I have a customer who was very happy with the wireless internet service that was their only option in a medium sized New Mexico town. Based on that good experience, they took satellite internet service at a new location. The users at the new location immediately complained that they could not perform their job duties due to the slowness of the internet connection. The fundamental problem is that the radio transmitting towers in the neighboring hills are only a few miles away, but the satellite is tens of thousands of miles away. Even at the speed of light, the distance affects the thoughput.
In some areas, satellite is the only option, but here is what satellite users may likely have to deal with...
In order for a two-way satellite service to perform
properly in conjunction with traditional terrestrial
networks (Internet, Intranet), satellite data networks must
employ special techniques to deal with the extra 44,600-
mile space segment of the connection. Without those
steps, the increased latency, the time required to
traverse the extra distance, means that TCP severely
limits performance.
The Internet relies on the Transmission Control Protocol
(TCP) to ensure packet delivery without errors. TCP
works by sending a certain amount of data, the "window
size," then waiting for the receiver to send an
acknowledgment of receipt. With TCP, the sender
cannot transmit more data until it has received an
acknowledgment. If an acknowledgment does not
arrive in a timely manner, TCP assumes the packet was
lost (discarded due to network congestion) and resends
it. When packets go unacknowledged, TCP also slows
the transmission rate to reduce congestion and to
minimize the need for retransmissions.
TCP/IP sessions start out sending data slowly. Speed
builds as the rate of the acknowledgments verifies the
network's capacity to carry more traffic. This is known
as slow-start, followed by a ramp-up in speed. The
speed of the connection builds until the sender detects
packet loss from a lack of an acknowledgment. This
allows TCP to achieve the fastest practical data transfer
rate for the conditions present on the network.
Terrestrial networks typically have round-trip latencies in
the range of 35 to 100 ms. Satellite networks, due to the
distance of geosynchronous satellites above the
equator, require 550 ms or more. Some satellite
connections have much higher latencies. Depending
upon the satellite hardware and subscription policy of the
service provider, latencies of 800 ms to as much at
2,000 ms or more can occur.
TCP interprets the additional satellite transit time as
network congestion. If uncorrected, this effect causes
the network to send all additional packets at the slowstart
rate.
Current satellite data networks employ a technique
referred to as TCP acceleration or IP spoofing to
compensate for the extra time required to transit the
space segment. Special equipment at the carrier's main
satellite hub appears to terminate the TCP session, so it
appears to the sender as the remote location. In
actuality the device at the satellite hub acts as a relay or
forwarder between the originating terrestrial location and
the remote satellite unit.
When the spoofing equipment receives Internet traffic
destined for a remote satellite location, it immediately
acknowledges receipt of the packet to the sender so
more data packets will follow promptly. This way the
sender never experiences the actual latency to the
remote site because acknowledgments return rapidly.
As a result, TCP moves out of slow-start mode quickly
and builds to the highest practical speed.
To prevent packets from being acknowledged twice, the
spoofing equipment suppresses acknowledgments from
the remote site. In this way, computers behind a satellite
link communicate seamlessly and efficiently with servers
on the terrestrial Internet.
IPsec VPNs not only encrypt the data portion of packets,
they also encrypt the TCP port number and IP address
of the sender's computer. (Think of TCP port as the
apartment number while the IP address is that of the
building.) Consequently, only the VPN software at the
remote site can decipher where packets originated and
acknowledge receipt of data.
Popular IPsec VPNs, therefore, defeat TCP acceleration
over satellite links because ground stations cannot
adjust the fields in the header when those fields are
encrypted. This situation requires that
acknowledgments transit the space segment twice (over
and back) and results in substantial performance
degradation. The impact on performance increases as
the latency rises.
Networking technology is improving constantly, so hopefully there will soon be a ingenious solution to this problem. But for now, bear in mind that wireless, wifi, cellular and satellite are not all the same thing. Try to do some research and speak with some existing satellite users in the area before you sign any contracts.
