02:37 - 
Quick Notes
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Quick Notes
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LTE - Radio Protocol Architecture
The radio protocol architecture for LTE can be separated into control plane architecture and user plane architecture as shown below:
At user plane side, the application creates data packets that are
processed by protocols such as TCP, UDP and IP, while in the control
plane, the radio resource control (RRC) protocol writes the signalling
messages that are exchanged between the base station and the mobile. In
both cases, the information is processed by the packet data convergence
protocol (PDCP), the radio link control (RLC) protocol and the medium
access control (MAC) protocol, before being passed to the physical layer
for transmission
User Plane
The user plane protocol stack between the e-Node B and UE consists of the following sub-layers:- PDCP (Packet Data Convergence Protocol)
- RLC (radio Link Control)
- Medium Access Control (MAC).
Packets received by a layer are called Service Data Unit (SDU) while
the packet output of a layer is referred to by Protocol Data Unit (PDU)
and IP packets at user plane flow from top to bottom layers.
Control Plane
The
control plane includes additionally the Radio Resource Control layer
(RRC) which is responsible for configuring the lower layers.
The
Control Plane handles radio-specific functionality which depends on the
state of the user equipment which includes two states: idle or
connected.
| Mode | Description |
|---|---|
| Idle | The user equipment camps on a cell after a cell selection or reselection process where factors like radio link quality, cell status and radio access technology are considered. The UE also monitors a paging channel to detect incoming calls and acquire system information. In this mode, control plane protocols include cell selection and reselection procedures. |
| Connected | The UE supplies the E-UTRAN with downlink channel quality and neighbor cell information to enable the E-UTRAN to select the most suitable cell for the UE. In this case, control plane protocol includes the Radio Link Control (RRC) protocol. |
The protocol stack for the control
plane between the UE and MME is shown below. The grey region of the
stack indicates the access stratum (AS) protocols. The lower layers
perform the same functions as for the user plane with the exception that
there is no header compression function for the control plane.
Let's have a close look at all the layers available in E-UTRAN Protocol
Stack which we have seen in previous chapter. Below is a more
ellaborated diagram of E-UTRAN Protocol Stack:
Physical Layer (Layer 1)
Physical Layer carries all
information from the MAC transport channels over the air interface.
Takes care of the link adaptation (AMC), power control, cell search (for
initial synchronization and handover purposes) and other measurements
(inside the LTE system and between systems) for the RRC layer.
Medium Access Layer (MAC)
MAC
layer is responsible for Mapping between logical channels and transport
channels, Multiplexing of MAC SDUs from one or different logical
channels onto transport blocks (TB) to be delivered to the physical
layer on transport channels, de multiplexing of MAC SDUs from one or
different logical channels from transport blocks (TB) delivered from the
physical layer on transport channels, Scheduling information reporting,
Error correction through HARQ, Priority handling between UEs by means
of dynamic scheduling, Priority handling between logical channels of one
UE, Logical Channel prioritization.
Radio Link Control (RLC)
RLC operates in 3 modes of operation: Transparent Mode (TM), Unacknowledged Mode (UM), and Acknowledged Mode (AM).
RLC
Layer is responsible for transfer of upper layer PDUs, error correction
through ARQ (Only for AM data transfer), Concatenation, segmentation
and reassembly of RLC SDUs (Only for UM and AM data transfer).
RLC
is also responsible for re-segmentation of RLC data PDUs (Only for AM
data transfer), reordering of RLC data PDUs (Only for UM and AM data
transfer), duplicate detection (Only for UM and AM data transfer), RLC
SDU discard (Only for UM and AM data transfer), RLC re-establishment,
and protocol error detection (Only for AM data transfer).
Radio Resource Control (RRC)
The
main services and functions of the RRC sublayer include broadcast of
System Information related to the non-access stratum (NAS), broadcast of
System Information related to the access stratum (AS), Paging,
establishment, maintenance and release of an RRC connection between the
UE and E-UTRAN, Security functions including key management,
establishment, configuration, maintenance and release of point to point
Radio Bearers.
Packet Data Convergence Control (PDCP)
PDCP
Layer is responsible for Header compression and decompression of IP
data, Transfer of data (user plane or control plane), Maintenance of
PDCP Sequence Numbers (SNs), In-sequence delivery of upper layer PDUs at
re-establishment of lower layers, Duplicate elimination of lower layer
SDUs at re-establishment of lower layers for radio bearers mapped on RLC
AM, Ciphering and deciphering of user plane data and control plane
data, Integrity protection and integrity verification of control plane
data, Timer based discard, duplicate discarding, PDCP is used for SRBs
and DRBs mapped on DCCH and DTCH type of logical channels.
