WO2008135640A1

WO2008135640A1 - A method and a device for error resilient communication

Info

Publication number: WO2008135640A1
Application number: PCT/FI2008/050254
Authority: WO
Inventors: Jarmo Holopainen
Original assignee: Jarmo Holopainen
Priority date: 2007-05-08
Filing date: 2008-05-08
Publication date: 2008-11-13
Also published as: FI20075326L; FI20075326A0

Abstract

The invention provides a method and a device for improving error resilience in data transmission over a communication link that has a lower quality than that required by a particular communication mode. Such circumstances may arise for example when a physical layer does not provide adequate connection quality for a higher layer communication protocol. According to the invention, the data units of such higher layer protocol are disassembled, and data is carried over the physical layer connection in other data units that are more robust to trans mission errors.

Description

A METHOD AND A DEVICE FOR ERROR RESILIENT COMMUNICATION

Technical field of the invention

The present invention relates in general to error resilient communication and in particular to improving error resilience in data transmission over a communication link, which has a lower quality than that required by a particular communication mode.

Background of the invention

Mobile telecommunications systems have become available for a vast number of people throughout the world. First generation mobile communication systems, such as the

Nordic Mobile Telephony (NMT) system, were based on analog technology. Digitalization of the mobile communications systems began with the introduction of second generation (2G) systems, such as GSM and IS-95

(CDMA) . At present, third generation (3G) mobile communication systems, such as UMTS/WCDMA, are becoming commonplace in urban areas.

Figure 1 depicts an outline of a UMTS Phase 1 network. As can be seen in the figure, the UMTS Phase 1 network comprises a GSM Base Station Subsystem BSS, a UMTS radio access network UTRAN, and a GSM core network CN. A GSM terminal communicates with a GSM base station (BS) over GSM air interface Um. A base station BS is connected to a Base Station Controller BSC by means of an Abis interface. The BSC is connected to a Mobile Switching Center through an A interface. Connections to an external circuit switched network, such as a Public Switched Telecommunications Network PSTN, may be processed by a Gateway Mobile Switching Center GMSC.

A UMTS terminal communicates with a UTRAN base station Node B over a UTRAN air interface Uu. A Node B is connected to a radio network controller RNC by means of an Iub interface. RNCs of the UTRAN are arranged to communicate through Iur interfaces.

As can be further seen in figure 1, at least some of the RNCs are connected to the circuit switched domain (MSC) and the packet switched domain (Switching GPRS Support Node SGSN and Gateway GPRS Support Node GGSN) through an IuCS and IuPS interface, respectively. Connections to an external packet data network PDN, such as the Internet, are routed via a GGSN.

A person skilled in the art is familiar with the UMTS architecture and appreciates that the circuit switched domain and the packet switched domain comprise a number of the network elements shown in figure 1 and that there are a number of other elements in a UMTS mobile communications network that are not shown in figure 1. Such elements may include e.g. a Home Location Register HLR and a Authentication Centre AuC. A mobile terminal may also be a dual-mode terminal that is capable of communicating with both GSM base stations BS and UTRAN Node Bs. The Abis interface connections of the GSM BSS may use El links that are implemented over landlines. Use of microwave links for Abis interface connections has been found useful due to lower capital, implementation, and operating expenses and increased network roll-out pace, especially in areas where landline connections are not readily available.

Suppose that a GSM network operator upgrades its GSM network to a UMTS Phase 1 network. In such a case, the network operator needs to inter alia roll-out a UTRAN radio access network, an outline of which is given in figure 1. Rolling-out a radio access network of a mobile communications network involves a number of commercial transactions, such as negotiating rental agreements for location sites for base station equipment. It means that when rolling-out the UTRAN network the operator may tend to utilize the location sites of the GSM for placing the UTRAN Node B equipment in order to save transaction costs and rental expenses. In striving for saving time and money the operator may also take advantage of existing transmission links as well, especially microwave links, the advantages of which were described above. Implementing e.g. the UTRAN Iub interface connections over the existing microwave links would dramatically speed up the UTRAN network roll-out.

