Telematica-MSQuestions

8/3/2019 Telematica-MSQuestions

http://slidepdf.com/reader/full/telematica-msquestions 1/8

Mobile and Sensor NetwoksNote: Please put your name inside square brackets [ name ] when adding your answer

below the corresponding question. 1. Describe advantages and disadvantages of multi-hop communications in wireless adhoc networks.

[Marcos] (Copied from AdHoc Network slides, #23)

Advantages :

1. Reduces interference.

2. Helps in saving transmit energy.3. Increases network capacity since the spatial domain can be reused for currentcommunications.

Disadvantages :

1. Increases the network traffic load.2. Increases traffic delivery delay.3. May increase the total energy expenditure.

2. Describe how QoS is provided through the IEEE 802.11e EDCA scheme (i.e., the

distributed access scheme). Also, describe the parameters used at the MAC layer for the channel access and how they should be set according to the level of traffic priority.

EDCA operates in CP (contention period)

it support 4 access categories, coresponding to 4 MAC-level queues (virtual STA).

Each AC has different CSMA/CA instances, AIFS, BO and TxOP_Limit (based onpriority) -> these parameters can be used to classify the traffic. The higher priority willhave smaller AIFS, more chances to send its packets...

If there is a collision (BO reachs 0 at the same time) among ACs in the same QSTA,then the higher priority AC will have TXOP to transfer the packet, the lower priority ACwill behave as it is an external collision.

Once the station has gained access to the medium, it can be allow to send more thanone packet without contending again (limited by TXOP_Limit). TXOP_Limit is not thesame for all ACs. SIFS (< DIFS) is used between packets + Burst ACK -> increasethroughput.



EDCA also provides the fairness among the same priority queue (independently of theframe size)

3. Discuss the fairness issue in wireless ad hoc networks that use the IEEE

802.11 standard at the MAC layer.

- Issue 1: Impact of node location.

+ Chain topology(A-B-C-D): A->B; C->D -> A fail to send to B (collision) -> A starts BO,then resend, then fail again. Throughput at A -> 0, throughput at C very high.

+ A->B, C->D, E->F; C can hear A, E -> never access the channel successfully

- Issue 2: 802.11 anomaly effect with CSMA/CA providing per-packet fairness: if thereare two sources (one fast, one slow) transmitting packets at the same length then both

sources have the same throughput but both low.

4. Define the concept of per- packet and of temporal fairness in channel access.Indicate the conditions under which per-packet and temporal fairness coincide.

[Marcos] (Copied from the slides)

• Per Packet Fairness: If two adjacent senders continuously attempt to send packets,each of them should send the same number of packets in the long run.

• Temporal Fairness: If two adjacent senders continuously attempt to send packets,each of them should transmit for the same amount of medium time. Per-packet and temporal fairness are equivalent if stations transmit packets of samesize and at the same data rate. In this case, all stations will also experience the samethroughput, otherwise:

● Same packet size but different rate: Per-packet fairness implies same throughputfor all stations; temporal fairness does not.

● Different packet size but same rate: Temporal fairness implies same throughputfor all stations;per-packet fairness does not.

● Different packet size and rate: hard to say

5. Describe the major problems/inefficiencies that affect the current IEEE 802.11p MACscheme, i.e., the current standard for vehicular networks.

[From slide Page 15-16]



1. Safety messages require low latency and reliable delivery (max latency: 20-200 ms,driver’s reaction time: 0.75-1.5 s). Although 802.11e supports QoS, it is not suitable for delivery of safety messages:

- The CSMA/CA mechanism implies unpredictable delays in accessing the channel,especially under high-load conditions (unacceptable for delay-sensitive applications).

- Collision probability increases as the number of MAC instances contending for thechannel increases. Note that safety messages can be generated by several vehicles atthe same time+ TDMA, FDMA or CDMA-based approaches are difficult to implement due to the needto assign time slots, frequencies or codes in a centralized manner in highly-dynamicscenarios+ The common time reference obtained through GPS allows nodes to besynchronized thus opening to distributed, TDMA-based MAC schemes2. Unreliable broadcast transmission (because of hidden terminal).

6. Describe the reactive and proactive approaches for traffic routing in wirelessad hoc networks. Also, highlight advantages and disadvantages of the twoapproaches.

[From slide 07-adhoc-routing, page 3]

The Proactive Approach: Based on distance vector and link-state mechanisms.Attempts to maintain consistent, up-to-date routing information from each node to everyother node in the network

- Respond to changes by propagating updates throughout the network- Good for connection-less traffic (send traffic to any node at any time)- Low latency, high overhead. It is preferred when time constraint is important.

The Reactive Approach: Routes are only created when desired by the source node.It has Route discovery protocol which ends either when a route is discovered or allpossible communication paths have been examined. Once a route is established, it’smaintained by some sort of route maintenance protocol.

- High latency, low overhead. It is suitable for mobile environment with low limitedbandwidth.

7. Describe how the reactive routing protocol for ad hoc networks, named DSR,works.

[From slide 07-adhoc-routing page 17]

The protocol consists of two phases:• Route discovery: Started by S when S needs to send data to D and doesn’t have



any route to D in cache- First, S sends RREQ to its neighbors (multicast or broadcast). RREQ has the

information of destination address and address of all traversed node.- If no reply by the “time to go”, S sends a new RREQ till a max no. of attempts

have been made.- When a node receives a RREQ, it must check to be sure that it does not

process it before (as a source node or intermediate/traversed node).- When RREQ reach destination or a node that knows a route to destination, it

will send back to the Source node a RREP packet including route record and cost intoRREP. RREP is sent by reversing the route followed by RREQ. If unidirectional, useany cached route to S. If none, send an RREQ and piggyback RREP.

