Introduction to MS-Aloha R. Scopigno, Networking Lab
[email protected] www.ms-aloha.eu 1 Slide 2 Introduction: Concepts
and Figures Introduction: Concepts and Figures First Proprietary
Mechanisms: RR-Aloha+ First Proprietary Mechanisms: RR-Aloha+
Proposed Extensions Proposed Extensions Simulative Settings
Simulative Settings The Final Version: MS-Aloha The Final Version:
MS-Aloha Proposed Extensions for Scalability Proposed Extensions
for Scalability RR-Aloha+ & MS-Aloha Simulations RR-Aloha+
& MS-Aloha Simulations Preemption and Conclusions Preemption
and Conclusions It works under mobility It simulatively overtakes
CSMA/CA Slide 3 Introduction: Concepts and Figures Introduction:
Concepts and Figures First Proprietary Mechanisms: RR-Aloha+ First
Proprietary Mechanisms: RR-Aloha+ Proposed Extensions Proposed
Extensions Simulative Settings Simulative Settings The Final
Version: MS-Aloha The Final Version: MS-Aloha Proposed Extensions
for Scalability Proposed Extensions for Scalability RR-Aloha+ &
MS-Aloha Simulations RR-Aloha+ & MS-Aloha Simulations
Preemption and Conclusions Preemption and Conclusions Slide 4 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 Requirements
for slotted Vanets Slide 5 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 (i) Slide 6 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(K+N)
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 MS-Aloha Base Mechanisms (ii) Slide 7 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
(i) Slide 8 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 MS-Aloha Format (ii) Slide 9 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 STI (8 bits) PSF (2)State (2) Slide 10
Time Efficiency The 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) Slide 11 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 Slide 12 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 Slide 13 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 (iv) Broadcast (100) Slide 14 Inter-packet time inside a
flow (Average on the 100 flows) Time between two consecutive
packets correctly received CSMA/CA saturation starts at