In 2G systems a minimum quality requirement for carrying voice circuits was assumed to be 10^~3 in terms of Bit Error Rate (BER) , and this requirement was followed in designing the microwave transmission links of the 2G systems. As opposed to e.g. GSM Abis interface, UTRAN Iub interface connections are implemented by using ATM connections in order to provide the required Quality of Service for different UMTS traffic classes. As ATM was originally designed for e.g. optical fiber or other high-quality medium as a transmission medium, ATM connections may require a better link quality (e.g. 10^~6 in terms of BER) than that provided by the current 2G microwave transmission links. This may result in poor performance of ATM connections over the Iub interfaces of UTRAN due to ATM traffic control and Quality of Service mechanisms. For example, some forms of data traffic may be very sensitive to data loss during data transmission, and so in order to carry such data traffic successfully over the Iub interface transmission links, a suitable UMTS traffic class along with a corresponding ATM circuit is assigned to said data traffic. When the quality of microwave link degrades e.g. due to weather conditions to such extent that the radio connection is lost, synchronization of El bit stream that is carried over the radio connection may be lost as well. Under such circumstances transmission of re-synchronization bit pattern may result in loss of cell delineation, which causes retransmission of ATM cells.

The poor performance of an ATM connection over the Iub interface may result in an unacceptable Quality of Service for traffic carried in the UMTS network, and in the absence of a solution to the problem network operators may be discouraged to roll-out 3G networks. Summary of the invention

It is the object of the present invention to solve or at least mitigate the problem described above. According to a first aspect the invention provides a method of providing digital data for transmission in an error resilient manner, wherein the method comprises the step of receiving digital data arranged into a first data unit having a first predetermined structure. The method is characterized by the steps of disassembling said first data unit, said first data unit comprising an ATM AAL2 packet; and arranging said digital data into a second data unit, said second data unit comprising a G.704 frame and having a second predetermined structure; and providing said second data unit for transmission over a physical transmission medium, said physical transmission medium comprising a microwave radio link.

According to an embodiment of the invention the first data unit comprises a header portion and a content portion, said content portion comprising said digital data, and the method further comprises the step of discarding the header portion before arranging the content portion into said second data unit.

In an embodiment the method further comprises the step of determining an indicator corresponding to the discarded header portion and arranging said indicator into the second data unit. The indicator may be associated with the content portion corresponding to the discarded header portion in the second data unit. In an advantageous embodiment of the invention the first data unit comprises an ATM AAL2 packet, the second data unit comprises a G.704 frame, and the physical transmission medium comprises a microwave radio link.

In an embodiment of the invention the method further comprises the steps of receiving the second data unit comprising said digital data, and extracting said digital data from the second data unit and arranging said digital data into a third data unit having the first predetermined structure.

In some embodiment s the method comprises the step of extracting the indicator from the second data unit and generating a header portion for the third data unit by means of the indicator, wherein said header portion corresponds to the discarded header portion. In some embodiments substitute data may be generated for data that is lost during transmission and a third data unit may be generated by means of the substitute data such that the third data unit has t he first predetermined structure.

According to a second aspect the invention provides a device for providing digital data for transmission in an error resilient manner, wherein the device comprises a processing unit comprising a first interface for receiving digital data arranged into a first data unit having a first predetermined structure, and a line interface unit comprising a second interface for providing processed data for transmission. The device is characterized in that the processing unit is adapted to disassemble said first data unit, said first data unit comprising an ATM AAL2 packet, and in that the line interface unit is adapted to arrange said digital data into a second data unit, said second data unit comprising a G.704 frame and having a second predetermined structure; and to provide said second data unit for transmission over a physical transmission medium, said physical transmission medium comprising a microwave radio link.

In an embodiment of the invention the processing unit is further adapted to determine an indicator corresponding to a header portion of the first data unit. The line interface unit (202) may be adapted to arrange said indicator into the second data unit, wherein said indicator is associated with a content portion of the first data unit.

