2.2.Layer 1: The Physical Layer
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Data on a computer network is represented as a binary expression.
Electrical voltage (on copper wiring) or light (carried via fiber-optic cabling) can represent these 1s and 0s.
The presence or the absence of voltage on a wire can represent a binary 1 or a binary 0, respectively.
The presence or absence of light on a fiber-optic cable can represent a 1 or 0 in binary.
An alternate approach to representing binary data is state transition modulation where the transition between voltages or the presence of light indicates a binary value.
Several standards for network connectors exist for example the TIA/EIA-568-B standard describes how an RJ-45 connector should be wired for use on a 100BASE-TX Ethernet network, as shown in Figure .
Examples of a physical topology include :
Bus
Ring
Star
Hub-and-Spoke
Full-Mesh
Partial-Mesh
For two networked devices to successfully communicate at the physical layer, they must agree on when one bit stops and another bit starts.
Two basic approaches to bit synchronization include :
Asynchronous
With this approach, a sender indicates that it is about to start transmitting by sending a start bit to the receiver. When the receiver sees this, it starts its own internal clock to measure the subsequent bits. After the sender transmits its data, it sends a stop bit to indicate that it has finished its transmission.
Synchronous
This approach synchronizes the internal clocks of both the sender and the receiver to ensure that they agree on when bits begin and end. A common approach to make this synchronization happen is to use an external clock (for example, a clock provided by a service provider), which is referenced by both the sender and the receiver.
The two fundamental approaches to bandwidth usage on a network are broadband and baseband.
Broadband technologies divide the bandwidth available on a medium into different channels.
Different communication streams are then transmitted over the various channels.
Baseband technologies, in contrast, use all the available frequencies
on a medium to transmit data.
Ethernet is an example of a networking technology that uses baseband.
TDM supports different communication sessions on the same physical medium by causing the sessions to take turns. For a brief period of time, defined as a time slot , data from the first session will be sent, followed by data from the second session. This continues until all sessions have had a turn, and the process repeats itself.
A downside to TDM is that each communication session receives its own time slot, even if one of the sessions does not have any data to transmit at the moment. To make a more efficient use of available bandwidth, StatTDM dynamically assigns time slots to communications sessions on an as-needed basis.
FDM divides a medium’s frequency range into channels, and different communication sessions transmit their data over different channels.
