PERFORMANCE ANALYSIS OF WIRELESS SYSTEMS IN

TELEMEDICINE

Hybrid Network for Telemedicine with Satellite and Terrestrial Wireless Links

M. Luglio and F. Zampognaro

University of Rome Tor Vergata, Via del Politecnico 1, 00156 Rome, Italy

Keywords: Telemedicine, hybrid networks, DVB-RCS, wireless communications.

Abstract: Telemedicine services represent a valuable opportunity to provide medical assistance ensuring high

flexibility and prompt set up and to significantly reduce costs. The use of hybrid networks based on

satellites and terrestrial wireless systems can be extremely advantageous in terms of flexibility, capillarity

and integration with modern medical equipment, in particular representing a suitable solution in case of

disasters. In the paper such an architecture is described and key performance for some reference

applications, evaluated through simulation, are shown and discussed.

1 INTRODUCTION

Nowadays telemedicine is gaining increasing

interest, in particular in remote and disaster-struck

areas where telecommunication infrastructure can be

respectively missing or compromised.

The aim of this paper is to show feasibility and

effectiveness of an hybrid network architecture for

telemedicine, through performance evaluation

carried out by simulations. The proposed hybrid

network, selected also for the Telesal project

(Arenaccio, Aversa and Luglio, 2006), is composed

of a satellite core network, interconnected with

terrestrial wireless tails using commonly available

and consolidated wireless technologies.

Satellite systems are extremely suitable to

represent the core infrastructure of the network for

their capability to provide data access ubiquitously

and in mobility over very large areas, including

remote or impervious locations where typically

terrestrial telecommunication infrastructures are not

present or economically not viable. To complement

such characteristics, terrestrial wireless systems can

be fruitfully utilized realizing the terrestrial tails to

ensure capillarity and to improve efficiency and

flexibility (Luglio and Vatalaro, 2002).

Performances of some reference telemedicine

applications will be evaluated using NS2 as

simulation tool (Fall and Varadhan, 2007).

In section 2 an overview of the involved

technologies is introduced, in section 3 the network

architecture is described along with some

telemedicine applications. In section 4 the

simulations setup is presented offering some the

simulation outputs summaries. Finally in section 5

conclusions are drawn.

2 TECHNOLOGY REVIEW

Different kinds of satellite configuration

(geostationary and low orbit) can be utilized in the

proposed hybrid architecture. As concerns terrestrial

systems in particular WiMax, WiFi and Bluetooth

looks the most suitable technologies for our scope.

2.1 Satellite Networks

Two kinds of satellite systems are considered:

a) wide band VSAT systems using a

geostationary (GEO) satellite and

b) narrow band global communication

system using a low orbit (LEO)

satellite constellation.

VSAT systems (Very Small Antenna aperture

Terminal) are characterized by the use of directional

fixed or steerable (to allow mobility) dish antennas

with a size of around 80-120 cm. They usually adopt

star or mesh topology, using GEO satellites, which

200

Luglio M. and Zampognaro F. (2008).

PERFORMANCE ANALYSIS OF WIRELESS SYSTEMS IN TELEMEDICINE - Hybrid Network for Telemedicine with Satellite and Terrestrial Wireless

Links.

In Proceedings of the First International Conference on Health Informatics, pages 200-204

 SciTePress

are suitable for multi-nation coverage area (usually

at continental level). They can offer uplinks of up to

2 Mbit/s, with downlink up to 40 Mbit/s.

Communications suffer of a physical delay of about

550 ms round trip, since the GEO satellite is placed

in a 36000 km orbit. Star systems need a double hop

to allow two terminals to directly communicate,

from the terminal to the star-centre (called HUB)

and from it to the other terminal, resulting in

physical delay of four times 125-130 ms. Mesh

systems, instead, allow direct communication

between two terminals without crossing the HUB

(physical delay of two times 125-130 ms).

For the specific scenario that have been

simulated, DVB-RCS standard (ETSI EN 301 790,

2003), developed for VSAT systems, has been

selected. The architecture referred to the standard

usually applies to star topology, with a central HUB

called NCC (Network Control Centre). Downlink

channel is broadcasted to all users using DVB-S

standards (ETSI EN 300 421, 1997), while return

channel is shared with a MF-TDMA technique.

DVB-RCS allows each terminal to negotiate

capacity requests on demand for transmission on the

shared return link according to pre-defined service

level agreement:

• volume based dynamic capacity (VBDC), to

issue bandwidth requests based on the actual

volume of traffic needed;

• rate based dynamic capacity (RBDC), to issue

bandwidth request based on the estimation of

transmission rate;

• constant rate assignment (CRA), to obtain

guaranteed bandwidth assignment.

