Wireless networks

Philippe Jacquet

Link layer: protocols in local area networks

• MAC/link address: – 6 octets

• Starts with 1: unicast• Starts with 0: multicast

• Internet address: – 4 octets (IPv4)

• First octets subnet address

– 16 octets (IPv6)

Couche 5Couche 5

Couche 4Couche 4

Couche 3Couche 3

Couche 2: LienCouche 2: Lien

Couche 1: PhysiqueCouche 1: Physique

Encapsulation

IP destination IP source data

IP packet

MAC receiver MAC emitter

MAC packet

Local protocol

• if IP address out of subnet– Toward gateway

• Otherwise address resolution protocol (ARP)– Get MAC address in correspondance

address– Otherwise launch an ARP request

gateway

Protocole ARP

• ARP Request

• ARP Reply

MAC FFFFFF MAC emitter IP destination

MAC receiver MAC emitter IP destination

emitter reply: correspondance

émetteur requête

ARPprotocol

Reverse ARP

• MAC → IP

DHCP

• IP addresses for mobile nodes (Dynamic Host Configuration Protocol)

Link/MAC: Multiple access protocols

• In wireless networks, medium channel is unique and must be shared– One or several of frequencies

Wireless Communication Architecture

• Access point architecture– Wifi infrastructure mode– GSM, UMTS– Wimax

• Ad hoc architecture– Mesh networks– Mobile ad hoc– Sensor networks

Multiple access protocols

• Frequency Division Multiple Access– Frequency set is split between users

• Time Division Multiple Access (TDMA)

time

frequencies

time

Wireless Access Protocols• Periodic TDMA

– Time slot periodically allocated to terminal in round robin.

– Examples: GSM, bluetooth.

timeslot

Wireless Access Protocols• Random access protocols

– More than two transmitters over one slot→ collision

• Collision detection (no ACK)• Collision resolution algorithm.

timeslot

Models of wireless reception

• Signal attenuation

• Minimal SNR for reception

1 5

2

34

€

r1

€

r2

€

r3

€

r4

€

r5

€

S(r) =S0

rα

€

receive 1 on z ⇔ SNR(1) =S(r1)

B + S(ri)i≠1

∑≥ K

€

z

Wireless ALOHA

€

1

TL2

• Transmitters come as a space-time Poisson process of rate per time unit and per area unit– Poisson rate is in– In dimension D it is in– In wired network D=0 (classic ALOHA)

€

1

TLD

€

ρ

Wireless ALOHA

• Signal level map

Wireless Aloha

• Reception areas

Reception area in urban environment

Area of correct reception

• Area where SNR>K around an emitter X

• Average size of order (homothetic principle)

X

€

σ(ρ) =σ (1)

ρ

€

σ(1) =K−

2

α sin2π

α

⎛

⎝ ⎜

⎞

⎠ ⎟α

2π for D = 2

Paradox of wireless Aloha:

– When D>0 average sum of correct reception remains the same for all

– Not true for D=0: classic unstable ALOHA

€

ρ

X

X

X X

Consequence on MANET theoretical performance

• N nodes on an area A, density

• Per node traffic rate

• Average neighbor size

€

σ ρ( ) A€

ν

€

λ

€

ρ =λν =λ NA

€

M =σ ρ( )ν =σ (1)

λ

Paradox of space capacity

• Average number of hops

• Net per node capacity

• Total network transport capacity€

β N

M

€

λ0 =λ

β N M=σ (1)

β

λ

N

€

Nλ 0 =O Nλ( )

MANET Capacity limit

€

M > logN

• But the network must be connected

• Total transport capacity€

max{λ} =σ (1)

logN

€

max{Nλ 0} =σ (1)

β

N

logN

Capacity paradox

• Transport Capacity increases with space and density when N increases.

• In D=0 (wired net)

– Transport Capacity is constant

€

λ0 = λ =σ (1)

N€

M = N

€

λ0N