Patent Application: US-66284096-A

Abstract:
a method of delievering data in a wireless communications network using a combination of collision sensing and collision avoiidance protocols . more precisely , if there are no hidden nodes detected in the network , a collision sensing protocol is used ; however , if there are hidden nodes , then a collision avoidance protocol is used . this invention also deals with methods of determining the presence or absence of hidden nodes .

Description:
in this section , we describe the algorithm which is used by every node of the network to detect the presence of hidden nodes . in this algorithm , each node of the network makes a decision regarding the use of csma or csma with rts / cts ( r - csma ) independently from other nodes . this is done by observing the frames transmitted in the network in a promiscuous way . the thrust of the algorithm is the following . if a node detects a frame with the source address s and destination address d , then s must be in range of the node and d might be hidden from the node . there are two different decision processes whether the detected node is a data frame ( packet ) or an acknowledgment frame . referring to fig3 in order to execute the algorithm , each node maintains a table ( 3 . 0 ) of lan addresses ( 3 . 3 ) which it knows about ( id 13 list ). each entry of this table has a timer ( 3 . 2 ) associated with it which is used for aging out entries . in addition , each entry i has a status attribute , s ( i ). s ( i ) can be either 1 or 0 . 1 ( or in range ) signifies station i to be in range . similarly , 0 ( or out of range ), signifies the station to be out of range . any station uses csma or r - csma mode . if all entries in the table have status 1 ( all are in range ), then the station is in csma mode . on the other hand , if any station in the table has a 0 status ( out of range ), then the station is in r - csma mode . it is important to note that this is a distributed algorithm . each node observes the frame exchanges on the lan and makes a decision on which mode of the mac to use . our assumption in describing this algorithm is that an immediate ack frame is transmitted by each station after receiving a correct packet from destination to that station and that this is done in the csma as well as in the r - csma . however , this algorithm can be implemented using the same approach when immediate hardware ack frames are not used ( to do so , the higher level in the protocol stack informs the mac layer if a packet has arrived correctly . the performance of the protcol with this approach is inferior to the one with hardware ack ). the algorithm for any node n can be described as follows . referring to fig1 after detection of a data frame , the source address ( s ) of the frame , if not already in the table , is added to the table ( 1 . 1 ). in any case , the status of s is marked as 1 ( see 1 . 2 ). ( 1 means that the network node is in range of the node n and 0 means potentially out of range ). regarding the destination address ( d ) of the frame , it is added to the table if it is not already in the table and if it is not a broadcast ( or group ) address ( see 1 . 3 ). in any case the status of d is marked as 0 ( see 1 . 4 ). one final issue is that only packets that are successfully received are used for this algorithm . the reason is that we would like to avoid identifying nodes as hidden if there is no ack transmitted due to a collision or erroneous transmission . referring to fig2 after detecting an ack frame , if the source address of the frame is not in the table , it is added ( see 2 . 1 ). in any case , the status of the source added is set to 1 ( see 2 . 2 ). in addition , if the destination address of the ack frame is not in the table , it is added ( see 2 . 3 ) and its status is marked as 0 ( see 2 . 4 ). as shown in fig3 at 3 . 2 , there is an aging timer associated to each table entry . after each entry which is added to or updated in the table , the associated aging timer is restarted . the aging timer is set to t1 . if the aging timer of an entry expires , the entry is removed from the table . the algorithm can be implemented in hardware or driver level . the cost of implementing it at the driver level is that the driver software is interrupted at every packet detection destined to any node . hence , the hardware implementation is preferable . referring to fig3 the table entries are anded ( 3 . 4 ) to get a final mode selection . the output of the and gate 3 . 4 and a signal from timer t2 is applied to a register ( 3 . 5 ) to determine the mode to be used . if the final mode is 1 , then cmsa is enabled , otherwise r - csma is used . the mode selector is used to set the protocol state machine . finally , the logic has a hysterics timer t2 which is used to avoid rapid switching between the csma and r - csma . that is , when a mode is selected , the mac protocol stays in that mode until the t2 has expired . in this approach each node takes an active role to determine the topology of the environment in which it communicates . a node , n , classifies its neighbor nodes to be a boundary node ( b ) or an internal ( i ). these classifications describe the correlation between node n and its neighbors . therefore , the detection of each boundary node corresponds to the detection of at least one hidden terminal ( node ) that n cannot communicate with or hear transmissions from . this relationship is maintained in a table ( see 4 . 