Transcript CS219: Advanced Topics in Internet Research

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
ATT Wireless GSM/GPRS Overlay
upgrade to E-GPRS or EDGE
W-CDMA-based tech.
Estimated Deployment
conclude year-end 2002
mid-2002
Late 2002
Cingular
Wireless
Year-end 2001
2002 & 2003
2002 & 2003
not determined
GPRS Overlay on GSM
EDGE Overlay on TDMA
EDGE overlay on GPRS/GSM
next step
Sprint PCS
CDMA 1X (release 0)
CDMA 1X (release A)
CDMA 1XEV-DO
CDMA 1XEV-DV
to early 2002
to early 2003
early 2003
2003~2005
Verizon
Wireless
CDMA 1X
CDMA 1XEV
next step
year-end 2001
following 1X rollout
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?
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