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    3 handoff management 3 handoff management Presentation Transcript

    • Handoff Management
    • Outline Handoff Basics Handoff Management Handoff Detection Handoff Assignment Example: Handoff in GSM Handoff Requirements and Goals Advanced Issues of Handoff Management Handoff Failure Radio Link Transfer Soft HandoffProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 2
    • Review:Signal Strength within Cells The contours may not be concentric and could be distorted by atmospheric conditions and topographical contoursProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 3
    • Handoff A most common reason that the handoff is conducted: When the phone is moving away from the area covered by one cell and entering the area covered by another cell, the call is transferred to the second cell to avoid call termination when the phone gets outside the range of the first cell The handoff process is of major importance within any cellular networks Failure for it to perform reliably can result in dropped calls, and this is one of the key factors that can lead to customer dissatisfactionProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 4
    • Basics of Handoff Management Handover (Europe) = handoff (North America) Handoff is the process that occurs when a mobile is “handed over” from one access point to another, i.e., the access point which the mobile is using changes Either handover or handoff can be abbreviated as HO hereafter Handoff Management (Handoff detection) How do you detect that you should handoff? (Handoff assignment) Who initiates handoff?Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 5
    • How HO Happens? As the MS moves away from the BS of the cell, the signal strength weakens and the radio connection of MS is changed to another adjacent cellProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 6
    • How HO Happens? (cont’d) By starting to look for a new BS before you need it, there is time to make a decision X3: Start looking for a new BS X5: Time to switch TL: Threshold for looking around TH: Threshold for handoffProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 7
    • Outline Handoff Basics Handoff Management Handoff Detection Handoff Assignment Example: Handoff in GSM Handoff Requirements and Goals Advanced Issues of Handoff Management Handoff Failure Radio Link Transfer Soft HandoffProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 8
    • “Ping-Pong” Effect The name of the phenomenon implies that the MS moves back and forth between the overlapped area of two adjacent cells Celli Cellj The solution is to allow MS continue maintain a radio link with the current BSi, until the signal strength from BSj exceeds that of BSi by some pre-specified threshold value E (shown as Xth)Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 9
    • How HO Happens? (cont’d) To avoid “ping-pong” effect, the MS continues to maintain a radio link with BSi until XthProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 10
    • Reality is More Complex Three common measurements of the channel: Word Error Indicator (WEI) Based on if the receiver is able to decode the received signal correctly Received Signal Strength Indication (RSSI) A measure of the received signal strength Quality Indicator (QI) Related to the signal to interference & noise ratio (S/I) Handoff may depend more reliably on WEI of the current channel rather than RSSI If WEI is good, then handoff is not performed However, it is necessary to accumulate WEI measurements over a period of time, whereas RSSI is known instantaneouslyProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 11
    • Reality is More Complex (cont’d) To make the handoff decision accurately and quickly, it is desirable to use both WEI and RSSI However, to use a simple policy for handoff detection can be feasible in practice For example, GSM Specification introduces 6 basic types of causes of HO with priority order Uplink quality Downlink quality Uplink level Downlink level MS-BS distance Power budget: To improve transmission quality in the lower power levelProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 12
    • Who Makes the HO Decision? NCHO (network controlled handoff) The network makes the decision Used in CT-2 Plus and AMPS MCHO (mobile controlled handoff) The mobile decides for itself Forward: the mobile initiates HO and sends the request to the new AP Backward: the mobile initiates HO and sends the request to the old AP Used in DECT, PACS and Mobile IP MAHO (mobile assisted handoff) The mobile provides data for the network to make the decision Used in GSM and IS-95 CDMAProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 13
    • NCHO Network Controlled Handoff The BS monitors the signal strength and quality from the BS The network uses multiple (current and surrounding) BSs to supervise the quality of all current connections by making measurements of RSSI The MSC makes the decision when and where to effect the handoff Drawback: Heavy network signaling traffic and limited radio resources at BSs prevent frequent measurements of neighboring links Long handoff time: up to 10 sec. or moreProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 14
    • MCHO Mobile Controlled Handoff The mobile decides for itself by monitoring signal strength and quality from the current and candidate BSs When it finds a better candidate, it initiates a handoff In MCHO, most of the work is done by the mobile (as it knows who it can hear, how well it can hear them, and can even consider its battery level, etc) Handoff time DECT: 100~500 ms. PACS: 20~50 ms.Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 15
    • MAHO Mobile Assisted Handoff The mobile provides data which the network uses to make the decision Essentially, it is a variant of NCHO but uses the mobile to help reduce the handoff time Example: In GSM, the MS transmits measurements twice a second ⇒ GSM handoff time ~ 1 sec. Note in both NCHO and MAHO, if the network cannot tell the mobile about the new channel / time slot / ... to use before the link quality has decayed too far, then the call may be terminatedProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 16
    • Handoff in GSM A handoff is decided by the network, not the MS BSS criteria Received signal level Channel quality Distance between MS and BTS Network operation criteria Current traffic load of the cell Ongoing maintenance work Link control for handoff preparation Periodically, the MS checks the downlink signal strength of its current BS and the neighboring BSs Uplink signal quality is monitored by the network side MS sends measurement report to its current BS and this report is to be evaluated for handoff decisions by the network sideProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 17
    • Handoff in GSM (cont’d) Handoffs include several procedures: measurements, decision and destination selectionProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 18
    • Handoff in GSM (cont’d) Use measurement results from the MS and BS To identify possible other BSs as targets for handovers To determine the optimal moment to execute the HO Objective: to keep (#HO per cell change) small Ideally, (#HO per cell change)=1 In reality, radio conditions are often not very stable when an MS leaves the BS => (#HO per cell change)=1.5~5Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 19
