TECHNICAL FIELD 
   The present invention relates to bandwidth management between the stations in a data transmission network of the Ethernet type and relates in particular to a method of allocating bandwidth to the stations of a Local Area Network such as an Ethernet network. 
   BACKGROUND 
   On local Area Networks (LANs) of the Ethernet type able to transmit data at speeds higher than one gigabit/s, an unlimited number of stations can be connected to a shared medium. To control the exchange of data between the stations connected on the shared medium, Ethernet uses a protocol called Carrier Sense, Multiple Access Collision Detect (CSMA/CD). The “Multiple Access” part means that every station is indeed connected to the shared medium forming a single data path. The “Carrier Sense” part means that before transmitting data, a station checks to see whether any other station is already transmitting. If the transmission medium appears to be idle, the station may begin to send data. However, two stations can start transmitting at the same time, causing a collision. When this occurs, each interfering station detects the collision. Hence, all stations attempt to transmit, back off, and try retransmissions at randomly selected later times, thus minimizing the chance of another collision. 
   Although Ethernet does not set an upper limit to the number of stations that can be connected on the same transmission medium, there are, in practice, drastic limitations. Generally speaking, as more users are added to a shared network, or as applications requiring more data are added, performance inevitably deteriorates. This is because the users become competitors in trying to use a common resource: the shared transmission medium. It is generally agreed that a moderately loaded 10 Mbps Ethernet shared by 30–50 users can sustain a throughput in the neighborhood of only 2.5 Mbps after accounting for packet overhead, inter-packet gaps, and collisions resulting in the use of the CSMA/CD protocol. Thus, although simple, CSMA/CD protocol is limited in its ability to take advantage of the intrinsic performance of the shared transmission medium i.e., 10 Mbps in this example. 
   Further increasing the number of users (and therefore packet transmissions) creates an even higher collision potential. Since collisions occur when two or more stations attempt to send information at the same time, when these stations realize that a collisions has occurred, they must, to obey the Ethernet standard, all shut off for a random time before attempting another transmission. This tends to add a considerable overhead, severely impacting performance. Consequently, the Ethernet mechanism collapses when the shared medium is overloaded. 
   One well-known solution to this problem is to segment traffic over independent, disjoint, smaller collision domains, which comes at the expense of having to put in place extra devices to allow communication between the independent pieces of the LAN thus created. This may be accomplished by using a bridge or a switch. For example, an eight-port high-speed switch can support eight Ethernets, each running at a full 10 Mbps and thereby interconnects more users on what appears to them as a single LAN. Such a solution, which creates a more expensive and complicated network, goes against the original objectives of the Ethernet LAN, which were to provide a very inexpensive solution, simple to manage for local communications over a campus or between the employees of a company dispersed over a group of buildings. 
   Another solution consists in implementing a token-passing mechanism of Token Ring LAN so that the physical Ethernet network becomes collision free and therefore can be used at higher rates. In this mechanism, a logical ring is formed between connected stations and a token is circulated among the connected stations that are part of the logical ring. Then, a station of the logical ring is permitted to transmit only while holding the token, thereby preventing collisions from happening. 
   Although the above system prevents collisions between stations, it does not prevent a station from monopolizing the bandwidth and preventing other stations from transmitting. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the invention is to provide a method of bandwidth management between the stations of an Ethernet network enabling each station of a logical ring to use a bandwidth which is determined according to predefined parameters. 
   Another object of the invention is to provide a method of ensuring a guaranteed bandwidth to each station of an Ethernet network, thereby making the network deterministic in terms of throughput and response time. 
   The invention therefore relates to a method of bandwidth management that ensures a guaranteed bandwidth to each station of a transmission network comprising a plurality of stations forming a logical ring upon which a token is circulated from station to station. Each station is allowed to transmit data to other stations over the transmission network upon receipt of the token while holding the token. During the time the station is transmitting data, a credit of data bytes allowed to be transmitted upon reception of said token is allocated each station. This credit is increased in proportion to the elapsed time since the preceding reception of the token. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be better understood by reading the following detailed description of the invention in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a schematic representation of stations of an Ethernet network organized as a logical ring wherein the invention may be implemented. 
       FIG. 2  illustrates the format of a token used in a logical ring according to  FIG. 1 . 
       FIG. 3  represents a flow chart of steps used in the method according to the invention. 
       FIG. 4  is a flow chart showing details of the step for decrementing the credit in a station used in the method illustrated in  FIG. 1 . 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   According to  FIG. 1 , an Ethernet LAN includes several stations  10 ,  12  and  14  physically connected on the shared medium  16  in the manner of a standard Ethernet LAN, but organized so that they form a logical ring  18 . This organization is achieved through the circulation of a token  20  which takes the form of a special Ethernet frame, forwarded  22  from one station to a next, e.g., from station  10  to station  12 . 
   It is worth noting here that not all stations connected on the same LAN segment need to participate into the collision-free ring  18  thus formed. The invention assumes that both types of protocols (collision and collision-free) may coexist at any given instant so that a station like  24  need not implement the new protocol while still being able to communicate with all the others connected on the shared transmission medium  16  using the regular collision protocol. 
   As represented in  FIG. 2 , the token may include the following information:
         An Ethernet Destination MAC address  26 , i.e. the MAC address of the next ring station.   An Ethernet Source MAC address  28 , i.e. the MAC address of the station sending the token.   A destination SAP (Service Access Point)  30  which, in the OSI model, is used to identify the individual application on a host which is sending a packet. A destination SAP address  30  having a HEX value of AA is however used here, which is the standard way of actually defining the type of application in the following SNAP (SubNetwork Access Protocol) field.   A source SAP  32 , i.e. the counterpart of the here above destination SAP. It takes a HEX value of AA too.   A control field  34  which takes a HEX value of 03 to indicate that this is a UI (Unnumbered Information) frame.       

