Patent Application
20080025339
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention that together with the description serve to explain the principles of the invention, wherein:
Reference will now be made to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The present invention is based on statistical multiplexing to exploit silence intervals found in VoIP data. Although statistical multiplexing approaches are well known, they may be subject to drawbacks, such as no guaranteed quality of service provision. Hence, an embodiment of the present invention provides a novel method that provides for the bandwidth efficiency of statistical multiplexing, preservation of quality of service and low signalling overhead.
Inter-clique contention is solved by assigning non-overlapping time windows to each clique 102. A clique's window duration is dynamically adjusted based on the transmission needs of the stations in that clique. In an embodiment of the present invention, required signalling is kept to a minimum and all signalling takes place among the stations and possibly server 110. Thus, no modification to access point 112 is required.
An embodiment of the invention provides a system map information that defines the information specifying which stations 104-108 belongs to a specific clique 102, along with the stations inter-frame spacing values, and the ordering of cliques 102. In the station map information, the station with the lowest inter-frame spacing value is referred to as “first” station of the clique. Conversely, the station with the highest inter-frame spacing value is referred to as “last” station of the clique.
A cycle in the system map information denotes the sequence of clique windows, wherein the cycle for: clique 102a, clique 102b, . . . , clique N denotes that window for clique 102a is first, the window for clique 102b is next and so on. The inter-frame spacing values are aligned to the start of the clique window. Furthermore, if the data has a periodicity requirement of X msec, where for example if X is equal to 20 msec for VoIP, then there is at least one cycle every X msec. A request-to-Send (RTS) signal from station Y, denoted by RTS(Y), shows that a request-to-send signal is transmitted to station Y. A clear-to-send (CTS) signal to station Y, denoted by CTS(Y), shows that a clear-to-send signal is addressed to station Y.
When a station, for example station 104a, joins the system or when it is going to start data transmission, station 104a obtains the system map information by querying a predefined address. The query is processed by SMI server 110, which returns the system map information. If there are existing cliques 102 in the system map information, station 104a determines if it can join clique 102a by listening to the request-to-send/clear-to-send signalling of all stations in each existing cliques. In an embodiment, if station 104a can hear the RTS(Y) and CTS (Y) of all the stations 104b-104x in clique 102a, station 104a will join clique 102a. If there are multiple “qualified” cliques, that is a clique where station 104a can hear the request-to-send/clear-to-send signalling of all stations in the clique, station 104a chooses of one of the qualified cliques. When station 104a joins, for example, clique 102a, station 104a contacts SMI server 110 to obtain an inter-frame spacing value. SMI server 110 assigns an inter-frame value that does not coincide with any of the existing inter-frame spacing values. SMI server 110 assigns the lowest available/unassigned inter-frame spacing value. It should be noted that depending on how inter-frame spacing values are managed, station 104a may or may not be the new “Last” station of clique 102a. SMI server 110 then distributes an updated system map information, including station 104a and its inter-frame spacing value, to all the stations 104-108.
Continuing with our example of station 104a, if there are no such existing qualified cliques where station 104a can hear the request-to-send/clear-to-send signalling of all stations in the clique, station 104a creates a new clique. To create a new clique, station 104a contacts SMI server 110 to obtain an inter-frame spacing value. Similar to when station 104a joins an existing clique, SMI server 110 assigns an inter-frame value that does not coincide with any of the existing inter-frame spacing values. Specifically, SMI server 110 assigns the lowest possible inter-frame spacing value among the ones that are currently unassigned and SMI server 110 distributes the updated system map information, including station 104a, to all existing stations.
Cliques 102 may need to be updated, if hidden terminal conditions change in the cliques over time, for example, due to mobility. If station 106a hears a CTS(106b), but did not hear RTS(106b) for station 106b in the same clique 102b, station 106a may decide to reinitiate its clique determination procedure by listening for the RTS(106) and CTS(106) of all stations in clique 102b. If station 106a cannot hear the RTS(106) and CTS(106) of all stations in clique 102b, station 106a may decide to leave the system.
