telecom fundamentals3

8/14/2019 Telecom Fundamentals3

1/37

Copyright 2003 P K Divakara

VSATVSAT

P K Divakaran


2/37

Copyright 2003 P K Divakaran

What is VSAT?What is VSAT?

Very Small Aperture Terminal (VSAT) is a

small fixed Earth station.

It provides a satellite based communication

network

The communication could be voice, data or

videoconferencing


3/37


VSATVSAT


4/37


Why VSAT?Why VSAT?

Irregular terrain, where terrestrial network

rollout is difficult

For remote/ rural area communications

Large geographic area to be covered

For data traffic on Point to Multipoint basis

To avoid Last mile problem


5/37


Applications of VSATApplications of VSAT

ATM Terminals (Banking)

Credit Authorisations

Play Win type of gaming terminals

To network farflung offices of a Company (Voice+ Data)

Inventory Management

Remote location connectivity

Remote process control/ Telemetry

Rural Telephony

Distance education/ Training


6/37


Components of a VSATComponents of a VSAT

systemsystemSpace segment

Satellite and the Transponders on it

(Transponders are leased by VSAT serviceproviders)

Ground Segment

VSAT Earth stations Hub & Control centre


7/37


Satellite CommunicationSatellite Communication

Earth Station

Satellite (Space Segment)

Hub & Control Centre

Earth

Statiouplink

Down

link


8/37


Space SegmentSpace Segment

Space segment

Satellites are generally owned by organisations like

ISRO, INTELSAT, EUROPESTAR etc Transponders are electronic repeaters in the satellite .

The functions of transponders are Signal reception from Earth station

Frequency translation- The frequency of the received signal is

translated to a different frequency called Downlink

frequency. This avoids frequency interference .

Amplification of signals for transmission on the Downlink


9/37



Space Segment

Each transponder has a Bandwidth of 40 Mhz.

Typically there are 12-18 Transponder in INSATsatellites

The Frequency band used in India are 6.725 Ghz to

7.025 GHz Uplink and 4.5 Ghz to 4.8 Ghz

Downlink . (Extended C Band)

Ku band (11/14 Ghz) has just been permitted for VSAT

in India. Ku band needs only small antenna size Earth

station


10/37



Extended C band. Unique to India

Ku Band

Uplink 14 Ghz to 14.5 Ghz

Downlink 10.95 Ghz to 11.7 Ghz

Susceptible to rain attenuation


11/37


Types of VSAT NetworksTypes of VSAT Networks

A

B

A & B are VSATs. For communicating between them , they

have to go thru the Hub.

Hub

STAR TOPOLOGY

Path thru Satellite


12/37


VSAT Star TopologyVSAT Star Topology

VSAT A


Hub & Control Centre

VSAT

Buplink

Down

link

Two hops thru satellite between A &B


13/37


Star TopologyStar Topology


14/37



Makes use of a Central Hub station for

communication between VSATs

Satellite double hop transmission is used

Time delay introduced due to double hop

transmission

Hence more suited for data applications

than voice


15/37



Smaller VSAT antenna size can be used (1.2m).Comparitively less costly VSATs

High Central Hub investment . Antenna size 5-11m

Hub is installed and maintained by the VSATOperator and shared by different VSAT users

(Shared Hub)Or the Hub could be for a Single large User

community (Dedicated Hub). For eg. VSATnetwork of National Stock Exchange with more

than 800 VSAT terminals


16/37


Mesh TopologyMesh Topology

A B

A & B are VSATs which communicate with each other directly

without going through a Hub. Only one Hop transmission

thru satellite is required

Thru satellite


17/37



VSAT A


Network Control

System

VSAT

Buplink

Down

link


18/37



The transmission between two VSATs isdirect thru a one hop transmission thru

satelliteThe Network control centre assists in

setting up the communication but is not partof the traffic path

One hop transmission is suited for voiceand video application because of less delays


19/37


VSAT TechnologiesVSAT Technologies

TDM/TDMA (Time Division Multiplex/

Time Division Multiple Access)

