1. 1G PERSONAL COMMUNICATION SYSTEMS: MOBILITY MANAGEMENT (PART II) Ian F. Akyildiz Broadband & Wireless Networking Laboratory School of Electrical and Computer Engineering Georgia Institute of Technology Tel: 404-894-5141; Fax: 404-894-7883 Email: ian@ece.gatech.edu Web: http://www.ece.gatech.edu/research/labs/bwn

2. IFA’2004 2 Cellular Network Architecture Location Register (Database) Mobile Switching Center MSC Backbone Wireline Network Base Station Controller Base Station Mobile Terminal Radio Network Cell

3. IFA’2004 3 Mobility Management Mobility Management Enables telecomm networks to Enables telecomm networks to –Locate roaming MSs for call delivery –Maintain connections as the MSs move between different cells Involves two operations Involves two operations –Location Management –Handoff Management Involves two types of mobility Involves two types of mobility –Terminal mobility –Personal mobility

4. IFA’2004 4 Types of Mobility Types of Mobility TERMINAL MOBILITY TERMINAL MOBILITY (Network should route calls to the MT (Network should route calls to the MT regardless of its point of attachment) regardless of its point of attachment) PERSONAL MOBILITY PERSONAL MOBILITY (Users should access the network wherever they are; UPT (Universal Pers. Tel #)) (Users should access the network wherever they are; UPT (Universal Pers. Tel #)) SERVICE PROVIDER MOBILITY SERVICE PROVIDER MOBILITY (Allow user to roam beyond regional networks). (Allow user to roam beyond regional networks).

5. IFA’2004 5 Mobility Management Location Management Location Management n Handoff Management Base Station MT A is receiving a call ! How will the network deliver the call to A ? A

6. IFA’2004 6 Location Management Call Delivery (Paging) Location Update (Registration)

7. IFA’2004 7 Cost Tradeoff Too Many Location Updates Too Few Location Updates Low Paging Costs High Update Costs High Paging Costs Low Update Costs

8. IFA’2004 8 Solution Local Areas (GSM) = Registration Areas (IS-41) Local Areas (GSM) = Registration Areas (IS-41) Registration Area Boundary Center Cell

9. IFA’2004 9 Handoff Types Intra-CellInter-Cell Soft Handoff Hard Handoff

10. IFA’2004 10 Mobility Management: Location Management Location management enables the system to track the locations of MTs between consecutive communications Location management enables the system to track the locations of MTs between consecutive communications Trade-off between the costs of location update and paging design optimal location management schemes to reduce the overall cost Trade-off between the costs of location update and paging design optimal location management schemes to reduce the overall cost LOCATION MANAGEMENT LOCATION REGISTRATION (UPDATE) CALL DELIVERY AUTHENTICATION DATABASE UPDATES DATABASE QUERIES TERMINAL PAGING

11. IFA’2004 11 Mobility Management: Handoff Management Handoff management: an MT keeps its connection active when it moves from one access point to another one Handoff management: an MT keeps its connection active when it moves from one access point to another one Four types of handoffs: Network Controlled Handoff (NCHO), Mobile Controlled Handoff (MCHO), Network Assisted Handoff (NAHO), and Mobile Assisted Handoff (MAHO) Four types of handoffs: Network Controlled Handoff (NCHO), Mobile Controlled Handoff (MCHO), Network Assisted Handoff (NAHO), and Mobile Assisted Handoff (MAHO) HANDOFF MANAGEMENT NEW CONNECTION GENERATION USER MOVEMENT NETWORK CONDITIONS RESOURCE ALLOCATION CONNECTION ROUTING DATA FLOW CONTROL INITIATION BUFFERING/SEQUENCING MULTICAST

12. IFA’2004 12 Location Management BACKBONE TELEPHONE NETWORK (HLR) Mobile Switching Center Visitor Location Register Mobile Terminal (MT) MSC VLR Local SignalingLong Distance Signaling (MSC) (VLR) Home Location Register

13. IFA’2004 13 Location Registration MT enters a new LA, and transmits location update to new BS BS forwards update to MSC, which queries VLR Does the MT have an existing record? New LA is under same VLR. VLR updates the LA ID # for the MT. Yes No VLR determines address of HLR, and sends location registration message HLR authenticates and registers MT by updating the VLR ID # for the MT. Then, HLR cancels former VLR.

