This article explores the fundamentals of the physical layer, various types of physical media like copper and fiber-optic cabling, and key metrics such as bandwidth, latency, throughput, and goodput.

Two OSI Layers (Physical layer 1 and Data Link layer 2) map to the TCP/IP Network Access layer.

The OSI physical layer provides the means to transport the bits that make up a data link layer frame across the network media.

Many international and national organizations, regulatory government organizations, and private companies are involved in establishing and maintaining physical layer standards.

Some of these standards organizations:

  • International Organization for Standardization (ISO)
  • American National Standards Institute (ANSI)/Telecommunications Industry Association (TIA)
  • International Telecommunication Union (ITU)
  • Institute of Electrical and Electronics Engineers (IEEE)
  • National telecommunications regulatory authorities including the Federal Communication Commission (FCC) in the USA and the European Telecommunications Standards Institute (ETSI)

In addition to these, there are often regional cabling standards groups that develop local specifications:

  • CSA (Canadian Standards Association)
  • CENELEC (European Committee for Electrotechnical Standardization)
  • JSA/JIS (Japanese Standards Association)

A physical connection can be:

  1. a wired connection using a cable;
  2. a wireless connection using radio waves.

Three types of media to interconnect devices:

  1. Metal wires within cables (electrical impulses);
  2. Glass or plastic fibers within cables (pulses of light);
  3. Wireless transmission (electromagnetic waves).

Role of Network Interface Cards (NICs)

Network interface cards (NICs) connect a device to the network.

Network adapter or network card are both alternate names for NICs. The NIC serves as an interface between a computer and the network.

Three functional areas of the physical layer

The physical layer standards address three functional areas:

  1. Physical Components
  2. Encoding
  3. Signaling

Physical Components

The physical components are electronic hardware devices (NICs), media (cable materials), and other connectors that transmit the signals that represent the bits. All these components are specified in standards associated with the physical layer.


Encoding or line encoding is the method or pattern used to represent digital information to provide a predictable pattern that can be recognized by both the sender and the receiver.


The way that bits are represented is called the signaling method. The physical layer must generate the electrical, optical, or wireless signals that represent the "1" and "0" on the media.

Metrics and Factors Affecting Data Transfer Rates

Different physical media support the transfer of bits at different rates. Data transfer is usually discussed in terms of bandwidth.

Bandwidth - the capacity at which a medium can carry data.

Digital bandwidth measures the amount of data that can flow from one place to another in a given amount of time and is typically measured in kilobits per second (kbps), megabits per second (Mbps), or gigabits per second (Gbps).

Physical media properties, current technologies for signaling and detecting network signals, and the laws of physics all play a role in determining the available bandwidth.

3 terms to measure the quality of bandwidth:

  1. Latency - the amount of time, including delays, for data to travel from one given point to another.
  2. Throughput - the measure of the transfer of bits across the media over a given period.
  3. Goodput - the measure of usable data transferred over a given period.

Goodput is always lower than throughput, which is generally lower than the bandwidth.

Metal Wires / Copper Cabling: Types and Uses

All copper media must follow strict distance limitations because of signal attenuation (reduction in the strength of a signal the farther it travels).

Two sources of signal attenuation:

  1. Electromagnetic interference (EMI) or radio frequency interference (RFI). Potential sources of EMI and RFI include radio waves and electromagnetic devices.
  2. Crosstalk - a disturbance caused by the electric or magnetic fields of a signal on one wire to the signal in an adjacent wire.

To counter the negative effects of EMI and RFI, some types of copper cables are wrapped in metallic shielding and require proper grounding connections.

To counter the negative effects of crosstalk, some types of copper cables have opposing circuit wire pairs twisted together, which effectively cancels the crosstalk.

There are three different types of copper cabling that are each used in specific situations.

Three main types of copper media used in networking:

  1. Unshielded Twisted-Pair (UTP) Cable
  2. Shielded Twisted-Pair (STP) Cable
  3. Coaxial Cable

Unshielded Twisted-Pair (UTP) Cables: Features and Applications

Unshielded twisted-pair (UTP) cabling is the most common networking media.

Cables are placed into categories based on their ability to carry higher bandwidth rates.

For example, Category 5 cable is used commonly in 100BASE-TX Fast Ethernet installations. Other categories include Enhanced Category 5 cable (5e), Category 6, and Category 7 and 8.

Category 5 supports 100 Mbps and Category 5e supports 1000 Mbps. Category 6 supports up to 10 Gbps. Category 7 also supports 10 Gbps. Category 8 supports 40 Gbps.

UTP cable consists of four pairs of color-coded wires that have been twisted together and then encased in a flexible plastic sheath. UTP cable is usually terminated with an RJ-45 connector. The ANSI/TIA-568 standard describes the wire color codes to pin assignments (pinouts) for Ethernet cables.

Different situations may require UTP cables to be wired according to different wiring conventions. This means that the individual wires in the cable have to be connected in different orders to different sets of pins in the RJ-45 connectors.

The following are the main cable types that are obtained by using specific wiring conventions:

  • Ethernet Straight-through - The most common type of networking cable. It is commonly used to interconnect a host to a switch and a switch to a router.
  • Ethernet Crossover - A cable used to interconnect similar devices. For example, to connect a switch to a switch, a host to a host, or a router to a router. However, crossover cables are now considered legacy as NICs use medium-dependent interface crossover (auto-MDIX) to automatically detect the cable type and make the internal connection.

Another type of cable is a rollover cable, which is used to connect a workstation to a router or switch console port.

Twisted-pairs protect the signal from interference and color-coded plastic insulation helps identify each pair.

Shielded Twisted-Pair (STP) Cables: Enhancing Noise Protection

The STP cable uses four pairs of wires, each wrapped in a foil shield, which are then wrapped in an overall metallic braid or foil.

Shielded twisted-pair (STP) provides better noise protection than UTP cabling.

STP cables combine the techniques of shielding to counter EMI and RFI, and wire twisting to counter crosstalk.

Coaxial Cables

Coaxial cable, which is used for cable TV and internet service and to attach antennas to wireless devices, uses several types of connectors including BNC, N type, and F type connectors.

Fiber-Optic Cabling: High-Speed Data Transmission

Optical fiber cable transmits data over longer distances and at higher bandwidths than any other networking media. Optical fiber is commonly used to interconnect network devices because fiber-optic cable can transmit signals with less attenuation and is completely immune to EMI and RFI.

Two types of fiber-optic cables:

  1. Single-mode fiber (SMF) (a very small core; it uses laser technology to send a single ray of light for a long-range, spanning hundreds of kilometers)
  2. Multimode fiber (MMF) (a larger core; it uses LED emitters to send light pulses; bandwidth up to 10 Gbps over link lengths of up to 550 meters)

Four types of fiber-optic cabling usage:

  1. Enterprise Networks - for backbone cabling applications and interconnecting infrastructure devices.
  2. Fiber-to-the-Home (FTTH) - providing always-on broadband services to homes and small businesses.
  3. Long-Haul Networks - connecting countries and cities.
  4. Submarine Cable Networks - reliable high-speed, high-capacity solutions for transoceanic distances.

There are four types of fiber-optic connectors: ST, SC, LC, and duplex multimode LC.