istituto superiore mario boella

Introduction to MS-AlohaIntroduction to MS-Aloha

R. Scopigno, Networking Lab – [email protected]

www.ms-aloha.eu1

Introduction: Concepts and FiguresIntroduction: Concepts and Figures

First Proprietary Mechanisms: RR-Aloha+First Proprietary Mechanisms: RR-Aloha+

Proposed ExtensionsProposed ExtensionsSimulative SettingsSimulative Settings

The Final Version: MS-AlohaThe Final Version: MS-Aloha

Proposed Extensions for ScalabilityProposed Extensions for ScalabilityRR-Aloha+ & MS-Aloha SimulationsRR-Aloha+ & MS-Aloha Simulations

Preemption and ConclusionsPreemption and Conclusions

It works under mobility

Based on reservationAimed at achieving determinism

Completely distributedInfrastructure 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

frequencyIn this study fixed at 200B, 10 Hz

Requirements for slotted VanetsRequirements for slotted Vanets

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

MS-Aloha Base Mechanisms MS-Aloha Base Mechanisms (i)(i)

The reservation of a slot is performed through two distinct phasesThe slot reservation through the FITrue slot occupation

In the period between slot(K) and slot(K+N) the channel is monitored to detect any reservation Check on slot and by FI analysisWhen slotK begins, the node transmits its packet if it

still has the reservation.Continuous monitoring to face mobility

MS-Aloha Base Mechanisms MS-Aloha Base Mechanisms (ii)(ii)

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 @5.9GHz, QAM16-1)• Frame: 100ms (10Hz application Rate)• Payload: 200 Bytes• If FI=12 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

MS-Aloha Format MS-Aloha Format (i)(i)

STI (8bit) Address1(48 bit) Address2(48 bit) Sequence Number (12bit) Fragment Number (4bit) FIbit(1bit)

STI: source identificationAddress1: source addressAddress2: destination addressSequenceNumber: field indicating the sequence number of each

packetFragmentNumber: used in case of frame fragmentationFIbit: bit indicating the presence of the FI before

the payload (sent in slot0 only)Payload:CRC: used to highlight any errors during

transmission

MS-Aloha Format MS-Aloha Format (ii)(ii)

FI fieldFI fieldFI: (Frame Information): Structure containing information

about the status of each slot

Each slot information is composed of:STI: the short identifier of the nodePSF (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

STI (8 bits) PSF (2) State (2)

Time EfficiencyTime EfficiencyThe Issue of Overhead The Issue of Overhead (i)(i)

The main concern is about the overhead implied by MS-AlohaThe overhead of MS-Aloha is fixedCSMA/CA introduces a protocol overhead too, but

it is variable and hard to be measuredComparison 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

account100-200 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 60m37dbm TX, -85dbm RX (benefits for CSMA)

The Issue of Overhead The Issue of Overhead (ii)(ii)Unicast (100)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

The Issue of Overhead The Issue of Overhead (iii)(iii)Unicast (200)Unicast (200)





The Issue of Overhead The Issue of Overhead (iv)(iv)Broadcast (100)Broadcast (100)


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

The Issue of Overhead The Issue of Overhead (v)(v)Broadcast (200)Broadcast (200)

MS-Aloha (224 slots, 200B Appl.Layer, 12Mbps)446s per slot (including guard-time)Payload_Time = 200*8/12Mbps = 133sOverhead_Time= 313s (3.756 bit_time @ 12Mbps)Overhead/Payload = 2,35η = 1/(1+2.35) ≈ 0,3 (including Ethernet-like Overhead)

CSMA/CA (200B Appl.Layer, 12Mbps) 8-50 Hz Appl. RateFrom interpacket time inside a flow to interpacket time in the air1.000-3.500 s IPT unicast; 500-5.000 s IPT broadcastPayload_Time = 200*8/12Mbps = 133sOverhead_Time= 867-3.367s unicast; 367-4.867s broadcast Overhead/Payload = 6.5-25 unicast; 2.7-36 broadcast η = 1/(1+{OH}) ≈ 0,13 0.04 unicast; 0,27 0.03 broadcast

The Issue of Overhead The Issue of Overhead (vi)(vi)

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.

