Physical Layer

Physical Layer Overview

The purpose of the Physical layer is to put digital bits on the media as encoded signals and to also receive encoded signals and turn them back into binary digits. Media at the Physical layer refers to either copper cables, fiber optic cables or wireless radio waves. Along with all the different types of cables the Physical layer also refers to the different connectors like RJ-45 connectors and ST/SC fiber optic connectors.

The Physical layer takes place in hardware as opposed to software, so instead of protocols and addressing the Physical layer is comprised of engineering standards defined by organizations like the IEEE, the ITU and the ISO.

Signaling

Signaling is changing bits in to a form that can be transmitted over distances and read by connectors on each end. In general terms, 1’s and 0’s are represented on the medium as variations in voltage, the presence or absence of light and changes in radio waves. In this way, 1’s and 0’s are signaled by changes in amplitude, frequency, and phase.

Two early signaling standards were Manchester Encoding (Ethernet) and Non-Return Zero (NRZ). NRZ uses the voltage on the wire as a 1 or 0. Since this is a very simple method of signaling it can only be used in low speed links. Manchester Encoding uses segments register a change in signal that goes up or down. If the change is down then it will be a 0 if the change is up it will be a 1.

Encoding

Encoding is used to improve efficiency and speed of data transmission. Code groups are used to encode bits into larger symbols prior to placing them on the media. For example, in the 4B/5B code group, four bit long codes are translated into five bit long symbols. One reason for this is that devices know that when they see a five byte symbol that doesn’t correspond to a four byte code or control code, the bits are an error or noise on the media. Another reason for this is that a long series of 1s could wear out or overheat media or network devices. Also, using code groups prevents data bits from accidentally matching a control signal, such as the bit pattern signaling the end of a frame.

Copper Media

The most commonly used network media uses copper wires to carry data between network devices. Copper media can refer to early ethernet implementations using coaxial cables like 10Base2 (Thinnet) and the predominant Fast Ethernet and Gigabit Ethernet using Cat5E UTP (unshielded twisted pair) cables. Unshielded twisted pair cables (UTP) use four twisted pairs of wires that are used for signaling and transmission, and coaxial cable uses a single copper conductor that is insulated by a shield. Cables used for networking all have requirements that are spelled out in Physical layer standards.

One problem with copper media is that it is susceptible to electromagnetic and radio interference from things such as motors, fluorescent lights, and radio transmitters. Interference problems can be solved by using different media, avoiding sources of interference when designing infrastructure, and properly handling and terminating cables. Unshielded twisted pair cables use the effect of “cancellation.” created by the twists in the cable pairs to resist electromagnetic interference.

Fiber Media

Fiber cabling uses glass or plastic fibers to let light signals travel from the source to the destination. Encoding schemes use light pulses for the signaling method. The speed with which light travels allows fiber optic cabling to deliver large data bandwidth rates and longer cabling runs. Downsides to fiber optic cabling is that it is more expensive than copper cabling and requires careful installation techniques to avoid sharp bends in the cable which will break the glass core. Because of its cost fiber cabling has been used mainly for backbones and vertical runs in networks. There are generally two types of fiber optic cabling, multimode cable and single mode cable. Single mode is more expensive, can be run farther distances, uses a laser as a light source, and has an 8 to 10 micron glass core. Multimode fiber uses a LED as its light signal, has a glass core of 50 to 60 microns, bounces the light inside of the cable, suffers from more light dispersion, and is cheaper than single-mode.

 

Wireless Media

Carries electromagnetic signals at radio and microwave frequencies and works well in open environments. Wireless media requires no physical access like copper cables and jacks, however, the easy open access that wireless provides also presents security risks.

  • IEEE 802.11 (WiFi) is considered a wireless LAN
  • IEEE 802.15 (WiPAN) is considered a wireless Personal Area Network, commonly known as “Bluetooth”
  • IEEE 802.16 (WiMAX) is considered a point-to-multipoint topology for wireless broadband access
  • 802.11a – 5 Ghz frequency, 54 Megabit per second,
  • 802.11b – 2.4 Ghz frequency, 11 Megabit per second,
  • 802.11g – 2.4 Ghz frequency, 54 Megabit per second,
  • 802.11n –  2.4 Ghz frequency, 100 Megabit per second

Media Connectors

EIA-TIA 568A and 568B are the unshielded twisted pair RJ-45 connector standards for wire colors used for pinouts for Ethernet straight-through and crossover cables. See the following diagrams:

A 568B “Straight Through” cable will have the following pin-outs on both ends of the cable

white/orange orange white/green blue white/blue green white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/orange orange white/green blue white/blue green white/brown brown

 

A 568A “Straight Through” cable will have the following pin-outs on both ends of the cable

white/green green white/orange blue white/blue orange white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/green green white/orange blue white/blue orange white/brown brown

 

A “Crossover” cable will have the 568B and the 568A standards on either ends of the cable
Notice: that pins 1&3 and 2&6 are crossed in a Fast Ethernet Crossover cable

white/orange orange white/green blue white/blue green white/brown brown
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
white/green green white/orange blue white/blue orange white/brown brown

Common Optical Fiber Connectors

Straight-Tip (ST) Trademarked by AT&T
-bayonet style connector used with multi-mode fiber
Subscriber Connector (SC)
– push-pull mechanism ensures positive insertion
– used with single-mode fiber
Lucent Connector (LC)
– small connector
– used with single-mode fiber
– supports multi-mode fiber

Common Fiber Termination and Splicing Errors
Misalignment
– media not precisely aligned together
End Gap
– Media does not completely touch
End Finish
– Dirt is present or the ends are not polished well enough

Optical Time Domain Reflectometer (OTDR)
– recommended test equipment
– injects a test pulse of light into the cable
– measures back scatter and reflection as a function of time
– calculates the approximate distance of detected faults

Field Test
– can be performed with a flashlight
– if light is visible at the other end cable is capable of passing light
– does not ensure performance
– used as a quick way to find broken fiber

Author: Dan

Dan teaches computer networking and security classes at Central Oregon Community College.

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