lec20 cellular

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Cellular Networks

Guest lecture by Li Erran Li, Bell LabsCOS 461: Computer Networks

4/18/2012 W 10-10:50am in Architecture N101

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Cellular Core Network



Cellular Networks Impact our Lives

More Mobile Connection

More MobileInformation

Sharing

More Mobile Users

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10100101010101010100101010101001010101

More InfrastructureDeployment



Mobile Data Tsunami ChallengesCurrent Cellular Technologies

• Global growth 18 times from2011 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 technologiesare inadequate

– Fundamental redesign of cellular networks is needed

Source: CISCO Visual Networking Index (VNI) Global Mobil DataTraffic Forecast 2011 to 2016

0

2

4

6

8

10

12

2011 2012 2013 2014 2015 2016

0.61.3

2.4

4.2

6.9

10.8

E x a b y t e s p e r M o n t h

Global Mobile Data Traffic Growth

2011 to 2016

Annual Growth 78%

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Outline

Goal of this lecture: understand the basics of currentnetworks

• Basic Architecture of LTE•

Access Procedure – Why no carrier sensing

• Connection Setup – Unlike WiFi, need to keep the same IP address at different

attachment points• Mobility Management• Power Management and Mobile Apps• Differences between 3G and LTE•

Conclusion

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Cellular Core Network

eNodeB 3 S-GW 2P-GW

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S-GW 1

eNodeB 1

eNodeB 2

Internet andOther IP Networks

GTP TunnelsUE 2

UE 1

LTE Infrastructure

MME/PCRF/HSS

• UE: user equipment• eNodeB: base station• S-GW: serving

gateway• P-GW: packet data

network gateway•

MME: mobilitymanagement entity• HSS: home subscriber

server• PCRF: policy charging

and rule function



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LTE Architecture (Cont ’d)

• eNodeB, S-GW and P-GW are involved insession setup, handoff,routing

UserEquipment (UE) Gateway

(S-GW)

MobilityManagement

Entity(MME)

Network Gateway(P-GW)

HomeSubscriber

Server(HSS)

Policy Control andCharging Rules

Function ( PCRF)

Station(eNodeB)

Base Serving Packet Data

Control Plane

Data Plane



Access Procedure

• Cell Search – Base station broadcasts

synchronization signals and cellsystem information (similar toWiFi)

– UE obtains physical layerinformation

• UE acquires frequency andsynchronizes to a cell

• Determine the start of thedownlink frame

• Determine the cell identity

• Random access to establish aradio link

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Base station

UE 2UE 1



Client Base station Core network

Step 1: random access request (pick one of 64 preambles)

Step 2: random access response

Step 3: transmission of mobile I D

Step 4: contention r esolution msg Only if UE is not known in Base station

Random Access

Adjust uplink timing

I f I D in msg matches UE I D, succeed.

I f coll ision, I D will not match!

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Base station

Random Access (Cont ’d)

UE 2UE 1

Why not carrier sensing likeWiFi?• Base station coverage is muchlarger than WiFi AP

– UEs most likely cannot heareach other

• How come base station can

hear UEs ’ transmissions? – Base station receivers are

much more sensitive andexpensive

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Connection Setup

• Session Requests – UE to base station – Base station to MME

• MME obtains subscriberinfo from HSS, selects S-GW and P-GW

– S-GW sends to P-GW• P-GW obtains policy

from PCRF

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S-GWUE P-GW

Session Request

MME



Connection Setup (Cont ’d)

• Session Response – Establishes GPRS

Tunnels (GTP) between

S-GW and P-GW,between S-GW and UE

– Base station allocatesradio resources to UE

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S-GWUE P-GW

MME

Session Response



Mobility Management

Handoff • Handoff without

change of S-GW – No change at P-GW

• Handoff with changeof S-GW or MME

• Inter-technologyhandoff (LTE to 3G)

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S-GWUE P-GW

MME



Mobility Management (Cont ’d)

Paging• If S-GW receives a

packet to a UE in IDLEstate, inform MME

• MME pages UEthrough base station

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S-GWUE P-GW

MME

RRC_I DL E Packet received

Paging Request

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Outline

• Basic Architecture of LTE• Access Procedure

– Why no carrier sensing• Connection Setup

– Unlike WiFi, need to keep the same IP address atdifferent attachment points

• Mobility Management• Power Management and Mobile Apps• Differences between 3G and LTE• Conclusion

