Packet Delay Times

Latency is defined as the amount of delay, measured in milliseconds, that occurs in a round-trip data transmission. Latency is unavoidable in any Internet connection and it can only be minimized using a better managed network. 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 (by routers or servers for example). The GEO satellites used for two-way Internet service are located approximately 23,000 miles above the equator. This means that 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 for a total round trip of over 90,000 miles. Depending on your latitude, this distance to the satellite could be even greater.

Each time a data packet ‘hops’ (i.e. handled by a device along the path) several milliseconds of latency are introduced. The physics involved account for approximately 550 milliseconds of latency, a limitation shared by all satellite providers. This latency can negatively affect the performance of many business applications and software that requires real-time user input.

In total the user may experience minimum packet roundtrip times (before experiencing a reaction after action) of 500ms or 0.5 seconds. In reality and due to the fact that the satellite or internet sites are under load, round trip times may increase up to 1200 or 1300 ms, in particular when TDMA network technology is getting used. 

SCPC technology is providing stable latencies however, up to the point when the satellite channel is becoming full. TDMA due its ATM type of packet delivery, already jitters and increases its packet latency on low loadings of the satellite channel.

This delay, or latency as it is often called, requires satellite Internet carriers to use special software acceleration techniques to deliver a high-speed satellite Internet connection. The delay cannot be influenced as it is physically determined by the distance between the satellite and the earth.

In case of burstive type protocols, such as HTTP and FTP clever caching technologies in satellite terminal and hub equipment, can provide acceleration through an intelligent prefetch.

It is a common perception that TCP does not work well over long-delay networks, particularly satellite. Although this is true in comparison to traditional T-1 or DSL lines, latency can be minimized to an extent where even most mission-critical applications can work perfectly over satellite.