The reduced complexity in the RAN is mirrored by a similar reduction in the core network, where the EPC (Evolved Packet Core) structure consists of five main nodes, although others may be required for backwards-compatibility purposes.
The MME handles control plane functions related to mobility management (authentication and security) and idle mode handling (location updates and paging), in which sense it is broadly analogous to the VLR (Visitor Location Register) or GMM (GPRS Mobility Management) functions found in legacy networks. The MME is also responsible for EPC bearer control, and so handles connection control signalling.
The S-GW and PDN (Packet Data Network) Gateway are broadly analogous to the SGSN (Serving GPRS Support Node) and GGSN (Gateway GPRS Support Node) found in R99 networks and perform user plane handling, switching/routing and interfacing functions. Unlike legacy systems, however, bearer control has been removed from these devices and resides with the MME.
The PCRF (Policy and Charging Rules Function) handles QoS and bearer policy enforcement and also provides charging and rating facilities.
Subscriber management and security functions are handled by the HSS (Home Subscriber Server), which incorporates the functions of the legacy HLR (Home Location Register) and which is already familiar from R5 elements such as the IMS (IP Multimedia Subsystem).
For backwards-compatibility purposes, SGSNs deployed to legacy parts of an operator’s network can be interfaced to both the MME (for mobility management) and the S-GW (for user plane flows).
The MME then provides legacy systems with an interface to the HSS, and the S-GW and PDN-GW assume the role previously performed by the GGSN.
The packet data services of legacy (GSM/GPRS, R99 and HSPA) networks and LTE/SAE systems can therefore interwork via a unified set of core network elements if required. The gateway elements form the EPC.
S1-MME
S1-U
MME
S-GW
IP Data Link Layer
S1-AP
SCTP
Physical Layer
Physical Layer UDP
IP Data Link Layer
User Plane PDUs
GTP–U
S1 Interface
S12AP
S1 Interface
Backhaul links to the core network are carried by the S1 interface. Following the general structure of the Iub interface which it replaces, traffic over the S1 is logically split into two types.
S1-U flows carry user plane traffic and S1-MME flows carry mobility management, bearer control and direct transfer control plane traffic.
Message structures for the S1-MME interface that operate between the eNB and the MME are defined by S1AP (S1 Application Protocol). The S1AP (S1 Application Protocol) performs duties that can be seen as a combination of those performed by R99 RANAP (Radio Access Network Application Part) and GTP-C (GPRS Tunnelling Protocol – Control plane).
To provide additional redundancy, traffic differentiation and load balancing, the S1-flex concept allows each eNB to maintain logical connections to multiple S-GWs and MMEs – there may therefore be multiple instances of the S1 interface per node.
The S1-U interface employs GTP-U to create and manage user-plane data contexts between the eNB and the S-GW.
Co-ordinated MME Pool and S-GW Service Area
E-UTRAN Tracking Areas served by Pools and Areas
Resilience Through Pooling Resilience Through Pooling
In common with ongoing developments within many existing 3G core networks, the EPC is designed to take advantage of the concept of ‘pooling’, specifically of MME and S-GW nodes.
The ‘S1-flex’ facility that allows each eNB in the E-UTRAN to be associated with multiple MMEs in the EPC allows those MMEs to be grouped into ‘pools’. Each pool will be responsible for the eNBs in one or more complete tracking areas.
This means that when an eNB selects the MME that will handle the Attach process for a UE, that MME can continue to serve that UE as long as it remains within the tracking areas associated with the MME’s pool. This reduces the requirement for MME relocation and consequently reduces the
network’s signalling load. Pooling also provides a measure of resilience for network services to the extent that, if one MME falls over, eNBs have a number of alternative devices to select. As with current implementations of the pooling concept, however, MME pooling does not protect the connections to UEs being served by a failed MME – when the MME fails all ongoing services supported by it fail too.
In the same way as an MME pool area comprises a set of cells within which a UE does not need to change the serving MME, an S-GW service area is a set of cells within which a UE does not need to change S-GW.
MME pools may overlap, and each MME pool area is identified by an MMEGI (MME Group Identifier).
S-GW Areas are also permitted to overlap.
Further Reading: 3GPP TS 23.401:3.1
MME
Evolved Packet Core ‘S’ Interfaces Evolved Packet Core ‘S’ Interfaces
In addition to the S1 interface connecting the E-UTRAN to the EPC, a broader range of ‘S’ interfaces have been defined to identify interconnections between EPC nodes and external nodes.
