The physical layer deals with using signals to represent bits for transmitting data across communication channels. These channels can be wired, involving cables, or wireless, without physical connections.
Figure 5.10 Physical layer.
There are a few concepts relating to channel that all higher layers need.
Bit Rate: It measures how fast data is sent, typically in bits per second ($\text{bps}$).
Bandwidth: The maximum data transfer rate over a medium.
<aside> <img src="/icons/map-pin_gray.svg" alt="/icons/map-pin_gray.svg" width="40px" /> Be careful of the letter case of the letter “B” as they can mean two different things:
As noted in the image below, one byte or $1\;\text{B}$ is equivalent to eight bits or $8\;\text{b}$
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Error Rate: The percentage of erroneous bits in received data, which varies between fiber and wireless channels.
Message Latency (Delay): The delay in sending a message across a channel, comprising transmission and propagation delay.
Refer to the figure below.
Figure 6.1 Message latency model.
This delay is related with the length of the packet ($M$ bits) and the transmission rate of the link ($R$) from router A to router B by $R$ bits/s. The transmission delay is:
$$ T_f = \frac{M}{R} \qquad [\text{s}] $$
This is the amount of time required to push (transmit) all of the packet’s bits into the link. This delay is typically on the order of microseconds to milliseconds in practice.
The time required to propagate from the beginning of the link to router B is the propagation delay. The bit propagates at the propagation speed of the link. The propagation delay is:
$$ T_c = \frac{\text{Link length}}{\text{Propogation Speed}} \qquad[\text{s}] $$
The propagation speed depends on the physical medium of the link.