Circuit-switched network

Details Circuit-switched network Circuit-switched network

1999-02-25

2004-05-04

Marcelo, Melvin (Department: 2666)

Multiplex communications

Pathfinding or routing

Combined circuit switching and packet switching

C370S352000, C370S393000

Reexamination Certificate

active

06731628

ABSTRACT:

PRIOR APPLICATIONS
This application claims priority from Japanese Applications No. 10-45693 filed Feb. 26, 1998, No. 10-45707 filed Feb. 26, 1998, No. 10-45744 filed Feb. 26, 1998, No. 10-45767 filed Feb. 26, 1998, No. 10-127762 filed May 11, 1998 and No. 10-127788 filed May 11, 1998, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to Synchronous Transfer Mode (STM) communications networks, and in particular to techniques for transferring data with an Internet Protocol (IP) address by way of an STM connection.
2. Description of Related Art
A feature of STM-based circuit-switched networks is that because signals on a physical channel are multiplexed into time slots and circuit-switched, the delay during communication is extremely short and there is no overhead such as the header required in Asynchronous Transfer Mode (ATM) where virtual paths and cells are employed. In a voice call, for example, quality problems arise and echo cancellers may be required if there is a delay of more than 20-30 ms. Not just in telephony, but also in video conferencing and other bidirectional services in general, it is preferable for delay to be short. From this point of view, STM is a transport mode which is well suited to digital telephone networks. However, a conventional STM-based circuit-switched network requires that data are transferred via an STM connection set up in advance from a source user terminal to a destination user terminal. Its applicability is therefore limited and such circuit-switched networks are only being considered for leased data circuits between large businesses.
On the other hand, in data communications based on an IP architecture; by writing the IP address, which is the identification number of a terminal, in the header of a packet and sending this to a network such as the Internet which supports IP, each router successively transfers the datagram without having to search for the destination corresponding to the address in question and without having to set up a connection between the two terminals which want to communicate. Thus no dedicated connection is required and data can be transferred by means of an extremely simple procedure. However, under this scheme delay is longer than in an STM-based circuit-switched network, and some overhead is necessary.
The present inventors have invented, and filed Application Ser. No. 09/205,612, a novel circuit-switched network which combines the advantages of an STM-based circuit-switched network with the advantages of data communications based on IP addresses. In the circuit-switched network disclosed in this prior patent application, burst data—i.e., a packet—which has been given a logical address such as an IP address, has its logical address analyzed within an STM network and is transferred through the network to the desired destination by means of the destination node E.164 address corresponding to the logical address of the packet's destination.
SUMMARY OF THE INVENTION
The present invention provides a novel circuit-switched network capable of transferring packets via an STM network using a different approach from that disclosed in the patent application outlined above.
A unique feature of the present invention is that it adds a further header to a packet which already has a header in which an IP address has been written, and thereby transfers the packet to the desired destination terminal via an STM network. The additional header contains routing information relating to the route up to the local switch serving the terminal constituting the destination indicated by the IP address, this information having been set in advance by the local switch serving the source terminal. A further feature of this invention is that, during this process, the transit switches relocate an arrived packet from one time slot to a different time slot on the basis of the routing information written in the packet header. It follows that dedicated connections are not set up in advance in the STM network. Instead, each time a switch receives a packet it sets up the time slot in which the packet will be carried at that point in time. As a result, high bit rate data transfer can be carried out as simply and easily as in data communications based on the IP architecture mentioned above, with the entire network acting as if it were an enormous router.
Namely, the present invention provides a circuit-switched network comprising a plurality of local switches each serving at least one terminal, and at least one transit switch connecting these local switches, these local switches and the at least one transit switch being connected via an STM network, wherein each local switch comprises: means for receiving a packet arriving from a terminal, said packet having a header in which an IP address has been written; a table in which is recorded routing information indicating which route should be used for transfer through the STM network to this IP address; means for adding to the packet a header in which is written the routing information corresponding to this IP address, said routing information having been obtained by looking up the table; and means which looks at this added header and transfers the packet in a time slot corresponding to said routing information.
The aforesaid at least one transit switch preferably comprises: means for looking at the header of the packet written in a time slot, the aforesaid routing information having been written in this header; and means for relocating the packet, in accordance with the result obtained by this look-up means, in a time slot corresponding to the routing information written in the header. This routing information is preferably time slot switching information indicating in which time slot the packet is to be written at each transit switch. The time slot switching information is thus information indicating the order of the time slot relocations required when a packet is transferred through the STM network, these relocations being carried out at each transit switch and at the local switch serving the destination terminal.
Preferably, each local switch and transit switch is also provided with means which, if the aforementioned time slot is busy, temporarily stores the packet until the time slot in question becomes idle. This ensures that a packet is not discarded when there is no idle time slot.
The temporary storage means can include means for investigating, at fixed time intervals, whether a time slot is idle or not. It can also include means which, if a time slot is not idle, makes a reservation so that the packet will be preferentially placed in that time slot when the slot becomes idle.
Temporary storage means can be provided for each input line. This avoids the situation where there is no idle time slot on the line used to transfer packets from the input lines to the temporary storage means. It also prevents a high volume of traffic from a particular input line to the temporary storage means hindering packet transfer from another input line to the temporary storage means.
A transit switch can include: means which obtains, from the routing information written in the header of data which has arrived in a time slot, the number of the output line to which this data should be forwarded; means for searching for an idle time slot among the time slots which can be used for an output line with this number; and means which relocates the arrived data in the idle time slot found by this search means.
In this case the search means can comprise means which uses the result of computing a hash function (which treats the output line number as a variable x) as the time slot number corresponding to that output line number. Alternatively, the time slot numbers can run in numerical order, and the output line numbers can be set in correspondence with these time slot numbers. The output line numbers as a whole run in numerical order, but each output line number is repeated the same number of times as the number of time slots allocated f

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