Istituto Superiore Mario Boella

1
Istituto Superiore Mario Boella
Introduction to MS-Aloha
R. Scopigno, Networking Lab scopigno_at_ismb.it www
.ms-aloha.eu
2

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

3

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

4
Requirements for slotted Vanets

• Based on reservation
• Aimed at achieving determinism
• Completely distributed
• Infrastructure would be a request too strong
• Dynamic clustering and master election would not
  scale
• It requires too much time and reacts slowly not
  compatible with MAC needs
• Preventing hidden terminal issue
• Frequent in urban area
• Supporting priority (preemption) for emergency
  messages
• Blocking must be prevented for such messages
• Efficiently support target length messages and
  typical frequency
• In this study fixed at 200B, 10 Hz

5
MS-Aloha Base Mechanisms (i)

• Each node who has obtained a slot appends to the
  slot its view of all the slots (FI)
• Against hidden station and to enable collision
  detection
• Potentially dangerous overhead
• Contention Phase (slot reservation)
• A node starts competing for slot assignment
  listening to
• Slot (free busy)
• N FIs coming from its neighbors
• The node transmits a data packet into a slot
  considered idle, together with its FIs

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MS-Aloha Base Mechanisms (ii)

• The reservation of a slot is performed through
  two distinct phases
• The slot reservation through the FI
• True slot occupation
• In the period between slot(K) and slot(KN) the
  channel is monitored to detect any reservation
• Check on slot and by FI analysis
• When slotK begins, the node transmits its packet
  if it still has the reservation.
• Continuous monitoring to face mobility

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MS-Aloha Format (i)

• Slot channel time space dedicated to a single
  host for data transmission.
• N number of slots within a single frame.
• FI (Frame Information) Structure containing
  information about the status of each slot.
• Required to prevent hidden station

• In this presentation
• Same Physical Layer of 802.11p (12Mbps, 10Mhz ch
  _at_5.9GHz, QAM16-1)
• Frame 100ms (10Hz application Rate)
• Payload 200 Bytes
• If FI12 bits per slot and Tg 1 us, then 224
  slots (of 446 us)
• Other setting (e.g. relaxed guard time) in other
  studies available in www.ms-aloha.eu

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MS-Aloha Format (ii)

• STI (8bit) Address1(48 bit)
  Address2(48 bit) Sequence Number (12bit)
  Fragment Number (4bit) FIbit(1bit)
• STI source identification
• Address1 source address
• Address2 destination address
• SequenceNumber field indicating the sequence
  number of each packet
• FragmentNumber used in case of frame
  fragmentation
• FIbit bit indicating the presence of the FI
  before the payload (sent in slot0 only)
• Payload
• CRC used to highlight any errors during
  transmission

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FI field

• FI (Frame Information) Structure containing
  information about the status of each slot
• Each slot information is composed of
• STI the short identifier of the node
• PSF (Priority Status Field) field indicating the
  priority of data transmitted in the slot. The
  values ranging from 1 to 3 (growing priority).
• STATE 2-bit flag indicating channel state

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Time EfficiencyThe Issue of Overhead (i)

• The main concern is about the overhead implied by
  MS-Aloha
• The overhead of MS-Aloha is fixed
• CSMA/CA introduces a protocol overhead too, but
  it is variable and hard to be measured
• Comparison by simulations in case of unicast
• Both broadcast and Unicast
• In Broadcast CSMA/CA does not involve backoff (no
  ACKs) ? no real OH
• The side effect of collisions should be taken
  into account
• 100-200 fixed nodes on two lanes
• Point-to-point full duplex traffic at variable
  application rate
• Peers in distinct Lanes
• Inter-Node-Dist 4m Inter-Peer-Dist 60m
• 37dbm TX, -85dbm RX (benefits for CSMA)

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The Issue of Overhead (ii)Unicast (100)

• Inter-packet time inside a flow (Average on the
  100 flows)
• Time between two consecutive packets correctly
  received

• CSMA/CA saturation starts at 15Hz
• variable, fixed on average, higher than MS-Aloha

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The Issue of Overhead (iii)Unicast (200)

• Inter-packet time inside a flow (Average on the
  100 flows)
• Time between two consecutive packets correctly
  received

