Patent Application: US-80013201-A

Abstract:
a method of handoff of a virtual channel connection of a mobile terminal in a wireless atm networks under the control of a connection manager is disclosed comprising the steps of : transmitting a sequence of handoff messages between the mobile terminal and the connection manager ; executing a sequence of operations by the connection manager for re - routing the virtual connections ; and wherein the re - routing of a downstream connection is scheduled using a timer .

Description:
the embodiment of the present invention concerns a method of performing handoff in a wireless atm network of the kind illustrated in fig1 and described above . in the described embodiment , in order to decide when handoff should take place , each mobile terminal keeps measuring the radio signal strength from access points within range and other parameters . like connection set up or release , handoff involves signalling between the terminals and the network . handoff may be backward or forward . in backward handoff , the terminal initiates the handoff signalling with the network through the current access point . in forward handoff , the mobile terminal initiates the signalling through the new access point . as will be apparent to one skilled in the art , the connection management and re - routing is carried out by a logical entity in the network called a connection manager cm . the terminal makes use of the connection manager services through signalling . to provide these services , the connection manager cm manages the network resources such as bandwidth , namespace and so on . in a distributed environment like corba , the connection manager cm can be considered as an object providing open interfaces for different connection management services . to realise the network services provided by the connection manager cm , the network resources need to be manipulated . this necessitates the access to network resources by the connection manager cm . for example , to set up an end - to - end connection between two terminals , the connection manager cm needs to set up connections across every switching node involved along the path including access points , if any and allocate the switch and access point resources required for the connection . the network nodes have interfaces for this purpose . the binding interface base defined for programmable networks in the ieee p1520 working group is an example [ 4 , 5 , 7 , 11 ]. to decide the route along which a connection needs to be set up , the connection manager cm makes use of the service of another server called a route manager rm which provides the optimal route between two terminals based the current points of attachment of the terminals . the route manager rm keeps track of the location of terminals as they may change location in a mobile network . the route information is returned as a set { a 1 , a 2 . . . a 1 } consisting of the network nodes involved in the connection . during handoff as illustrated in fig2 a crossover switch cs is first determined based on the current location cl and new location nl of the mobile terminal mt . the crossover switch cs is the switch up to which the connections from the corresponding ( connected ) terminal ct to the new and old locations of the mobile terminal mt are common . the connection from the crossover switch to the mobile terminal mt is what is re - routed . the part of the connection from the crossover switch cs to the mobile terminal mt at the new location nl is called a make segment ms ( via access point ap 2 ) and that from the crossover switch to the old location is called a break segment bs ( via access point ap 1 ). the connection re - routing involves setting up the connection along the make segment ms followed by deleting the connection along the break segment bs . the various operations involved in connection re - routing during handoff are the following : the signalling involved in backward handoff is shown in fig3 . the messages are processed either by the access points ap or connection manager cm or both and a response is sent back . only the successful cases of the signalling are shown . in backward handoff , the mobile terminal initiates the handoff signalling with the current access point whereas in forward handoff , the mobile terminal initiates handoff signalling with the new access point . in backward handoff , the handoff detachment ( from the current access point ) and the handoff attachment ( to the new access point ) follow the handoff request . the access point relays the handoff signalling messages between the mobile terminal and the connection manager cm . there are various approaches for re - routing the connections during hand - off as will be apparent to one skilled in the art such as virtual channel extension , anchor - based re - routing , dynamic crossover switch based re - routing and so on and the approach used is essentially a tradeoff between network resource usage and performance . the virtual connections ( vcs ) from the mobile terminal to the network consists of upstream ( used by the mobile terminal for transmission ) as well as downstream ( used by mobile terminal for reception ) connections . the data on these connections are called upstream and downstream data respectively . the backward handoff used in this example takes place from access point ap 1 to access point ap 2 . at time t r , the mobile terminal sends out a handoffrequest message to connection