Wireless Networking

EE290T Spring 2002 Puneet Mehra [email protected]

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

  1.

2.

3.

4.

5.

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[2 2+i slot_time x rand()] x Modification: backoff_time = P J 2+i where P J 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 “Evaluation of Quality of Service Schemes for IEEE 802.11 Wireless LANs”. Anders Lindgren 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 IEEE Network 2001.

“Bluetooth: An Enabler for Personal Area Networking”. Per Johansson 2001.

et Al . IEEE Network 2000.

et Al.

et Al. et Al.

IEEE IEEE Network