In an embodiment of the invention the second interface of the line interface unit is adapted to receive a third data unit having the second predetermined structure, and the processing unit is adapted to extract digital data from the third data unit and to arrange said digital data into a fourth data unit having the first predetermined structure. The processing unit may be adapted to extract an indicator from the third data unit and for generating a header portion for the fourth data unit by using the indicator, wherein said header portion corresponds to a discarded header portion.

In an embodiment of the invention the processing unit is adapted to generate substitute data for data that is lost during transmission. T he processing unit may be adapted to arrange the substitute data into the fourth data unit.

The invention involves several advantages over prior art. For example, the invention enables mobile network operators to utilize existing 2G microwave transmission links for e.g. UTRAN Iub transmission, which reduces costs and required time for 3G network roll-out. The invention can also be used for compensating adverse effects of a low-quality transmission link.

In the following advantageous embodiments of the present invention are explained in detail with reference to the appended drawings. A person skilled in the art appreciates that the embodiments are described for illustrative purposes only and are not intended to limit the scope of the claims to said embodiments. For example, the invention may be used in any application where a communication link quality is not adequate for the communication protocol in use.

A skilled person also appreciates that an aspect of the invention may be used in combination with one or more of the disclosed embodiments.

Brief description of the drawings

Figure 1 depicts an outline of a UMTS Phase 1 mobile communication network. Figure 2 illustrates a device according to the invention .

Detailed description of the drawings

Figure 1 shows inter alia the Abis interfaces between the BSC and the GSM base stations as well as the Iub interfaces between the RNC and the Node B's. Even though figure 1 shows the Abis and Iub interface connections as separate links, implementing the Abis and Iub interfaces over shared physical transmission media to a feasible extent involves many technical and economical advantages, as is described hereinbefore.

The present invention is explained with reference to an Iub interface of a UTRAN network, where an ATM connection is implemented over an El link using a microwave radio link as a transmission medium, but the invention is equally applicable to any arrangement, where a lower layer may not be capable of providing adequate communication quality for a higher layer communication protocol. Such circumstances typically arise when a communication connection comprises a wireless leg, such as a microwave or Bluetooth link, but may also arise due to poor wired connections, such as modem connections.

Figure 2 depicts a basic configuration of a device 200 according to the present invention. The device 200 comprises a processing unit 201 and a line interface unit 202. A person skilled in the art appreciates that the device 200 may also comprise other components, such as memory chips and further interface ports .

The processing unit 201 comprises a first interface 204, through which the processing unit 201 is arranged to receive digital data supplied to the processing unit for processing, wherein the received digital data is arranged into first data units having a first predetermined structure. In a preferred embodiment of the invention, the digital data is received in the form of ATM cells, which carry application data arranged into AAL2 (ATM Adaptation Layer 2) packets. As shown in figure 3, an ATM cell comprises a 5-byte header portion 30 and a 48-byte payload portion 31 , which form a 53-byte ATM cell.

The AAL2 protocol is designed for transmission of low- rate, variable length packets of delay-sensitive applications. AAL2 is described in detail in ITU-T specification 1.363.2, which is incorporated herein by reference, and is therefore not further described herein.

The first interface 204 may comprise a port according to the UTOPIA (Universal Test and Operations Physical Interface for ATM) physical interface specification (e.g. af-phy-0017.000 and af-phy-0039.000, incorporated herein by reference) by The ATM Forum, or any other standard or proprietary physical interface suitable for receiving digital data from other processing units via connection 203, such as a system internal bus. The processing unit 201 further comprises means for communicating with the line interface unit 202. Figure 2 depicts as an example two ports 205 and 206 connected to the line interface unit 202 through connections 207 and 208, respectively. The ports 205 and 206 may be for example serial ports, and the connections 207 and 208 may use e.g. PCM buses.

The processing unit 201 further comprises means for data processing that are not shown in figure 2. Such data processing means may include e.g. digital signal processor (s) , application-specific integrated circuits, and internal processor memory means, as well as program code stored and executed in said means.

