WO2001073991A1

WO2001073991A1 - Method and apparatus for switching connections in fixed networks using the cdma-technique

Info

Publication number: WO2001073991A1
Application number: PCT/FI2001/000303
Authority: WO
Inventors: Lassi HIPPELÄINEN
Original assignee: Nokia Networks Oy
Current assignee: Nokia Oyj
Priority date: 2000-03-29
Filing date: 2001-03-29
Publication date: 2001-10-04
Anticipated expiration: 2002-09-29
Also published as: FI20000723A0; FI20000723A7; AU4840501A; FI20000723L

Abstract

A method, a switching element and a bridge element are disclosed for switching connections in a fixed network. Data related to a certain connection is received from a first fixed transmission medium (101) and transmitted to a second fixed transmission medium (104). There is established a certain code (803), and the data received from the first fixed transmission medium is coded (111, 805) using said code and a certain code division multiple access method so that coded data results. The coded data is transmitted (111, 211) to a shared medium (110), which connects the first fixed medium and the second fixed medium. The signal in the shared medium is decoded (114, 805) using said code and the resulting decoded data is transmitted (114) to the second fixed transmission medium.

Description

METHOD AND APPARATUS FOR SWITCHING CONNECTIONS IN FIXED NETWORKS USING THE CDMA-TECHNIQUE

The invention relates in general to switching connections. In particular the invention relates to switching data streams whose bandwidth changes dynamically and that require guaranteed quality of service.

Conventional switches in the telephony networks have been designed to deliver speech in real time. Digitized speech is usually transmitted in a format that requires 64 kbps transmission capacity. The delay the switching causes is constant and small enough for real time applications. The quality of service is guaranteed by reserving a certain 64 kbps transmission line for a certain call in the beginning of the call.

Recently real time applications that require more transmission capability have emerged. For example, video conferencing and video streams are used more and more. The conventional telephony network is not optimal for delivering these services, because more than one 64 kbps transmission line is needed to transmit the data stream. It is possible to reserve many lines for a certain connection, but in the switches each 64 kbps line is processed separately. Knowledge that they all relate to a certain connection cannot be utilized.

Another new application is browsing the WWW, where the required transmission capabilities are very different in the opposite directions. In the downlink direction, towards the user, images and media clips, for example, are transmitted. In the uplink direction the transmitted data is mainly addresses of the WWW-pages. This data requires much less transmission capacity than the data in the downlink direction. In a conventional circuit switched network, it is easiest to handle data streams that require the same transmission capacity to both directions.

In addition to circuit switched networks, real time data, such as video streams, is also transmitted in the packet switched networks, but the problem with packet networks is that the network does not usually guarantee quality of service. The delays of separate consecutive packets may have so much jitter that it affects, fo example, the quality of the video. It is also possible that, in a case of overload, the routers of a packet network do not have time to process all the packets that arrive. Some packets may therefore be lost. This also deteriorates the quality of service.

Code division multiple access (CDMA) methods are currently used in radio access networks as a method to share a common radio frequency band between many connections. In a CDMA method, each connection has a specific code sequence. Using this code sequence the data flow related to a certain connection is spread into a spread signal which uses frequencies of a larger bandwidth than the original narrowband signal. Many spread signals can be transmitted using a shared medium, and the signal in the shared medium resembles white noise. The code sequences of different connections must be chosen so that they do not correlate and the code sequence of a specific connection has to autocorrelate. When processing the seemingly white noise signal transmitted in the shared media with a code sequence that correlates with the code sequence used in coding a certain narrowband signal, the resulting narrowband signal is, in an optimal situation, the same as the original narrowband signal. Possible transmission errors may cause some differences. The receiver has to know the transmitters code sequence and the code sequences must be synchronized. The code sequence is in the following description called the CDMA code. The terms coding and decoding in this description refers to CDMA coding and CDMA decoding.

Recently, switching in optical networks has been suggested to be done using CDMA methods. For example, Optical-CDMA and CodeStream have been developed by Commercial Technologies Corporation. Optical fibers are widely used in the backbone networks, for example as trunk between telephone exchanges, but very seldom the cables connecting a household to a network or the cables within a building are optical fibers. _;

The problem with the prior art switching methods and elements for fixed networks is that it is not possible to handle various dynamically changing bandwidths and simultaneously provide a guaranteed quality of service. Packet switching is insensitive to the user data rates that are transmitted in the connections, but it cannot easily provide guaranteed quality of service. Circuit switching, on the other hand, can offer guaranteed quality of service by reserving a certain transmission capacity for a certain connection, but current switching elements are not designed for switching user data rates higher than 64 kbps. CDMA, which is rather insensitive to user data rates, is applied in the optical networks only. The end-user is very seldom connected directly to an optical network.

