LTE Network Architecture
The high-level network
architecture of LTE is comprised of following three main components:
·
The User Equipment (UE).
·
The Evolved UMTS
Terrestrial Radio Access Network (E-UTRAN).
·
The Evolved Packet Core
(EPC).
The evolved packet core
communicates with packet data networks in the outside world such as the
internet, private corporate networks or the IP multimedia subsystem. The
interfaces between the different parts of the system are denoted Uu, S1 and SGi
as shown below:
The User Equipment (UE)
The internal
architecture of the user equipment for LTE is identical to the one used by UMTS
and GSM which is actually a Mobile Equipment (ME). The mobile equipment
comprised of the following important modules:
·
Mobile
Termination (MT) : This handles all the
communication functions.
·
Terminal
Equipment (TE) : This terminates the
data streams.
·
Universal
Integrated Circuit Card (UICC) : This is also known as the SIM card for LTE equipments. It runs
an application known as the Universal Subscriber Identity Module (USIM).
A USIM stores user-specific data very similar to 3G SIM card. This keeps
information about the user's phone number, home network identity and security
keys etc.
The E-UTRAN (The access
network)
The architecture of
evolved UMTS Terrestrial Radio Access Network (E-UTRAN)
The E-UTRAN handles the
radio communications between the mobile and the evolved packet core and just
has one component, the evolved base stations, called eNodeB or eNB. Each eNB is a base station that controls the
mobiles in one or more cells. The base station that is communicating with a
mobile is known as its serving eNB.
LTE Mobile communicates
with just one base station and one cell at a time and there are following two
main functions supported by eNB:
·
The eBN sends and
receives radio transmissions to all the mobiles using the analogue and digital
signal processing functions of the LTE air interface.
·
The eNB controls the
low-level operation of all its mobiles, by sending them signalling messages
such as handover commands.
Each eBN connects with
the EPC by means of the S1 interface and it can also be connected to nearby
base stations by the X2 interface, which is mainly used for signalling and
packet forwarding during handover.
A home eNB (HeNB) is a
base station that has been purchased by a user to provide femtocell coverage
within the home. A home eNB belongs to a closed subscriber group (CSG) and can
only be accessed by mobiles with a USIM that also belongs to the closed
subscriber group.
The Evolved Packet Core
(EPC) (The core network)
The architecture of
Evolved Packet Core (EPC) has been illustrated below. There are few more
components which have not been shown in the diagram to keep it simple. These
components are like the Earthquake and Tsunami Warning System (ETWS), the
Equipment Identity Register (EIR) and Policy Control and Charging Rules
Function (PCRF).
Below is a brief
description of each of the components
·
The Home Subscriber
Server (HSS) component has been carried forward from UMTS and GSM and is a
central database that contains information about all the network operator's
subscribers.
·
The Packet Data Network
(PDN) Gateway (P-GW) communicates with the outside world ie. packet data
networks PDN, using SGi interface. Each packet data network is identified by an
access point name (APN). The PDN gateway has the same role as the GPRS support
node (GGSN) and the serving GPRS support node (SGSN) with UMTS and GSM.
·
The serving gateway
(S-GW) acts as a router, and forwards data between the base station and the PDN
gateway.
·
The mobility management
entity (MME) controls the high-level operation of the mobile by means of
signalling messages and Home Subscriber Server (HSS).
·
The Policy Control and
Charging Rules Function (PCRF) is a component which is not shown in the above
diagram but it is responsible for policy control decision-making, as well as
for controlling the flow-based charging functionalities in the Policy Control
Enforcement Function (PCEF), which resides in the P-GW.
The interface between
the serving and PDN gateways is known as S5/S8. This has two slightly different
implementations, namely S5 if the two devices are in the same network, and S8
if they are in different networks.
Functional split between
the E-UTRAN and the EPC
Following diagram shows
the functional split between the E-UTRAN and the EPC for an LTE network:
2G/3G Versus LTE
Following table compares
various important Network Elements & Signaling protocols used in 2G/3G abd
LTE.
|
2G/3G
|
LTE
|
|
GERAN and UTRAN
|
E-UTRAN
|
|
SGSN/PDSN-FA
|
S-GW
|
|
GGSN/PDSN-HA
|
PDN-GW
|
|
HLR/AAA
|
HSS
|
|
VLR
|
MME
|
|
SS7-MAP/ANSI-41/RADIUS
|
Diameter
|
|
DiameterGTPc-v0 and v1
|
GTPc-v2
|
|
MIP
|
PMIP
|
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