Overview of Wireless Networking

Overview of Wireless Networking

• Wireless Link Characteristics

• Services and Applications

Overview

• Fundamental issues and impact– wireless– mobility

• For each layer in the protocol stack– A subset of design requirements– Design challenges/constraints– Possible design options

Wireless Channel Characteristics

• Radio propagation– Multipath, fade, attenuation, interference

& capture– Received power is inversely proportional

to the distance: distance-power gradient•Free space: factor 2•Inbuilding corridors or large open indoor

areas: <2•Metal buildings: factor 6•Recommended simulation factors: 2~3

for residential areas, offices and manufacturing floors; 4 for urban radio communications

Wireless Channel

• Wireless transmission is error prone

• Wireless error and contention are location dependent

• Wireless channel capacity is also location dependent

Link-Level Measurements

• Measurements taken from 802.11b-based MIT Roofnet

• Focus:– Explore reasons for loss– mainly on long outdoor links

Roofnet: multihop wireless mesh

1 kilometer

Using omni-direction antenna

+ Easy to deploy

+ Provide high connectivity

- Don’t allow engineered link quality

Lossy radio links are common

1 kilometer

1-30%

30-70%

70-100%

Broadcast packet delivery probability

Delivery prob. uniformly distributed

Node Pair

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Packet

Delivery

Pro

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> two-thirds of linksdeliver less than 90%

ImplicationsProtocols should exploit intermediate-

quality links• Link-quality-aware routing (ETX, LQSR)• 802.11 transmit bit-rate selection• Multicast data distribution• Opportunistic protocols (OMAC, ExOR)

• An emerging research direction

Hypotheses for intermediate delivery probability

1. Marginal signal-to-noise ratios2. Interference: Long bursts3. Interference: Short bursts (802.11)4. Multi-path interference

Methodology: Link-level measurements of packet loss

• Goal: all-pairs loss rates• Each node broadcasts for 90 seconds• All other nodes listen• Raw link-level measurements:

– No ACKs, retransmissions, RTS/CTS– No other Roofnet traffic– No 802.11 management frames– No carrier sense

Hypothesis 1: Marginal S/N

• Simplified model for packet loss:– P(delivery) = f(signal/noise)– Signal strength reflects attenuation – Noise reflects interference

• Perhaps marginal S/N explains intermediate delivery probabilities

Delivery vs. S/N with a cable and attenuator

Signal-to-noise ratio (dB)

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pro

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Laboratory

Delivery vs. S/N on Roofnet

S/N does not predict delivery probability for intermediate-quality links

Signal-to-noise ratio (dB)

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Roofnet

Laboratory

Hypothesis 2: long bursts of interference

Bursty noise might corrupt packets without affecting S/N measurements

A B

Loss over time on two different Roofnet links

The top graph is consistent with bursty interference. The bottom graph is not.

avg: 0.5stddev: 0.28

avg: 0.5stddev: 0.03

Del

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Time (seconds)

Most links aren’t burstyC

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Std dev of one-second delivery averages

Hypothesis 3: short bursts of interference (802.11)

AB

• MAC doesn’t prevent all concurrent xmits• Outcome depends on relative signal levels• Hypothesis: When a nearby AP sends a packet, we lose a

packet.

Methodology: record non-Roofnet 802.11 traffic

• Goal: measure non-Roofnet traffic• Before the broadcast experiments• Each node records all 802.11 traffic

No correlation between foreign traffic observed and packets lost

Non-Roofnet packets observed per second (before the experiment)

Exp

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pac

kets

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Hypothesis 4: Multi-path interference

Reflection is a delayed andattenuated copy of the signal

A

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A channel emulator to investigate multi-path effects

Sender Receiver

delay attenuation

Reflection causes intermediate packet loss

Del

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Delay of second ray(nanoseconds or feet)

Roofnet links are long

Link distance (feet or nanoseconds)

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It’s reasonable to expect delays >500 ns

Summary

• Most Roofnet links have intermediate loss rates

• S/N does not predict delivery probability

• Loss is not consistent with bursty interference

• Multi-path is likely to be a major cause

Satellites• Geostationary Earth Orbit (GEO)

Satellites– example: Inmarsat

SAT

ground stations

Satellites• Low-Earth Orbit (LEO) Satellites

– example: Iridium (66 satellites, 2.4 Kbps)

SAT

ground stations

SAT

SAT

constellation

Satellites

• GEO– long delay: 250-300 ms propagation

delay

• LEO– relatively low delay: 40 - 200 ms– large variations in delay - multiple

hops/route changes, relative motion of satellites, queueing

Wireless Connectivity - Characteristics

• Transmission errors– Wireless LANs - 802.11, Hyperlan– Cellular wireless– Multi-hop wireless– Satellites

• Low bandwidth– Cellular wireless– Packet radio (e.g., Metricom)

• Long or variable latency– GEO, LEO satellites– Packet radio - high variability

• Asymmetry in bandwidth, error characteristics– Satellites (example: DirectPC)

Mobility

• Why mobility?– 30~40% of the US workforce is

mobile (Yankee group)– Hundreds of millions of users are

already using portable computing devices and more than 60% of them are prepared to pay for wireless access to the backbone information

Mobility• Four types of activities for a typical office

work during a workday:– Communication (fax, email)– Data manipulation (word processing,

directory services, document access & retrieval)

– Information access (database access and update, internet access and search)

– Information share (groupware, shared file space)

• Question: how does mobility affect each of the above activities?

