WO2001091378A1

WO2001091378A1 - Method and apparatus relating to packet data communications

Info

Publication number: WO2001091378A1
Application number: PCT/SE2001/000973
Authority: WO
Inventors: André VAN HOUWELINGEN
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2000-05-23
Filing date: 2001-05-04
Publication date: 2001-11-29
Also published as: SE0001984L; SE516564C2; SE0001984D0; AU2001256906A1

Abstract

A method and apparatus are provided for improving packet data communications in a point to multi-point communication system (1) having a primary node (BS) and secondary nodes (SU1-SU5). Data packets, which are to be transmitted from the primary node to one or more of the secondary nodes, are stored by the primary node. A time interval for transmitting a first data packet (111) from one of the secondary nodes to the primary node is established. After establishing the time interval for the transmission of the first data packet, a second data packet (113) is selected from the stored data packets. The transmission of the second data packet over a first downlink is timed so that an acknowledgement packet (115) acknowledging a receipt of the first data packet can be transmitted promptly over the first downlink from the primary node after the receipt of the first data packet.

Description

METHOD AND APPARATUS RELATING TO PACKET DATA COMMUNICATIONS

TECHNICAL FIELD OF THE INVENTION

The present invention pertains to the field of methods and apparatuses relating to packet data communications, and more particularly to the part of this field where the packet data communications are performed in a point to multi-point system.

BACKGROUND AND RELATED ART

Most wireless communication systems include a land system and one or more subscriber units. The land system typically includes one or more (radio) base stations, which provide wireless communication services to the subscriber units . Each base station serves at least one geographical area, often referred to as a cell. Subscriber units that are positioned in the cell(s) communicate with the base station via a radio interface. The subscriber units communicate, via the base station(s), with other subscriber units in the wireless system or with, other communication units connected to communication networks having connections to the wireless communication system.

Many wireless communication systems used today were originally designed for speech communication, i.e. telephony. The switching technology used in is therefore typically so called circuit switching, which is also used in the conventional PSTN (Public Switched Telephony Network), i.e. an ordinary fixed telephone network. This means that there is a dedicated two-way connection set up between two communicating units continuously for the whole duration of a call, even if no information (e.g. speech) is in fact transferred. Circuit switching is suitable for applications having need for continuous flows of information, which are delay sensitive but not bit error sensitive, e.g. speech and video communication. Today, however, there is an increasing interest in data communication, e.g. electronic mail, Internet, file transfers et cetera. For many forms of data communication circuit switching is, however, not the optimal solution. Packet switching is an alternative form of switching, which is often more suitable for data communication. The information, that is to be transferred through a packet data network employing packet switching, is divided into packets, which need not be of equal length. Each data packet contains an address indicating its destination. When the data packet reaches a node of the packet data network, the packet is sometimes checked for bit errors, e.g. by means of cyclic redundancy check (CRC) , that might have occurred during transmission, and a retransmission is requested if bit errors are detected. Today, however, it is not unusual for the error check to be performed first when the packet has reached its destination. The error checking and the retransmission do, however, cause delays in the transferring of information. If no bit errors are detected, the address of the packet is read and the packet is forwarded to a next node. This is repeated until the packet reaches its destination. At the destination the packets are collected, and the original information is assembled from the packets . In packet switching there is no dedicated connection (channel) for each user. The transfer of information requires only as much (or little) of the channel resources (e.g. bandwidth or time) as is necessary. Packet switching is normally not the best choice for speech and video communication, because of the delays involved. Instead, packet switching is highly suited for most forms of data communications, thanks to the high accuracy that results from error checking and retransmission.

Wireless packet data networks have also been designed. The wireless packet data networks are normally designed as overlay systems on the existing PLMN, e.g. CDPD (Cellular Digital Packet Data) for the AMPS and D-AMPS systems and GPRS (General Packet Radio Service) for the GSM system. However, there are also wireless communication systems, which have been designed more exclusively for packet data communications, e.g. mobitex, Hiperlan/2 or systems according to the IEEE 802.11 wireless LAN standard. In wireless packet data systems, the base stations are normally connected (directly or via an intermediate node such a mobile services switching centre) to a PDBN (Packet Data Backbone Network) . The PDBN comprises one or more node(s) for performing the packet switching within the land system. The PDBN is normally connected to one or more communication networks, e.g. Internet, Intranets, PSTN, ISDN or mobile networks. The PDBN may also be connected to individual computers (e.g. PCs) .

