Mobile and sensor networks - Appunti, esercizi svolti, domande teoriche

B-MAC

Contention based, minimalistic protocol; has configurable options, minimizes idle listening, has periodic sensor data transfer

CSMA (no RTS/CTS) + optional ACK + optional LPL (Low Power Listening)

Easier to implement than S-MAC

CCA (Clear Channel Assessment): main feature of B-MAC

1. before transmitting, a random BO value is drawn and then the channel is sampled
2. if the sample > noise floor the channel is idle, transmitter can send immediately;
3. if the sample < noise floor transmitter has to take other 5 samples
4. if in those 5 samples there is no outlier the channel is busy another BO is needed
5. if in those 5 samples exists at least one outlier channel is idle, transmitter can send immediately

noise floor is updated when the channel is known to be clear

LPL (Low Power Listening): minimizes listening cost

Node wakes up, turns on its radio, samples the channel using CCA

• If energy is detected node powers up

receives the packet→ Node is back to sleep after having received a packet, after a timeout→ Preambles > check intervals are used receiver is woken up and can participate to the communication; large preambles are preferred to more frequent checks Suitable for single hop applications doing data samplingo Sampling rate defines optimal check interval

802.15.4• Devices can be:o FFD (Full Function Device): can be found in any topology, is PAN (Personal Area Network) coordination capable, talks to other devices, has a complete protocol seto RFD (Reduced Function Device): can be found only in certain topologies, can’t become PAN, talks only to other networks coordinator, has a reduced protocol

There are three main topologies:o Star: 10 m range, has a unique PAN that must be FFDo P2P: mesh network, every node must be able to root; any device can communicate with any device (ah-hoc + self organizing); multiple hops are used to route messageso Cluster-tree: the PAN is

also the cluster head and devices form a tree-like network around it; quite similar to BT Scatternet; increases area coverage but increases also message latency

• Physical packet field: preamble + start packet delimiter + physical header + PSDU (data)
• MAC layer: header of 7 bytes, has CRC to detect correct demodulation
• Coordinator can determine whether to work in beacon-enabled mode or not

Beacon-enabled mode: 15 equally spaced slots + beacon; PAN periodically sends beacon for synchronization; time is slotted; BO period start aligned with the beacon start; superframe is divided into CP and CFP.

PAN can store messages sent to sleeping nodes and deliver them once awake. To save energy, sensors' RF are powered down whenever it's possible: the coordinator can define idle periods within the superframe by sending everyone to sleep or individual nodes can enter a LP state at any time (given that energy gain > cost of entering LP); in this last case, a receiver

won’t hear any incoming transmissions

No beacon-enabled mode: no beacon is issued by the PAN, BO periods are not synchronized, uses unslotted CSMA/CA

• Poll-based transfer: the coordinator receives data for a sleeping node B coordinator notifies all nodes with a beacon saying “B has an incoming data packet” B wakes up and sends a data request to the coordinator the coordinator replies sending data

Slotted CSMA/CA

• Parameters:
• NB (inner bock): times the CSMA/CA was required to BO while attempting to transmit; NB has to be smaller than MaxCSMABackoff. NB resets if MaxCSMABackoff is reached and increments whenever a BO expires.
• CW (inner block): contention window length, defines the number of BO periods that are needed to be clear before a transmission can start
• BE (inner block): backoff exponent, related to how many periods a device must wait before attempting to access the channel again; BE has to be smaller than MaxBE
• NF (outer block): times a

resets if MaxFrameRetry is reached and increments whenever an expected ACKis not received

• Transmission procedure:
  • Wait for a random BO time
  • If the channel is busy, wait for a random BO
  • If the channel is idle, transmit

No RTS/CTS, ACKs are optional

ACKs are sent after SIFS from the frame reception end

→ → → → →Transmission sequence is: BO CCA data1 SIFS ACK SIFS

Past exams theory questions (to be answered in 600 characters including spaces)

• Bluetooth
  • Cheap, requires less energy than Wi-Fi, flexible and reconfigurable system. Organised in piconets(1 master and up to 7 slaves) grouped into scatternets; 3 power classes, has fast frequencyhopping over 79 MHz spectrum (AFH possible to reduce interferences and use only goodchannels). Many types of links: SCO, eSCO, ACL. Connection oriented.
• B-MAC
  • Contention based protocol for sensor networks, simpler than S-MAC. CSMA based, no CTS/RTS, optional ACK, LPL (low power listening) possible.

