The document describes the LTE protocol stack, which contains a user plane and control plane. It divides the protocol stack into layers for the radio network and transport network. The physical layer transfers data and performs error detection. The MAC sublayer maps transport channels to logical channels and handles scheduling. The RLC layer provides different reliability modes for data transfer. The PDCP layer performs header compression and ciphering. The RRC layer controls handovers, paging, and radio bearer setup. Transport protocols like IP, UDP, and GTP are used in the fixed network.

2. LTE PROTOCOL STACK
Shoeb Ahmed(2017420001)
Sardar Patel Institute Of Technology
Mumbai-400058, India
Abstract—Abstract-: Protocol architecture made for interfaces
in the LTE network, contains user plane and control plane. For
network infrastructure, protocols are divided into two layers,
radio network layer and transport network layer and for air
interface, Non Access Stratum and access stratum. Transport
channels from physical layer mapped into logical channels in
medium access control sublayer through medium access control
protocol data units. Data is send to packet data convergence
protocol sublayer from radio link control sublayer by transparent
mode, unacknowledged mode and acknowledged mode depending
on accuracy. Packet data convergence protocol sublayer transfer
user plane and control plane data to application layer after
integrity protection. Radio resource control layer broadcast
system information while application protocols control signalling
messages. A complete description of protocol architecture is
presented in this paper. Paper explains the structure of protocol
data units. Specifications are defined in general approach.
I. INTRODUCTION
Protocol architecture is designed for reducing complexity
of networks and communication between devices working on
different algorithms. A program or hardware is made for each
layer to obey the protocols so that they can communicate with
each other and different implementations are possible for each
layer. Interfaces follow the protocol stack to minimize the
information that passed through between a pair of layers.
II. PROTOCOL STACK LAYER
The protocol stack is an implementation of a computer
networking protocol stack. The terms are often used inter-
changeably. Severely speaking, the suite is the definition of
the protocol, and the stack is the software execution of them.
Individual protocols within a stack are often designed with a
single function in mind. This modularization makes design and
estimation easier. Since each protocol module usually commu-
nicate with two others, they are commonly imaginary as layers
in a stack of protocols. The lowest protocol always deals with
”low-level”, physical interaction of the hardware. Each higher
layer adds more features. User applications generally deal only
with the top most layers.
A. Physical Layer
Data is transferred by physical layer. Data exchanged be-
tween Medium Access Control (MAC) layer and physical layer
called Transport Block (TB) is exchanged per transmission
time interval of 1 ms. Function of Physical layer are
1. For Physical Uplink Shared Channel (PUSCH) 1 TB and
for Physical Downlink Shared Channel (PDSCH), upto 2 TB
are delivered to physical layer.
2. 24 bit Cyclic Redundancy Check (CRC) for detection of
burst errors in message,
3. TB error indication to higher layers
4. Convolutional coding in forward error correction (FEC),
5. Physical layer support Hybrid Automatic Repeat Request
(HARQ) as combination of both process CRC and FEC.
6. Interleaving is used if errors within code become more than
error correcting capability of burst errors.
7. Data is modulated according to modulation scheme decided
by MAC scheduler, which can be QPSK, 16QAM and 64
QAM.
8. Mapping to physical resource
9. MAC scheduler partly configures mapping from assigned
resource blocks to the available number of antenna ports.
B. Medium Access Control Sublayer
Data link layer contains Medium Access Control (MAC),
Radio Link Control (RLC), Packet Data Convergence Control
(PDCP) sublayers.
For data transfer and radio resource allocation MAC sublayer
transport channels map control plane and user plane infor-
mation into control and traffic channels respectively, called

3. logical channels. Two MAC entities are defined in E-UTRA on
each side UE and E-UTRAN, that perform different functions.
In MAC architecture, MAC sublayer multiplex MAC Service
Data Unit (SDU) from multiple logical channels onto transport
block-
1. To be delivered to physical layer on transport channels.
2. Multiplexing and demultiplexing of MAC SDUs from one
or different logical channels onto TB to be delivered to or
from physical layer on transport channels.
3. Schedule information reporting
4. Error correction through hybrid automatic repeat request
(HARQ)
5. Priority handling between UEs by dynamic scheduling
6. Priority handling between logical channels of one UE
7. Logical channel prioritization
8. Transport format selection.
C. Radio Link Control (RLC)
The RLC layer is used to format and transport traffic
between the UE and the eNB. RLC provides three different re-
liability 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 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 broad-
casting 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 conditions.RLC offers services to PDCP in form of
radio bearer. These radio bearers are mapped to EPS bearers
in EPC.
Some Function of RLC are
1. RLC also responsible for re-segmentation of RLC data
PDUs(only for AM data transfer)
2. Recording of RLC data PDUs (only for UM and AM data
transfer)
3. Duplicate detection (only for UM and AM data transfer)
4. Protocol error detection (only for AM data transfer)
D. Packet Data Convergence Protocol (PDCP)
In the user-plane the PDCP layer is responsible for com-
pressing/decompressing the headers of user plane IP packets
using Robust Header Compression (ROHC) to enable efficient
use of air interface bandwidth. This layer also 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.
E. Radio Resource Control (RRC)
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.
F. Non-Access Stratum(NAS)
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.
G. Fixed Network Transport Protocols
• Each interface in the fixed network uses standard IETF
transport protocols.
• These interfaces use protocols from layers 1 to 4 of the usual
OSI model
• The transport network can use any suitable protocols for
layers 1 and 2, such as Ethernet
• Every network element is then associated with an IP address,
and the fixed network uses the Internet Protocol (IP) to route
information from one element to another across underlying
transport network

