Mobility is one of the most of import characteristics of a radio communicating system. Normally, uninterrupted service is achieved by back uping handoff ( or handover ) from one cell to another. Handover is defined as a procedure, in which a MN displacement from the air interface provided by another BS. It is frequently initiated either by traversing a cell boundary or by impairment in quality of the signal in the current channel. Handoff is divided into two wide categories-hard and soft handoffs. They are besides characterized by “ interruption before brand ” and “ do before interruption. ” In difficult handoffs, current resources are released before new resources are used ; in soft handoffs, both bing and new resources are used during the handoff procedure. Ill designed handoff strategies tend to bring forth really heavy signaling traffic and, thereby, a dramatic lessening in quality of service ( QoS ) . The ground why handoffs are critical in cellular communicating systems is that neighbouring cells are ever utilizing a disjoint subset of frequence sets, so dialogues must take topographic point between the nomadic station ( MS ) , the current helping base station ( BS ) , and the following possible BS. Other related issues, such as determination devising and precedence schemes during overloading, might act upon the overall public presentation.
The ground for HO can be assorted:
RSS is non plenty for keeping proper connexion at the border of the cell.
BS capacity is full and more traffic is pending.
Faster or cheaper web is available ( in the instance of perpendicular handover ) .
HO procedure occurs is two phases:
Fig. 3.1 handover scenario [ ]
3.2 Qos in WI-FI and WiMAX handover:
The mobility with quality of service in wireless webs is a world of all time closer. The demand for mobility while pass oning becomes more and more necessary. Technologies for radio webs with interoperability develops the construct of “ communications anytime anyplace ” . Interoperability uses the handover to guarantee the mobility of a nomadic terminus through assorted communicating substructures, supported by different engineerings of radio webs. Maintain the connexion between base Stationss and nomadic terminus is the key of the client ‘s satisfaction, with the warrant of continuity of services and applications running, traveling between different cells, utilizing the engineering of radio communications to supply better coverage in the place where is. The purpose of this survey is to analyze how the mobility between different entree engineerings in radio webs can be optimized and what their behaviour in scenarios of interoperability. To analyze the quality of the service of mobility, we analyze the clip required for the handover and the bing loss of bundles, the latter being a really of import factor to fulfill the user.
3.3 Different types of handovers:
Handovers can be divided into difficult and soft handover:
A difficult handoff is one in which the channel in the beginning cell is released and merely so the channel in the mark cell is engaged. Thus the connexion to the beginning is broken before the connexion to the mark is made-for this ground such handoffs are besides known as break-before-make. Difficult handoffs are intended to be instantaneous in order to minimise the break to the call. A difficult handoff is perceived by web applied scientists as an event during the call.
A soft handoff is one in which the channel in the beginning cell is retained and used for a piece in analogue with the channel in the mark cell. In this instance the connexion to the mark is established before the connexion to the beginning is broken, therefore this handoff is called make-before-break. The interval, during which the two connexions are used in parallel, may be brief or significant. For this ground the soft handoff is perceived by web applied scientists as a province of the call, instead than a brief. When a call is in a province of soft handoff the signal of the best of all used channels can be utilised for the call at a given minute or all the signals can be combined to bring forth a clearer transcript of the signal. The latter is more advantageous, and when such combine is performed both in the downlink and the uplink the handoff is termed as softer. Softer handoffs are possible when the cells involved in the handoff have a individual cell site, termed as softer. Softer handoffs are possible when the cells involved in the handoff have a individual cell site.
3.4 horizontal handover and Vertical Handover:
3.4.1 Horizontal handover
The chief concern of horizontal handover is to keep ongoing service although the alteration of IP reference due to the motion of a nomadic node. Keeping ongoing service is done by concealing the alteration of IP reference ( e.g. , Mobile IP ) or dynamically updating the changed IP reference ( e.g. , mSCTP ) .
To conceal the alteration of IP reference during the motion of a nomadic node, Mobile IP keeps two types of IP reference ; one permanent IP reference ( Home reference ) might be used above conveyance bed and one mutable IP reference ( Care-of reference ) might be used under conveyance bed. mSCTP could dynamically update the IP reference during the ongoing service.
The bulk of proposed handover mechanism powers be included in horizontal handover because they focus on maintain ongoing service even though merely the IP reference is changed.
3.4.2 Vertical handover
Vertical handover is happened when a nomadic node moves across heterogenous entree webs. Differently from horizontal handover, the used entree engineering is besides changed every bit good as IP reference, because the nomadic nodes moves different entree web which uses different entree engineering. In this instance, the chief concern of perpendicular handover is to keep ongoing service although non merely the alteration of IP references but besides the alteration of web interfaces, QoS features, and etc.
For illustration, a appropriately equipped laptop might be able to utilize both a high velocity radio LAN and a cellular engineering for Internet entree. Wireless LAN connexions by and large provide higher velocities, while cellular engineerings by and large provide more omnipresent coverage. Thus the laptop user might desire to utilize a radio LAN connexion whenever 1 is available, and to ‘fail over ‘ to a cellular connexion when the radio LAN is unavailable. Vertical handoffs refer to the automatic failover from one engineering to another in order to keep communicating. This is different from a ‘horizontal handoff ‘ between different radio entree points that use the same engineering in that a perpendicular handoff involves altering the informations link bed engineering used to entree the web. Vertical handoffs among a scope of wired and wireless entree engineerings including WiMAX can be achieved utilizing Media independent handover which is standardized as IEEE 802.21.
