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Transcript
Outline for This Lecture • Reality Check of Wireless Network Usage • Overview of Wireless and Mobile Networking CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 Wireless Channel • Wireless transmission is error prone • Wireless error and contention are location dependent • Wireless channel capacity is also location dependent CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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? CS211/Fall 2003 10/08 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? CS211/Fall 2003 10/08 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? CS211/Fall 2003 10/08 Protocol Stack • Draw the entire protocol stack – For each component/layer • Some requirements • Issues to address • Possible design options CS211/Fall 2003 10/08 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) CS211/Fall 2003 10/08 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.) CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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? CS211/Fall 2003 10/08 Network Layer • Constraint: – Unaware hosts running IP – Route management for mobile hosts needs to be dynamic – A backbone may not exist (ad-hoc network) CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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 CS211/Fall 2003 10/08 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? CS211/Fall 2003 10/08