Chapter 3 : Physical Layer

 PURPOSE OF THE PHYSICAL LAYER

      All data being transferred over a network must be represented on a medium by the sending node and interpreted on a medium by the receiving node. The physical layer is responsible for these functions. This section explores the physical layer.

Wireless access point (WAP) A network device that provides connectivity of wireless clients to connect to a data network. A wireless AP uses radio waves to communicate with the wireless NICs in the devices and other wireless access points.

Network interface card (NIC) Computer hardware, typically used for LANs, that allows a computer to connect to some networking cable. The NIC can then send and receive data over the cable at the direction of the computer. 

International Organization for Standardization (ISO) An international standards body that defines many networking standards and that created the OSI model. 

Telecommunications Industry Association/ Electronic Industries Association (TIA/EIA) An organization that develops standards that relate to telecommunications technologies. Together, the TIA and the Electronic Industries Alliance (EIA) have formalized standards, such as EIA/TIA-232, for the electrical characteristics of data transmission.

International Telecommunications Union (ITU) A United Nations (UN) agency responsible for issues that concern information and communication technologies. 

American National Standards Institute (ANSI) A private nonprofit organization that oversees development of standards in the United States. 

Institute of Electrical and Electronics Engineers (IEEE) An international, nonprofit organization for the advancement of technology related to electricity. IEEE maintains the standards defining many LAN protocols.

Physical Components

The physical layer standards address three functional areas: 

• Physical components are the electronic hardware devices, media, and other connectors that transmit the signals representing bits. Hardware components such as NICs, interfaces and connectors, and cables (including cable materials and cable designs) are all specified in standards associated with the physical layer. The various ports and interfaces on a Cisco 1941 router are also examples of physical components with specific connectors and pinouts based on standards.
• Encoding is a method of converting a stream of data bits into a predefined “code.” Codes are groupings of bits used to provide a predictable pattern that can be recognized by both a sender and a receiver. In other words, encoding is a method or pattern used to represent digital information. This is similar to how Morse code encodes a message using a series of dots and dashes.
• Signaling The physical layer must generate the electrical, optical, or wireless signals that represent the 1s and 0s on the media. The way that bits are represented is called the signaling method. The physical layer standards must define what type of signal represents a 1 and what type of signal represents a 0. This can be as simple as a change in the level of an electrical signal or optical pulse.

Bandwidth The rated throughput capacity of a given network medium or protocol. Bandwidth is listed as available or consumed data communication resources expressed in bits per second.

A combination of factors determines the practical bandwidth of a network:  

• The properties of the physical media 
• The technologies chosen for signaling and detecting network signals 

Bandwidth Terminology

Terms used to measure the quality of bandwidth include: 

• Latency refers to the amount of time, including delays, for data to travel from one point to another.
• Throughput is the measure of the transfer of bits across the media over a given period of time.
• Goodput is the measure of usable data transferred over a given period of time. Goodput is throughput minus traffic overhead for establishing sessions, acknowledgments, encapsulation, and retransmitted bits. Goodput is always lower than throughput, which is generally lower than the bandwidth.

Characteristics of Copper Cabling 

Copper cabling is the most common type of cabling used in networks today. In fact, copper cabling is not just one type of cable. There are three different types of copper cabling that are each used in specific situations.

Networks use copper cabling because it is inexpensive and easy to install, and it has low resistance to electrical current. However, copper cabling is limited by distance and signal interference. 

Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent. However, the farther the signal travels, the more it deteriorates. This is referred to as signal attenuation. For this reason, all copper media must follow strict distance limitations, as specified by the guiding standards. 

The timing and voltage values of electrical pulses are also susceptible to interference from two sources:

- Electromagnetic interference (EMI) or radio frequency interference (RFI): EMI and RFI signals can distort and corrupt the data signals being carried by copper media. Potential sources of EMI and RFI include radio waves and electromagnetic devices, such as fluorescent lights or electric motors.

- Crosstalk: Crosstalk is a disturbance caused by the electric or magnetic fields of a signal on one wire to the signal in an adjacent wire. In telephone circuits, crosstalk can result in hearing part of another voice conversation from an adjacent circuit. Specifically, when an electrical current flows through a wire, it creates a small, circular magnetic field around the wire, which can be picked up by an adjacent wire. 

Unshielded Twisted-Pair (UTP)

Unshielded twisted-pair (UTP) cabling is the most common networking medium. UTP cabling, terminated with RJ-45 connectors, is used for interconnecting network hosts with intermediary networking devices, such as switches and routers.

