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
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Cellular Networks Impact our Lives
More Mobile Connection
More MobileInformation
Sharing
More Mobile Users
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101010010000101100101010101010010101001010101010101011010
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More InfrastructureDeployment
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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
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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
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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
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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
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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
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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
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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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Courtesy: Harish Vishwanath
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
Courtesy: Harish Vishwanath
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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
Courtesy: Harish Vishwanath
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)