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Explore the effects of directional antennas on routing in ad hoc networks. Analyze how directional antennas influence routing updates, spatial re-use advantages, mobility impact, and MAC layer interaction.
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Questions on what is of interest • Transmission of Routing Updates or Control Messages – directional ? omni-directional ? • What are the effects of the extension of range ? – Recap : Directional Range > Omni-directional range. • Is the additional spatial re-use providing any advantages ? More stable routes ? • What is the impact of mobility ? • Is there any interaction between the MAC and the routing layers ? If so what is it ? • What might be other challenges ?
[8] R.Roy Choudhury and N.H.Vaidya, “Impact of Directional Antennas on Ad Hoc Routing”, Proceedings of IFIP Personal and Wireless Communications, 2003. • Link: http://www.crhc.uiuc.edu/wireless/groupPubs.html
Objective and Strategies • Evaluate the impact of the use of directional antennas on ad hoc routing – in other words to answer the questions that we brought up. • Steps that they take: • Perform simulations to understand how the presence of directional antennas affects routing. • Use the insights gained to propose changes to existing routing strategies (or design new strategies as might be appropriate). • Analyze the effects of the new schemes.
Reported Related Work • This is a relatively new area – only three papers so far. • One of the papers simply looks at performance enhancements due to the use of directional antennas on on-demand routing. • A second paper looks at computing maximally disjoint routes (useful due to spatial re-use – no interference between routes.) • Ram’s paper simply uses a link state routing protocol.
Antenna Model Used • Cone-Sphere Model. • N beams – so the angular width of each beam is 2P/N. • Two modes of operation – Omni & Directional – node can toggle between modes. • Directional gain Gd > Omnidirectional Gain Go. • Thus, a node has two neighborhoods : • Directional Neighborhood. • Omni-directional Neighborhood.
Is this a tongue twister or what ? Directional-Omni (DO) and Omni-Omni (OO) Neighbors • B is a DO neighbor of A if it can receive A’s directional transmissions even if B is in the omni mode. • B is a OO neighbor of A if it can receive A’s omni transmissions. • Note that the OO neighbors are a sub-set of the DO neighbors.
The MAC • Of course not the MAC shown • We need a MAC that can use directional antennas. • The authors use DiMAC – their own MAC protocol. • RTS/CTS handshake is directional. • At each node, DiMAC maintains what is called a directional NAV (network allocation vector) table – used in other similar MAC protocols. • This helps tabulate the direction from which a RTS or a CTS is received from each neighbor.
MAC protocol continued • Each node uses the lookup table and determines the direction of the table – transmits RTS in that direction. • The recipient node listens omni-directionally. • It figures out the beam on which the RTS was received and sends out the CTS in that direction. • Nodes that overhear RTS or CTS or both defer transmissions for the proposed duration of the data transfer. • Protocol suffers from the deafness problem
Deafness • A node say Node C, attempts to initiate a dialogue with another node (say Node A). • However, Node A is talking to someone else (say Node B) and is beamformed in the direction of B. • Node C’s signals are not received by Node A – it is deaf to Node C’s signals. • Node C interprets the the absence of a reply to be a collision. • Repeats this transmission multiple times. Finally drops packet. • Can be potentially treated as a route failure.
Routing: DSR over DMAC • RECAP: DSR is an on-demand routing protocol for ad hoc networks; so far assumes the presence of omni-directional antennas • Sequence: RREQ, RREP when route is not available in cache. Use the route until it fails. Failure communicated using an RERR message. • Source routing.
Route Discovery -- Sweeping • Omni-directional broadcast is emulated by sequentially transmit the packet in each direction. • This can increase delay – if N directions, each sweep takes N times the time taken by an omni-directional transmission. • But, sweeping can reach the DO neighbors.
