VSAT Tutorial: Architecture, Components, Access, and Protocols

VSAT (Very Small Aperture Terminal) technology plays a crucial role in satellite communication, providing reliable connectivity for a variety of applications, including remote internet access, enterprise networking, and military communications. This tutorial on VSAT covers its network architecture, system components (RF transceiver, LNA, PA, modem, antenna), access techniques, and protocols used for satellite communication.

VSAT, or Very Small Aperture Terminal, is a type of satellite communication system that uses small, ground-based antennas to transmit and receive data from satellites in geostationary or low earth orbits. Typically, the antenna size ranges from 0.75 meters to 2.4 meters, making it ideal for remote and mobile communication.

VSAT is used for many applications which include Internet access, distance education, telemedicine, telephony, banking, video conferencing, and more. It is mainly used where traditional terrestrial communication infrastructure is not available or practical. The interface to the satellite is through a disc antenna, and various frequency bands are used for its operation. The most common frequency bands used in VSAT include C band, Ku band, and Ka band.

VSAT System Components

As shown in the figure, a VSAT system is composed of two main parts: ODU (Outdoor Unit) and IDU (Indoor Unit). ODU and IDU are interfaced using a cable. ODU consists of an LNA, PA, Transceiver, Antenna, and OMT. IDU consists of a Modem, MUX-DEMUX, and interface to telephone/PC/EPABX.

VSAT tutorial - VSAT system overview

RF Transceiver

The RF Transceiver is basically a frequency converter. It consists of two parts: a frequency up-converter and a frequency down-converter. The up-converter converts the modulator frequency (usually either 70/140 MHz) to satellite frequency bands (C, Ku, Ka) as per the satellite in use for a particular location/application. The down-converter does the reverse of the up-converter.

In addition to frequency conversion, both provide gain to the signal as per the VSAT link budget, and the same needs to be adjusted as per the need of the link using attenuation settings available in the RF transceiver in both up/down links.

RF Power Amplifier (PA)

The RF Power Amplifier is the unit which provides power amplification without any frequency change before the signal is transmitted to the antenna and consecutively to the air.

RF LNA

The RF LNA is the low-noise amplifier used in VSAT. As the signal is received by the antenna, it is composed of noise as well as a useful information signal part. In addition, it is very low in power level. The LNA’s job is to boost this low level of signal to a sufficient level considering less amplification of the noise part, so that adequate C/N is maintained.

Satellite Modem

The satellite modem provides two major functionalities in the VSAT. The first one is that it makes the link reliable by adding forward error correction functionality using various techniques such as convolution coding/turbo coding, and so on. The second one is that it does the task of modulation and demodulation.

There are various modulation techniques used in the modem; the most popular among them are QPSK/8PSK.

Antenna

The antenna is basically an electro-magnetic frequency transducer. It sends and receives EM waves of various frequency bands. Antenna diameter and aperture vary from band to band. Hence, C band antenna design is different from Ku band. The signal to be transmitted will be provided to the antenna by the Power Amplifier. The signal received by the Antenna is fed to the LNA/LNB through OMT as shown in the figure.

VSAT Network Architecture

VSAT networks consist of three main components: a central hub station, a satellite, and multiple remote VSAT terminals.

VSAT network architecture is the way a Hub station and/or VSATs are interfaced with a satellite to provide the service. There are five main topologies: broadcast, point to point, point to multipoint (star), mesh, and hybrid.

The satellite itself operates as a relay, receiving signals from the ground terminals and transmitting them to the designated locations, ensuring long range connectivity and minimal signal loss.

Let’s understand each of these VSAT topologies.

VSAT network architecture topologies

• Broadcast: There is a single broadcasting station interfaced with a satellite, and the satellite will relay signals to all the VSATs. Here, a broadcasting station-satellite-all VSATs link exists.
• Point to Point: Two VSATs communicate via satellite using a dedicated assigned channel. So here, a VSAT1-Satellite-VSAT2 dedicated link exists.
• Star: There are three entities: a hub station (usually with a larger antenna), VSATs, and a Satellite. All the communications between VSATs happen through the Hub station. Hence, if VSAT1 and VSAT2 need to communicate, then the link is VSAT1-satellite-Hub-Satellite-VSAT2. Hence, two-hop communication is needed to communicate between any two VSATs in the network.
• Mesh: VSATs can communicate with one another directly, and no Hub station is needed. But each VSAT needs to be complex owing to more functionalities required similar to the Hub station. Also, antenna specifications need to be different than the star type of topology.
• Hybrid: This is the combination of both star and mesh types. Here, a few of the VSATs communicate via the Hub, and a few can communicate directly with one another.

VSAT Access Techniques

These techniques define how multiple VSAT terminals share the same satellite transponder frequency band without interference. The most commonly used access techniques in VSAT are as follows.

1. FDMA (Frequency Division Multiple Access): In FDMA, different VSAT terminals are allocated distinct frequency bands within the same transponder. Each terminal transmits on its unique frequency, enabling simultaneous communication. However, it requires high-frequency stability and bandwidth management.
2. TDMA (Time Division Multiple Access): In TDMA, multiple terminals share the same frequency channel but transmit at different time slots. This technique is efficient for data bursts and reduces bandwidth wastage, making it suitable for applications with variable data rates.
3. DAMA (Demand Assigned Multiple Access): DAMA dynamically allocates bandwidth to VSAT terminals based on demand, ensuring optimal use of the satellite’s capacity. It is ideal for networks with varying traffic patterns, as it assigns resources only when needed, reducing idle channel time.

VSAT Network Protocols

Protocols play a critical role in establishing and managing communication links in VSAT networks. Some of the commonly used protocols include:

• TCP/IP (Transmission Control Protocol/Internet Protocol): Standard suite for internet and intranet communications, facilitating data exchange between remote VSAT terminals and central hubs.
• MPLS (Multiprotocol Label Switching): Used in enterprise networks for efficient routing and network segmentation, ensuring better performance and security.
• ATM (Asynchronous Transfer Mode): Supports data, voice, and video transmission over satellite links with high reliability and quality of service (QoS).
• DVB-S2 (Digital Video Broadcasting - Satellite - Second Generation): A standard for satellite transmission that offers improved bandwidth efficiency, error correction, and flexibility, commonly used for video broadcasting and IP data transmission.

VSAT NMS Protocol

The VSAT Network Management System (NMS) protocol is crucial for monitoring, controlling, and managing the entire VSAT network from a central location. It allows network operators to oversee remote VSAT terminals, hub stations, and the communication links in real-time, ensuring smooth operations and quick fault resolution.

The NMS protocol facilitates remote configuration, status monitoring, performance analysis, and diagnostics of VSAT components, such as RF transceivers, modems, and antennas. It uses standard management protocols like SNMP (Simple Network Management Protocol) or proprietary protocols tailored to specific network architectures to collect performance data and alarms from VSAT terminals.

This centralized management capability improves network efficiency, reduces operational costs, and enhances service quality by providing operators with detailed visibility and control over the entire VSAT network.

To know more on NMS (Network Management System) basics for star configuration VSAT including NMS protocol used for local and remote NMS.

Conclusion

Understanding VSAT network architecture and its system components is essential for optimizing satellite communication performance. With a solid grasp of RF transceivers, LNAs, PAs, modems, and antennas, along with access techniques and protocols, you can design and deploy more efficient VSAT networks. This tutorial serves as a foundational resource for those looking to enhance their knowledge of satellite communication and leverage VSAT technology in various fields.