cellular networks and mobile computing coms 6998-10, spring 2013

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Cellular Networks and Mobile Computing COMS 6998-10, Spring 2013. Instructor: Li Erran Li ( lierranli@cs.columbia.edu ) http://www.cs.columbia.edu/ ~lierranli/coms6998-10Spring2013/ 3 / 5 /2013: Radio Resource U sage P rofiling and Optimization. Announcements. Course evaluation due! - PowerPoint PPT Presentation


Narrowing the Beam: Lowering Complexity in Cellular Networks by Scaling Up

Cellular Networks and Mobile ComputingCOMS 6998-10, Spring 2013Instructor: Li Erran Li (lierranli@cs.columbia.edu)http://www.cs.columbia.edu/~lierranli/coms6998-10Spring2013/3/5/2013: Radio Resource Usage Profiling and Optimization1AnnouncementsCourse evaluation due!Project description due on March 25

2Review of Previous LectureWhat are the physical layer technologies in 3G and LTE?

Code Division Multiple Access (CDMA) Use of orthogonal codes to separate different transmissionsEach symbol or bit is transmitted as a larger number of bits using the user specific code SpreadingSpread spectrum technologyThe bandwidth occupied by the signal is much larger than the information transmission rateExample: 9.6 Kbps voice is transmitted over 1.25 MHz of bandwidth, a bandwidth expansion of ~1004Courtesy: Harish VishwanathUMTS Physical LayerLTE Physical LayerThe key improvement in LTE radio is the use of OFDMOrthogonal Frequency Division Multiplexing2D frame: frequency and timeNarrowband channels: equal fading in a channelAllows simpler signal processing implementationsSub-carriers remain orthogonal under multipath propagation

One resource elementOne resource block12 subcarriers during one slot (180 kHz 0.5 ms)

One OFDM symbolOne slot12 subcarrierstimefrequencyFrame (10 ms)Subframe (1 ms)Slot (0.5 ms)Time domain structure 5Wide-Area Cellular Networks - Design Choices Review of Previous Lecture (Contd)What are the mobility protocols used in cellular networks?

Mobility Protocol: GTPSGWPDN GWS5eNodeBS1-CPMMES1-US11SGiHSSMSCRNCIuCSNodeBIubSGSNIuPSGTPUEGTPGTPGnCourtesy: Zoltn Turnyi 7Wide-Area Cellular Networks - Design Choices Mobility Protocol: Proxy Mobile IP (PMIP)SGWPDN GWS5eNodeBS1-CPMMES1-US11SGiHSSGTPUEPMIPEPC Evolved Packet CoreNon-3GPP Access

(cdma2000, WiMax, WiFi)S2PMIPCourtesy: Zoltn Turnyi 8Wide-Area Cellular Networks - Design Choices Review of Previous Lecture (Contd)Is carrier sensing multiple access (CSMA) used in cellular networks?

Base stationRandom Access

UE 2UE 1Why not carrier sensing like WiFi?Base station coverage is much larger than WiFi APUEs most likely cannot hear each otherHow come base station can hear UEs transmissions?Base station receivers are much more sensitive and expensive

10Time-frequency resource on which random-access preamble is transmitted on the PRACH channel10Review of Previous Lecture (Contd)What are the problems of current LTE network architecture?

Cellular Core Network

eNodeB 3S-GW 2P-GW12

S-GW 1eNodeB 1 eNodeB 2Internet andOther IP Networks

GTP TunnelsUE 2UE 1LTE Data Plane is too CentralizedUE: user equipmenteNodeB: base stationS-GW: serving gatewayP-GW: packet data network gatewayData plane is too centralizedScalability challenges at P-GW on charging and policy enforcement!Verizon claims that, at its current rate of traffic growth, which is roughly doubling each year, it will reach the capacity thresholds on both its 3G EV-DO and LTE networks beginning in some markets by the end of 2013, and across its entire network by the end of 2015. Unless it can get new spectrum i.e., the cable operators 20 MHz of AWS spectrum its customers connection speeds and service quality will start suffering.

1213LTE Control Plane is too DistributedProblem with Inter-technology (e.g. 3G to LTE) handoffProblem of inefficient radio resource allocation User Equipment (UE)Gateway (S-GW)Mobility Management Entity (MME)Network Gateway (P-GW)Home Subscriber Server (HSS)Policy Control and Charging Rules Function (PCRF) Station (eNodeB)BaseServingPacket Data Control PlaneData PlaneNo clear separation of control plane and data plane13Review of Previous Lecture (Contd)What can we do to design a better cellular network for the future?

