UPLINK Technology Group - VSAT Terms Glossary

VSAT Terms Glossary

AZIMUTH - The horizontal angle that the dish must turn to face a particular satellite. (Also see Elevation and Skew).

BACKHAUL- A terrestrial circuit that connects customer premises equipment to a central office, or a customer office to the Network Operations Center.

C-BAND - One of two common satellite frequency bands (Ku-band is the other). C-band earth stations use the 6 GHz frequency band to transmit and the 4 GHz frequency band to receive.

DNS – “Domain Name Server” (or System or Service), pronounced as three letters. An Internet service that translates domain names into IP addresses.

Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.datastormusers.com translates to 63.161.122.77.

The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.

EIRP - Equivalent Isotropic Radiated Power. A measure of the strength of the signal radiated by an antenna. On the downlink EIRP varies over the footprint.

ELEVATION - The vertical angle a dish must raise to point at a particular satellite.

FDMA - Frequency Division Multiple Access. FDMA is a technique used to allow more than one earth station to share the bandwidth of a satellite transponder. Each earth station is assigned a specific carrier or set of carriers for transmission over a specific portion of the bandwidth.

GEOSTATIONARY SATELLITE - A satellite orbiting the Earth 22,300 miles above the equator. The orbiting velocity matches the Earth's rotational speed, thereby making the satellite appear stationary with respect to Earth. Satellite positions match the degrees of longitude on the Earth beneath the satellite.

GEOSYNCRONOUS SATELLITE – A satellite that is 22,300 miles above the Earth, but is not necessarily centered over the Equator.

IP ADDRESS - Internet Protocol, pronounced as two separate letters. IP specifies the format of packets, and the addressing scheme used on the Internet. Every single piece of hardware on the Internet has its own unique IP address.

IP by itself is something like the postal system. It allows you to address a package and drop it in the system, but there's no direct link between you and the recipient. TCP/IP (pronounced as 5 letters), on the other hand, establishes a connection between two hosts so that they can send messages back and forth for a period of time.

IP addresses are in the form of a 32-bit numeric address written as four numbers separated by periods. Each number can be zero to 255. For example, 10.249.101.24 could be an IP address.

Within a LAN, you can assign IP addresses at random as long as each one is unique; addresses which are public to the Internet must be within assigned ranges in order to avoid duplication. The authorities that assign public Internet addresses have designated certain ranges as never to be used on the Onternet; by convention, those are normally used as private addresses on a LAN. The ranges for private addresses are all addresses starting with 10 (e.g. 10.200.44.36), addresses between 172.16.0.0 and 72.31.255.255, and addresses between 192.168.0.0 and 192.168.255.255.

Addresses such as www.groundcontrol.com are converted to their numeric equivalent by DNS servers (Domain Name Servers).

Kbps - Kilobits-per-second

KBps– Kilobytes per second

KA BAND - (Pronounced "Kay-ay band") A satellite frequency band that uses 30 GHz signals to transmit and 20 GHz signals to receive.

KU BAND - The 10/14 GHz frequency band.

LATENCY (in Satellite communication) - Internet traffic travels at the speed of light. That means that a request from a web-surfer in California to a web server in New York takes about 0.03 seconds (30 milliseconds) to make the round trip. In practice, because the request may pass through a dozen or more routers and switches, each with some delay, it will average about 90 milliseconds for a good connection.

With satellite connections the distances are so vast that even light speed isn't fast enough to not be noticeable. The satellites used are 22,000+ miles above the equator, so the round trip in on the order of 95,000 miles from North America. That means there will be a round trip of over 500 milliseconds, not counting the normal Internet switching and routing. The satellite switches are also relatively slow (they route the signal between up and downlinks), so that the fastest possible connection is about 650 milliseconds.

This time lag is called latency, and is used to explain some of the problems for such Internet uses as VoIP and real-time gaming.

LEO - Low Earth Orbiting Satellite. These satellites (normally used with satellite telephone service) orbit the planet many times a day. A satellite dish must move across the sky to track the orbiting satellite.

LINK BUDGET - a calculation based on satellite power, earth station power and antenna size which describes the parameters of a satellite communications link.

Mbps – Mega bits per second.

MBps – Mega bytes per second.

NOC - "Network Operations Center". The location of the super large satellite dishes that is the central hub of traffic for satellite provider.

PING – A utility to determine whether a specific IP address is accessible, and how long it takes to communicate with that address. It is also used to describe the action of using the utility. (On Windows XP, click on “Start” then “Run” and type in “CMD” and press return. The DOS line prompt allows you to type the address you wish to ping. The command would be typed (without the quotes) as: “ping 66.180.30.212”.

Ping works by sending a packet to a specified address and waiting for a reply.

POLARIZATION (Cross Pol) – Satellite dishes transmit and receive on horizontal and vertical planes. These planes must be lined up with the orbiting satellite perfectally, or the signal is quickly lost. The process of cross polarization is the process of aligning the satellite dish to be aligned with the satellite.

TDM - Time Division Multiplexing, a satellite channel divided into time slots for transmission from a central hub to a group of VSAT sites. Individual time slots can be addressed specifically to one or more VSATs. Usually used as the access protocol for the outbound channel (hub to VSAT) of a VSAT system.

TDMA - Time Division Multiple Access, a contentious access satellite channel which is divided into time slots for sharing between VSATs at different sites usually for connection to a hub. Usually used as the access protocol for the inbound channel (VSAT to hub) of a VSAT system.

TRANSPONDER SKEW - The rotation of a dish around its center point. Seen as a clockwise or counter-clockwise rotation when facing the front of the dish. Skew is needed to align the antenna with the polarization of the satellite signal when the dish is not located on the same longitude as the satellite. When a dish is west of the satellite, the skew is a negative number, and from the front of the dish the left edge will be higher than the right. When the dish is east of the satellite it will have a positive skew, with the left edge lower than the right edge.

STAR NETWORK - A network in which data flows to and from a central hub. With a Satellite Network, all customer transmitted traffic sent to the orbiting satellite is routed down to the NOC where it enter the Internet from a single point. (As a note, a network that connects one user satellite dish to any other user dish is called a Point-To-Point satellite network. In this case, the router is in the orbiting satellite).

TRANSPONDER - The receivers of the orbiting satellite that translates incoming broadcasts, amplifies the signal and rebroadcasts it down to the NOC (Network Operations Center).

VOIP - Voice over IP, pronounced as four letters. A category of hardware and software that enables people to use the Internet as the transmission medium for telephone calls. For users who have fixed-price Internet access, Internet telephony can provide low-cost, or even free, phone service.

VoIP does not have the same quality as a typical phone line, and for Internet users the low upload speeds and high latency make telephony a less-than-wonderful experience. A variety of hardware and software products that buffer and enhance the voice signal are available, but none have shown to be better than mediocre at this time.

VSAT - Very Small Aperture Terminal, business premises earth stations, term used to describe almost any direct to business satellite earth station, one-way or interactive. Typically however, it is used for data systems. Can use TDM/TDMA, SCPC or DAMA access schemes.

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