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Power Management: LTE• UE runs radio resource

control (RRC) statemachine

• Two states: IDLE,CONNECTED

• Discontinuous reception

(DRX): monitor onesubframe per DRX cylce;receiver sleeps in othersubframes

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Courtesy:Morley Mao

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Power Management: UMTS

• State promotions have promotion delay• State demotions incur tail times

Tail Time

Tail Time

Delay: 1.5sDelay: 2s

Channel RadioPower

IDLE Notallocated

Almostzero

CELL_FACH Shared,Low Speed

Low

CELL_DCH Dedicated,High Speed

HighCourtesy: Feng Qian

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Example in Detail: RRC State Machinefor a Large Commercial 3G Network

Promo Delay: 2 SecDCH Tail: 5 sec

FACH Tail: 12 sec

DCH : High Power State (high throughput and power consumption)FACH : Low Power State (low throughput and power consumption)

IDLE: No radio resource allocated

Tail TimeWaiting inactivity timers to expire

Courtesy: Feng Qian

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Example in Detail: Pandora Music

Problem : High resource overhead of periodic audience measurements (every 1min)

Recommendation : Delay transfers and batch them with delay-sensitive transfers

Courtesy: Feng Qian

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•

IDLE: procedures based on reception ratherthan transmission

– Reception of System Information messages –

Cell selection registration (requires RRC connectionestablishment) – Reception of paging messages with a DRX cycle

(may trigger RRC connection establishment) – Location and routing area updates (requires RRC

connection establishment)

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Why Power Consumptions of RRC Statesso different?

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•

CELL_FACH: need to continuously receive(search for UE identity in messages on FACH),data can be sent by RNC any time

–

Can transfer small data – UE and network resource required low – Cell re-selections when a UE moves – Inter-system and inter-frequency handoff possible – Can receive paging messages without a DRX cycle

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UMTS RRC State Machine (Cont’d)

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•

CELL_DCH: need to continuously receive, andsent whenever there is data

– Possible to transfer large quantities of uplink anddownlink data

– UE and network resource requirement is relativelyhigh

– Soft handover possible for dedicated channels andInter-system and inter-frequency handover possible

– Paging messages without a DRX cycle are used forpaging purposes

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UMTS RRC State Machine (Cont’d)

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GGSN

SGSN

RNC

Node B eNodeB

RNC functions moved toeNodeB.

• No central radio controller node• OFDM radio, no soft handover

• Operator demand to simplify

Mobility Management EntityMME(not user planefunctions)

Control plane/user plane split forbetter scalability

• MME control plane only• Typically centralized and pooled

PGWSGW

P DN Gate WayServing Gate Way

PGW/SGW• Deployed according to traffic

demand• Only 2 user plane nodes (non-roaming case)

•

Functional changes compared to the current UMTSArchitecture

LTE vs UMTS (3G): Architecture22

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Physical Layer: UMTS

Simultaneous meetings in different rooms(FDMA)

Simultaneousmeetings in thesame room atdifferent times(TDMA)

Multiple meetings in thesame room at the same time(CDMA)

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Courtesy: Harish Vishwanath

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Code Division Multiple Access (CDMA)• Use of orthogonal codes to separate differenttransmissions• Each symbol or bit is transmitted as a larger number of bits using the user specific code – Spreading• Spread spectrum technology

– The bandwidth occupied by the signal is much

larger than the information transmission rate – Example: 9.6 Kbps voice is transmitted over 1.25

MHz of bandwidth, a bandwidth expansion of

~100

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Physical Layer: UMTS (Cont ’d)



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Physical Layer: LTE (Reverse link OFDM)

User 1

User 2

User 3

Efficient use of spectrum by multipleusers

Sub-carriers transmitted by differentusers are orthogonal at the receiver

- No intra-cell interference

CDMA uplink is non-orthogonalsince synchronization requirement is~ 1/W and so difficult to achieve

Users are carrier synchronized to the base

Differential delay betweenusers

’

signals at the baseneed to be small comparedto symbol duration

W


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Typical Multiplexing in OFDMA

Each color represents a userEach user is assigned afrequency-time tile whichconsists of pilot sub-carriers anddata sub-carriersBlock hopping of each user ’ s tilefor frequency diversity

Time

Typical pilot ratio: 4.8 % (1/21)for LTE for 1 Tx antenna and9.5% for 2 Tx antennas


Pilot sub-carriers

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•

UMTS has CELL_FACH – Uplink un-synchronized

• Base station separates random access transmissions andscheduled transmissions using CDMA codes

• LTE does not have CELL_FACH – Uplink needs synchronization

• Random access transmissions will interfere with

scheduled transmissions

LTE vs UMTS (3G): Physical Layer

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Conclusions• LTE 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• Challenges

–

P-GW central point of control, bad for contentdistribution, and scalable policy enforcement – Mobile video will be more than half of the traffic – Needs lots of spectrum (spectrum crunch)

lec20 cellular

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