Mobility Manager Subscriber Information BasePolicy and Charging Rule FunctionNetwork Operating System: CellOSInfra-structure Routing Cell AgentRadio HardwarePacket Forwarding HardwareCell AgentRadio Resource ManagerPacket Forwarding HardwareCell AgentCellSDN Architecture15DPI to packet classification based on application SCTP instead of TCP to avoid head of line blockingOffloading controller actions, e.g. change priority if counter exceed thresholdTranslates policies on subscriber attributes to rules on packet header Central control of radio resource allocationCell AgentRadio HardwarePacket Forwarding HardwareCell AgentPacket Forwarding HardwareCell AgentCellSDN Virtualization16Slicing Layer: CellVisorNetwork OS (Slice 1)Network OS (Slice 2)Network OS (Slice N)Slice semantic space, e.g. all roaming subscribers, all iPhone usersOutlineIntroduction Network CharacteristicsRRC State InferenceRadio Resource Usage Profiling & OptimizationBiayo Su and Ashwin Ramachandran on radio resource profiling (15min) Network RRC Parameters Optimization Xin Ye and Nan Yan on RadioJockey (15min)Conclusion

IntroductionTypical testing and optimization in cellular data network

Little focus has been put on their cross-layer interactionsMany mobile applications are not cellular-friendly.The key coupling factor: the RRC State MachineApplication traffic patterns trigger state transitionsState transitions control radio resource utilization, end-user experience and device energy consumption (battery life)

RRCState Machine?

Courtesy: Feng Qian et al.18Network characteristics 4GTest on Androidhttp://mobiperf.com/4g.htmlMeasures network performance with the help of 46 M-Lab nodes across the world 3,300 users and 14,000 runs in 2 months 10/15/2011 ~ 12/15/2011

4GTest user coverage in the U.S.Courtesy: Junxian Huang et al.To make such large-scale measurement, we developed and published an app called 4GTest in Android market.In order to make sure that the Internet path does not become the bottleneck, 4GTest chooses the closest M-Lab nodes to connect to, which are distributed across the world.Within 2 months of deployment, we collect data from more than 3000 usersBased on this figure, we can see that our data set has a reasonably good coverage in the United States.19Downlink throughputLTE median is 13Mbps, up to 30MbpsThe LTE network is relatively unloadedWiFi, WiMAX < 5Mbps median

Comparing downlink throughput, LTEs median throughout is 13Mbps, and for some users, it can be up to 30Mbps.We need to notice that LTE network is currently relatively unloaded due to its new coming.For WiFi and WiMAX, their median downlink throughput is smaller than 5Mbps. And 3G networks are lower.20Uplink throughputLTE median is 5.6Mbps, up to 20MbpsWiFi, WiMAX < 2Mbps median

For uplink throughput, LTEs median throughput is 5.6Mbps, which is also much higher than that of WiFi or WiMAX.21RTTLTE median 70msWiFi similar to LTEWiMAX higher

For RTT, LTE and WiFi have similar median value of 70ms, WiMAX and 3G networks have higher RTTs, and the median value is between 100ms and 200ms.22The RRC State Machine for UMTS NetworkState promotions have promotion delayState demotions incur tail times

Tail TimeTail TimeDelay: 1.5sDelay: 2sChannelRadio PowerIDLENot allocatedAlmost zeroCELL_FACHShared, Low SpeedLowCELL_DCHDedicated, High SpeedHigh

Courtesy: Feng Qian et al.State machine the standard for all UMTS carriers transitions & paras can change23Example: RRC State Machinefor a Large Commercial 3G Network

Promo Delay: 2 SecDCH Tail: 5 secFACH Tail: 12 secDCH: High Power State (high throughput and power consumption)FACH: Low Power State (low throughput and power consumption)IDLE: No radio resource allocated

Tail Time: waiting inactivity timers to expireCourtesy: Feng Qian et al.24Why State Promotion Slow?Tens of control messages are exchanged during a state promotion.

RRC connection setup: ~ 1secRadio Bearer Setup: ~ 1 sec+Figure source: HSDPA/HSUPA for UMTS: High Speed Radio Access for Mobile Communications. John Wiley and Sons, Inc., 2006.Example of the State Machine Impact:Inefficient Resource Utilization

FACH and DCHWasted Radio Energy34%Wasted Channel Occupation Time33%A significant amount of channel occupation time and battery life is wasted by scattered bursts.State transitions impact end user experience and generate signaling load.Analysis powered by the ARO tool Courtesy: Feng Qian et al.A huge problem26RRC state transitions in LTE

Courtesy: Junxian Huang et al.27RRC state transitions in LTE


No radio resource allocated

Low power state: 11.36mW average power

Promotion delay from RRC_IDLE to RRC_CONNECTED: 260msCourtesy: Junxian Huang et al.28RRC state transitions in LTE


Radio resource allocated

Power state is a function of data rate: 1060mW is the base power consumptionUp to 3300mW transmitting at full speedCourtesy: Junxian Huang et al.29RRC state transitions in LTE

Continuous ReceptionSend/receive a packetPromote to RRC_CONNECTEDReset TtailCourtesy: Junxian Huang et al.30RRC state transitions in LTE

Ttail stopsDemote to RRC_IDLEDRXTtail expiresCourtesy: Junxian Huang et al.31Tradeoffs of Ttail settingsTtail settingEnergy Consumption# of state transitionsResponsivenessLongHighSmallFastShortLowLargeSlowCourtesy: Junxian Huang et al.Ttail is a very important parameter for both 3G and 4G networks.It incurs tradeoffs between UE energy consumption, signaling overhead in the network quantified by the # of state transitions here, and responsiveness

When Ttail is set to be a long value, UE consumes more energy, with smaller # of state transitions and fast responsiveness since UE spends more time in the RRC_CONNECTED state and there is less promotion delays.When Ttail is set to be a short value, UE consumes less energy, incurring larger number of state transitions and more promotions delaysLater in this


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