Agenda. What are we looking at? Introduction. Aim of the project. IP Routing
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1 Agenda Handoffs in Cellular Wireless Networks: The Daedalus Implementation & Experience by Shrinivasan Seshan, Hari Balakrishnan Randy H. Katz A short presentation by Aishvarya Sharma Dept of Computer Science TKK, Finland Introduction to the problem Purpose of the experiment Design Implementation Performance Analysis Overheads Conclusions and issues Introduction Focus is fast changing, if not already to mobile networking. Power, Mobility & Security remain the key issues Mobile networks suffer from dynamicity. Handoffs are expensive yet essential. Not many systems are operational that provide smooth handoffs with negligible latencies and data losses. What are we looking at? Standard IP routing in Internet is static in nature and seldom changes. Routing tables cannot be changed that frequently. Not a scalable solution. High delay jitter and data loss results are unacceptable for multimedia and for protocols like TCP/IP. Multimedia applications and reliable protocols adapt to long term delays and packet losses. But don t work well with frequent changes. IP Routing Data from mobile hosts (MH) to corresponding hosts (CH) use normal IP routing Trouble is for the downlink data that is to be sent to MH. Two stage delivery Packet delivery to Home Agent (maintains the location of the MH) Forwarding packets from HA to MH using either encapsulation, loose source routing or dynamic addresses. Aim of the project Design a handoff mechanism with low latencies and with minimum data loss Latencies less than ms Typical video is 3 frames/s, with one frame being worth ms. Uses multicast to send packets to nearby base stations together with intelligent buffering. Attempt to keep it similar to IETF Mobile IP routing. 1
2 IETF Mobile IP in brief IETF Mobile IP Routing Encapsulation Each mobile host is assigned a home network, that includes a home agent. Each time the routing changes, the home agent is notified. If the mobile moves far away from it s home agent, it leads to considerable delays Packets during handoffs are either lost or are delayed resulting in poor performance Multicast-based mobile IP routing Routing in proposed solution Uses multicast & intelligent buffering Achieves low latency handoff and reduced packet loss Three steps 1. Delivery of packet to home agent 2. Determine the physical location of mobile host. 3. Deliver packets from home agent to mobile host. Delivering of packet from HA to MH Determining current location of MH Each MH is assigned a long term IP corresponding to it s home location and a temporary multicast address. HA intercepts packets by proxy ARP Neighboring BS join the multicast group, but NOT the MH Each BS broadcasts a beacon MH keeps track of signal strength/quality, etc. MH decides which cell to join and future candidates. MH configures the multicast routing between HA and other BSs. BS for the cell containing the MH & neighboring cells as determined by the MH joins the IP group 2
3 Handoff messaging BS #1 MH BS #2 Handoffs PKT 1 beacon PKT2 buffer request pkt3 Handoff Latency Stronger Beacon Forward request PKT3 PKT4 Mobile initiated List of last few packets received by MH is given to new BS. To let BS determine which packets are already delivered. IETF uses Unicast instead of Multicast MH never joins the Multicast group, but controls which BS join the group and receive packets. Intelligent buffering help in low latencies and low data loss while keeping buffer size small. Idea is to use fixed network to maximum and use the scarce radio resources as less as possible Other Optimizations To reduce latencies VC Trees Splits wireless networks into regions Small regions lead to more handoffs, large regions put a load on the root nodes. Group-based routing Three layer hierarchy of MHs, BSs and supervisor systems. Static multicast groups Both define static regions where handoffs occur smoothly. Implementation WaveLAN 915 MHz Ethernet-like wireless radio Shared 2Mb/s CSMA/CA MAC-Layer Handoffs are between the two base stations connected over Ethernet Implementation 2 Consists of four modules Encapsulator At home Agent Decapsulator At Base Station Beaconing system Route analyzer At MH Modifications to the OS network stack Encapsulator 1/2 Resides at the home Agent When MH leaves home location, it initializes the home agent with a pre-defined multicast address. When MH is away, HA uses proxy ARP to respond to all ARP requests for MH and returns own MAC address so that all messages for MH are intercepted by him. HA kernel contains information about each MH (for which it s responsible) IP Address, Multicast address and TTL information. 3
