1 Lecture on Wireless Measurements & Modeling Prof. Maria Papadopouli University of Crete ICS-FORTH

Agenda Introduction on Mobile Computing & Wireless Networks Wireless Networks - Physical Layer IEEE MAC Wireless Network Measurements & Modeling Location Sensing Performance of VoIP over wireless networks Mobile Peer-to-Peer computing 2

Empirical measurements Can be beneficial in revealing – deficiencies of a wireless technology – different phenomena of the wireless access & workload Impel modelling efforts to produce more realistic models & synthetic traces Enable meaningful performance analysis studies using such empirical, synthetic traces and models  Highlight the ability of empirical-based models to capture the characteristics of the user-workload and provide a flexible framework for using them in performance analysis

Propagation Models One of the most difficult part of the radio channel design Done in statistical fashion based on measurements made specifically for an intended communication system or spectrum allocation Predicting the average signal strength at a given distance from the transmitter 4

Signal Power Decay with Distance A signal traveling from one node to another experiences fast (multipath) fading, shadowing & path loss Ideally, averaging RSS over sufficiently long time interval excludes the effects of multipath fading & shadowing  general path-loss model: P(d) = P 0 – 10n log 10 (d/d o ) n: path loss exponent P(d): the average received power in dB at distance d P 0 is the received power in dB at a short distance d 0 5 _ _

Signal Power Decay with Distance In practice, the observation interval is not long enough to mitigate the effects of shadowing  The received power is commonly modeled to include both path-loss & shadowing effects, the latter of which are modeled as a zero-mean Gaussian random variable with variance σ sh in the logarithmic scale, P(d), in dB can be expressed: P(d) ~ N (P(d), σ sh 2 ) This model can be used in both line-of-sight (LOS) & NLOS scenarios with appropriate choice of channel parameters 6 _

7 Monitoring Depending on type of conditions that need to be measured, monitoring needs to be performed at Certain layers Spatio-temporal granularities Monitoring tools – Are not without flaws – Several issues arise when they are used in parallel for thousands devices of different types & manufacturers: Fine-grain data sampling Time synchronization Incomplete information Data consistency

8 Monitoring & Data Collection Fine spatio-temporal detail monitoring can  Improve the accuracy of the performance estimates but also  Increase the energy spendings and detection delay Network interfaces may need to Monitor the channel in finer & longer time scales Exchange this information with other devices

9 Challenges in Monitoring (1/2) Identification of the dominant parameters through – sensitivity analysis studies Strategic placement of monitors at – Routers – APs, clients, and other devices Automation of the monitoring process to reduce human intervention in managing the Monitors Collecting data

10 Challenges in Monitoring (2/2) A ggregation of data collected from distributed monitors to improve the accuracy while maintaining low overhead in terms of Communication Energy Cross-layer measurements, collected data spanning from the physical layer up to the application layer, are required

11 Wireless Networks – Are extremely complex – Have been used for many different purposes – Have their own distinct characteristics due to r adio propagation characteristics & mobility e.g., wireless channels can be highly asymmetric and time varying Note: Interaction of different layers & technologies creates many situations that cannot be foreseen during design & testing stages of technology development

12 Empirically-based Measurements Real-life measurement studies can be particularly beneficial in revealing – deficiencies of a wireless technology – different phenomena of the wireless access and the workload Rich sets of data can – Impel modeling efforts to produce more realistic models – Enable more meaningful performance analysis studies

13 Unrealistic Assumptions – Models & analysis of wired networks are valid for wireless networks – Wireless links are symmetric – Link conditions are static – The density of devices in an area is uniform – The communication pairs are fixed – Users move based on random-walk models

14 Wireless Access Parameters Traffic workload – In different time-scales – In different spatial scales (e.g., AP, client, infrastructure) – In bytes, number of packets, number of flows, application-mix Delays – Jitter and delay per flow – Statistics at an AP and/or channel User mobility patterns Link conditions Network topology

15 Traffic Load Analysis As the wireless user population increases, the characterization of traffic workload can facilitate More efficient network management Better utilization of users’ scarce resources Application-based traffic characterization

16 Hourly Session arrival rates

17 Traffic Load at APs Wide range of workloads that log-normality is prevalent In general, traffic load is light, despite the long tails No clear dependency with type of building the AP is located exists – Although some stochastic ordering is present in Tail of the distributions Dichotomy among APs is prominent in both infrastructures:  APs dominated by uploaders  APs dominated by downloaders As the total received traffic at an AP ↑ – There is also ↑ in its total traffic sent – ↓ in the sent-to-received ratio

18 Traffic load at APs Substantial number of non-unicast packets Number of unicast received packets strongly correlated with number of unicast sent packets (in log-log scale) Most of APs send & receive packets of relatively small size Significant number of APs show rather asymmetric packet sizes – APs with large sent & small receive packets – APs with small sent & large receive packets Distributions of the number of associations & roaming operations are heavy-tailed Correlation between the traffic load & number of associations in log-log scale

