1. Field of the Invention
The invention relates to data communication systems and methods of operation. More particularly, the invention relates to a delay-oriented traffic scheduling method for the control of such communication between an Asynchronous Transfer Mode (ATM) network and a synchronous transfer mode network.
2. Background Art
Recently, ATM and wireless technologies have gained a high visibility as they provide efficient and economical answers to requirements raised by new telecommunication services and by new end-user behaviours.
ATM has been recognised as the common base on which different types of services and network can operate. ATM technology can efficiently combine the transmission of speech, video, hi-fi sound (what is commonly called the multimedia traffic) and computer data into the wired network. Furthermore, ATM has proven to scale well from very high speed network infrastructure (the information highways) to customer premises networks. One of the great advantages of the ATM technology is the fact that it can guarantee some level of service when an ATM connection is set up. Such guarantees can correspond to transmission rate, transmission latency and information loss. Guarantees can be achieved mainly because the ATM architecture assumes that the transmission media are almost error free.
Wireless technology is currently becoming more and more successful as it allows mobile end-users to remain connected to their network and applications while being freed from a wired attachment. Different approaches are currently proposed for wireless communications: they differentiate by the services and the coverage they provide. Wireless Wide Area Networks (WAN) like the so-called Global System For Mobile Communications (GSM) provide nation-wide coverage to the end-user, with modem equivalent transmission speeds. Wireless Data Packet networks provide nation-wide coverage to the end-user, with the Cellular Digital Packet Data (CDPD) transmission services. Finally, Wireless Local Area Networks (LAN) provide establishments wide coverage with conventional LAN (e.g. Ethernet) equivalent communication services.
By marrying these two technologies, wireless ATM networks provide ATM benefits to users working with portable terminals, supporting multimedia applications for mobile users.
But the wireless and ATM technologies significantly differ on several important transmission characteristics:
the wireless channel is a low quality channel in the sense that the typical Bit Error Rate (BER) falls in the range 10.sup.-3 -10.sup.-5, whereas the wired ATM transmission media provide a BER in the range of 10.sup.-9, PA0 the nature of the electromagnetic waves is such that any receiver within range of a transmitter beam can get enough energy to receive information sent by the transmitter. Thus, the wireless channel topology must be seen as point-to-multipoint. On the contrary, the wired ATM links have a point-to-point topology, PA0 the only technically and economically viable approach with a wireless channel is to use it in half-duplex mode whereas full-duplex transmissions are commonly used on wired ATM networks.
Some known techniques are intended to fill the gap between the wireless and ATM technologies.
A first solution based on high speed point-to-point radio links may meet the requirements of a Wireless ATM network in term of channel capacity, but cannot provide a satisfying answer to other needs. The main limitation is the point-to-point topology preventing wireless end-users to be mobile. In addition, the scheduling policies commonly used on such radio links have not been built to meet the Quality of Service (QoS) requirements of ATM connections.
A solution based on the conventional wireless LAN technique is to some extent in line with some requirements of a wireless access to an ATM network without addressing all of them. Such a solution disclosed in patent publication EP 709 982 is directed to a Medium Access Control (MAC) protocol for wireless access of a plurality of remote stations on a LAN. This system gives a solution to the support of conventional LAN traffic, but cannot guarantee all the QoS requirements imposed by any type of ATM traffic. In addition to this limitation, the design point of traditional wireless LAN products is optimised for conventional LAN traffic (based on large packets: 1.5 Kbytes or more) and therefore presents very poor efficiency figures if packets are replaced by ATM cells.
The best solution is described in the European patent application 96 480047.8 which relates to a MAC protocol for wireless radio frequency access for a plurality of ATM mobile terminals to an ATM access point. This protocol is based on a time division structure in which time is slotted, and time slots are grouped into variable length time frames consisting of downlink time slots and uplink time slots. The variable length time frames consists of three periods (DOWN, UP.sub.-- RESERVED, UP.sub.-- CONTENTION). The first period, the DOWN period, is the downlink channel which is used exclusively for data transfer from the Access Point to the Mobile Terminals. The following period, the UP.sub.-- RESERVED period, is the uplink channel that is used for contention-free data transfer from the Mobile Terminals to the Access Point. The allocation of the time slots in the DOWN and UP.sub.-- RESERVED periods is performed by the Access Point, and depends on the service class and QoS parameters of each established ATM connection between the Mobile Terminals and any ATM station according to different priority levels derived from the ATM contract parameters. But, such a system does not take into account the cell delay tolerance depending on the arrival time of the cells. In other words, this system takes only the priority to each connection into account based on its service class, but not the delay constraints of the individual connections in the order of slot allocation per connection.