   The five above fields are the standard Ethernet MAC and LLC header fields. 
   Two following fields are specific to the logical ring.
         A SNAP 5-byte header field introducing a new Ether-type, i.e. a collision-free Ethernet ring  36 .   A 1-byte token field  38  to help manage the circulation of the token (however, this is not mandatory), and which takes a default HEX value of 00.       

   Therefore, the token is a standard Ethernet frame uniquely identified through its SNAP field  36 . Its sole possession, by a station, thus gives implicit permission to use the shared transmission medium on which functional frames can be placed before the token is passed to the next station in sequence. 
   Hence, logical ring  18  may be thought of as a list of stations pertaining to the ring. Actually, each station needs only to hold a record of the next and previous station identifiers in the form of their MAC addresses. Thus, when a station like station  10  has the token, it is allowed to transmit functional frames destined to another station (if it has indeed something to transmit) while holding the token. The transmission is achieved by placing the functional frames on the shared transmission medium  16  irrespective of the mode of propagation (unicast, multicast or broadcast) so that the receiving station(s), which, are listening, can catch it. At completion of transmission, station  10  that currently holds the token, must forward it  22  through the shared transmission medium, to the next station  12  in sequence of the logical ring, using the MAC address. 
   One aspect of the invention is to determine for each station the credit of data bytes this station may transmit whenever it receives the token which is circulating from station to station over the logical ring. 
   Credit allocated to a station depends upon specific parameters of the station, which are:
         CIR, standing for Committed Information Rate, which is the guaranteed transmission rate in bits/second that the station can use to transmit data.   TB, standing for Transmission Burst, which is the maximum of data bytes the station may transmit on the network upon reception of the token.       

   Referring to  FIG. 3 , at the initial time, the station initializes three variables (step  40 ).
         C: Credit, which is equal to the number of data bytes the station is allowed to transmit upon reception of the token;   TRTT: Token Round Trip Time, which is the time in seconds taken for the token to circulate around the logical ring since the preceding reception of the token by the station; and   T: which is the time when the token is received by the station.       

   Accordingly, C is set to TB, which is the absolute maximum of data bytes which may be transmitted; TRTT is set to 0, since the station does not know a preceding time of a token reception; and T is set to the clock time. 
   After the initialization of the station parameters, the station waits for the token (step  42 ) and loops back as long as the token is not received. Upon receiving the token, the station notes the reception time T R , and sets the variable TRTT to T R −T (representing the last token round trip time) and sets the variable T to T R  (step  44 ). 
   Then, the processing unit of the station computes (step  46 ) an intermediary variable as follows:
 
 C   1   =C +( CIR )( TRTT )/8
 
which means that the credit of the station can be increased by a number of bytes which is proportional to the token round trip time of the token from the preceding reception by the station. At this step, it is checked whether the value of C 1  is greater than TB (step  48 ). If so, C is set to TB (step  50 ) since TB is the maximum of bytes which are allowed to be transmitted. If not, the value of C is set to C 1  (step  52 ).
 
   Finally, the station transmits its data frames over the network and decrements its credit C (step  54 ) according to the substeps represented in  FIG. 4 . 
   First of all, it is determined whether the station has at least one frame to be transmitted (step  60 ). In such a case, it is checked whether the size FS of the frame to be transmitted is less than or equal to the credit C (step  62 ). If so, the frame is transmitted (step  64 ) and the credit is decremented and set to the new value:
 
 C=C−FS (step 66)
 
   Then, the process loops back to the step of determining whether there is a frame to be transmitted (step  60 ). The process is repeated as long as there is a frame to be transmitted and if there is a credit sufficient to transmit the frame. When there is no frame to be transmitted or if the size of the frame exceeds the credit C, the process of frame transmission is ended (step  68 ). 
   It must be noted that the two parameters CIR and TB are specific to each station and therefore are predefined for the station. Thus, on a 10 Mbps Ethernet LAN, a server could be configured with a CIR=1 Mbps whereas the other stations could be configured each with CIR=100 Kbps. But, the sum of all configured CIR should not exceed the bandwidth on the network in order to guarantee the CIR for each station and not to oversubscribe the network. 
   Likewise, the predetermined maximum value of TB is configurable on each station. This parameter should allow a station that has not transmitted for a certain time to transmit several frames with only one token. However, the value of TB must not be too high to avoid generating a peak of traffic. A good value for TB is 3×1500 bytes (1500 bytes corresponds to the MTU on Ethernet) which allows the transmission of 3 large Ethernet frames.
A method of bandwidth management over a transmission network comprising a plurality of stations forming a logical ring that circulates a token from station to station. Each station is allowed to transmit data to other stations over the transmission network when it receives the token. The station is allowed to transmit C bytes, where C is a credit. The credit increases in proportion to the time spent since the preceding reception of the token by the station.