19 In general, the traffic load is light  long tails RARE EVENTS OF LARGE SIZE

20 Wide-range of workloads As total received traffic at an AP ↑  its total traffic sent ↑

21 APs with small amount of received traffic and large amount of sent traffic As total received traffic at an AP ↑  its total traffic sent/received ↓

22 The number of unicast received packets strongly correlated with the number of unicast sent packets (in log-log scale)

23 Asymmetry in the sizes of sent & received packets at an AP Majority of APs with small sent and received packet sizes

24 Correlation between traffic load & # of associations

25 Application-based Traffic Characterization  Using port numbers to classify flows may lead to significant amounts of misclassified traffic due to: – Dynamic port usage – Overlapping port ranges – Traffic masquerading Often peer-to-peer & streaming applications: – Use dynamic ports to communicate – Port ranges of different applications may overlap – May try to masquerade their traffic under well-known “non- suspicious” ports, such as port 80

26 Desirable Properties for Models – Accuracy – Tractability – Scalability – Reusability – “Easy” interpretation

Related work Rich literature in traffic characterization in wired networks – Willinger, Taqqu, Leland, Park on self-similarity of Ethernet LAN traffic – Crovela, Barford on Web traffic – Feldmann, Paxson on TCP – Paxson, Floyd on WAN – Jeffay, Hernandez-Campos, Smith on HTTP  Traffic generators for wired traffic – Hernandez-Campos, Vahdat, Barford, Ammar, Pescape, … P2P traffic – Saroiu, Sen, Gummadi, He, Leibowitz, … On-line games – Pescape, Zander, Lang, Chen, … Modelling of wireless traffic – Meng et al.

Dimensions in Modeling Wireless Access Intended user demand User mobility patterns – Arrival at APs – Roaming across APs Channel conditions Network topology

Mobility models Group or individual mobility Spontaneous or controlled Pedestrian or vehicular Known a priori or dynamic Random-walk based models – Randway model in ns-2 Markov-based model

A Very Simple Channel Model Idle Busy P ii P bb P bi P ib A channel can be in the idle or busy state Markov-based model allows us to determine: How much time the system spends in each state Probability of being in a particular state  In real rife, there is non-stationarity due dynamic phenomena Compute stationary probabilities Gilbert model

Main approaches for traffic generation Packet-level replay – An exact reproduction of a collected trace in terms of packet arrival times, size, source, destination, content type  Reflects specific traffic conditions – Suffers from arbitrary delays e.g., interrupts, service mechanisms, scheduling processes  difficult to incorporate feedback-loop characteristics Source-level generation  Allows the underlying network, protocol, & application layer to specify & control the packet arrival process – Simplest example: infinite source model

Our approach  Inspired by the source-level (or network independent) modelling Assumptions: 1.Client arrivals at an infrastructure (initiated by humans) at a large extent are not affected by the underlying network technology 2.Very low % of packet loss at the network layer  flow arrivals & sizes approximate intended user traffic demand

1230 Sessio n Wired Network Wireless Network Router Internet User A User B AP 1 AP 2 AP3 Switch disconnection Flow time Events Arrivals t1 t2t3t7t6t5t4

Traffic Demand Parameters Session – arrival process – starting AP Flow within session – arrival process – number of flows – size (in bytes) Captures interaction between clients & network Above packet-level analysis

Wireless infrastructure & acquisition 26,000 students, 3,000 faculty, 9,000 staff in over 729-acre campus 488 APs (April 2005), 741 APs (April 2006) SNMP data collected every 5 minutes Several months of SNMP & SYSLOG data from all APs Packet-header traces: – Two weeks (in April 2005 and April 2006) – Captured on the link between UNC & rest of Internet via a high- precision monitoring card

Related modeling approaches  Flow-level modeling by Meng [mobicom ‘04] No session concept Weibull for flow interarrivals Lognormal for flow sizes AP-level over hourly intervals  Hierarchical modeling by Papadopouli [wicon ‘06] Time-varying Poisson process for session arrivals BiPareto for in-session flow numbers & flow sizes Lognormal for in-session flow interarrivals Sessions capture the non-stationarity of traffic workload

Modeling methodology 1.Selection of models (e.g., various distributions) 2.Fitting parameters using empirical traces 3.Evaluation and comparison of models Visual inspection e.g., CCDFs & QQ plots of models vs. empirical data Statistical-based criteria e.g., QQ/simulation envelopes, Kullback-Liebler divergence Systems-based criteria e.g., throughput, delay, jitter, queue size 4.Validation of models 5.Generalization of models

Synthetic trace generation

Synthetic traces based on empirical ones Generate session arrivals within each session: generate number of flows for each flow: generate flow arrivals & sizes based on specific models Session arrivals: using hourly, building-specific empirical traces Flow-related data: using empirical traces of different spatial scales original data from the real-life infrastructure Produced by this process:

Model validation  Use empirical data from different tracing periods April 2005 & 2006 spatial scales AP-level < building-level < building-type-level < network-wide traffic AP campus-wide wireless infrastructures UNC, Dartmouth  Do the same distributions persist across these traces ?  Compare their performance (empirical traces: “ground truth”) YES!

