Network capacity and performance

When we are dealing with Packet switching-based networks, the connection speed depends on multiple factors:

Bandwidth

Bandwidth refers to the width of the frequency range used by the transmission system; the wider the range, the more information it can transmit. This frequency is measured in hertz.

When expressed in bps (bits per second), it is called bit rate, which is the amount of bits per second that a transmission medium can potentially transmit.

Throughput

If the bit rate is the amount of bits per second that a link guarantees to transmit, the throughput is the amount of bits per second that a link actually transmits.

It can vary but will always be less than or equal to the bit rate of the corresponding link.

If a path crosses multiple links with different throughputs, the total throughput is the minimum of the throughputs encountered. The link with the lowest throughput is called the bottleneck, since is the link that is slowing all the rest of the data down.

Latency

A packet traveling across the network accumulates delays. Latency (or delay) defines the time it takes for an entire message to reach its destination from the moment the first bit leaves the source.

There are four types of delays, and the latency will consequently be the sum of these delays.

Transmission delay

When a source device sends a packet, it must transmit the packet’s bits onto the transmission line one by one, which causes a delay for the last bits of a packet that have to wait to be sended.

The transmission delay is calculated as $t_2-t_1$, where $t_1$ is the moment when the first bit is transmitted, and $t_2$ is when the last bit is transmitted. Since this time depends on the packet length and the bit rate of the connection, we have that:

$\text{Transmission delay}=\frac{\text{Packet length (in bits)}}{\text{Bit rate (in bits per second)}}$

Propagation delay

Propagation delay is the time it takes for a bit to travel from point $A$ to point $B$ in a transmission medium.

This time depends on the distance between the two points and the propagation speed of the medium (very close to the speed of light). The formula for propagation delay is:

$\text{Propagation delay}=\frac{\text{Distance}}{\text{Propagation speed}}$

Elaboration delay

Processing delay is the time required by a router (or a terminal system) to receive the packet, decapsulate it, perform error detection, and deliver the packet to the output port or the higher-level protocol (in the case of a terminal).

Queueing delay

Queueing delay is typical of routers, as they are equipped with an input queue to store packets waiting to be processed and an output queue to store packets waiting to be transmitted. This delay is the time a packet spends waiting in the input queue and the output queue.

Note

The number of bit the link can contains is the result of $\text{rate}\cdot \text{delay}$.

Packet loss

If the router’s input queue is full, incoming packets are lost.

This means that the lost packets must be retransmitted, and the frequency of this operation affects communication performance.

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tags:#computer-networking resources: 1 - Introduzione.pdf