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Wireless Internet Routing
Review of Wireless Networking (with Routing in Mind)
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Review of Wireless Networking
Architecture of wireless networks
Wireless PHY
Wireless MACo 802.11
2
PHY: physical layer / MAC: medium access control (layer)
Architecture of Wireless Networks
Keywords:o Single-hop vs. multi-hop
o Infrastructure based vs. point to point or ad-hoc/mesh
Most wireless networks today have a single wireless hop
3
Single-Hop Wireless Networks
Cellular networks:
GSM, 3G, 4G (IP based)
Wireless LAN
PAN/BAN (Bluetooth)
Generally infrastructure based (access point or base station),
not much (wireless) routing necessary
4
network infrastructure
PAN: personal area network / BAN: body area network
Cellular Networks
5
Mobile
Switching Center
Public telephonenetwork, andInternet
Mobile
Switching Center
Wireless
Typical Base Station (BS):connects mobiles into wired network
Wi-Fi (802.11) Networks
6
Internet
hub, switchor router
AP
AP
Typical Access Point (AP)
Wireless
Single-Hop Networks with No Infrastructure
Bluetooth “piconet”, master/slave structure
Simple peer to peer connections
7Pictures: Wikipedia
Multi-Hop Wireless Networks
Wireless infrastructureo Mesh networks
Client mobility, not infrastructure
Wireless routing becomes necessary
8
AP
APAP
APAP
AP
AP
AP
AP
Mesh Networks with Internet Access
AP
APAP
AP
AP
Internet
9
Internet
Wireless infrastructure (backbone)o Static backbone
Gateways to the Interneto Eg: Freifunk, Seattle Wireless
Clients connects to APo Transparent wireless network
o Mobility, roaming
Multi-Hop Wireless Networks
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No infrastructure: ad-hoc networko MANET: Mobile Ad Hoc Networks
o Sensor networks
o VANET
Origins: packet radio networks (1978, “Advances in packet radio technology”)
Nodes: transmit, receive, forward
Nodes can be mobile
Wireless routing becomes necessary
Master/Slave Multi-Hop Wireless Network
Bluetooth (“Scatternet”)o Master is a slave
No infrastructure
11Pictures: Wikipedia
Why Multi-Hops Wireless Networks ?
Ease of deployment
Speed of deployment
Decreased dependence on infrastructure
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Because There are Many Applications
Personal area networkingo Cell phone, laptop, ear phone, wrist watch
Military environmentso Soldiers, tanks, planes
Civilian environmentso Mesh networkso Taxi cab networko Meeting roomso Sports stadiumso Boats, small aircrafto Environmental monitoring
Emergency operationso Search-and-rescueo Policing and fire fighting
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There are Many Variations
Fully Symmetric Environmento All nodes have identical capabilities and responsibilities
Asymmetric Capabilitieso Transmission ranges and radios may differ o Battery life at different nodes may differo Processing capacity may be different at different nodeso Speed of movement
Asymmetric Responsibilitieso Only some nodes may route packets o Some nodes may act as leaders of nearby nodes (e.g., cluster
head)
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There Really are Many Variations
Traffic characteristics may differ in different ad hoc networkso Bit rate
o Timeliness constraints
o Reliability requirements
o Unicast / multicast / geocast
o Host-based addressing / content-based addressing / capability-based addressing
May co-exist (and co-operate) with an infrastructure-based network
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Many, Many, Many Variations…
Mobility patterns may be differento People sitting at an airport loungeo New York taxi cabso Kids playingo Military movementso Personal area network
Mobility characteristicso Speedo Predictability
o Direction of movemento Pattern of movement
o Uniformity (or lack thereof) of mobility characteristics among different nodes
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Standardisation Effort: MANET@IETF
MANET IETF working group (http://www.ietf.org/html.charters/manet-charter.html)o “standardize IP routing protocol functionality suitable for
wireless routing application within both static and dynamic topologies with increased dynamics due to node motion or other factors”
o Work on unicast routing protocol, multicast forwarding and neighbor discovery
o More later…
RFCs: 2501, 3561, 3626, 3684, 4728, 5449
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Wireless Networks Architecture: Summary
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Single-hop Multiple hops
Infrastructure(e.g., APs)
Noinfrastructure
host connects to base station (WiFi,WiMAX, cellular) which connects to
larger Internet
no base station, noconnection to larger Internet (Bluetooth,
ad hoc nets)
host may have torelay through several
wireless nodes to connect to larger Internet: mesh net
no base station, noconnection to larger
Internet. May have torelay to reach other a given wireless node
MANET, VANET
This course: routing in multi-hop wireless networks (MANET/Ad hoc networks and mesh networks)
Wireless Network Specifics and Challenges
Dynamic topology: client and/or node mobility, node failures, time-varying linkso Mobility: route loss, packet loss, network partition
o Mesh network: dynamic backbone, clients roaming
Wireless PHY and linko Time-varying
o Low reliability (packet loss)
o Limited bandwidth
o Limited transmission range
o Shared and broadcast medium (security)
o Bidirectional and unidirectional links, asymmetric links
Energy-constrained operationo Especially for ad hoc networks
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Wireless Physical Layer
Unlike a wired connection!