The object of the invention is to present a new flexible method for switching connections in traditional fixed networks. A further object is to present a method that guarantees the quality of service and that enables the switching of connections of various bandwidths. Even further objects are to present a method that is scaleable and distributable, that can be used with existing cables and networks and that can be used to construct a complete switching architecture.

These and other objects of the invention are achieved by using a code division multiple access method in switching connections.

The method according to the invention is a method for switching connections, where data related to a certain connection is received from a first fixed transmission medium and transmitted to a second fixed transmission medium, and it is characterized in that

- a certain code is established, • - the data received from the first fixed transmission medium is coded using said code and a certain code division multiple access method resulting in coded data,

- the coded data is transmitted to a shared medium, which connects the first fixed medium and the second fixed medium,

- the signal in the shared medium is decoded using said code and - the resulting decoded data is transmitted to the second fixed transmission medium.

A switching element according to the invention is a switcMng element which comprises

- means for receiving data from a first fixed transmission medium and

- means for transnύtting data to a second fixed transmission medium, and it is characterized in that it further comprises

- a shared medium,

- means for estabhshing a certain code,

- coding means for coding received data using a certain code division multiple access method and said code, - means for transmitting coded data to the shared medium,

- means for receiving the signal in the shared medium and

- decoding means for decoding the signal in the shared medium using said code.

The invention further relates to a bridge element, which has means for receiving data from a first shared medium and means for transr tting data to a second shared medium and which is characterized in that it further comprises

- means for decoding the signal received from the first shared medium using at least one code division multiple access code,

- means for coding the decoded data using at least one code division multiple access code resulting in the data transmitted to the second shared medium, and - a transmission medium connecting the means for decoding and means for coding. In a method according to the invention, the data received from a certain medium, for example from a cable, is coded using a connection-specific code and a code division multiple access (CDMA) method. The coded data related to all mcoming connections is transmitted to a shared medium. When the CDMA code that was used to code data related to a specific connection is used to process the signal in the shared medium, ihe connection-specific data can be separated from the seemingly white noise signal in the shared medium.

Data related to a certain connection is transmitted towards a switching element using input medium and further from the switching element using output medium. In a method according to the invention, all the input media and output media, for example cables, are connected via encoders and decoders to a same shared medium. Because all the input and output media are connected to each other by the same shared medium, data from any input medium can be transmitted to any output medium. In the method according to the invention, the shared medium thus acts as a switch. For each connection that at a certain time passes a switching element, according to the invention a connection-specific CDMA code has to be established.

In CDMA methods the bits of the CDMA code are called chips in order to separate the chips of the CDMA code from the user data bits. The chip rate in a system according to the invention is a design parameter that sets an upper limit for the user data rate of the connections transmitted via said system. Typically the encoders transmit coded data to the shared medium with a certain transmission rate. If the user data rate in a certain connection is smaller than the transmission rate, an encoder can transmit coded data in bursts and spend some time in an idle state between the bursts.

For the switching method to work properly, the combination of the shared medium and the coders and decoders in a switching element according to the invention is preferably linear. This sets requirements to the output of the coders, to the shared media and to the input of the decoders. The combined amplitude of the signals,; which the coders produce, should not exceed the maximum input of the decoders. In addition, the shared medium should not distort the inserted signal, for example by causing harmonic distortion.

The switching method and element according to the invention comprise at least the following advantages. The connections that are switched need not to have the same transmission capacity, as in conventional circuit switching elements. The switching method and element may process connections with varying bandwidths, so the bandwidth of the connections may be adjusted dynamically and continuously.

The switching method and element according to the invention support both guaranteed throughput and statistical multiplexing. As long as the coded data transmitted by the encoders does not exceed the capacity of the shared medium| there is typically no need for flow control. As the amount of transmitted coded data increases, the transmission rate of the encoders usually has to be limited. Typically this done using queuing algorithms, such a Fair Queuing. Many queuing algorithms are based on tokens: the transmission of a certain amount of data costs a token to the encoder, and when the encoder runs out of tokens, it can no longer transmit data before it receives new tokens. New tokens are allocated to the encoders according to a queuing algorithm, for example, and the total number of tokens allocated at a time may either correspond to the maximum capacity of the shared medium (guaranteed throughput) or it may exceed the maximum capacity of the shared medium (statistical multiplexing). In statistical multiplexing one trusts that the probability of all encoders tjansnήtting at the same time is small. The estimated probability of simultaneous transmission affects the total number of tokens.