Channel is declared busy or idle through CCA procedure: before transmitting a random back off is performed and then a sample is taken from the channel; if sample < floor noise channel is idle and transmission is possible. If sample > floor noise, 5 more samples are taken: if all 5 samples confirm the first comparison channel is busy, if there is at least 1 outlier channel is idle. LPL minimizes listen cost by implementing wakeup/sleep mode and long preambles.

Comparison between Bluetooth and BLE:

• BLE consumes less energy than BT and has a much lower peak current consumption;
• BLE transmits over 40 channels (3 reserved to advertising), 2 MHz each while BT uses 79 channels, 1 MHz each;
• BT can implement AFH, BLE devices are awake only when a connection is initiated hence they are used for non-continuous streaming;
• BLE security is easier to break.

S-MAC:

Contention based protocol for sensor networks, improves energy efficiency but increases latency and lowers fairness, all sensors.

1. Deliver to a sink node that collects samples/connects to the internet. Can use coordinated sleep (radio off when asleep) to cope with idle listening, overhearing avoidance by sleeping when neighbours talk, adaptive listening to reduce multi-hop latency due to periodic sleep, collision avoidance.
2. 4 main issues of D2D networks:
   • TOPOLOGY NETWORK: arrangement of nodes and links may change due to node movement or link failure; topologies are flat, tree, cluster-based.
   • CHANNEL ACCESS: limited range, packet lost when errors occur, listen before transmit, BO in case of collision, hidden terminals.
   • MULTI-HOP COMMUNICATION: each node becomes a router; needs a coordinator and algorithm to favour co-op; problem with users sharing device performances.
   • ENERGY CONSUMPTION: small batteries to increase portability.
3. S-MAC trade offs:
   • Coordinated sleep helps improving energy consumption (radio is turned off when sleeping) but increases latency (since duty cycle is reduced to 10%).
   • In order to work as bridges,

nodes laying between two differently scheduled groups need to stay awake for both the schedules, hence consuming more energy (fairness problem).

• Paging in Bluetooth

Process to form connections between BT devices and define roles; message is unicast to a selected listener and sender has an estimate of listener's clock. When device to be paged receives a paging msg it responds acknowledging reception, pager replies with FHS packet, paged device sends ACK back, pager starts data exchange.

• When are packets discarded in the unslotted protocol?

Packets are discarded if the number of time the CSMA/CA was required to back off (NB) exceeds a max number (MaxCSMABackoff) or if the number of times a packet has been transmitted (NF) exceeds a max number of attempts (MaxFrameRetry).

• LPL in B-MAC

Minimizes idle listening: node wakes up, turns on radio and samples channel using CCA. If energy is detected the node turns on to receive packet; after receiving or timeout node goes back to sleep.

Large preambles (preferred to frequent checks) are used to wake receiver up and have it ready; LPL is suitable for single hop applications doing data sampling.

• Inquiries: Asynchronous procedure to discover new devices and exchange neighbours MAC and clock. No synchronisation between 2 or more entities, sender stops repeating inquiry procedure when it has contacted a sufficient number of nodes. Sender transmits IAC over two hops and waits for feedback; random BO used by listeners to avoid collision of replies.
• Beacon-enabled mode: Superframe made of 15 equally spaced slots + beacon; PAN periodically starts beacons to synch the network; time is slotted: BO periods begin with beacon start. Superframe divided into CFP and CP and PAN can store messages addressed to a sleeping node and deliver them when it wakes up. When saving energy a receiver won't ear any transmission: a solution is to use poll-based transfer.
• Frequency hopping in Bluetooth: Bluetooth uses FH as a mean to

Per evitare interferenze e collisioni, vengono utilizzati tutti e 79 i canali in modo equo. In BT, AFH (Adaptive Frequency Hopping) può essere implementato: una mappa indica i buoni canali RF che possono essere utilizzati per la trasmissione (canali selezionati tramite un algoritmo specifico), mentre i canali cattivi vengono rimappati per diventare nuovamente utilizzabili. I buoni canali non sono influenzati dalle interferenze WiFi.

LEACH (Low-Energy Adaptive Clustering Hierarchy) è un protocollo basato su programmazione per reti di sensori, con CDMA tra i cluster e TDMA all'interno dei cluster. I nodi comunicano solo con il cluster head, che a sua volta comunica solo con la stazione base. Poiché il cluster head consuma più energia, viene adottata la rotazione del cluster head per aumentare l'equità del consumo. Un cluster head può essere considerato come un master nelle reti BT.

Le topologie Bluetooth prevedono che i nodi Bluetooth si riuniscano in piconet, dove...