CMPE 257 Spring CMPE 257: Wireless and Mobile Networking Spring 2002 Week 9

CMPE 257 Spring Announcements Project status update. Midterm grading. Project demos and poster session. Demos: May 31 – June 3. Poster session: June 4. Project deliverables. Well-documented code. README file explaining the contents of each file, etc. Short write-up (3-5 pages) describing architecture, code structure, demo, lessons learned, etc.

CMPE 257 Spring Today Location Management.

CMPE 257 Spring Why is location management needed? In wired networks, hosts don’t move. Constant association between host (id, address) and its location. In mobile wireless networks, hosts can move. Host id/address no longer provides location information. Need location tracking mechanism to deliver information destined to host.

CMPE 257 Spring Location databases Home Location Register (HLR). Visitor Location Register (VLR).

CMPE 257 Spring Home location register (HLR) One HLR for each network. User permanently associated to a HLR. HLR keeps user profiles for each user. Location information. Services subscribed. Billing information. User profiles can be centralized in HLRs. Problems?

CMPE 257 Spring Visitor location register (VLR) VLR stores information about MHs visiting the area. Number and placement of VLRs vary. One per network. Tradeoffs?

CMPE 257 Spring Location lookup When A wants to communicate with B, A needs to find B’s location. Response time is important! In HLR-only systems: A queries B’s HLR. IN HLR-VLR systems: Query local VLR first.

CMPE 257 Spring Location management Standards: Interim Standard 41 (IS-41). North America. GSM MAP. Europe. Both use 2-level location database hierarchy

CMPE 257 Spring Location management tasks Location registration. Call delivery.

CMPE 257 Spring Location registration Update location databases (HLR and VLRs). MH authentication when location info available.

CMPE 257 Spring More on location registration… MH performs location update. Coverage area split into registration (location) areas (RAs or LAs). RA consists of several cells under same MSC. VLR covers a number of RAs.

CMPE 257 Spring Location registration procedure MH moves to new cell and sends location update to new BS. BS informs MSC which contacts VLR. VLR updates user profile for MH. If new RA belongs to same VLR, update profile with new RA info. Else, VLR contacts MH’s HLR and updates MH’s HLR’s location information. HLR authenticates MH and sends ACK to new VLR; HLR also de-registers MH with old VLR Old VLR sends an ACK.

CMPE 257 Spring Call delivery 2 steps: Finding current VLR. Locating the MH current cell.

CMPE 257 Spring Call delivery procedure Caller contacts callee’s MSC through nearby BS. MSC finds address of callee’s HLR and contacts HLR. HLR finds callee’s current VLR and MSC. Connection is set up between caller and callee’s MSC. Polling to find where callee is within RA (paging). Callee responds.

CMPE 257 Spring Issues [Akyildiz96] Location registration and call delivery are expensive. Signaling traffic.

CMPE 257 Spring Solutions Still keeping the 2-level database model. Caching MH’s VLR. Pointer forwarding: setting pointers from previous VLR to new VLR. Local anchor: use “nearby” VLR to receive location updates from MH. HLR keeps pointer to local anchor. “Localizes” signaling traffic.

CMPE 257 Spring Solutions (Cont’d) Deeper hierarchies. Tree of location databases. Leaf databases contain information on local MHs. Higher level databases contain pointers (MH id+database id) to next lower level database storing profile or pointer to lower level. In the worst case, query travels all the way to root, down the appropriate subtree. Partitioned databases. Groups of location databases. No location update if MH moves within same partition.

CMPE 257 Spring HiPER [Jannink97] Life-long numbering. Same number irrespective of provider/location. Hierarchical location database. No concept of a “home site” (HRL/VRL). Leaf databases store user profiles in a zone. Higher-level databases store pointers to lower- level. Root stores pointer to every user. Scalability?

CMPE 257 Spring Locating a user… When A calls B, query for B’s location propagates up the hierarchy from A’s zone to first database containing pointer to B; then, down that subtree. Drawbacks? When a user moves, its record is sent to the appropriate leaf database; databases along the way to the least common ancestor for old and new zones are updated. How expensive is this?