14. IFA’2004 14 Location Registration BS keeps broadcasting Location Area (LA) ID#. BS keeps broadcasting Location Area (LA) ID#. MT listens to broadcast and will perform a location update when: MT listens to broadcast and will perform a location update when: –Powering up –Crossing LA boundaries –After a defined period of time

15. IFA’2004 15 Location Registration

16. IFA’2004 16 Call Delivery Incoming call for roaming MT reaches an MSC The calling MSC determines the address of the MT’s HLR, and sends a location request message to the HLR. The HLR sends a route request message to the VLR, which forwards the message to its MSC The MSC gives the MT a Temporary Local Directory Number, and forwards the TLDN back to the HLR The HLR forwards this message to the calling MSC, which sets up a route to the MT at its current MSC. Finally, the current MSC tells all of the BSs in the MT’s LA to send a polling signal to page the MT. When the MT responds, the call is connected.

17. IFA’2004 17 Call Delivery

18. IFA’2004 18 How is a Cell Phone Call made? When a mobile originates a call, a call initiation request is sent on the control channel to BS. When a mobile originates a call, a call initiation request is sent on the control channel to BS. With this request the mobile transmits its tel number (MIN; With this request the mobile transmits its tel number (MIN; Mobile ID Number), electronic serial number (ESN) and tel number of the called party. Mobile ID Number), electronic serial number (ESN) and tel number of the called party. Base station receives this data and sends it to the MSC. Base station receives this data and sends it to the MSC. MSC validates the request make connection to the called party through the PSTN and instructs base station and mobile to use an idle forward and reverse voice channel to allow conversation to begin. MSC validates the request make connection to the called party through the PSTN and instructs base station and mobile to use an idle forward and reverse voice channel to allow conversation to begin. (in AMPS -> 10-60 voice channels; one control channel in each cell base station). (in AMPS -> 10-60 voice channels; one control channel in each cell base station).

19. IFA’2004 19 Research Areas in Location Management Database Architectures Database Architectures Paging Techniques Paging Techniques Multi-network location management Multi-network location management Location Area Design Location Area Design

20. IFA’2004 20 Database Architectures Centralized Centralized –Increase database hierarchy –Cache user locations at switching points –Replicate user profiles at more than one database –Use pointers to follow a path of VLRs to the MT’s current location Distributed Distributed –Database Trees –Partitioning

21. IFA’2004 21 Paging Techniques Blanket paging Blanket paging –Paging the MS in all cells belonging to an LA simultaneously. Advantage: The delay of the response to paging is kept at a minimum. Advantage: The delay of the response to paging is kept at a minimum. Disadvantage: Paging has to be done in several cells. Disadvantage: Paging has to be done in several cells. Closest-cells first Closest-cells first –The cell where the MS was last seen is paged first followed by subsequent equidistant ring of cells. –Several rings may be polled simultaneously in a paging cycle to keep delay low. Sequential paging Sequential paging –Subsequent pages are performed in most likely locations based on past history and distance.

22. IFA’2004 22 Paging (MSC) VLR

23. IFA’2004 23 Location Area Design Tradeoff Tradeoff –Location Updates versus Terminal Paging Goal: Improvements to tradeoff Goal: Improvements to tradeoff Geographical Geographical Fixed versus Dynamic Fixed versus Dynamic User-based versus Global definition User-based versus Global definition Network-specific Network-specific

24. IFA’2004 24 Dynamic Location Update Schemes Movement-based Movement-based –The MT performs an update each time it crosses a certain movement threshold, where one movement is made by crossing a cell boundary. Distance-based Distance-based –The MT performs an update when its distance from the cell where it performed its last update surpasses a certain distance threshold. Time-based Time-based –The MT performs an update at a constant time threshold, deltaT.

25. IFA’2004 25 Example A MT is moving through the cellular network (R= km) as shown in the figure at a rate of 30km/hour. A MT is moving through the cellular network (R= km) as shown in the figure at a rate of 30km/hour. Label the cell ID’s where the MT will perform its updates for: Label the cell ID’s where the MT will perform its updates for: –Movement-based (T=3) –Distance-based (T=6km) –Time-based (30 minutes)

26. IFA’2004 26 Example Figure A MT is moving through the cellular network (R= km) at a rate of 30km/hour. A MT is moving through the cellular network (R= km) at a rate of 30km/hour. Where will updates be performed for: Where will updates be performed for: –Movement-based (T=3) –Distance-based (T=6km) –Time-based (30 minutes) A D C B H G F L K P N S R Q M J I E O

