Internet Insight: Getting Data Up and Down

Compared with terrestrial communications, satellite links can be tricky to manage.

Compared with terrestrial communications, satellite links can be tricky to manage. For instance, satellite service providers must deal with the weather. Heavy rain or snow can slow transmissions and increase the number of dropped packets. Fortunately, new algorithms can minimize weather problems.

Another problem is distance. A signal travels from a users desktop to a network switch to a very small aperture terminal—a satellite dish—that exchanges information with a satellite orbiting the planet 22,300 miles up. Each time a data packet is shipped, it travels from the sending system to the satellite and then down to a receiving dish.

The minimum time to make the trip is 0.25 seconds. However, the framing, queuing and switching that TCP/IP relies on to ensure that packets arrive at the proper destination can increase that time to as much as 0.5 seconds.

TCP/IP was designed for networks where delays are measured in milliseconds, rather than tenths of a second. So, satellite delays can disrupt the acknowledgements and handshaking that are at the core of the network protocol. When a signal becomes garbled, TCP/IP slows down its transmission rate. In a satellite network, that can mean transmissions crawl.

However, vendors have developed workarounds for the problem. These include sophisticated error-checking mechanisms that compensate for weather problems. Packet spoofing fools a sending system into thinking it has received acknowledgements for shipped packets and helps avert transmission delays.

Satellite service vendors are now using these networking techniques to deliver broadband IP services offering performance comparable to that of land lines.