The base station and the associated subscriber units constitute a kind of point to multi-point system, where the base station is a primary node and the subscriber units are secondary nodes . Naturally, only a limited amount of bandwidth can be utilised when the base station is to provide packet data services to the associated subscriber units . Since there are several subscriber units which are to share the packet data services provided by the base station, it is desirable to use the available communication resources efficiently, so that the packet data services provided to the subscriber units are of good quality.

SUMMARY OF THE INVENTION

The present invention addresses the problem of improving packet data communications in a point to multi-point communication system having a primary node and secondary nodes, the nodes performing packet data communications over an uplink and a downlink, wherein at least the primary node is arranged for acknowledging receipt of a data packet by transmitting an acknowledgement packet.

The above-stated problem is solved in short by presenting ways and means for an improved co-ordination of the transmission of data and control information in the point to multi-point communication system. An object of the invention is thus to provide improved packet data communication in a point to multi-point communication system, and the invention includes methods as well as devices for achieving this object.

The above-stated problem is solved in somewhat more detail according to the following. Data packets which are to be transmitted from the primary node to one or more of the secondary nodes are stored by the primary node. A time interval for transmitting a first data packet from one of the secondary nodes to the primary node is established. After establishing the time interval for the transmission of the first data packet, a second data packet is selected from the stored data packets . The transmission of the second data packet over the first downlink is timed so that an acknowledgement packet acknowledging a receipt of the first data packet can be transmitted promptly over the first downlink from the primary node after the receipt of the first data packet. A retransmission of the first data packet is avoided. The opposite flows of data packets over the first uplink and the first downlink are, therefore, better organised, providing efficient full duplex packet data communications .

A main advantage of the invention is that available communication resources are used more efficiently, thereby providing users of the system packet data communications of higher speed and quality.

The invention will now be described further using preferred embodiments and referring to the drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram of a communication system including WLAN.

Figure 2 is block diagram of a base station in the WLAN. Figure 3 is block diagram of a typical subscriber unit in the WLAN.

Figure 4 is a diagram of a structure of a RTS packet.

Figure 5 is a diagram of a structure of a CTS packet.

Figure 6 is a diagram of a structure of a data packet.

Figure 7 is a diagram of a structure of an ACK packet.

Figure 8 is number of time diagrams illustrating a co-ordination packet data communications in the WLAN.

PREFERRED EMBODIMENTS

In figure 1 is shown a block diagram of a WLAN 1 (Wireless Local Area Network) according to an embodiment of the invention. The WLAN 1 includes a (radio) base station BS and a number of subscriber units SU1-SU5, e.g. computers having associated radio communication equipment, radio telephones (fixed or mobile) or wireless screen phones et cetera. The base station BS and the subscriber units SU1-SU5 are arranged for performing packet data communications using a first radio channel rcl and a second radio channel rc2. The first radio channel rcl serves as an uplink, i.e. as a communication link for packet data communications from the subscriber units SU1-SU5 to the base station BS. The subscriber units SU1-SU5 take turn to use the first radio channel rcl. The second radio channel rc2 is used by the base station BS for packet data communications to any one of the subscriber units SU1-SU5. The base station BS is connected to a PDBN 4 (Packet Data Backbone Network) via a data communication link 3. The data communication link 3 can, for example, be a land line connection (e.g. an optical fibre or a co-axial cable) , an xDSL connection or a microwave link. DSL stands for Digital Subscriber Line; the x is used indicate the different variations of DSL, such as ADSL, HDSL, and VDSL. Communications over the data communication link 3 are effected using an appropriate packet data protocol, such as Ethernet (a widely used local area network technology which is now standardised in IEEE 802.3), ATM (Asynchronous Transfer Mode) or IP (Internet Protocol) . The PDBN 4 is connected to number of communication networks. In the particular example of figure 1, the communication networks connected to the PDBN are: Internet 5; a PSTN 6 (Public Switched Telephone Network), i.e. a conventional fixed telephone network; an ISDN 7 (Integrated Services Digital Network); and a CN 8 (Corporate Network), e.g. an Intranet. Consequently, the subscriber units SU1-SU5 can be engaged in packet data communications with each other via the base station BS, or, via the base station BS and the PDBN 4, with other communication units (not shown) connected to the communication networks 5-8.

