Multiplex communications – Pathfinding or routing – Combined circuit switching and packet switching
Reexamination Certificate
1999-09-02
2003-06-24
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Pathfinding or routing
Combined circuit switching and packet switching
C370S401000
Reexamination Certificate
active
06584098
ABSTRACT:
This invention relates to telecommunications systems, and in particular to telecommunications systems capable of carrying both voice and data.
Telecommunications systems have been developed for carrying many different types of traffic. For the purposes of the present invention, these can be grouped into two different basic types of telephony system, known as “circuit-switched” and “packet-switched”.
In a circuit-switched system, a connection between source and destination is established at the beginning of a call, and reserved for the exclusive use of that call, for the duration of the call. The reserved resources may be a complete physical telephone line, but for most parts of the system it is likely to be a timeslot in a time division multiplex system and/or an allocated part of the spectrum in a (radio) frequency-division, or (optical) wavelength-division, multiplex.
In a packet-switched system, data to be transmitted from one point to another is formed into short elements (known as packets) which are each handled separately, and routed according to the availability of network resources at the time of the transmission of the individual packet. This allows a large number of individual data messages to be sent simultaneously over any particular leg of the network, by interleaving packets of different calls over that leg. It is also possible to route different parts of the data (i.e. different packets) by different parts of the network, if there is insufficient capacity on any one route for the entire message. Each data packet carries an individual signalling overhead indicating the destination of the packet, so that at each node in the network the packet can be routed towards its ultimate destination. It also carries a sequence number, to identify its position within the complete message, so that the receiving party can re-assemble the packets in the correct order at the receiving end, and can identify whether any packets have failed to arrive.
Various transaction protocols exist, such as “TCP/IP” (Transport Control Protocol/Internet Protocol), illustrated in
FIG. 11
, which shows the headers to be found in an individual packet. The initial Internet Protocol (IP) Header
110
(typically 20 bytes) defines the destination, the source, and information such as the transmission protocol to be used. There follows further header information
111
according to the indicated transmission protocol, which in this case is “TCP” (Transmission Control Protocol). This comprises a further 20 bytes, which includes information indicating which file transfer protocol is to be used—for example SMTP (Small Message Transfer Protocol), FTP (File Transfer Protocol) or HTTP (HyperText Transfer Protocol). Further header information
112
follows, specific to the indicated protocol. The remainder of the packet comprises the information to be conveyed, known as the “payload”
115
.
It is known, for example from International Patent Application no. WO95/31060, and U.S. Pat. No. 5,729,544, to select a circuit-switched or packet-switched routing for a packetised message, according to the message transfer protocol indicated in the TCP header
111
. This allows short messages using the “SMTP” protocol to be packet-switched, whereas lengthy messages such as large computer files using the “FTP” protocol can be sent over a circuits-switched route. The greater amount of processing required to set up a circuit-switched link, as opposed to that required to transmit individual packets, is offset by the fact that the processing for a circuit-switched link only has to be done once.
However, this arrangement takes no account of the information content of the data to be transmitted. Certain types of information content are inherently more suitable for circuit-switching, and others are more suited to packet-switching. In particular, these information can be grouped into two principal classes, referred to herein as “delay-intolerant” traffic, and “corruption-intolerant” traffic.
Traditional voice telephony is “delay-intolerant”. This class also includes such types of traffic as live video links etc. For such calls it is important that the time taken for the traffic to travel from source to destination remains constant, and as short as possible. This requirement is more important than the completeness of the data. For example, in a digitised voice signal there is, from the listener's point of view, considerable redundancy in the signal, so the loss of some digital information in the voice signal can be tolerated whilst still providing an acceptable signal quality at the receiving end. However, a delay in transmission, particularly if it is not constant, can be very distracting and make conversation difficult.
In contrast, digital data signals representing text, numerical data, graphics, etc. can be transmitted with considerable variation in the length of time different parts of the data take to get from the source to the destination. In some cases different parts of the signals may be delayed by such differing amounts that the data may not arrive in the same order that it was transmitted, but the original data can be reconstructed if the order in which it is transmitted can be determined. This is achieved by labelling each packet with a position label, indicating its position in the sequence. In such transmissions the completeness of the data is more important than the time it takes to get to its destination, so it is referred in this specification to as “corruption-intolerant”.
Corruption-intolerant data are preferably carried by means of a packet-switching system. The system transmits each packet as a self-contained entity and reliability of transmission takes priority over speed, so the loss of an individual packet is unlikely. If such a loss does occur, it can be identified by a gap in the sequence of position labels, and its retransmission can be requested.
However, packet-switching is inappropriate for delay-intolerant call traffic. This is firstly because there is no certainty that each packet will take the same route and therefore take the same amount of time. Furthermore, such traffic tends to be of a more continuous nature, ill suited to the intermittent nature of a packet-switched system. The division of the call into packets, (requiring each packet to have its own addressing overhead), adds a significant data overhead to the call. This also adds to the amount of processing overhead that is required to route each packet through the system. For such types of call traffic the point-to-point “circuit-switched” system of conventional telephony is more appropriate, because in such a system resources are reserved end-to-end throughout the duration of the call.
A circuit-switched system cannot offer efficient connectionless packet-switched transmission. Likewise it is difficult for packet-based systems to support delay-intolerant applications with the same quality of service as traditional circuit-switched telephony systems provide. From a network operator's point of view it is more efficient to route corruption-intolerant (delay-tolerant) calls by way of a packet-switching system and delay-intolerant calls by way of a circuit-switching system. However, an individual user may wish to use one terminal connection for both types of transmission. The prior art system already discussed only distinguishes between protocols generally used for large file sizes (e.g. HTTP and FTP), and those for small files (SMTP). These do not relate to the information content of those files. In particular, it is possible to generate a voice signal or other delay-intolerant bit stream on, for example, a computer, and transmit it as a data stream by way of a data terminal. A particular example is the use of the “Internet” for carrying voice and video messages. If the communications system handles such a call as a conventional data call, the voice or picture quality perceived at the remote end can suffer from having been packet-switched rather than circuit-switched. Conversely, handling data over a circuit-switched system is both in
British Telecommunications Public Limited Company
Yao Kwang Bin
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