Model evaluation Create synthetic data based on models Analysis with metrics not explicitly addressed by the models – Statistical-based aggregate flow arrival count process aggregate flow interarrival (1 st & 2 nd order statistics) System-based: performance of an IEEE LAN traffic load and queue size in various time scales per-flow & hourly aggregate throughput per-flow delay and jitter  Compare their performance (empirical traces: “ground truth”)

Modeling in Various Spatio-temporal Scales Sufficient spatial detailScalableAmenable to analysis Hourly AP    Network-wide    Objective Scales  Tradeoff with respect to accuracy, scalability & reusability

Scalability vs. Accuracy: Flow Interarrivals EMPIRICAL BDLG(DAY) BDLGTYPE(DAY) NETWORK(TRACE) Spatial /Temporal Scales

Scalability vs. Accuracy: Number of Flow Arrivals in an Hour EMPIRIC AL BDLG(DA Y) NETWORK(TRACE) BDLGTYPE(TRACE)

Model evaluation Create synthetic data based on models Analysis with metrics not explicitly addressed by the models – Statistical-based aggregate flow arrival count process aggregate flow interarrival (1 st & 2 nd order statistics) System-based: performance of an IEEE LAN traffic load and queue size in various time scales per-flow & hourly aggregate throughput per-flow delay and jitter  Compare their performance (empirical traces: “ground truth”)  Dominant parameters ? Impact of application mix?

Wireless Networks Router Internet User A AP Switch User B Scenario User C Various traffic, channel conditions Network monitor Collected traces Statistical Analysis Testbed Deploymen t Monitoring & Data collection Certain Protocol Evaluation at AP Iterative process Network monitor Select testbed Protocol evaluation Cross validation

Simulation/Emulation testbed TCP flows UDP Wired clients: senders Wireless clients: receivers

Hourly aggregate throughput EMPIRICAL BIPARETO-LOGNORMAL Fixed flow sizes & empirical flow arrivals (aggregate traffic as in EMPIRICAL) Pareto flow sizes, empirical flow arrivals FLOW SIZE—FLOW (INTER)ARRIVAL BIPARETO-LOGNORMAL-AP Impact of flow size

Per-flow throughput EMPIRICAL BIPARETO-LOGNORMAL-AP BIPARETO-LOGNORMAL Fixed flow sizes & empirical number of flows FLOWSIZE—FLOWARRIVAL Pareto flow sizes & uniform flow arrivals Pareto flow sizes due to large % of small size flows (= MSS)

Impact of application mix on per-flow throughput AP with 85% web traffic AP with 50% web & 40% p2p traffic AP with 80% p2p traffic TCP-based scenario

UDP traffic scenario  Wireless hotspot AP  Wireless clients downloading  Wired traffic transmit at 25Kbps  Total aggregate traffic sent in CBR & in empirical is the same Empirical: 1.4 Kbps Bipareto-Lognormal-AP: 2.4 Kbps Bipareto-Lognormal: 2.6 Kbps Large differences in the distributions

Conclusions Model validation over two different periods (2005 and 2006) over two different campus-wide infrastructures (UNC & Dartmouth) BiPareto captures well the flow sizes over heavy & normal traffic AP using statistical-based metrics using system-based metrics hourly aggregate throughput per-flow delay per-flow throughput

Conclusions (con’t) Accuracy: our models perform very close to the empirical traces popular models deviate substantially from the empirical traces Scalability: same distributions at various spatial & temporal scales group of APs per bldg addresses scalability-accuracy tradeoffs Accurate and scalable models of wireless demand

Conclusions (con’t)  Application mix of AP traffic mostly web: very accurate models both web & p2p : models are ok  mostly p2p: large deviations from empirical data  Modelling P2P traffic is challenging due to the increased number, diversity, complexity & unpredictability in user interaction  Both flow size and flow interarrivals Impact of various parameters

Revisiting modelling approach Physical meaning of the models and their parameters Client profile – e.g., depending on the application-mix, amount of traffic Group mobility Multiple network interfaces Cooperative client models Dependencies among traffic demand & network conditions – Impact of underlying network conditions on application & usage patterns

UNC/FORTH web archive  Online repository of models, tools, and traces – Packet header, SNMP, SYSLOG, synthetic traces, …  Free login/ password to access it  Simulation & emulation testbeds that replay synthetic traces for various traffic conditions Mobile Computing University of Crete/FORTH 

57 Application-based traffic characterization Most popular applications: web browsing and peer-to-peer (81% of the total traffic). Most users are also dominated by these two applications. Network management & scanning: responsible for 17% of the total flows. While building-aggregated traffic application usage patterns appear similar, the application cross-section varies within APs of the same building. Most wireless clients appear to use the wireless network for one specific application that dominates their traffic share. File transfer flows (e.g., ftp and peer-to-peer) are heavier in the wired network than in the wireless one. There is a dichotomy among APs, in terms of their dominant application type and downloading and uploading behavior.