Figure 2 is a block diagram describing the base station BS in more detail according to a preferred design. The base station BS includes a (radio) transceiver 21 having a transmitter 23 and a receiver 25. The transmitter 23 is arranged for transmitting radio signals on the second radio channel rc2 (downlink) . In the particular embodiment of figure 2, the transmitter 23 includes an up-converter 29 and a D/A-converter 27. The up-converter is connected to an antenna 35 and to the D/A-converter 27. The D/A- converter 27 is also connected to a packet data control unit 37 and arranged for receiving digitally modulated base band signals from the packet data control unit 37. The digitally modulated base band signals include data packets and communication control packets, which are to be sent from the base station to the subscriber units using the second radio channel rc2. In a manner well known to the skilled person, the up-converter is arranged for providing a frequency shift of an analogue signal from the D/A-converter from a base band frequency range to a frequency range associated with the second radio channel rc2. In the particular example of figure 2, the receiver 25 includes a down- converter 33 connected to the antenna 35. The down-converter 33 is arranged, in a manner well known to the skilled person, for frequency shifting radio signals received by the antenna 35. The received radio signals are frequency shifted from a frequency range associated with the first radio channel rcl to the base band frequency range. The receiver 25 includes also an A/D- converter 31, which is connected to the down-converter 33 and the packet data control unit 37. The packet data control unit 37 is arranged for receiving and demodulating digital signals from the A/D-converter 31. The digital signals include data packets as well as communication control packets . The packet data control unit 37 is also connected to a buffer system 39 having one or more buffers in which data packets, which are to transmitted to one or more of the subscriber units SU1-SU5, are temporarily stored.

In addition to the examples given above, the subscriber units SU1-SU5 may also be communication providers of a more general nature. Figure 3 is block diagram describing in detail one form of subscriber unit with a capability to provide communication services to many forms communication equipment. The subscriber unit of figure 3 includes a transceiver 42 having a transmitter 45 and a receiver 47. The design and function of the transmitter

45 in figure 3 are similar to the design and function of the transmitter in figure 2. Naturally, however, the transmitter 45 is arranged for transmitting on the first channel rcl (uplink) . The transmitter 45 includes an up-converter 29, which is connected to an antenna 57 and a D/A-converter 49. The D/A- converter is also connected to a packet data control unit 59, which is arranged for controlling packet data functions of the subscriber unit in figure 3. The design and function of the receiver 47 of the subscriber unit in figure 3 are similar to the design and function of the receiver 25 in figure 2.

Naturally, however, the receiver 47 is arranged for receiving radio signals on the second radio channel rc2 (downlink) . The receiver 47 includes a down-converter 55, which is connected to the antenna 57 and to an A/D-converter 53 , the A/D-converter 53 being also connected to the packet data control unit 59. Furthermore, the packet data control 59 unit is connected to a number (in this example three) of CPEs 63 (Customer Premises

Equipment) via associated data links 61. The CPEs 63 include equipment for communicating with the packet data control unit 59 over the data links 61. The data links may be wireless links as well as "wire" connections. In principle, any appropriate data communication protocol may be used, e.g. Ethernet, POTS (Plain

Old Telephone Service) , ISDN, WLAN-protocols , short-range wireless connections (e.g. based on the Bluetooth specification or similar) et cetera. The CPEs 63 can be virtually any kind customer equipment, e.g. personal computers, fixed telephones, mobile telephones, screen phones, TV or video, home appliances

(such as a fridge or a freezer) et cetera.

