VSAT FAQs - latency, spoofing, shared / dedicated bandwidth

Spoofing & Latency

What is spoofing?

Why is spoofing important? The entire Internet is based on TCP/IP. TCP (Transmission Control Protocol) manages and controls transmissions using IP (Internet Protocol). TCP sends data and looks for acknowledgments (receipts) sent back from the receiving end to indicate that everything was received. If the acknowledgments are not received, TCP resends the packets and slows down its transmission speed for future data. TCP expects these acknowledgments to be received within a certain time frame. Because of the long round-trip (90,000+ miles) that the packets must travel over the satellite link and back, the acknowledgments are delayed by several hundred milliseconds. If uncorrected, this delay would cause TCP to throttle back its speed dramatically.
Spoofing is accomplished by special NOC equipment that causes TCP acknowledgments to be returned to the sender very quickly. It does this by spoofing (pretending to be the remote site) and acknowledging the packets instantly, at the same time as it forwards the packets to the remote site. TCP sees rapid acknowledgments and therefore ramps up its speed quickly.

It is in this manner that multi-megabit speeds are made possible over satellite.

Latency: in plain English

Latency is the amount of delay, measured in milliseconds, found in a round-trip data transmission. Not directly related to speed, latency can be an issue with all networks including satellites.
Latency is caused by several factors, including the number of times the data is handled along the transmission path, for instance, by a router or server. Each time a data packet is handled by a device along the path (called a "hop") several milliseconds of latency are induced.

More importantly in the satellite world, latency is caused by the distance that the signal must travel. The satellites used for two-way Internet service are located approximately 23,000 miles above the equator.

This means a round-trip transmission travels 23,000 miles to the satellite, 23,000 miles from the satellite to the remote site, and then as the TCP/IP acknowledgment is returned, another 46,000 miles on the return trip. That's a total round trip of about 92,000 miles. Even at the speed of light, this accounts for more delay (in milliseconds) than found in a terrestrial network.

Most Internet applications including web browsing, email, FTP etc., work in their normal manner even when traversing this long distance and user experiences are very positive. Certain applications, such as Voice Over IP are affected but also work. Some applications, such as online gaming are not recommended.

Citrix, and terminal emulators without local-echo, can also be affected by latency depending on the underlying application and configuration. If you have an application that is particularly sensitive to latency, it is highly recommended that customers check with their software vendor to confirm how specific applications are affected.