Elements of a wireless ntework
- wireless hosts
laptop,smartphone
run application
may be staionary(non-mobile) or mobile(wireless does not always mean mobility) - base station
typically connexted to wired network
relay-responsible for sending packets between wired host(s) in its “area”(e.g.,cell towers,80.11 access points) - wireless link
typically used to connect mobile(s) to base station
also used as bcakbone link
multiple access protocol coordinate link access
various data rates,transmission distance - infrastructure mode
base station connects mobile into wired network
handoff:mobile changes base station providing connection into wired network - ad hoc mode
ad hoc mode
no base stations
nodes can only transmit to other nodes withink link coverage
nodes organize themselves into a network:route among themselves
Wireless network taxonomy
single hop | multiple hops | |
---|---|---|
infrastructure(e.g.,APs) | host connexts to base station(WiFi,WiMAX,cellular)which connects to larger Internet | host may have to relay through several wireless nodes to connects to larger Internet:mesh net |
no infrastructure | no base station,no connection to larger Internet(Bluetooth,ad hoc nets) | no base station,no connection to larger Internet.May have to relay to reach other relay to reach other a given wireless node MANET,VANET |
Mobility
spectrum of mobility,from the network
perspective:
- vocabulary
home network:permanent “home” of mobile
permanent address:address in home network,can always be used to reach mobile
home agent:entity that will perform mobility functions on behalf of mobile,when mobile is remote
permanent address:remains constant
care-of- address:address in visited network
visited network:network in which moobile currently
correspondent:wants to communicate with mobile
foreign agent:entity in visited network that performs mobility functions on behalf of mobile
Mobility:approaches
- let routing handle it:
routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
routing tables indicate where each mobile located
no changes to end-systems - let end-systems handle it:
indirect routing:communication from correspondent to mobile goes through home agent,then forwared to remote
direct routing:correspondent gets foreign address of mobile,sends directly to mobile
⚡️Mobility:registration
⚡️Mobility via indirect routing
- mobile uses two addresses:
permanent address:used by correspondent(hence mobile location is transparent to correspondent)
care-of -address:used by home agent to forward datagrams to mobile - foreign agent functions may be done by mobile itself
- traingle routing:
correspondent-home-network-mobile
inefficient when correspondent,mobile are in same network.
Indirect routing:mobving between networks
suppose mobile user moves to another network
- registers with new foreign agent
- new foreign agent registers with home agent
- home agent update care-of -address for mobile
- packets continue to be forwarded to mobile(but with new care-of-address)
mobility,changing foreign networks networks transparent:on going connections can be maintained!
⚡️Mobility via direct routing
overcome traingle routing problem
non-transparent to correspondent:correspondent must get care-of-address from home agent
Accomodating mobility with direct routing
anchor foreign agent:FA in first visited network
data always routed first to anchor FA
when mobile moves:new FA arranges to have data forwarded from old FA(chaining)
Mobile IP
- RFC 3344
- has many features we’ve seen:
home agents,foreign agents,foreign-agent regitration,care-of-address,encapsulation(packet-within-a-packet) - three components to standard:
indirect routing of datagrams
agent discovery
registration with home agent
Mobile IP:indicate routing
Mobile IP:agent discovery
agent advertisement:foreign/home agents advertise service by broadcasting ICMP messages(typefield=8)
Mobile IP:registration example
Wireless,mobility:impact on higher layer protocols
- logically,impact should be minimal…
best effort service model remains unchanged
TCP and UDP can (and do) run over wireless,mobile - …but performance-wise
packet loss/delay due to bit-error(discarded packets,delays for link-layer retransmissions),and handoff
TCP interprets loss as congestion,will decrese congestion window un-necessarily
delay impairments for real-time traffic
limited bandwidth of wireless links