It’s All about Latency

A few years ago, my wife and I decided to move out of our apartment and into a house about 4 miles away.  We owned a 1999 GMC Jimmy SUV at the time and figured we could move all of our boxes using the Jimmy without renting a U-haul.  The move took about 10 round trips and 10 hours.  We used a very terrestrial-like means of moving: logging short round trip mileage!  If we had moved to another city about 300 miles away or so it would have take the same amount of trips (10), but much longer than 10 hours (probably weeks using the GMC Jimmy method).  And there lies the rub.  The Jimmy represents your bandwidth/speed and your capability of carrying and delivering data packets.  The mileage, however, represents part of what effects latency (round trip time or distance).  The speed of the Jimmy is the same either way, the capability to carry boxes (data packets) is the same, but the distance travelled is the difference.   Satellite links have huge travel time and terrestrial networks have very low travel time by comparison.

Now lets talk about the boxes or data packets.  They contain information critical to a client and the host.  For example, when you type in the web address for yahoo.com, data packet is sent to Yahoo’s servers requesting their main website.  In return, Yahoo sends links, images, and formatting back to your computer, allowing you to view their web page.  Unfortunately, it is not one transmission back and forth but many smaller transactions so that your computer and Yahoo’s server can manage the data and keep track of the data in an efficient manner.  The Jimmy could have moved us into our new place in one trip, if we had just a few boxes, but since we had many boxes, it took multiple deliveries and round trips.  So latency is not only effected by mileage but the amount of data packets and deliveries (IP transactions) that occur in a given route between your computer and the host (in the previous example Yahoo was the host).  The more handshakes a host requires due to the protocol being used, the higher the latency and the more lag you experience.

A very solid explanation about latency and application testing

Satellite networks are considered high-latency networks because the round trip distance exceeds 100,000 miles for a single data packet transaction.  Most terrestrial based networks never exceed 5,000 miles for a single data packet transaction.  The good news is that satellite communication technology has come a long way to lower latency through spoofing, caching, and TCP/IP acceleration.  Your Internet applications will still work over satellite but they will just have more lag time, but only in seconds, not minutes for most applications.  If you have a specific Internet application that is proprietary to your business or personal needs, we recommend testing the application or protocol using one of NIASAT’s demo/trial programs or by contacting NIA and setting up testing at one of our satellite service testing offices.

-Josh Golding
NIA, Inc.

Did you know?: The first geosynchronous communication satellite, Syncom 2, was launched in 1963. The world’s first commercial communication satellite, called Intelsat I (nicknamed Early Bird), was launched into synchronous orbit on April 6, 1965 (Excerpt from Wikipedia).

Further Reading:

http://www.starlancs.com/EducateMe/educate_vsat_latency.html

http://compnetworking.about.com/od/speedtests/a/network_latency.htm

The footprint of a communications satellite is the ground area that its transmitter/receiver offers coverage, and determines the satellite dish diameter required to receive each transmitter/receiver’s, or transponder, signal. There is usually a different map for each transponder (or group of transponders) as each may be aimed to cover different areas of the ground. Footprint maps usually show either the estimated minimal satellite dish diameter required or the signal strength in each area measured in decibel Watts, or dBW. (Wikipedia)

What is the easy answer?  Satellite Signal Coverage Area, or where the signal “touches down”.  Here are some sample maps of satellite coverage areas, or FOOTPRINTS, from Intelsat and iDirect.

http://www.intelsat.com/flash/coverage-maps/covmaphome.htm

Horizons-1 @ 127.0 Degrees East

IS-25 @ 328.5 Degrees East