When a station leaves the system or when it stops data transmission, it notifies SMI server 110, which updates the system map information and distributes the updated system map information to all stations 104-108. The inter-frame spacing value assigned to the leaving station, for example 106a, is returned to a pool of unassigned inter-frame spacing values. If the leaving station was “first” station in the clique, the next ranked station becomes the new “first” station. If the leaving station was “Last” station in the clique, the preceding ranked station becomes the new “last” station.
A station that has data to transmit will listen for clear-to-send signal of the “first” station of the clique to which the sending station belongs and start its inter-frame spacing timer at receipt of that clear-to-send signal. A station that is first station of its clique and that has data to transmit will listen for clear-to-send signal of the “last” station of the preceding clique. When that clear-to-send signal of the “last” station of the preceding clique is heard, the first transmitting station in the next clique issues a request-to-send signal. In an embodiment of the invention, the first station of the first clique will listen for the clear-to-send signal of the “last” station in the last clique. In one embodiment of the invention, the first station sends the request-to-send signal right away. In another embodiment of the invention, the first station sends the request-to-send signal after a predetermined amount of time to provide time for other traffic.
At its normally scheduled time, as determined by its inter-frame spacing value, a station that is the last station of its clique transmits its data and issues a request-to-send signal. If the station that is the last station of its clique does not have data, it issues only a request-to-send signal.
If a clique is a single member clique, meaning that there is only one station in the clique, then to avoid confusion for the cliques that follow the single member clique on how to interpret the clear-to-send signals addressed to the station in the single member clique, stations of the clique following the single member clique start transmission of a request-to-send signal only after hearing two clear-to-send signals addressed to the station of the single member clique. To address the scenario where the station in the signal member clique has failed, without updating the SMI map, the stations in the clique following the single member clique start a timer after the last clear-to-send signal that is send on the medium. After expiration of the timer, if there is no activity on the medium, the stations in the clique following the single member clique may start transmission of a request-to-send signal.
As noted above, a new station listens for request-to-send signals and “clear-to-send signals” of all stations in each clique to determine which cliques are “qualified” cliques. Thus, request-to-send and clear-to-send signals must be issued from other stations, which are not “first” or “last” stations of a clique, to allow a new station to determine whether nor not to join a clique, and also to enable cliques updates. Various approaches may be implemented for enabling the other stations to listen for request-to-send signals and clear-to-send signals. For example, each station may issue a request-to-send signal every N cycles for cycle i, i+N, i+2N, etc. where i is chosen randomly by the station. In another example, a station may issue a request-to-send signal upon request from server 110. Server 110 runs its algorithm for issuing the request.
In the basic form described above, failure of a “Last” or “First” station will have a system wide impact. To address the single failure point issue, if a failure occurs at the “First” station, such that no clear-to-send signal is heard for the first station, the next ranked station in the clique takes over and issues a request-to-send signal. If failure is confirmed after m cycles, server 110 updates the system map information as if the failed station left the system. If, however, failure occurs at the “Last” station, such that no clear-to-send signal is heard for the last station, the First station of the next clique takes over and issues a request-to-send signal. If failure is confirmed after m cycles, server 110 updates the system map information as if the failed station left the system. Server 110 is assumed to be reliable enough and as such is implemented with the appropriate amount of redundancy. However, an additional fallback is for the stations 104-108 to operate without communicating with server 110, to overcome the case when stations 104-108 cannot reach server 110.
The present invention has the advantages of both contention free and low overhead mechanisms. It eliminates contention and therefore the cost of collision and retransmission attempts, while providing guaranteed delay quality of service with virtually no signalling overhead. Therefore, the system can operate very close to its maximum capacity. The present invention also overcomes the hidden terminal problem and it does not require modifications to the access points. Furthermore, because contention is eliminated, this present invention is also very relevant for the WiFi mesh case. It is known that contention based access can result in serious throughput issues in mesh case. Although there can still be a hidden terminal problem if non-enhanced terminals coexist in the system, this problem may be addressed by assigning separate channels to non-enhanced terminals.
It should be appreciated by one skilled in art, that the present invention may be utilized in any device that implements the network availability information described above. The foregoing description has been directed to specific embodiments of this invention. It will be apparent; however, that other variations and modifications may be made to the described embodiments, with the attainment of some or all of their advantages. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