0 of fig4 ), the system topology table . if it is determined that there are no hidden nodes which can communicate with a station , then that station is labeled as internal . a station , i , is internal to another station , j , if j can hear transmission of all i &# 39 ; s neighbor ( adjacent ) nodes . the presence of hidden nodes is detected through boundary nodes . hence , each boundary node has dependencies ( e . g . waiting for ack ) on at least one hidden node . based on this table all dependencies for each source entry must be eliminated before that station is marked as internal . at transmission time each node checks the status ( 4 . 1 ) of the destination in its table and dynamically transmits csma with collision avoidance ( or reservation csma , r - csma ) if the status is b , or pure csma if the destination is an i node . if the destination is not found in the table , then the node sends the packet using an r - csma scheme . the node will switch to pure carrier sensing when the destination is declared as i . there are also additional table maintenance techniques such as water mark phase out and event based table updates added to ensure entry validation and movement in the environment . watermark is a node phase out which can be described as a predefined count ( 4 . 2 ). the count is incremented or reset ( as described below ) based on the table entries and , once reached that node is dropped from the table . this is as a result of a station leaving the environment . event based updates are implemented to ensure that movement is reflected and that rules for updating the table and modifying the status are followed . referring to fig4 the system topology table ( t - table ) has four columns : nodes , 4 . 3 , ( source address of the transmitted packet ), status 4 . 1 , ( b for boundary and i for internal ), dependency 4 . 4 , { d0 , d1 , . . . , dn } destination addresses which are associated with a node , and watermark , or count ( c (. )), 4 . 2 . dependency is defined as a set of elements , which contain the address of hidden nodes . count is a watermark counter which is updated on each table access . the count is reset to zero on each update to a specific node . additionally all other watermarks for entries in the table are incremented by one . when the watermark reaches a predefined number , that entry ( s ) is removed from the table . refer to fig5 . each node monitors all packets transmitted ( except for broadcast packets ). for each packet it extracts the source , s , and the destination , d , of the packet . the source node , s , is inserted in the table , 5 . 1 , ( if it is not already there ). the status entry of s is set to b , s ( s )= b , ( 5 . 2 ). the destination , d , is added to the dependency list for s ( 5 . 2 ), d ( s )= d ( s ) u { d }, ( 5 . 2 ). this signifies s to be a boundary node ( b ), and d is added to its dependency list . if the destination d is not in the table , there is nothing more left to be done ( 5 . 3 ). the algorithm does not try to track hidden nodes . in fact , hidden nodes are purged from the table as soon as they are detected . if status of d is internal ( s ( d )= i ), then the d is purged from the table ( 5 . 4 ). the rationale is that if the packet is an ack or a cts packet , then d has most possibly moved out of sight and is purged . if the packet is a data packet or an rts , then when the ack or cts is generated ( which is done at the mac level and is very fast ), it is put back in the table as an i . if ( s ( d )= b ), then the dependency list of d is tested ( 5 . 5 ). if d ( d ) has more than one element , then s is removed from this list ( see 5 . 6 ) ( if s is not in the list , then there is nothing else to be done ). on the other hand , if d ( d ) has only one element which is s ( i . e . d ( d )={ s }), then the status of d is changed to i and its dependency list is set to null ( see 5 . 7 ). after that the dependency list of s , the source node , is tested ( see 5 . 8 ). if d ( s ) has more than 1 element , then { d } is removed from d ( s ) ( see 5 . 9 ). if d ( s ) has only one element which is d , then status of s is changed to i and its dependency list is set to null ( see 5 . 10 ). refer to fig6 . in theory , the dependency list for a table entry , s , can become very large . in fact , it contains the hidden nodes for which s is the gateway . in practice , the number of entries in d ( s ) can be limited to 2 , and it can be implemented by a shift register ( 6 . 