Typical UnresolvedTypical UnresolvedIssues of Other Slotted SolutionsIssues of Other Slotted Solutions

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 hopsIn 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 periodicallySimulations 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

1. Memory Refresh1. Memory Refresh

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

2. Signaling Semantic 2. Signaling Semantic (i)(i)

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

2. Signaling Semantic 2. Signaling Semantic (ii)(ii)

Trasmission Order

Slot 0:

Slot 1:

Slot 2:

nodes receiving from

nodes receiving from

Slot 3:

During slot 3, node will send acknowledgement about into slot 2 of its FI

FI

Example: Why an Additional Bit is RequiredExample: Why an Additional Bit is Required

Trasmission Order

Slot 0:

Slot 1:

Slot 2:

Receive from

Receive from

Slot 3:

FI

Slot 4:

The node notices the collision and send slot 2 as free on its FI

So the nodes sense a collision status affecting slot 2, then set it as free (Busy=0), while the nodes do not change the slot 2 status

Busy = 0

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.

Example: Why an Additional Bit is RequiredExample: Why an Additional Bit is Required

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 Label+Slot 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 A’s 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

3. Scalability of STI Label Space 3. Scalability of STI Label Space (i)(i)

3. Scalability of STI Label Space 3. Scalability of STI Label Space (ii)(ii)

MS-AlohaMS-AlohaTwo 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 coverageCausing 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 directionsThe number of collisions grows highThe effect is disruptive if slot re-use is hindered

Among the causes: slot state forwarding with no limitations on the number of hops

Limitation of Limitation of FIFI Forwarding ForwardingSo 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 overWhen 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 eitherThis solutions have been demonstrated, by simualtions, to:Decrease the logical radius of propagation of a slot reservation Improve of resource re-use.

Busy Busy 2-Hop Free

Improving Slot Re-Use Improving Slot Re-Use (i)(i)Slot re-use can be further improved setting a higher threshold

on minimum reception powerIf 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 AInstead, acting on the transmitted power would affect the SNRFurther improvement by introducing a mechanism which

regulates the THR dynamically THR defined on each node separately based on its perceptionBlocking completely preventedSimulations show that it worksEffects on slot reuse (increased)Effects on Packet Delivery Rate (PDR)

Lowered at higher distances but kept high close to the transmitter

Improving Slot Re-Use Improving Slot Re-Use (ii)(ii)

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

• 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

Simulation: SettingsSimulation: SettingsThe MS-Aloha has been implemented on NS-2Most 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 synchronizationThe 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 R=1Km) 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

In order to quantify results the following metrics adoptedPDR (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-AlohaSlot 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)

Simulation: MetricsSimulation: Metrics

Simulations: PDRSimulations: PDR

Whatever the threshold MS-Aloha achives a higher PDR than CSMA/CA and a negligible worsening where reception is already low

Higher thresholds in MS-Aloha force slot re-use which cause interferences and worsen PDR but quite far fram the transmitter: MS-Aloha preserves time/space-coordination

Simulations: CollisionsSimulations: CollisionsMulti-hop FI forwarding vs 2-hop

Number of CollisionsMultiHopNumber of Collisions

2-Hop

Only collisions due to mobility!

Preemption Preemption (i)(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 fieldWhenever 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-assistance, 4-entertainment

Preemption Preemption (ii)(ii)Questions to be answered:Can preemption help saturate the channel?Does preemption work also under saturation? Can it really gain

channel accessSeveral simulations. Following results achieved with:858 nodes, average speed 80km, TX power: 2 dbm5x5 grid (150m distance); 2-lane roadsApplication rate at 30HzWith and without preemptionWith preemption each nodes tries to have a High-Priority slot

and a Low-Priority slotResults (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

Preemption Preemption (iii)(iii)Collisions without preemptionCollisions at slot0 with preemption

Thank You Thank You for for

Your Kind AttentionYour Kind Attention

R. Scopigno, Networking Lab – [email protected]

www.ms-aloha.eu41

istituto superiore mario boella

Documents

slot information

slot assignment

fitrue slot occupationin

frame information

priority of data

number of slots

msalohaproposed extensions

sequence number