The WLAN 1 is point to multi-point system, where the base station BS constitutes a primary node controlling the packet data communications over the radio channels rcl and rc2. Consequently, when one of the subscriber units is to transmit a data packet using the first radio channel rcl, the transmission must be allowed by the packet data control unit 37 in the base station BS . Furthermore, an appropriate time interval for the transmission must be established. Below is set out the steps normally used in WLANs for controlling the transmissions of data packets from the subscriber units SU1-SU5.

- The subscriber unit transmits a RTS packet (Request To Send) on the first radio channel rcl (uplink) . The RTS packet indicates to the base station BS that the subscriber unit requests to transmit a data packet of a particular duration or size.

- After receiving the RTS packet, the base station BS is arranged for evaluating the request and, if finding the request acceptable, for transmitting a CTS packet (Clear To Send) on the second radio channel rc2. The CTS packet indicates that the transmission of the data packet is allowable and establishes the time interval for transmitting the data packet .

- After receiving the data packet, the base station BS is arranged for transmitting an ACK packet (Acknowledgement) to the subscriber unit having transmitted the data packet. The ACK packet indicates to the subscriber unit the receipt, by the base station BS, of the transmitted data packet and whether any bit errors have occurred in the transmission of the data packet. If the subscriber unit does not receive the ACK packet in good time after the transmission of the data packet or if bit errors have occurred, the subscriber unit is arranged for performing a retransmission of the data packet.

Figure 4 is a diagram illustrating a typical structure of the RTS packet. The RTS packet in figure 4 includes a synchronisation sequence 71, an address field 73 and a request field 75. The address field 73 includes a transmitter address identifying the subscriber unit transmitting the RTS packet. The address field also includes a receiver address identifying the base station BS as the receiver of the RTS packet. The request field includes a request to transmit a data packet of particular size or duration.

Figure 5 is a diagram illustrating a typical structure of the CTS packet. The CTS packet in figure 5 includes a synchronisation sequence 77, an address field 79 and an allowance field 81. The address field 79 includes a transmitter address identifying the base station BS. The address field 79 also includes a receiver address identifying the subscriber unit having transmitted the RTS packet. The allowance field 81 includes an indication that a transmission of a data packet is allowed. The allowance field also establishes an actual time interval when the subscriber unit is allowed to transmit the data packet. All subscriber units receive the CTS packet, and consequently, all the subscriber units are informed of which one of the subscriber unit has been allowed to transmit the data packet and also of when the data packet will be transmitted.

In figure 6 is a diagram illustrating a typical structure of the data packet . The data packet in figure 6 includes a synchronisation sequence 83, a message type field 85, an address field 87, a data field 89 and an error detection field 91. The message type field includes information identifying the message type and also a priority associated with the data packet . The address field 87 includes a transmitter address identifying the node (base station or subscriber station) transmitting the data packet. The address field 87 also includes a receiver address identifying the receiving node (base station or subscriber unit) . The data in the data field 89 may in principle represent any kind of information, such as text (e.g. e-mail) , data base information, pictures, sound (e.g. voice or music), program code, data relating to distributed game playing, financial transactions et cetera. The error detection field 91 includes an error detecting code, e.g. a CRC (Cyclic Redundancy Check) or similar.

Figure 7 is a diagram illustrating a typical structure of the ACK packet. The ACK packet in figure 7 includes a synchronisation sequence 93, an address field 95 and an acknowledgement field 97. The address field 95 includes a transmitter address identifying the base station BS and a receiver address identifying the subscriber unit having transmitted the data packet .

The above-indicated steps for controlling and effecting packet data transmissions from the subscriber units SU1-SU5 has hitherto been used in half duplex systems. The WLAN 1 in figure 1 is however a full duplex system, in the sense that transmissions over the first radio channel rcl (uplink) and transmissions over the second radio channel rc2 (downlink) can be performed simultaneously. The first channel rcl is used when transmitting data packets and communication control packets (RTS and ACK) from subscriber units SU1-SU5 to the base station BS . The second radio channel rc2 is used when transmitting control packets from the base station BS to the subscriber stations SU1- SU5, e.g. CTS packets and ACK packets acknowledging the receipt of data packets transmitted from the subscriber units SU1-SU5. However, the second radio channel rc2 is also used when transmitting data packets from the base station to the subscriber units SU1-SU5. In order to make efficient use of the radio channels rcl and rc2 there must be an appropriate co- ordination of the packet data communications over the radio channels rcl and rc2.

