modelling machine type communicationin ieee 802.11ah networks

Modelling Machine Type Communicationin IEEE 802.11ah Networks

Institute for Information Transmission ProblemsRussian Academy of Sciences

Moscow, Russia

Alexander KrotovEvgeny Khorov Andrey Lyakhov

IEEE International Conference on Communications - Workshop on 5G & Beyond - Enabling Technologies and Applications. London, UK, June, 2015

2

Swarm of sensor STAs

High collision rate

State-of-the-art wireless networks (e.g. Wi-Fi) do not support swarms of sensors involved into Internet of things interaction

Evgeny Khorov, Alexander Krotov, Andrey Lyakhov. Modelling Machine Type Communication in IEEE 802.11ah Networks

802.11ah Restricted access window

Key Idea: to divide STAs into small groups and spread their access attempts over a long period of time

To support IoT scenarios, IEEE 802 is developing a novel amendment to the Wi-Fi standard: IEEE 802.11ah.Among other modifications, it describes RAW, new channel access method, which allows the AP to allocate some time interval called RAW slot to a group of STAs.Inside each RAW slot, the STAs use legacy EDCA channel access

𝐶𝑊 𝑖

i15 31 63127

511

255

1023

0 1 2 3 4 5 6 7

54 213σ Data

ACKSIFS

AIFS

Backoff

Empty slot

Successful slot

Select backoff

RAW start

Data AckTimeout

Collision slot

Select new backoff

Collision

EDCA

3

𝑁

𝑀

RAW

ScenarioLet the AP select a group of M sensor STAs and estimate that N of M STAs have data.More precisely, let each of N STAs have one data frame at the beginning of the RAW slot.What length of the RAW slot is enough? TaskFor a given duration of the RAW slot find the probability that:A. Marked STA successfully transmits its frame;B. All N STAs successfully transmit their frames.


How to choose values of RAW parameters?

General Problem

• No hidden stations• Frames are not corrupted due to noise• Lengths of all frames are equal

Stations are working

synchronously

Assumptions

Problem Statement

4

Trick

1𝐶𝑊 𝑟

…𝐶𝑊 𝑟

𝑟>0

1𝐶𝑊 0

…𝐶𝑊 0

𝑟=0

Probability of transmission in slot after retries:

𝑷𝒓 (𝒕 ,𝒓 )={ 1−∑𝑖=0

𝑡 −1

Pr (𝑇𝑋 ,𝑖 ,𝑟 ) ,𝑟=0

∑𝑖=0

𝑡 −1

Pr (𝐶 ,𝑖 ,𝑟 −1 )−∑𝑖=0

𝑡 −1

Pr (𝑇𝑋 ,𝑖 ,𝑟 ) ,𝑟>0

Attempt failed Attempt has not occurred

Probability of reaching slot with retry counter :

For each transmission attempt leads to a collision:

Trick: Since almost does not depend on , use for any value of Evgeny Khorov, Alexander Krotov, Andrey Lyakhov. Modelling Machine Type Communication in IEEE 802.11ah Networks

Let us number EDCA virtual slots inside RAW: t.Probability of transmitting in slot if STA reached it with retry counter 𝐏𝐫 (𝑻𝑿∨𝒕 ,𝒓 )=𝑷𝒓 (𝑻𝑿 ,𝒕 ,𝒓 )

𝑷𝒓 (𝒕 ,𝒓 )

A. the marked STA successfully transmits its frame

Transition Processes

𝑡𝑡+1

c s r c+

1c+1r+1s+

1

c s r

absorbing states

𝜋𝑒

𝜋𝑐+¿¿

𝜋𝑐−

𝜋 𝑠−𝜋 𝑠

+¿¿

Empty slot

Collision without marked STA

Collision with marked STA

Successful transmission of another STA

Markov chain A Markov chain B

…absorbing

states

cs

cs

c+1s

cs+1

c=1N

𝜋𝑒

𝜋𝑐

𝜋 𝑠

c=0N

…

Let𝑝=Pr (𝑇𝑋|𝑡 ,𝑐 , 𝑠 )=∑𝑟=0

𝑐

Pr (𝑇𝑋|𝑡 ,𝑟 ) Pr (𝑡 ,𝑐 ,𝑠 ,𝑟 )

∑𝑟=0

𝑐

Pr (𝑡 ,𝑐 ,𝑠 ,𝑟 )Transmission attempt probability for non-

marked STA:


Tasks: For :a given RAW slot duration find the probability thatB. all N STAs successfully transmit their frames

+: marked STA transmits-: marked STA does not transmit

𝑐: number of collisions𝑠: number of successful slots𝑟: retry counter

𝑡𝑡+1

6

6

PDF

CDF


Numerical ResultsProbability that the marked STA successfully transmits its frame

Probability that all N STAs successfully transmit their frames.

7

By the beginning of the RAW slot each of 1000 STAs has

a packet with probability 0.3


For N stations

Solution:Split stations into several groups

Splitting into 50 groups minimizes channel time in this scenarioSlot duration ~20 ms

Required RAW slot durationRequired delivery probability: 0.9

For N=Bin(M=1000, p=0.3) stations

More than 1 second(exceeds protocol limit)

8

Useful ReferencesThis paperEvgeny Khorov, Alexander Krotov, Andrey Lyakhov.Modelling Machine Type Communication in IEEE 802.11ah networks.In Proc. of IEEE International Conference on Communications - Workshop on 5G & Beyond - Enabling Technologies and Applications. London, UK, June, 2015

An example of usage of this model Dmitry Bankov, Evgeny Khorov, Andrey Lyakhov. The Study of the Centralized Control Method to Hasten Link Set-up in IEEE 802.11ah Networks. In Proc. of the 21th European Wireless Conference, Budapest, Hungary, May, 2015

The model can be also applied to study and manage many new mechanisms developed in .11ah and described in Evgeny Khorov, Andrey Lyakhov, Alexander Krotov, Andrey Guschin. A survey on IEEE 802.11ah: an Enabling Networking Technology for Smart Cities. Computer Communications, Volume 58, March 2015, pp. 53–69, 2015.

modelling machine type communicationin ieee 802.11ah networks

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