- Route Caching: S caches the route to D contained in the RREP. For each D,more than one route can be cached, thus S can perform traffic routing over severalpossible routes

- Data is sent from S to D following the discovered route.• Route Maintenance : While using a route to D, S can detect if the route is notlonger valid and, in case, send an error message. Upon route failure, S may useanother route(if it knows it) or start a new route discovery.

- Along a source route, each node transmitting a packet is responsible for correcttransmission confirmation -> An ACK is needed

– MAC or link-layer ACKs can be used – Overhearing next node forwarding the packet (passive ACK: A hears

B sending to C)

– Use of DSR-ACKs: the node transmitting the packet can require anACK (which may follow a different route back)- If a node does not receive ACK for some times, it will mark link to next hop asbroken, remove from caches all route including that link, and finally send RERRto all sources that sent a packet to be routed over that link.- Nodes overhearing RERR remove from their cache all routes including thebroken link.- Source node can use an alternative route from its cache if it has any; otherwiseit starts a new route discovery.

8. Consider examples of wireless network scenarios where the behavior of the nodesand their interaction can be studied by using Game Theory. In particular, select (at your choice) one among: (i) the forwarder’s dilemma, (ii) the joint packet forwarding, and (iii)the jamming game. Then, describe the problem you selected, define the type of gamewith which the behavior of the nodes can be represented and the node payoffs. Finally,



comment on the existence of the Nash equilibrium point(s).

Forwarder’s dilemma: R1 <-> P1 <-> P2 <-> R2

- Players: p1, p2 need to send to R1, R2

- Decision: Forward, Reject

- Strategy: J(F,F) = (1-c,1-c), J(F,R)=(-c,1), J(R,F)=(1,-c), J(R,R)=(0,0)

- Type: simultaneous, non-cooperative game, non-zero sum game.

- Nash equilibrium: (R,R)

Joint packet forwarding: S <-> P1 <-> P2 <-> D

- Players: p1, p2 forward packet from S->D

- Decision: Forward, Reject

- Strategy: J(F,F) = (1-c,1-c), J(F,R)=(-c,0), J(R,F)=(0,0), J(R,R)=(0,0)

- Type: simultaneous, non-cooperative game

- Nash equilibrium: (R,R), (F,F)

The jamming game:

9. Consider a Bluetooth scatternet, composed of two piconets. Indicate when the twopiconets interfere with each other and with which probability such an occurrence takesplace.

[Marcos]

10. Consider the Bluetooth technology. Describe how the inquiry and pagingprocedures work.

[Andrea]--> from slides



Inquiry :Used to discover other devices and exchange synchronization information (HW address andclock)This allows two nodes to agree on a common channel-hopping sequence (for paging):Address used to select the sequence

Clock used to select the sequence phase• Asymmetric procedure• Listener and sender hop using the same sequence but the sender hops faster than thelistener Paging :• Used to establish connections and define roles• Steps similar to Inquiry, BUT Paging message is unicast to a selected listener (ID packetuses the DAC) listener does not need to back-off The sender has also an estimate of the listener’s clock it is enabled to communicate with thelistener almost instantaneously

11. List the low- power operational modes at which a Bluetooth node can work, andbriefly describe the behavior of the node in each of these operational states.

[Andrea]--> from slides● Sniff:

○ Defined for slaves only, the slave listens for Dsniff slots every Tsniff andfor a Nsniff attempt number of times

○ The master issues a sniff command indicating the Tsniff period, the Dsniff,and Nsniff

○ The slave remains synchronized to the piconet and maintains its MACaddress

○ Listens to the channel while consuming little energy○ The master can force the slave into sniff mode; both the slave and the

master can request to put the connection in sniff mode or to move back toactive mode

● Hold:

○ Master and slave agree on the hold time duration. Both of them canrequest hold mode

○ Tx/rx in one piconet/inquiry/paging/scanning and in hold state in the other ○ ACL link can be put in hold mode also to save energy (if so, the unit in

hold turns off its receiver)



○ Maintains its MAC address○ Returning from hold, it must listen for the master and resync its clock

offset

● Park:○ The master can force a slave in park mode; either the slave or the master

can require to put the connection in park mode○ The master can force the park mode to one slave at a time○ The node does not participate in piconet activity and does not have a MAC

address (PM_ADDR for the unpark master initiated procedure, AR_ADDRfor the unpark slave initiated-procedure)

○ Remains synchronized to the piconet waking up periodically to listen to thebeacons from the master

○ Returning from park, it must listen for the master and resync its clock

offset○ A slave can send an unpark request in a proper half- slot after a broadcast

packet sent by the master to poll parked slaves. The slave can unparkwhen it receives an ack by the master

12. Consider the MAC layer of wireless sensor networks and describe how the B--MACprotocol works.

[Marcos]

It’s a contention-based protocol, uses a simplified CSMA without RTS/CTS. Its maingoal is to minimize the idle listening periods (and cost), so it implements Low Power Listening (LPL).

Low-Power Listening principles:

● Node periodically wakes up, turns radio on and samples channel using CCA:○ Wake up time fixed○ “Check time” variable

● If energy is detected (i.e. other node is transmitting its “preamble”), the nodepowers up in order to receive the packet● Then, the node will go back to sleep if:

○ After a packet is received○ After a timeout



13. Describe how the standard channel access scheme (IEEE 802.15.4) for sensor networks works, when the network is not beacon--enabled.

[Marcos] {Copied from slides 121 - 123}

When the network is not beacon enabled the channel access scheme is an un-slotted

CSMA/CA :● Backoff periods aren’t synchronized between devices.● No RTS/CTS● ACK is optional● When a sensor wants to transmit:

○ Wait for a random period of time○ If the channel is busy wait for another random period of time.○ If the channel is idle transmit○ ACKs are sent without using CSMA/CA

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