MANET:%20Performance

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MANET Performance

• Reference Performance comparison of two
  on-demand routing protocols for ad hoc networks
  Perkins, C.E. Royer, E.M. Das, S.R. Marina,
  M.K. IEEE Personal Communications, Volume 8
  Issue 1, Feb. 2001 Page(s) 16 28
  (AdHocUnicast-4.pdf)

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DSR

• Using source routing
• The sender knows the complete hop-by-hop route to
  the destination
• These routes are stored in a route cache
• The data packets carry the source route in the
  packet header
• Sending a data packet
• 0. To a destination for which it does not already
  know the route
• 1. Route discovery
• Flooding the network with route request (RREQ)
  packets

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DSR (cont)

• Each node receiving an RREQ rebroadcasts it,
  unless it is the destination or it has a route to
  the destination in its route cache
• Such a node replies to the RREQ with a route
  reply (RREP) packet that is routed back to the
  original source
• RREQ and RREP packets are also source routed
• The RREQ builds up the path traversed across the
  network
• The RREP routes itself back to the source by
  traversing this path backward
• The route carried back by the RREP packet is
  cached at the source for future use

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DSR (cont)

• 2. If any link on a source route is broken
• The source node is notified using a route error
  (RERR) packet
• The source removes any route using this link from
  its cache
• A new route discovery process must be initiated
  by the source if this route is still needed
• 3. For any forwarding node
• Caches the source route in a packet it forwards
  for possible future use (aggressive use of source
  routing)

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DSR (cont)

• Optimizations
• 1. Salvaging
• An intermediate node can use an alternate route
  from it own cache when a data packet meets a
  failed link
• 2. Gratuitous route repair
• A source node receiving an RERR packet piggybacks
  the RERR in the following RREQ
• This helps clean up the caches of other nodes in
  the network that may have the failed link in one
  of the cached source routes

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DSR (cont)

• 3. Promiscuous listening
• When a node overhears a packet not addressed to
  itself, it checks whether the packet could be
  routed via itself to gain a shorter route
• If so, the node sends a gratuitous RREP to the
  source of the route with this new better route
• It also helps a node to learn different routes
  without directly participating in the routing
  process

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AODV

• To maintain routing information
• Uses traditional routing tables, one entry per
  destination
• Uses sequence numbers maintained at each
  destination to determine freshness of routing
  information and to prevent routing loops
• A routing table entry is expired if not used
  recently
• A set of predecessor nodes is maintained for each
  routing table entry
• Indicating the set of neighboring nodes which use
  that entry to route data packets
• These nodes are notified with RERR packets when
  the next hop link breaks
• Each predecessor node, in turn, forwards the RERR
  to its own set of predecessors, thus effectively
  erasing all routes using the broken link

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AODV (cont)

• Optimization
• Control the RREQ flood in the route discovery
• Initially, expanding ring search to discover
  routes to an unknown destination
• Increasingly larger neighborhoods are searched to
  find the destination
• The search is controlled by the TTL field in the
  IP header of the RREQ packets

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DSR vs. AODV

• 1. By the virtue of source routing
• DSR has access to a significantly greater amount
  of routing information than AODV
• For example, in DSR, using a single request-reply
  cycle, the source can learn routes to each
  intermediate node on the route in addition to the
  intended destination
• Promiscuous listening of data packet
  transmissions
• AODV can gather only a very limited amount of
  routing information
• This usually causes AODV to rely on a route
  discovery flood more often, which may carry
  significant network overhead

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DSR vs. AODV (cont)

• 2. Route caching
• DSR replies to all requests reaching a
  destination from a single request cycle
• The source learns many alternate routes to the
  destination ? saves route discovery floods
• In AODV, the destination replies only once to the
  request arriving first and ignores the rest
• The routing table maintains at most one entry per
  destination

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DSR vs. AODV (cont)

