Physical Layer Basics

Purpose of the Physical Layer

• A physical connection to a local network must be established before network communications occur.
• This can be a wired connection using a cable or a wireless connection using radio waves.
• Network Interface Cards (NICs) connect a device to a network and are used for wired connections.
• Wireless Local Area Network (WLAN) NICs are used for wireless connections.
• The physical layer provides the means to transport bits that make up a data link layer frame across network media.
• It accepts a complete frame from the data link layer, encoding it as a series of signals for transmission onto the local media.
• Encoded bits comprising a frame are received by either an end or intermediate device.

Physical Layer Characteristics

• Physical layer standards address three functional areas: physical components, encoding, and signaling.
• Physical components include hardware devices, media, and connectors transmitting signals representing bits.
• Encoding converts a stream of bits into a format recognizable by the next device in the network path, providing predictable patterns.
• Signaling is the method by which bit values "1" and "0" are represented on the physical medium or carrier signal, varying based on the medium.
• Bandwidth is the capacity at which a medium can carry data.

Bandwidth Terminology

• Digital bandwidth measures how much data can flow from one place to another in a given amount of time.
• Bits per second (bps) is the fundamental unit of bandwidth.
  • 1 Kbps = 1,000 bps = 103 bps
  • 1 Mbps = 1,000,000 bps = 106 bps
  • 1 Gbps = 1,000,000,000 bps = 109 bps
  • 1 Tbps = 1,000,000,000,000 bps = 1012 bps
• Latency is the amount of time, the delays, for data to travel from one point to another, measured in seconds or milliseconds.
• Throughput is the measure of transfer of bits across media over a given period. It's limited by available bandwidth.
• Goodput is the measure of usable data transferred over a given time period and is equal to throughput minus traffic overhead.

Copper Cabling

• Data is transmitted on copper cables as electrical pulses, but signal deteriorates over distance due to attenuation.
• Copper media must follow strict distance limitations.
• Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI) distorts and corrupts data signals carried by copper media. Shielding is used counter.
• Crosstalk causes disturbance from electric or magnetic fields on one wire affecting the signal on an adjacent wire.
• Opposing circuit wire pairs are twisted together to cancel crosstalk.

Types of Copper Cabling

• Unshielded Twisted Pair (UTP)
  • Most common networking media.
  • Termination with RJ-45 connectors.
  • Interconnects hosts with intermediary network devices.
  • An outer jacket protects the copper wires from physical damage
  • Twisted pairs protect signal from interference and color-coded plastic insulation electrically isolating each wire.
• Shielded Twisted Pair (STP)
  • Offers better noise protection than UTP
  • More expensive and harder to istall than UTP
  • Terminated with RJ-45 connectors that interconnect hosts with intermediary network.
  • Outer jacket for physical protection, a braided or foil shield to protect from EMI/RFI, a foil shield for wires, and color-coded insulation.
• Coaxial Cable consists of:
  • An outer cable jacket for physical protection
  • A woven copper braid or metallic foil, acting as the second wire and a shield
  • A layer of flexible plastic insulation
  • A copper conductor transmitting the electronic signals.
  • It is used in wireless installations like connecting to wireless devices and cable internet installations.

UTP Cabling Standards

• Ethernet Straight-through cables have both ends wired to T568A (host to network device)- or T568B standard.
• Ethernet Crossover cables have one end wired to T568A and the other to T568B (host-to-host, switch-to-switch, or router-to-router).
• Rollover cables are a Cisco Proprietary cable for connecting a host serial port to a router or switch console port.

Fiber-Optic Cabling

• Fiber-optic cabling is used in four types of industry: enterprise, fiber-to-the-home (FTTH), long-haul, and submarine cable networks.
• It transmits data over greater distances and higher bandwidths with less attenuation, is immune to EMI/RFI, and interconnects network devices.
• Light pulses encode the bits on flexible, thin strands of glass (smaller than a human hair).

Types of Fiber Media

• Single-Mode Fiber
  • Core is very small and employs expensive lasers.
  • Suitable for long-distance application.
• Multimode fiber:
  • Core is larger and less expensive LEDs.
  • LEDs transmit at different angles.
  • Up to 10 Gbps over 550 meters is possible.

Fiber-Optic Cabling

• Yellow jackets are for single-mode fiber cables, while orange or aqua jackets indicate multimode fiber cables.
• Singlemode fiber has a core diameter of 9μm, propagates light in a straight line, and is used for long distances (km).
• Multimode fiber has a core diameter of 50 μm or 62.5 μm, propagates light in multiple modes, and is used for shorter distances (up to a few hundred meters).
• Optical fiber is used for backbone cabling for high-traffic, point-to-point connections and interconnecting buildings.

Wireless Media

• Wireless media carry electromagnetic signals that represent the binary digits of data using radio or microwave frequencies.
• Wireless concerns include coverage area limitations due to construction materials and local terrain, interference.
• IEEE 802.11 wireless NIC devices use Carrier/Sense Multiple Access/Collision Avoidance (CSMA/CA).
• IEEE 802.15 is the Wireless Personal Area Network (WPAN) standard.
• WiMAX is based on IEEE 802.16
• Wireless LAN requires wireless access points to concentrate signals and connect to copper infrastructure and wireless NIC adapters for communication on each end network host.