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: ; Fax: Web:

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

IFA’ 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

IFA’ 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).

IFA’ 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

IFA’ Location Management Call Delivery (Paging) Location Update (Registration)

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

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

IFA’ Handoff Types Intra-CellInter-Cell Soft Handoff Hard Handoff

IFA’ 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

IFA’ 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

IFA’ 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

IFA’ 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.

IFA’ 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

IFA’ Location Registration

IFA’ 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.

IFA’ Call Delivery

IFA’ 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 -> voice channels; one control channel in each cell base station). (in AMPS -> voice channels; one control channel in each cell base station).

IFA’ 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

IFA’ 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

IFA’ 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.

IFA’ Paging (MSC) VLR

IFA’ 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

IFA’ 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.

IFA’ 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)

IFA’ 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

IFA’ 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

IFA’ 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

IFA’ 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

IFA’ 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.

IFA’ 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.

IFA’ 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.

IFA’ Select cell i on left of boundary Select cell j on right of boundary Ideal Boundary Cell i Cell j Signal strength (in dB) Signal Strength

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

IFA’ 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.

IFA’ 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

IFA’ 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.

IFA’ 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!!!!!

IFA’ 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.

IFA’ 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)

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

IFA’ 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.

IFA’ 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

IFA’ Handoff Initiation— What criteria should cause handoff?

IFA’ 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

IFA’ 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.

IFA’ 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.

IFA’ 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.

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

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

IFA’ 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

IFA’ 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.

IFA’ Problem Time(s) BS BS BS BS 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

IFA’ Problem Time(s) BS BS BS BS 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

IFA’ Problem Time(s) BS BS BS BS 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

IFA’ Problem Time(s) BS BS BS BS 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

IFA’ Problem Time(s) BS BS BS BS 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

IFA’ Problem: Solution

IFA’ ExerciseTime(s) BS BS BS BS 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?