Adapted from: Computer Networking, Kurose/Ross! 1DT066". Distributed
Information Systems". " Chapter 6! Wireless, WiFi and mobility! Adapted from:
Computer ...
1DT066" Distributed Information Systems"
"
Chapter 6! Wireless, WiFi and mobility!
Adapted from: Computer Networking, Kurose/Ross!
Elements of a wireless network! wireless hosts! ! ! !
network infrastructure
Adapted from: Computer Networking, Kurose/Ross!
pp 539-544!
!
laptop, smartphone! run applications! may be stationary (nonmobile) or mobile! May roam or migrate!
Wireless, Mobile Networks
6-2
Elements of a wireless network! base station! ! !
network infrastructure
Adapted from: Computer Networking, Kurose/Ross!
connected to wired network! relaying- responsible for sending packets between wired network and wireless host(s) in its radio area (blue circle)! " e.g., 3G/cell towers, 802.11 access points !
Wireless, Mobile Networks
pp 539-544!
6-3
Infrastructure mode! infrastructure mode! !
!
network infrastructure
Adapted from: Computer Networking, Kurose/Ross!
pp 539-544!
base station connects mobiles into wired network! handoff: mobile changes base station!
Wireless, Mobile Networks
6-4
Ad hoc mode! ad hoc mode! ! No (wired) base stations! ! nodes can only transmit to other nodes within radio reach! ! nodes organize themselves into a network: route only among themselves!
Adapted from: Computer Networking, Kurose/Ross!
pp 539-544!
Wireless, Mobile Networks
6-5
Wireless Link Characteristics ! important differences from wired link ….! !
" decreased signal strength: radio signal attenuates as it propagates through matter (path loss)! " interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) are shared by other devices (e.g., microwave oven). Electrical devices, such as electrical motors, interfere as well.! " multipath propagation: radio signal reflects off objects, the ground, atmosphere, etc. Reflections arrive at destination at slightly different times! ! …. make communication across (even a point to point) wireless link much more difficult compared to a wire.! Adapted from: Computer Networking, Kurose/Ross!
pp 545!
Wireless, Mobile Networks
6-6
Characteristics of selected wireless links!
Data rate (Mbps)
200 54
802.11n 802.11a,g
5-11
802.11a,g point-to-point
802.11b
4
4G: LTWE WIMAX 3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
1
802.15
.384
2.5G: UMTS/WCDMA, CDMA2000
.056
2G: IS-95, CDMA, GSM
Indoor
Outdoor
10-30m
50-200m
Adapted from: Computer Networking, Kurose/Ross!
pp 543!
Mid-range outdoor
Long-range outdoor
200m – 4 Km
5Km – 20 Km Wireless, Mobile Networks
6-7
IEEE 802.11 Wireless LAN! 802.11b! ! 2.4 GHz unlicensed spectrum! ! up to 11 Mbps!
802.11a ! " 5-6 GHz range! " up to 54 Mbps!
802.11g ! " 2.4-5 GHz range! " up to 54 Mbps! 802.11n: multiple antenna! " 2.4-5 GHz range! " up to 200 Mbps!
! ! !
all use the radio sharing protocol CSMA/CA for multiple access! all have base-station and an ad-hoc network configuration mode! Adapted from: Computer Networking, Kurose/Ross!
pp 552!
Wireless, Mobile Networks
6-8
802.11 LAN architecture! ! Internet
wireless host communicates with base station! " base station = access point (AP)!
! hub, switch or router
Basic Service Set (BSS) (aka cell ) in infrastructure mode contains:! " wireless hosts! " access point (AP): base station!
BSS 1
BSS 2 Adapted from: Computer Networking, Kurose/Ross!
pp 553-554!
Wireless, Mobile Networks
6-9
802.11: Host association to AP! ! host:
must associate with an Access Point!
" Host scans radio channels, listening for beacon frames containing AP’s name (SSID) and MAC address! " Host selects (the best) AP to associate with! " may perform authentication to get access! " will typically run DHCP to get IP address in AP’s subnet!
!
Adapted from: Computer Networking, Kurose/Ross!
pp 555-557!
Wireless, Mobile Networks 6-10
IEEE 802.11: Sharing the radio channel! ! !
Many nodes can independently chose to send at the same time! 802.11: Carrier Sense Multiple Access – host senses (listen) radio channel if busy before transmitting! " Don’t transmit and collide with ongoing transmission by other node!
!
802.11: difficult to detect a collision!! " difficult to receive (sense collisions) when transmitting due to weak received signals ! " can t sense all collisions in any case: hidden terminal, ! " goal: avoid collisions: CSMA/C(ollision)A(voidance)!
A
B
A
C
C C s signal strength
A s signal strength
B
space Adapted from: Computer Networking, Kurose/Ross!
Wireless, Mobile Networks 6-11
pp 557-560!
IEEE 802.11 MAC Protocol: CSMA/CA! 802.11 sender 1 if sense channel idle for DIFS then
sender
transmit entire frame (no CD) 2 if sense channel busy then DIFS start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, Draw this figure! repeat 2
802.11 receiver - if frame received OK
receiver
data
SIFS
ACK
return ACK after SIFS (ACK needed due to hidden terminal problem) Adapted from: Computer Networking, Kurose/Ross!
pp 557-560!
Wireless, Mobile Networks 6-12
What is mobility?! !
spectrum of mobility, from the network perspective:!
no mobility
mobile wireless user, using same access point
Roaming - high mobility
mobile user, connecting/ disconnecting from network using DHCP.
mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
Adapted from: Computer Networking, Kurose/Ross!
pp 581!
Wireless, Mobile Networks 6-13
Mobility: vocabulary! home network: permanent home of mobile (e.g., 128.119.40/24)
home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote
wide area network
permanent address: address in home network, can always be used to reach mobile e.g., 128.119.40.186
Adapted from: Computer Networking, Kurose/Ross!
pp 582!
Wireless, Mobile Networks 6-14
Mobility: more vocabulary! permanent address: remains constant (e.g., 128.119.40.186)
visited network: network in which mobile currently resides (e.g., 79.129.13/24)
care-of-address: address in visited network. (e.g., 79,129.13.2)
wide area network
foreign agent: entity in visited network that performs mobility functions on behalf of mobile.
correspondent: wants to communicate with mobile Adapted from: Computer Networking, Kurose/Ross!
Wireless, Mobile Networks 6-15
pp 582!
Mobility: registration! visited network
home network
2
1
wide area network
foreign agent contacts home agent home: this mobile is resident in my network
mobile contacts foreign agent on entering visited network
end result:! ! foreign agent knows about visiting mobile! ! home agent knows location of mobile! Adapted from: Computer Networking, Kurose/Ross!
pp 583-588!
Wireless, Mobile Networks 6-16
Mobility via indirect routing! home agent intercepts packets, forwards to foreign agent
foreign agent receives packets, forwards to mobile
home network
visited network
3
1
wide area network 2
4
correspondent addresses packets using home address of mobile
Adapted from: Computer Networking, Kurose/Ross!
mobile replies directly to correspondent
pp 583-588!
Wireless, Mobile Networks 6-17
Indirect Routing: comments! ! 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! ! triangle
routing: correspondent2home2network2mobile! " inefficient when ! Correspondent and Mobile ! are in same network.!
Adapted from: Computer Networking, Kurose/Ross!
pp 583-588!
Wireless, Mobile Networks 6-18