Table 3.1: Changed points in horizontal/vertical handover [ 3 ]
Could be changed
QoS parametric quantity
Could be changed
It is noted that the any handover mechanisms for horizontal handover could non straight be used for perpendicular handover. Because handover mechanisms for horizontal handover have merely the ability to work out the job of the alteration of IP reference, they could non keep ongoing service when web interfaces or QoS features are changed. To back up perpendicular handover, the modifying of bequest Mobile IP is needed [ 20 ] . The chief capablenesss of perpendicular handover as compared to horizontal handover are as follows ;
Use of different entree engineerings
Use of multiple web interfaces
Use of multiple IP references
Use of multiple ( mutable ) QoS parametric quantities
Use of multiple web connexions ( multi-homing characteristics )
Horizontal handover assume that the nomadic node has merely individual web interface, individual IP reference, and individual web connexion at a clip. Break-before-make in horizontal handover is that a nomadic node can do a new web connexion merely after unpluging the old connexion.
If we use multiple web interfaces, multiple IP references, and multiple web connexions for perpendicular handover, we can develop MM mechanism based on make-before-break. So, the latency clip of handover could be decreased or eliminated absolutely.
Table 3.2: Capabilities in horizontal/vertical handover [ 20 ]
IP reference at a clip
Single IP reference
Multiple IP reference
QoS parametric quantity
Because MM mechanisms for horizontal handover assume that the nomadic node has merely individual web interface, individual IP reference, and individual web connexion at a clip, there are many limitations to develop MM mechanisms such as break-before-make. Break-before-make in horizontal handover is that a nomadic node can do a new web connexion merely after unpluging the old connexion.
If we use multiple web interfaces, multiple IP references, and multiple web connexions for perpendicular handover, we can develop MM mechanism based on make-before-break. Make-before-break is that a nomadic node can do a new web connexion before unpluging the old connexion. So, the latency clip of handover could be decreased or eliminated absolutely. Besides, perpendicular handover mechanism based on soft handover could be available
3.5 How to make up one’s mind which type of handover suitable:
For the nomadic device the stairss to execute a horizontal ( within a same web type ) or perpendicular ( across heterogenous web ) handover can be classified into three logical undertakings shown in Figure 1 which in chronological order are, one ) “ when and why ” should the device passage to a new web, two ) “ where ” should the device passage to, and three ) “ how ” should the device passage between webs to keep connectivity and session continuity. The “ when and why ” is triggered when the nomadic device receives an indicant that a passage should take topographic point. These can include external conditions like signal debasement or web congestion, find of a more suited web supplying lower cost, higher bandwidth or better energy efficiency, or be user or web initiated. The “ where ” measure is when the nomadic device selects the following web to link to, and the “ how ” defines how the device performs the passage, e.g. either making a horizontal or perpendicular handover.
The web can ease the nomadic device web choice by supplying a web map depicting the presence and feature of webs available in the locality of the nomadic device. Additionally the web itself can originate a handover petition when for illustration it detects that the burden on one of the entree webs is approaching capacity. Watch the picture below to hear research worker Vijay Kesavan discoursing the engineering behind a WiFi/WiMAX handover.
“ When and why? ” to handover. The end is to supply accurate and prognostic nexus bed information about an at hand debasement or loss of signal to get down a handover – for illustration a make-before-break operation. A three measure procedure predicts with a high degree of truth the signal quality way. We
Smooth out the signal features,
Predict the signal degree in the close hereafter and
Perform a tendency analysis, concentrating on methods that provide truth with a low calculation operating expense.
We use exponential norm for smoothing, “ Straight Line ” for anticipation and Fast Fourier Transform for tendency analysis on both the short and long term window. We predict link bed debasement on WiFi and WiMAX with approximately 90 % truth and supply this indicant about 800 milliseconds to 1 2nd before the loss happens thereby supplying a important lead clip to take action.
Figure 3.2: System Architecture [ 1 ]
“ Where? ” to handover to? Once indicant of an at hand disjunction is received, the following determination is to choose another suited web to link to. In the radio environment the ability to detect webs depends on the sensing and propinquity of the signal beginning and the environment, e.g. interferers, obstructions, etc. Mobile devices sporadically scan to “ seek ” radio webs on each of their interfaces therefore devouring power. This is where the information learned from the web on the bing affiliated channel, e.g. a web information map depicting the presence and features of other webs, helps the nomadic device better its energy efficiency by scanning merely on appropriate interfaces at specific geographic locations. To measure the best web to travel to, a cost map taking into history web degree ( e.g. bandwidth, hold, security, etc. ) , platform degree ( e.g. battery burden, thermic, etc. ) , user ‘s penchants ( e.g. pecuniary cost, operator, etc. ) and web penchants defined by web operator/virtual web operator/enterprise is used.
“ How? ” to handover to? This is the executing portion of the handover procedure. There are many methods available for managing session handover, e.g. Mobile IP ( MIP ) , Proxy Mobile IP, Session Initiation Protocol ( SIP ) , Voice Call Continuity ( 3GPP VCC ) . We used a SIP based A/V application.
Figure 3.3: Heterogeneous Handover with prognostic triggers clocking diagram [ 1 ]
Using the optimisations mentioned earlier we can see in Figure-2 that we can get down a make-before-break where the two wirelesss are at the same time active with adequate lead clip to back up seamless handover for multimedia Sessionss. ( It should be noted that hallmark has non been taken into history here ) . Our execution is based on the IEEE 802.21 Draft specification for both client and web constituents ( Information Server and Command Server ) , but this is merely one option among many to accomplish this.