DAMA (Demand Assignment Multiple

Access


20/37


TDM/TDMATDM/TDMA

Star Topology

Double hop Satellite system

Suited for interactive data applications

speed upto 19.2 kbps

All VSATs in the system share satellite

resources on a Time slot basis


21/37


TDM/TDMATDM/TDMA

Communications method VSATs to HubInbound Routes using

TDM/TDMA on a carrierTime slots assigned to each VSAT for transmission

on the Inbound RoutesSeveral VSATs share an Inbound Route

Hub to VSATs Outbound Route on aCarrier

Time synchronisation very critical


22/37


Types of TDMATypes of TDMA

Fixed Assigned TDMA Each VSAT assigned a Fixed Time Slot for

transmission on the In Route

Used when traffic on all Vsats are balanced

Slotted Aloha VSATs can access any Time Slot at any time on the In

Route randomly

Suitable for short bursty transmissions

Need to have collission resolution mechanismbetween VSAT transmissions


23/37


Types of TDMATypes of TDMA

Dynamic Reservation

Switch between a Slotted Aloha mode to

Fixed Assigned Mode depending on trafficgrowth

Performed dynamically depending on traffic

profile so that VSATs have an assured amount

of Time slots for transmission


24/37


DAMADAMA

Mesh topology (Single hop)

On Demand a pair of available Satellite Carrier

frequencies are assigned between two VSATs bythe Network Control System

After use, the Carrier frequencies are returned acommon pool

Ideally suited for Voice / data traffic in batchmode

Optimum utilisation of Satellite resources


25/37


DAMADAMA

Network Control System (NCS) Receives requests for Frequency Channel from a VSAT

over a shared DAMA common signalling channel

NCS checks the validity of the request and allocatespair of Carrier frequencies between the RequestingVSAT and the Destination VSAT

After the call, NCS returns the frequencies to a

common poolNCS only performs control functions and System

performance monitoring and is NOT a part of theTraffic path


26/37


PAMAPAMA

Pre Assigned Multiple Access (PAMA)

Mesh Topology

VSATs are pre allocated designated Carrierfrequencies

Equivalent to Terrestrial Leased connections

No call set up delays For constant use for high traffic volumes


27/37


VSAT TerminalsVSAT Terminals

Constituent Parts

Outdoor Unit

Indoor Unit


28/37


Outdoor UnitOutdoor Unit

Antenna System Feed horn

Directs Transmitted power towards the antenna dish( Reflector) and collects the Received power from itMounted on the antenna frame at its focal point by

means of support arms.

Antenna Dish (Reflector)Varies in size from 1.2 m diameter onwardsProvides Signal Gain in both Transmit & Receive

directions


29/37


VSATVSATRFT

Feed horn


30/37


Outdoor Unit (ODU)Outdoor Unit (ODU)

RFT

The Radio Frequency Trans Receiver unit

Connected to Feed Horn and mounted on theAntenna Frame


31/37


RFTRFT

IF from

Indoor Unit(70 Mhz)

Up Converter(70 Mhz to

C Band)

Power

Amplifier

Feed HornLow Noise

Amplifier

(LNA)

Down

Converter ( C Band to

70 Mhz IF)

IF to IndoorUnit (70 Mhz)

Trans

Receive


32/37


Indoor UnitIndoor Unit

Consists of

Modulators

Modulates the User Data/ signal to an IF of 70Mhz towards the RFT

Demodulators

Demodulates the IF received from RFTDetermines the Access Scheme (TDMA,

DAMA etc)


33/37


Indoor Unit (IDU)Indoor Unit (IDU)

Interface towards the user end equipment

like LAN, PABX, Routers, Multiplexors


34/37


IDUIDU

Access

Control&

User

Interface

Modulators

Demodulators

OD

U

Customer

Equipment

LAN, PABX etc


35/37


Regulatory IssuesRegulatory Issues

CUG

Space Segment

Licence fee


36/37


VSAT Service Providers inVSAT Service Providers in

IndiaIndiaHughes Escorts Communications Ltd

HCL Comnet

Comsat Max

Bharti Broadband

Telstra Vcom


37/37


VSAT equipment VendorsVSAT equipment Vendors

Gilat

Hughes

Viasat

telecom fundamentals3

Documents