• CSMA/CA saturation starts at 10Hz
• variable, fixed on average, higher than MS-Aloha

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The Issue of Overhead (iv)Broadcast (100)

• Inter-packet time inside a flow (Average on the
  100 flows)
• Time between two consecutive packets correctly
  received

• CSMA/CA saturation starts at 15Hz
• variable, fixed on average, higher than MS-Aloha

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The Issue of Overhead (v)Broadcast (200)

• Inter-packet time inside a flow (Average on the
  100 flows)
• Time between two consecutive packets correctly
  received

• CSMA/CA saturation starts at lt10Hz
• variable, fixed on average, higher than MS-Aloha

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The Issue of Overhead (vi)

• MS-Aloha (224 slots, 200B Appl.Layer, 12Mbps)
• 446?s per slot (including guard-time)
• Payload_Time 2008/12Mbps 133?s
• Overhead_Time 313?s (3.756 bit_time _at_ 12Mbps)
• Overhead/Payload 2,35
• ? 1/(12.35) 0,3 (including Ethernet-like
  Overhead)
• CSMA/CA (200B Appl.Layer, 12Mbps) 8-50 Hz Appl.
  Rate
• From interpacket time inside a flow to
  interpacket time in the air
• 1.000-3.500 ?s IPT unicast 500-5.000 ?s IPT
  broadcast
• Payload_Time 2008/12Mbps 133?s
• Overhead_Time 867-3.367?s unicast 367-4.867?s
  broadcast
• Overhead/Payload 6.5-25 unicast 2.7-36
  broadcast
• ? 1/(1OH) 0,13 ? 0.04 unicast 0,27 ? 0.03
  broadcast

16

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

17

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

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Typical UnresolvedIssues of Other Slotted
Solutions

• TDMA algorithms are usually for fixed or slowly
  varying topologies
• Fixed networks (RR-Aloha) or free-space
  (line-of-sight), low density and slowly varying
  mutual positions (STDMA)
• Even if standard the may NOT be suitable (!)
• They do not fit the requirements of dynamic
  environments such that of Vanet
• A node can appear suddenly due to obstructions
• Hidden terminals are much more frequent than in
  free space
• The density of nodes is so high to make hidden
  collisions more frequent
• These have a direct impact on the efficiency and
  the quality of the services
• MS-Aloha solves these issues with a first set of
  proprietary mechanisms
• Mechanisms first published under the name of
  RR-Aloha functions
• Three tricks memory refresh, signaling
  semantics, scalability of label space
• The properness of the solutions has been
  validated through simulations.

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1. Memory Refresh

• Simulations highlight the first simple, yet
  unresolved issue concern the refreshing rate for
  the information on channel state
• In case the information is not refreshed, once a
  slot j is assigned to node M, the slot state
  would be frozen
• The slot would be continuously announced busy
  also if the node gets switched-off
• Additionally the information would jump too many
  hops
• In a vehicular environment, the same would happen
  if the node M got far from the radio range of its
  previous neighbourhood
• Moreover M would announce fallacious information
  - based on a radio range which is not actual
• On each node the memory needs to be refreshed
  periodically
• Simulations involving node mobility highlight
  this as the primary cause of inefficient slot
  allocation
• It is shown to works if information is refreshed
  once per MS-Aloha Period
• Additionally information on slot j is refreshed
  when the elapsed time has reached the position j

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2. Signaling Semantic (i)

• In DTDMA and MS-Aloha the problem of hidden
  terminal is counteracted by message broadcasting
  with FI
• In case of fixed nodes, each node expects
  confirmation of slot assignements by all the
  nodes in its neighbourhood
• The assignement is result a logic AND among
  received Fis
• If the ad-hoc network is continuously changing it
  is hard to know what one's neighbourhood is like
• Not all the nodes can be required to be always
  connected to confirm
• If a new node switches on, it 0 states in the
  FI will reset all the connections
• The information carried by FI is managed by a
  logical OR
• The semantic is changed conflicting FIs - rather
  than acks drive changes

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2. Signaling Semantic (ii)

