Why Latency Matters to Mobile Backhaul - O3b Networks

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Why Latency Matters to Mobile Backhaul For instance, the receiver advertised a maximum window size of 10 segments. The transmitter will send the 10 segments (at a variable rate described in the congestion avoidance) and will then stop until ACK is received. For each ACK (if window size stays constant) received, the transmitter may send another segment. The algorithm may advertise a variable window size depending on the actual state of the receiver (i.e. buffers filling or emptying), allowing it to transmit more or fewer segments and vary the actual throughput. Congestion Control Flow control enables the two end devices to regulate the transmission throughput, but it does not take into account the status of the network links between them. Congestion control mechanisms are required to regulate the speed of the data transmission and the quality of the network link. Using a slow start mechanism, the sender begins to transmit TCP segments at a low rate, gradually increasing the number of segments simultaneously transmitted until the full window size is reached or congestion is detected. In case of congestion, an avoidance mechanism will drastically decrease the rate and initiate the step increase again, creating a saw-tooth effect. Various TCP flavors (Reno, Vegas, Cubic, Compound) have variations in the algorithm used to define the increasing and decreasing rate, based on a maximum segment size and/or queuing delay. Other optimizations include retransmission of selected segments and fast recovery to increase the transmission rate more quickly. TCP maximum throughput estimation TCP maximum throughput can be estimated using a formula based on TCP RENO (RFC2001) as described by Padhye et. al. in "Modeling TCP Throughput: a Simple Model and its Empirical Validation" The model used is the following one: ⎛ ⎜Wmax B( p) ≈ min , ⎜ RTT RTT ⎝ 2bp 3 + T min 1, 0 1 ( ) ( ) 3 bp 2 3 p 1+ 32 p ⎟⎟ 8 ⎠ ⎞ www.o3bnetworks.com 18
Why Latency Matters to Mobile Backhaul Where B(p): approximate model of TCP throughput [packet/s] Wmax: maximum window buffer size of receiver [packets] RTT: Round Trip Time [sec] b: number of packets that are acknowledged by a received ACK p: probability that a packet is lost T0: time-out for re-transmitting an unacknowledged (lost) packet [sec] RTT should take into account the additional delay due to serial transmission of the packet (especially for low rate interfaces) and various TCP receive window sizes depending on the Operating System of the end-device. TCP transfer duration estimation Overall duration of file transfer can be approximated using the formula given by N. Dukkipati et al in “An argument for Increasing TCP’s Initial Congestion Window” S being the size of the file C being the bottleneck link-rate RTT being the two-way end-to-end latency being 1.5 if ACK are delayed of 2 else. Init_cwnd is the initial congestion windows, generally no more than three segments or about 4kB. The transfer time calculated does not take into account packet loss (no packet loss) for simplicity. Packet loss further increases the duration of transfer due to retransmission of error packets and congestion control mechanisms. www.o3bnetworks.com 19
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Why Latency Matters to Mobile Backhaul - O3b Networks

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