Characteristicso Time-varying channel: propagation, connectivity and available
capacity
o Shared medium: interference, collisions
Consequences for the upper layers: links exhibito Time-varying behavior: connectivity, rate
o Low reliability: packets are lost (typical 10^-2 PER)
o Smaller bandwidth than wired counterpart (10 to 100 Mbit/s)
o Asymmetric / bidirectional and unidirectional links
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PHY or PHY? Radio Waves vs Bits
Depends whom you talk to!
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BPF LNADemodulation
Decoding
CodingModulation
Amp
Waveforms(analog PHY)
Waveforms (analog PHY) Bits (digital PHY)
Bits (digital PHY)
From MAC
To MAC
Propagation is Time-Varying
Because of mobility or mobile environment
Path loss:o Free space: Friis,
o Real life: logarithmic, 1/rα
Shadowingo Obstacles: buildings, trees
Fadingo Multipath
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Pr/Pt Very slow
Slow
Fast
PrPt
d=vt
v
Pictures: Ganesan CS691aa 07, Reddy 07
d
Received Power is not Uniform
Reasonso Propagation
o Antenna gain
o Hardware issues
23Pictures: Ganesan CS691aa 07
Received Power is not Uniform
Connectivity is not a unit-disc
24
RSSI: low power RSSI: high power
Pictures: Ganesan CS601aa 07
Received Power is Time-Varying
25Pictures: Vyas 06
Rate of a Link Depends on the Received Power
R = f(SNR) SNR=Pr/(I+N)
Link rate is time-varying
26
10 20 30 40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
Detour: Interference
Where does I come from?
Remember! We have a shared medium: “INTERFERENCE”
In-system interferenceo APs in the same building
External interferenceo Bluetooth on 802.11
o Microwave oven on DECT phone
And N?
Thermal noise in the hardware
27
Packet loss variability as function of the distance
28Pictures: Ganesan CS691aa 07
Typical Network Behavior: Variable Packet Loss
Typical Network Behavior: Reception Rate Changes Over Time
29Pictures: Ganesan CS691aa 07
Links are Asymmetric
Node A can transmit to node B but node B cannot transmit to node A
Achtung! Signal propagation (path loss, shadowing, fading) is not asymmetric!
But hardware is not perfecto Frequency mismatch
o Variation of transmission power
o AGC changing the thresholds
o Different noise levels and noise floors
And links are time-varying
And shared
30AGC: automatic gain control
Wireless Physical Layer Summary
From a routing point of view
Links are asymmetric
Links are not reliableo Fading, interference
Links are time-varyingo Connectivity (neighbors or reachable nodes)
o Rate
o Delivery probability or packet error rate (PER)
Broadcast by default: overhearing
31
Wireless Medium Access Control (MAC)
Remember! We have a shared medium
MAC: arbitrate (point to point) transmissions between nodes
Compared to classic Etherneto Collision detection is very hard
o No full duplex (requires extra transceivers)
o Not a single rate guaranteed
32
Typical Issues a MAC Must Solve
Competing nodes
Hidden terminal
Exposed node
33
Hidden Terminal
34
AB
C
Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each othermeans A, C unaware of their
interference at B
A B C
A’s signalstrength
space
C’s signalstrength
Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other
interfering at B
Exposed Terminal
35
BA
C
D
IEEE 802.11: multiple access
Avoid collisions: 2+ nodes transmitting at same time
802.11: CSMA - sense before transmittingo Don’t collide with ongoing transmission by other node
802.11: no collision detection!o Difficult to receive (sense collisions) when transmitting due
to weak received signals (fading)
o Can’t sense all collisions in any case: hidden terminal, fading
o Goal: avoid collisions: CSMA/C(ollision)A(voidance)
36
AB
CA B C
A’s signalstrength
space
C’s signalstrength
IEEE 802.11 MAC Protocol: CSMA/CA
802.11 sender1 if sense channel idle for DIFS then
transmit entire frame (no CD)2 if sense channel busy then
start random backoff timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random backoff interval, repeat 2
802.11 receiver- if frame received OK
return ACK after SIFS (ACK needed due to hidden terminal problem)
37
sender receiver
DIFS
data
SIFS
ACK
Avoiding collisions (more)
Idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of
long data frames
Sender first transmits small request-to-send (RTS) packets to BS using CSMAo RTSs may still collide with each other (but they’re short)
BS broadcasts clear-to-send CTS in response to RTS
CTS heard by all nodeso sender transmits data frame
o other stations defer transmissions
38
avoid data frame collisions completely using small reservation packets!
