LTE – Cellular Telecommunications Case Study
Overview
• Network Architecture
• Background
• Air interfaces
• Network protocols
• Application: Messaging • Research
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LTE – Network Architecture
Core-Network (EPC)
Internet
SGi
MME
S6a
S5 S11
S1-u
P-GW
MME
S-GW
S1-c
RAN
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LTE – Core Network Elements (logical nodes)
Mobility Management Entity (MME) – a control plane node of EPC responsible for
• Connection/release of bearers to a UE
• UE IDLE to ACTIVE transitions
• Handles security keys (and NAS and AS layer security and authentication)
• Terminates Non-Access Stratum (NAS) signaling (between CN and UE)
• Management of tracking area list
• Selection of P-GW and S-GW
• Selection of MME for H/O with MME change
Serving Gateway (S-GW) – a user plane node of EPC responsible for connection RAN to EPC
• Mobility anchor for UE moving between eNBs (also anchor for 3GPP legacy technologies)
• Store of information and statistics associated with billing
Packet Data Network Gateway (PDN-GW/P-GW) – user plane node connects EPC to Internet
• Allocates IP address per UE
• QOS enforcement (policy controlled by PCRF)
• Mobility anchor for non-3GPP legacy technologies (CDMA2000, 3x, etc.)
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LTE – EPS Protocols
Stream Control Transmission Protocol (SCTP)
• Commonly used protocol for exchange of control messages within Evolved Packet System (EPS)
• Different than TCP in that it represents several streams within a connection – as opposed to TCP which represents a sequence of bytes
• Unidirectional – pair for bidirectional
• Overcomes head of line blocking associated with TCP – no order constraints in SCTP
• SCTP allows multihoming – reach destination using multiple network addresses
• Message level framing characteristics offers better transmission efficiencies GPRS Tunneling Protocol (GTP)
• Used for packet routing within EPS
• GTP-U and GTP-C
• IP packets are encapsulated and tunneled
• GTP tunnel at each node identified by tunnel end point identifier (TEID), IP address, UDP port
• Transport bearer id’d by: src TEID, dest TEID, src IP, dest IP, UDP port numbers
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LTE – Radio Access Network
Evolved UMTS Terrestrial Radio Access Network (E-UTRAN)
• Includes ONLY the base station – eNodeB (eNB)
• No central node – FLAT hierarchy used = flat architecture
• Flat architecturefewer nodeslower latency (5ms)
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LTE – Radio Access Network
eNodeB
• Radio bearer control
• Radio admission control
• UL/DL radio resource scheduling and allocation
• IP header compression and encryption of user data
• Selection of an MME during UE attachment (NNSF – to select which MME would be associated with a particular UE based on information received from the UE)
• Routes user plane toward S-GW
• Scheduling and transmission of paging messages, broadcast info, and public warnings
• Measurement and reporting configuration
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LTE – Protocol Architecture (Stack)
Protocol Stack – 2 types
1. Control plane for transfer of control information
2. User plane for transfer of user data
Control Plane
1. Access Stratum (AS) – protocols run between the UE and eNB • Layer 3
• Radio Resource Control (RRC)
• Layer 2
• Packet Data Convergence Protocol (PDCP)
• Radio Link Control (RLC)
• Medium Access Control (MAC)
• Layer 1
• Physical Layer
2. Non Access Stratum (NAS) – protocols run between UE and MME; layer above AS
• EPS Mobility Management (EMM) – used for attach, detach, TAU, UE Id, UE auth, security mode control (EMM connection-related services are AKA EPS Connection Management (ECM))
• EPS Session Management (ESM) – used for activation, deactivation, modification of default or
dedicated EPS bearers
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LTE – RAN Protocol Architecture
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LTE – RAN Protocol Architecture
Control Plane
User Plane
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LTE – RAN Protocol Architecture (DL)
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LTE – E-UTRAN Protocol Architecture
Radio Resource Control (RRC)
• Responsible for handover (HO) functions – decision, transfer UE context
• Periodicity of Channel Quality Indicator (CQI)
• Setup and maintenance of radio bearers Packet Data Convergence Protocol (PDCP)
• Compression of IP header to reduce number of pits to transmit OTA interface – based on Robust Header Compression (ROHC)
• Ciphering for control plane
• Integrity protection of transmitted data
• In-sequence delivery and duplicate removal for HO
• One PDCP entity per radio bearer configured for a terminal. Radio Link Control (RLC)
• Segmentation and concatenation of PDCP packets
• Retransmission and guarantees in-sequence delivery to higher layers
• Error Correction via Auto Repeat Request (ARQ)
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LTE – E-UTRAN Protocol Architecture
Medium Access Channel (MAC)
• Multiplexing of logical channels
• Responsible for satisfying QoS
• UL/DL scheduling (located in eNB)
• HARQ retransmissions
• Provides services RLC in the form of logical channels Physical Layer (PHY)
• Modulation/demodulation, coding/decoding, MIMO techniques, etc.