CMPE 257 Spring Replication Replicate user profile at other databases in the hierarchy. Tradeoff: lookup latency versus update and storage cost. “Lazy” consistency. Where to replicate? Locality of calls and mobility. Also use intermediate nodes in the tree.

CMPE 257 Spring Replication parameters Number of replicas. Bound on storage requirements and/or number of updates. Replication cost-benefit. Local call-to-mobility ratio (LCMR). Benefits: number of local calls to user. Cost: number of moves during given time period. R i,j = C i,j /M i. Min and max threshold.

CMPE 257 Spring More parameters… Highest hierarchical level user profile may be replicated. If site j replicates user i’s profile, all ancestors of j will also be replicas. L sets upper bound on replication level.

CMPE 257 Spring Off-line replica allocation Inputs: database topology and estimated LCMR. Output: replication plan which is sent to the databases. 1 st. phase: bottom-up traversal. For each user i, assign it to replica j if LCMR ij >= R max. If n=N, additional replicas below L with largest LCMR ij -R min assigned to user i.

CMPE 257 Spring Evaluation Implemented their own event-driven simulator (Pleiades). User movement models. User calling models. Simulations. Model of the Bay Area geography and demographics. Compared several location management techniques: HLR/VLR, centralized, caching, full replication, simple hierarchy (no replication), hiper.

CMPE 257 Spring Location Management by [Prakash96] System model: cellular architecture. Cells, base stations, MHs, location servers. Node wishing to communicate with MH needs to find MH’s location (cell). Once location is determined, info sent to BS (over wired network), who relays to MH. BS co-located with location servers.

CMPE 257 Spring Motivation Distribute location management load evenly across location servers. Avoid “hot spots”.

CMPE 257 Spring Problem statement “Which location servers will store an MH’s location?”  Single location server?  Multiple statically assigned location servers?  Multiple location servers based on MH’s location?  Multiple location servers based on location and identity.  Location servers change as MH moves.  MHs in same cell will map to different sets of servers.

CMPE 257 Spring Problem statement (cont’d) “Given an MH and its current location (given by the current BS), determine set of location servers given the MH’s id and its location.”, or h: BSxMH -> S BS. Function h determines the read set (replicas to query when trying to locate MH) and write set (replicas to update when MH moves) for MH. Multiple id’s assigned to popular MHs. Why?

CMPE 257 Spring Assigning location servers Mapping MH id to virtual id. “Cold” MHs have single id. “Hot” MHs have multiple (two) ids. Use hash function to map MH’s virtual id and its BS id to set of BSs. Quorum construction: partitioning BSs into sets. Small size (update load) and symmetric (load balancing).

CMPE 257 Spring “Active” location management Previously described systems rely on MHs to update their location. Some more recent systems take a proactive approach: detect user and figure out location (positioning).

CMPE 257 Spring Location for the Active Office [Ward97] Indoor sensor system that tracks location of: people (active badge), equipment (equipment tags), etc. Requirements: accurate (within 15cm), 3 dimensions, scalable (number of objects locatable, area covered), cost. RF communication.

CMPE 257 Spring System components Transmitters attached to every locatable object. Matrix of receiver elements in all rooms where objects are to be tracked. Controller which polls one mobile object at a time.

CMPE 257 Spring Operation Periodically, mobile node is polled. Polled mobile broadcasts signal. Controller synchronizes receivers, who listen for some time to detect the peak of mobile’s transmission. Controller polls receivers for the measured time interval between the sync signal and the signal peak (if any).

CMPE 257 Spring Distance computation Time measured by receiver composed of: time to transmit the polling signal+time to transmit pulse (function of distance being calculated)+processing time. Distance between mobile and receiver calculated. Empirically computed speed of sound in the room and service times.

CMPE 257 Spring Position calculation Triangulation using 4 receivers to determine a point in 3 dimensional space as estimate of position. In this particular set up, since all receivers are in the ceiling, only 3 distances required. Extra reported distances can be used for higher accuracy.

CMPE 257 Spring Evaluation Experiments with prototype show 95% of readings within 14cm accuracy. Even better accuracy for averaged readings. Addresses limit number of trackable objects. Large number of receivers and ultrasound nature of transmission from mobile proved to pay off regarding accuracy. Power savings mode minimizes maintenance. Low interference levels from office equipment.