The gateways and the MME are the main new nodes in the EPC. They are interconnected via the S5 and S11 interfaces.
The SGi interface provides a connection to the operator’s IP-based services. It is likely that this will include services managed through the IMS. In this respect the S6a interface connects the MME to the HSS, and the S7 interface provides access from the PCRF to the PDN-GW (Packet Data Network Gateway).
The S3 and S4 interfaces provide connectivity into the EPC from legacy 2G/3G SGSNs (Serving GPRS Support Nodes). However, the UTRAN may be connected directly to the EPC via the S12 interface.
WLANs (Wireless Local Area Networks) or WiMAX (Worldwide Interoperability for Microwave Access) can be supported through the EPC via the S2 interface. This would require connectivity to the MME, which is provided by interfaces and interworking functions not shown in this diagram.
Modulation and
7.8 19.4 32.4 64.8 129.6
10.4 25.9 43.2 86.4 172.8
Channel bandwidth
Source: WCDMA for UMTS: 4th Edition – Holma & Toskala (Ed) Data Rates and Services
The potential peak data rates of 100 Mbit/s and more are only available when employing channel bandwidths of 20 MHz, which are difficult to find in most countries’ crowded radio environments.
Even then, the fastest data rates will only be achievable on links that use advanced antenna techniques such as MIMO (Multiple Input Multiple Output). 2x2 MIMO, where both transmitter and receiver use two separate antennas to carry parallel streams of data over the same channel, is required for data rates of up to 170 Mbit/s, while future versions of E-UTRA that promise data rates of up to 360 Mbit/s would require 4x4 MIMO.
The data rates for E-UTRA variants up to 2x2 MIMO in a 20 MHz channel are shown in the diagram.
These data rates assume error coding rates of 1/2, 3/4 and 4/4, which are not currently defined in the specifications and so should only be considered to be an example of what is generically achievable with the technology.
Data rates of 100 Mbit/s or more will provide users with access to almost any Internet or
communications service currently available, from movie downloads and database access down to simpler communications activities such as making a telephone call or sending a text message.
The capacity allocation method employed by E-UTRA has more in common with HSPA than R99 UMTS. There is no dedicated channel in LTE, meaning that bandwidth is shared between users in a flexible, on-demand way.
This flexibility, coupled with the high data rates, makes E-UTRA very attractive.
Although E-UTRA’s theoretical ability to provide one user in a cell with a 100 Mbit/s connection has been much discussed, network operators are more excited about the possibility of providing 1 Mbit/s connections to 100 simultaneous users in one cell.
Non-Access
Stratum (NAS) Non-Access
Stratum (NAS)
RRC RRC
PDCP PDCP
RLC RLC
MAC MAC
Physical Layer Physical Layer
User Equipment eNB Evolved Packet Core
E-UTRA Protocols E-UTRA Protocols
In line with other aspects of E-UTRA, the air interface protocol stack has been designed to reduce complexity.
Whereas an R99/HSPA-enabled Node B employs a protocol stack with a variety of RLC and MAC instances, an E-UTRA eNB employs a protocol stack with just one instance of each layer.
The extent of the air interface protocol stack has also been reduced. In previous incarnations of UMTS some layers operated between the UE and the Node B, while most extended all the way to the RNC.
With the elimination of the RNC, all air interface protocols in E-UTRA operate between the UE and the eNB.
PDN Connectivity Services
PDN-GW PDN Connectivity Service (PCS)
EPS Bearer Evolved Packet
System
Packet Data Network
PDN Connectivity Services
The EPS is designed to provide IP connectivity between a UE and a PDN (Packet Data Network).
The connection provided to a UE is referred to as a PCS (PDN Connectivity Service).
This consists of an EPS bearer that connects the UE to an Access Point in a PDN-GW (PDN Gateway) and traverses both the E-UTRAN and the EPC. The PDN-GW routes traffic between the EPS bearer and the external PDN.
The EPS bearer, in turn, carries one or more SDF (Service Data Flow) between the UE (User Equipment) and external data services.
If a UE requires additional connectivity that is only available via a different PDN-GW Access Point, then additional PDN Connectivity Services may be established in parallel.
Further Reading: 3GPP TS 23.401:4.7.1
Connection Hierarchies UE
eNB S-GW PDN-GW Application
Server PDN Connectivity Service (PCS)
Radio Bearer ID
C-RNTI LCID
EPS Bearer ID (EBI)
GTP TEID GTP TEID
S1-U S5/S8 SGi
Connection Hierarchies
To quote 3GPP TS 23.401 (4.7.1), ‘The Evolved Packet System provides IP connectivity between a UE and a PLMN external packet data network. This is referred to as a PDN Connectivity Service. The PDN Connectivity Service supports the transport of one or more Service Data Flows’.