Figure 8 includes three time diagrams illustrating the base station's BS co-ordination of the packet data communications over the first radio channel rcl (uplink) and the second radio channel rc2 (downlink) .

In the example of figure 8, the second subscriber unit SU2 has made a request to transmit a first data packet 111. The base station BS has then established a time interval when the second subscriber unit is to transmit the second data packet 111. The lower diagram in figure 8 illustrates the transmission of the first data packet 111 from the second subscriber unit SU2 during the established time interval, i.e. between a time point tl and time point t2. The first data packet 111 has been received in full by the base station BS at a time point t3. Naturally, the time point t2 precedes the time point t3 due to the finite propagation speed of radio waves .

After having established the time interval for the transmission of the first data packet 111, the base station BS is arranged for selecting a second data packet 113 from the buffer system 39. In the particular example of figure 8, the selected second data packet 113 is to be transmitted to the first subscriber unit SUl. The base station BS is arranged for timing a transmission of the second data packet 113 so that the base station BS, after having received the first data packet 111, can promptly transmit an ACK packet 115 to the second subscriber unit SU2 , so as to avoid an unnecessary retransmission of the first data packet 111 from the second subscriber unit SU2. Preferably, the base station is arranged for timing the transmission of the second data packet 113 so that it has been transmitted from the base station BS before the time point t3 , but preferably as close to the time point t3 as possible . In the example of figure 8, the ACK packet 115 is received in full by the second subscriber unit SU2 at a time point t4. In order to simplify the timing of the transmission of the second data packet 113 , the base station BS is preferably arranged for selecting the second data packet 113 so that a duration of the second data packet 113 is equal to or less than the a duration

(t2-tl) of the first data packet 111. In the particular example of figure 8, the second data packet 113 has duration which is less than the duration of the first data packet 111.

In an embodiment where the base station BS is arranged for monitoring transmission delays between the base station BS and the subscriber units SU1-SU5, the base station BS can easily calculate the time point t3 and base the timing of the transmission of the second data packet on this calculation. However, in the example of figure 8, a different approach is taken. In figure 8, the timing of the transmission of the second data packet 113 is such that the transmission of the second data packet 113 ends simultaneously with the transmission of the first data packet 111. The timing approach in figure 8 is advantageous, since it can be used also when the base station BS is not arranged for monitoring the transmission delays . After receiving the second data packet at time point t5, the first subscriber unit SUl is arranged for transmitting an ACK packet 117 to the base station BS. The transmission of the ACK packet 117 is shown in the middle diagram in figure 8. The ACK packet 117 is received in full by the base station BS at a time point t6. Due to the specific timing of the transmission of the second data packet 113, the first subscriber unit SUl can effect the transmission of the ACK packet 117 promptly after having received the second data packet 114, thereby avoiding an unnecessary retransmission of the second data packet 113. Alternatively, the base station BS may be arranged to provide the second data packet 113 with an indication (preferably in the message type field 85) that the second data packet 113 is to be regarded as a data packet which requires no acknowledgement, in which case the fist subscriber unit will not transmit the ACK packet 117.

When selecting the second data packet 113 from the buffer system 39, the base station BS may be arranged to take the priorities of the stored data packets into account. Consequently, if there are several suitable candidates, which can be selected from the buffer system 39 as the second data packet 113, the base station BS is arranged for selecting the candidate which has the highest priority. Furthermore, the buffer system 39 may be arranged so that data packets having the same priority are stored in the same buffer. In this case the base station BS is arranged for first searching the buffer associated with the highest priority for a suitable candidate. If no suitable candidate is found in the highest priority buffer, the base station BS is arranged for searching in turn the lower priority buffers until a suitable data packet has been found and selected. This organisation of the buffer system naturally makes the selecting of second data packet 113 faster.

