Vertical Handover Decision Algorithm for Heterogeneous Cellular-WLAN Networks Zsolt Alfred POLGAR, Andrei Ciprian HOSU, Zsuzsanna Ilona KISS, Mihaly VARGA
Outline WiFi connectivity in NGN networks; System model & problem formulation; The VHO decision algorithm; Evaluation methodology & test scenario; Simulation results; Conclusions. 2
WiFi connectivity in NGN networks Public WiFi networks represent a viable solution for ensuring ubiquitous wireless connectivity; Usage of unlicensed bands makes possible free access; New standards, like 802.11n, 802.11ac, allow large transfer rates; Up to 600Mbps on 2.4GHz and 2600Mbps on 5GHz. High gain outdoor units allow extended coverage; 14dBi omnidirectional antennas allow reach up to 1 mile. Traffic offloading of the 3G/4G networks could be achieved; Solution feasible especially in urban environment. 3
WiFi connectivity in NGN networks WiFi coverage in Spain an example (Gowex net.) 4
WiFi connectivity in NGN networks WiFi coverage in Madrid an example (Gowex net.) 5
WiFi connectivity in NGN networks WiFi coverage in Romania and in Cluj an example The RDS network; 6
System model & problem formulation System model: Heterogeneous networking environment including: Cellular networks, providing large coverage; WLAN/WiFi networks; Offload the traffic passing through the 3G/4G networks. The WLANs have overlapping coverage. The mobile terminals are equipped with cellular and WiFi interfaces and run the following algorithms: The VHO decision algorithms; VHO triggering and selection of the VHO target network; The VHO execution algorithms; Seamless routing over the heterogeneous networks. 7
System model & problem formulation The coupling between the heterogeneous networks is implemented by specially designed gateways: The Service Continuity Gateways (SCG), located in the Internet. The coupling infrastructure includes also: The Central Database (CD): It stores the network state and traffic related information for each wireless network. The Connectivity Support Server (CSS): It updates the CD and controls the access of the users to this database. Virtual communication channels (IP tunnels) are established between the mobile terminals and SCG. 8
System model & problem formulation The system model considered: 3G/4G Cell User User IP communication tunnel Gateway Support Server User User User WiFi AP User User User WiFi AP User IP communication tunnel WiFi AP WiFi AP User IP communication tunnel IP tunnel Ubiquitous connectivity infrastructure Internet 9
System model & problem formulation Problem formulation: The mobile terminal has to decide when to use the WiFi connectivity; The speed of the terminal has to be low (speed< 1-2 m/s). The mobile terminal has to select the best WiFi network for transmission, if several networks are available: A VHO decision algorithm which allows selecting the best WiFi network is necessary; The complexity of the algorithm should be low Selection of the WiFi network is performed based on the network and traffic information and possibly the cost; This information is available in the Central Database of the coupling infrastructure. 10
The VHO decision algorithm The users travelling with vehicular speed are connected to cellular networks; WiFi connectivity is not appropriate for high bit rate connectivity in vehicular scenarios. The users travelling with pedestrian speed try to connect to WLANs, if possible; It is considered that: The cost of the Mbyte transmitted in the WLANs is significantly lower than the cost of the Mbyte in cellular networks; Enough transmission capacity is available in the WLANs. 11
The VHO decision algorithm NO 3G Connectivity Speed < thr_p thr_p pedestrian speed threshold» 1m/s thr_v vehicular speed threshold» 3m/s thr_l WiFi signal level variation threshold» 50% 3G-WLAN and WLAN-WLAN VHO decision is taken by the user terminal. VHO decision taken based on: Received Signal Strength (RSS); NO VHO is possible YES Detection of available WLANs through scan operation The network list is empty? NO YES Acquire from the CD the network and traffic parameters characterizing all WLANs in the current geographical position Utility function computation for each detected network NO WLAN connection is used? YES YES Available Transmission Rate (ATR); Target WLAN selection and 3G-WLAN or WLAN-WLAN handover execution WLAN-3G handover execution 3G Connectivity Possibly, the cost of the network usage. Measurement of the network and traffic parameters in the active WLAN YES NO Update the CD with the newest measurement results Speed > thr_v NO RSS variation > thr_l 12
The VHO decision algorithm User Support Server signaling and data exchange Timer period 3G connectivity Speed < thr Scan WLANs Connect to best WLAN (WLAN-x) Monitor WLAN-x & start timer Monitor WLAN-x & start timer WLAN-x parameters below thr Scan WLANs Connect to best WLAN (WLAN-y) User Server Send CI query (WLAN list) Retrieve CI (WLAN list) Send CI data (WLAN list) Prepare CI (WLAN list) Send CI update (WLAN-x) Update stored CI Send CI update (WLAN-x) Update stored CI Send CI query (WLAN list) Retrieve CI (WLAN list) Send CI data (WLAN list) Prepare CI (WLAN list) CD CI Context Information = Network State and Traffic information 13