The line interface unit 202 comprises a second interface 209 for supplying data processed by the device 200 for transmission over a transmisison medium. Only one second interface 209 is shown in figure 2, but a skilled person appreciates that the line interface unit 202 may comprise a plurality of second interfaces 209. In an embodiment of the invention a portion of the second interfaces 209 may be used for sending data and another portion may be used for receiving data. The data is supplied to a transmission unit either directly or via a further processing unit if need be. In an advantageous embodiment of the present invention the data is supplied via the second interface 209 to a microwave radio unit for transmission over a microwave radio link. Thus, the device 200 may be used e.g. in conjunction with the RNC and Node B elements shown in figure 1 at the endpoints of the Iub interface connections, where the Iub interface connections are implemented over microwave radio links.

In a similar manner as the processing unit 201, the line interface unit 202 further comprises means for data processing that are not shown in figure 2. Such data processing means may include e.g. digital signal processor (s) , application-specific integrated circuits, and internal processor memory means, as well as program code stored and executed in said means.

The line interface unit 202 is preferably capable of constructing G.704/E1 frames from data processed by the processing unit 201. The structures of G.704 frames and multiframes are shown in figure 4. A G.704 frame 40 contains 32 time slots, each consisting of 8 bits. Thus, the length of a G.704 frame is 256 bits. A G.704 multiframe 41 consists of 16 frames and 4096 bits. The G.704 protocol is described in detail in ITU-T G.704 Recommendation, which is incorporated herein by reference .

The second interface 209 may be a physical port comprising a suitable connector, such as RJ-45 or BNC.

As was mentioned earlier, the device 200 according to the invention may be used e.g. at the endpoints of a Iub interface connection implemented over a microwave radio link. Preferably, the device 200 is capable of two-way operation, i.e. the device is arranged to process data received from e.g. a processing unit of a RNC or Node B via the first interface 204 as well as data received from a Iub interface via the second interface 209. In said embodiment, the device is further arranged to pass on data to the Iub interface via the second interface 209, as well as data to a processing unit outside the device via the first interface 204 after processing in the device 200. In another embodiment, the device 200 is adapted for one-way operation, which means that e.g. a RNC or a Node B requires two devices: one for inbound traffic and one for outbound traffic. The device 200 may be realized for example as a media adapter card, which can be mounted in a card slot of an RNC or a Node B.

As was described above, the processing unit 201 is arranged to receive digital data contained in first data units having a first predetermined structure e.g. from an external processing unit via the first interface 204. Preferably, but not necessarily, the first data units are AAL2 packets placed in ATM cells . The processing unit 201 disassembles the first data units and extracts at least application data from said data units. Typically, the application data is carried in the content portions of the first data units, but some useful application or application-related data may also be carried in the header portions of the first data units. The processing unit 201 may also discard the header portion (s) of the first data units. If the processing 201 unit is arranged to discard the header portion of a first data unit, the processing unit 201 preferably determines an indicator corresponding to the header portion prior to discarding said first data unit. Once the processing unit 201 has extracted application data from the first data units and possibly determined indicators for discarded header portions, the processing unit 201 forwards the application data and the indicators to the line interface unit 202 for further processing. The processing unit 201 may also keep the original header portions and forward them to the line interface unit 202 in stead of generating and forwarding indicators .

The processing unit 201 may determine an indicator for each discarded header, or one indicator may represent a plurality of headers, i.e. in an advantageous embodiment of the invention an indicator generated by the processing unit 201 may represent one ATM cell header 30 and/or AAL2 packet header or a plurality of ATM cell headers 30 and/or AAL2 packet headers.

The processing unit 201 determines the indicators by means of a suitable algorithm. For example, an indicator may be an interpolation or extrapolation of other indicator (s) and/or header portion values, a linear or non-linear prediction calculated from other indicator (s) , header portion values, and/or their predictions, or a value calculated by means of a hash function. Advantageously, an indicator value that reduces the number of bits is generated.

