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 5 Couche 4 Couche 3 Couche 2: Lien Couche 1: Physique

Encapsulation IP destinationIP sourcedata IP packet MAC receiverMAC 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 FFFFFFMAC emitter IP destination MAC receiverMAC 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. time slot

Wireless Access Protocols Random access protocols –More than two transmitters over one slot→ collision Collision detection (no ACK) Collision resolution algorithm. time slot

Models of wireless reception Signal attenuation Minimal SNR for reception

Wireless ALOHA 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)

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

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 X X XX

Consequence on MANET theoretical performance N nodes on an area A, density Per node traffic rate Average neighbor size

Paradox of space capacity Average number of hops Net per node capacity Total network transport capacity

MANET Capacity limit But the network must be connected Total transport capacity

Capacity paradox Transport Capacity increases with space and density when N increases. In D=0 (wired net) –Transport Capacity is constant