Such an assignment scheme is called DAMA

(Demand Assignment Multiple Access) and it is

used to share the same upload channel dynamically

and efficiently among several terminals. According

to the request policy, different cost may by charged

by the satellite operator.

On the other hand LEO constellations are

composed of several satellites at low orbit (between

700 and 1500 km), which are in continuous

movement with respect to terrestrial Earth surface.

The system is designed to maximize the probability

of user-satellite line of sight even at high latitudes

and handover functionalities must be implemented

in order to keep connection when changing serving

satellite. LEO terminals use omnidirectional

antennas and offer limited bit rate, usually

dimensioned for voice communications (similar to

GSM). Latency is limited to a few ms, but variable

in time and affected to big spikes due to the

handover execution.

Globalstar has been selected for the simulation

campaign (http://www.globalstar.com/en/), due to its

common availability in Europe.

2.2 Terrestrial Wireless Networks

To realize the terrestrial component PANs (Personal

area networks), LANs (local area networks) or

WANs (wide area networks) concepts can be

adopted. The first two are usually associated to

license free bands (IMS), with data throughput

ranging from 1 up to tens of Mbit/s (with a coverage

from few meters to some tens of meters). In

particular Bluetooth (IEEE Std 802.15.1) and Wi-Fi

(IEEE Std. 802.11) are representative technologies

of PAN and LAN, respectively. A WAN is instead

capable of long range coverage with higher

throughput and it usually works either in licensed or

free bands. WiMAX (IEEE Std. 802.16e-2005) is an

example of WAN with allocation of commercial

frequency bands around 3.5 GHz. HIPERLAN

(ETSI EN 300 652, 1998) represents another

example of WAN system working in the unlicensed

band of 5.4 GHz.

For the purpose of our hybrid network proposal,

only the license-free LAN and PAN technologies

will be included, leaving to a future study a more

comprehensive integration of WAN, LAN and PAN

networks together with the satellite segment.

Wi-Fi is a widespread wireless technology that

provides wired-LAN-like connection service to

mobile devices in the range of around 100 m.

Maximal bandwidth available on Wi-Fi variants

ranges from 11 Mbit/s (standard 802.11b) to 54

Mbit/s (standard 802.11a or 802.11g). So far the

infrastructure mode, with a central base station

(called Access Point), has been widely deployed in

most cases, although Wi-Fi foresees an ad-hoc direct

connectivity. A set of base stations can serve up to

128 user terminal each, guaranteeing local mobility.

Newer standard 802.11i and 802.11e are defining

respectively stronger algorithms for security

(WPA2) and QoS at MAC layer.

Bluetooth is a PAN ad-hoc wireless system

which allows terminals to flexibly and

autonomously configure themselves to communicate

without a pre-existing infrastructure in a peer-to-

peer fashion. Bluetooth Standard version 1.1 is the

actual reference implemented in commercial

products such as headsets, GPS devices, etc. It is

designed to offer a total 1 Mbit/s data rate with a

coverage of 10 meters maximum. When Bluetooth

terminals get close enough, they can cluster into a

piconet and temporarily designate one master unit to

PERFORMANCE ANALYSIS OF WIRELESS SYSTEMS IN TELEMEDICINE - Hybrid Network for Telemedicine with

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coordinate transmissions with up to seven slave

units. The time needed to join a piconet and start

service is in the order of some seconds.

Bluetooth is based on packet transmission and

frequency hopping (FH) technologies to provide

channelization among different piconets within the

same area, to form the so called scatternets. Each

Bluetooth service has a pre-defined QoS profile to

announce during setup, and it is accepted in the

piconet only if there are enough resources.

3 TELEMEDICINE NETWORK

The set up of a telecommunications network as

support to telemedicine can be extremely difficult in

remote or in disaster-struck areas. For instance, the

installation of a single dedicated point to point radio

link to restore or deliver GSM communication

channels can take several hours. In this context, the

use of satellite terminals can shorten this time to a

few minutes, thanks to the intrinsic broad coverage

of a satellite service. The core satellite network can

be complemented by terrestrial wireless tails

composed of Bluetooth piconets and Wi-Fi links.

3.1 Architecture

The proposed architecture is shown in Figure 1.

Connection between satellite terminal and HUB is

assumed to be alternatively realized with either

DVB-RCS or Globalstar.

