IMPLEMENTING ATM VIRTUAL CHANNELS IN A PACKET NETWORK

Abstract: The invention relates to a method for transmitting speech or some other data (D) over the air interface in a telecommunication system comprising at least an air interface (Um) and a packet radio network. The data (D) to be transmitted are made into transport frames (TF). The transport frames (TF) are split in transport packets (TP) having a header and a payload portion. The transport packets (TP) are assembled into radio packets (RP) that are transmitted over the air interface (Um). The method is characterized by incorporating several transport packets (TP), or parts thereof, in at least some of the radio packets (RP). For as efficient as possible bandwidth utilization, the transmission packets (XP) are packed by removing redundant parts from them. The packing is carried out by removing redundant parts from the information that originates from the header and/or payload portion of the transport packets (TP).

Description: IMPLEMENTING ATM VIRTUAL CHANNELS IN A PACKET NETWORK BACKGROUND OF THE INVENTION The present invention relates to a method according to claim 1 for transmitting data over the air interface in a telecommunication system which comprises at least one air interface and a packet network. The radio connection used in conventional digital mobile communication systems is circuit-switched, which means that the radio resources allocated to a subscriber are kept allocated to the connection for the entire duration of the call. A new service planned for digital mobile communication systems is referred to as a packet radio service. The packet radio service planned for the GSM system, namely GPRS (General Packet Radio Service), is described in the ETSI recommendations TC-TR-GSM 02.60 and 03.60. The packet radio service in the D-AMPS system, used in the United States, is referred to as CDPD. By means of a packet radio service, the user of a mobile station MS may be allocated a packet radio connection that utilizes radio resources efficiently. In a packet switched connection, radio resources are allocated only for the duration of data transmission. The data to be transmitted are assembled into fixed length packets. (The last packet in a message is usually shorter than the other ones.) Once such a packet has been sent over the air interface Um, and the transmitting party has no packets to be sent immediately thereafter, the radio resource may be released for use by other subscribers. Figure 1 shows the parts essential to the present invention in the telecommunication system, and the protocols required i.e. practices for transmitting data packets as in prior art over the air interface Um of the radio network in a GPRS system. The invention is not, however, restricted to the GPRS packet radio system operating in conjunction with the GSM network. An applications program of a computer PC establishes a connection according to, for example, the Internet Protocol IP. A PPP (Point to Point Protocol) is an adaptation layer which allows replacing different network layer protocols, such as the IP layer, with other network layer protocols, like OSI, X.25 or CLNP (Connectionless Network Protocol). A GLP (GPRS Link Protocol) and MAC (Medium Access Control) carry out the functionality of the OSI model second layer when connecting to the physical layer, i.e. the air interface. In the exemplary case of Figure 1 , the physical layer is the air interface Um between the mobile station MS and the base station BSS. The broken line around the PC and the MS signifies that the computer and the mobile station may be integrated to form a fixed unit that contains a data processing part and a radio part. In wired packet networks, the use of ATM technique is becoming more common. As a transfer technique for a broadband ISDN network, an Asynchronous Transfer Mode (ATM) has been developed. In ATM data transfer, data are transferred in fixed-length packets consisting of 53 bytes, referred to as ATM cells. In each cell, the cell header takes up five bytes while the remaining 48 bytes are payload, i.e. actual information. ATM cells are specified in the CCITT Recommendation 1.361 and CCITT Draft Recommendation 1.150. In simple terms, the user information to be transferred is cut into fixed-length bit strings, and each bit string is inserted into the information field of the ATM cell. The number of bit strings in a time unit indicates the transfer capacity required by the user. In addition, a header, to be described in closer detail below, is added to the information field, whereby a fixed length ATM cell having 53 bytes is created. The cell is an independent data carrying unit as it indirectly comprises information on the receiving party's address on the basis of which the receiving party may be found in the network. In the ATM network, a plurality of different service criteria has been defined for which it is possible to set a large number of different kinds of parameters. Such criteria include response time, bit error ratio, and the probability of packet loss. The ATM is a connection oriented packet network, which means that connections are set up and terminated according to standardized protocols. A connection between two parties via an ATM network is referred to as an ATM Virtual Channel. The advantages of ATM include, to mention but one, flexible providing of