CSC 290e Mobile Computing Wireless LANs 10 October 2001 Reference: Wireless Communications and Networks - Stallings Chapter ------------------------ ------------------ Wireless LANs Applications * sustitute for traditional wired LANs, lower installation cost, ease of relaocation of components * buildings with large open areas - manufacturing plants, stock exchange trading floor, warehouses * historical buildings, small offices, * inherently mobile settings * cross building interconnect Styles: nomadic access - wireless link between LAN hub and mobile data terminal ad hoc networking - peer-to-peer network, no infrastructure group of employees temporarily meeting Requirements: throughput - efficiently use the wireless meduim to maximize capacity' number of nodes - support hundred of nodes in multiple cells connection to backbone LAN - common case. ad hoc networks less common service area - typically 100 to 300m battery power consumption - issue for battery-powered stations want protocols with less traffic to reduce power consumption transmission robustness and security - easy to eavesdrop, noisy environments collocated network operation - 2 or more wLANs in same area license-free operation - don't want to have to buy a frequency band handoff/roaming - MAC should allow users to move from one cell to another dynamic configuration - dynamic and automated addition, deletion, relocation of end systems without disruption to other users Technologies Infrared (IR) LAN - limited to a single room, light does not go through walls Spred spectrum LANs - mostly in ISM band (industrial, scientific, and medical) to no FCC licensing is required 900MHz, 2.4 GHz, 5.7 GHz access with CSMA IR - remote controls can achieve high bandwidths can use ceiling reflection to achieve coverage of a room more easily secured than microwave equipment relatively inexpensive and simple - subject to background radiation interference from sunlight and indoor lighting transmission techniques: directed beam - focused/aimed (e.g., remote control) can be km in range (good for cross building) omnidirectional - single base station within line of site of all other stations acts as a repeater, can be received by all in area diffused - good for ad hoc networks transmitters focused and aimed at a point on a diffusely reflecting ceiling. This is then reradiated and picked up by all in the area we will come back to radio-based LANs ------------------------ Structure of Wireless LAN base stations, wireless nodes, fixed infrastructure How to get message from one wireless node to another? use the fixed infrastructure to do the routing Works when nodes stay within their home network why does it break when a host leaves its home network? fixed routing infrastructure can't handle that movement it thinks your ip address should have only one location if you change your location, the infrastructure can't figure out where you are Mobile IP is one mechanism that allows you to move to a new network and remain connected. More on this in a few days ------------------------ Ad Hoc Networks no infrastructure, no centralized administration Scenarios it might be useful: robots on Mars robots in unknown area short duration meetings disaster relief workers If all close together, no problem, just talk directly If multiple hops to destination, need a routing mechanism unlike the fixed network, ad hoc networks cannot rely on the fixed infrastructuer to do the routing DSR - dynamic source routing --- no periodic routing requests - important because sending/receiving periodic messages consumes power - low overhead when topology not changing assume all nodes will participate in the routing process Overview: each packet contains the "source route" each host maintains a "route cache" a host can initiate "route discovery" to send a packet to an unknown host need to do "route maintenanace" to monitor integrity of the route NOT a reliable protocol. e.g., if a host needs to do a route discovery, it is perfectly acceptable for it to drop the packet that triggered the discovery, and expect a higher layer in the protocol stack to retransmit Route Discovery: broadcast a route request contents: source (who is doing the discovery) target of the route discovery (who you want to find) route record - hops visited so far by the route request processing on receipt 1. if the pair has recently been seen, drop 2. if this host is in the route record, drop 3. if target = this host, return copy of the route in a route reply 4. else, append this host's id to route record, re-broadcast loops not possible because of (2) avoids broadcast storms because of (1) reply can be sent along reverse of route record, or in case of non-bi-directional links, the reply can piggyback on a route request back to the original source Route Maintenance: traditionally done with periodic transmissions route discovery does not have periodic transmissions do hop-by-hop ack at data link layer to early detect failures passive acks - if you hear someone retransmit it, treat it as an ack send a route error packet to the source or store the error, and start a new route discovery (shorter) Optimizations: treating the route cache as a tree, and using partial routes a->b->c->e |->d contains routes for a->e a->d and also a->c can add entries when forwarding data packets when you are an intermediate hop can add entries when forwarding a route reply can promiscuiously receive messages and build route cache can answer a route request from your cache what if receive multiple hosts reply? can cause a collision in the medium can result in routes of different length solution: delay slightly before returning a cached value delay period is based on the number of network hops in the route to be returned, and listen for other replies while waiting can create loops! a->b->c->d If b needs the route to d, does a route request, and a replies, then the route would be b->a->b->c->d solution: don't send a route if it contains loops