An application client on the disaster-struck area

is assumed to be reached directly by the satellite

terminal, or being part of a Bluetooth piconet. In

both cases the connection with the Satellite Terminal

can be wired or realized via a Wi-Fi wireless link

(dashed line). Nodes of different segments are

connected with Ethernet cables or with internal bus

if integrated in the same hardware. In all cases

connectivity is implemented at IP layer to leverage

on IP built-in routing functionalities and address

resolution.

Security and QoS must also be carefully

considered in hybrid networks offering telemedicine

services. QoS is a key issue for real time services,

and must be offered end-to-end along the whole data

path. This means that each segment must be

coordinated centrally for its specific QoS

management setup. Security and encryption, usually

available for each technology independently, must

be ensured also end-to-end, due to the sensitivity of

data transmitted. End-to-end QoS and security could

be handled at IP layer, since it can be considered too

complex an adaptation of different QoS and security

procedures offered by the different technologies at

layer 2. Solutions like DiffServ and secure tunneling

(VPNs) could be adopted.

Figure 1: Network Architecture.

3.2 Reference Applications

Two kinds of applications, both real time and non

real time, will be tested over the proposed network.

Table 1 shows a list of these applications with the

most significant characteristics. The last two rows

show dimensions of representative files which can

be transferred by non real time telemedicine

applications.

Table 1: Telemedicine applications.

Real time

Application Protocol Codec Bitrate

Voice call RTP G729 8÷12 kbit/s

Video call RTP MPEG4 >384 kbit/s

Non Real time

Application

Data

Protocol Size Raw Size

compressed

Radiography FTP 5.7

Mbytes

380 kbytes

ECG trace FTP 90 kbytes 45 kbytes

4 SIMULATIONS

Simulations have been performed using NS2

platform. The architecture introduced in section 3.1

has been set up and verified with the help of NAM

(NS2’s visual output), as shown in Figure 2.

Application clients 4-6 have Bluetooth connectivity

to the Home Gateway. The Home Gateway and the

Application clients 7-8 have a Wi-Fi and wired

Ethernet connectivity to the satellite terminal,

respectively.

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Figure 2: NAM output of NS2 simulation.

Traffic sources originated from Application

Clients are compatible with the applications listed in

Table 1. All the traffic is delivered from the

application clients to the satellite terminal via a

wired or wireless link, through the satellite (in

alternative DVB-RCS or Globalstar) up to the

application server at the other end of the network.

The application server is representing the operative

centre for emergency handling. TCP New Reno has

been used as transport protocol for file transfer and

standard UDP for real time traffic.

For simulations using DVB-RCS as satellite

technology, a return link capacity of 512 kbit/s with

the correct physical delay has been considered. For

DAMA capacity requests, RBDC has been simulated

according to (Roseti and Kristiansen, 2006) while

CRA consists in a granted capacity, similar to a

SCPC service (Single Channel Per Carrier, unshared

uplink channel). For all the other technologies

involved (Bluetooth, WiFi, Globalstar), common

operational values as seen in section 2 have been

used in NS2 simulated links.

Non real time application performances are

summarized in Table 2. For this class of applications

the performance index considered is the average

time needed by the application on a Bluetooth node

to send one data file (including reception and

acknowledgement) to the application server at the

other end of the network.

Table 2: Non Real Time Average Data Transfer Time.

Data Globalstar DVB-RCS

w/ RBDC

requests

DVB-RCS

w/ CRA

requests

Radiography

RAW

Not

performed

140s 115.6s

Radiography

Compressed

333.5s 16s 12.5s

ECG trace

RAW

79.6s 6.9s 4.9s

ECG trace

Compressed

45.9s 4.8s 3.4s

Figures clearly show the differences in

performances between GEO and LEO satellite

system, and also between the two different request

policies for DVB-RCS. The use of satellite to

transfer medical data of limited size is acceptable

under all conditions, while bigger size data transfer

is not practical for narrow band Globalstar satellite

system.

For real time applications, two different setup

have been considered for the two alternative GEO

and LEO satellite systems:

• When using DVB-RCS, a video call with five

simultaneous voice calls with higher quality

profile (12 kbit/s each) have been initiated from

the Bluetooth nodes 4-6. Other two voice calls

have been initiated from Application clients 7

and 8

• Over the Globalstar system, only one voice call

originated by a Bluetooth node could be

performed at the minimum codec rate (8 kbit/s)

In both cases the packet error rate has been

verified to remain under 1%.

