Wireless Networking

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Wireless Networking

• EE290T Spring 2002
• Puneet Mehra
• pmehra_at_eecs.berkeley.edu

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Topics

• Supporting IP QoS in GPRS
• QoS Differentiation in 802.11
• 802.11 and Bluetooth Coexistence
• Bluetooth

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Supporting IP QoS in the General Packet Radio
Service

• GPRS enhancement for GSM infrastructure to
  support packet-switched service
• Limitations in architecture
• Can only differentiate QoS on basis of IP address
  of mobile station (MS) not on per-flow basis
• GPRS core uses IP tunnels which makes
  implementation of IP QoS difficult
• Proposed Solutions
• IntServ approach
• DiffServ approach

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GPRS architecture

• GSNs have GPRS-compliant protocol stack.
• Supporting GSNs attach to MS, Gateways attach to
  Net
• QoS profile assigned to every MS, but
• No QoS in the network core -gt possible congestion
• IP tunnels used between GGSN and SGSN
• So RSVP/Diffserv TOS bit unavailable to
  intermediate nodes

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IntServ Approach to QoS

• Establishing QoS across Core
• Uses RSVP tunneling. Original messages pass
  through, but then additional state set up as
  needed.
• GGSN coordinates all reservations since it sees
  non-encapsulated packets.
• Mapping RSVP QoS to GPRS QoS
• Use either UpdatePDPContextRequest
  ChangePDPContextRequest messages, as well as
  ModifyPDPContextRequest messages.
• Requires significant changes to GGSN, but other
  nodes just need RSVP functionality

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DiffServ Approach to QoS

• GGSN assigns incoming traffic to a specific PHB
  (figure 6)
• To provide QoS over MS lt-gt SGSN link, each MS
  has multiple IPs.
• Each IP has own GPRS QoS and gets mapped to a
  given PHB class (can be done at connect time or
  on demand).
• Requires significant changes to all components.

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Simulation Environment

• Random handoffs w/ A1 getting most traffic
• Fast-moving and Slow-moving MS users modeled
• Traffic reflected occasional rush hour
  frequency
• 300,400 500 MSs simulated for 4 hour periods

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Results

• Low Percentage of failed reservations
• With 500 MSes, only 3.6 failed reservations
• Low signaling overhead due to addition of RSVP
  signaling
• RSVP signaling was lt2.5 of total traffic
• Overall Good scalability due to RSVP aggregation
• Get even better performance if modify the RSVP
  refresh interval

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Evaluation of Quality of Service Schemes for IEEE
802.11 Wireless LANS

• 802.11 has 2 different MAC schemes
• Distributed Coordinator Function (DCF)
• Point Coordinator Function (PCF)
• 4 Schemes Tested for Differentiation
• PCF mode
• Distributed Fair Scheduling
• Blackburst
• Enhanced DCF

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802.11 Distributed MAC scheme

• Carrier Sense Multiple Access with Collision
  Avoidance (CSMA/CA) algorithm.
• The Steps
• First Sense the Medium.
• If Idle for DIFS time period, send frame.
• Else - do exponential random backoff involving
  multiple of minimum contention window (CW)
• Each time medium is idle for DIFS, window
• If(window 0) transmit frame

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Differentiation Methods

• 802.11e Enhanced DCF
• Different minimum contention window
• Higher priority has smaller window
• Different interframe spaces
• Use Arbitration IFS some multiple of DIFS time
  period
• Packet Bursting station can send multiple
  frames, for certain time limit, after gaining
  control of medium
• PCF
• Centralized, polling-based mechanism involving
  the base station.
• Time consists of Contention Free Periods, when
  only polled station access medium.

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Differentiation Methods Cont.

• Distributed Fair Scheduling (DFS)
• Backoff interval dependent on weight of sending
  station.
• Blackburst
• High priority stations try to access medium at
  constant intervals.
• Enter a blackburst contention period, where a
  station jams the channel for time proportional to
  how long it has been waiting.
• Synchronization between high-priority flows leads
  to little wasted bandwidth due to contention

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Simulation Results

• Simulations carried out in ns-2 with background
  cross traffic
• EDCF and blackburst provided best service to
  high-priority flows, especially with high loads,
  but starved best-effort
• Blackburst had best medium utilization
• PCF performed worst, and EDCF is, distributed,
  and offers better performance
• DFS offered better service differentiation while
  avoiding starving low-priority flows when network
  load is high

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Differentiation mechanisms for IEEE 802.11

• DCF Details
• Hidden Node Problem
• Solution optional RTS/CTS scheme w/
  fragmentation_threshold
• Network Allocation Vector (NAV) used to do
  virtual carrier sensing get transmission
  duration from RTS/CTS frame info
• Different Inter Frame Spacing (IFS)
• MAC ACK packets use Short IFS (SIFS) instead of
  DIFS

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QoS Differentiation in DCF

• Backoff increase function
• Each priority level has a different backoff
  increment function
• Different DIFS
• Each priority has a different DIFS
• Maximum frame length
• Each priority has a different maximum frame that
  can be transmitted at once