The switching element can support dynamic bandwidth allocations. The queuing algorithm can issue tokens to specific paths per request, as long as the limiting condition (guaranteed throughput or statistical multiplexing) is not exceeded. The_{ token allocation rate is negotiated using a resource reservation protocol, such as RSVP.

Switching delay in a switching method and element according to the invention is predictable and constant. The delay is caused by the coding, decoding and the delay of the medium that is used in the switching element. Constant delay is an advantage when data is transmitted for real time applications: jitter in the delay, as for example in packet switched networks, may cause, for example, a video stream to flow non- uniformly. The switching method and element according to the invention may thus offer quality of service, unlike current packet switched networks.

In the shared medium there is data from all the connected media. Because the data is coded with a code specific to each connection, just by detecting the signal in the shared medium it is not possible to reveal transmitted data, as it is in, for example, certain local area networks. An eventual eavesdropper needs to know the code) which was used in coding the data, and have access to decoding equipment. This enhances the information security of the shared media. The switching element according to the invention can be scaled by adding more coders and decoders to it. In conventional routers that switch data packets it is more difficult to increase the routing capacity. The limit for increasing capacity of a switching element according to the invention is the noise tolerance in decoding. In CDMA methods, the signals for other receivers are seen as noise or interference. If the noise level is high, decoding of the signal may introduce errors to the transmitted data. The system can also be distributed: the switching medium may extend, for example, from one building to another. Here the limiting factor is keeping the power level correct in spite of losses in the shared medium. ₍

\ The switching element and method according to the invention may employ a synchronous CDMA technique, where the chip clocks of the transmitters in a system are synchronized, or an asynchronous CDMA technique, where the chip clocks of the transmitters may drift. The throughput of a system employing asynchronous CMDA technique is typically somewhat smaller than that of a system employing synchronous CMDA technique, but in an asynchronous system there is no need to keep track of the chip clocks.

Similarly as in packet switched networks, it is possible to bypass a faulty switch, if there is an alternative route in the network. This can be done without breaking the connection, although finding and activating an alternative route may cause some extra delay. A further advantage of the switching method and element according to the invention is that they can be used to replace conventional circuit switching elements. In addition, the radio access networks of the third generation cellular networks employ CDMA techniques, so the hardware for coding and decoding most probably becomes more efficient and less expensive as the 1hird generation cellular networks become more widespread.

The invention will now be described more in detail with reference to the preferred embodiments by the way of example and to the accompanying drawings where

Figure 1 shows a schematic drawing of a switching element according to a first preferred embodiment of the invention,

Figure 2 shows a schematic drawing of a switching element according to a third preferred embodiment of the invention,

Figure 3 shows a schematic drawing of a bridge element according to a fourth preferred embodiment of the invention, Figure 4 shows a schematic drawing of network consisting of switching elements according to the invention,

Figure 5 shows a schematic drawing of a parallel switching element according to a fifth preferred embodiment of the invention,

Figure 6 shows schematic drawings of multipath switching elements according to a sixth preferred embodiment of the invention,

Figure 7 shows a schematic drawing of a multicasting method according to the invention, and

Figure 8 shows a flowchart of a switching method according to the invention.

The same reference numerals are used for corresponding parts in the figures.

When describing the invention in detail, the input and output transmission media are here treated as unidirectional, for the sake of clarity.

Figure 1 shows a schematic drawing of a switching element 100 according to a first preferred embodiment of the invention. In the situation presented in Figure 1, certain transmission media 101, 102 103 and 104 are connected to the switching element 100. The transmission media can be, for example, twisted pair cable.

The switching element 100 comprises a shared medium 110 that is linear. This shared medium can be, for example, a line inside an integrated circuit, a trace on a printed circuit board or a coaxial cable, depending on the application. The trans- mission media are connected to the shared medium 110 through devices 111-114 that are responsible for the CDMA functionality and transnήtting and receiving signal to and from the shared medium. The devices comprise coders, which perform CDMA coding, and transmitter for transnήτting the coded data to the shared medium 110. Further, they comprise receivers for receiving the signal in the shared medium 110 and decoders for decoding the signal using a certain CMDA code. The coding/- decoding functionality can also be implemented in a separate device than the transmission of signals using the shared medium 100.