27. IFA’2004 27 Answer-Movement-based A D C B H G F L K P N S R Q M J I E O T = 3, 2 h = 3 Update at S and G

28. IFA’2004 28 Answer-Distance-based A D C B H G F L K P N S R Q M J I E O Update at O, and near the M/G border T=6km T = 6, 2 h = 3

29. IFA’2004 29 Answer-Time-based A D C B H G F L K P N S R Q M J I E O Update only at M

30. IFA’2004 30 Group Problem Design a location update and paging scheme. Design a location update and paging scheme. –Provide a diagram with numbered steps. –Explain how your scheme reduces the signaling overhead.

31. IFA’2004 31 Handoff The transfer of a mobile terminal’s active connection(s) from one channel to another. Hard handoffs vs. soft handoffs – –Hard handoff: break old connection, then form new connection. – –Soft handoff: Connect to several BSs simultaneously. In CDMA, handoff does not change the physical channel, it just changes the BS that handles the channel.

32. IFA’2004 32 SIGNAL STRENGTH Cellular systems depend on the radio signals received by an MS throughout the cell and on the contours of signal strength emanating from the BSs of two adjacent cells i and j. Cellular systems depend on the radio signals received by an MS throughout the cell and on the contours of signal strength emanating from the BSs of two adjacent cells i and j.

33. IFA’2004 33 Select cell i on left of boundary Select cell j on right of boundary Ideal Boundary Cell i Cell j -60 -70 -80 - 90 - 100 -60 -70 -80 -90 -100 Signal strength (in dB) Signal Strength

34. IFA’2004 34 Signal strength contours indicating actual cell tiling. This happens because of terrain, presence of obstacles and signal attenuation in the atmosphere. -100 -90 -80 -70 -60 -70 -80 -90 -100 Signal Strength (in dB) Cell i Cell j Signal Strength (2)

35. IFA’2004 35 SIGNAL STRENGTH Signal strength goes down as a mobile terminal moves away from the BS. Signal strength goes down as a mobile terminal moves away from the BS. As the mobile terminal moves away from the BS of the cell, the signal strength weakens and the so-called HANDOFF occurs. As the mobile terminal moves away from the BS of the cell, the signal strength weakens and the so-called HANDOFF occurs. This implies a radio connection to another adjacent cell. This implies a radio connection to another adjacent cell.

36. IFA’2004 36 BS i Signal strength due to BS j E X1X1 Signal strength due to BS i BS j X3X3 X4X4 X2X2 X5X5 X th MS P min P i (x) P j (x) P z (x) (for z=1,2) denote the power received at MS from BS z. By looking at the variation of signal strength from either base station it is possible to decide on the optimum area where handoff can take place. Handoff Region

37. IFA’2004 37 HANDOFF REGION At X 1 the received signal from BS j is close to 0 and the signal strength at the mobile terminal could be primarily attributed to BS i. At X 1 the received signal from BS j is close to 0 and the signal strength at the mobile terminal could be primarily attributed to BS i. Similarly, at distance X 2 the signal from BS i is negligible. Similarly, at distance X 2 the signal from BS i is negligible. To receive and interpret signals correctly at mobile, the received signal must be at a minimum power level P min (X 3 and X 4 ), i.e., between X 3 and X 4 the mobile terminal can be served either by BS i or BS j. To receive and interpret signals correctly at mobile, the received signal must be at a minimum power level P min (X 3 and X 4 ), i.e., between X 3 and X 4 the mobile terminal can be served either by BS i or BS j.

38. IFA’2004 38 HANDOFF REGION The area between X 3 and X 4 is called. The area between X 3 and X 4 is called HANDOFF AREA or HANDOFF REGION. Where to perform HANDOFF depends on many factors. Where to perform HANDOFF depends on many factors. * Do handoff at X 5 where two BSs have equal signal strength. * Do handoff at X 5 where two BSs have equal signal strength. HARD HANDOFF!!!!! HARD HANDOFF!!!!!

39. IFA’2004 39 HANDOFF REGION * Avoid “Ping-Pong Effect”, if the mobile moves back and forth between BS i and BS j * Avoid “Ping-Pong Effect”, if the mobile moves back and forth between BS i and BS j * SOLUTION  SOFT HANDOFF!!!!: * SOLUTION  SOFT HANDOFF!!!!: Continue to maintain both links with BS i and BS j until the signal strength from BS j exceeds that of B i by some pre-specified threshold value E as shown by point X in Figure. Continue to maintain both links with BS i and BS j until the signal strength from BS j exceeds that of B i by some pre-specified threshold value E as shown by point X in Figure.

