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 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.
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.
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
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
A 568A “Straight Through” cable will have the following pin-outs on both ends of the cable
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
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
– media not precisely aligned together
– Media does not completely touch
– 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
– 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