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Backoff Increase Function

• Original backoff_time Floor22i x rand() x
  slot_time
• Modification backoff_time PJ2i where PJ is
  the priority factor. Larger value leads to longer
  delay/lower throughput
• Results
• Provides differentiation for UDP, but large
  ratios lead to instability
• No effect for TCP. Assume that AP is responsible
  for sending TCP-ACKs -gt since senders ended up
  waiting for ACK from AP and there was no
  contention for RTS messages

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DIFS differentiation

• Stations with higher priority have smaller DIFS
  interval
• Results
• Works well for UDP flows
• AP priority determines effect on TCP
  differentiation (since it sends ACKs)
• Can give UDP priority over TCP. How? By changing
  priority of AP.

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Maximum Frame Length (MFL)

• Priority due to size of maximum transmittable
  data unit
• Results
• Throughput proportional to MFL
• Ratios dont affect system stability
• Can prioritize TCP or UDP traffic

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Results of Channel Errors

• All Approachs
• Channel errors lower data rate
• Backoff Time Approach
• Prioritization dependent on channel (Bad!)
• Maximum Frame Length
• During channel errors, large packets more likely
  to be corrupted -gt smaller differentiation

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Wi-Fi (802.11b) and Bluetooth Enabling
Coexistance

• Bluetooth WiFi Basics
• Bluetooth - short range cable replacement tech. 1
  Mb/s data rate
• WiFi - wireless LAN tech operating at 11Mb/s
  (actually up to 22Mb/s now)
• Both Operate in 2.4 GHz Range
• Bluetooth (uses FHSS) transmit high energy in
  narrow band for short time
• WiFi (Uses DSSS) wider bandwidth with less
  energy
• Sharing spectrum -gt interference

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Interference Overview

• Noise at Receiver
• In-band noise noise in frequencies used (harder
  to filter)
• Out-of-band noise
• Types of Noise
• White (Gaussian) evenly distributed across band
• Colored specific behavior in time/frequency
• To coexist
• Receivers must deal with in-band colored noise
  but designed assuming only white noise

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Interference Experiments

• Experimental Setup
• Used laptop w/ Wi-Fi and bluetooth cards
• Results
• Wi-Fi stations less than 5-7m from AP suffered
  more than 25 degradation in presence of cubicle
  environment

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More Results
Bluetooth Throughput reduction due to Wi-Fi
interference
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Interference-Reduction Techniques

• Regulatory and standards
• Eg Allow bluetooth to only hop over certain
  range
• Usage and Practice
• Limit simultaneous usage to avoid interference
• Technical Approaches
• Limit bluetooth power for short-range devices
• Use other frequencies (5 GHz HiperLan and
  802.11a)
• Much more RF power required
• Shorter Range
• Appears to be an open research area

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Bluetooth An Enabler for Personal Area Networking

• Personal Area Network (PAN)
• Electronic devices seamlessly interconnected to
  share info (perhaps even constantly online)
• Characteristics
• Distributed Operation
• Dynamic network topology (assume mobile nodes)
• Fluctuating Link Capacity
• Low Power Devices

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Bluetooths role in PAN

• Piconets
• Adhoc networks formed by nodes
• Master/Slave semantics with polling of data
• Scatternet
• Interconnection of piconets.
• Nodes may be in several piconets at once, serving
  as gateways

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Routing Issues

• Packet Forwarding in Bluetooth
• Bluetooth Network Encapsulation Protocol (BNEP)
  ethernet-like interface for IP
• Scatternet forwarding use BNEP broadcast
  messages and ad-hoc routing techniques

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Scheduling Issues

• Intrapiconet Scheduling (IRPS)
• Schedule for polling slaves in piconet
• Interpiconet scheduling (IPS)
• Scheduling a nodes time between multiple
  piconets.
• Main challenge make sure that node is available
  in piconet when master wants to communicate

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IPS Framework

• Rendez-vous Point Algorithms Proposed for IPS
• nodes communicate when slave/master will meet (in
  time) to exchange data
• Main Issues
• How to decide on the RP, and how strict is the
  commitment
• How much data to exchange during RP
• RP timing
• can be periodic or pseudo random
• Window exchange
• Static or dynamic

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References

• Supporting IP QoS in the General Packet Radio
  Service. G. Priggouris et Al. IEEE Network 2000.
• Evaluation of Quality of Service Schemes for
  IEEE 802.11 Wireless LANs. Anders Lindgren et
  Al. IEEE LCN 2001.
• Differentiation mechanisms for IEEE 802.11.
  Imad Aad and Claude Castelluccia. IEEE Infocom
  2001.
• Wi-Fi (802.11b) and Bluetooth Enabling
  Coexistence. Jim Lansford et Al. IEEE Network
  2001.
• Bluetooth An Enabler for Personal Area
  Networking. Per Johansson et Al. IEEE Network
  2001.