In a switching device according to the first preferred embodiment each decoder has its own CDMA code and each decoder can decode the signal in the shared medium using this one CDMA code. Correspondingly, at most one connection at a time is carried in each output medium 103, 104 in a switching device according to the first preferred embodiment of the invention. The connection 120, for example, which is established between the input medium 101 and the output medium 104, prevents the connection 121 to be established simultaneously, unless the connections are, for example, multiplexed in time or packet data connections, where each data packet has a header indicating the connection to which it belongs. There is need to prevent the coder/transmitters related to connections using a same receiver from transnritting information at the same time. If they both use the same CDMA code of the receiver, the coded data related to the two connections is summed in the shared medium.

For example, in the connection 120 in Figure 1, the CDMA code specified for the receiver/decoder device 114 is taken into use in the coder in the coder/transmitter device 111. The receiver/decoder device 113 uses a different code than the receiver/decoder device 114 to decode the signal in the shared medium, and therefore it sees the signal intended for the receiver/decoder device 114 as noise or interference. Only those receiver/decoder devices that use the same code as a certain coder/transmitter device can decode the data flow.

It is typical to CDMA arrangements that exactly the same code functions both as the encoding code and the decoding code. However, in order to preserve generality one must note that there are coding arrangements where the encoding and decoding codes are different and constitute a code pair (or key pair), so that a signal encoded with the encoding code of a code pair can only be decoded with the decoding code of the same code pair. The general expression "code" can therefore be understood to cover both the single code which is used both for encoding and decoding, and a code pair where an encoding code is used for encoding and the corresponding decoding code is used for decoding. ;

When the CDMA codes are determined based on the output transmission medium or, in other words, based on the destination of the connection, there should be a way to control that only one transmitter at a time sends data to a certain receiver. Otherwise the data flows that are coded using the same code are summed in the shared medium, and the receiver/decoder device receives the summation data flow. A specific logical entity may control that only one coder/transmitter device at a time is transmitting data to a certain receiver/decode device. The same entity may also be responsible for selecting the correct CDMA code as a response to a routing request of a certain coder/transmitter device.

In a switching element according to a second preferred embodiment of the invention, the coders and decoders can simultaneously code/decode data using more than one CDMA codes. It is therefore possible that each output/input medium carries several connections. A distinct CDMA code is selected for each connection or for data packets related to a certain connection. A switching device according to the first preferred embodiment of the invention may act, for example, as a conventional telephone switch, and a switching device according to the second preferred embodiment of the invention may act, for example, as a packet data router.

Figure 2 presents a switching element 200 according to a third preferred embodiment of the invention. In Figure 2, the transmission media 201a, 201b and 201c are connected via coder/transmitter/receiver/decoder devices 211a, 211b and 211c to a first shared medium 110a. The devices 211 are typically located, for example, in one building. Similarly, transmission media 202a, 202b and 202c are connected via coder/transmitter/receiver/decoder devices 212a, 212b and 212c to a second shared medium 110b.

Arrow 220 presents a connection established via devices 211b and 211c that are connected to the same shared medium. Because the devices 211 connected to shared medium 101a are located near each other, for example in a same building, the losses in the shared medium are negligible. Each device 211 typically transmits coded data to the shared medium with a similar amplitude, and at the receivers the amplitudes of the transmitted signals are about the same. The CDMA method works well in this case.

Arrow 221 presents a connection established via devices 211a and 212a. These devices are, for example, not in a same building, and therefore, if the shared media 110a and 110b were directly connected to each other, the signal transmitted, for example, by device 212a could be weaker than the signal transmitted by devices 211b or 211c. This could cause transmission errors. In the switching element 200 according to the third preferred embodiment of the invention, the shared media 110a and 110b are connected to each other using an amplifier 213. This way the signal in the shared medium 110b is amplified properly before it is transmitted to the shared medium 110a, and vice versa.

Preferably the amplifier 213 passes through only that part of the signal in the shared meciium 110a; 110b that is intended to travel to the other shared medium. This can be achieved, for example, by implementing a band pass filter to the amplifier. If the destination transmission medium of a connection or destination of a data packet is connected to the same shared medium than the input transmission media, the devices 211b and 212c, for example, use a certain first frequency band when transnήtτmg/receiving coded data. Similarly, if the destination transmission medium of a connection or of a data packet is connected to the other shared medium, the transmitting device, for example 211a, uses a certain second frequency band, which is used in the second shared medium 101b. The amplifier 213 passes to the second shared medium 110b only the signal in the second frequency band and to the first shared medium 110a the signal in the first frequency band. Furthermore, the receivers in ihe devices 211 and 212 may also use a band pass filter. This way the signals, which are intended to the receiving devices connected to the other shared medium and transmitted by the devices connected to the same shared medium, are not seen as interference.