• If channel state are managed by AND, 1 bit is
  enough to describe channel state
• Only if all FI agree on the assignment, the busy
  state is confirmed
• If a collision is detected, it is announced just
  by free message (thanks to AND logic)
• In steady state it may work with mobility and
  OR it gets ambiguous ? example follows
• In order to solve this issue, the STATE subfield
  is extended to two bits
• 2 bits allows to ditinguish the following slots
  free, busy and collision
• An additional variation in the semantic
  collisions require an explicit indication
• Simulations show that the overhead and latences
  introduced by the additional bit are negligible
  while make the VANET stable

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Example Why an Additional Bit is Required
Trasmission Order
Slot 0
Slot 1
Slot 2
During slot 3, node will send
acknowledgement about into slot 2 of its FI
FI
nodes receiving from
nodes receiving from
Slot 3
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Example Why an Additional Bit is Required
In the next FIs, the nodes which have detected a
collision will send slot2 status as free of its
FI This way the collision notification gets
missed! The remaining nodes will send an ack
about slot2 assignment, without detecting
properly the collision, also due to the OR
operation.
Trasmission Order
Slot 0
Slot 1
So the nodes sense a
collision status affecting slot 2, then set it as
free (Busy0), while the
nodes
do not change the slot 2 status
Slot 2
The node notices the collision and send slot
2 as free on its FI
Receive from
FI
Receive from
Slot 3
Busy 0
Slot 4
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3. Scalability of STI Label Space (i)

• 8-bit labels STI used to identify each node
  inside the communication area 256 possible
  values
• STI are used to identify what node is using each
  reserved node
• STI are used instead of MAC addresses (typically
  48-bit wide numbers) to avoid excessive overheads
  in the FIs
• In urban areas the label space may be a very
  strong limit
• However the same label can be re-used in
  different slots
• The purpose of STI is collision detection -
  different nodes using the same slot
• LabelSlot ? Node Identification
• Still statistically not-negligible event of two
  hidden terminals chosing the same slot and the
  same STI
• Scalability finally solved assigning STI a
  temporary meaning
• STI changed by the nodes directly receiving from
  node A into STI
• They know also As MAC and compute a new STI
  based on STI and MAC
• The nodes which do not receive from A just know
  STI. The other know that STI and STI represent
  the same node A
• At next period the STI is changed by A into
  STI and so on. Collision are, soon or later,
  detected

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3. Scalability of STI Label Space (ii)
26

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

27

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

28
MS-Aloha

• Two main issues can still hinder the exploitation
  of MS-Aloha in a VANET scenario
• The scalability of the protocol (number of
  available free slots)
• Its capacity to strongly react to changing
  conditions due to mobility
• Simulations show that the unconstrained multihop
  forwarding of channel state is harmful
• Slot reservation is extended beyond the bounds of
  wireless coverage
• Causing resource waste and slot depletion
• Mobility introduces a not negligible probability
  of getting closer to nodes which have been
  assigned the same slot
• This becomes more relevant when nodes move in
  opposite directions
• The number of collisions grows high
• The effect is disruptive if slot re-use is
  hindered
• Among the causes slot state forwarding with no
  limitations on the number of hops

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Limitation of FI Forwarding

• So far the channel-state is described by two bits
  (State)
• Only 3 states are used (free 00, busy 10,
  collision 01)
• One free configuration (say 11)
• The free configuration can be exploited to keep
  trace of number of hops the information is
  forwarded over
• When some information on slot reservation is not
  directly detected, it is announced as 2-hop
  (11)
• Nodes which receive it they know that they should
  not use the slot but should not forward this
  information either
• This solutions have been demonstrated, by
  simualtions, to
• Decrease the logical radius of propagation of a
  slot reservation
• Improve of resource re-use.

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Improving Slot Re-Use (i)

• Slot re-use can be further improved setting a
  higher threshold on minimum reception power
• If the received power is lower than a given
  threshold THR the message IS considered for
  MS-Aloha but does not contribute to the FI
  messages
• It conceptually corresponds to lowering the
  radius of cluster of nodes which perceive a slot
  x assigned to a node A
• Instead, acting on the transmitted power would
  affect the SNR
• Further improvement by introducing a mechanism
  which regulates the THR dynamically
• THR defined on each node separately based on its
  perception
• Blocking completely prevented
• Simulations show that it works
• Effects on slot reuse (increased)
• Effects on Packet Delivery Rate (PDR)
• Lowered at higher distances but kept high close
  to the transmitter