1 ). the entries in the shift register are 0 , and 1 , with 1 being the oldest element as shown above . the function of this shift register is similar to a fifo queue . in addition , there is no duplicate entry for the same node in this queue . hence , the differences between this implementation and a typical shift register are : upon entry of new elements d into the table the register shifts to the right adjusting the content , adding d →( 0 ) [( i ) means the content of register element i ]. additionally , the elements in the fifo can be removed by shifting out or resetting the element based on the location of the elements and the operation performed on the register . there are no duplicate entries in this set . therefore , each update will check ( 0 ) and if d =( 0 ), then the entry is ignored . if d =( 1 ) then ( d ) is shifted into the register . additionally , in the removal process if element 0 was to be removed then element 1 will shift into 0 . if element 1 was the target then there is no change to element 0 . what has been illustrated are the minimum requirements . the shift register or fifo can be expanded to the nth elements , creating a linear sparse matrix . the same basic rules apply to the entry , shifting , and removal of the elements . refer to fig7 . detection of an ack frame initiates a table cleanup as follows . for every node , n , in the table ( see 7 . 1 ) which is a boundary node , i . e . s ( n )= b ( see 7 . 2 ), the elements of it dependency list ( d in d ( n )), are checked . see 7 . 3 . if s ( d )= i ( 7 . 4 ), then d is removed from the dependecny list of n ( see 7 . 5 ). if d is a boundary node ( see 7 . 4 ), then the water count of d is compared with the water count of n ( see 7 . 8 ). if d is an older entry , then this might indicate that d has moved out of the network , in which case d is removed from the table ( see 7 . 9 ). in this section , we describe algorithm iii where every node of the network detects the presence of hidden nodes . in this algorithm , each node of the network makes a decision regarding the use of csma or csma with rts / cts ( r - csma ) independently from other nodes . this is done by observing the frames transmitted in the network in a promiscuous way . this algorithm is basically the same as algorithm i : distributed algorithm for detection of hidden nodes , with the following differences : the r - csma or csma is done in a per - node manner . that is , any packet transmission is done in the csma mode if there is no hidden node from the node which is communicating to the destination of the packet transmission . if hidden nodes are present , then r - csma is used . instead of a one dimensional status table , a two dimensional table is used . the status table is a two dimensional table in which there is one row and one column associated with each node in the network and where each entry is a single bit ( see fig8 ). after a node is discovered , a row and column is added to the table corresponding to that node . let us assume that we are looking at the table of node n . the table entry [ i , j ]= 1 means that the frame transmissions of node i to j can be heard by n , and similarly , table entry [ i , j ]= 0 means that the frame transmissions of node i to j cannot be heard by n . so , to make a decision as to send frames to a node i in csma or r - csma , we need to look at all entries of the row i and column i . if there is an entry where [ i , k ]= 1 and [ k , i ]= 0 , then k is hidden from node n . however , if for all values of k , we have [ i , k ]= 1 , and [ k , i ]= 1 , then i does not communicate with any node which is hidden from n . so , to communicate to node i , all [ i , k ] and [ k , i ] entries of the table are exclusive - nored , and finally , the results are anded to get the final mode selection bit ( 1 for csma and 0 for r - csma with node i ). in fig1 , the exclusive - nor circuits 11 . 2 , operating on the corresponding entries in table 11 . 1 , and the outputs of the circuits 11 . 2 are applied to and gate 11 . 1 . the output of the and gate then determines the access mode to be used . in the following we describe the algorithm where ( s , d ) represents a frame with source s and destination d . [ s , d ] refers to the location [ s , d ] of the status table containing a single bit . refer to fig9 . after detection of a data frame , the source address ( s ) of the frame is added to the table ( see 9 . 1 ). in any case , the status of [ s , d ] is marked as 1 ( 1 means that the network node is in range of the node n and 0 means potentially out of range ). see 9 . 2 . regarding the destination address ( d ) of the frame , it is added to the table if it does not exist and if it is not a broadcast ( or group ) address , see 9 . 3 . in any case , the table entry [ d , s ] is set to 0 . see 9 . 2 refer to fig1 . after detecting an ack frame , if the source address of the frame is not in the table , it is added , see 10 . 1 . in any case , the status of [ s , d ] added is set to 1 , see 10 . 2 . in addition , if the destination address of the ack frame is not in the table , it is added and [ d , s ] is marked as 0 , see 10 . 3 and 10 . 4 . as an example , we have shown the status table for a 5 nodes ( 1 , 2 , 3 , 4 , 5 ) at node 6 . the logic diagram for the r - csma / csma with respect to node 4 is drawn in fig1 . if the output of the and gate 11 . 1 is 1 then csma is used otherwise r - csma . also , the table ( 11 . 0 ) is initialized to 0 at the beginning or when the table entries are aged out . similar to the above cases , each table entry is aged out . 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