Figure 3 presents a way to combine two switching elements according to the invention. In Figure 3 there are two switching elements according to the first preferred embodiment of the invention. The components of one of the switching elements are labeled with letter a, and the ones of the other with letter b. Each switching element in Figure 3 comprises the shared medium 110 and transmission media 101 and 102. Each of these transmission media is connected to the share^ medium using a coder/transmitter/receiver/decoder device 111, 112. If the devices 111, 112 can code/decode using one CDMA code, the switching elements 110a and 110b in Figure 3 may connect, for example, telephones in a certain building. If the transmission media 111; 112 are connected to the shared medium 110 with devices that can use simultaneously several CDMA codes, then the switching elements may function, for example, as local area network segments to which terminals are connected. Connections between terminals connected to the same segment may be established, for example, using a method according to the fourth preferred embodiment of the invention.

In Figure 3 the two switching elements according to the first or second preferred embodiment of the invention are connected to each other with a bridge element 300 according to a fourth preferred embodiment of the invention. This bridge element comprises two coder/transmitter/receiver/decoder devices 311, 312, which are connected to each other with a transmission medium 302. The coder/transmitter/, receiver/decoder devices 311, 312 belonging to the bridge element 300 according to the fourth preferred embodiment of the invention can perform coding/decodhig using several CDMA codes simultaneously.

When a connection is to be established between two endpoints that are not connected to the same shared medium, then the bridge element 300 is involved in switching. If, for example, a connection from a terrninal reachable through transmission medium 101a is to be established to a terminal reachable through transmission medium 102b, then a common CDMA code is arranged between the coder/transmitter device Ilia and the receiver/decoder device 311 and between the coder/transmitter device 312 and the receiver/decoder device 112b. The two CDMA codes need not be equal, they can be selected independently. i

In the switching element 200 according to the Ihird preferred embodiment of the invention the filter/amplifier 213 separates the two shared media 110a and 110b. The separation is preferably based on the frequency band using which coded data is transmitted within the switching element. When two shared media 110a and 110b are connected to each other with a bridge element 300 according to the fourth preferred embodiment of the invention, it is possible to use the same frequencies in both shared media.

Figure 4 presents a network 400 of switching elements according to the second preferred embodiment of the invention. The switching elements 100a, 100b and 100c, are connected to each other with transmission media. At the edges of the network 400 there are ingress/egress switching elements 401 and 402 according to the invention. Let us consider, for example, a packet data connection, which is to be established via the network 400. Typically each data packet contains a header stating, for example, the destination network address. In conventional packet data networks, each router decides independently to which router it sends a certain datii packet.

In a network 400 it is advisable to set up a path across the network for each connection. The path may pass, for example, the switching elements 100b and 100c. The ingress switching element 401 may hand out a label for a connection and set up a path for the connection. A specific protocol may be used in setting up the path. The path setup typically involves a definition of a CDMA code corresponding to the label in each switching element 100, which belongs to the network 400 and via which the path passes. The CDMA codes may be selected independently in each switching element. Each switching element along the path decodes the signal in the shared medium of the switching element using the CDMA code specific for the connection until the connection is torn down.

Each data packet belonging to a connection may be given, typically in the ingress switching element, a label identifying the connection. The switching elements 100" within the network 400 use only the label when processing the data packets. The; label typically dictates the CDMA code, which is used in each switching element, when coding the data packets. After setting up the path and setting proper CDMA codes to suitable coders/decoders within the switching elements 100, the data packets can be switched fast over the network 400.

The network 400 may be, for example, a network carrying IP (Internet protocol) data packets or it may be an ATM (Asynchronous Transfer Mode) network. An example of a method for label-based switching is Multi-Protocol Label Switching (MPLS), using which it is possible to hand out labels to, for example, IP packets or ATM cells and implement switching on the second protocol layer.

A method corresponding to Figure 4 is thus such that an identifier (or label) identifying a connection is established and the identifier is added to each data packet related to a connection. For a connection, a path through a network is established and each switching element along the path is notified of the identifier. In each switching element along the path for the connection, a code for coding and decoding is established and the signal in the shared medium of each switching element is decoded using the established code in said switching element until the connection is torn down.

The code, using which a data packet is coded in a switching element, is selected for a data packet based on the identifier in the data packet.

