Multiplex communications – Data flow congestion prevention or control – Flow control of data transmission through a network
Reexamination Certificate
1998-07-02
2001-10-02
Kizou, Hassan (Department: 2662)
Multiplex communications
Data flow congestion prevention or control
Flow control of data transmission through a network
C370S230000, C370S252000, C709S233000
Reexamination Certificate
active
06298042
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a packet switching apparatus adapted to determine and control an allowed transmission rate to be informed to a transmitting terminal in a packet switching network in which a band management is done by feedback control, and a method of controlling the allowed transmission rate.
2. Description of the Related Art
As one of the communication systems in which feedback control is done, an available bit rate (ABR) service in an asynchronous transfer mode (ATM) network has been proposed. The ABR service is disclosed in the document “ATM forum Traffic Management Specification Version 4.0 R11” (Shirish S. Sathaye, March 1996). In the ABR service, an allowed cell rate (ACR) being a transmission cell rate at a terminal is controlled by a feedback control in which a resource management (RM) cell is used.
FIG. 4
is a schematic block diagram showing the configuration of an ATM network for an ABR service. The operation of the ABR service as in the document “ATM forum Traffic Management Specification Version 4.0 R11” will be described herebelow with reference to FIG.
4
.
In
FIG. 4
, an ATM network is generally indicated with a reference
401
. The ATM network
401
incorporates an ATM switch
402
and has connected thereto a transmitting terminal
403
and receiving terminal
404
. For sending data to the receiving terminal
404
, the transmitting terminal
403
mixes a forward resource management (FRM) cell in a data cell. Upon reception of the data cell through the ATM switch
404
, the receiving terminal
404
will change the FRM cell to a backward resource management (BRM) cell and send it back to the transmitting terminal
403
.
When the FRM cell sent from the transmitting terminal
403
and the BRM cell sent back from the receiving terminal
404
are passed through the ATM switch
402
, the ATM cell
402
will write into either the FRM or BRM cell or into both an explicit rate (ER) indicative of how much congested the connections from the transmitting terminal
403
to the receiving terminal
404
are currently. The ER is a maximum transmission rate at which such connections routed through the ATM switch
402
are allowable without any congestion and which is currently allowed for the transmitting terminal
403
. When the connections through the ATM switch
402
are totally a low load, a higher ACR is allowable for the transmitting terminal
403
. Therefore, the ER value calculated by the ATM switch
402
will be a large one. On the contrary, when such connections are a larger load, the ATM switch
402
will calculate a smaller ER value. The transmitting terminal
403
will increase or decrease the ACR based on an ER written in the BRM cell sent from the ATM switch
402
to effect a data communications at a rate within the range of increased or decreased ACR.
As mentioned above, the transmitting terminal
403
decreases the ACR because of a received ER value which is lower. In addition, the ACR is decreased due to an initialization thereof. More particularly, if a length of time for which the transmitting terminal
403
sends no FRM cell exceeds an ACR decrease time factor (ADTF), the ACR will be decreased to an initial cell rate (ICR). As described in the aforementioned document “ATM forum Traffic Management Specification Version 4.0 R11”, this initialization is an operation effected by the transmitting terminal
403
controlling itself. One of the FRM cell sending conditions is that one FRM cell is sent for a predetermined number of data cells. Thus, since an extremely small number of FRM cells is sent when the transmitting terminal
403
has no further data to send, the ACR initialization will be effected. The purpose of this ACR initialization is for the transmitting terminal
403
to resume with an ICR, not any ACR, a data transmission of which it has once been in pause, in a predetermined time from the pause, thereby preventing the network from being abruptly applied with a large load.
FIG. 5
is a schematic block diagram of an ATM switch in the ATM network, showing the role of the ATM switch. In the network illustrated in
FIG. 5
, transmitting terminals
502
equal in peak cell rate (PCR) to each other are connected to an ATM switch
504
to links
503
, respectively, each having a linkage capacity which is same as the PCR of the transmitting terminals
501
and
502
, they are multiplexed by an ATM switch
504
, and connected to an ATM network
505
via another link
503
. Of the transmitting terminals, one
501
will always send data at a full ACR as the actual transmission rate while the other
502
will send data initially at the full ACR, then at a rate extremely low (LCR) but at which the ACR is not initialized by the transmitting terminal itself, and at the full ACR again in a predetermined time after that.
FIG. 6
is a time chart showing the operations of the transmitting terminal
502
shown in FIG.
5
.
FIG. 6
shows values of an ER received by the transmitting terminal
502
, and an ACR and an actual transmission rate, respectively, of the terminal
502
. First, when the transmitting terminals
501
and
502
are continuously sending data, the ER for no congestion of the ATM switch
504
will take a half of a PCR value (will be referred to simply as “PCR” hereinafter), that is, “PCR/2”. Accordingly, the ACR of the transmitting terminals
501
and
502
having received that ER will also be “PCR/2”. The transmitting terminals
501
and
502
will send data at an actual transmission rate “PCR/2”.
In this condition, the actual transmission rate of the transmitting terminal
502
will be reduced to “LCR” at a time t0. The “LCR” is a very small value at which however the ACR will not be initialized. As the actual transmission rate is thus decreased, the ATM switch
504
will be applied with little load for the data transmission by the transmitting terminal
502
. Therefore, the ATM
504
will calculate a new ER. Conventionally, the ER will be equal to “PCR” when calculated by the commonly used ER calculation method stated in “Explicit Rate Indication for Congestion Avoidance; ERICA)” in the aforementioned document “ATM forum Traffic Management Specification Version 4.0 R11”. The ATM switch
504
will inform the new ER thus calculated thereby to the transmitting terminals
501
and
502
.
It should be noted for
FIG. 6
that the ER value of “PCR” is informed to the transmitting terminal
502
at a time t2 a predetermined time after the time t0 at which the actual transmission rate of the transmitting terminal
502
has decreased to “LCR”. This is because for a time period from the reduction of the actual transmission rate of the transmitting terminal
502
until the terminal
502
is informed of an ER reflecting the reduced actual transmission rate, a propagation delay is caused by a distance between the transmitting terminal
502
and ATM switch
504
.
The transmitting terminal
501
has an actual transmission rate equal to “PCR” since the received new ER will lead to an ACR which takes a value “PCR”. On the other hand, the actual transmission rate of the transmitting terminal
502
is very low being “LCR”, but the ACR will be maintained at “PCR” because the received ER has a value of “PCR”.
Next, assume that a factor having reduced to “LCR” the actual transmission rate of the transmitting terminal
502
has been eliminated at a time t5 as shown in FIG.
6
. Thus, the actual transmission rate of the transmitting terminal
502
will be raised up to the full ACR. The ACR is not be initialized but maintained at “PCR”, so that the actual transmission rate will abruptly jump up to an ACR value, that is, “PCR”. Therefore, both the transmitting terminals
501
and
502
will send data at the actual transmission rate “PCR” after the time t5 and so the amount of data arriving at the ATM switch
504
and ATM network
505
will exceed the capacity (equal to “PCR”) of the link
503
connecting the ATM switch
504
and ATM network
505
to each other. Thus, a serious congestion will take place at
Iwasaki Takashi
Murase Tutomu
Foley & Lardner
Kizou Hassan
NEC Corporation
Pezzlo John
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