• 3. Stale routes in the cache
• Current spec. of DSR does not contain any
  explicit mechanism to expire stale routes
• Stale routes, if used, may start polluting other
  cache
• Some stale entries are indeed deleted by route
  error packets, but promiscuous listening and node
  mobility ? more caches are polluted by stale
  entries
• AODV has a much more conservative approach than
  DSR
• When faced with two choices for routes, the
  fresher route (based on destination sequence
  number) is always chosen
• Also, if a routing table entry is not used
  recently, the entry is expired
• Determination of a suitable expiry time is
  difficult

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DSR vs. AODV (cont)

• 4. Route deletion (using RERR) activity
• Is also conservative in AODV
• By way of a predecessor list, the error packets
  reach all nodes using a failed link on its route
  to any destination
• In DSR, a route error simple backtracks the data
  packet that meets a failed link
• Nodes that are not on the upstream route of this
  data packet but use the failed link are not
  notified promptly

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DSR vs. AODV (cont)

• Goal of the simulation
• Determine the relative merits of the aggressive
  use of source routing and caching in DSR, and the
  more conservative routing table and
  sequence-number-driven approach in AODV

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Simulation Model

• Based on NS-2
• MAC layer protocol
• DCF of IEEE 802.11
• RTSCTS for unicast data
• Broadcast data packets and RTS control packets
  are sent using physical carrier sensing
• Radio model
• Luccent WaveLAN (2Mbps)
• 250m radio range

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Simulation Model (cont)

• AODV and DSR
• RREQ packets are treated as broadcast packets in
  the MAC
• RREP and data packets are all unicast packets
  with a specified neighbor as the MAC destination
• RERR packets
• Are broadcast in AODV
• Use unicast transmissions in DSR
• Send buffer 64 packets
• Contains all data packets waiting for a route,
  but no reply has arrived yet
• Packets are dropped if they wait in the send
  buffer for more than 30s

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Simulation Model (cont)

• Interface queue
• All packets (data and routing) sent by the
  routing layer are queued at the interface queue
  until the MAC layer can transmit them
• Maximum size of 50 packets
• Two priorities routing packets get higher
  priority than data packets
• Traffic models
• Traffic sources CBR
• Random source-destination pair
• 512-byte data packets

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Simulation Model (cont)

• Mobility model
• Random waypoint model
• From a random location to a random destination
  with a randomly chosen speed (uniformly
  distributed between 0 20 m/s)
• Once the destination is reached, another random
  destination is targeted after a pause
• Pause time affects the relative speeds of the
  mobiles
• Two field configurations
• 1500m x 300m with 50 nodes
• 2200m x 600m with 100 nodes

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Performance Metrics

• Packet delivery fraction
• The ratio of the data packets delivered to the
  destination to those generated by the CBR sources
• Average end-to-end delay of data packets
• Includes all possible delays caused by
• Buffering during route discovery latency
• Queuing at the interface queue
• Retransmission delays at the MAC
• Propagation and transfer times

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Performance Metrics (cont)

• Normalized routing load
• The number of routing packets transmitted per
  data packet delivered at the destination
• Each hop-wise transmission of a routing packet is
  counted as one transmission
• Normalized MAC load
• Routing, ARP, control (RTS, CTS, ACK) packets
  transmitted by the MAC layer for each delivered
  data packet
• Consider both routing overhead and MAC control
  overhead
• Also accounts for transmissions at every hop

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Varying Mobility and of Sources

• 50 node experiments
• Packet rate for 10, 20, 30 traffic sources 4
  packets/s
• Packet rate for 40 traffic sources 3 packets/s
• 100 node experiments
• Packet rate for 10, 20 sources 4 packets/s
• Packet rate for 40 sources 2 packets/s

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Simulation results (50 nodes)

