ATM system with ABR function

Details ATM system with ABR function ATM system with ABR function

1998-10-29

2002-04-16

Ton, Dang (Department: 2661)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S236100, C370S253000

Reexamination Certificate

active

06373844

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ATM (Asynchronous Transfer Mode), in particular, to an ATM system with ABR (Available Bit Rate) function for optimally using a network resource and a method for deciding a transmission rate of ATM cells.
2. Description of the Related Art
FIG. 1
shows the structure of a VS/VD (Virtual Source/Virtual Destination)
11
disclosed in “ABR with Virtual Source/Virtual Destination for ATM Wide Area Network” (Tsutomu Murano et. al., B-719, 1996 Convention, The Institute of Electronics, Information and communication Engineers, Japan, Aug. 30, 1996) (referred to as Paper [1]).
Assuming that a network shown in
FIG. 2
has been structured, the operation of the VS/VD will be described. In
FIG. 2
, solid lines represent links (transmission lines). SW
1
, SW
2
, SW
3
, and SW
4
represent switches (exchanges). The switches SW
1
and SW
2
are connected with one link. Likewise, the switches SW
3
and SW
4
are connected with one link. A terminal
11
is connected to a terminal
12
through the switches SW
1
and SW
2
. A terminal
31
is connected to a terminal
32
through the switches SW
3
and SW
4
. A terminal
41
is connected to a terminal
42
through the switches SW
3
and SW
4
. A terminal
21
is connected to a terminal
22
through the switches SW
3
, SW
4
, SW
1
, and SW
2
.
An area between the terminals
11
and
12
is referred to as segment
1
. An area between the terminals
21
and
22
is referred to as segment
2
. An area between the terminals
31
and
32
is referred to as segment
3
. An area between the terminals
41
and
42
is referred to as segment
4
. In each segment, congestion information of a downstream area is fed back and thereby a transmission rate of an upstream area is decided. Since the segment
2
includes the four switches SW
3
, SW
4
, SW
1
, and SW
2
, the feedback loop becomes long. Thus, the feedback control becomes difficult.
To solve such a problem, the VS/VD is disposed in the middle of the segment
2
so as to divide the segment
2
into segment
2
a
and
2
b
. Thus, as with the segments
1
,
3
, and
4
, each of the segments
2
a
and
2
b
includes only two switches.
Returning to
FIG. 1
, the upstream segment and the downstream segment of the VS/VD
11
is the segment
2
a
and the segment
2
b
, respectively. The circuit shown in
FIG. 1
is divided into an upstream segment circuit portion and a downstream segment circuit portion. In each of the segments
2
a
and
2
b
, a transmission bit rate is decided corresponding to the situation of the network.
A data cell and an RM (Resource Management) cell are input from the upstream segment through a relevant transmission line. Each cell is a packet with a fixed bit length (for example, 53×8 bits).
Next, with reference to
FIGS. 5A
,
5
B,
5
C, and
5
D, ATM cells will be described. In
FIGS. 5A
,
5
B,
5
C, and
5
D, the ATM cells are categorized as data cells and RM (Resource Management) cells. A data cell is composed of a header portion of five bytes and a user information portion of 48 bytes. An RN cell is used for resource management. The header portion of the data cell is the same as the header portion of the RN cell.
FIG. 5C
shows the format of the RM cell. As shown in
FIGS. 5C and 5D
, the RM cell comprises five bytes the header portion, one byte of RM protocol ID for identifying the RM cell, eight bits of a message type field in which contains DIR (direction bit in RM cell which DIR=0 is a forward direction and DIR=1 is backward direction), BN (BECN bit in RM cell), CI(congestion indication bit in RM cell), NI(no increase in RM cell), RA(resource allocation bit in RM cell), and three bits reserves.
The upstream segment circuit portion terminates an FRM (Forward Resource Management) cell corresponding to the destination behavior defined in ATM Forum Traffic Management Specification Version 4.0 (S.S. Satheye, April
1996)
and folds back a BRM (Backward Resource Management) cell.
On the other hand, the downstream segment circuit portion terminates a BRM cell corresponding to the source behavior defined in ATM Forum Traffic Management Specification Version 4.0 (S.S. Satheye, April 1996), decides a transmission rate of a data cell to be transmitted to the downstream segment, and generates a new FRM cell.
To absorb the difference between the transmission rate of the upstream segment and the transmission rate of the downstream segment, a VS buffer is disposed in the downstream segment circuit portion.
In other words, the upstream segment circuit portion shown in
FIG. 1
has a cell identifying portion
12
and an RM cell folding portion
13
. The cell identifying portion
12
identifies a cell received from the upstream segment. The RM cell folding portion
13
folds back an FRM cell as a BRM cell to the upstream segment. At this point, DIR =“1”, CI=“1” or “0”, NI =“
1
” or “0”, and RA =“1” or “0” of a message type field (seventh byte in a RM cell) of the BRM cell shown in
FIG. 5D
are set. The data cell is temporarily stored in the VS buffer
17
of the downstream segment circuit portion.
The downstream segment circuit portion has a BRM cell terminating portion
14
, a rate controlling portion
15
, an FRM cell generating portion
16
, and the VS buffer
17
. The BRM cell terminating portion
14
terminates a BRM cell received from the downstream segment and extracts congestion information (ER, CI, and NI) from the payload of the cell. The rate controlling portion
15
calculates a rate information for ACR (Allowed Cell Rate) corresponding to the congestion information. The rate controlling portion
15
controls the output timing of the VS buffer
17
corresponding to the ACR.
The rate information is placed as CCR (Current Cell Rate) in the payload of an FRM cell generated by the FRM generating portion
16
. The resultant cell is transmitted to the downstream segment through the VS buffer
17
. At this point, DIR=“0” and BN =“0” of the message type field (bit
7
) of the FRM cell shown in
FIG. 5D
are set. Since the VS/VD
11
controls a short feedback loop, when a switch of the VS/VD
11
is designed with a limited buffer amount of the VS buffer
17
, the VS/VD
11
can be used for a network system that grows.
In addition, since the VS buffer
17
absorbs temporary congestion due to a statistical fluctuation of the applied load, the switch does not need to control terminals due to the congestion. Thus, the use rate of the network is improved.
However, in the VS/VD disclosed in Paper [1], since the feedback loop is cut by the VS/VD, the following problems take place.
In
FIG. 2
, it is assumed that the link connecting the switches SW
1
and SW
2
and the link connecting the SW
3
and the SW
4
have a transmission capacity of 150 Mbps each. When the transmission capacity of the line connecting the switches SW
1
and SW
2
is equally divided by the segment
1
and the segment
2
b
, the transmission capacity of 75 Mbps can be assigned to each segment.
Likewise, when the transmission capacity of the link connecting the switches SW
3
and SW
4
is equally divided by the segment
2
a
, the segment
3
, and the segment
4
, the transmission capacity of 50 Mbps can be assigned to each segment.
Considering the VS/VD disposed between the segment
2
a
and the segment
2
b
, even if the transmission capacity of 75 Mbps is assigned to the downstream segment
2
b
, since cells are input from the upstream segment
2
a
at 50 Mbps, the transmission capacity of 25 Mbps of 75 Mbps assigned to the segment
2
b
is not used.
In other words, in the system disclosed in Paper [1], since the allowed cell rate ACR of the downstream segment is decided regardless of that of the upstream segment, if the transmission rate of the upstream segment is lower than the transmission rate of the downstream segment, part of the allowed cell rate ACR assigned to the downstream segment is not used. Thus, from a point of view of the entire net

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