Within the EPS, user connectivity is provided via the EPS Bearer, which is analogous to the PDP Context provided by legacy 3GPP PS networks. The EPS Bearer tunnels user traffic between the PDN-GW APN and the UE via the S-GW and eNB and is, in reality, a concatenation of connections over three successive interfaces:
a GTP-U tunnel over the S5 interface (PDN-GW to S-GW), identified by a GTP TEID
a GTP-U tunnel over the S1-U interface (S-GW to eNB), also identified by a GTP TEID
an E-UTRAN RB (Radio Bearer) on the LTE Uu interface (eNB to UE)
LTE Radio Bearers are identified by the C-RNTI (Cell-specific Radio Network Temporary Identifier) and LCID (Logical Channel ID). There is a one-to-one mapping between an RB and an EPS Bearer.
Each EPS Bearer can support multiple SDF, although as QoS in the EPC is applied on a ‘per bearer’
level, all SDFs sharing the same EPS Bearer will also share the same QoS (Quality of Service). UEs can be assigned multiple EPS Bearers with different QoS levels to support a varied service set.
Further Reading: 23.401:4.7; 36.300
EPS Bearers and E-RABs UE
eNB S-GW PDN-GW Application
Server PDN Connectivity Service (PCS)
EPS Bearer ID
E-RAB ID EPS Bearer ID
Radio Bearer ID
S1-U S5/S8 SGi
EPS Bearers and E-RABs
An EPS Bearer (and the LTE RB that it maps to) carries traffic across the E-UTRAN and the EPC between the UE and the PDN-GW.
An E-RAB (and the RB that it maps to) carries traffic between the UE and the S-GW over the E-UTRAN, which may involve journeys across both X2 and S1 interfaces.
The E-RAB therefore travels over a subsection of the route traversed by the EPS Bearer and there is a one-to-one mapping between one pair of connections.
To simplify the identification of connections, a paired EPS Bearer and E-RAB share the same 8-bit identifier; although only the least significant 4 bits of the ID are active.
Further Reading: 23.401:4.7; 36.300:8.2; 36.413:9.2.23 (E-RAB ID); 29.274:8.8 (EPS Bearer ID)
Connection Identifiers
S-GW MME
EPS Bearer
PDN-GW UE
Data Radio Bearer
Connection Identifiers
The EPS Bearer ID is assigned by the MME upon bearer establishment.
It uniquely identifies an EPS Bearer for one UE accessing via the E-UTRAN.
The EPS Bearer ID is a one-octet string, which in theory means that each UE can have up to 256 EPS Bearers associated with it per MME. However, the relevant specifications currently indicate that the most significant 4 bits of the ID should be set to 0, which limits the number of EPS Bearers per UE to 16.
The EPS Bearer travels between the UE and the PDN-GW; during handovers it may also extend over the X2 interface between source and target eNBs.
When travelling over the S1 and X2 interfaces, there is a one-to-one mapping between the EPS Bearer and the E-RAB (E-UTRAN Radio Access Bearer) and between the identities assigned to each of those entities.
Further Reading: 3GPP TS 23.401:5.2.1
Transport Identities
eNB UE S1AP ID MME UE S1AP ID
S1-U GTP Tunnel S-GW
MME S1-MME S1-AP Context
Tunnel Endpoint IDs (TE-ID) X2-U (GTP TE-Ids)
X2-C (eNB UE X2AP ID)
Transport Identities
To allow the S1 and X2 protocols to identify the UEs that form the endpoint of each transport tunnel, terminals are assigned identities that are unique within the eNBs or MMEs that support those endpoints.
The UE S1AP ID and MME S1AP ID are unique within the eNB and MME respectively that are handling the E-RAB/EPS Bearer to an Attached UE. The IDs are simple numerical identifiers (24 bits in the eNB and 32 bits in the MME) and are not associated with a specific instance of the S1 interface in each device. An eNB can therefore support a maximum of 224 (16.7 million) UE S1 connections and an MME 232 (4.3 billion).
The UE X2AP ID performs the same basic function as the S1-related identities, but for the X2 interface. The X2 is optional and is only used to pass handover-related traffic between source and target eNBs, so the X2AP ID will only be created as required when a handover is initiated. The ID is 12 bits long and provides a maximum of 4096 UE X2 handover identities per eNB.
