Why Latency Matters to Mobile Backhaul - O3b Networks

Why Latency Matters to Mobile Backhaul
The increase in latency of 420ms between O3b’s MEO solution and a GEO
satellite has a significant impact on interactive services.
As demonstrated, revenue loss is a significant consequence of the quality
degradation of a users’ experience caused by download delays. End users
perceive latency increases of a few hundred milliseconds in a negative
way, which translates into a loss of business for providers and advertisers.
Users are unaware whether the delay in response time is due to a slow
server or a satellite link. The end result in both cases is the same: a poor
user Quality of Experience.
TCP in a nutshell
The Internet uses the Transmission Control Protocol (TCP) to provide end-toend
services. In particular, TCP ensures the error free sequenced delivery of
data transmitted from a source to a destination.
Data is sent by the transmitter to the receiver in fragments called TCP
segments. A maximum size is defined for the segments. The receiver will
confirm the correct reception of sent TCP segments by acknowledging them
(sending the transmitter a response “ACK message” with a correct
sequence number). If an ACK message is not received or if it is not correct,
the transmitter will return the message ensuring a reliable transfer. At the
end of the session, it is correctly closed by exchange of FIN messages.
The key factor in this context is that TCP will only send a limited amount of
data before it needs an acknowledgement. The amount of data is governed
by the TCP buffer size, which in Windows 7 is 64 kbytes. If 64 kbytes of
data has been sent but the acknowledgements have not yet been received,
TCP will wait for an acknowledgement before it sends another packet. This
acknowledgement mechanism is what limits the transmission rate over
high latency links.
Non interactive data services
The major complexity of TCP comes from its flow and congestion control
mechanisms. The TCP flow control mechanism adjusts the TCP window size
(the number of segments that can be sent and not yet be acknowledged)
depending on the status of the end devices and congestion/latency on the
link during the transfer.
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