Abstract but only Packet Switching (PS) data paths. Also,

Abstract

LTE (Long Term Evolution)
is the most leading wireless broadband technology. Also, is the first 3GPP
wireless standard full IP-based. Due to its chance to reach very high throughput,
and efficient end-to-end (E2E) QoS treatment is
needed in order to guarantee a good QoS. In this paper, a comprehensive study
on LTE network is done. The performance metrics used for the evaluation are delay,
load, traffic dropped and the throughput. The results are obtained from links
between mobiles UEs and eNodeBs in our scenario. Different parameters are
considered in all scenarios simulated using OPNET
Modeler software tool.

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Keywords: LTE; Delay; Load; Throughput;
OPNET.

I.     Introduction

Long Term Evolution
(LTE) is a Third Generation Partnership Project (3GPP) standard for wireless
transmission systems. It is also used to indicate the 4th Generation
(4G) of wireless systems. It is characterized by innovative features, as 1:

·       DL data rate: up to 100 Mbps.

·       UL data rate: up to 50 Mbps.

·       BW: (1.4, 3, 5, 10,15, 20) MHz

·       Access Techniques: OFDMA (Orthogonal Frequency Division
Multiplexing Access) in DL.                SC-OFDM
(Single Carrier Orthogonal Frequency Division Multiplexing) in UL.

·       Low latency:    
5 MHz

·       Optimal mobility support

 

Due to all these important features, Long Term Evolution is
providing good performances, especially for services like VoIP, video
conference, multiplayer games, multimedia messaging, etc. All these
applications require wider BW, smaller delay, higher throughput and recent
transmission methods that will give higher spectral efficiency and good
quality.

LTE provides a full IP-based wireless standard. It means that
all (E2E) connection between LTE network and end user are using IP protocol as
communication transport protocol. For this reason, Circuit Switching (CS) data
paths are not present in LTE system but only Packet Switching (PS) data paths.
Also, LTE standard provides a unique QoS-aware mechanism for E2E service
delivering, to guarantee minimum QoS requirements for its service delivered to
the end user 2.

The rest of this paper is organized as follows: Section II
will include system description that will summarize the network architecture including
QoS management. Section III will describe the QoS Assessment of LTE systems.
Section IV will discuss the simulation
platform of the proposed LTE network include configuration parameters and the simulation
results and analysis. Section V will conclude the paper.

II.  SYSTEM
DESCRIPTION

 

A.  LTE Network Architecture

The LTE system based on flat IP architecture, where all the
network entities are connected thorough IP. The network can be divided into two
parts as:

·       Radio Access Network (RAN)

·       Evolved NodeB
(eNodeB); which serves the UE forms the RAN whereas Evolved Packet Core (EPC)
represents the Core Network (CN) part of the network.

 

These
network elements have their role either in signaling traffic (control plane),
user data traffic (user plane) or both. Figure 1 shows the overall network
architecture and the interfaces through which the network elements are
connected. 3

 

 

 

 

 

Figure 1: Overall network
architecture of LTE 4.

The different entities of LTE network and their primary
functionalities are listed below 5:

·       User Equipment (UE)

It represents the mobile terminal of the system. There are
variety of terminal starting from handheld smart phones till bulky laptops.

·      
Evolved NodeB (eNodeB)

It is responsible for major radio related operations of the
system. This has evolved from the traditional NodeB of 3G system which
performed minimal functionality of Radio Network Controller (RNC). However, in
LTE the functionality of RNC has been decentralized to many eNodeBs.

·       Mobility Management Entity (MME)

It is the key control node for LTE network which primary
functions are tracking user location, paging procedures, activation /
deactivation of bearer channels and inter-network handover 6.

·       Serving Gateway (S-GW) 

It acts as a gate way for user plane traffic. It also acts as
a mobility anchor for the users moving from one eNodeB to another. This entity
is responsible for legal (valid) interception.

·       Packet Data Network (PDN) Gateway (P-GW)

They are the login point for external IP network to the LTE
network. This is responsible for policy enforcement and charging support. P-GW
also acts as an anchor during non-3GPP inter- network mobility.

·       Home Subscriber Support (HSS)

It is the central
data base server for the LTE system. It contains all identity and subscription
related information for the home users. This entity replaces the Home Location
Register (HLR) and Authentication Center (AuC) of 3G system.

·       Policy Charging and
Rules Function (PCRF)

It is a logical entity which is responsible for decision making
based on the policies and rules defined. It is also responsible for enforcing
them to work together with P-GW.

B.  QoS management

Reference 7 shows
that the LTE system look forward to QoS management; which based on two main
elements:

·       Evolved Packet System (EPS) bearers

 EPS bearers are connection-oriented virtual
transmission channels carried out on a single Packet Data Network (PDN)
connection among two or more end-points. Bearer; is just a virtual concept. It defines how
the UE data is treated when it travels across the network. Network might treat
some data in a special way and treat others normally. Some flow of data might
be provided guaranteed bit rate (GBR) while other may face low transfer.  In short, bearer is a set of network
parameter that defines data specific treatment  
e.g. Person A will always get at least 512 Kbps download speed on
his LTE phone while for Person B; there is no guaranteed bit rate and
might face extremely bad download speed at times.

