Transmission time
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In telecommunication networks, the transmission time is the amount of time from the beginning until the end of a message transmission. In the case of a digital message, it is the time from the first bit until the last bit of a message has left the transmitting
node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines, ...
. The packet transmission time in seconds can be obtained from the ''packet size'' in bit and the
bit rate In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
in
bit/s In telecommunications and computing, bit rate (bitrate or as a variable ''R'') is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s), often in conjunction w ...
as: :Packet transmission time = Packet size / Bit rate Example: Assuming 100 Mbit/s Ethernet, and the maximum packet size of 1526 bytes, results in :Maximum packet transmission time = 1526×8 bit / (100 × 106 bit/s) ≈ 122 μs


Propagation delay

The transmission time should not be confused with the propagation delay, which is the time it takes for the first bit to travel from the sender to the receiver (During this time the receiver is unaware that a message is being transmitted). The
propagation speed The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, ...
depends on the physical medium of the link (that is,
fiber optics An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means to ...
,
twisted-pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
copper wire, etc.) and is in the range of 2\times10^8 meters/sec for copper wires and 3\times10^8 for wireless communication, which is equal to the
speed of light The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. The ratio of actual propagation speed to the speed of light is also called the
velocity factor The velocity factor (VF), also called wave propagation speed or velocity of propagation (VoP or of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibr ...
of the medium. The propagation delay of a physical link can be calculated by dividing the distance (the length of the medium) in meter by its propagation speed in m/s. :Propagation time = Distance / propagation speed Example: Ethernet communication over a UTP copper cable with maximum distance of 100 meter between computer and switching node results in: :Maximum link propagation delay ≈ 100 m / (200 000 000 m/s) = 0.5 μs


Packet delivery time

The ''packet delivery time'' or latency is the time from when the first bit leaves the transmitter until the last is received. In the case of a physical link, it can be expressed as: :Packet delivery time = Transmission time + Propagation delay In case of a network connection mediated by several physical links and forwarding nodes, the network delivery time depends on the sum of the delivery times of each link, and also on the packet queuing time (which is varying and depends on the traffic load from other connections) and the processing delay of the forwarding nodes. In wide-area networks, the delivery time is in the order of milliseconds.


Roundtrip time

The round-trip time or ping time is the time from the start of the transmission from the sending node until a response (for example an ACK packet or ping ICMP response) is received at the same node. It is affected by packet delivery time as well as the data processing delay, which depends on the load on the responding node. If the sent data packet as well as the response packet have the same length, the roundtrip time can be expressed as: :Roundtrip time = 2 × Packet delivery time + processing delay In case of only one physical link, the above expression corresponds to: :Link roundtrip time = 2 × packet transmission time + 2 × propagation delay + processing delay If the response packet is very short, the link roundtrip time can be expressed as close to: :Link roundtrip time ≈ packet transmission time + 2 × propagation delay + processing delay


Throughput

The
network throughput Network throughput (or just throughput, when in context) refers to the rate of message delivery over a communication channel, such as Ethernet or packet radio, in a communication network. The data that these messages contain may be delivered ove ...
of a connection with flow control, for example a
TCP TCP may refer to: Science and technology * Transformer coupled plasma * Tool Center Point, see Robot end effector Computing * Transmission Control Protocol, a fundamental Internet standard * Telephony control protocol, a Bluetooth communication s ...
connection, with a certain window size (buffer size), can be expressed as: :Network throughput ≈ Window size / roundtrip time In case of only one physical link between the sending and transmitting nodes, this corresponds to: :Link throughput ≈ Bitrate × Transmission time / roundtrip time The ''message delivery time'' or ''latency'' over a network depends on the message size in bit, and the network throughput or effective data rate in bit/s, as: :Message delivery time = Message size / Network throughput


See also

* Minimum-Pairs Protocol * End-to-end delay


References

*Behrouz A. Forouzan, Sophia Chung Fegan, ''Data communications and networking''. McGraw-Hill Higher Education, 2007 - 1134 pages. *Kurose, J.F. & Ross, K.W. (2005) ''Computer Networking—A top-down approach featuring the internet.'' Pearson Education Inc. {{ISBN, 0-321-26976-4 Data transmission