The VHO decision algorithm The estimated ATR of the wireless link can be computed as: est f (RSS) gives the average bit rate of the WiFi connection as a function of the RSS. ChBPF Channel Busy Period Fraction: Represents the fraction of the active time when the WiFi channel is used for transmission; It can be measured by passive monitoring of the WiFi link. Other WiFi link s ATR evaluation mechanism: The Probe Gap Model (PGM): ATR f ( RSS ) (1 BLER) (1 ChBPF ) Based on sending pairs of packets with predefined gaps. 14
The VHO decision algorithm The WiFi link s BLER can be evaluate as follows: Based on the evaluation of the Signal to Interference and Noise Ratio (SINR) of the WiFi link; By counting the ACK/NACK messages received at the MAC layer; By measuring the 802.11 beacon frame loss ratio. The parameters used in target network selection have different measurement units; Normalization is necessary; max-min normalization: v (x min(x )) / (max(x ) min(x )) ij ij ij ij ij i i i x ij is the value of the j-th parameter in the i-th network; v ij is the normalized value of x ij. 15
The VHO decision algorithm The utility function of a network defined as: M is the number of parameters considered; w j is the weighting factor of parameter j; The weights represent the importance of the attributes; The weight vector w can be computed by solving: The pairwise comparison matrix B (MxM): It indicates how many times more important or dominant one element is over another. Its elements are the b ij comparisons between the i-th and j-th parameter. λ is the eigenvalue of B and I is the identity matrix; The weight vector is the eigenvector w [ w1,..., w ] T M of B corresponding to maximum eigenvalue λ max. C i M j 1 w v j ij ( B I) w 0 16
Evaluation methodology & test scenario A real test site which replicates a real scenario was used to evaluate the proposed VHO algorithm: The mobile terminal is moving in the coverage area of a 3G network and of several WLANs; 3G-WiFi and WiFi-WiFi VHO operations are taking place. The RSS and the ATR parameters of the WLANs composing the het. network were measured; The GPS coordinates of the measurement points were recorded; The data obtained were fed into a system level simulator developed in the UCONNECT FP7 project. 17
Evaluation methodology & test scenario The test site located in the TUCN campus AP1 AP2 AP1 AP2 GPS module 3G interface 1 SMR 3G interface 2 AP5 Bus WiFi AP WiFi interface 1 WiFi interface 2 AP3 AP4 User device 2 User device 1 Power supply AP3 18
Latitude Evaluation methodology & test scenario Coverage of the test site WiFi APs; RSS levels [dbm] AP2-65 46.757 46.7569-70 46.7568-75 46.7567-80 46.7566-85 46.7565-90 46.7564 46.7563 23.5968 23.5967 23.5966 23.5965 23.5964 23.5963 Longitude 23.5962 23.5961 23.596 46.7562 23.5959 19
Evaluation methodology & test scenario WLAN background traffic test scenarios: AP Scenario 1 Scenario 2 Scenario 3 Cost of MByte AP1 6Mbps 10Mbps 1Mbps 0.1 AP2 8Mbps 10Mbps 2Mbps 0.2 AP3 10Mbps 10Mbps 10Mbps 0.15 AP4 1Mbps 1Mbps 8Mbps 0.25 AP5 0Mbps 0Mbps 0Mbps 0.3 ChBPF versus RSS for different background traffic Experimental results; ChBPF is influenced by the value of RSS. 20
21 Simulation results VHO decision and weighting methods: VHO decision based on ATR, RSS, and utility function; Four sets of weights are considered. Utility function computed based on ATR, RSS and cost. Weighting method index 1 2 3 4 Algorithm Weights ATR RSS Cost ATR based 1 0 0 RSS based 0 1 0 ATR+RSS based 0.4 0.6 0 ATR+RSS+Cost based 0.3 0.6 0.1 ATR based 1 0 0 RSS based 0 1 0 ATR+RSS based 0.5 0.5 0 ATR+RSS+Cost based 0.33 0.33 0.33 ATR based 1 0 0 RSS based 0 1 0 ATR+RSS based 0.6 0.4 0 ATR+RSS+Cost based 0.6 0.3 0.1 ATR based 1 0 0 RSS based 0 1 0 ATR+RSS based 0.55 0.45 0 ATR+RSS+Cost based 0.45 0.35 0.2
Simulation results Average achievable transfer for test Scenario 1 The algorithm which takes into consideration the cost of each AP can present lower performance than the algorithms which neglect the cost. If the cost is considered the network with the highest achievable rate is not always selected. 22
Simulation results Average achievable transfer for test Scenario 2 ATR based decision has the best performance; RSS based decision has the worst performance; Utility based decision has performance similar to ATR based decision; Utility based decision reduces the effects of ATR meas. imprecisions. 23
Simulation results Average achievable transfer for test Scenario 3 Similar results were obtained in all scenarios. ATR based decision is outperforming other methods if its meas. precision is high enough. Utility based decision in which the ATR has a higher weight has slightly better performance. 24
Conclusions VHO decision algorithms which consider network state and traffic information perform significantly better than classical VHO decision algorithms which consider only the RSS/coverage parameter. An advanced heterogeneous network architecture is necessary for acquisition and dissemination of the network state and traffic information. Computer simulations show that the ATR parameter based target network selection offers the largest transfer rate for the user. If the ATR measurement precision is lower, a combined ATR-RSS based decision can be used with similar results. 25
Thank you for your attention!! 26