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.
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.
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,