Collision Avoidance: RTS-CTS exchange
39
APA B
time
DATA (A)
reservation collision
defer
802.11: advanced capabilities
Rate Adaptation
base station, mobile dynamically change transmission rate (physical layer modulation technique) as mobile moves, SNR varies
40
QAM256 (8 Mbps)
QAM16 (4 Mbps)
BPSK (1 Mbps)
operating point
1. SNR decreases, BER increase as node moves away from base station
2. When BER becomes too high, switch to lower transmission rate but with lower BER
10 20 30 40SNR(dB)
BE
R
10-1
10-2
10-3
10-5
10-6
10-7
10-4
802.11: advanced capabilities
Power Management
node-to-AP: “I am going to sleep until next beacon frame”o AP knows not to transmit frames to this node
o node wakes up before next beacon frame
beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sento node will stay awake if AP-to-mobile frames to be sent;
otherwise sleep again until next beacon frame
41
Wireless MAC Summary
From a routing point of view
Links are not reliableo Collisions
Links are time-varyingo Delay
42
Wireless Internet Routing
This course: routing in MANET/ad hoc networks and mesh networkso Internet? = Gateway(s) wireless/wired
Challenges of routing in wireless networkso High time variability
o Dynamic topology
o Links: unreliable, asymmetric, time-varying
43
Backup Slides
More on 802.11
GSM network architecture
44
802.11 LAN architecture
45
Wireless host communicates with base station base station = access point
(AP)
Basic Service Set (BSS)(aka “cell”) in infrastructure mode contains: wireless hosts
access point (AP): base station
ad hoc mode: hosts only
BSS 1
BSS 2
Internet
hub, switchor router
AP
AP
802.11: Channels, association
802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencieso AP admin chooses frequency for APo interference possible: channel can be same as that chosen by
neighboring AP!
host: must associate with an APo scans channels, listening for beacon frames containing AP’s
name (SSID) and MAC addresso selects AP to associate witho may perform authentication [Chapter 8]o will typically run DHCP to get IP address in AP’s subnet
46
802.11: passive/active scanning
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AP 2AP 1
H1
BBS 2BBS 1
122
34
Active Scanning:
(1) Probe Request frame broadcast from H1
(2)Probes response frame sent from APs
(3)Association Request frame sent: H1 to selected AP
(4)Association Response frame sent: H1 to selected AP
AP 2AP 1
H1
BBS 2BBS 1
1
23
1
Passive Scanning:(1) beacon frames sent from APs(2)association Request frame sent:
H1 to selected AP (3)association Response frame sent:
H1 to selected AP
frame
controlduration
address
1
address
2
address
4
address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seq
control
802.11 frame: addressing
48
Address 2: MAC addressof wireless host or AP transmitting this frame
Address 1: MAC addressof wireless host or AP to receive this frame
Address 3: MAC addressof router interface to which AP is attached
Address 4: used only in ad hoc mode
Internetrouter
AP
H1 R1
AP MAC addr H1 MAC addr R1 MAC addr
address 1 address 2 address 3
802.11 frame
R1 MAC addr H1 MAC addr
dest. address source address
802.3 frame
802.11 frame: addressing
49
frame
controlduration
address
1
address
2
address
4
address
3payload CRC
2 2 6 6 6 2 6 0 - 2312 4
seq
control
TypeFrom
APSubtype
To
AP
More
fragWEP
More
data
Power
mgtRetry Rsvd
Protocol
version
2 2 4 1 1 1 1 1 11 1
duration of reserved transmission time (RTS/CTS)
frame seq #(for RDT)
frame type(RTS, CTS, ACK, data)
802.11 frame: more
50
hub or switch
AP 2
AP 1
H1 BBS 2
BBS 1
router
802.11: mobility within same subnet
H1 remains in same IP subnet: IP address can remain same
switch: which AP is associated with H1?o self-learning (Ch. 5): switch will
see frame from H1 and “remember” which switch port can be used to reach H1
51
GSM Network Architecture
Structure of a GSM network (from Wikipedia)
52