Why Latency Matters to Mobile Backhaul - O3b Networks

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Why Latency Matters to Mobile Backhaul Distinct Queries/User Query Refinement Revenue/User Any Clicks Satisfaction Time to Click (increase in ms) 50ms – – – – – – 200ms – – – -0.3% -0.4% 500 500ms – -0.6% -1.2% -1.0% -0.9% 1200 1000ms -0.7% -0.9% -2.8% -1.9% -1.6% 1900 2000ms -1.8% -2.1% -4.3% -4.4% -3.8% 3100 Figure 8 – Bing table of latency impact for user search service Phil Dixon (Shopzilla.com) presented results from the Shopzilla site redesign at the Velocity 2009 Conference. He illustrated that a reduction in page load time from 7 to 2 seconds resulted in a 25% increase in page views and a 7-12% increase in revenue. Below, a mobile subscriber is using a HSPA session over a satellite backhaul to access Internet data services. Figure 9 - End to end latency for data services using O3b backhaul Figure 10 - End to end latency data services using GEO satellite backhaul www.o3bnetworks.com 10
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. www.o3bnetworks.com 11
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Why Latency Matters to Mobile Backhaul - O3b Networks

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