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

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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

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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

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network infrastructure

PAN: personal area network / BAN: body area network

Cellular Networks

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Mobile

Switching Center

Public telephonenetwork, andInternet

Mobile

Switching Center

Wireless

Typical Base Station (BS):connects mobiles into wired network

Wi-Fi (802.11) Networks

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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

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AP

APAP

APAP

AP

AP

AP

AP

Mesh Networks with Internet Access

AP

APAP

AP

AP

Internet

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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

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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

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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

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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

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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

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Typical Issues a MAC Must Solve

Competing nodes

Hidden terminal

Exposed node

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Hidden Terminal

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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

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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)

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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)

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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

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avoid data frame collisions completely using small reservation packets!

Collision Avoidance: RTS-CTS exchange

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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

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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

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Wireless MAC Summary

From a routing point of view

Links are not reliableo Collisions

Links are time-varyingo Delay

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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

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Backup Slides

More on 802.11

GSM network architecture

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802.11 LAN architecture

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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

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802.11: passive/active scanning

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AP 2AP 1

H1

BBS 2BBS 1

122

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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

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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

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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

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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

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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

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GSM Network Architecture

Structure of a GSM network (from Wikipedia)

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