- Millions of daily travelers use wireless networks on public transit like trains, subways, and buses to stay connected while commuting. This demand for high-capacity wireless is increasing.
- To support this demand, wireless network operators decrease cell sizes by deploying more access points (APs), but moving between smaller cells requires efficient handover techniques.
- The standard 802.11r roaming protocol is unable to efficiently handle vehicles moving between APs at speeds like 25 mph, as handovers fail and link quality decreases.
1. WIFI Goes to Town
Rapid Picocell Switching for Wireless
Transit Networks
Directed by :
Belasker Adel
Ismail Yaich
2. •Every day Millions of daily traveler
in urban centers that use different
means of transports : by train , light
rail , underground rail , vehicles .. are
using wireless networks to keep
them connected outdoor and along
the way .
•This number of traveler is increasing hugely
which implies a great demand for high-capacity
wireless networks serving transportation that
allows users to surf the web , watch videos ,
make audio calls .. all while traveling .
3. •A solution used by Wireless
networks operators in order to
support this demand of high-
capacity is to decrease cells size .
4. •But moving to smaller
cells needs a performant
« Handover« technique to
serve the client roaming
(moving) from an Access
Point AP to another .
5. •The standard Roaming WIFI Protocol
used nowadays is 802.11r Fast
Roaming, it allows clients switch to a
nearby AP without loosing connection .
6. •That’s why the WIFI standard 802.11r is unable
to handle efficiently vehicular clients .
•But we tested this current Handover solution (802.11r) on a client that drives from
AP1 to AP2 at a speed of 25 mph and it shows that the handover fails and the link
quality is decreasing significantly because switching decision is too slow .
7. Solution
•As an alternative to 802.11r protocol a research team from Princeton
University introduced the WIFI Goes To Town solution which is the first
WIFI roadside hotspot networks designed to operate in vehicular speeds .
8. •It is a deployment of an inexpensive array
of WIFI APs , called Pico Cells , along a
roadside with a central controller that
controls tightly the Pico Cells array .
•It enables APs to make delivery decisions to
the vehicular clients at millisecond level .
•It introduces a quick method on how
to decide which AP should delivers the
packet to the client .
9. •In order to accomplish this goals WIFI Goes To Town team
worked on writing some advanced algorithms and techniques
then implementing it into kernel level .
13. Fonctionnement du Système
• Uplink
AP1 AP2 AP3
Controler (deduplicates data)
Même
BSSID
(@ MAC)
Packages to send
14. Fonctionnement du Système
• Uplink problem:
AP1 AP2
data ack
ack
Solution:
Block ACK forwarding,
AP2 envoie un pacquet Block
ACK à AP1 pour que AP1 ne
renvoie pas les données au
client.
Block
ack
15. Fonctionnement du Système
• Downlink
Qui va envoyer?
AP3
AP2
controler
pacquets
pacquets pacquets
Le contrôleur décide qui
est le routeur qui va
envoyer en se basant sur
un algorithme de
switching.
On calcule à chaque fois
le SNR d’un pacquet émis
par le client vers les APs
pour savoir la qualité du
signal reçue et puis on
choisie le taux le plus
élevé ainsi le routeur
adéquat.
SNR = signal / bruit (db)
16. Fonctionnement du Système
• Downlink example
AP3
AP2
controler
pacquets
switch
start
ACK
Lorsque un client en
mouvement et reçoie
des pacquets de AP2 et
puis le controleur
décide de faire un
switch vers AP3 suivant
notre algorithme, AP2
envoie un pacquet start
vers AP3 et puis AP3
envoie un ack vers le
controler.
Finalement AP2 indique
à AP3 les pacquets
blocqués pour qu’il
sache quels pacquets il
doit continuer à
envoyer vers le client.