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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:1. First Sense the Medium.2. If Idle for DIFS time period, send frame.3. Else - do exponential random backoff involving
multiple of minimum contention window (CW)4. Each time medium is idle for DIFS, window—5. 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 = PJ
2+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.