The one way average delay of RTP packets

delivery from source to destination, measured at the

two ends of the network, has been used as

performance index for the real time applications.

The averaged delay values are shown in Table 3.

Table 3: Real Time average packets delay (one way).

Data Globalstar DVB-RCS

w/ RBDC

requests

DVB-RCS

w/ CRA

requests

Voice Call 0.27s 0.48s 0.36s

Video Call n.a. 0.61s 0.35s

Globalstar and DVB-RCS with CRA profiles

show similar performances for the voice call average

delay. In fact, in both cases there is no need for

explicit bandwidth requests which introduce an

additional access delay. RBDC request policy has a

bigger average delay compared to the other two

cases, because periodic requests based on estimated

rate must be issued by the terminal to the NCC, thus

increasing the perceived delay by an additional

factor. RBDC is usually associated to a cheaper

contract with satellite operator in comparison with

CRA.

Jitter has resulted limited when simulation

adopted the Globalstar network and the DVB-RCS

with CRA profile. In particular some significant

variations are present during LEO satellite handover,

which was not simulated in our NS set up and

usually occurs every 20 minutes in average.

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Instead, when DVB-RCS system adopts RBDC

access scheme, significant jitter variations are

observed. Please note that RBDC allocation

mechanism is not standardized by DVB and the

reported effect might vary depending on the selected

implementation.

In particular the jitter is due to voice calls

packets pattern, a small packet each 10 ms,

according to the codec standards. Small packets can

trigger the request of a bigger capacity of the

simulated DVB-RCS system which remains

assigned to the terminal for a longer time (in this

simulation 100 ms), resulting in temporary extra

capacity. As consequence, the time needed for the

delivery of packets decreases, resulting in a lower

perceived rate needed. This vicious loop makes

RBDC requests oscillating, together with system

capacity assigned. This affects packet delivery delay

too, which is shown in Figure 3 for a voice call.

Figure 3: Packet delay oscillations on RBDC requests.

A proper de-jitter buffer (Ferrari and Verma,

1991) must be designed at receiver side in order to

compensate the packet arrival latency variations for

this case.

5 CONCLUSIONS

The paper describes a hybrid network for

telemedicine applications adopting satellite networks

and wireless technologies of different kinds. Both

sample real time and non real time applications has

been run in a simulated scenario including all the

described network links, to assess performances.

The results obtained has proven the feasibility of

such an hybrid network including satellite links. In

particular positive results has been obtained with

both GEO and LEO systems, taking into account

limitations of LEO narrow band capacity. The main

differences between different systems and the use of

different request policies for DVB-RCS has been

discussed.

REFERENCES

S. Arenaccio, F. Aversa and M. Luglio, 2006. A satellite

based wireless network for telemedicine: the Telesal

Project. In 12

Ka and Broadband Communications

Conference, Naples, Italy, pp.749-754.

M. Luglio, F. Vatalaro, 2002. The Use of Wireless

Technology for Telemedicine. In Symposium on

Telemedicine In Care Delivery, Technology and

Application, pp.353-359

K. Fall and K. Varadhan, 2007. The NS Manual,

http://www.isi.edu/nsnam/ns/doc/index.html

ETSI EN 301 790, 2003, Digital Video Broadcasting

(DVB); Interaction Channel for Satellite Distribution

System, V1.3.1.

ETSI EN 300 421, 1997, Digital Video Broadcasting

(DVB); Framing structure, channel coding and

modulation for 11/12 GHz satellite service, V1.1.2.

ETSI EN 300 652, 1998, Broadband Radio Access

Networks (BRAN) - HIgh PErformance Radio Local

Area Network (HIPERLAN) Type 1, v1.2.1

IEEE Std 802.15.1, 2002, Wireless MAC and PHY

Specifications for Wireless Personal Area Networks

(WPANs™)

IEEE Std 802.11, 1999, Wireless LAN Medium access

control (MAC) and Physical Layer (PHY)

Specifications.

IEEE Std 802.16e-2005, Air Interface for Fixed

Broadband Wireless Access Systems- Physical and

MAC Layers for Combined Fixed and Mobile

Operation in Licensed Bands.

C. Roseti and E. Kristiansen, 2006, TCP behaviour in a

DVB-RCS environment. In Proceedings of 24th AIAA

International Communications Satellite Systems

Conference (ICSSC), San Diego.

D. Ferrari and D. Verma, 1991. Buffer Space Allocation

for Real-Time Channels in a Packet Switching

Network. Technical report, Information Computer

Science Institute, Berkeley, California.

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