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Improving Slot Re-Use (ii)

• The average does not change much
• Slot re-use is also a statistical event the
  point is to make it possible
• However potentially still scalable
• Less blocked nodes and for less time
• More unused slot

Slot Reuse -96 dbm 1.968 -86 dbm 2.040 -80
dbm 2.174 Sent Packets CSMA/CA 100
() MS-Aloha -96 92,50 MS-Aloha -86
94,75 MS-Aloha -80 99,50 () far from
saturation
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Simulation Settings

• The MS-Aloha has been implemented on NS-2
• Most simulations use MS-Aloha set as follows
• each slot lasts 0.447 ms
• 224 slot per frame (the overall frame takes about
  0.1sec)
• Packet generation rate of 10Hz
• Also other settings adopted
• 200 slots and over 78.5 µs guard time relaxed
  synchronization
• The simulation adopts the following scenarios
• Simulation lasts 2000 sec.
• Nakagami model was used to model propagation and
  urban grid with corner obstruction (extra
  attenuation)
• Transmitted power 7dbm or 20 dbm
• Wireless reception sensitivity -96dbm
• 400-900 nodes (speed in the range 50-120Km)
• Circular topology (radius R1Km) with four lanes
  or
• Urban topology with grid 150m blocks and
  750m-wide map
• In all the simulations MS-Aloha performs better
  than (or as well as) CSMA/CA in terms of PDR and
  determinism

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Simulation Metrics

• In order to quantify results the following
  metrics adopted
• PDR (Packet Delivery Rate) function that shows
  how much a node is likely to receive a packet
  varying the distance from the transmitting node
• Suitable for both MS-Aloha and CSMA/CA
• In CSMA/CA it is affected with high congestion
• Mean Collisions the average number of collisions
  detected on the same slot, over the whole
  simulation and all the nodes
• Suitable for only MS-Aloha
• Slot Re-Use number of times a slot is re-used by
  different nodes (at a given time).
• Suitable for aloha MS-Aloha
• See previous slides
• Determinism is hardly measured but it is
• Close to 100 for MS-Aloha (fixed delays and high
  PDR, only affected by slot collisions)
• Lower in CSMA/CA, due to unpredictable delay
  (non-deterministic transmission time due to
  collision avoidance) and lower PDR (non-
  deterministic reception)

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Simulations PDR
35
Simulations CollisionsMulti-hop FI forwarding
vs 2-hop
Only collisions due to mobility!
36

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

37

• Introduction Concepts and Figures
• First Proprietary Mechanisms RR-Aloha
• Proposed Extensions
• Simulative Settings
• The Final Version MS-Aloha
• Proposed Extensions for Scalability
• RR-Aloha MS-Aloha Simulations
• Preemption and Conclusions

38
Preemption (i)

• Preemption as an additional solution against
  channel blocking
• Acting on service differentiation and aimed at
  QoS guarantee
• Each station accesses the channel with a
  priority, variable in 1-4 (2 bits)
• The priority is announced in a subfield of the FI
  field
• Whenever a node with higher priority needs to
  transmist, it can override a node with lower
  priority
• E.g. Node 1 can transmit in slot 5 even if it is
  already occupied by node 2, if node 2 has a lower
  priority
• In a possible practical scenario nodes have the
  highest priority only for emergency messages
• Normal access uses 3 lower classes
• E.g. 1-emergency 2-channel-access 3-assistan
  ce, 4-entertainment

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Preemption (ii)

• Questions to be answered
• Can preemption help saturate the channel?
• Does preemption work also under saturation? Can
  it really gain channel access
• Several simulations. Following results achieved
  with
• 858 nodes, average speed 80km, TX power 2 dbm
• 5x5 grid (150m distance) 2-lane roads
• Application rate at 30Hz
• With and without preemption
• With preemption each nodes tries to have a
  High-Priority slot and a Low-Priority slot
• Results (2.000 sec of simulated time)
• Transmitted packets with preemption-34.360 w/o
  preemption-18.028
• With preemption HP packets 17.980 LP packets
  16.378

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Preemption (iii)
41

• Thank You for Your Kind Attention

R. Scopigno, Networking Lab scopigno_at_ismb.it www
.ms-aloha.eu