manager cm through access point 1 ( ap 1 ). connection manager cm then decides the access point to which handoff should take place and instructs this access point ap 2 to start buffering the downstream data , and then responds to the handoffrequest message . the mobile terminal then sends a handoffdetachment message at time t d through access point ap 1 and the connection manager cm responds to this . after successful detachment from access point ap 1 , the terminal sends out a handoffattachment message through access point ap 2 and connection manager cm responds to it . once the attachment is successful , the data transfer between the terminal and the network takes place through ap 2 . the three messages handoffrequest , handoffdetachment , and handoffattachment and their response or confirmation from the connection manager cm pass through the respective access points and hence the access points are aware of these events . the connection manager cm is aware of all connections that are active at the mobile terminal . the details of the connection manager cm &# 39 ; s actions ( cm processing 1 - 3 in fig3 ) in response to the messages are explained below : decide or select the access point ( new ap ) to which handoff should take place for every connection originating or terminating at the mobile terminal mt , invoke the route manager and obtain the optimal route between the corresponding terminal ct and the new access point . compute the crossover switch and the make and break segments assign connection identifiers ( i . e ., vpi / vci values ) for individual links on the make segment instruct the new access point ap to start buffering of downstream data schedule the set up of make segments of downstream connections using a timer schedule the deletion of break segments of downstream connections using a timer the processing at the access point ap 2 ( ap processing of fig3 ) is as follows : a feature of the described embodiment is that the time at which the connections are re - routed is controlled using a timer . as mentioned above , the re - routing consists of two major operation for each virtual connection that is active : ( i ) setting up the connection along the make segment ( ii ) releasing the connection along the break segment . the mobile terminal mt will be sending application data through access point ap 1 until it sends out the handoffdetachment message at time t d . the upstream data in transit will go through the virtual connection before the break segment of this connection is deleted by connection manager cm ( connection manager cm processing - 2 ). from time t d onwards , the upstream data is stored in the mobile terminal mt , until the attachment to ap 2 is completed . the earliest time at which the make segment of the downstream connection will be set up is t ′ d . the mobile terminal mt will be receiving application data until the terminal receives the response to its handoffdetachment message . the downstream data during that period may also be buffered at ap 2 , depending on the time at which its make segment is set up . once the mt gets attached to ap 2 , the upstream as well as downstream data transmissions resume . though ap 2 is instructed to start buffering the data after receiving the handoffrequest message by connection manager cm , the actual number of atm cells buffered at ap 2 depends on the time elapsed from the instant at which the make segment of the downstream connection is set up till the time at which the attachment to ap 2 takes place . 1 . set up the make segment of a downstream connection after startbuffering instruction is given to ap 2 to make sure that buffers have been allocated 2 . carryout the deletion of the break segment and set up of the make segment of a downstream connection after receiving the handoffdetachment message from the mobile terminal . 3 . delete the break segment of an upstream connection before setting up the make segment of the upstream connection , to avoid virtual connection merging . since , the re - routing of an upstream connection is well controlled , neither cell loss nor cell duplication occurs in this connection . however , the probability of cell loss or duplication in a downstream connection is high depending on the time at which the make segment is set up . after receiving the handoffdetachment message , the break segment of a downstream connection is deleted at time t = t ′ d + t . the value of t is chosen in such a way that the downstream data in transit will be completely received by the mt before receiving the response to its handoffdetachment message . as shown in fig4 if the make segment of the downstream connection is set up before deleting the break segment of the downstream connection , then cell duplication occurs and if it is done after deleting its break segment , cell loss occurs . when set up and deletion both occur at time t , this results in zero cell loss and zero cell duplication . the amount of cell loss / duplication depends on the set up time as well as the bandwidth and other qos parameters associated with the virtual connection . for example , the behaviour of two virtual connections , vc 1 and vc 2 , having different bandwidths is shown in the fig4 ( vc 1 is having higher bandwidth compared to vc 2 ). if the application using a virtual connection needs strictly zero cell loss , the set up should happen at time t . if the application can tolerate cell loss up to level c , then the set up can be done by time t 1 