Non Access Stratum (NAS) Protocols
The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME.
NAS
protocols support the mobility of the UE and the session management
procedures to establish and maintain IP connectivity between the UE and a
PDN GW.
LTE - Layers Data Flow
Below is a logical digram of E-UTRAN Protocol layers with a depiction of data flow through various layers:
Packets received by a layer are called Service Data Unit (SDU) while
the packet output of a layer is referred to by Protocol Data Unit (PDU).
Let's see the flow of data from top to bottom:
IP
Layer submits PDCP SDUs (IP Packets) to the PDCP layer. PDCP layer does
header compression and adds PDCP header to these PDCP SDUs. PDCP Layer
submits PDCP PDUs (RLC SDUs) to RLC layer.
PDCP Header Compression:
PDCP removes IP header (Minimum 20 bytes) from PDU, and adds Token of
1-4 bytes. Which provides a tremendous savings in the amount of header
that would otherwise have to go over the air.
- RLC layer does segmentation of these SDUS to
make the RLC PDUs. RLC adds header based on RLC mode of operation. RLC
submits these RLC PDUs (MAC SDUs) to the MAC layer.
RLC Segmentation: If an RLC SDU is large, or the available radio data rate is low (resulting in small transport blocks), the RLC SDU may be split among several RLC PDUs. If the RLC SDU is small, or the available radio data rate is high, several RLC SDUs may be packed into a single PDU. - MAC layer adds header and does padding to fit this MAC SDU in TTI. MAC layer submits MAC PDU to physical layer for transmitting it onto physical channels.
- Physical channel transmits this data into slots of sub frame.
LTE - Communication Channels
The
information flows between the different protocols are known as channels
and signals. LTE uses several different types of logical, transport and
physical channel, which are distinguished by the kind of information
they carry and by the way in which the information is processed.
- Logical Channels: : Define whattype of information is transmitted over the air, e.g. traffic channels, control channels, system broadcast, etc. Data and signalling messages are carried on logical channels between the RLC and MAC protocols.
- Transport Channels: Define howis something transmitted over the air, e.g. what are encoding, interleaving options used to transmit data. Data and signalling messages are carried on transport channels between the MAC and the physical layer.
- Physical Channels: Define whereis something transmitted over the air, e.g. first N symbols in the DL frame. Data and signalling messages are carried on physical channels between the different levels of the physical layer.
Logical Channels
Logical
channels define what type of data is transferred. These channels define
the data-transfer services offered by the MAC layer. Data and
signalling messages are carried on logical channels between the RLC and
MAC protocols.
Logical channels can be divided into control
channels and traffic channels. Control Channel can be either common
channel or dedicated channel. A common channel means common to all users
in a cell (Point to multipoint) while dedicated channels means channels
can be used only by one user (Point to Point).
Logical channels
are distinguished by the information they carry and can be classified in
two ways. Firstly, logical traffic channels carry data in the user
plane, while logical control channels carry signalling messages in the
control plane.
Transport Channels
Transport channels define how and with
what type of characteristics the data is transferred by the physical
layer. Data and signalling messages are carried on transport channels
between the MAC and the physical layer.
Transport Channels are
distinguished by the ways in which the transport channel processor
manipulates them.
Physical Channels
Data and signalling messages are carried on
physical channels between the different levels of the physical layer
and accordingly they are divided into two parts:
- Physical Data Channels
- Physical Control Channels
Physical data channels
Physical
data channels are distinguished by the ways in which the physical
channel processor manipulates them, and by the ways in which they are
mapped onto the symbols and sub-carriers used by Orthogonal
frequency-division multiplexing (OFDMA). Following table lists the physical data channels that are used by LTE:
The transport channel processor composes several types of control
information, to support the low-level operation of the physical layer.
Physical Control Channels
The transport channel processor
also creates control information that supports the low-level operation
of the physical layer and sends this information to the physical channel
processor in the form of physical control channels.
The
information travels as far as the transport channel processor in the
receiver, but is completely invisible to higher layers. Similarly, the
physical channel processor creates physical signals, which support the
lowest-level aspects of the system.
The base station also transmits two other physical signals, which
help the mobile acquire the base station after it first switches on.
These are known as the primary synchronization signal (PSS) and the
secondary synchronization signal (SSS).
Summary
1.) Uu-interface
2.) Control Plane
3.) User Plane
4.) Packet Handle
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