Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-03-17
2002-04-30
Yao, Kwang B. (Department: 2664)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S395310, C370S395510
Reexamination Certificate
active
06381244
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a connectionless communication method for the connectionless transmission of user data. More particularly, the invention relates to a datagram-type connectionless communication method in which a cell (also referred to as a “leading cell” below) which includes the address of a destination terminal and the like is inserted at the head of a cell stream transferred with an attached prescribed line identifier, and the network transmits a user cell to the destination terminal upon referring to the leading cell.
(a) Connection-oriented Communication and Connectionless Communication
There are two methods of connecting terminals, namely a connection-oriented method and a connectionless method. As shown in
FIG. 31A
, the connection-oriented communication method involves dialing a number, for example, verifying the transmitting side (A system) and receiving side (B system) prior to the start of communication so as to ring the other party's bell, establishing a path between the two sides and then performing data communication via the established path. This method is advantageous in that reliable communication is possible via a connected line and in that it is simple to transfer information after the connection is established. However, since making the connection takes time, this method is not efficient for the transmission of data of short duration.
The connectionless communication method, on the other hand, as shown in
FIG. 31B
, involves collecting transmission data into packets, attaching destinations to the packets and then sending the packets to a network, whereupon the network checks the destination of each packet one by one and transmits each packet to the destination terminal. Since making a connection with this method does not take time, even short-duration data can be sent efficiently.
In order to make possible multimedia transmission which includes the transmission of video, ATM (Asynchronous Transfer Mode) technology has been developed in association B-ISDNs and a variety of communication services have been realized. A B-ISDN is capable of providing both connection-oriented and connectionless services.
(b) Connection-oriented Communication in ATM Networks
FIG. 32
is a diagram useful in describing connectionless communication in an ATM network. Shown in
FIG. 32
are ATM terminals A~D, ATM exchanges EX
1
~EX
4
, a network NWK composed of a group of ATM exchanges, and a network management center NMC. In a case where terminal A is to perform connection-oriented communication with terminal C, the following procedure for establishing a VCC (Virtual Channel Connection) is required:
(1) Terminal A issues a request for communication with terminal C to the network management center NMC via a common signal line (a VCC for signaling).
(2) The network management center NMC responds to the request from terminal A by performing computation to determine that the VCC between the terminals A and C via the ATM exchanges EX
1
, EX
2
, EX
3
, EX
4
, in the order mentioned, is the most advantageous route, and by instructing each ATM exchange to establish the VCC.
(3) The ATM exchange EX
1
notifies terminal A and the ATM exchange EX
4
notifies terminal C of line identifiers VPI, VCI used in communication.
(4) Terminal A disassembles data into cells, attaches the line identifiers of which it has been notified to the cells and then sends the cells to the line. Each of the exchanges EX
1
~EX
4
exchanges and transmits the cells while changing the line identifiers of the cells. Terminal C accepts the cells, which have the specified line identifiers, transmitted from the exchange EX
4
and assembles the cells into data.
(c) Connectionless Communication in ATM Networks
(c-1) Problem Which Arises When Connectionless Communication is Used in ATM Networks
The header of an ATM cell contains only virtual line identifiers such as VCI and VPI. That is, the header does not contain an address indicating the destination terminal. Consequently, providing a connectionless communication service such as electronic mail is accompanied with difficulties. For example, if the average downloaded file size on the WWW (Worldwide Web) is two kilobytes and the ATM switch transmits at a rate of 155 megabits per second, then this is the size of a file that can be downloaded in 100 microseconds. However, since an ATM switch requires 10 milliseconds to be connected, 99% is the overhead for the connection. In other words, if an ATM network of the connection-oriented transfer type is used as a transmission path for implementing a connectionless communication service, a delay occurs owing to the time needed to establish the connection.
Solutions to this problem include a PVC (Permanent Virtual Circuit) in the ATM layer and a datagram in a higher layer (e.g., IP layer).
(c-2) PVC Technique
The PVC technique involves establishing end-to-end PVCs beforehand between all of the terminals A~F that are capable of communicating, as shown in FIG.
33
. However, this method is disadvantageous in that PVC resources are wasted. In particular, this method cannot be employed in a large-scale network (the Internet, for example) in which communication is performed between an unspecified number of terminals.
(c-3) Datagram Technique
With the datagram technique, as shown in
FIG. 34
, PVCs (datagram VCCs: VCC
1
~VCC
7
) are established beforehand between the terminals A~F and whichever of the exchanges EX
1
, EX
2
is nearest to them, and between the mutually adjacent exchanges EX
1
and EX
2
. An IP packet of a higher layer is assembled from ATM cells by each exchange and switching processing is executed upon observing the terminating-side terminal address that has been entered in the IP packet header. It should be noted that a “datagram” is the unit in which messages are handled in accordance with the IP (Internet Protocol). A datagram technique is a method of providing a destination address in the header of a packet, deciding, on each occasion, the transfer route for each packet and then transmitting the packet.
FIG. 35
is a diagram showing the relationship between an IP packet (IP datagram) and ATM cells. Here P represents an IP packet, which is composed of a header PH and transmission data DT. The header PH includes a variety of information, such as a source address SA and destination address DA. The IP packet P is partitioned into a number of ATM cells CL
1
~CL
n
, and a header HD is added onto each cell at the beginning thereof. Line identifiers (VPI/VCI) included in the headers of the cells CL
1
~CL
n
have identical values.
FIG. 36
is a diagram showing the structure of an exchange which implements the conventional datagram approach. Shown in
FIG. 36
are an exchange
1
, an ATM switch unit
2
, and an IP router
3
. (1) A switching able
2
a
of the ATM switch unit
2
stores the correspondence between the input line identifiers (input VPI/VCI) and output line identifiers (output VPI/VCI), and an ATM switch
2
b
generally switches a cell, which arrives from the VCC, to a prescribed route based upon the content of the switching table
2
a
. (2) If a cell arrives from a datagram VCC, the ATM switch unit
2
b
delivers this cell to the IP router
3
. A packet assembler
3
a
in the IP router
3
assembles ATM cells into an IP packet and delivers the IP packet to a routing controller
3
c
. (3) A routing table
3
b
stores the correspondence between terminal addresses and output-destination datagram VCCs. Accordingly, the routing controller
3
c
refers to the routing table
3
b
to obtain an output-destination datagram VCC that corresponds to a destination terminal address indicated by the header of the entered IP packet and notifies the ATM switch
2
b
to perform the switching of one packet.
In this datagram approach involving a higher layer (the IP layer), it is required that the above-mentioned processing operations (2) and (3) be performed whenever a cell arrives from the datagram VCC. An additional problem is that assembling the packet in step (2) above takes time.
Anothe
Inoue Hideki
Kimura Takeshi
Nagano Hiroshi
Nishimura Takashi
Sekiya Naohide
Rosenman & Colin LLP
Yao Kwang B.
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