different services. All the bandwidths, for example, in the ATM network are equally natural within the capacity of the physical layer (with modern technology, between 1.5 - 622 Mbps). In "A possible scenario for wireless ATM" by Jouni Mikkonen, published in Mobile Communication International, February 1996, pp. 59 - 61 , it is suggested that the ATM network be extended over the air interface to a mobile station. The aforementioned article does not directly take a stand as to how the ATM cells should be transported over the air interface. In straightforward implementation of the ATM technique, each ATM cell is carried over the air interface as an individual radio packet. Such a solution is shown in Figure 2. Messages produced by the adaptation layer are transformed into fixed-length ATM cells by adaptation layers AAL (ATM Adaptation Layer) and SAAL (Signalling ATM Adaptation Layer). In the latter case, the SAAL is preceded by an adaptation layer according to the ITU protocol Q.2931. The Q.2931 is an ITU signalling protocol. Among other things, it establishes and releases connections by transmitting Setup and Release messages. Packing ATC cells as such in packets of a packet radio network reduces the available bandwidth as each of the protocol layers inserts their own headers to the packets. This is illustrated by Figure 3, in which D (Data) signifies payload, H (Header) refers to an ATM cell header, and R (Radio) denotes a radio packet header. In addition, the efficiency of bandwidth use is impaired by the fact that the lengths of packets used on the various layers are not multiples of each other, which means that all the packets cannot be filled up. As such, ATM cells are not well suited for transmission units in a packet network because it would be uneconomical to allocate and release radio resources for each 53-byte-long ATM cell. Another problem may also be seen in that present-day packet radio networks are not capable of offering isochronous ATM connections. BRIEF SUMMARY OF THE INVENTION It is therefore an object of the invention to develop a method such that the aforementioned ATM virtual channels may be implemented via a packet radio network so as to solve the problems mentioned above. The method should be as flexible as possible. This means that the method may be taken in use in present-day systems with few changes, but on the other hand it is to allow flexible expansion with the packet radio systems of the next generation in mind. The objects of the invention are achieved by a method which is characterized by what is set forth in the characterizing part of claim 1. The preferred embodiments of the invention are set forth in the dependent claims. The invention is based on assembling ATM cells, or advantageously only their payloads, into larger transmission units for transmission over the air interface. With this arrangement, it is possible to optimize the transmission capacity of the air interface, a bottleneck in the transfer, and to facilitate repacking the ATM cells for transmission in a wired network. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING In the following, the invention will be described in closer detail in conjunction with the preferred embodiments and with reference to the accompanying drawings, in which: Figure 1 shows the parts essential to the present invention, and the protocols required for transmitting data packets as in prior art over the air interface Um of the radio network in a GPRS system; Figure 2 shows a system according to Figure 1 and the protocols required when the ATM network is extended to a mobile station; Figure 3 illustrates sending packets over the air interface by ATM technique as in prior art; Figure 4 illustrates the technique of the present invention for extending an ATM network to a mobile station; and Figure 5 illustrates sending packet over the air interface by ATM technique according to the invention. DETAILED DESCRIPTION OF THE INVENTION Referring to Figures 4 and 5, a protocol stack is according to the invention supplemented with an adaptation layer below the ATM layer (closer to the physical layer), the function of such an adaptation layer being to form Radio Packets (RP) from the Transport Packets (TP) so that several transport packets TP or at least parts thereof are incorporated in at least some of the radio packets RP. In the case of an ATM network, the transport packets are ATM cells. In the case of a GPRS packet radio network, the radio packets are GPRS packets having the approximate length of 300 bytes. According to the preferred embodiment of the invention, the ATM cells produced by a communication program of a computer are assembled for the radio path into a larger Transmission Packet (XP), whose size is chosen so that the maximum allocation time of the radio path will not be exceeded. In a so-called "third generation" i.e. UMTS mobile communication system, the optimum size of a packet transferred on the radio path might be approximately 3000 bytes, which corresponds to the payload of approximately 60 ATM cells. For long data transmissions, the radio resource is allocated for as long a time as possible on the radio path in question. With this method, the net bandwidth is considerably better than if every ATM cell were to be transferred over the air interface as an individual radio packet. According to the preferred