Multiplex communications – Data flow congestion prevention or control – Control of data admission to the network
Reexamination Certificate
1998-12-28
2002-08-20
Ngo, Ricky (Department: 2732)
Multiplex communications
Data flow congestion prevention or control
Control of data admission to the network
C370S234000, C370S236100, C370S395100
Reexamination Certificate
active
06438107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a communications network which transmits packets and contains a plurality of services, and more specifically to an apparatus and method of controlling the cell transfer rate in an ATM (asynchronous transfer mode) network containing a plurality of services such as an ABR (available bit rate) service, a UBR (unspecified bit rate) service, and so on.
2. Description of the Related Art
Currently, in an ATM forum and the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), an ABR service is considered a method of applying the data switch technology in the ATM to a high-speed data communications network such as a LAN (local area network), a WAN (wide area network), and so on.
To easily accommodate a conventional LAN, a UBR service in which the quality is not defined is also considered.
Described below is the ABR service considered in the ATM forum and the ITU-T.
In the communications using the ABR service, a resource management cell (RM cell) is used to notify a terminal of the information about a network resource. A source terminal transmits an RM cell each time it transmits the predetermined number of user data cells. The RM cell reaches a destination terminal through a network, and is looped back to the source terminal.
On the other hand, an ATM switch in the network notifies the source terminal of the information in the network by writing the resource information (band information and congestion information) in the ATM switch to an RM cell passing through the ATM switch. Upon receipt of the RM cell to which the band information and the congestion information, the source terminal re-computes the allowed cell rate (ACR) according to the information, and transmits the cell at a rate equal to or lower than the ACR.
When the source terminal establishes an ABR connection at the start of the communications, it declares through the network the peak cell rate (PCR), which is the highest transmission rate, and the minimum cell rate (MCR), which is the lowest request rate, and negotiates for their values. As a result of the negotiation, the source terminal cannot transmit a cell at a rate higher than the determined PCR. On the other hand, the network guarantees the source terminal a cell transmission rate equal to or higher than the MCR determined as a result of the negotiation. Accordingly, in the ABR service, the ACR of the source terminal is variable in the range from the MCR to the PCR (MCR≦ACR≦PCR).
Using the ABR service in the above described operation, the network can avoid congestion or recover from the congestion, and the terminal units can transmit cells at a high transmission rate when the resources of the network are available.
Recently, the operations of the source terminal and the destination terminal communicating with each other through the ABR service are considered targets to be standardized in the ATM forum. Described below is the outline of the operations.
FIG. 1A
is a chart showing the concept of the flow control in the ABR service.
In
FIG. 1A
, according to the arrow indicating the forward flow A, an ABR source terminal
101
transmits a cell forward (from source to destination) at a rate equal to or lower than the ACR at each point. At this time, the ABR source terminal
101
transmits one RM cell
114
to a switch (ATM switch)
102
each time a predetermined number of user data cells
112
are transmitted. Upon receipt of an RM cell
114
F from the switch
102
, the ABR source terminal
101
adds a predetermined value to the value of the ACR if the congestion indicator (CI) bit (not shown in
FIG. 1A
) in the RM cell
114
B is set to 0 (non-congestion), and if the no-increase (NI) bit is set to 0 (not specified). On the other hand, if the above described CI bit is set to 1 (congestion), then the ABR source terminal
101
subtracts the predetermined value from the ACR value. Furthermore, the ABR source terminal
101
compares the value of the explicit cell rate (ER) written in the RM cell with the ACR value obtained in the above described computation after the above described operations, and defines a smaller value as a new ACR. At this time, the ACR value should satisfy MCR≦ACR≦PCR.
In
FIG. 1A
, according to the arrow indicating the backward flow B, an ABR destination terminal
103
terminates the user data cell
112
transmitted from the source terminal, loops back the received RM cell
114
F, and transmits it as the backward (from destination to source) RM cell
114
B to the ABR source terminal
101
. In this case, if the explicit forward congestion indication (EFCI) bit indicating the congestion in the user data cell
112
A received immediately before receiving the RM cell
114
F is set to 1, then the ABR destination terminal
103
sets the CI bit in the looped-back RM cell
114
F to 1, and the RM cell
114
is transmitted as a backward RM cell
114
B after the setting.
Described below is the function of the ATM switch in the ABR service.
The operations of the network for realizing the ABR service, that is, the ATM switch, can be performed in an EFCI mode and an ER mode.
In the EFCI mode, the ATM switch sets the EFCI bit in the user data cell passing through the ATM switch to 1 when congestion exists, and passes the cell.
In the ER mode, the ATM switch computes the ER explicitly indicating the transmission rate to the source terminal based on the resources and the congestion state in the ATM switch, and writes the obtained value to the forward or backward RM cell passing through the ATM switch. At this time, the ATM switch compares the ER value in the RM cell with the ER value computed by the ATM switch itself, and writes a smaller ER value to the RM cell.
In addition to the RM cell transmitted from the source terminal, the ATM switch or the destination terminal can generate an RM cell, and transmit it to the source terminal.
In addition to the above described mode function, the functions referred to as ABR VS/VD (virtual source/virtual destination) are assumed as operations of the ATM switch for realizing the ABR service. When the VS/VD functions are realized, a VD function
122
a
of looping back upstream the forward RM cell transferred from upstream as a backward RM cell is provided at the upstream of an ATM switch
122
as shown in FIG.
1
B. At the downstream of the ATM switch
122
(ABR destination terminal
123
), a VS function
122
b
for generating and terminating an RM cell is provided.
Thus, in the VS/VD functions, when the ATM switch
122
having the VS/VD functions loops back, generates, and terminates an RM cell for an ABR connection, the control loop of the ABR connection is divided (segmented) into a plurality of control loops as shown in
FIG. 1B
, not a simple loop of an ABR source terminal
121
, an ATM switch network, an ABR destination terminal
123
, an ATM switch network, and an ABR source terminal
121
.
In this case, the VD function
122
a
provided in the ATM switch
122
virtually simulates the function of a destination terminal. Similarly, the VS function
122
b
provided in the ATM switch
122
virtually simulates the function of a source terminal.
Thus, the control loop between the ABR source terminal
121
and the ABR destination terminal
123
is divided into a plurality of control loops by the ATM switch
122
. In each control loop, the feedback control of the transfer rate using an ER value is performed in parallel so that the response of the feedback can be improved. As a result, the network can more quickly avoid and recover from congestion, and the terminal can communicate cells at a higher rate when the resources of the network are available.
The following functions are required to realize the above described VS/VD functions.
(1) ABR Source/Destination operations prescribed by the ATM forum traffic management 4.0 (essential functions).
a) generating and terminating an RM cell (function to be realized by the VS).
b) computing ACR according to the information set in the received backward RM ce
Nakamichi Koji
Somiya Toshio
Fujitsu Limited
Katten Muchin Zavis & Rosenman
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