• For 50 node experiments
• 1. The packet delivery fractions for DSR and AODV
  are very similar with 10 20 sources (Fig. 1a
  1b)
• With 30 40 sources, AODV outperforms DSR by
  about 15 (Fig. 1c, 1d) at lower pause time
  (higher mobility)
• For higher pause times (lower mobility), DSR has
  a better delivery fraction than AODV
• 2. Delays performance of both protocol is similar
  to that with delivery fraction
• Almost identical delays with 10 20 sources
  (Fig. 2a, 2b)
• With 30 40 sources, AODV has about 25 lower
  delay than DSR (Fig. 2c, 2d) for lower pause
  times. But for higher pause times, DSR has better
  (30 40 lower) delay than AODV

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Simulation results (50 nodes)

• For 50 node experiments
• 3. In all cases, DSR demonstrates significantly
  lower routing load than AODV (Fig. 3), usually by
  a factor 2-3, with the factor increasing with a
  growing number of sources
• DSRs normalized routing load is fairly stable
  with an increasing number of sources, even though
  its delivery and delay performance get
  increasingly worse
• 4. AODV has similar or slightly lower MAC load
  than DSR (Fig. 4) for lower pause times
• As the pause time is increased, the MAC load
  comparison goes against AODV
• With increase in pause time, MAC load remains
  almost steady for AODV, while it decreases
  significantly for DSR

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Simulation results (100 nodes)

• For 100 node experiments
• 1. When the number of sources is low, the
  performance (delivery fraction delay) of DSR
  and AODV is similar regardless of mobility
• 2. With large numbers of sources, DSR delivers
  better performance under low-mobility conditions
• However, AODV starts outperforming DSR for
  high-mobility scenarios
• 3. DSR always demonstrates a lower routing load
  than AODV
• Major contribution to AODVs routing overhead is
  from route request, while route replies
  constitute a large fraction of DSRs routing
  overhead

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Simulation results (100 nodes)

• AODV has more route requests than DSR, and the
  converse is true for route replies
• The relative routing load differences will be
  much smaller if the comparison is made in terms
  of bytes, reasons
• 1. DSR uses large routing packets
• 2. DSR data packets carry routing information
• 4. Comparison of MAC load goes against DSR except
  under low-mobility conditions
• Note that MAC load computation takes into account
  both the routing and control packets at the MAC
  layer.
• When only control packets were considered, we
  have seen that AODV always has lower load than DSR

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Simulation results (effect of loading)

• Mobility Zero pause time (highest mobility)
• Y-axis (throughput) represents the combined
  received throughput at the destination of the
  data sources
• X-axis (offered load) combined sending rate of
  all data sources
• With 10 sources
• 1. DSRs throughput starts saturating only at an
  offered load of around 400 kbps (Fig. 7a)
• This is due to a poor packet delivery fraction
• 2. AODVs throughput increases further along,
  starting to saturate around 700 kbps

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Simulation results (effect of loading)

• 3. AODV always has lower average delay than DSR
  (Fig. 7c) until the point where DSR begins to
  saturate
• Comparison of delays beyond that point does not
  provide any useful insight since DSR loses more
  than half the packets
• 4. AODV generates higher routing load in kbps
  than DSR (Fig. 7a)
• The routing load comparison in packets after
  normalization (Fig. 8a) also show similar
  behavior
• 5. However, AODV has lower MAC load than DSR
  (Fig. 8c)

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Simulation results (effect of loading)

• With 40 sources (Fig. 7b 7d)
• The qualitative scenario is similar to 10
  sources, but the quantitative picture is very
  different
• Both AODV and DSR saturate much earlier, AODV
  300 kbps, DSR 200 kbps
• AODV has a better delay characteristic than DSR
• AODV has a higher normalized routing load and
  lower normalized MAC load than DSR

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Observations

• A. Routing load and MAC overhead
• 1. DSR almost always has a lower routing load
  than AODV
• The difference is often significant (by a factor
  of up to 3) if the routing load is presented in
  terms of packet counts
• Presenting routing loads in terms of bytes is
  less impressive (at most about a factor of 2)
• By virtue of aggressive caching, DSR is more
  likely to find a route in the cache, and hence
  resorts to route discovery less frequently than
  AODV
• But DSR generates more replies and errors

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Observations (cont)

