EEL 6591 Wireless Networks Mobility Management Wireless Information Networking Group (WING) EEL 6591 Wireless Networks Mobility Management

Outline Handoff management Location management handoff strategies Wireless Information Networking Group (WING) Outline Handoff management handoff strategies Location management location update paging call delivery

Mobility Management General network architecture Wireless Information Networking Group (WING) Mobility Management General network architecture

Wireless Information Networking Group (WING) Handoff Basics The ultimate purpose is to support the call without any interruption Performance metrics for design call blocking probability handoff blocking probability call dropping probability handoff rate duration of interruption delay handoff probability

Handoff Basics Hard handoff Soft handoff Softer handoff Wireless Information Networking Group (WING) Handoff Basics Hard handoff MS connects with one BS only at any time instant Break before connect common in TDMA systems Soft handoff MS can use multiple BSs for transmit/receive similar to diversity technique mostly for CDMA systems Softer handoff handoff between sectors in one cell (channel handoff)

Handoff Initiation Relative signal strength Wireless Information Networking Group (WING) Handoff Initiation Relative signal strength choose the BS with the strongest signal at all times disadvantage: too many unnecessary handoffs Relative signal strength with threshold switch only if the received signal drops sufficiently weak (a threshold is used) and the other BS has stronger received signal P1<Ph and P2>Ph, switch to BS2 ping-pong effect: switch back and forth

Handoff Initiation Relative signal strength with hysteresis Wireless Information Networking Group (WING) Handoff Initiation Relative signal strength with hysteresis switch only if the new BS is sufficiently stronger than the old BS P2- P1>h, switch BS2 Relative signal strength with hysteresis and threshold switch only if the current signal level drops below a threshold and the target BS is stronger than the current BS by a hysteresis margin (h) Prediction techniques switch based on the future value of received signal

Handoff Initiation Two approaches for averaging Wireless Information Networking Group (WING) Handoff Initiation Two approaches for averaging window averaging: using window size w sk = mk+mk-1+…+mk-w+1=sk-1 +mk-mk-m mk=sk/w leaky-bucket integration (exponential average) sk=a sk-1+mk, a<1 the forgetting factor more general approach: a(z)sk=b(z)mk (filtering) smoothing technique overcome certain irregular and fast variations

Handoff Strategies Link transfer types (types of handoffs) Wireless Information Networking Group (WING) Handoff Strategies Link transfer types (types of handoffs)

Handoff Strategies Mobile-controlled handoff (MCHO) Wireless Information Networking Group (WING) Handoff Strategies Mobile-controlled handoff (MCHO) popular in low-tier system, such as PCS MS monitoring the strength and quality of signal from serving BS, when some handoff criteria are met, the MS checks the “best” candidate BS for a traffic channel and launches a handoff ongoing measurements and processing of measured data triggering decision searching for new channel execution of automatic link transfer

Handoff Strategies-MCHO Wireless Information Networking Group (WING) Handoff Strategies-MCHO

Handoff Strategies Network-controlled handoff (NCHO) Wireless Information Networking Group (WING) Handoff Strategies Network-controlled handoff (NCHO) BS monitors the strength and quality of signal from MS, when signal is weak, the network arranges for handoff (assigns a BS) BS asks surrounding BSs to also measure signals from MS and report the measurements MSC issues commands for signal strength measurements while the serving BS supervises the quality of all connections (no direct link between BSs)

Handoff Strategies Mobile-Assisted Handoff (MAHO) a variation of NCHO Wireless Information Networking Group (WING) Handoff Strategies Mobile-Assisted Handoff (MAHO) a variation of NCHO network asks MS to measure the strength and quality of signals from surrounding BSs (pilot signals) and to report them back to serving BS so that handoff decision can be made GSM is using MAHO: efficient for TDMA system One further twist: all measurements can be used to predict the location of the mobile

Handoff Strategies-NCHO/MAHO Wireless Information Networking Group (WING) Handoff Strategies-NCHO/MAHO

Handoff Failures Reasons for handoff failures Wireless Information Networking Group (WING) Handoff Failures Reasons for handoff failures No channel is available at target BS handoff is denied by the network for lack of resources: available resource is less than the requested, or rerouting fails it takes too long to setup the handoff after handoff initiation target link fails in some way during the execution of handoff

Intersystem Handoff Handoff between two MSCs (dashed: signaling) Wireless Information Networking Group (WING) Intersystem Handoff Handoff between two MSCs (dashed: signaling)

Wireless Information Networking Group (WING) Mobility Management Location management or location tracking: keeping track of mobile users in the system Two-level hierarchy Home Location Register (HLR): a database storing mobile users’ subscription information directory service profile information current information validation period Visitor Location Register (VLR): temporarily storing subscription for mobile users who visits the area

Mobility Management Two basic operations Wireless Information Networking Group (WING) Mobility Management Two basic operations Registration (location update): a process that a mobile user informs the system about its location update the location information in HLR location info: the VLR’s ID the MS is currently visiting Paging (location tracking): a process that the system locates the MS MSC associated with the VLR for the MS directs all BSs to broadcast a message for the MS paging procedure sequential paging parallel paging selective paging

Registration Process VLR-HLR-VLR signaling Wireless Information Networking Group (WING) Registration Process VLR-HLR-VLR signaling