• Provides services to the MAC layer in the form of transport channels
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LTE – Radio Bearers
Radio Bearers
• •
Transports control signaling or user data packets with common QoS between UE and eNB
There are two types:
• To carry signaling: Signaling Radio Bearer (SRB)
• To carry user data: These are associated with an EPS bearer
Signaling RB User Data RB
Defined Radio Bearers
UE
S1 Bearer
• SRB1: RRC Signaling with high priority
• SRB2: RRC signaling and NAS signaling (lower priority)
• Best Effort: AKA as the default EPS Bearer
• GBR: Guaranteed Bit Rate
• VoIP: RB to carry VoIP
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eNodeB
S-GW
LTE – Radio Bearers
Radio Bearers
• •
Each bearer is identified by the Logical Channel ID (LCID)
Each bearer is associated with QoS parameters:
• Max bit rate and guaranteed bit rate • VoIP or non-VoIP
• H-ARQ usage
UE
Established during Attach Procedure
SRB1
SRB2
Default (Best effort)
Default vs. Dedicated Bearer
Established after Attach Procedure Stream or VoIP
• Default is able to carry various types of traffic (i.e. no filter) without QoS. Typically created during the Attach procedure
• Dedicated bearer carries specific data flow identified by the Traffic Flow Template (TFT) with a given QoS – voice, streaming – can be established during the Attach procedure or after (on- demand)
eNodeB
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LTE – Radio Bearers
Radio Bearers
• Transports control signaling or user data packets with common QoS between UE and eNB
Signaling Radio Bearer (SRB)
• Transport RRC messages and NAS messages on the control plane SRB0
• No ciphering or integrity protection
• Used during RRC connection establishment
• Sets up SRB1
• Transports RRC msgs using CCCH logical channel
• Carries:
RRCConnectionRequest message
RRCConnectionSetuup message
RRCConnectionReject message
RRCConnectionReestablishmentRequestRequest message RRCConnectionReestablishment message
RRConnectionReestablishmentReject message 14
LTE – Radio Bearers
SRB1
• Transports RRC messages using DCCH logical channel
• May or may not have piggy-backed NAS messages
• Includes ciphered, not ciphered, integrity protected, non-integrity protected
• Used for security activation
• SRB1 carries carries the RRC messages in RRC_CONNECTED state
• Carries (partial list):
RRCConnectionSetupComplete message
RRCConnectionReconfiguration(Complete) message
RRCConnectionEstablishment(Complete) message
RRCConnectionRelease message
SecurityModeCommand message
MeasurementReport message
UECapabilityEnquiry message
ULHandoverPreparationTransfer message
UEInformationRequest(Response) 15
Piggybacked NAS Messages:
DLInformationTransfer message ULInformationTransfer message
LTE – Radio Bearers
SRB2
• Transports RRC messages using DCCH logical channel (different than SRB1)
• Lower priority than SRB1
• Always established after SRB1
• Ciphering and integrity protection used
• eNB sends the RRCConnectionReconfiguration messageto UE to establish SRB2
• Message includes RadioResourceConfigDedicated IE which contains SRB-Identity assigned
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LTE – Radio Bearers
Data Radio Bearer (DRB)
• Transport user plane data using DTCH logical channel
• Ciphering is used for DRBs but not integrity
• DRBs always used after security activation
• A UE can have a maximum of eight DRBs established for each PDN
• Identified uniquely by DRB-identity
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