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Directional Antennas in Mobile Ad Hoc Networks:. MAC and Routing Issues Prepared by: Rima Khalaf Behnam Rezaei. Based on the work of:. N. Vaidya, Y.b. Ko, and V. Shankarkumar, Texas A&M University; “Medium Access Control Protocols for Directional Antennas in Ad Hoc Networks”

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directional antennas in mobile ad hoc networks

Directional Antennas in Mobile Ad Hoc Networks:

MAC and Routing Issues

Prepared by:

Rima Khalaf

Behnam Rezaei

based on the work of
Based on the work of:
  • N. Vaidya, Y.b. Ko, and V. Shankarkumar, Texas A&M University; “Medium Access Control Protocols for Directional Antennas in Ad Hoc Networks”
  • A. Nasipuri et. Al , University of Texas, San Antonio; “A Medium Access Protocol for Ad hoc Networks with Directional Antennas”
  • A. Nasipuri et. Al, University of Texas, San Antonio; “On Demand Routing Using Directional Antennas in Mobile Ad Hoc Networks”
  • R. Bagrodia, J. Martin, A. Ren, M. Takai, Computer Science Department, UCLA Directional Virtual Carrier Sensing for Directional Antennas in Mobile Ad Hoc Networks”
rts cts handshake in 802 11
RTS/CTS Handshake in 802.11

A

B

C

D

E

RTS

RTS

CTS

CTS

DATA

DATA

ACK

ACK

rts cts in ieee 802 11
RTS/CTS in IEEE 802.11
  • RTS and CTS contain proposed duration of

data transmission

  • All in-range nodes MUST wait for this duration before transmitting
  • Advantage: Elimination of Hidden terminals
  • Disadvantage – Wastage of network capacity (D cannot send anything to E)
mac protocols for directional antennas can be classified in 2 categories
MAC Protocols for Directional Antennas can be Classified in 2 Categories:
  • MAC Protocols for networks where the nodes possess location information (by use of GPS for instance)
  • MAC Protocols where nodes do not have location information but depend on the Angle of Arrival of the Received Signal
vaidya et al model
Vaidya (et al.) Model
  • Each node knows its exact location and the location of its neighbors.
  • Each node is equipped with 4 directional antennas.
vaidya scheme 1
Vaidya Scheme 1
  • The node uses a directional antenna to send the RTS (D-RTS), whereas CTS Packets are sent Omni-directionally.
  • Data Packets and Acks are sent directionally
  • Any other node that hears the CTS ONLY blocks the antenna on which the CTS was received.
slide11
Other nodes CAN transmit

A B  C xD  E

A  B C  D xE

  • All DRTS may not get an OCTS reply

(D & E in the above scenarios cannot send OCTS if anyone sends them a DRTS because one of their antennas is blocked)

  • Control packets may collide

A B- -> C  D E

problem with scheme 1
Problem with Scheme 1
  • What if A wants to talk to B? A has no way to know that B is busy

A possible scenario of collisions with DRTS packets

vaidya scheme 2
Vaidya Scheme 2
  • The node uses two types of RTS namely, Directional RTS and Omni-directional RTS according to the following rule:
  • I) if non of the directional antennas of the node are blocked the node will send an omni-directional RTS.
  • II) Otherwise the node will send a DRTS provided that the desired directional antenna is not blocked. If the desired antenna is blocked the node will defer until that antenna becomes unblocked.
  • The CTS,Data and ACK are the same as before.
performance
Performance

Throughput in Kb/s

directional virtual carrier sensing r bagrodia et al
Directional Virtual Carrier Sensing (R. Bagrodia et. Al)
  • Main Advantage: No additional hardware is required to get location information.
  • Each node estimates the position of its neighbors by the “Angle of Arrival” information, i.e. by noting the antenna which received the highest signal.
  • Operation can be summarized in 3 main processes:
1 aoa angle of arrival caching
1. AOA(Angle of Arrival) Caching
  • Each node caches an estimated angle of arrival from neighboring nodes even if the signal is sent to it or not.
  • When that node has data to send, it searches its cache for an angle of arrival information, if the AOA is found, the node will send a directional RTS, otherwise, the RTS is send omni-directionally.
1 aoacaching cntd
1. AOACaching (Cntd ….)
  • The node updates its AOA information each time it receives a newer signal from the same neighbor.
  • It also invalidates the cache in case if it fails to get the CTS after 4 directional RTS transmissions.
  • The CTS is sent directionally.
2 beam locking and unlocking
2. Beam Locking and Unlocking
  • When a node gets an RTS, it locks its beam pattern towards the source to transmit the CTS.
  • The transmitting node only locks its pattern once it received the CTS.
  • Beam Patterns are formed at both the transmitter and the receiver to maximize signal power.
example

(2) CTS

(3) DATA

(4) ACK

(1)RTS

Example
  • Node A has data to transmit to node B and finds an AOA field for B in its cache.
  • The AOA field is currently a little outdated since B has moved since the last update.