ü

Default Bearer: When LTE UE attaches to the
network for the first time, it will be assigned default bearer which remains as
long as UE is attached. Default bearer is best effort service. Each default bearer
comes with an IP address. UE can have additional default bearers as
well. Each default bearer will have a separate IP address. QCI 5 to 9 (Non-
GBR) can be assigned to default bearer.

 

 

 

 

 

 

       

Figure
2: Default Bearer Non-guaranteed bit rate 7.

ü

Dedicated Bearer: To put it
simple, dedicated bearers provides dedicated tunnel to one or more specific
traffic (i.e. VoIP, video etc). Dedicated bearer acts as an additional bearer
on top of default bearer. It does not require separate IP address due to the
fact that only additional default bearer needs an IP address and therefore
dedicated bearer is always linked to one of the default bearer established
previously. Dedicated bearer can be GBR or non-GBR (whereas default bearer can
only be non-GBR). For services like (VoLTE) we need to provide better user
experience and this is where Dedicated bearer would come handy.

 

 

 

 

       

 

 

Figure 3: Dedicated Bearer network
architecture of LTE 7.

·       Quality of Service Class Identifiers (QCI).

QCI is a scalar representing a set of packets forwarding
treatments (e.g., scheduling weights, admission thresholds, queue management
thresholds, etc). LTE specifies a number of standardized QCI values with
standardized characteristics, which are preconfigured for the network elements.
This ensures multivendor deployments and roaming. The mapping of standardized
QCI values to standardized characteristics is shown below in Table (1), and the
QoS attributes associated with the LTE bearer in Table (2). 8

Table
1:
shows list of different QCIs. 8.

QCI

Resource Type

Priority

Packet
Delay Budget

Packet Error
Loss Rate

Example Services

1

GBR

2

100 ms

0,01

Conversational
Voice

2

4

150 ms

0,001

Conversational
Video
 (live streaming)

3

3

50 ms

0,001

Real-Time Gaming

4

5

300 ms

0,000001

Non-
Conversational Video
 (buffered streaming)

5

Non-GBR

1

100 ms

0,000001

IMS Signaling

6

6

300 ms

0,000001

Video
 (Buffered Streaming)
TCP-based
 (e.g., web, e-mail, chat, FTP,
 point- to-point file sharing,
progressive video, etc.)

7

7

100 ms

0,000001

Voice, Video
(Live Streaming),
Interactive Gaming

8

8

300 ms

0,001

Video
(Buffered Streaming), TCP-based
(e.g., web, e-mail,
chat, FTP, point- to-point file sharing, progressive video,
etc.)

9

9

0,000001

 

QCI Attribute

Description

Resource

Type Guaranteed Bit Rate vs. Non-
Guaranteed Bit Rate

Packet Delay Budget

Maximum acceptable E2E delay between the UE and
the PDN-GW

Packet Error Loss Rate

Maximum acceptable rate of IP
packets that are not successfully received by the PDCP layer

Allocation Retention Priority

Value assigned for scheduling
when capacity is reached, with “1” being highest level

Table 2: LTE QCI attributes giving a
brief description. 8.

III.   QOS
ASSESSMENT

Reference 9 shows; the various performance QoS
parameters used for analysis:

1)   Delay:

Delay is the
time of generation of packet by source to destination reception. So, it is the
time (in seconds) taken by packet to go across the network. All the delay in
the network are called packet E2E delay.

2)   Load:

This
represents the total load submitted to LTE layers by all higher layers in all
LTE eNodes of the network. It can be represented in bits/sec and packets/sec.

3)   Throughput:

Presents the
amount of successful data transferred from one location to another over a
specific period of time.

4)   E2E delay:

E2E delay less
200 ms is considered acceptable. It is the most important parameter in video conferencing
because it affects the QoS and degrades the system performance

5)   Packet Delay Variation:

Variation in
packet is seen which are transmitted, but not received by receiver in intended
time. It can degrade the performance of system. Low packet delay variation is
important.

IV.   SIMULATION
PLATFORM

OPNET modeler version 17.5 simulator is a packet based event driven dynamic
system level simulator which accurately and efficiently simulate the behavior
of various types of real world networks. It has an enormous library and a
Graphical User Interface (GUI) with a “user friendly” sense. It was
available under license. We will use it to simulate the proposed
LTE network to evaluate its performance. To measure the QoS parameters for video
conferencing and VoIP on LTE network, scenario shown in the figure 4 is used. Simply,
the eNodeBs are configured with non-overlapping 3 MHz FDD PHY profiles so that
there won’t be any interference between the cells. Accordingly, the eNodeBs are
configured to allow inter-frequency handovers. Each UE has an IP traffic flow
to the server (uplink). Each IP traffic flow has 1,1 Mbits/sec. The eNodeB_1 is
configured to be high loaded, eNodeB_3 is configured with low load; while
eNodeB_2 is configured to fail at t = 200 sec, and remain failed
until the end of the simulation.