Possible scenarios of mobility

• Scenario 1: user logs out from computer 1, moves to computer 2 and logs in– Should the user see the same workspace?

• Scenario 2: different devices for different networks

• Scenario 3: user docks a laptop, works in a networked mode for a while, then disconnects and works in the standalone mode for a while, and then docks back– In stand-alone mode

• What kind of activities can the user do?• What cannot be done?• Can we provide an illusion of connectivity in this

case?• Can we automatically re-integrate the work he has

done while disconnected when he finally reconnects to the network server?

Impact of Mobility• Scenario 4: a user has a notebook with a

wireless connection, connects to a remote host via network 1, shuts down connections, connects to the remote host via network 2, continues to work– Is the disconnection between network

migration necessary?– When can we make the disconnection

transparent to users? When we cannot?– What are the key issues to ensure seamless

network migration?– Is it really important or users do not care

about the automatic process? For what applications? What to change for the applications?

Protocol StackLook at:

1. Applications/Services

2. OS issues

3. Middleware (skip):1. Transcoding

Application Layer

Middleware and OS

Transport Layer

Network Layer

Link Layer & Below

Issues in building services in mobile networking environments

• Mobility induced issues:– Seamless services: service migration– Location services: location itself is a service

• Heterogeneity induced issues:– Hardware diversity

• Client devices & different networks– Software diversity

• System software: OS, networking protocols• Application software

• Wireless induced issues:– Time-varying network connectivity: disconnection, partial

connection, full connection

Possible services for mobile environments

• Location service• Location transparent services

– Hide locations from users• Location dependent services

– Services “local” to a geographic location– Not available globally

• Location aware services– Services are globally available, but multiple

instantiations of the same service are a function of locations

– Service adapts to a location

Issues in Operating Systems• Energy-efficient scheduler• File systems for disconnected operation due

to mobility and disconnected wireless links– access the same file as if connected– retain the same consistency semantics for

shared files as if connected– availability and reliability as if connected– ACID (atomic/recoverability, consistent, isolated/

serializablity, durable) properties for transactions

• Constraints:– disconnection and/or partial connection– low bandwidth connection– variable bandwidth and latency connection– connection cost

Next Step:

Networking Issues

Physical/MAC Layer

• Requirements: – Continuous access to the channel to transmit a

frame without error– Fair access to the channel: how is fairness

quantified?– Low power consumption– Increase channel throughput within the given

frequency band

• Constraints:– Channel is error prone– Channel contention and error are location

dependent– Transmission range is limited (but also enables

channel reuse)– Shared channel (hidden/exposed station problem)

Physical/MAC Layer

• Possible options:– Physical layer:

• Narrow band vs wide band: direct sequence, frequency hopping, OFDM

• Antenna technology: smart antenna, directional antenna, MIMO

• Adaptive modulation

– MAC layer• Multiple access protocols (CSMA/CA, MACAW,

etc.)• Frame reservation protocols (TDMA, DQRUMA,

etc.)

Link Layer• Requirements:

– Error sensitive application•A reliable link abstraction on top of

error-prone physical channels– Delay sensitive application

•A bounded delay link abstraction on top of error-prone channels

• Constraints:– Errors in the channel– Spatial congestion– Link capacity is changing (PHY multi-

rate option)

Link Layer• Possible options at the link layer

– Windowing to provide error and flow control– Combating error:

•Proactive: error correction via e.g. FEC•Reactive: error detection+retransmission,

ARQ•Channel-state prediction+channel swapping

– Fairness options: long term vs short term, deterministic vs probabilistic, temporal vs throughput•All links are treated equal•Users in error prone or congested location

suffer

Network Layer• Requirements:

– Maintain connectivity while user roams– Allow IP to integrate transparently with

roaming hosts• Address translation to map location-

independent addressing to location dependent addressing

• Packet forwarding• Location directory

– Support multicast, anycast– Ability to switch interfaces on the fly to

migrate between failure-prone networks– Ability to provide quality of service: what

is QoS in this environment?

Network Layer

• Constraint:– Unaware hosts running IP– Route management for mobile hosts

needs to be dynamic– A backbone may not exist (ad-hoc

network)

Network Layer• Possible options:

– Mobile IP and its variants• Two-tier addressing (location independent

addressing <-> location dependent addressing)

• A smart forwarding agent which encapsulates packets from unware host to forward them to MH

• Location directory for managing location updates

– Ad hoc routing•Shortest path, source routing,

multipath routing

Transport Layer• Requirements:

– Congestion control and rate adaptation• Doing the right thing in the presence of

different packet losses– Handling different losses (mobility-induced

disconnection, channel, reroute)– Improve transient performance

• Constraints: – Typically unware of mobility, yet affected by

mobility– Packet may be lost due to congestion, channel

error, handoffs, change of interfaces, rerouting failures

– Link-layer and transport layer retransmit interactions

Transport Layer

• Options:– Provide indirection– Make transport layer at the end hosts

ware of mobility– Provide smarts in intermediate nodes

(e.g. BS) to make lower-layer transport aware

– Provide error-free link layers