4 Encapsulator 2/2 Decapsulator 1/2 Routing from CH to MH is done at each BS. On the primary BS, the packets are decapsulated and sent over the air to the MH. Other BSs buffer the packets instead. ~ 1 ms - buffer for x3 the latency time (< 3 ms ) For a data rate of 1.4 Mbps, and each packet of ~ 144 bytes, it implies storing around 12 packets Scans all multicast packets to identify the ones that are for the MH the BS is responsible for. It either buffers the packets (if just listening) or forwards the packets to the MH MH sends in the IP IDs of the last 3 packets received by it before sending a request to a new BS to transmit. Daemon listens to route analyzer (MH) via TCP connection for control signaling Decapsulator 2/2 Beaconing System 1/2 Runs on both MH and BS Primary responsibility to provide location information of MH to Route Analyzer. NIC card measures the signal strength, background noise and quality of received packet. But since this can be done only on reception of a packet, each BS periodically sends a beacon packet. (analogous to BCCH in GSM). Each beacon packet sent over UDP/IP contains IP address of wired and Wireless interfaces of the BS. A beacon frequency of 1 beacon/s yields in 1% degradation in bandwidth Beaconing System 1/2 Route Analyzer Determines the route of packets to mobile host. Two modules Information Retrieval Module Determines the neighbors of the MH Route Control Module Determines which BS is best for routing data Next best BS for a handoff Uses hysteresis 15% better signal strength for a handoff Nearby BS are requested for buffering Uses 3 type of policies 1. All BS in range to buffer packets 2. Fixed number of BS buffering at all times 3. Adapts the number of buffering BSs to rate of MH s motion. 4
5 Performance Analysis 1/4 Performance Analysis 2/4 Handoff Distance between BSs Handoff Latency No. of lost packets Multicast-based with buffering hops 1 hop 2 hops 3 hops Throughput achieved for TCP transfers to mobile host at various handoff frequencies In absence of handoffs, peak output is 1.45Mbps, 1% less than achieved from point-to-point. This is attributed to WaveLAN MAC-level effects. Measured with average transfer of 8 Mbytes of data Multicast but with no buffering No Multicasting and no buffering hops 1 hop 2 hops 3 hops hops 1 hop 2 hops 1-15 ms 1-15 ms 1-15 ms 15-2 ms ms 2-25 ms hops 25-3 ms 4-5 Performance Analysis 3/4 Performance Analysis 4/4 Handoff timings without multicasting and buffering stations at distance three Timing of handoffs in case of multicasting with buffering Protocol Overheads 1. Routing of packets to BS Triangle Routing Not the best route between MH and CH. Reasonable only if HA is near the best route. Alternative: Allow CH to transmit to BS too. Difficult! Multicast Implementation Experiment used DVMRP Better alternatives are available, with special support for mobility. Protocol Overheads 2/2 2. Buffering of Packets Consumes memory and bandwidth Avoid buffering at BS the MH does not visit. Physical layout Degree of overlap High vs. low Mobility Patterns Trade off between network resource consumption and improved handoff performance. Wired network layout of BS Additional network bandwidth by multicasting is acceptable if BSs are on the same physical broadcast-based network. So BS that tend to buffer packets together should be placed closer to each other. 5
6 Problems faced with WaveLAN Conclusions Implements CSMA/CA MAC-layer protocol Adjacent BS contend for the link based on CSMA/CA. Under heavy load, adjacent BS conflict and are denied access for much longer periods, even though if they had packets to send. This continues even when the adjacent BS becomes the primary BS. BS backs off to a point that it remains silent even if it should be transmitting. This leads to a latency of 2-95 ms, but not very often. Multicasting coupled with buffering at BS helps in achieving latencies of and with zero packet loss. Video playback and TCP don t notice any disruptions. Security can be an issue as MH needs to know the network topology. Should we always trust adjacent BS in the network? References Handoffs in Cellular Wireless Networks- The Daedalus Implementation and Experience vey/split/node49.html HO2.pdf 6
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