Network Protocols: Physical & Data Link Layer Essentials

Posted on May 23, 2025 in Technology

Physical Layer Protocols

1. Slower Wireless Network Performance

What is a reason for slower network performance on a wireless network?

  • The performance is based on the distance from a Wireless Access Point (WAP). The further, the weaker.
  • This can mean less bandwidth or no wireless connection.

2. Data Flow Process from Source to Destination Node

Explain in detail each step of the process that data undergoes from a source node to a destination node.

  1. The user data is segmented by the transport layer, placed into packets by the network layer, and further encapsulated into frames by the data link layer.
  2. The physical layer encodes the frames and creates the electrical, optical, or radio wave signals that represent the bits in each frame.
  3. These signals are then sent on the media, one at a time.
  4. The destination node physical layer retrieves these individual signals from the media, restores them to their bit representations, and passes the bits up to the data link layer as a complete frame.

3. Basic Forms of Network Media

There are three basic forms of network media. List and define each.

  1. Copper cable: The signals are patterns of electrical pulses.
  2. Fiber-optic cable: The signals are patterns of light.
  3. Wireless: The signals are patterns of microwave transmissions.

4. Encoding or Line Encoding

What is encoding or line encoding?

  • It 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 the sender and the receiver.

5. Digital Bandwidth Definition

Define digital bandwidth.

  • It means to measure the amount of data that can flow from one place to another in a given amount of time.

6. Factors Determining Practical Network Bandwidth

What factors determine the practical bandwidth of a network?

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

7. Factors Influencing Throughput

What factors influence throughput?

  • The amount of traffic.
  • The type of traffic.
  • The latency created by the number of network devices encountered between source and destination.

8. Goodput Explained

Explain what goodput is.

  • Goodput is the measure of usable data transferred over a given period of time.
  • Goodput is throughput minus traffic overhead for establishing sessions, acknowledgments, and encapsulation.

Network Media

9. Sources of Interference

List and explain the two different sources of interference.

  • 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.

10. Countering EMI and RFI Effects

What can be done to counter the negative effects of EMI and RFI?

  • Copper cables are wrapped in metallic shielding and require proper grounding connections.

11. Countering Crosstalk Effects

What can be done to counter the negative effects of crosstalk?

  • Copper cables have opposing circuit wire pairs twisted together, which effectively cancels crosstalk.

12. Main Types of Copper Media

List the three main types of copper media used in networking.

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

13. Limiting Crosstalk in UTP Cable

List and explain how cable designers can limit the negative effect of crosstalk in UTP Cable.

  • Cancellation: Designers now pair wires in a circuit. When two wires in an electrical circuit are placed close together, their magnetic fields are the exact opposite of each other. Therefore, the two magnetic fields cancel each other and also cancel out any outside EMI and RFI signals.
  • Varying the number of twists per wire pair: To further enhance the cancellation effect of paired circuit wires, designers vary the number of twists of each wire pair in a cable. UTP cable must follow precise specifications governing how many twists or braids are permitted per meter (3.28 feet) of cable. Notice in the figure that the orange/orange white pair is twisted less than the blue/blue white pair. Each colored pair is twisted a different number of times.

14. TIA/EIA-568 Cabling Standards

TIA/EIA-568 stipulates the commercial cabling standards for LAN installations. The cabling elements defined include:

  1. Cable types
  2. Cable lengths
  3. Connectors
  4. Cable termination
  5. Methods of testing cable

15. Main UTP Cable Types

List and define the three main UTP cable types.

  1. 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.
  2. 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.
  3. Rollover: A Cisco proprietary cable used to connect a workstation to a router or switch console port.

16. Fiber-optic Cabling Industry Applications

List and describe how Fiber-optic cabling is now being used in four types of industry.

  1. Enterprise Networks: Used for backbone cabling applications and interconnecting infrastructure devices.
  2. Fiber-to-the-Home (FTTH): Used to provide always-on broadband services to homes and small businesses.
  3. Long-Haul Networks: Used by service providers to connect countries and cities.
  4. Submarine Cable Networks: Used to provide reliable high-speed, high-capacity solutions capable of surviving in harsh undersea environments up to transoceanic distances.

17. Fiber-optic Cable Classifications

Fiber-optic cables are broadly classified into two types. List and describe both.

  1. Single-mode fiber (SMF): Consists of a very small core and uses expensive laser technology to send a single ray of light, as shown in Figure 1. Popular in long-distance situations spanning hundreds of kilometers, such as those required in long haul telephony and cable TV applications.
  2. Multimode fiber (MMF): Consists of a larger core and uses LED emitters to send light pulses. Specifically, light from an LED enters the multimode fiber at different angles. Popular in LANs because they can be powered by low-cost LEDs. It provides bandwidth up to 10 Gb/s over link lengths of up to 550 meters.

