Why Latency Matters to Mobile Backhaul - O3b Networks

Why Latency Matters to Mobile BackhaulThe increase in latency of 420ms between O3b’s MEO solution and a GEOsatellite has a significant impact on interactive services.As demonstrated, revenue loss is a significant consequence of the qualitydegradation of a users’ experience caused by download delays. End usersperceive latency increases of a few hundred milliseconds in a negativeway, which translates into a loss of business for providers and advertisers.Users are unaware whether the delay in response time is due to a slowserver or a satellite link. The end result in both cases is the same: a pooruser Quality of Experience.TCP in a nutshellThe Internet uses the Transmission Control Protocol (TCP) to provide end-toendservices. In particular, TCP ensures the error free sequenced delivery ofdata transmitted from a source to a destination.Data is sent by the transmitter to the receiver in fragments called TCPsegments. A maximum size is defined for the segments. The receiver willconfirm the correct reception of sent TCP segments by acknowledging them(sending the transmitter a response “ACK message” with a correctsequence 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 theend 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 ofdata before it needs an acknowledgement. The amount of data is governedby the TCP buffer size, which in Windows 7 is 64 kbytes. If 64 kbytes ofdata has been sent but the acknowledgements have not yet been received,TCP will wait for an acknowledgement before it sends another packet. Thisacknowledgement mechanism is what limits the transmission rate overhigh latency links.Non interactive data servicesThe major complexity of TCP comes from its flow and congestion controlmechanisms. 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 thelink during the transfer.www.o3bnetworks.com 11