FI101921B - Implementation of ATM virtual channels in a packet network - Google Patents

Description

101921 Implementing ATM virtual channels in a packet network

The invention relates to a method according to the preamble of claim 1 for transmitting data over an air interface in a telecommunication system comprising at least one air interface and a packet network.

The conventional radio connection used in digital mobile communication systems is circuit-switched, which means that the radio resources reserved for the subscriber are kept reserved for that connection throughout the call. The packet radio service is a new service designed for digital mobile communication systems. The General Packet Radio Service (GPRS) packet radio service designed for the GSM system is described in ETSI Recommendations TC-TR-GSM 02.60 and 03.60. The packet radio service of the US D-AMPS system is called CDPD.

With the help of the packet radio service, the user of the mobile station MS can be provided with a packet-based radio connection that efficiently utilizes radio resources. In a packet-switched connection, radio resources are reserved only when data is being transmitted. The data to be transmitted is compiled into packets of a certain size. (The last packet of a message is usually shorter than the others.) When such a packet is transmitted over the air interface Um and the transmitting party does not have 20 immediately following packets to transmit, the radio resource can be released for use by other subscribers.

Figure 1 shows the essential parts of a telecommunication system according to the invention and the necessary protocols, i.e. the practices for transmitting data packets over the air interface Um of the radio network in the GPRS system according to the prior art. However, the invention is not limited to a GPRS packet radio system operating in connection with a GSM system.

The PC application of the PC connects, for example, according to the Internet Protocol (IP) policy. PPP (Point to Point Protocol) is an adaptation layer that allows various network layer protocols, such as the IP layer, to be replaced by other network layer protocols, such as OSI, X.25 or CLNP (Connectionless Network Protocol). GLP (GPRS Link Protocol) and MAC (Medium Access Control) implement the Layer 2 functionality of the OSI model by 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 dashed line surrounding the computer PC and the mobile station MS means that the computer and the mobile station can also be integrated into a fixed unit with a data processing part and a radio part.

The use of ATM technology is becoming more common in wired packet networks. Asynchronous Transfer Mode ATM (Asynchronous Transfer Mode) has been developed as a * 5 broadband ISDN transmission technology. In ATM communication, information is transmitted in standard-sized 53-byte packets called ATM cells. In each cell, five bytes are the cell header and the remaining 48 bytes are the payload, or actual information. ATM cells are specified in CCITT Recommendation 1.361 and CCITT Draft Recommendation 10 1.150. Simplified, the user information to be transmitted is broken into fixed-length bit strings, and each bit string is placed in the information field of the ATM cell. The number of bit strings per unit time describes the transmission capacity required by the user. In addition, a header is described in the information field, which will be described in more detail below, in which case a standard 53-byte-long 15 ATM cell is created. A cell is an independent data transport unit because it indirectly contains information about the recipient's address, based on which the recipient can be found on the network. In the ATM network, several different service criteria are defined, for which a large number of different parameters can be set. Such criteria include e.g. response time, bit error rate, and probability of packet loss.

20 ATM is a Connection Oriented packet network, which means that connections are established and terminated according to standardized practices. Contact between the two parties through the ATM network is called an ATM virtual channel (Virtual Channel). The advantages of ATM include the fact that different services can be provided flexibly. For example, all 25 bandwidths in an ATM network are equally natural within the capacity of the physical layer (with current technology between 1.5 and 622 Mbps).

In the article Jouni Mikkonen: “A possible scenario for wireless ATM”,

Mobile Communications International, February 1996, pp. 59-61, it is proposed to extend the ATM network over the air interface to a mobile station. These 30 articles do not directly address how ATM cells should be transported across the air interface.

In a straightforward implementation of ATM technology, each ATM cell is transported over the air interface as its own radio packet. Such a solution is shown in Figure 2. The messages generated by the application layer are converted into fixed length 35 ATM cells by the adaptation layers AAL (ATM Adaptation Layer) and SAAL (Signaling ATM Adaptation Layer). In the latter case, the matching layer SAAL is preceded by an matching layer according to ITU practice Q.2931.

3 101921 Q.2931 is an ITU signaling protocol. Se mm. establish and disconnect by sending Setup and Release messages.

As such, packeting ATM cells into packets in a packet radio network degrades the available bandwidth because each protocol layer 5 associates its own headers with the packets. This is illustrated in Figure 3, where D (Data) stands for payload, H (Header) stands for ATM cell header, and R (Radio) stands for radio packet header. In addition, the efficiency of bandwidth usage is impaired by the fact that the lengths of packets used on different layers are not multiples of each other, so that not all packets are filled.

As such, ATM cells are ill-suited as packet radio network transmission units because it would be uneconomical to allocate and release a radio resource for each 53 byte ATM cell. Another problem is that isochronous ATM connections cannot be provided in the current packet radio network.

The object of the invention is thus to develop such a method that the above-mentioned ATM virtual channels can be implemented via a packet radio network in such a way that the above-mentioned problems can be solved. The method should be as flexible as possible. This means that the method can be introduced in existing systems with minor modifications, but on the other hand it should allow flexible expansion for next generation packet radio systems.

The objects of the invention are achieved by a method characterized by what is stated in the characterizing part of claim 1. Preferred embodiments of the invention are the subject of dependent claims 25.