4. • LTE supports both IPv4 and IPv6 for this task
• In the EPC, support of IPv4 is mandatory and support of
IPv6 is recommended
• The radio access network can use either or both of the two
protocols
H. Internet Protocol(IP)
The Internet Protocol (IP) is the essential convention in the
Internet layer and has the undertaking of conveying packets
from the source to the destination focused around the IP
addresses in the packet headers. It characterizes tending to
strategies that are utilized to mark the datagram with source
and destination data uniquely. LTE supports mobiles that are
using IP version 4 (IPv4), IP version 6 (IPv6) and Dual stack
IPv4/IPv6
Dual stack IPv4/IPv6: The most direct approach to making
IPv6 nodes compatible with IPv4 nodes by maintaining a
complete IPv4 stack. A network node that supports both IPv4
and IPv6 is called a dual stack node. A dual stack node
configured with an IPv4 address and an IPv6 address can
have both IPv4 and IPv6 packets transmitted. For an upper
layer application supporting both IPv4 and IPv6, either TCP
or UDP can be selected at the transport layer, while IPv6 stack
is preferred at the network layer.
I. User Datagram Protocol(UDP)
User Datagram Protocol (UDP) is one of the communica-
tions protocols that are used over the internet protocol (IP).
UDP helps in exchanging messages between devices and offers
limited service. UDP does not provide sequence number to the
packets and so cannot reassemble the packets at the other end.
This means that it does not make sure that the entire message
has reached the destination and whether it is in the right order.
UDP is generally preferred in time sensitive applications, as
to wait for retransmitted packets is not an option in real time
system.
J. Stream Control Transmission Protocol (SCTP)
The Stream Control Transmission Protocol (SCTP) is a
transport layer protocol and it ensures that there is reliable
and in order transmission of messages. SCTP is used for
simultaneous transmission of multiple streams of data between
connected source and destination
K. Diameter
Diameter is an authentication, authorization, and accounting
protocol (AAA) for computer networks and is a replacement
of RADIUS (Remote Authentication Dial-In User Service)
protocol. Diameter protocol provides unlimited scalability to
enable growth, secure and reliable transmission of packets.
L. GPRS Tunneling Protocol version 2 (GTPv2-C)
GPRS Tunneling Protocol version 2 (GTPv2-C) is used for
signaling between Mobility and Management entity (MME)
and Serving Gateway (S-GW); and between Serving Gateway
(S-GW) and PDN Gateway (P-GW). GTPv2 is part of GTP
family which is IP-based communications protocols. GTP is
present in GSM, UMTS and now in LTE to provide general
packet radio service (GPRS). This protocol is used to activate
a session, to deactivate the same session, to update a session
and to check Quality of Service (QoS) for a particular session.
M. GPRS Tunneling Protocol version 1 (GTPv1-U
GTPv1-U is used to carry user data between the Evolved
Node B (eNB) and the S-GW. The user data which is trans-
mitted in form of packets can be in IPv4, IPv6, or PPP format.
This protocol is being used since no changes are required for
LTE as compared to earlier 3GPP releases
N. X2AP
The X2AP protocol is used in the X2 interface and helps
in handling the UE mobility within eNBs. It provides the
functions like mobility management and load management
between the different eNB.
III. CONCLUSIONS
Protocol architecture is described with introduction of func-
tions of layers and structure of protocol data units. Because
of separation of radio network functionality and transport
network functionality new technologies and methods can be
used so that different architectures are possible for different
manufacturers.
Radio access technology has witnessed rapid innovation, es-
pecially in the LTE domain. The driving force to further
develop LTE towards LTE–Advanced - LTE Release10 is set to
provide higher bitrates in a cost efficient way and, at the same
time, completely fulfill the requirements set by ITU for IMT
Advanced, also referred to as 4G. The IMT Advanced systems
(4G) shall provide best-in-class performance attributes such
as peak and sustained data rates and corresponding spectral
efficiencies, capacity, latency, overall network complexity and
quality-of-service management.
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