Mobility Management Traditional mobile communication applications were in two-way voice communication, text, s and remote file downloading. The emerging applications in video streaming, sensor networking, telemedicine and surveillance are expected to dominate and shape the next generation of mobile communication systems. One critical feature that enables the ubiquitous communication is the mobility management - which is perceived to provide continuous constant quality of service even under very harsh and unexpected conditions. Basic mobility management operations include location update as mobile units move around and location lookup as mobile units are wanted.
Players Nokia Erricsson Cisco Nortel Siemens train system door/0,2241,hq_en_0_67428_rArNrNrNrN,00. html door/0,2241,hq_en_0_67428_rArNrNrNrN,00. html
Mobility issues Radio resource management Location info management Security Temporary loss of connectivity with movement Scarce resources : Small devices, low battery power, small CPU, less memory, light weight,…. React to sudden change in environment due to bandwidth and other resource changes ????
Location Management Location management schemes are based on users' mobility and incoming call rate characteristics. The network mobility process has to face strong antagonism between its two basic procedures: location update (or registration) and paging. The location update procedure allows the system to keep location knowledge more or less accurately in order to find the user. Location registration also is used to bring the user's service profile near its location. The paging process by the system sends paging messages in all cells where the mobile terminal could be located. A network must retain information about the locations of endpoints in the network in order to route traffic to the correct destinations.
Location tracking… Location tracking (also referred to as mobility tracking or mobility management) is the set of mechanisms by which location information is updated in response to endpoint mobility. It is important to differentiate between the identifier of an endpoint and its address (i.e., where the endpoint is located). Mechanisms for location tracking provide a time- varying mapping between the identifier and the address of each endpoint. Most location tracking mechanisms may be perceived as updating and querying a distributed database (the location database) of endpoint identifier-to-address mappings.
Two operations of LM The paging operation is performed by the cellular network. When an incoming call arrives for a mobile station, the cellular network will page the mobile station in all possible cells to find out the cell in which the mobile station is located so that the incoming call can be routed to the corresponding base station. This process is called paging. The number of all possible cells to be paged is dependent on how the location update operation is performed. The location update operation is performed by an active mobile station.
Paging schemes Blanket paging in GSM : Paging the MS in all the cells within a LA (location area) simultaneously. If the LA update is correct, in the very first paging cycle, the MS will receive a paging request & respond to it. Here the delay of the paging response is kept to a minimum. The disadvantage is that paging has to be done in several cells with the same LA! Closest-cells first: The cell where the MS was last seen is paged first followed by subsequent rings of cells that are equidistant from this cell in each paging cycle. Also called sequential paging.
Location Tracking (update) Has two components: (1) determining when and how a change in a location database entry should be initiated; and (2) organizing and maintaining the location database. In cellular networks, endpoint mobility within a cell is transparent to the network, and hence location tracking is only required when an endpoint moves from one cell to another. Location tracking typically consists of two operations: (1) updating (or registration), the process by which a mobile endpoint initiates a change in the location database according to its new location; and (2) finding (or paging), the process by which the network initiates a query for an endpoint's location (which also may result in an update to the location database). Most location tracking techniques use a combination of updating and finding in an effort to select the best trade-off between update overhead and delay incurred in finding. Specifically, updates are not usually sent every time an endpoint enters a new cell, but rather are sent according to a predefined strategy so that the finding operation can be restricted to a specific area..
Location Tracking Location management methods are most adapted and widely used in current cellular networks, e.g., GSM, IS-54. IS-95, etc. The location management methods are broadly classified into two groups. The first group includes all methods based on algorithms and network architecture, mainly on the processing capabilities of the system. The second group contains the methods based on learning processes, which require the collection of statistics on subscribers' mobility behavior, for instance.
Location update algorithms Two types: Static Dynamic A location update scheme can be classified as either global or local. A location update scheme is global if all subscribers update their locations at the same set of cells, and a scheme is local if an individual subscriber is allowed to decide when and where to perform location update. A local scheme is also called individualized or per-user based.
In GSM, LM…. Home & visiting databases are called Home Location Register (HLR) & Visiting Location Register (VLR) respectively. When the MS observes a change in the LA identity, it transmits a location update to MSC through its BS. The MSC contacts its VLR with the location update. VLR does nothing if it serves the old & the new LA. If the VLR has no info about the MS, it contacts the HLR of the MS via a location registration message. The HLR authenticates and acknowledges the location registration, updates its own database, and sends a message to the old VLR to cancel the registration.
Cost of the two operations The total location management cost is the sum of the location update cost and the paging cost. There is a trade-off between the location update cost and the paging cost. If a mobile station updates its location more frequently (incurring higher location update cost), the network knows the location of the mobile station better. Then the paging cost will be lower when an incoming call arrives for the mobile station. Therefore both location update and paging costs can not be minimized at the same time. However, the total cost can be minimized or one cost can be minimized by putting a bound on the other cost. For example, many researchers try to minimize the location update cost subject to a constraint on the paging cost. Ex: If there is no delay constraint, the cells can paged sequentially in order of decreasing probability, which will result in the minimal paging cost. If all cells are paged simultaneously, the paging cost reaches the maximum while the paging delay is the minimum.
