Wireless Networking

1. Wireless Networking EE290T Spring 2002 Puneet Mehra pmehra@eecs.berkeley.edu

2. Topics • Supporting IP QoS in GPRS • QoS Differentiation in 802.11 • 802.11 and Bluetooth Coexistence • Bluetooth

3. 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

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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

10. 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

11. 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.

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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.

18. 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

19. 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

20. 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

21. 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

22. 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

23. More Results Bluetooth Throughput reduction due to Wi-Fi interference

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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.