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Outline for This Lecture

• Reality Check of Wireless Network Usage

• Overview of Wireless and Mobile Networking

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Comparisons of 3G and 802.11• Coverage

– 3G: large coverage– 802.11: small

• Throughput– 802.11: up to 11/54 Mbps– 3G: up to 2 Mbps

• Cell size and density– 802.11: several hundred feet– 3G: up to several kilometers

• Applications supported:– 802.11: mainly data, but may support VoIP– 3G: data plus voice in 1XEVDV

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Measurements on 802.11 WLAN• Mobile host is prevalent, but

mobile flows are not• Network usage is highly dependent

on applications• Highly nonstationary traffic pattern

– Days and evenings– Workdays and weekends

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Migration to 3G (Src: Wireless Week Research)• Carrier Network Technology Estimated DeploymentATT Wireless GSM/GPRS Overlay conclude year-end 2002 upgrade to E-GPRS or EDGE mid-2002 W-CDMA-based tech. Late 2002

Cingular GPRS Overlay on GSM Year-end 2001Wireless EDGE Overlay on TDMA 2002 & 2003 EDGE overlay on GPRS/GSM 2002 & 2003 next step not determined

Sprint PCS CDMA 1X (release 0) to early 2002 CDMA 1X (release A) to early 2003 CDMA 1XEV-DO early 2003 CDMA 1XEV-DV 2003~2005

Verizon CDMA 1X year-end 2001Wireless CDMA 1XEV following 1X rollout next step not determined

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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

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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

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Wireless Channel

• Wireless transmission is error prone

• Wireless error and contention are location dependent

• Wireless channel capacity is also location dependent

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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

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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)– Sharing of information (groupware, shared file

space)

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

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Mobility• 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 network – 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 (s)he has done

while disconnected when (s)he finally reconnects to the network server?

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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?

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Protocol Stack• Draw the entire protocol stack

– For each component/layer• Some requirements• Issues to address• Possible design options

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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:– Interference, fade, multi-path, and signal attenuation cause

the channel to be error prone– Channel contention and error are location dependent– Channel capacity is fluctuating– Transmission range is limited (but also enables channel

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

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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.)

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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 due to modulation

techniques

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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

– A few possible definitions of fairness: long term vs short term, deterministic vs probabilistic, temporal vs throughput

• All users are treated equal• Users in error prone or congested location suffer

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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

– Provide connection to packet flow as opposed to datagram (connection oriented networks)

– 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?

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Network Layer

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

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

network)

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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)

– Connection-oriented mobility support• Multicast• Finding the first branch point and rerouting packets

– Ad hoc routing• Shortest path, source routing, multipath routing

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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 is 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

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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

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Operating Systems• Requirements:

– Provide the same environment to the user whether mobile (partially connected) or on the backbone network: same files, same context, ability to run same programs, access the same databases, servers & services, retain the same ID

– Provide an abstraction of the environment for the aware application to adapt intelligently

• Constraints:– Scheduling limited CPU resources & limited

energy– Limited disk, memory– Partial connectivity

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File Systems• Requirements:

– 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/serializable, durable) properties for transactions

• Constraints:– Disconnection and/or partial connection– Low bandwidth connection– Variable bandwidth and latency connection,– Connection cost

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File systems• Four major aspects of disconnected

or partially connected operations:– Hoarding: what to pre-fetch– Consistency: what to keep consistent

when connectivity is partial– Emulation: how to operate when

disconnected– Conflict resolution: how to resolve

conflicts• Many choices within each aspect

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Applications/Services

• A few questions for application designs:– How much to know about mobility (dynamic

state)?– How much to control the activity of OS?– How to structure the interaction btw. App

and systems– How to write location-aware applications?– What kind of filtering, data retrieval, and

control support to be provided at the backbone?