The line interface unit 202 receives the application data and the indicators (or the header portions, if indicators were not generated) via connections 207 and 208 and arranges the application data and preferably the indicators into second data units having a second predetermined structure. If indicators are not available, the header portions are arranged into the second data units along with the application data. The second data units are preferably, but not necessarily, G.704 frames 40, 41. The application data and indicator bits can be inserted into the G.704 frames 40, 41 in sequence, or a suitable interleaving scheme may be used. The application data and the indicator bits are preferably arranged into the G.704 frames 40, 41 such that there is an association between an indicator and corresponding application data, i.e. the content portion of a first data unit and indicator (s) generated to replace the discarded header portion of the first data unit are placed into the G.704 frames 40, 41 such that their position with respect to each other is known. The line interface unit 202 then passes on the G.704 frames 40, 41 for transmission to a receiving end. In a preferred embodiment of the invention, the line interface unit 202 may provide the G.704 frames 40, 41 directly to a microwave radio unit for transmission over a Iub interface, or the frames may be processed further in another processing unit prior to transmission over the Iub interface.

As is described above, the device 200 is preferably capable of two-way operation such that e.g. a device 200 mounted in a RNC and Node B is capable of sending and receiving data over the Iub interface, i.e. the devices 200 of the RNC and the Node B can exchange data over the Iub interface. Thus, the device 200 can preferably operate both as a sending and a receiving device. For example, the device 200 mounted in the RNC may receive data processed by the device 200 of the Node B through the second interface 209 of the device 200 of the RNC. In transmission over the Iub interface the data is carried in second data units having the second predetermined structure. As described above, the second data units are preferably G.704 frames 40, 41. The line interface unit 202 may be adapted to disassemble the G.704 frames and extract application data and preferably indicators from the G.704 frames, or the line interface unit 202 may forward the frames to the processing unit 201 for disassembly as well as application data and indicator extraction.

When the device 200 operates as a receiving device, the processing unit 201 arranges the application data into data units having the first predetermined structure. Preferably, the processing unit 201 arranges the application data into ATM AAL2 packets. If the processing unit 201 receives indicator (s) representing discarded header portions of first data units that were received, processed, and provided for transmission according to the invention by a device 200 operating in a send mode, the processing unit 201 of the receiving device generates header portions for the data units by means of the indicator (s) . The processing unit 201 uses corresponding algorithms and index or hash tables that were used in generating the indicator (s) . If the sending device 200 has not generated indicators but transmitted original header portions instead, the processing unit 201 of the receiving device uses said header portions. Preferably, the processing unit 201 generates ATM AAL2 headers for the ATM AAL2 packets.

The processing unit 201 of a device 200 operating in receive mode may also be adapted to generate substitute data for data that is lost during transmission. The lost data may represent application data, indicator data, or header data, if indicators were not generated, or combinations of said data. The processing unit 201 may use said substitute data to fill in missing parts of the data units that are composed in the processing unit 201, when the device 200 operates as a receiving device. The substitute data may represent a header portion or a content portion of the composed data units, or both. In an advantageous embodiment of the invention, the processing unit 201 is capable of generating substitute data, that can fill in the missing or corrupted portions of ATM AAL2 packets that are reconstructed in the processing unit 201 when the device 200 operates as a receiving device.

Thus, when the device 200 according to the invention operates as a sending device, it disassembles first data units received from other processing units, extracts at least application data from said data units and places at least said application data into second data units having a structure other than that of the first data units. Preferably, the device 200 also generates indicator (s) representing the header portions of the first data units and places said indicators into the second data units as well. A device 200 operating as a receiving device reconstructs data units corresponding to the first data units from the application data and indicator (s) carried in the second data units and extracted thereform.