Emerging issues in LM… Database architecture for 3G & 4G..(access to db and management of queries to reduce delay) Reduce load on a centralized db (such as HLR), local caches of the MS can be maintained…..used by Mobile IP. Alternate location update strategies & paging algorithms are investigated. Traffic modeling to investigate the performance.. ?????
What is Handoff??? Involves entire gamut of actions required to handle an ongoing connection when the mobile terminal moves from one point of access to another. Handoff is v important cos of the cellular architecture used for spectrum utilization. Reference: 4190/ pdf
Handoff effects In cellular n/w involving voice, you hear a audible click when handoff takes place (changing point of access from one BS to another BS) In WLAN, packets are lost when handoff changes the point of access from one AP to another. Additional congestion control mechanisms required. Causes ping-pong effect due to several handoffs between two BS back & forth: has toll on user quality reception & network load.
Types of handoffs Hard handoff (further has: Intra and Inter cell handoffs): A firm decision is made when to handoff & has no simultaneous connection between two or more stations. Soft handoff (further has: multiway soft handoffs & softer handoffs): A conditional decision is made whether to handoff or not. Depending on the pilot signal from 2 or more BSs, eventually a hard decision is made. In the interim period user has simultaneous traffic with all candidate BSs.
A hard handoff …. In a hard handoff, the link to the prior BS is terminated before or as the user is transferred to the new cells BS; the MS is linked to no more than one BS at any given time. Hard handoff is primarily used in FDMA and TDMA,where different frequency ranges are used in adjacent channels in order to minimize channel interference. So when the MS moves from one BS to another BS, it becomes impossible for it to communicate with both BSs (since different frequencies are used).
When handoff should be initiated?
Handoff initiation-Performance evaluation.. It is assumed that the signal is averaged over time, so that rapid fluctuations due to the multipath nature of the radio environment can be eliminated. Figure shows a MS moving from one BS (BS1) to another (BS2). The mean signal strength of BS1 decreases as the MS moves away from it. Similarly, the mean signal strength of BS2 increases as the MS approaches it. Relative Signal Strength: This method selects the strongest received BS at all times. The decision is based on a mean measurement of the received signal. This method is observed to provoke too many unnecessary handoffs, even when the signal of the current BS is still at an acceptable level.
Relative Signal Strength with Threshold: its relative value is compared to the signal strengths of the two BSs at the point at which they are equal. If the threshold is higher than this value, say T1 in Figure, this scheme performs exactly like the relative signal strength scheme, so the handoff occurs at position A. If the threshold is lower than this value, say T2 in Figure, the MS would delay handoff until the current signal level crosses the threshold at position B. In the case of T3, the delay may be so long that the MS drifts too far into the new cell. This reduces the quality of the communication link from BS1 and may result in a dropped call. Also causes co-channel interference to users. The scheme may create overlapping cell coverage areas. A threshold is not used alone in actual practice because its effectiveness depends on prior knowledge of the crossover signal strength between the current and candidate BSs.
Network-Controlled Handoff The network makes a handoff decision based on the measurements of the MSs at a number of BSs. In general, the handoff process (including data transmission, channel switching, and network switching) takes 100–200 ms. Information about the signal quality for all users is available at a single point in the network that facilitates appropriate resource allocation. Network-controlled handoff is used in first- generation analog systems such as AMPS (advanced mobile phone system), TACS (total access communication system), and NMT (advanced mobile phone system).
Mobile-Assisted Handoff In a mobile-assisted handoff process, the MS makes measurements and the network makes the decision. In the circuit-switched GSM (global system mobile), the BS controller (BSC) is in charge of the radio interface management…. means allocation and release of radio channels and handoff management. The handoff time between handoff decision and execution in such a circuit-switched GSM is approximately 1 second.
Mobile-Controlled Handoff In mobile-controlled handoff, each MS is completely in control of the handoff process. This type of handoff has a short reaction time (on the order of 0.1 second). MS measures the signal strengths from surrounding BSs and interference levels on all channels. A handoff can be initiated if the signal strength of the serving BS is lower than that of another BS by a certain threshold.
Handoff schemes Based on traffic models
Handoff Schemes in Single Traffic Systems In this scheme: we have non-priority, priority, and queuing handoff schemes for a single traffic system such as either a voice or a data system. we assume that a system has many cells, with each having S channels. The channel holding time has an exponential distribution with mean rate. Both originating and handoff calls are generated in a cell according to Poisson processes, with mean rates O and H, respectively. We assume here a system with a homogeneous cell. The focus is on a single cell (called the marked cell). Newly generated calls in the marked cell are labeled originating calls (or new calls). A handoff request is generated in the marked cell when a channel holding MS approaches the marked cell from a neighboring cell with a signal strength below the handoff threshold.