Call Delivery Procedure Wireless Information Networking Group (WING) Call Delivery Procedure Call delivery or call termination

SS7 Signaling for Roaming Management Wireless Information Networking Group (WING) SS7 Signaling for Roaming Management Common Channel Signaling (CCS), e.g., SS7 The signaling network (SS7) is overlaying over the wireless networks to handle the control signaling exchanges

Location Update Schemes Wireless Information Networking Group (WING) Location Update Schemes Registration area (RA) or location area (LA): a group of cells for location management purpose Registration is needed when the MS is power on/off the MS moves out of the registration area active location update is used De-registration is sometimes performed if MS moves out, the memory at the old VLR will be reclaimed back through de-registration

Location Update Schemes Wireless Information Networking Group (WING) Location Update Schemes Three dynamic location update schemes time-based: periodic location update movement-based: an MS performs a location update whenever it makes a predefined number of movements across cell boundaries (can be detected by MS using BS switches) distance-based: an MS performs a location update when its distance from the cell where it performed the last location exceeds a predefined value Performance: distance-based>movement-based >time-based

Wireless Information Networking Group (WING) Terminal Paging Paging under delay constraints: choose a paging scheme within the delay constraints, selective paging may be used paging all cells at the same time: delay is the minimum, but too much paging traffic paging one cell at a time: minimum in signaling traffic, but delay is too long selective paging: divide the cells in the paging area into groups, then page each group at a time, this can control the delay and the signaling traffic

Tradeoff of Signaling Traffic Wireless Information Networking Group (WING) Tradeoff of Signaling Traffic Observations more frequent location updates shrink the uncertainty region of MS location, which leads fewer paging cost less frequent location updates result in larger uncertainty region, which leads to more paging cost Tradeoff analysis minimize (location cost+paging cost) some papers by Ho, Akyildiz & Lin (‘97), Li et al (‘00), Fang (‘01)

Reduction of Signaling Traffic Wireless Information Networking Group (WING) Reduction of Signaling Traffic Observation: signaling traffic mostly occurs between MS and HLR, HLR is the bottleneck! Idea: reduce the signaling traffic to HLR Dynamic hierarchical database architecture add a new level between the two-level mobility data base architecture in IS-41 or GSM MAP: Directory Register (DR) storing location pointer Three types of DR local pointer: at local DR pointing to the current MSC of MS direct remote pointer: at remote DR pointing to current MSC indirect remote pointer: at remote DR pointing to current DR

Reduction of Signaling Traffic Wireless Information Networking Group (WING) Reduction of Signaling Traffic Per-user location caching caching the MS and location binding at STP (Signal Transfer Point): (M,VLR), VLR is the recently known location of M cache hit means signal traffic saving (no need to query HLR) User profile replication user profiles are replicated at selected local mobility databases when call arrives, such databases will be searched first

Reduction of Signaling Traffic Wireless Information Networking Group (WING) Reduction of Signaling Traffic Pointer forwarding Instead of reporting a location change to HLR every time an MS moves out a LA, a pointer is setup at the old VLR, pointing to the new VLR A threshold is set in order to avoid long chain of pointers Local anchoring A VLR close to the MT is selected as its local anchor, location changes are reported to local anchor instead of HLR

Reduction of Signaling Traffic Wireless Information Networking Group (WING) Reduction of Signaling Traffic Two location algorithm (TLA) in mobility databases (HLR/VLR), use two entries in location information field: current VLR and previous VLR location update: done only when the new VLR is NOT the two VLRs in the location information entry Similar to the one-step pointer forwarding

Location Prediction (Tracking) Wireless Information Networking Group (WING) Location Prediction (Tracking) Location profiling it is per-user-based, home-way-work e.g., 7:30-8:30am, on the way, 8:30am-12:00pm, at work, 12:00-1:00pm, lunch, 1:00-5:30pm, at work, … probability distribution model: a user is at place i with probability p(i) Use the past history or measurements Measurements (power) from at least three BSs can determine the MS’s location Kalman’s filtering technqiues can be used to track a user (Liu, Bahl and Chlamtac)

Location Prediction (Tracking) Wireless Information Networking Group (WING) Location Prediction (Tracking) Two-level pointer forwarding scheme add another level of database between HLR and VLR mobility anchor (MA): a group of RAs form a regional area of interest handled by an MA, which acts as a “local HLR” At MA level, we form a chain of pointers while the original pointer is used 3G systems are a natural three-level hierarchy: HLR-GLR-VLR GLR: gateway location register

Geolocation FCC mandate Wireless Information Networking Group (WING) Geolocation FCC mandate E-911 service: wireless systems should make 911 service possible--quickly locate an MS at reasonable time with reasonable accuracy Geolocation study: locate a user in a system GPS power levels angle of arrival (AOA) time difference of arrival (TDOA) a special issue on IEEE Communications Magazine

Further Reading Pollini’s paper (course webpage) Wireless Information Networking Group (WING) Further Reading Pollini’s paper (course webpage) Lin & Chlamtac, Chap. 2, 3, 4 Lin’s paper in Wiley International Journal on Wireless Communications and Mobile Computing A paper by Ma and Fang in IEEE Transactions on Mobile Computing (to appear)