40. IFA’2004 40 Handoff Control NCHO (Network-Controlled Handoff) NCHO (Network-Controlled Handoff) –All close-by BSs monitor signal strength from the mobile terminal. –MSC collects data from BSs, decides best candidate BS for the mobile terminal, and initiates the MS’s handoff (CT-2, AMPS). –Results in heavy signaling load, handoff delay of many seconds. MAHO (Mobile-Assisted Handoff) MAHO (Mobile-Assisted Handoff) –MT monitors signal strength from nearby BSs and reports the measurements back to the BS/MSC (twice per second). –MSC decides best candidate BS and initiates the handoff (GSM) MCHO (Mobile-Controlled Handoff) MCHO (Mobile-Controlled Handoff) –MT monitors signal strength from nearby BSs, decides best candidate BS, and initiates handoff (DECT)

41. IFA’2004 41 Handoff Management (Detection & Decision) Channel Assignment Radio Link Transfer

42. IFA’2004 42 Handoff Management Initiation (Detection/Decision) Initiation (Detection/Decision) –The user, the network, or changing channel conditions detect the need for handoff. New connection generation (Channel Assignment) New connection generation (Channel Assignment) –The network must find new resources for the handoff call –The network must also perform any needed routing operations. Data flow control (Radio Link Transfer) Data flow control (Radio Link Transfer) –Delivery of the data from the old path to the new path is maintained according to agreed-upon service guarantees.

43. IFA’2004 43 Handoff Initiation A balance of user movement versus network conditions A balance of user movement versus network conditions Goals: Goals: –Keep user connected –Minimize network signaling –Minimize “ping-pong” handoffs

44. IFA’2004 44 Handoff Initiation— What criteria should cause handoff?

45. IFA’2004 45 Handoff Management Handoff in cellular telephony: Handoff in cellular telephony: –Transfer of a voice call from one BS to another Handoff in WLANs: Handoff in WLANs: –Transfer of a connection from one AP (Access Point) to another Handoff in hybrid networks: Handoff in hybrid networks: –From a BS to another, from an AP to another, from a BS to an AP, or vice versa

46. IFA’2004 46 Handoff Decision Time Algorithms Traditional algorithms employ thresholds Traditional algorithms employ thresholds Channel measurements: Channel measurements: –Received Signal Strength (RSS) Measures the co-channel interference power and noise Measures the co-channel interference power and noise –Alternatively to RSS or in conjunction: Path loss Path loss Carrier-to-interference ratio (CIR) Carrier-to-interference ratio (CIR) Signal-to-interference ratio (SIR) Signal-to-interference ratio (SIR) BER BER Block error rate (BLER) Block error rate (BLER) Symbol error rate (SER) Symbol error rate (SER) Etc. Etc.

47. IFA’2004 47 Handoff Decision Time Algorithms Goals: Goals: –Keep user connected –Minimize network signaling –Minimize “ping-pong” handoffs In order to avoid the ping-pong effect, additional parameters are used such as hysteresis margin, dwell timers, and averaging windows. In order to avoid the ping-pong effect, additional parameters are used such as hysteresis margin, dwell timers, and averaging windows.

48. IFA’2004 48 Handoff Decision Time Algorithms Received Signal Strength (RSS): The BS whose signal is received with the largest strength is selected. Received Signal Strength (RSS): The BS whose signal is received with the largest strength is selected. RSS + Threshold: If the RSS of a new BS exceeds that of the current one and the signal strength of the current BS is below a threshold. RSS + Threshold: If the RSS of a new BS exceeds that of the current one and the signal strength of the current BS is below a threshold. RSS + Hysteresis: If the RSS of a new BS is greater than that of the old BS by a hysteresis margin. RSS + Hysteresis: If the RSS of a new BS is greater than that of the old BS by a hysteresis margin. RSS + Hysteresis + Threshold: If the received signal strength of a new BS exceeds that of the current one by a hysteresis margin and the signal strength of the current BS is below a threshold. RSS + Hysteresis + Threshold: If the received signal strength of a new BS exceeds that of the current one by a hysteresis margin and the signal strength of the current BS is below a threshold. Algorithms + Dwell Timer: A timer is started at the instant when the condition in the algorithm is true. The handoff is performed is the condition continues to be true until the timer expires. Algorithms + Dwell Timer: A timer is started at the instant when the condition in the algorithm is true. The handoff is performed is the condition continues to be true until the timer expires.

