Outline Wireless Ad Hoc & Sensor Networks (Wireless Sensor Networks – Part II) • MAC protocols • Naming and Addressing • Time Synchronisation (self studying part not included in the exam) • Summary Summary WS 2010/2011 WS 2010/2011 Prof. Dr. Dieter Hogrefe Dr. Omar Alfandi 2 MAC protocols MAC protocols – Energy Problems • Medium access in WSNs is difficult mainly because of • Recall: Transceiver consumes a significant share of energy – Impossible (or very difficult) to send and receive at the same time – Interference situation on the receiver is important, but can be – Sending is costly; Receiving costs are often almost the same different from the observed situation by the sender – Idling is cheaper, but about as expensive as receiving – High error rates for signalling packets High error rates for signalling packets – Sleeping costs almost nothing, but results in a deaf node Sleeping costs almost nothing but results in a “deaf” node • Requirements • Derived energy problems regarding the MAC protocol – High throughput, low overhead, low error rates g g p , , – Collision : Waste of effort when two packets collide Collision : Waste of effort when two packets collide – Energy efficiency, handle switched off devices – Overhearing : Waste of effort when receiving a packet that was directed at another destination – Protocol overhead : Waste of effort due to MAC-related overhead – Idle listening : Waste of effort when waiting for incoming packets, but nobody is sending but nobody is sending 3 4
MAC protocols – Main Options MAC protocols – Centralised Medium Access • Idea: A central station controls, when a node may Wireless medium access the medium access – Example: Polling, centralised computation and assignment of time slots (TDMA) • Simple and efficient, but burden to the central station p , Centralised Centralised Distributed Distributed • Not feasible for large WSNs, but if network is divided into smaller groups, this approach can be useful – e.g. compare Bluetooth piconets Schedule- Contention- Schedule- Contention- based based based based Usually, only distributed medium access is considered! Fixed Demand Fixed Demand assignment g assignment g assignment g assignment g 5 6 MAC protocols – Distributed Medium Access MAC protocols – Main Options • Schedule-based protocols Wireless medium – TDMA component provides a schedule regulating which access participant may used which resource at which time ti i t d hi h t hi h ti • Typical resource : frequency band (with given CDMA-code) • Implicit idle listening avoidance mechanism p g Centralised Centralised Distributed Distributed – Schedule can be fixed or computed on-demand • Sometimes mixed – Collisions, Overhearing and Idle listening are no issues C lli i O h i d Idl li t i i Schedule- Contention- Schedule- Contention- – BUT: time-synchronisation is needed based based based based • Contention-based protocols • Contention based protocols – Risk of colliding packet is deliberately taken Fixed Demand Fixed Demand – Mechanisms to avoid/reduce collisions required ( often random) q ( ) assignment g assignment g assignment g assignment g 7 8
Contention-based MAC protocols I Contention-based MAC protocols II • Basic options • Receiver informs potential interferers – ALOHA not good in most cases – While a reception is on-going • By sending out a signal indicating the reception (Busy tone protocol) – Listen before talk (CSMA) • Problem: cannot use the same channel on which the reception • BUT: sender is not knowing what is going on at the receiver takes place used separate channel for signalling might destroy packets despite listening g y p p g – Before a reception is on-going • Besides, receiver needs possibility to inform possible • Can use the same channel senders in its neighbourhood upcoming transmission • Receiver itself needs to be informed • Receiver itself needs to be informed “shut them up” for this duration – Recall: • Hidden Terminal Problem Hidden Terminal Problem • Exposed Terminal Problem A B C D 9 10 Contention-based MAC protocols – S-MAC I Contention-based MAC protocols – S-MAC II Listen Listen Listen Listen • Only in the ‘listen periods’ SYNCH RTS CTS • Problem period period data will be exchanged – In WSNs most of the time nothing happens, i.e. there is only a Sleep l low data rate d t t – Sending & receiving S di & i i Period Wake-up – For low data rates, MACA’s idle listening is unsuitable • SYNCH period Duty-Cyle = • Idea: use ‘rendez-vous’ mechanism • Idea: use rendez-vous mechanism Listen period length/ Listen period length/ • • Synchronisation of the sleep schedule by Synchronisation of the sleep schedule by Wakeup period length exchanging schedule tables with neighbours – turn off nodes and ensure that neighbouring nodes turn on simultaneously to allow packet exchange • Forming of virtual clusters “synchronised islands” • Proposal of this approach: S-MAC • Data transfer – S-MAC = S ensor- MAC • Perform RTS/CTS P f RTS/CTS – S-MAC is energy efficient and provides collision avoidance and overhearing • Transfer data, ACK Virtual clusters • Not affected nodes change to sleep mode • Not affected nodes change to sleep mode 11 12
Contention-based MAC protocols – T-MAC Contention-based MAC protocols – Preamble Sampling • In S-MAC the ‘listen period’ is of fixed length • Until now, periodic sleep was used to synchronise the sleep and wake up phases of the nodes • Problem • Alternative: Don’t try to explicitly synchronise nodes – What happens when there is no traffic? nodes have to stay awake unnecessarily – Regularly sample the medium to check for activity, rest of the time change to sleep mode time change to sleep mode • Idea Idea • Use long preambles to ensure that receiver stays awake – Prematurely go back to sleep mode when there is no traffic for a certain time (timeout) ( ) to catch the actual packet to catch the actual packet adaptive duty cycle – e.g. WiseMAC Wake-Up Preamble WUP Data Sender Sender • Implementation of this idea: T-MAC – T imeout- MAC ACK Receiver Rx Tx – As S-MAC, but uses timeout to reduce listen period As S MAC but uses timeout to reduce listen period Sleep Period Wake Up Wake Up 13 14 Contention-based MAC protocols – B-MAC I Contention-based MAC protocols – B-MAC II • B-MAC combines several of the mentioned approaches • Low Power Listening – Tries to provide practically relevant solutions – Preamble sampling – Uses the clear channel assessment techniques to decide • Clear Channel Assessment whether there is packet arriving when node wakes up – Adapts to noise floor by sampling channel when it assumed to – Timeout puts node back to sleep if no packet arrived Timeout puts node back to sleep if no packet arrived be free be free • B-MAC does not have – Samples are exponentially averaged result used in gain control – Synchronisation y – For actual assessment when sending a packet, look at five – RTS/CTS channel samples channel is assumed as free, when even a • BUT: single sample is significantly below noise single sample is significantly below noise – This results in simpler and slimmer implementation – Optional: Random back-off, if channel is found busy – Clean and simple interface • Optional: Immediate link layer ACKs for received packets p y p • Currently, B-MAC is considered as default MAC protocol 15 16
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