Cellular NetworksGuest lecture by Li Erran Li, Bell LabsCOS 461: Computer Networks4/18/2012 W 10-10:50am in Architecture N101*Cellular Core Network

Cellular Networks Impact our Lives*

Mobile Data Tsunami Challenges Current Cellular Technologies Global growth 18 times from 2011 to 2016

AT&T network:Over the past five years, wireless data traffic has grown 20,000%At least doubling every year since 2007

Existing cellular technologies are inadequateFundamental redesign of cellular networks is neededSource: CISCO Visual Networking Index (VNI) Global Mobil Data Traffic Forecast 2011 to 2016*

OutlineGoal of this lecture: understand the basics of current networks

Basic Architecture of LTEAccess ProcedureWhy no carrier sensingConnection SetupUnlike WiFi, need to keep the same IP address at different attachment pointsMobility ManagementPower Management and Mobile AppsDifferences between 3G and LTEConclusion

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Cellular Core NetworkeNodeB 3S-GW 2P-GW*S-GW 1eNodeB 1 eNodeB 2Internet andOther IP NetworksGTP TunnelsUE 2UE 1LTE InfrastructureMME/PCRF/HSSUE: user equipmenteNodeB: base stationS-GW: serving gatewayP-GW: packet data network gatewayMME: mobility management entityHSS: home subscriber serverPCRF: policy charging and rule function

*LTE Architecture (Contd)eNodeB, S-GW and P-GW are involved in session setup, handoff, routingUser 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 Plane

Access ProcedureCell SearchBase station broadcasts synchronization signals and cell system information (similar to WiFi)UE obtains physical layer informationUE acquires frequency and synchronizes to a cellDetermine the start of the downlink frameDetermine the cell identity

Random access to establish a radio link

*Base stationUE 2UE 1

ClientBase stationCore networkRandom AccessAdjust uplink timingIf ID in msg matches UE ID, succeed.If collision, ID will not match!*

Base stationRandom Access (Contd)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

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Connection SetupSession RequestsUE to base stationBase station to MMEMME obtains subscriber info from HSS, selects S-GW and P-GWS-GW sends to P-GWP-GW obtains policy from PCRF*S-GWUEP-GWSession Request MME

Connection Setup (Contd)Session ResponseEstablishes GPRS Tunnels (GTP) between S-GW and P-GW, between S-GW and UEBase station allocates radio resources to UE*S-GWUEP-GWMMESession Response

Mobility ManagementHandoffHandoff without change of S-GWNo change at P-GWHandoff with change of S-GW or MMEInter-technology handoff (LTE to 3G)*S-GWUEP-GWMME

Mobility Management (Contd)PagingIf S-GW receives a packet to a UE in IDLE state, inform MMEMME pages UE through base station*S-GWUEP-GWMMERRC_IDLEPacket receivedPagingRequest

OutlineBasic Architecture of LTEAccess ProcedureWhy no carrier sensingConnection SetupUnlike WiFi, need to keep the same IP address at different attachment pointsMobility ManagementPower Management and Mobile AppsDifferences between 3G and LTEConclusion

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Power Management: LTEUE runs radio resource control (RRC) state machineTwo states: IDLE, CONNECTEDDiscontinuous reception (DRX): monitor one subframe per DRX cylce; receiver sleeps in other subframes*Courtesy:Morley Mao

Power Management: UMTSState promotions have promotion delayState demotions incur tail times

Courtesy: Feng Qian*

ChannelRadio PowerIDLENot allocatedAlmost zeroCELL_FACHShared, Low SpeedLowCELL_DCHDedicated, High SpeedHigh

Example in Detail: RRC State Machinefor a Large Commercial 3G NetworkDCH: High Power State (high throughput and power consumption)FACH: Low Power State (low throughput and power consumption)IDLE: No radio resource allocatedCourtesy: Feng Qian*

Example in Detail: Pandora MusicProblem: High resource overhead of periodic audience measurements (every 1 min)Recommendation: Delay transfers and batch them with delay-sensitive transfersCourtesy: Feng Qian*

IDLE: procedures based on reception rather than transmissionReception of System Information messages Cell selection registration (requires RRC connection establishment) Reception of paging messages with a DRX cycle (may trigger RRC connection establishment)Location and routing area updates (requires RRC connection establishment)

*Why Power Consumptions of RRC States so different?

