SAE (System Architecture Evolution): With the very high data rate and low latency requirements for 3G LTE, it is necessary to evolve the system architecture to enable the improved performance to be achieved. One change is that a number of the functions previously handled by the core network have been transferred out to the periphery. Essentially this provides a much "flatter" form of network architecture. In this way latency times can be reduced and data can be routed more directly to its destination.
Mobility Management Entity, MME: The MME is the main control node for the LTE SAE access network, handling a number of features:
- Idle mode UE tracking
- Bearer activation / de-activation
- Choice of SGW for a UE
- Intra-LTE handover involving core network node location
- Interacting with HSS to authenticate user on attachment and implements roaming restrictions
- It acts as a termination for the Non-Access Stratum (NAS)
- The SAE MME acts the termination point for ciphering protection for NAS signaling. As part of this it also handles the security key management. Accordingly the MME is the point at which lawful interception of signalling may be made.
- Paging procedure
- The S3 interface terminates in the MME thereby providing the control plane function for mobility between LTE and 2G/3G access networks.
- The SAE MME also terminates the S6a interface for the home HSS for roaming UEs.
It can therefore be seen that the SAE MME provides a considerable level of overall control functionality.
Serving Gateway, SGW: The Serving Gateway, SGW, is a data plane element within the LTE SAE. Its main purpose is to manage the user plane mobility and it also acts as the main border between the Radio Access Network, RAN and the core network. The SGW also maintains the data paths between the eNodeBs and the PDN Gateways. In this way the SGW forms a interface for the data packet network at the E-UTRAN.
Also when UEs move across areas served by different eNodeBs, the SGW serves as a mobility anchor ensuring that the data path is maintained.
PDN Gateway, PGW: The LTE SAE PDN gateway provides connectivity for the UE to external packet data networks, fulfilling the function of entry and exit point for UE data. The UE may have connectivity with more than one PGW for accessing multiple PDNs.
NAS: Non Access Stratum
RRC: Radio Resource Control
PDCP: Packet Data Convergence Protocol
RLC: Radio Link Control
MAC: Medium Access Control
PHY: Physical Layer
In the control-plane, the NAS protocol, which runs between
the MME and the UE, is used for control-purposes such as network
attach, authentication, setting up of bearers, and mobility management. All NAS messages are ciphered and integrity protected by the
MME and UE. The RRC layer in the eNB makes handover decisions
based on neighbor cell measurements sent by the UE, pages for
the UEs over the air, broadcasts system information, controls UE
measurement reporting such as the periodicity of Channel Quality
Information (CQI) reports and allocates cell-level temporary identifiers to active UEs. It also executes transfer of UE context from the
source eNB to the target eNB during handover, and does integrity
protection of RRC messages. The RRC layer is responsible for the
setting up and maintenance of radio bearers.
In the user-plane, the PDCP layer is responsible for
compressing/decompressing the headers of user
plane IP packets using Robust Header Compression
(ROHC) to enable efficient use of air interface bandwidth.This layer performs ciphering of both user
plane and control plane data. Because the NAS messages are carried in RRC, they are effectively double
ciphered and integrity protected, once at the MME
and again at the eNB.
The RLC layer is used to format and transport traffic
between the UE and the eNB. RLC provides three
different reliability modes for data transport- Acknowledged Mode (AM), Unacknowledged Mode (UM), or
Transparent Mode (TM). The UM mode is suitable
for transport of Real Time (RT) services because such
services are delay sensitive and cannot wait for retransmissions. The AM mode, on the other hand,
is appropriate for non-RT (NRT) services such as file
downloads. The TM mode is used when the PDU sizes are known a priori such as for broadcasting system
information. The RLC layer also provides in-sequence
delivery of Service Data Units (SDUs) to the upper
layers and eliminates duplicate SDUs from being delivered to the upper layers. It may also segment the
SDUs depending on the radio condition
Furthermore, there are two levels of re-transmissions for providing reliability, namely, the Hybrid
Automatic Repeat reQuest (HARQ) at the MAC
layer and outer ARQ at the RLC layer. The outer
ARQ is required to handle residual errors that are
not corrected by HARQ that is kept simple by the
use of a single bit error-feedback mechanism. An
N-process stop-and-wait HARQ is employed that
has asynchronous re-transmissions in the DL and
synchronous re-transmissions in the UL. Synchronous HARQ means that the re-transmissions of
HARQ blocks occur at pre-defined periodic intervals. Hence, no explicit signaling is required to
indicate to the receiver the retransmission schedule. Asynchronous HARQ offers the flexibility of
scheduling re-transmissions based on air interface
conditions.