Mobility: Scanning • In mobile scenarios, the direction of a particular neighbor (as indicated in the table) could become stale. • Scanning is used to address this. • Scan HELLO packets are transmitted sequentially on each antenna beam. • When a node receives such a HELLO message it responds using the same (appropriate) antenna beam. • Note: Neighbor discovery is more complex now.
Partial Scanning • Scanning can be expensive. • Typically if communication is regular the node might not have moved far. • Partial scan is where a node searches for a lost neighbor using only “K” beams adjacent to the beam that was previously in use for that neighbor. • Reduces overhead how does one choose K ? • However, for bursty communications this might deteriorate to a “scan”.
Simulations and Performance • Use of Qualnet for simulations. • CBR Traffic • 1500 x 1500 square region. • Directional DSR (including sweeping and directional routes) and DiMAC. • Several scenarios considered. • Metrics of interest are Route Discovery latency (RDL) and throughput.
Intuitive Thoughts • RREQ messages get delayed due to Sweeping as one might expect. • Shorter routes might be discovered due to extension of range. • Deafness might be a problem – how critical ?
Studying Route Discovery Latency (RDL) • Smaller beamwidth longer range. • If the distance of separation between the source and destination is small not much to be gained – the directional hop count and omni hop count are almost same. • Smaller hop count offset by increase due to sweeping delay. • At larger distances the advantage of the higher transmission range dominates.
Dependent on node density as well. • Intuitively as density becomes higher one might expect to find more DO neighbors – so one might expect hop count to decrease thereby improving performance. • Not the case ! Interference due to side lobes increases the possibility of collisions. • Performance enhancements not significant. • Unclear from the paper : Why does omni transmissions not have the same problem which is, why is the degradation more significant in the case of directional schemes ? Is this an artifact of the protocols themselves ?
Throughput • Even though hop-count is expected to decrease throughput does not go up significantly. • This is because due to sweeping delays, the optimal path may not be found. • The nearest neighbor may not be the first one to be found – so sub-optimal paths may be discovered earlier.
Delayed Route Reply Optimization • Replying to all RREQs is not a good thing. • First – increase in overhead. • Second, when destination is responding to the first RREQ, it might miss others (directional schemes) – remember RREQ is broadcast. • Unclear from paper: Is RREQ missed in between ? • So wait for a pre-specified time T before sending an RREP – T = r * Tsweep; r is a system parameter. • Tsweep is the time taken to finish a full sweep.
The authors also find that deafness can create significant problems when linear topologies are used. • This causes the directional schemes to in fact fall somewhat below the omni schemes in linear topologies in the presence of multiple flows. • Refer to the paper for the details. • In random topologies however, significant benefits are seen – shorter routes, higher spatial re-use. Partitions are prevented.
Effect of transmission range. • As we proceed from omni, performance increases. • However, as we increase the transmission range (reduce beamwidth), sweeping delays increase and the problems due to deafness exacerbate. Thus, shortest RREQ routes are never received in spite of the delayed optimization. • Artifact of the protocols (especially sweeping). • At extremely small beamwidths, performance degrades.
Routing Overhead • New metric: • SNumber of Control packets X Area blocked by each packet • SNumber of data packets a = ------------------------------------------------ • Intuitively, network capacity consumed by each control packet is proportional to the interference region caused by the packet. • Initially it is seen that the sweeping overhead much higher than omni transmissions as one might expect.
Selective Forwarding Optimization Figure from [8] • Do not forward RREQ in the direction received. • Forward control packets on those beams that are diagonally opposite to the beam with which the control packet was received. • Reduces overhead significantly but still higher than DSR.
Impact of Mobility • Partial Scanning used. • Seems to work fairly well. • Ultimately, the use of the techniques seem to demonstrate promise in terms of using directional antennas. • However, many of the schemes can be energy intensive – especially the sweeping part. • Open questions on how to make these schemes improve energy efficiency to a greater extent – already benefits in terms of reduced collision rate, increased throughput, reduced transmissions due to shorter routes.
In terms of receiving abstracts for your talks: Just Kidding !!! Thanks!