18. Dispersion in Fiber Optics

One of the highlighted differences between multimode and single-mode fiber is the amount of dispersion. What is dispersion?

  • It refers to the spreading out of a light pulse over time.
  • The more dispersion there is, the greater the loss of signal strength.

19. Wireless Network Devices

What two network devices are required for a wireless network to function?

  • Wireless Access Point (AP): Concentrates the wireless signals from users and connects to the existing copper-based network infrastructure, such as Ethernet. Home and small business wireless routers integrate the functions of a router, switch, and access point into one device as shown in the figure.
  • Wireless NIC adapters: Provide wireless communication capability to each network host.

Data Link Layer Protocols

20. Data Link Layer Responsibilities in the OSI Model

List what the data link layer of the OSI model is responsible for:

  1. Allowing the upper layers to access the media.
  2. Accepting Layer 3 packets and packaging them into frames.
  3. Preparing network data for the physical network.
  4. Controlling how data is placed and received on the media.
  5. Exchanging frames between nodes over a physical network media, such as UTP or fiber-optic.
  6. Receiving and directing packets to an upper layer protocol.
  7. Performing error detection.

21. Data Link Layer Sublayers

The data link layer is divided into two sublayers. List and describe both.

  1. Logical Link Control (LLC) – This upper sublayer communicates with the network layer. It places information in the frame that identifies which network layer protocol is being used for the frame. This information allows multiple Layer 3 protocols, such as IPv4 and IPv6, to utilize the same network interface and media.
  2. Media Access Control (MAC) – This lower sublayer defines the media access processes performed by the hardware. It provides data link layer addressing and access to various network technologies.

Media Access Control

22. Media Access Control Method Explained

What is meant by the term media access control method?

  • Media access control is the equivalent of traffic rules that regulate the entrance of motor vehicles onto a roadway.
  • The absence of any media access control would be the equivalent of vehicles ignoring all other traffic and entering the road without regard to the other vehicles.

23. Network Topology Types

The topology of a network is the arrangement or relationship of the network devices and the interconnections between them. LAN and WAN topologies can be viewed in two ways. List and explain both.

  • Physical topology – Refers to the physical connections and identifies how end devices and infrastructure devices such as routers, switches, and wireless access points are interconnected. Physical topologies are usually point-to-point or star.
  • Logical topology – Refers to the way a network transfers frames from one node to the next. This arrangement consists of virtual connections between the nodes of a network. These logical signal paths are defined by data link layer protocols. The logical topology of point-to-point links is relatively simple while shared media offers different access control methods.

24. Half-duplex and Full-duplex Communication

Explain the following terms in detail.

  1. Half-duplex communication – Both devices can transmit and receive on the media but cannot do so simultaneously. The half-duplex mode is used in legacy bus topologies and with Ethernet hubs. WLANs also operate in half-duplex. Half-duplex allows only one device to send or receive at a time on the shared medium and is used with contention-based access methods.
  2. Full-duplex communication – Both devices can transmit and receive on the media at the same time. The data link layer assumes that the media is available for transmission for both nodes at any time. Ethernet switches operate in full-duplex mode by default, but can operate in half-duplex if connecting to a device such as an Ethernet hub.

25. Access Control Methods for Shared Media

Some multi-access networks require rules to govern how devices share the physical media. List and explain the two basic classes or types of access control methods for shared media.

  1. Contention-based access – All nodes operating in half-duplex compete for the use of the medium, but only one device can send at a time. However, there is a process if more than one device transmits at the same time. Ethernet LANs using hubs and WLANs are examples of this type of access control. Figure 1 shows contention-based access.
  2. Controlled access – Each node has its own time to use the medium. These deterministic types of networks are inefficient because a device must wait its turn to access the medium. Legacy Token Ring LANs are an example of this type of access control.

26. CSMA/CA vs. CSMA/CD

Explain how CSMA/CA differs from CSMA/CD.

  • CSMA/CA uses a method similar to CSMA/CD to detect if the media is clear.
  • CSMA/CA also uses additional techniques.
  • CSMA/CA does not detect collisions but attempts to avoid them by waiting before transmitting.

27. Data Link Layer Frame Parts

Each data link layer frame type has three basic parts:

  1. Header
  2. Data
  3. Trailer

28. Common Data Link Layer Protocols

List common data link layer protocols.

  1. Ethernet
  2. 802.11 Wireless
  3. Point-to-Point Protocol (PPP)
  4. HDLC
  5. Frame Relay