Shielded Twisted-Pair (STP)

Shielded twisted-pair (STP) provides better noise protection than UTP cabling. However, compared to UTP cable, STP cable is significantly more expensive and difficult to install. Like UTP cable, STP uses RJ-45 connectors.

Coaxial Cable

Coaxial cable, or coax for short, gets its name from the fact that there are two conductors that share the same axis.

coaxial cable consists of the following: 

1. The entire cable is covered with a cable jacket to prevent minor physical damage. 

2. The insulating material is surrounded by a woven copper braid, or metallic foil, that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, also reduces the amount of outside electromagnetic interference. 

3. A layer of flexible plastic insulation surrounds a copper conductor. 

4. A copper conductor is used to transmit the electronic signal.

The numbers in identify some key features of coaxial cable: 

    1. Outer jacket 

    2. Braided copper shielding 

    3. Plastic insulation 

    4. Copper conductor 

Although UTP cable has essentially replaced coaxial cable in modern Ethernet installations, the coaxial cable design is used in the following situations: 

• Wireless installations: Coaxial cables attach antennas to wireless devices. The coaxial cable carries radio frequency (RF) energy between the antennas and the radio equipment. 
• Cable internet installations: Cable service providers provide internet connectivity to their customers by replacing portions of the coaxial cable and supporting amplification elements with fiberoptic cable. However, the wiring inside the customer’s premises is still coax cable. 

fiber-optic cable A physical medium that uses glass or plastic threads to transmit data. A fiber-optic cable consists of a bundle of these threads, each of which is capable of transmitting data into light waves.

Fiber-optic cabling is now being used in four types of industry: 

• Enterprise networks: Fiber is used for backbone cabling applications and for interconnecting infrastructure devices. 
• Fiber-to-the-home (FTTH): Fiber is used to provide always-on broadband services to homes and small businesses. 
• Long-haul networks: Service providers use fiber to connect countries and cities. 
• Submarine cable networks: Fiber is used to provide reliable high-speed, high-capacity solutions capable of surviving in harsh undersea environments at up to transoceanic distances. Search the internet for “submarine cables telegeography map” to view various maps online.

WIRELESS MEDIA

Wireless media carry electromagnetic signals that represent the binary digits of data communications using radio wave or microwave frequencies.

Wireless media provide the greatest mobility options of all media. Wireless is now the primary way users connect to home and enterprise networks, and the number of wireless-enabled devices continues to increase. 

These are some of the limitations of wireless: 

• Coverage area: Wireless data communication technologies work well in open environments. However, certain construction materials used in buildings and structures, as well as the local terrain, can limit the effective coverage. 
• Interference: Wireless is susceptible to interference and can be disrupted by such common devices as household cordless phones, some types of fluorescent lights, microwave ovens, and other wireless communications. 
• Security: Wireless communication coverage requires no access to a physical strand of cable. Therefore, devices and users not authorized for access to the network can gain access to the transmission. Network security is a major component of wireless network administration. 
• Shared medium: WLANs operate in half-duplex, which means only one device can send or receive at a time. The wireless medium is shared among all wireless users. Many users accessing the WLAN simultaneously results in reduced bandwidth for each user.

Although wireless is increasing in popularity for desktop connectivity, copper and fiber are the most popular physical layer media for deployment of intermediary network devices, such as routers and switches.

Types of Wireless Media

The IEEE and telecommunications industry standards for wireless data communications cover both the data link and physical layers. In each of these standards, physical layer specifications are applied to areas such as the following:

• Data-to-radio signal encoding 
• Frequency and power of transmission 
• Signal reception and decoding requirements 
• Antenna design and construction

These are the wireless standards:

• Wi-Fi (IEEE 802.11) A wireless LAN (WLAN) technology that uses a contention-based protocol known as CSMA/CA. The wireless NIC must first listen before transmitting to determine if the radio channel is clear. If another wireless device is transmitting, the NIC must wait until the channel is clear. Wi-Fi, which is a trademark of the Wi-Fi Alliance, is used with certified WLAN devices based on the IEEE 802.11 standards.

• Bluetooth (IEEE 802.15) A wireless personal area network (WPAN) standard that uses a device pairing process to communicate over distances from 1 to 100 meters. 

• WiMAX (IEEE 802:16) Worldwide Interoperability for Microware Access, a wireless standard that uses a point-to-multipoint topology to provide wireless broadband access. 

• Zigbee (IEEE 802.15.4) A specification used for lowdata-rate, low-power communications. It is intended for applications that require short ranges, low data rates, and long battery life. Zigbee is typically used for industrial and Internet of Things (IoT) environments such as wireless light switches and medical device data collection.