Sencore AFN Series User Manual page 28

devices on the same fast switch on a LAN, the RTT should be almost 0. Within the
Continental US, RTT over the Internet can vary depending on the link and distance but
can be in the 60 to 100 ms range. Transoceanic RTT can be 60-200 ms depending on
the route. RTT is used as a guide when configuring Bandwidth Overhead and Latency.
To find the RTT between two devices, the ping command can be used.
○ For example: ping 198.51.100.20
○ Response (RTT = 6.633 ms)
■ 56 data bytes 64 bytes from 198.51.100.20: seq=1 ttl=64 time=6.633 ms
● RTT Multiplier - The RTT Multiplier is a value used in the calculation of SRT Latency. It
reflects the relationship between the degree of congestion on a network and the Round
Trip Time. As network congestion increases, the rate of exchange of SRT control
packets (as well as retransmission of lost packets) also increases. Each of these
exchanges is limited by the RTT for that channel, and so to compensate, SRT Latency
must be increased. The factor that determines this increase is the RTT Multiplier, such
that:
○ SRT Latency = RTT Multiplier * RTT
○ The RTT Multiplier, then, is an indication of the maximum number of times SRT
will try.
● Packet Loss Rate - Packet Loss Rate is a measure of network congestion, expressed
as a percentage of packets lost with respect to packets sent.
○ Constant loss refers to the condition where a channel is losing packets at a
constant rate. In such cases, the SRT overhead is lower bound limited, such that:
**Minimum Bandwidth Overhead = 1.65 * Packet Loss Rate**
○ Burst loss refers to the condition where a channel is losing multiple consecutive
packets, up to the equivalent of the contents of the SRT latency buffer. In such
cases, the SRT overhead is lower bound limited, such that:
■ Minimum Bandwidth Overhead = 100 ÷ RTT Multiplier
○ Burst losses that last longer than the SRT Latency will result in stream artifacts.
SRT Latency should always be set to a value above the worst case burst loss
period.
○ Burst losses that last longer than the SRT Latency will result in stream artifacts.
SRT Latency should always be set to a value above the worst case burst loss
period.
● Bandwidth Overhead - The control packets associated with an SRT stream do, of
course, take up some of the available bandwidth, as do any media packet
retransmissions. When configuring an SRT stream, a Bandwidth Overhead value needs
to be specified to allow for this important factor.
○ The portion of audio and video content in the stream is determined by their
respective bit rate settings, which are configured on the audio and video
encoders themselves. SRT Bandwidth Overhead is calculated as a percentage of
the A/V bit rate, such that the sum of the two represents a threshold bit rate,
which is the maximum bandwidth the SRT stream is expected to use.
○ The SRT Bandwidth Overhead is assigned as a percentage, based in part on the
quality of the network over which the stream will be traversing. Busier networks
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AFN Platform – User Manual