Figure 5 shows a way to increase the capacity of a switching element according to the invention by presenting a parallel switching element according to a fifth preferred embodiment of the invention. Figure 5 shows how transmission media 101 and 104 may be connected to each other using multiplexer/demultiplexers 501a; 501b and several coder/transmitter/receiver/decoder devices. The multiplexer/demultiplexer 501a, for example, divides the data to several coder/transmitter devices, which each transmit the coded data further using a distinct shared medium. Figure shows in a way of example eight coder/transmitter/receiver/decoder devices 111, 112 which are connected pairwise to four shared media 110a, 110b, 110c and HOd. The coded data is decoded by a certain number of decoders and the decoded data is combined to a single data flow in the multiplexer/demultiplexer 501b.

In the parallel switching element presented in Figure 5, all the coder/transmitter and receiver/decoder device pairs may use a same CDMA code simultaneously. This is because each device pair uses separate shared medium. If the coding and decoding device are capable of using several CDMA codes simultaneously and code/decode information related to several data flows simultaneously, the connection-specific CDMA codes may also be the same for each coding/decoding device pair.

It is also possible to use only one shared medium and still divide the mcoming data flow to several coder/transmitter devices and construct the data flow from the output of many receiver/decoder devices. In this case, each device pair related to a certain connection has to use different CDMA codes. Using more coding and decoding devices and a single shared medium, the capacity of the switching element is limited by the signal-to-interference ratio of CDMA decoding. The use of more than one shared media within a switching element allows the increase of capacity even further.

It is possible to increase the rehability and robustness of a switching element according to the invention. This can be done, for example, by connecting each transmitter and/or receiver to the shared medium more than once. An example is presented in Figure 6A, where a transmission medium 101 is connected to one coder/transmitter device 111. This coder/transmitter is connected to the shared medium 110 in three points. The receiver/decoder device 112 is in Figure 6A connected to the shared medium 110 only at one point, but it could be also connected to the shared medium in several points. The coded data arrives to the receiver/decoder along many paths and a RAKE receiver, which can receive multipath signals arriving along many paths and having different delay properties^ can combine the components in the decoding process. It is also possible to connect a transmission medium to many coder/transmitter devices, and each of these uses the same CDMA code. If one of the connection points between the coder/transmitter 111 and shared medium 110 or, if there are many coders, one of the coder/transmitters breaks, the coded data can still be received.

Also the shared medium can be distributed to increase rehability and robustness. For example, in Figure 6B the three shared media 110a, 110b, and 110c can be in different buildings, bridged with filter/amplifiers 213 a and 213b, as described in the fourth preferred embodiment of the invention. The coder/transmitter device 111 transmits the same signal to all three shared media. (For clarity's sake Figure 6B shows only one coder/transmitter.) The receiver/decoder device 112 is a RAKE receiver that combines the signals even though their different paths have different delays. In this example losing the direct connection 103, or either of the inter-, mediate shared media 110a and 110b, or either of the connecting amplifiers 213a and 213b, will not interrupt the operation, because two thirds of the signal still remains. The loss of a path may be due to a malfunction, an external incident, or a planned maintenance break.

The redundancy can easily be generalized to more than three paths. The same switching element may also provide different levels of robustness for different services, so that only a subset will use the indirect paths. Planning the robustness will be easier, if each of the intermediate shared media 110a, 110b, and 110c have been allocated individual frequency bands.

Figure 7 presents how multicasting can be done when switching elements according to the invention are used. Multicast functionality is described here only on a general level, more detailed information about multicast transmissions can be found, for example, in Internet RFC's (Request For Comments) 1112 and 2236. ^'>

In Figure 7, four switching elements according to the invention and terminals connected to the switching elements construct a network. The switching elements are connected with bridge elements 300a, 300b and 300c. A server 701 is connected to the shared medium 110a with input medium and the coder/transmitter device Ilia. Terminal 702 is connected to the same shared medium with the receiver/- decoder device 113 a. Similarly, the terminal 703 is connected to the shared medium 110c with the receiver/decoder device 113c, and the terminal 704 is connected to the shared medium HOd with the receiver/decoder device 113d. The coder/trans- mitter and receiver/decoder devices connecting the server and terminal to the shared medium in Figure 7 can be devices that use one CDMA code at a time, but they may also be devices capable of processing data using several CDMA codes simultaneously.