• AODVs routing load was dominated by RREQ packets
  (90 of all routing packets)
• DSRs routing load was dominated by RREP packets,
  due to multiple replies from the destination
  (roughly 50)
• In terms of absolute numbers, DSR always
  generated more RREP and RERR packets (factor 24)
  than AODV, but significantly fewer RREQ packets
  (up to an order of magnitude for high mobility)
• 2. Higher MAC load for DSR for high mobility
  and/or high traffic load
• RREP is unicast in AODV DSR RTS/CTS/Data/Ack
• RREQ is broadcast (not use any additional MAC
  control packets)
• RERR unicast in DSR, but broadcast in AODV

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Observations (cont)

• Further experiments for route MAC load
• Fig. 9 shows detailed statistics at the
  application layer, the routing layer, and the MAC
  layer
• 100 nodes
• 40 CBR sources, rate 2 packets/sec
• Packet size 512 bytes

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• unicast

unicast
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R-unicast
Data

R-broadcast
ACK
Data

CTS
ACK
CTS
RTS
RTS
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Observations (cont)

• B. Effect of mobility
• High mobility
• Link failures happen very frequently
• Trigger new route discovery in AODV
• The reason of DSR is mild and causes route
  discovery less often (the route discovery is
  delayed in DSR until all cached routes fail
• But the chances of the caches being stale is
  quite high in DSR. The cache staleness and high
  MAC overhead together result in significant
  degradation in performance for DSR. This effect
  is more severe with large numbers of sources and
  for larger networks

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Observations (cont)

• Low mobility
• The possibility of link failures is low
• Nodes usually get clustered with low mobility ?
  network congestion in certain regions ? causes
  link layer feedback to report link failures
• Such spurious link failures lead to new route
  discoveries in AODV
• DSR is largely unaffected by this problem. DSR
  caches are nearly up to date for low-mobility
  cases
• Also, AODV timer-based route expiry mechanism
  could result in unnecessary route invalidations
• A combination of nodes with different mobility
• Hard to predict the relative performance of AODV
  and DSR

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Observations (cont)

• C. Packet delivery and choice of routes
• DSR aggressive use of route caching
• Comparatively poorly in delivery fraction and
  delay in more stressful situation (larger numbers
  of nodes, sources, and/or higher mobility)
• Perform better in less stressful situations
• Picking stale routes ? consumption of additional
  network bandwidth, possible pollution of caches
  in other nodes
• Significant improvement of DSR
• Cache expiry using suitable timeouts
• Wider propagation of routes errors

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Observations (cont)

• D. Delay and choice of routes
• Correlation between the end-to-end delay and
  number of hops is usually small (correlation
  coefficient less than 0.1), except at very low
  load
• Buffering and queuing delay, time to gain access
  to the radio medium in a single congested node
  are often large
• In AODV, the destination replies only to the
  first arriving RREQ. This favors the least
  congested route instead of the shortest route
• In DSR, the destination replies to all RREQs,
  making it difficult to determine the least
  congested route

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Observations (cont)

• DSR always had a shorter average path length than
  AODV (1530 shorter), even though AODV often
  has less delay

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Observations (cont)

• E. Effect of loading of the network
• Network capacity is poorly utilized by the
  combination of 802.11 MAC and on-demand routing
• Instantaneous network capacity is roughly 7 times
  the nominal channel bandwidth (2Mbps) for zero
  pause scenario with 100 nodes
• The delivered throughput to the application was
  at most about 2 3 of the network capacity
• With more unicast routing packets, DSR suffers
  from this phenomenon more than AODV

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Conclusion

• General observation
• Delay and throughput DSR outperforms AODV in
  less stressful situations
• Aggressive use of caching, and lack of any
  mechanism to expire stale routes or determine the
  freshness of routes
• AODV outperforms DSR in more stressful
  situations
• Routing load DSR generates less routing load
  than AODV
• MAC layer load DSRs apparent savings on routing
  load did not translate to an expected reduction
  on real load on the network