embodiment, the ATM cell headers are not transmitted on the radio path, but only the payloads of the cells. This alternative is based on the notion that most of the information in the ATM cell header may be regenerated after transmission over the air interface. More specifically, the ATM cell header in the User Network Interface (UNI) consists of the following bits: flow control 4 bits virtual path indicator (VPI) 8 bits virtual channel indicator (VCI) 16 bits payload type 3 bits cell loss priority 1 bit header check sequence 8 bits total 40 bits During a long data transmission, most, possibly even all the bits in the header are the same. Still, with some codecs, for example, some frames are more important than others. This information may be included in the field "cell loss priority". In case this field changes, the check sequence of the header changes. As regards parts that change on a cell to cell basis, like the check sequence, the information elements that change in headers of each ATM cell may be sent over the radio path and the unchanged information elements are generated at the receiving side. In implementations in which the header bits rarely change, it may be advantageous to send a separate data field, indicating that the header bits change between two successive frames. The information that originates from the payload of the ATM cells may be packed separately by some algorithm that removes redundancy, e.g. by Lempel-Ziv algorithm. The packing algorithms work the better the more data they have at their disposal at a time. Packing one ATM cell at a time does not provide a good packing result. In interactive embodiments the data at least partly consist of information entered by the user. To keep the response times reasonable, it is in such a case advantageous to carry out time supervision which takes care that the transmission packets XP are transmitted at the latest when a predetermined time T has elapsed from incorporating the first transfer packet TP in the transmission packet XP in question. The time T is chosen so that the O 97/48211 PCΪYFI97/00344 user will not find the delay annoying. A suitable time might be within 0.1 - 0.5 s, advantageously 0.2 s. It is obvious for a person skilled in the art that due to advancements in technology the basic idea of the invention may be implemented in many different ways. The invention and its embodiments are not restricted to the examples described above but they may vary within the scope of the claims.

Claims: CLAIMS 1. A method for transmitting data (D) over the air interface in a telecommunication system which comprises at least one air interface (Um) and a packet network, the method comprising the steps of: - forming transport frames (TF) from the data (D) to be transmitted; - splitting the transport frames into transport packets (TP) having a header and a payload portion; - assembling the transport packets (TP) into radio packets (RP) which are transmitted over the air interface (Um) characterized by - incorporating a plurality of transport packets (TP), or at least parts thereof, in at least some of the radio packets (RP). 2. A method as claimed in claim 1, characterized by - assembling the transport packets (TP) into transmission packets (XP) by incorporating several transport packets (TP) or at least parts thereof in at least some of the transmission packets (XP); - packing the transmission packets (XP) by removing redundant parts from them; and - assembling the packed transmission packets (XP) into radio packets (RP). 3. A method as claimed in claim 2, characterized in that the packing of the transmission packets (XP) is carried out by removing redundant parts from the information that originates from the header of the transport packets (TP). 4. A method as claimed in claim 2 or 3, characterized in that the packing of the transmission packets (XP) is carried out by removing redundant parts from the information that originates from the payload portion of the transport packets (TP). 5. A method as claimed in any one of claims 2 - 4, characterized in that as large as possible a size is chosen for the transmission packets (XP), but no larger than the payload of the radio packet (RP). 6. A method as claimed in any one of claims 2 - 5, characterized in that the transmission packets (XP) are transformed into radio packets and are transmitted, at the latest, when a predetermined time (T) has elapsed from incorporating the first transport packet (TP) in the transmission packet (XP) in question. 7. A method as claimed in any one of claims 2 - 6, characterized in that the transmission packets (XP) are transmitted upon detecting that the transport packet (TP) incorporated in the transmission packet (XP) is the last transport packet formed from the transport frame (TF) in question. 8. A method as claimed in any one of claims 1 - 7, characterized in that the packet network is an ATM network whereby the transport packet (TP) is an ATM cell and the transport frame (TF) is a frame of an ATM adaptation layer. 9. A method as claimed in any one of claims 1 - 7, characterized in that the radio packet (RP) is a radio packet of the GPRS system or the UMTS system.

Inventors: OLKKONEN MIKKO, OLKKONEN, MIKKO
Applicants: NOKIA TELECOMMUNICATIONS OY, OLKKONEN MIKKO, OLKKONEN, MIKKO
Application year: 1997
Main classification: H04L12
IPC codes: H04L12, H04Q7, H04Q11