The 4-byte GTP TEID (Tunnel Endpoint ID) is used in the EPS the same way as it is in legacy networks. Each device that supports a GTP tunnel refers to it in terms of the TEID assigned to the tunnel plus the IP address and UDP port number of the interface that handles it. TEIDs are assigned by the receiving side of each connection and are exchanged using S1AP during tunnel establishment.
Further Reading: 3GPP TS 23.401:5.2; 36.413:9.2.3; 29.274 (GTPv2-C); 36.41x (S1); 36.42x (X2)
Default and Dedicated Bearers
Initial or Default EPS Bearer
S-GW PDN-GW
UE
eNB
Subsequent or Dedicated EPS
Bearer
Internet IMS Both Bearers routed
via same APN Both Bearers share
same IP address
Default and Dedicated EPS Bearers
Each UE will establish an initial or default EPS Bearer as part of the attach process. This will provide the required ‘always on’ IP connectivity to the UE and may be to a default APN (Access Point Name), if one is stored in the user’s subscriber profile, or to an APN selected by the network.
In networks that interconnect to an IMS, the default bearer allows the UE to perform SIP registration and thereafter to provide a path for session initiation messaging. In these circumstances, the data rate and QoS assigned initially to the default bearer is commensurate with the expected low level of SIP-based traffic flow, but these parameters can be modified to accommodate the requirements of application traffic flows when a connection is established.
If a UE has a requirement to establish an application connection whose QoS or data rate demands are incompatible with those currently assigned to the default bearer (but which can still be routed through the current APN), the PDN-GW or PCRF may initiate the establishment of an additional EPS Bearer to carry the new traffic flow. Any additional bearers assigned to a UE in addition to the default bearer are termed dedicated bearers and will be identified by different EPS Bearer/E-RAB and radio bearer IDs.
A UE may have more than one PDN Connectivity Service running if it has connections established through more than one APN/PDN-GW. In that case, there will be one Default Bearer and an optional number of Dedicated Bearers created for each PCS. The 4-bit EPS Bearer ID limits the total number of bearers established for one UE to sixteen (numbered 0 to 15).
Further Reading: 3GPP TS 23.401:4.7.2
Section 1 Glossary
1xEV-DO 1x Evolution – Data Only
3GPP 3rd Generation Partnership Project
A1AP A1 Application Protocol
APN Access Point Name,
CDMA Code Division Multiple Access
C-RNTI Cell-specific Radio Network Temporary Identifier EDGE Enhanced Data rates for Global Evolution
eNB Evolved Node B
EPC Evolved Packet Core
E-RAB E-UTRAN Radio Access Bearer
E-UTRA Evolved Universal Terrestrial Radio Access.
E-UTRAN Evolved Universal Terrestrial Radio Access Network
GGSN Gateway GPRS Support Node
GMM GPRS Mobility Management
GSM Global System for Mobile Communications GTP-C GPRS Tunnelling Protocol – Control plane GTP-U GPRS Tunnelling Protocol – User plane
HLR Home Location Register
HSDPA High Speed Downlink Packet Access
HSPA High Speed Packet Access.
HSS Home Subscriber Server
HSUPA High Speed Uplink Packet Access
IMS IP Multimedia Subsystem
LCID Logical Channel ID
LTE Long Term Evolution
MAC Medium Access Control
MIMO Multiple Input Multiple Output
MME Mobility Management Entity
OFDMA Orthogonal Frequency Division Multiple Access
PCRF Policy and Charging Rules Function
PCS PDN Connectivity Service
PDCP Packet Data Convergence Protocol
PDN Packet Data Network.
PDN-GW Packet Data Network Gateway
PDU Protocol Data Unit
QoS Quality of Service
RAN Radio Access Network
RANAP Radio Access Network Application Part
RB Radio Bearer
RLC Radio Link Control
RNC Radio Network Controller
RNSAP Radio Network Subsystem Application Protocol
RRC Radio Resource Control
RRM Radio Resource Management
S1AP S1 Application Protocol
SAE System Architecture Evolution SC-FDMA Single Carrier FDMA
SCTP Stream Control Transmission Protocol
SDF Service Data Flow
SGSN Serving GPRS Support Node
S-GW Serving Gateway
TCP Transmission Control Protocol
TEID Tunnel Endpoint ID
UDP User Datagram Protocol
UE User Equipment
UMTS Universal Mobile Telecommunications System VLR Visitor Location Register
WiMAX Worldwide Interoperability for Microwave Access
WLAN Wireless Local Area Network
X2AP X2 Application Protocol