ATM cell multiplexer and scheduling method therefor

Details ATM cell multiplexer and scheduling method therefor ATM cell multiplexer and scheduling method therefor

2000-05-05

2004-03-30

Ho, Duc (Department: 2665)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S395310, C370S395400

Reexamination Certificate

active

06714546

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an ATM multiplexer equipped with a plurality of cell assembler/disassemblers, and a scheduling method therefor. Scheduling refers to controlling transmission time allocations for each cell assembler/disassembler when an ATM cell transmitted from at least 1 connection established in each cell assembler/disassembler is multiplexed and transmitted to an ATM network over one ATM line.
2. Description of the Related Art
In recent years, an ATM multiplexer has been required to interwork over an ATM network traffic that accords with a variety of communications protocols, such as voice, frame relay (FR), and HDLC (High-level Data Link Control Procedure). To realize these kinds of requirements, the ATM cell multiplexer has to implement traffic control in accordance with the service categories of each communications protocol to preserve the quality of service (QOS) thereof.
The following 2 methods exist for an ATM cell multiplexer to implement traffic control. In the first method, an ATM cell multiplexer is provided with an ATM switch, and makes use of the traffic control functions of the ATM switch. In the second method, an ATM cell multiplexer is not provided with an ATM switch, and cell streams between an ATM cell assembler/disassembler (CLAD) and ATM buses are scheduled. Because, in general, an ATM switch is costly to manufacture and requires complex control, an ATM cell multiplexer, which employs the above-mentioned second method not using an ATM switch, is generally provided for a market which requires an inexpensive, compact product.
FIG. 8
is a diagram illustrating the principle of an ATM cell multiplexer in which an ATM switch is not used. In
FIG. 8
,
4
CLAD
11
are connected in parallel to an ATM bus
14
and transmission authority is assigned by an ATM bus scheduler
12
allocating time slots (TS) to each CLAD in accordance with the volume of traffic of each CLAD
11
connection. An ATM cell, which is transmitted from a CLAD
11
during an allocated time slot, is sent to an ATM network by way of a line controller
13
.
FIG. 9
is an example of a constitution of an ATM bus scheduler
12
. In
FIG. 9
, the ATM bus scheduler
12
has a schedule table
201
comprising a plurality of time slots, in which the identification data of each CLAD
11
(CLAD-ID) is set. Now, when a schedule table reader
202
reads out the schedule table
201
, an ATM bus controller
203
controls an ATM bus
14
such that transmission authority is assigned to a connection of a CLAD
11
of the identification data set in a time slot of this schedule table
201
. That is, the identification data set in each time slot in a schedule table is data for allocating transmission time to each CLAD
11
. Furthermore, a schedule management main controller
204
of CLAD
11
controls the schedule table reader
202
and ATM bus controller
203
.
FIG. 10
is a diagram for illustrating a schedule table
201
. In
FIG. 10
, a schedule table
201
is constituted from 3 tables
201
a
,
201
b
,
201
c
. In general, a plurality of tables is prepared in accordance with the types of service categories available. For example, when the 3 types of service categories of CBR (Constant Bit Rate), VBR (rt-VB (real time-Variable Bit Rate), nrt-VBR (non-real time-Variable Bit Rate)), and UBR (Unspecified Bit Rate) are available, in table
201
a
, which is prepared as the main table, there is set identification data (CLAD-ID) corresponding to a CBR connection and a VBR connection. Now, for a CBR connection, there is allocated a traffic volume time slot equivalent to the PCR (Peak Cell Rate), and for a VBR connection, there is allocated a traffic volume time slot equivalent to the SCR (Sustainable Cell Rate). Further, in table
201
b
, which is prepared as a sub-table of table
201
a
, there is set identification data (CLAD-ID) corresponding to a VBR connection. Then, a time slot corresponding to the traffic volume of the remainder of the traffic volume assigned to table
201
a
(that is, PCR-SCR) is allocated. Furthermore, in table
201
c
, which is prepared as an additional sub-table of table
201
b
, there is set identification data (CLAD-ID) corresponding to a UBR connection, and a time slot corresponding to the traffic volume thereof is allocated.
Now then, an ATM bus scheduler
12
assigns during 1 cycle (for example, 0.4 seconds) transmission authority to connections corresponding to identification data set in 1536TS prepared in table
201
a
. At this time, a time slot of a connection that is not in the process of communications, and a time slot in which a connection is not set (in the figure, “No transmission authority”) are allocated to a connection having transmission authority in sub-table
201
b
. Furthermore, when identification data (CLAD-ID) of a connection having transmission authority is not set in sub-table
201
b
, a time slot is assigned to a connection having transmission authority in table
201
c
. By preparing a plurality of schedule tables with priorities that differ in accordance with quality of service (QOS) like this, it is possible to maintain each quality of service. That is, if a connection pertains to a service category which is required to be stricter about time delays, transmission is assigned to such connection based on a schedule table with higher priority.
In addition, to preserve the quality (QOS) of a connection of a service category that is strict about time delays (for example, either CBR or rt-VBR), the transmission authority of such a connection must be allocated at equal intervals for 1 cycle's worth of time slots (for example, 1536 slots) that are set in table
201
a
, the main table. For example, when the traffic volumes of a plurality of connections are the same, transmission authorities for a plurality of connections can be assigned in equal intervals by shifting by a predetermined number the time slot allocated to each connection. Conversely, when the traffic volumes of a plurality of connections differ, the intervals of time slots to which transmission authorities have been assigned will differ, respectively, even if the time slot allocated to each connection is shifted by a predetermined number, thus generating time slots with overlapping transmission authorities. In a case such as this, overlap will be avoided by shifting the transmission authority of any one of the connections to a previous or subsequent time slot. However, as explained above, for the connection of a service category that is strict about time delay, it is desirable that discrepancies in transmission authority spacing be as small and as few as possible.
Therefore, conventionally, a main table
201
a
, in which transmission authority intervals for a plurality of established connections are set to be as equal as possible, is prepared beforehand.
However, when a main table
201
a
is set beforehand as in the above-mentioned prior art, the following problem occurs when a new connection is established in accordance with installing an additional CLAD. That is, when a new connection is a connection of a service category that is strict about time delays, the transmission authority of this connection must be assigned at intervals that are as equal as possible, but, in the main table
201
a
, this connection has to be allocated to an empty time slot that has yet to be assigned transmission authority. An empty time slot does not always exist at equal intervals, and the larger the number of connections set in the main table
201
a
, the less likely it is that the time slot will be equally spaced. Therefore, there occur situations in which it is impossible to assign an equally spaced transmission authority to a newly established connection.
Conversely, suppose there is a situation, in which a main table
201
a
is not prepared beforehand as described above, but rather, a main table
201
a
is set by allocating a time slot to a connection each time in accordance with the number of connections established. In a case such as this, the foll

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