49. IFA’2004 49 Traditional Handoff Algorithms What kind of handoff is Happening in A, B, C, D?

50. IFA’2004 50 Sample RSS seen by MS traveling in a straight line between them

51. IFA’2004 51 Performance of Handoff Algorithms Performance measures (related to voice connections): Performance measures (related to voice connections): –Call blocking probability –Handoff blocking probability –Delay between handoff request and execution –Call dropping probability Objective: Minimize unnecessary handoffs Objective: Minimize unnecessary handoffs Overlooked issues: Overlooked issues: –Throughput maximization –Maintaining QoS guarantees during and after handoff

52. IFA’2004 52 Generic Handoff Management Process (1)Decision to handoff is made (network-controlled, mobile-assisted or controlled) (2)MT registers with visiting database via a handoff announcement (3)New visiting database communicates with home database for authentication and subscriber profile (4)Home database responds with authentication. Both databases are updated. (5)Home database communicates with old visiting database to clear registration information for the MT (6)The old visiting database flushes or redirect packets to the new visiting database and removes the MT form its list.

53. IFA’2004 53 Problem Time(s)02.557.51012.51517.520 BS1-47-57-52-55-60-62-60-65-66 BS2-59-56-55-54-52-51-49 - 60.5 -52 BS3-70-72-75-70-58-50 -62-75 BS4-72-71-65-60-55-53-50-49-56 Show the handoff times for: a)RSS b)RSS + threshold of -60 dBm c)RSS + hysteresis of 10 dB d)RSS + hysteresis of 5 dB + threshold of -55 dBm

54. IFA’2004 54 Problem Time(s)02.557.51012.51517.520 BS1-47-57-52-55-60-62-60-65-66 BS2-59-56-55-54-52-51-49 - 60.5 -52 BS3-70-72-75-70-58-50 -62-75 BS4-72-71-65-60-55-53-50-49-56 Show the handoff times for: a)RSS b)RSS + threshold of -60 dBm c)RSS + hysteresis of 10 dB d)RSS + hysteresis of 5 dB + threshold of -55 dBm

55. IFA’2004 55 Problem Time(s)02.557.51012.51517.520 BS1-47-57-52-55-60-62-60-65-66 BS2-59-56-55-54-52-51-49 - 60.5 -52 BS3-70-72-75-70-58-50 -62-75 BS4-72-71-65-60-55-53-50-49-56 Show the handoff times for: a)RSS b)RSS + threshold of -60 dBm c)RSS + hysteresis of 10 dB d)RSS + hysteresis of 5 dB + threshold of -55 dBm

56. IFA’2004 56 Problem Time(s)02.557.51012.51517.520 BS1-47-57-52-55-60-62-60-65-66 BS2-59-56-55-54-52-51-49 - 60.5 -52 BS3-70-72-75-70-58-50 -62-75 BS4-72-71-65-60-55-53-50-49-56 Show the handoff times for: a)RSS b)RSS + threshold of -60 dBm c)RSS + hysteresis of 10 dB d)RSS + hysteresis of 5 dB + threshold of -55 dBm

57. IFA’2004 57 Problem Time(s)02.557.51012.51517.520 BS1-47-57-52-55-60-62-60-65-66 BS2-59-56-55-54-52-51-49 - 60.5 -52 BS3-70-72-75-70-58-50 -62-75 BS4-72-71-65-60-55-53-50-49-56 Show the handoff times for: a)RSS b)RSS + threshold of -60 dBm c)RSS + hysteresis of 10 dB d)RSS + hysteresis of 5 dB + threshold of -55 dBm

58. IFA’2004 58 Problem: Solution

59. IFA’2004 59ExerciseTime(s)02.557.510 BS1-50-58-60-65-62 BS2-55-53-57-65-54 BS3-72-70-55-70-60 BS4-78-75-68-60-57 Show the handoff times for: a)RSS BS1-BS?-BS?-BS?-BS? b)RSS + threshold of -60 dBm BS1-BS?-BS?-BS?-BS? c)RSS + hysteresis of 5 dB BS1-BS?-BS?-BS?-BS?