The shared media 110a, 110b, 110c and HOd are connected to each other, as shown in Figure 7, using bridge elements according to the fourth preferred embodiment of the invention. The coder/transmitter/receiver/decoder devices 311a and 313a construct one bridge element 300a with the transmission medium connecting them. Similarly, coder/transmitter/receiver/decoder devices 311b and 313b construct a second bridge element 300b, and coder/transmitter/receiver/decoder devices 311c and 313 c construct a third bridge element 300c.

In multicast, a sender may send a data flow that is received by many receivers. The data flow sent by the server 701 and coded by the coder/transmitter device Ilia may be received by the terminal 702, if the receiver/decoder device 113 a uses the same CDMA code as the coder/transmitter device Ilia when decoding the data flow. The terminal 703 can receive the same data flow, if it is transmitted over the bridge elements 300a and 300b. When the terminal 704 requests to join the multicast group, the receiver/decoder device 311c of the bridge element 300c can be adjusted to use the same CDMA code as the coder/transmitter device 313 a and receiver/decoder device 311b are already using when processing the data flow. Thereafter the coder/decoder device 313c and the receiver/decoder device 133d are configured to use a common CDMA code, and the terminal 704 can receive the multicast transmission.

If a terminal connected to a shared medium, where the multicast transmission is already present in a coded form, wishes to begin listening to the transmission, only the receiver/decoder device through which the terminal is connected to the shared medium has to be configured to use the same CDMA code as is used in coding the multicast transmission. In Figure 7, in the shared media 110a, 110b, 110c and HOd the data flows related to the multicast need not be coded using the same CDMN code. The CDMA code may be chosen independently in each network segment. Hie CDMA codes may also be the same in each shared medium, and in this case the CDMA codes may be multicast group specific.

Figure 8 presents a flowchart of a switching method 800 according to the invention. In step 801, a new connection between two endpoints is to be established through a switching element according to the invention. In step 802 the correct output medium and the related receiver/decoder device are selected. This is done, for example, using a routing table during the set up of a connection through a switching element according to the invention or during the set up of a path through a network consisting of switching elements accordmg to the invention.

If the transmission media connected to a switching element according to the invention carry only one connection at a time, the CDMA code used for a certain connection can be determined using, for example, the identity of the receiver/- decoder device that is connecting the selected output medium to the switching element. The single connection may be a multiplexed connection that carries data related to several applications. It is also possible that the CDMA code is connection specific.

In step 803 the CDMA code using which the data is coded before it is transmitted to the shared medium is determined. The CDMA code may be fetched, for example, from a routing table where a CDMA code for each receiver/decoder and output medium may be specified. In step 804 the CDMA code is taken into use in the coder/transmitter device that connects the input medium to the shared medium.

hi step 805 the coder uses the determined CDMA code when coding the data related to the connection in question. The selected decoder also uses the same CDMA code,

In the switching method 800 in step 805 the coder/transmitter device codes the mcoming data with a CDMA code and transmits the coded data to the shared medium. The receiver/decoder device decodes the signal it detects in the shared medium using the selected CDMA code. The decoded data, which in the absence of transmission errors and decoding errors is equal to the mcoming data flow, is transmitted further to the output transmission medium.

The invention does not restrict the selection of the code division multiple access method that can be used in connection with the invention. Further, the CDMA method may be synchronous or asynchronous.

The user data that is transmitted in the connections that are switched according to the invention may be packet based, for example data packed in Internet Protocol packets, or it may be, for example, digitized voice or speech flow which is presented in a format according to the telephony network standards. The invention, does not restrict either the protocols using which data is transmitted in the input and output media.

In the figures each input medium is connected to the shared medium using a separate coder/transmitter device. It is also possible to connect many input media to a single coder/transmitter device. The decoding may also be done in one device, and thereafter each decoded data flow is transmitted to the proper output medium. In these cases, each of the input and output media may also carry either one connection at a time or many simultaneous connections.

Claims

1. A method for switching connections in a fixed network, where data related to a certain connection is received from a first fixed transmission medium (101) and transmitted to a second fixed transmission medium (104), characterized in that - a certain code is established (803),

- the data received from the first fixed transmission medium is coded (111, 805) using said code and a certain code division multiple access method resulting in coded data,

- the coded data is transmitted (111, 211) to a shared medium (110), which connects the first fixed medium and the second fixed medium, f

- the signal in the shared medium is decoded (114, 805) using said code and

- the resulting decoded data is transmitted (114) to the second fixed transmission medium.

2. A method according to claim 1, characterized in that the code is determined based on the second fixed transmission medium.