for vc 1 and time t 2 for vc 2 . on the other hand , if the application cannot tolerate cell duplication above level c , then the set up has to be done only after time t ′ 1 for vc 1 and time t ′ 2 for vc 2 . the set up is thus controlled using a timer with a timeout value depending on the qos and other parameters . this way , the described embodiment can minimize the buffer requirements at ap 2 , by exploiting the application requirements . using the timer based mechanism , the make segments of different virtual connections can be set up at different times depending on the requirements of the applications . this way , the buffering at access point ap 2 can be optimised . once the connection manager cm receives the handoffdetachment message , a timer is started with a time out value x . the make segment set up takes place when the timer expires . the timeout value can be decided based on the application requirements . for example , if the application using the virtual connection can neither tolerate cell loss nor cell duplication , then the time out value should be exactly equal to t , in order to coincide with the deletion of the break segment . on the other hand , if an application cannot tolerate any cell loss but can tolerate some amount of cell duplication , then the timeout value should be less than t . if the application is able to tolerate cell loss , the set up can be scheduled for a time after t , depending on the amount of cell loss the application can tolerate . in general , the timeout value will be x = min ( d , d ) where d is the time elapsed between receiving the handoffdetachment message and the handoffattachment message ( see fig3 ), d is the time out value decided based on the application characteristics which can be found out by experimentation . when there are multiple number of virtual connections , each can use an appropriate value of d . in this case , the make segment set up occurs by time d . this need not be the case always and one may use other values as well . if the connection set up procedure experiences a delay of s , then the timeout value should be x - s . the value of d can be determined based on the application characteristics , and the quality of service guaranteed for the virtual connection . this information can be conveyed to the connection manager cm in the handoffrequest message so that the timer can be programmed in advance . if there are n nodes ( i . e . switches and access points ) involved in the make segment , to have a more precise control over the set up time , the connection at the cross - over switch can be set up at the timer controlled instant and the connections at the remaining ( n − 1 ) nodes can be set up in advance immediately after receiving the handoffdetachment message . in this case , the atm cell data flow on the make segment will take place only after the connection at the crossover switch cs has been set up . in practice , the mobile terminal will continue to receive downstream data for a short period even after receiving the response to its handoffdetachment message . this can continue until the mobile terminal switches the radio frequency ( rf ) carrier from ap 1 to ap 2 . the described embodiment can exploit this fact in scheduling the virtual connection re - routing . as a result , the deletion of the break segment and the set up of the make segment can be delayed further , based on an estimate of the time taken by the mobile terminal to switch the rf carrier . as a result , the buffering at ap 2 can be further reduced . although the embodiment described uses backward handoff , the invention is equally applicable to rerouting connections using forward handoff as will be apparent to one skilled in the art upon reading this specification . although the embodiment described has illustrated the technique of the present invention applied to virtual channel connections to / from a mobile terminal , the technique is equally applicable to rerouting virtual path connections to / from the mobile terminal . the following references and patents are background reading for the skilled man , although not strictly necessary for the understanding of the invention and are incorporated herein by reference : 1 . r . r . pillai , w . wang , l .- k . seng , b . jose , he sha , r . agrawal , and m . ranganath , “ implementation and performance evaluation of an open control architecture for wireless atm networks ,” proceedings of 1999 ieee second conference on open architectures and network programming ( openarch &# 39 ; 99 ), mar . 26 - 27 , 1999 , new york , pp . 45 - 58 . 3 . biswas et al ., “ the ieee p1520 standards initiative for programmable network interfaces ” ieee communications magazine , october 1998 , pp . 64 - 70 ( also see http : l / www . ieee - pin . org ) programming interfaces for networks — atm switch service interface , ieee / wg p1520 / ts / atm - 018 , new york , mar . 28 - 29 , 1999 . programming interfaces for networks — proposal for standardising the qgsmp protocol , ieee / wg p1520 / ts / atm / 002r1 princeton , jan . 18 - 19 , 1999 programming interfaces for networks — proposal for standardization of atm binding interface base for wireless access , ieee / wg p1520 / ts / atm011 , new york , mar . 28 - 29 , 1999 . programming interfaces for networks — proposal for standardization of u - interface objects for atm , ieee / wg p1520 / ts / atm - 012 , new york , mar . 28 - 29 , 1999 . 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