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Wireless Networking. EE290T Spring 2002 Puneet Mehra pmehra@eecs.berkeley.edu. Topics. Supporting IP QoS in GPRS QoS Differentiation in 802.11 802.11 and Bluetooth Coexistence Bluetooth. Supporting IP QoS in the General Packet Radio Service.

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Wireless Networking

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Wireless Networking

EE290T Spring 2002

Puneet Mehra

pmehra@eecs.berkeley.edu


Topics

  • Supporting IP QoS in GPRS

  • QoS Differentiation in 802.11

  • 802.11 and Bluetooth Coexistence

  • Bluetooth


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


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 -> possible congestion

  • IP tunnels used between GGSN and SGSN

    • So RSVP/Diffserv TOS bit unavailable to intermediate nodes


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


DiffServ Approach to QoS

  • GGSN assigns incoming traffic to a specific PHB (figure 6)

  • To provide QoS over MS <-> SGSN link, each MS has multiple IP’s.

  • 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.


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


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 <2.5% of total traffic

  • Overall Good scalability due to RSVP aggregation

    • Get even better performance if modify the RSVP refresh interval


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


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


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.


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


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


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


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


Backoff Increase Function

  • Original: backoff_time = Floor[22+i x rand()] x slot_time

  • Modification: backoff_time = PJ2+i 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 -> since senders ended up waiting for ACK from AP and there was no contention for RTS messages


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.


Maximum Frame Length (MFL)

  • Priority due to size of maximum transmittable data unit

  • Results

    • Throughput proportional to MFL

    • Ratios don’t affect system stability

    • Can prioritize TCP or UDP traffic


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 -> smaller differentiation


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 -> interference


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


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


More Results

Bluetooth Throughput reduction due to Wi-Fi interference


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


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


Bluetooth’s 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


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


Scheduling Issues

  • Intrapiconet Scheduling (IRPS)

    • Schedule for polling slaves in piconet

  • Interpiconet scheduling (IPS)

    • Scheduling a node’s time between multiple piconets.

    • Main challenge: make sure that node is available in piconet when master wants to communicate


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


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.


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