3. A method according to claim 1, characterized in that the step of establishing a certain code comprises the substep of establishing a connection-specific code for the data related to a certain connection when it is allowable to transmit data related to more than one connection at a time in at least one of the first fixed transmission medium and the second fixed transmission medium.

4. A method according to claim 3, characterized in that the decoded data is transmitted to the second fixed transmission medium and a third fixed transmissioii medium. -

5. A method according to claim 1, characterized in that - the data received from the first fixed transmission medium is divided into more than one data flows,

- each data flow is coded using a different code,

- each coded data flow is transmitted to said shared medium,

- the signal in the shared medium is decoded using said different codes, - the decoded data is combined to a single data flow which is transmitted to the second fixed transmission medium.

6. A method according to claim 1, characterized in that

- the first fixed transmission medium and the second fixed transmission medium are connected using more than one shared media (110a, 110b, 110c, HOd), - the shared media are distinct from each other,

- the data received from the first fixed transmission medium is divided (501) into more than one data flows,

- each data flow is coded (Ilia, 111b, 111c, Hid) using the same code, - at most one of the coded data flow is transmitted through each shared media,

- the signal in each shared media is decoded (112a, 112b, 112c, 112d) using the same code, and

- the decoded data is combined (502) to a single data flow which is transmitted to the second fixed transmission medium.

7. A method according to claim 1, characterized in that

- an identifier identifying a connection is established,

- the identifier is added to each data packet related to a connection,

- for a connection a path through a network is established and each switching element along the path is notified of the identifier, ^? - a code for coding and decoding is established in each switching element along the path for the connection,

- the signal in the shared medium of each switching element is decoded using the established code in said switching element until the connection is torn down, and

- a code, using which a data packet is coded in a switching element, is selected for a data packet based on the identifier in the data packet.

8. A method according to claim 1, characterized in that

- a code is established for a multicast connection and

- in each switching element the data related to the multicast connection is coded and decoded using the same code.

9. A switching element (100), which comprises

- means for receiving data from a first fixed transmission medium (101) and - means for txansrmtting data to a second fixed transmission medium (104), characterized in that it further comprises >

- a shared medium (110), - means for estabnshing a certain code,

- coding means for coding received data (111) using a certain code division multiple access method and said code,

- means for transmitting coded data to the shared medium,

- means for receiving the signal in the shared medium and - decoding means for decoding (114) the signal in the shared medium using said code.

10. A switching element according to claim 9, characterized in that at least one of the means for coding data comprises means for coding data related to more than one connections using a different code for each connection.

11. A switching element according to claim 9, characterized in that at least one of the means for decoding data comprises means for decoding data related to more than one connections using a different code for each connection.

12. A switching element (200) according to claim 9, characterized in that it comprises at least two shared media (110a, 110b), in that it further comprises

- means for selecting a part of the signal (213) in a first shared medium, - means for amplifying (213) said part of the signal, and

- means for transmitting (213) the amplified signal to a second shared medium, and in that first means of the means for transrnitting the coded data to a shared medium and of the means of receiving the signal in the shared medium are connected to a first shared medium and second means of said means are connected to a second shared medium.

13. A switching element (200) according to claim 12, characterized in that the means for coding received data and means for hansnntting the coded data, which are connected to the first shared medium, use a first frequency band and the means for coding received data and means for transmitting the coded data, which are connected to the second shared medium, use a second frequency band, which is outside the first frequency band, and in that the means for selecting a part of the signal in a shared medium is an electrical frequency band filter.

14. A switching element (500) according to claim 9, characterized in that it comprises at least two shared media and in that it further comprises - means (501) for dividing received data to coding means, each of which coding means is connected to a separate shared medium, and

- means (502) for multiplexing decoded data, which is decoded by at least two decoding means, into one decoded data.

15. A switching element (600a) according to claim 9, characterized in that at least one of the coding means is connected to the shared medium in at least two points, and in that at least one of the decoding means is a RAKE-decoding means capable of detecting various transmission delays.

16. A switching element (600b) according to claim 15, characterized in that at least one of the coding means is connected to at least two shared media and the RAKE-decoding means decodes the coded data in the two shared media.

17. A bridge element (300), which has means for receiving data from a first shared medium (110a) and means for transnntting data to a second shared medium (100b), characterized in that it further comprises

- means for decoding (311) the signal received from the first shared medium using at least one code division multiple access code,

- means for coding (312) the decoded data using at least one code division multiple access code resulting in the data transmitted to the second shared medium, and

- a transmission medium (301) connecting the means for decoding and means for coding.