Wireless networks two components to be considered in deploying applications and protocols wireless links ; mobile computing they are NOT the same thing! wireless vs. wired links lower bandwidth; higher error rates mobile computing a device dynamically changes its access point to Internet over time temporary IP addr, dynamically variable network topology Wireless Network Taxonomy single (or last) hop vs. multi hop (ad hoc) B B FH B single hop: computation/communication mainly carried out by the fixed infrastructure s only communicate through the Bs ad hoc: s must act as routers
Wireless Network Taxonomy W PAN: Wireless Personal Area Network IEEE 802.15 standards heavily based on the Bluetooth specification short range of communications (0 to 30 mt.) wireless connection of personal devices microphones, handheld device with PC, badge W LAN: Wireless Local Area Network IEEE 802.11 standard (Wi Fi) larger communication range (up to 100 150 mt.) network infrastructure where wiring is not possible Physical Layer infrared: short distance, highly directional radio frequencies: omnidirectional propagation s equipped with either uni or omni directional antennas signal subject to interferences (from other sources in the same frequency range), attenuation (with distance), distortion, multi path fading?
Physical Layer IM: Industrial/cientific/Medical band in 2.400 2.484 GHz does not require licensing to be used adopted by both Bluetooth and Wi Fi (!) transmission policies that provide robustness (to noise) and security direct sequence spread spectrum frequency hopping spread spectrum 1 3 2 4 F F Discussion omnidirectional propagation + shared frequency range all the s in the source range receive multiple neighbour sources interfere with each other policies needed to arbitrate the access to the broadcast medium and avoid collisions collisions both inside a cell, and amongst cells coordinator inside a cell (e.g. the B, the Bluetooth master) to control the access usually s do not send and receive contemporarily sender and receiver must be synchronized to exchange data
MAC Layer TDMA: slotted channel used for instance by Bluetooth unused slot = wasted resources (same for FDMA) or: slot reservation (separate data/control channels) piconet M M 1 M 2 M Time MAC Layer CDMA: orthogonal codes do not interfere similar to graph colouring problem network configuration: which code to use inside each cell (same as FDMA) when a moves into a new cell, it must be reconfigured to use the appropriate code
CMA: hidden/exposed stations hidden station: A and C both sense the channel and then they transmit collision at B A B C exposed station: C senses the channel and detects the B transmission (to A) s C does not send (to D) resource waste A B C D solution: temporarily reserve the medium Multiple Access with Collision Avoidance MACA is used for instance in 802.11 (optional) it solves both hidden st. and exposed st. problems refrain from sending till end of CT possibly refrain from sending till end of frame A B C D RT (B, C, 512B) CT (512B) data refrain from sending till end of frame RT short frame: lower cost in retransmitting it rather than a (longer) data frame
Mobility support so far, discussed only wireless issues moblity: a can connect to the network from different locations at different times; it is able to retain its network connectivity while moving impact on network protocols and applications addressing in different cells routing location roaming support (i.e. continuous service provisioning) Mobility support periodic monitoring of the neighbourhoods (beaconing) I am here! Who can hear me? e.g. inquiry in Bluetooth in single hop networks: handoff procedure B 1 Please, give me the status info for B 2 (reconfigure ) I was with B 1 before
Roaming support sessions maintained in spite of movements difficult: e.g. think at reliable only once multicast B 1 source B 2 session re direction mobility tracking (positioning) movements monitored for advanced configuration of the next location (e.g. for Qo) Mobile IP: home servers Impact on higher layers TCP performance degrades in wired networks: loss due to congestion shrink window and send more slowly in wireless networks: loss due to network unreliability send more aggressively, and more quickly! difficult to estimate retransmission timer how should TCP deal with heterogeneous (i.e. partly wired and partly wireless) connections? a lot of ongoing research work
Research issues technologies interoperability (e.g. Bluetooth with 802.11) routing in ad hoc networks IETF manet WG proactive vs. reactive ; flat vs. hierarchical context awareness vs. transparency should the service provided to a depend on the context (e.g. network status) or not? location awareness vs. transparency (positioning) possibly dependent on the application (e.g. weather forecast, parking availability )