In a preferred embodiment of the invention, a device 200 operating as a sending device, extracts application data and/or application-related data from ATM AAL2 packets, arranges said data into G.704 frames and provides said frames for transmission over Iub interface implemented over a microwave radio link. As G.704 frames are less sensitive to transmission errors caused by e.g. weather conditions than ATM cells, the inventions provides for a more error robust method and device for carrying e.g. UMTS application data over microwave transmission links, that were designed to meet the requirements of 2G systems, than running ATM connections as such over said microwave transmission links.

As appreciated by a person skilled in the art, the invention may have various embodiments within the scope of the appended claims. For example, the processing unit 201 and the line interface unit 202 may be implemented as a single physical unit or a plurality of physical units. The number of the first interfaces 204 and the second interfaces 209, as well as the internal communication ports 205, 206 may also vary according to specific design needs.

Claims

Cl aims :

1. A method of providing digital data for transmission in an error resilient manner, wherein the method comprises the step of: receiving di gital data arranged into a first data unit having a first predetermined structure, characterized in that the method further comprises the steps of: disassembling said first data unit, said first data unit comprising an ATM AAL2 packet; and arranging said digital data into a second data unit, said second data unit comprising a G.704 frame and having a second predetermined structure; and providing said second data unit for transmission over a physical transmission medium, said physical transmission medium comprising a microwave radio link.

2. A method according to claim 1, wherein the first data unit comprises a header portion and a content portion, said content portion comprising said digital data, and wherein the method further comprises the step of discarding the header portion before arranging the content portion into said second data unit.

3. A method according to claim 2, wherein the method further comprises the step of determining an indicator corresponding to the discarded header portion and arranging said indicator into the second data unit.

4. A method according to claim 3, wherein the indicator is associated with the content portion corresponding to the discarded header portion in the second data unit.

5. A method of receiving digital data provided for transmission in an error resilient manner according to any of claims 1 to 4, wherein the method comprises the steps of: receiving the second data unit comprising said digital data; and extracting said digital data from the second data unit and arranging said digital data into a third data unit having the first predetermined structure.

6. A method according to claim 5, wherein the method further comprises the step of extracting the indicator from the second data unit and generating a header portion for the third data unit by means of the indicator, wherein said header portion corresponds to the discarded header portion.

7. A method according to claim 5, wherein the method further comprises the step of generating substitute data for data that is lost during transmission.

8. A method according to claim 7, wherein the method further comprises the step of generating a third data unit having the first predetermined structure by means of the substitute data.

9. A device (200) for providing digital data for transmission in an error resilient manner, wherein the device comprises: a processing unit (201) comprising a first interface (204) for receiving digital data arranged into a first data unit having a first predetermined structure; a line interface unit (202) comprising a second interface (209) for providing processed data for transmission, characterized in that the processing unit (201) is adapted to disassemble said first data unit, said first data unit comprising an ATM AAL2 packet, and in that the line interface unit (202) is adapted to: arrange said digital data into a second data unit, said second data unit comprising a G.704 frame and having a second predetermined structure; and to provide said second data unit for transmission over a physical transmission medium, said physical transmission medium comprising a microwave radio link .

10. A device according to claim 9, wherein the processing unit (201) is further adapted to determine an indicator corresponding to a header portion of the first data unit.

11. A device according to claim 10, wherein the line interface unit (202) is further adapted to arrange said indicator into the second data unit, wherein said indicator is associated with a content portion of the first data unit.

12. A device according to claim 9, wherein the second interface (209) of the line interface unit (202) is adapted to receive a third data unit having the second predetermined structure and wherein the processing unit

(201) is further adapted to extract digital data from the third data unit and to arrange said digital data into a fourth data unit having the first predetermined structure .

13. A device according to claim 12, wherein the processing unit (201) is further adapted to extract an indicator from the third data unit and for generating a header portion for the fourth data unit by using the indicator, wherein said header portion corresponds to a discarded header portion.

14. A device according to claim 12, wherein the processing unit (201) is further adapted to generate substitute data for data that is lost during transmission .

15. A device according to claim 14, wherein the processing unit (201) is adapted to arrange the substitute data into the fourth data unit.