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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
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
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
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