The invention is based on the fact that ATM cells or, according to a preferred alternative, substantially only the payloads of the cells are combined into larger transmission units for transmission via the air interface. This arrangement optimizes the throughput of the bottleneck air interface and facilitates the packaging of ATM cells for retransmission in a wired network.

The invention will now be described in more detail in connection with preferred embodiments, with reference to the accompanying drawings, in which: Figure 1 shows key parts of a telecommunication system according to the invention and necessary protocols for transmitting data packets over a radio network air interface Um in a GPRS system; 4 101921

Figure 2 shows the system of Figure 1 and the protocols required when an ATM network is extended to a mobile station;

Figure 3 shows the transmission of packets over an air interface by ATM technology according to the prior art; and Figure 4 shows a technique according to the invention for extending an ATM network to a mobile station; and

Figure 5 shows the transmission of packets over an air interface by ATM technology according to the invention.

10

Referring to Figs. at least parts of them. In the case of an ATM network, the transport packets are ATM cells. In the case of a GPRS packet radio network, radio packets are GPRS packets of about 300 bytes in length.

According to a preferred embodiment of the invention, the ATM cells formed by the computer connection program 20 are combined for a radio path into a larger transmission packet XP (Transmission Packet), the size of which is selected so that the maximum reservation time of the radio path is not exceeded. In the so-called "third generation" or UMTS mobile communication system, the optimal size of a packet to be transmitted over a radio path could be about 3000 bytes, which corresponds to a payload of about 60 ATM cells. with this method, the net bandwidth is significantly better than if each ATM cell is transported over the air interface as its own radio packet.

According to a preferred alternative, the headers of the ATM cells 30 are not transmitted on the radio path, but only the payloads of the cells. This option is based on the observation that most of the data in the ATM cell header can be regenerated after transmission over the air interface. More specifically, the ATM cell header at the user network interface UNI (User Network Interface) consists of the following bits: 35 flow control 4 bits virtual path indicator (VPI) 8 bits virtual channel indicator (VCI) 16 bits 5 101921 payload type 3 bits cell loss priority 1 bit header check sequence 8 bits total 40 bits 5

During a long data transfer, most, possibly even all, bits in the header are the same. However, for example, with some codecs, some frames are more important than others. This information can be included in the “cell loss priority” field. As this field changes, the 10-heading check sequence also changes. For cell-to-cell variable portions, such as a check sequence, variable information elements for each ATM cell header can be transmitted over the radio path and fixed information elements are generated on the receiving side. In applications where the header bits change infrequently, it may be advantageous to send a separate data field indicating that the header bits change between two consecutive frames.

The information derived from the payload of ATM cells can be separately compressed by some redundancy removal algorithm, such as the Lempel-Ziv algorithm. Compression algorithms work better the more data they have at a time. Packing one 20 ATM cell at a time does not produce a good compression result.

In interactive applications, the data consists at least in part of information entered by the user. In this case, in order to keep the response times reasonable, it is advantageous to implement time control, which ensures that the transmission packets XP are transmitted at the latest when a predetermined time T has elapsed since the first transport packet TP was combined with that transmission packet XP. The time T is chosen so that the user does not perceive the delay as annoying. A suitable time may be in the range of 0.1 to 0.5 s, preferably 0.2 s.

It will be apparent to one skilled in the art that as technology advances, the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above but may vary within the scope of the claims.

Claims (9)

A method for transmitting data (D) over an air interface in a telecommunications system, comprising at least one air interface (Um) and a packet network, in which method 5. data (D) to be transmitted is transmitted to transport frames (TF); - the transport frames are divided into transport packages (TP), which have a label and a payload part; - transport packets (TP) are designed to radio packets (RP) transmitted over the air interface (Um); It is known that - several transport packages (TP) or at least parts of them are connected to at least some of the radio packages (RP). A method according to claim 1, characterized in that - transport packets (TP) are designed for transmission packets (XP), such that several transport packets (TP) or at least parts of them are connected to at least some of the transmission packets (XP); - the shipping packages (XP) are packaged by removing redundant parts from them; and - the packaged transmission packets (XP) are designed into radio packets 20 (RP). Method according to claim 2, characterized in that the packing of transmission packets (XP) is carried out by removing redundant parts from the information derived from the label of the transport packet (TP). 25 Method according to claim 2 or 3, characterized in that the packing of the transmission packets (XP) is carried out by removing redundant parts from the information derived from the payload portion of the transport package (TP). Method according to any of claims 2-4, characterized in that the size of the transmission packet (XP) is selected as large as possible, but at the most as great as the utility of the radio packet (RP). Method according to any of claims 2-5, characterized in that the transmission packets (XP) are converted to radio packets and are transmitted at the latest when a predetermined time (T) has elapsed since the first transport packet (TP) was connected to said transmission packet (XP). 101921 Method according to any of claims 2-6, characterized in that the transmitted packet (XP) is sent when it is observed that the transport packet (TP) connected to the transmission packet (XP) is the last of the transport frame (TF) formed. 5 Method according to any of claims 1-7, characterized in that the packet network is an ATM network, the transport package (TP) being an ATM cell and the transport frame (TF) being a frame in an ATM matching layer. Method according to any of claims 1 to 7, 10, characterized in that the radio packet (RP) is a radio packet in a GPRS system or UMTS system.