Survey Topic: WiFi On The Move Presented by - Abhinav Tekumalla (atekumal) Bahula Gupta (bahulag)

Transcription

1 Outline Survey Topic: WiFi On The Move Presented by - Abhinav Tekumalla (atekumal) Bahula Gupta (bahulag) WiFi on the move : Challenges VanLAN ViFi Cabernet HSPDA/ WiMax WiFi on the move Providing WiFi connectivity to moving vehicles. Supports broadband data rates Operates in unlicensed band 2.4 / 5 GHz band Cheaper as compared to cellular networks Ubiquitous deployment of inexpensive networks Challenges Wireless environment changes rapidly as vehicle moves WiFi range is limited: order of 10s of meters Due to the mobile nature of the client devices, frequent Handoffs are required AP 2 AP 1 WiFi Access in Vehicles through WiMax/HSPDA Goal: To provide WiFi access in vehicles using an Access Point connected to the WiMax/HSPDA network. Access Point in the vehicle acts as a gateway between the WiMax and WiFi communication methodologies Ideal to be implemented on buses and trains Experiment performed in Korea over WiBro network WiFi access in vehicles using HSPDA/ WiMax Internet HSPDA/ WiMax AP Bus/ Train 1

2 VanLAN Goal: Analyze wireless link characteristics between a base station and a vehicle in an urban environment. Contributions: Proposed a way to predict performance using real time measurements of BRR (Beacon Reception Rate) Features of VanLAN Modified beacons of 500 Bytes in size are transmitted every 100ms. Channel 1 and Channel 11 are used A session starts when a beacon is received and ends when no beacon is received for a fixed time Two time thresholds are used : Meta and Mini Meta session threshold = 60 secs Mini session threshold = 2 secs Prediction of Gray Periods Occurrence of Gray Periods is quite complex and highly unpredictable Poor connectivity periods can arise even close to the BS Paper suggests two prediction techniques: 1.Using Current Measurements 2.Using data aggregated over multiple trips on the same route. RSSI (Received Signal Strength Indicator) and BRR (Beacon Reception Rate) can be used to gauge the strength of the connection Using Current Measurements A lower BRR is often followed by a Gray Period There is no threshold to reliably gauge the onset of a Gray Period Present of a threshold may lead to false positives or false negatives Using Long Term History Data aggregated over a period of time might be used to predict the regions of Gray Period If most Gray Periods occur at consistent locations, for instance, because of some obstructions, then this method can be used to predict the regions. For other Gray Periods, it is hard to predict Interference and wireless variability in the environment are often responsible for Gray Periods Contributions Defined and measured the occurrence of Gray periods in vehicular WiFi access The proposed methods are not very accurate in predicting the occurrence of Grey Periods. In practice, Gray Periods are very unpredictable since they are a function of the wireless environment Were able to identify regions of good connectivity up to a certain extent 2

3 ViFi Goal: Develop a protocol which relies on macrodiversity - using multiple base stations simultaneously This coverts hard handoff to a smooth handoff. Uses probabilistic algorithm for coordination between multiple base stations(access points) Protocol Overview The vehicle designates one of the base stations as an anchor The base station with the highest BRR is selected by the vehicle as the anchor Anchor primary access point for any packets directed to and from the vehicle All other nearby base stations are designated as auxiliary base stations Protocol Implementation The source transmits a packet and the destination broadcasts an ACK upon receipt of the packet If the auxiliary does not hear the ACK in a certain time frame, it probabilistically relays the packet. If the destination receives the relayed packet and it has not already sent an ACK, it broadcasts an ACK If the source does not receive the ACK within a retransmission interval, it retransmits the packet Salvaging When a vehicle moves out of range, but the old anchor still has packets for the vehicle New anchor identifies old anchor using beacon sent by vehicle Upon request from the new anchor, the old anchor transfers the unacknowledged packets to it The new anchor then sends these packets to the vehicle ViFi Anchor Positives: As the handoff becomes smooth, there will be longer sessions and lesser disruptions in communication Negatives: This protocol requires a vehicle to detect multiple access points at any given point on its path which is not very feasible Auxiliaries 3

4 Cabernet Goal: Developed a new client side protocol as an enhancement to TCP Specifically built for fleeting and intermittent connections Features: QuickWiFi CTP (Cabernet Transport protocol) CTP - Cabernet Transport Protocol Capable of distinguishing packet losses due to congestion from packet losses over wireless link The changes made in the protocol are on the client (vehicle) side and not in the existing APs. Internet host must be aware that the client is using CTP protocol Client Side IP Address Abstraction CTP exposes a reliable socket API to the application When handoff occurs, the client reconnects using a network independent unique identifier CTP makes the handoff transfer seamless by hiding the IP address changes from the application The connection does not break Congestion Control CTP reacts only to congestive losses at the AP s end and not to losses occurring due to wireless losses CTP sender sends probe packets periodically to the AP, lack of response is interpreted as congestion at AP CTP adjusts the transmission rate depending on probe response from AP This rate adjustment helps increase uplink throughput as compared to TCP QuickWifi A client side feature of Cabernet that drastically reduces connection establishment time Fully automates the following features : AP scanning AP selection Association DHCP negotiation Address resolution End to end connection establishment Reduces time required to establish connection to ~350ms Features of QuickWiFi Associates with the first AP whose beacon is heard Limits number of retries and therefore reduces retry time period to the order of milliseconds Ping through test internet connection Connection loss monitoring if no transmissions for 500 milliseconds, scanning is resumed to associate with new AP 4

5 Positives: In fleeting connections, IP address abstraction can reduce the time required for handoff The time required to establish a connection is drastically reduced using QuickWiFI Negatives: If the DHCP server takes long to respond, the client keeps on trying to establish a connection Maximum bit rate used is 11Mbps, which would mean a lower throughput. Average downlink throughput of WiMax: 2799 Kbps Average uplink throughput of WiMax: 500 Kbps Meant to be shared by multiple WiFi devices Results in very low throughput per device Can be a viable option only when WiMax can support larger bandwidths Problems considered solved ViFi is a good model to smoothen the handoffs which are very frequent in case of a vehicular communication Cabernet reduces the connection time establishment time and also provides for a smooth handoff. If better downlink rates are provided, WiMax can be a strong contender Open ended research options The environment is changing and unpredictable, the onus is on developing a protocol that can provide acceptable performance under these condition The quality of connection is variable due to the Gray periods which cannot be predicted The ideal protocol should be able to transfer large amounts of data in small and variable connection times References Understanding WiFi-based Connectivity From Moving Vehicles, IMC, October 2007 Interactive WiFi Connectivity for Moving Vehicles, ACM Sigcomm 2008 Cabernet: Vehicular Content Delivery Using WiFi, Mobicom 2008 A Measurement Study on Internet Access in Vehicular Wi-Fi Networks - Younghyun Kim, Jaeduck Ko, Wonjung Kim, Sangheon Pack 5