CELL_FACH: need to continuously receive (search for UE identity in messages on FACH), data can be sent by RNC any timeCan transfer small dataUE and network resource required lowCell re-selections when a UE movesInter-system and inter-frequency handoff possibleCan receive paging messages without a DRX cycle

*UMTS RRC State Machine (Contd)

CELL_DCH: need to continuously receive, and sent whenever there is dataPossible to transfer large quantities of uplink and downlink data UE and network resource requirement is relatively highSoft handover possible for dedicated channels and Inter-system and inter-frequency handover possible Paging messages without a DRX cycle are used for paging purposes

*UMTS RRC State Machine (Contd)

GGSNSGSNRNCFunctional changes compared to the current UMTS ArchitectureLTE vs UMTS (3G): Architecture*

Physical Layer: UMTSSimultaneous meetings in different rooms (FDMA)Simultaneous meetings in the same room at different times (TDMA) Multiple meetings in the same room at the same time (CDMA)*Courtesy: Harish Vishwanath

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 ~100*Courtesy: Harish VishwanathPhysical Layer: UMTS (Contd)

Orthogonal Frequency Division Multiple Access (OFDM)Closely spaced sub-carriers without guard band Each sub-carrier undergoes (narrow band) flat fading- Simplified receiver processing Frequency or multi-user diversity through coding or scheduling across sub-carriers Dynamic power allocation across sub-carriers allows for interference mitigation across cells Orthogonal multiple accessFrequencyNarrow Band (~10 Khz)Wide Band (~ Mhz)T large compared to channel delay spreadSub-carriers remain orthogonal under multipath propagationT1*Courtesy: Harish VishwanathPhysical Layer: LTE

Physical Layer: LTE (Reverse link OFDM)User 1User 2User 3 Efficient use of spectrum by multiple users Sub-carriers transmitted by different users are orthogonal at the receiver- No intra-cell interference CDMA uplink is non-orthogonal since synchronization requirement is ~ 1/W and so difficult to achieve Users are carrier synchronized to the base Differential delay between users signals at the base need to be small compared to symbol durationW*Courtesy: Harish Vishwanath

Typical Multiplexing in OFDMAEach color represents a userEach user is assigned a frequency-time tile which consists of pilot sub-carriers and data sub-carriersBlock hopping of each users tile for frequency diversityTime FrequencyTypical pilot ratio: 4.8 % (1/21) for LTE for 1 Tx antenna and 9.5% for 2 Tx antennas*Courtesy: Harish VishwanathPilot sub-carriers

UMTS has CELL_FACHUplink un-synchronizedBase station separates random access transmissions and scheduled transmissions using CDMA codesLTE does not have CELL_FACHUplink needs synchronizationRandom access transmissions will interfere with scheduled transmissions

*LTE vs UMTS (3G): Physical Layer

ConclusionsLTE promises hundreds of Mbps and 10s msec latency

Mobile apps need to be cellular friendly, e.g. avoid periodic small packets, use push notification services

Roaming and inter-technology handoff not covered

ChallengesP-GW central point of control, bad for content distribution, and scalable policy enforcementMobile video will be more than half of the trafficNeeds lots of spectrum (spectrum crunch)*

***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.

**Primary synchronization sequence: 3SSS: 168A total of 504 cell identities*Time-frequency resource on which random-access preamble is transmitted on the PRACH channel*Time-frequency resource on which random-access preamble is transmitted on the PRACH channel*Primary synchronization sequence: 3SSS: 168A total of 504 cell identities*Primary synchronization sequence: 3SSS: 168A total of 504 cell identities*Primary synchronization sequence: 3SSS: 168A total of 504 cell identities*Primary synchronization sequence: 3SSS: 168A total of 504 cell identities*State machine the standard for all UMTS carriers transitions & paras can change**First explain the appProblem recommendation: make annimation*Dedicated channels can be used for both CS and PS connections HSDPA and HSUPA can be used for PS connections Soft handover possible for dedicated channels and HSUPA Inter-system and inter-frequency handover possible Paging Type 2 messages without a DRX cycle are used for paging purposes

**Dedicated channels can be used for both CS and PS connections HSDPA and HSUPA can be used for PS connections Soft handover possible for dedicated channels and HSUPA Inter-system and inter-frequency handover possible Paging Type 2 messages without a DRX cycle are used for paging purposes

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