A

B

B

example cont d

(2) CTS

(3) DATA

(4) ACK

(1)RTS

Example (cont’d)
  • Node B senses the RTS from node A, and then adapts its antenna pattern to maximize the gain for the signal coming from node A.
  • Node A locks its antenna pattern after the reception of the CTS from B.
3 dnav setting
3. DNAV Setting
  • DNAV (Directional Network Allocation Vector) instead of NAV used in the classical 802.11
  • DNAV also contains direction information of neighboring nodes. It is updated each time the physical layer provides new information about the location of a neighboring node.
3 dnav setting1
3. DNAV Setting

`

  • DNAV reserves the channel only in specified range of directions.
  • The algorithm selectively excludes directions included in DNAV for transmission in which the node may cause interference with other transmissions in progress.
  • Meanwhile it allows transmitting frames along other free directions.
performance1
Performance

PDR and throughput of the network in the mobility scenario (without the physical CS in the IEEE 802.11).

PDRs with and without the physical CS

performance cntd
Performance Cntd.

Effects of the physical CS in mobility scenarios

Mix of omni-directional and directional antennas

performance cntd1

Channel frequency

2.4 [GHz]

Signal reception

BER based(with DBPSK modulation)

Data rate

2 [Mbps]

Noise figure

10.0 [dB]

TX power

15.0 [dBm]

TX power (directional)

0.0 [dBm]

RX threshold (RXT)

-81.0 [dBm]

CS threshold (CST)

-91.0 [dBm]

AOA cache expiration time

2 [s]

Table 1: Set of parameters used in the simulation.

Performance Cntd.

: PDR and throughput of the network in the no mobility scenario (without the physical CS in the IEEE 802.11).

Set of parameters used in the simulation

problems with bagrodia et al
Problems with Bagrodia et al.
  • Basically this scheme suffers from the same problem as Vaidya’s scheme. This is a result of the directional RTS,i.e. a node can not know that a neighboring node is busy if that node sent a directional RTS to another neighbor.
on demand routing with directional antennas nasipuri et al
On Demand Routing with Directional Antennas (Nasipuri et al)
  • Motivation: Trying to reduce the routing overhead associated with the flooding of Route Request Packets associated with on demand protocols such as AODV and DSR.
  • Directional antennas are exploited to limit the flooding to a specific region of the network thus reducing routing overhead,
underlying mac protocol
Underlying MAC Protocol
  • There is no use in modifying a routing algorithm for directional antennas if the underlying MAC protocol itself was not modified.
  • The MAC protocol used is very similar to IEEE 802.11 but the RTS and CTS are sent omni-directionally, but data is sent directionally.
  • 2 Routing Protocols were proposed:
routing protocol 1
Routing Protocol 1
  • If a node S has a packet to send to node D, it transmits a query packet on the directional antenna which it had been using earlier to communicate with D.
  • Assuming that D has not moved too far since the last communication instant, the flooding is restricted to the region containing the last known location to D.
routing protocol 1 cont d
Routing Protocol 1 (cont’d)
  • If D has moved out of range, and S did NOT receive a Route Reply from D after a suitable timeout period, then S will generate a Route Request on all antennas (Omni- Directional), this time flooding the whole network.
  • Result: a decrease in Routing Overhead at the cost of increased latency.
drawbacks of protocol 1
Drawbacks of Protocol 1
  • The angular span of the antenna in S that was used to transmit data packets on the first hop of the of the last valid route to D may not necessarily include D.
  • This happens if the first intermediate node on the path to D and the node D do not lie within the angular span of the same directional antenna of S.
  • Solution: Routing Protocol 2 …
routing protocol 2
Routing Protocol 2
  • At the end of every successful route discovery, the source records the directions of the antennas to be used on every hop of the newly discovered route.
  • The information is made available to the source if each node on the route adds to the header of the Route Reply packet, the antenna identifier which it used to receive the packet while forwarding it back to the source.
routing protocol 2 cont d
Routing Protocol 2 (Cont’d)
  • This allows the source to get a rough estimate about the angular location of the destination by simply counting the number of times each antenna was used on the route that has been found.
performance tradeoffs
Performance Tradeoffs
  • If directional route search was successful from the first attempt, then both protocols have the advantage of remarkably reducing routing overhead.
  • However, as node mobility increases, the chances of finding a route from a directional search drastically decrease.
performance tradeoffs cntd
Performance Tradeoffs Cntd.
  • Another disadvantage is that a directional route request may find not always find the shortest route.
  • This happens if the destination is not included in the search zone but an intermediate node which received the request a route to the destination and positively responds.
finally

Finally …

Comparisons, Performance Tradeoffs, Proposed modifications, and conclusions …

performance comparison

RTS

CTS

Data

Ack.

802.11

Omni

Omni

Omni

Omni

Nasipuri

Omni

Omni

Directional

Directional

Vaidya 1

Directional

Omni

Directional

Directional

Vaidya 2

Dir./Omni

Omni

Directional

Directional

Bagrodia

Dir/Omni

Directional

Directional

Directional

Performance Comparison
proposed modifications
Proposed modifications
  • What seems interesting to improve here is routing based on the random algorithms and probability of giant component when performing random routing.
  • It seems that directional antennas can be used to model biological inspired connectivity and routing.
  • Better use of Route Requests with antenna direction information
conclusion
Conclusion
  • We have presented 4 different MAC protocols, and one routing protocol that take advantage of the spatial reuse capabilities offered by directional antennas.
  • All of the discussed protocols deliver optimum performance under no mobility conditions. Their performance degrade drasticaly as mobility increases
conclusions cntd
Conclusions Cntd.
  • The use of directional antenna in ad hoc and large scalable networks is still unclear due to technical and physical difficulties of implementing directional antennas on each node.
  • Those protocols are designed to increase network throughput at the cost of some increased design complexity …
  • … Which brings us back to the popular saying …
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