    • Outline Handoff Basics Handoff Management Handoff Detection Handoff Assignment Example: Handoff in GSM Handoff Requirements and Goals Advanced Issues of Handoff Management Handoff Failure Radio Link Transfer Soft HandoffProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 20
    • Handoff Requirements Requirements from a handoff scheme are: Latency: the time required to effect the handoff should be appropriate for the rate of mobility of the mobile terminal, as well as the nature of data transferred Scalability: the handoff procedure should support handoffs within the same cell, between different base stations in the same or in different networks Minimal drop-off and fast recovery Quality of service should be maintained or re-negotiatied after the handoff is completed Minimal additional signalingProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 21
    • Handoff Goals Minimal impact on traffic Making a handoff at the “right” time Tolerance / adaptation for congestion and capacity The new and old cells may have different levels of utilization, available bandwidth, ... Efficiency The handoff should result in improved efficiency (in terms of traffic, energy consumption, reduced interference, …) The handoff process should try to minimize the resources it consumes Improve availability Handoff should result in using a BS which provides better bandwidth, lower cost, lower delay, low delay variance, ...Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 22
    • Handoff Goals (cont’d) The mobile should be able to use the maximum set of BSs (which may involve changing spreading code, modulation, coding, or changing to a different radio module) to achieve a better system optima Fast handoff Optimal BTS selection Reduced signaling overhead Increased speech quality Avoidance of far-away-cell effect Consideration of adjacent-cell capacity Avoidance of ping-pong effect Reduced handover failureProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 23
    • Outline Handoff Basics Handoff Management Handoff Detection Handoff Assignment Example: Handoff in GSM Handoff Requirements and Goals Advanced Issues of Handoff Management Handoff Failure Radio Link Transfer Soft HandoffProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 24
    • Handoff Failure What happens if there are insufficient resources in the new BS? Existing calls cannot be handed over ⇒ forced terminations New calls are blocked Goals of channel assignment To achieve high spectrum utilization To maintain a given service quality To use a simple algorithm To require a minimum number of database lookups Unfortunately, it is hard to do all of these at once!Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 25
    • Channel Assignment Several schemes are introduced to reduce the number of forced terminations, at the cost of increased blocking or decreased efficiency, including: Nonprioritized scheme (NPS) A handoff call is treated the same as a new call Reserved channel scheme (RCS) To reserve some resources for handoffs Queuing priority scheme (QPS) To exploit the handoff area (i.e., cell overlap) to queue up mobiles waiting for handoff Subrating scheme (SRS) To downgrade an existing call in the new cell and to share resources with the call being handed over (e.g., by changing a full- rate to a half-rate CODEC)Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 26
    • Radio Link Transfer When performing handoff, it is sometimes necessary to transfer radio link among system components In the GSM network, several handoff types can be enumerated: Intra-cell HO Inter-cell or inter-BS HO Inter-BSC HO Inter-MSC HO Intersystem HO between two PCS networks (?!)Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 27
    • Preview:Part of the GSM Architecture BTS: base transceiver station BSC: base station controller MSC: mobile switching centerProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 28
    • Different HO Types in the GSMNetworkProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 29
    • Intra-cell & Inter-cell HOs Intra-cell HO For administrative reasons or because of channel quality (channel- selective interferences) Inter-cell HO Because of weak signal field strength and bad channel quality Also for administrative reasons (e.g., for traffic load balancing)Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 30
    • Flows of HO within One MSC When both cells are connected to the same MSC, the mobile can signal that it is going to change cells (by providing measurements), then the MSC can set up the correct resources deliver traffic in the new cell This often involves setting up a bridge to copy traffic to both the old and new channelsProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 31
    • Inter-MSC HO When the two cells are connected to different MSCs, the situation is more complexProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 32
    • Inter-MSC HO (cont’d) Anchor-based approach Anchor MSCProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 33
    • What Happens If the MobileMoves Again?Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 34
    • What Happens If the MobileMoves Again? (cont’d) Note that the call always goes via the so-called anchor MSC (in this case MSC1) to relay MSCs This is because the phone attached to the PSTN knows nothing about mobility and the originating exchange thinks the call is still in existence Without path minimization, the chain of chunks between MSCs could continue to grow as long as the call lasts and the mobile keeps moving to new MSCs With voice calls, the call duration is generally rather limited, but with data communication it could continue for a very long timeProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 35
    • Hard Handoff vs. Soft Handoff Hard handoff Break-before-make The mobile connects only to a single BS at a time Soft handoff Make-before-break The mobile receives from & transmits to multiple BSs simultaneously The signal of the best of all connected channels is utilized Generally used in CDMA systemsProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 36
    • Hard Handoff X XProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 37
    • Soft Handoff XProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 38
    • Softer Handoff Softer handoff: In soft handoff, the network and the mobile combine all the signals to produce a clearer copy of the signal in both the up and down links Received Power Total at the MS Cell A Cell B DistanceProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 39
    • WCDMA Handoffs Softer handoff is more advantageous to soft handoff while soft handoff is more advantageous to hard handoff. However, not all handoffs in CDMA systems can be performed as softer handoffs Example: In WCDMA systems, we have the following scenarios: Softer handoff The handoff is within one cell between different sectors Soft handoff The handoff is between different BSs Hard handoff The handoff is between different frequencies or between WCDMA and GSMProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 40
    • Soft Handoff in Multiple Forms Some CDMA systems use very precise link level timing to enable the signals from multiple BSs to arrive additively at the mobile, thus leading to a physically stronger signalProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 41
    • Advantages of Soft Handoff Advantages of utilizing soft handoff include: Reduces number of call drops Increases the overall capacity Mobile transmit power is reduced Voice quality near the cell boundaries are improvedProf. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 42