In the examples above, the co-ordination of packet data communications over a pair of radio channels rcl and rc2 is performed in a WLAN. Naturally, this form of co-ordination can be used also in other forms of wireless communication systems, for example in short range wireless communication systems (e.g. based on the Bluetooth specification or similar) . In fact, the co-ordination is not limited to wireless systems but has general applicability to point to multi-point packet data communication system having at least a first uplink and a first downlink which can be used simultaneously, thereby providing full duplex capabilities. The uplink(s) and downlink(s) may be based on "wire" connections, such as optical fibres, co-axial cables et cetera.

Claims

1. A method for use in packet data communications in a point to multi-point communication system (1) having a primary node (BS) and secondary nodes (SU1-SU5) , the nodes having means (25,45) for communicating over at least a first uplink and means (23,47) for communicating over at least a first downlink, the method comprising: establishing a time interval for transmitting a first data packet (111) over the first uplink from one of the secondary nodes to the primary node; and transmitting the first data packet from the secondary node during the established time interval, the method characterised by: storing data packets which are to be transmitted from the primary node to on or more of the secondary nodes; selecting a second data packet (113) from the stored data packets ; and timing a transmission of the second data packet over the first downlink so that an acknowledgement packet (115) acknowledging a receipt of the first data packet can be transmitted over the first downlink from the primary node promptly after the receipt of the first data packet.

2. A method according to claim 1, wherein the selecting of the second data packet (113) includes that the second data packet is selected to have a duration which less than or equal to a duration of the first data packet (111) .

3. A method according to any one of claims 1 or 2, wherein the selecting of the second data packet (113) includes that the second data packet is selected having regard to a priority ranking of the stored data packets.

4. A method according to any one of the claims 1, 2 or 3, wherein the timing of the second data packet (113) includes that the transmission of the second data packet is timed so that the transmission of the second data packet ends simultaneously with the transmission of the first data packet (111) .

5. A method according to any one of the claims 1 to 4, wherein the method further includes : providing the second data packet (113) with an indication that no acknowledgement is to be effected after a receipt of the second data packet .

6. A method according to any one of the claims 1 to 5, wherein the first uplink is a first radio channel (rcl) .

7. A method according to any one of the claims 1 to 6, wherein the first downlink is a second radio channel (rc2) .

8. A primary node (BS) for a point to multi-point packet data communication system (1) , the primary node comprising: means (23) for effecting packet data transmissions over at least a first downlink to a number of secondary nodes (SU1-SU5) ; means (25) for receiving packet data information transmitted to the primary node from the secondary nodes over at least a first uplink; means (37) for establishing a time interval for transmitting a first data packet (111) over the firsts uplink from one of the secondary nodes to the primary node, characterised in: that the primary node comprises means (39) for storing data packets which are to be transmitted from the primary node to one or more of the secondary nodes; that the primary node comprises means (37) for selecting a second data packet (113) from the stored data packets; and that the primary node comprises means (37) for timing a transmission of the second data packet over the first downlink so that an acknowledgement packet (115) acknowledging a receipt of the first data packet can be transmitted over the first downlink from the primary node promptly after the receipt of the first data packet.

9. A primary node according to claim 8, wherein the means for selecting the second data packet (113) is arranged so that the second data packet is selected to have a duration which less than or equal to a duration of the first data packet (111) .

10. A primary node according any one of the claims 8 or 9, wherein the means for selecting the second data packet (113) is arranged so that the second data packet is selected having regard to a priority ranking of the stored data packets .

11. A primary node according to any one of the claims 8, 9 or 10, wherein the means for timing the transmission of the second data packet (113) is arranged so that the transmission of the second data packet is timed so that the transmission of the second data packet ends simultaneously with the transmission of the first data packet (111) .

12. A primary node according to any one of the claims 8 to 11, wherein the primary node further comprises : means (37) for providing the second data packet (113) with an indication that no acknowledgement is to be effected after a receipt of the second data packet.

13. A primary node according to any one of the claims 8 to 12 , wherein the first uplink is a first radio channel (rcl) .

14. A primary node according any one of the claims 8 to 13 , wherein the first downlink is a second radio channel.

15 . A point to multi-point communication system including a primary node according to any one of the claims 8 to 14 .