. By setting the polling at a time close to the occurrence of a peak rate of information generations, all information can be transmitted even if a burst is generated. With this method, however, polling operations at few bursts will be all wasted so that the transmission band cannot be used with a high degree of efficiency. In order to solve this problem, the polling is set at a time close to an average value of information generations to absorb fluctuations in information generation interval and to avoid congestion. In this case, however, a buffer memory with a very large size is required. In order to solve this buffer-size problem, there was invented a dynamic allocation technique capable of allocating band portions with a high degree of efficiency in a system with a transmission quantity varying dynamically as disclosed in Japanese Patent Laid-open No. Hei 10-242981. This technique is referred to as a conventional method.
In the conventional method, the SLT requests each ONU that the ONU notify the SLT of the amount of information stored in a buffer owned by the ONU by using an ascending signal. The SLT makes such a request by using a descending PLOAM cell. The amount of information stored in a buffer owned by each ONU is referred to as an in-buffer-information amount. In accordance with the descending PLOAM cell, the ONU retrieves the in-buffer-information amount and generates a request for a band portion for transmission of the information. The requested band portions is to be allocated to the ONU by the SLT on a polling basis. The request for the band portion to be allocated on a polling basis is multiplexed with ascending information in an ascending signal, which is then transmitted to the SLT. The SLT extracts the in-buffer-information amount from the request for the band portion to be allocated on a polling basis received from each ONU through a transmission line. Then, the SLT allocates a fixed-band portion determinately allocated on a polling basis to each ONU initially. The SLT then assigns a portion of a shared band allocated on a polling basis as described later to a free time slot. Subsequently, polling information is mixed with descending information in a descending signal transmitted to each ONU. A procedure to allocate a portion of a shared band on a polling basis is explained briefly as follows. First of all, a band unit is allocated to ONUs on a polling basis, that is, allocated to ONUs each having an in-buffer-information amount of at least equal to a threshold value of 0. The threshold value is incremented by 1 and a band unit is again allocated to ONUs on a polling basis, that is, allocated to ONUs each having an in-buffer-information amount of at least equal to a threshold value of 1. Thereafter, each time the threshold value is incremented, the same procedure is executed for the updated threshold value. As the threshold value reaches such a large value that there are no longer ONUs to which a band unit is to be allocated on a polling basis, the threshold value is restored to 0 to start the procedure from the beginning. As a result of the processing, a polling count of an ONU, that is, the number of polling operations in which a shared band unit is allocated to the ONU or the number of shared band units allocated to the ONU, is proportional to the in-buffer-information amount plus 1. Assume for example that the in-buffer-information amounts of ONU 1 and ONU2 are 50 and 300 respectively whereas the total size of the shared band is 1,000 units. In this case, the number of shared-band units allocated to ONU 1 is 143 (=1,000×51/352) and the number of shared-band units allocated to ONU 2 is 857 (=1,000×301/352).
However, the conventional method has the following 3 problems.
1: The Size of a Band Portion or the Number of Band Units Allocated to Each ONU Greatly Vibrates with the Lapse of Time.
As described above, the number of band units allocated by the SLT to each ONU is proportional to the most recent in-buffer-information amount plus 1. Thus, almost no band is allocated to an ONU with a small in-buffer-information amount. However, an ONU with a small in-buffer-information amount is not in a state of communicating no information. The small in-buffer-information amount is caused by the fact that a band portion previously allocated thereto by the SLT is greater than the magnitude of the actual communication. Since an ONU with a small in-buffer-information amount is not polled by the SLT, the in-buffer-information amount increases abruptly. When the in-buffer-information amount increases abruptly, the SLT allocates a sufficiently large band portion to the ONU. Thus, the in-buffer-information amount decreases abruptly. These operations to allocate band portions to ONUs are carried out at intervals of updating an allocated band portion. As a result, the size of a band portion allocated to an ONU fluctuates greatly.
FIG. 55
is a diagram showing tabular results of allocation based on polling in the conventional system obtained by simulation. In the simulation, the total ban

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