Multiplex communications – Channel assignment techniques – Adaptive selection of channel assignment technique
Reexamination Certificate
1999-03-03
2003-08-26
Vincent, David (Department: 2732)
Multiplex communications
Channel assignment techniques
Adaptive selection of channel assignment technique
C370S447000
Reexamination Certificate
active
06611529
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a contention scheme for access to a transmission medium.
BACKGROUND OF THE INVENTION
Access to a shared medium in local area networks such as Ethernet® networks is typically realized by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) protocol. This protocol provides for a station on the network to wait for the medium to become idle before transmitting its data packet. If more than one station transmits at the same time, a collision occurs. Upon detecting a collision, the transmitting stations abort their transmissions and transmit so-called jam signals, each jam signal being of equal duration. After transmitting the jam signals, the stations enter a so-called backoff mode in which each station waits for a randomly chosen time interval before attempting retransmission. Because the waiting time is randomly chosen, the probability of a second collision is reduced.
Such contention-based networks were originally developed primarily for non-real-time applications such as data. More recently, they have been employed to support real-time applications such as voice and video. However, using contention-based networks to carry real-time traffic may be problematic because of such networks' inability to guarantee a maximum, or bounded, delay service and thus their inability to guarantee a desired level of service quality for real-time applications. As a result, techniques have been developed that give priority to applications with real-time constraints.
One such technique is taught in our co-pending U.S. patent applications (Ser. No. 08/792,327 now U.S. Pat. No. 6,014,037 filed Jan. 30, 1997 and entitled “A Wireless Lan Distributed Access Procedure Providing Priority For Voice Transmissions”; Ser. No. 08/923,301 now U.S. Pat. No. 6,359,899 filed Sep. 04, 1997 and entitled “Priority Access For Real-Time Traffic In Contention-Based Networks”; Ser. No. 08/923,302 now U.S. Pat. No. 5,982,779 filed Sep. 04, 1997 and entitled “Priority Access For Real-Time Traffic In Contention-Based Networks”). Specifically, upon transmitting a packet and detecting a collision with existing network traffic, a real-time station contends for access to the medium by transmitting a so-called blackburst signal for a predetermined duration of time. That predetermined duration of time is determined as a function of the level of access priority that it is desired for that station to have vis-à-vis other real-time stations. For example, it may be a function of the amount of time that the real-time station had to wait for the transmission medium to become idle after the data packet became ready for transmission. Thus, the real-time station that was waiting the longest has the longest blackburst signal.
This being so, the real-time station that was waiting the longest is given priority in transmitting its data packet. Having the longest blackburst, the real-time station that was waiting the longest finds that at some point during its blackburst transmission its blackburst is the only signal being transmitted on the medium—meaning that other real-time stations whose waits were shorter completed their blackburst transmissions. The real-time station thus “wins” the contention and accesses the medium to transmit a data packet thereon following the termination of the blackburst. On the other hand, a real-time station “loses” the contention if that station finds that, upon termination of its blackburst, another signal is being transmitted on the medium—meaning that another real-time station whose wait was longer is still transmitting its blackburst. The losing real-time station then waits for the transmission medium to once again become idle. Upon perceiving the medium to be idle, the real-time station transmits a packet. If yet another collision occurs, the real-time station again transmits its blackburst. However, in this instance the duration of the blackburst will be longer than before as a result of the increase in the amount of time that the real-time station had to wait to access the transmission medium. Assuming no other real-time station had to wait longer, the real-time station's blackburst is longer than other blackburst signals. Thus the real-time station finds the medium to be idle following its blackburst transmission and transmits its data packet. In this way, a station that had previously lost the contention does ultimately gain access to the medium.
In the embodiments disclosed in the cited patent applications, the transmission network is configured as a bus. However, often the network is a so-called star network in which one or more stations are connected to a “hub”. A hub is a device that serves as the functional equivalent of a bus in that it communicates the signals transmitted by one station to at least one other station. In addition, if a collision occurs, the hub propagates the collision event throughout the network by transmitting a jam signal. The hub also enforces certain rules to guard against errant station behavior. In particular, the hub keeps track of the values of a number of parameters for every station, one of which being the length of each collision, L
ec
. If, for any station, the value of this parameter reaches its maximum value set by the hub, the hub “shuts down” the offending station, or “isolates” it from the network, i.e., the hub does not communicate the data transmitted by that station to any other station(s).
The above-described hub actions create problems for a station that has won a blackburst contention, referred to hereinbelow as a “winning station.”. First, the contention itself, independent of its duration, causes the hub to transmit a jam signal whereupon any data packet that the winning station transmits immediately upon winning the contention is not communicated to any other station(s). That is, another station receives the jam signal rather than the transmitted data packet. In addition, the winning station may have transmitted its blackburst signal for a significant period of time during which it collided with the blackburst signals of other real-time stations. As a result, that station may have its value for the L
ec
parameter exceed the maximum allowed by the hub and thus would be shut down by the hub. However, the station, having won the contention and unaware that it has been shut down, proceeds to transmit its packet, which, unfortunately, is never received by another station. Finally, if, for example, two stations that are participating in a blackburst contention both have their values for the L
ec
parameter exceed the maximum, both stations are shut down by the hub before either station can determine whether it has priority over the other station. As a result, when the stations are reinstated into the network, both stations will commence transmission and another contention will occur.
SUMMARY OF THE INVENTION
According to the principles of the invention, a station whose transmission is colliding with the transmission of another station and which heretofore would have transmitted a data packet immediately following its colliding transmission—as, for example, in a blackburst contention as described above—suspends transmission immediately upon termination of the collision and, if necessary, at a particular point(s) in time during the collision. The station recommences transmission after a predetermined non-zero duration of time, or “time notch”, during which the medium is idle. More specifically, that station suspends its transmission when the collision terminates and at other points in time as may be necessary to prevent the duration of its colliding transmission from causing that station's value of the L
ec
parameter to exceed the maximum allowed by the hub. To this end, a station may illustratively time the duration of the collision and insert time notches as appropriate. An alternative approach that is easier to implement for a station involved in a blackburst contention is for that station to cause the occurrence of a time notch after the transmission of every so-called black slot tha
Krishnakumar Anjur Sundaresan
Sobrinho Joao Luis
Trotter John Andrew
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