 

 

 

 

 

 

 

 

Figure 4: The proposed LTE
network.

A.   
LTE
CONFIGURABLE PARAMETERS:

Entire LTE network is modeled following parameters listed below in Table
3.

LTE Network Settings

Network Node

Parameter Description

Parameter Value(s)

User Equipment

Operating power

20 dBm

Pathloss model

Okumura-Hata model

Channel Model

Path Loss, Shadowing and Fast Fading

Shadow fading deviation

5 dB

Receiver sensitivity

-200 dBm

HO detection timer

5 s

eNodeB

Handover Type

Inter-Frequency

LTE bandwidths

5 MHz

Duplex mode

FDD

eNodeB antenna gain

15 dBi

Transmit / Receive antennas

2

Tx. power

43 dBm

Receiver sensitivity

-200 dBm

Link (PPP DS3)

Traffic environment

Traffic loaded

Traffic Model

IP_Traffic_Flow

Server

Server Configuration

Sun Ultra 10 333MHz 1CPU 1Core

Table 3: LTE Network Settings.

 

B.   
SIMULATION
RESULTS AND ANALYSIS

At first, we will
define the most important performance metrics that will be used for the evaluation.
Reference 10 state
that; Throughput, for simulations done in OPNET;
the difference between the traffic sent from a source and traffic received at
the intended destination is known as “Traffic Dropped”. It can be represented in
bits/sec or packets/sec. It could be due to mainly the fading wireless channel and also due
to different network effects such as congestion, interference etc., which could
occur at different layers. The throughput is traffic sent minus the dropped
traffic divided by traffic sent, be shown in figure 5 (a, b) to clarify all
parameters.

Figure
5: Performance metrics has multiple components that are referred to from left
to right. (a) Throughput of all eNodeBs, (b) Traffic sent, Traffic received for
eNodeB_1 and eNodeB_3.

From figure 5 (a), we can analyze that in case of
eNodeB_1 the data transferred over all simulation
time period; is higher than eNodeB_3. But for eNodeB_2 is down after specific
time period (during the eNB failure).

Traffic sent; is
the amount of data sent by eNB to UE. Traffic rate should be high so that even
if data is lost or delayed on the way, not much loss will occur. From the
figure 5 (b), it is clearly analyzed that eNodeB_1 sent more traffic than eNodeB_3.
Traffic received; is the amount of data received by the mobile
station. From the figure 5 (b), it is clearly analyzed that eNodeB_1 receives
more traffic than eNodeB_3.

When eNodeB_2
fails, the UEs attached to eNodeB_2 start scanning the other eNodeBs. Then
based on their distance, they get associated with either eNodeB_1 or eNodeB_3. As
notice in figure 6 (a, b) that Mobile_0_2_2 attaches to eNodeB_1 and Mobile_0_2_4
attaches to eNodeB_3 after eNodeB_2’s failure.

 

Figure 6: Mobile has multiple
components that are referred to from left to right. (a) Mobile _0_2_2 attaches
to eNodeB_1, (b) Mobile_0_2_4 attaches to eNodeB_3.

 

After this switch
we see in figure 7 (a, b) respectively; a big increase in the average of Packet E2E delay. This is expected;
because due to the newly attached UEs, eNodeB_1’s PUSCH (Physical Uplink Shared
Channel) utilization hits 100%. The same doesn’t hold for eNodeB_3; because,
only Mobile_0_2_4 attaches to eNodeB_3 after eNodeB_2’s failure. This effect of
new UEs can be seen below “PUSCH Utilization of eNodeBs”. Similarly,
“Delay of eNodeBs”, we see that the delay of eNodeB_1 increases after
eNodeB_2’s failure shown in figure 7 (c).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7: LTE network multiple components that are
referred to from left to right. (a) Average of Packet E2E delay for both
Mobile_0_2_2 and Mobile_0_2_4, (b) PUSCH utilization for
ala eNodeBs, (c) Delay of eNodeB_1 and eNodeB_3.

V.  CONCLUSION

Purpose of this work is to evaluate a
comprehensive analysis end-to-end QoS for LTE network; while focusing on
influence of eNodeB failure during simulation, and allow inter-frequency handovers
done. An efficient analysis of QoS performance metrics is presented. It based
on Throughput, Traffic sent and received, PUSCH utilization, LTE delay and
packet E2E delay. When eNodeB_2 failed, we present a very continuous behavior of
handover
process and the association of new UEs. In future, QoS parameters should be
improved to get minimum delay and maximum throughput over different scenarios and
schemes.

ACKNOWLEDGEMENT

The author would like to give an extended
acknowledgement to OPNET Team at NTI
(National Telecommunication Institute) for using OPNET Modeler software for
educational and research activities.

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