The Physical Layer

1. The Physical Layer Chapter 2

2. The purpose of the physical layer is to transport a raw bit stream from one machine to another. Various physical media can be used for the actual transmission. Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and maintenance. Media are roughly grouped into guided media, such as copper wire and fiber optics, and unguided media, such as radio and lasers through the air. The most common type of media is copper cable. The most common types of copper cabling are twisted-pair and coaxial and coaxial cabling.Another type of LAN connection media is fiber-optic cable. Consisting of a number of glass or high-grade plastic optical strands surrounded by a tough cloth-and-plastic wrap, fiber-optic cables resemble coaxial cables from the outside. Wireless transmissions use radio waves or infrared light to transmit data.

3. Transmission Media There are two categories of transmission media: Guided media Unguided media • Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only. • Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media. • There re three basic types of Guided Media: • Twisted pair • Coaxial cable • Optical fiber

4. Guided Transmission Data • Magnetic Media • Twisted Pair • Coaxial Cable • Fiber Optics

5. Twisted pair • The wires in Twisted Pair cabling are twisted together in pairs. • Each pair would consist of a wire used for the positive data signal and a wire used for the negative data signal. • Any noise that appears on one wire of the pair would occur on the other wire • The noise appears on both wires; it cancels itself. • Cables with a shield are called Shielded Twisted Pair and commonly abbreviated STP. Cables without a shield are called Unshielded Twisted Pair or UTP. Twisting the wires together results in a characteristic impedance for the cable. Typical impedance for UTP is 100 ohm for Ethernet 10BaseT cable.

6. Unshielded Twisted Pair (UTP) Cable

7. UTP or Unshielded Twisted Pair cable is used on Ethernet 10BaseT and can also be used with Token Ring. It uses the RJ line of connectors (RJ45, RJ11, etc...) • STP or Shielded Twisted Pair is used with the traditional Token Ring cabling .

8. Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP.

9. 2. Coaxial cable • Coaxial Cable consists of two conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. • An insulating protective coating called a jacket covers the outer conductor.  The outer shield protects the inner conductor from outside electrical signals. • The distance between the outer conductor (shield) and inner conductor plus the insulating material determine the cable properties or impedance. • Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet.

10. Coaxial Cable

11. Coaxial Cable A coaxial cable.

12. 3. Optical fiber • Optical Fiber consists of thin glass fibers that can carry information at frequencies in the visible light spectrum and beyond. • The typical optical fiber consists of a very narrow strand of glass called the Core. Around the Core is a concentric layer of glass called the Cladding. • A typical Core diameter is 62.5 microns (1 micron = 10-6 meters). • Typically Cladding has a diameter of 125 microns. • Coating the cladding is a protective coating consisting of plastic, it is called the Jacket

13. Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

14. Fiber Optic Cable

15. An important characteristic of fiber optics is Refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it "bends" as it passes from one medium to the other. • If the angle of incidence is small, the light rays are reflected and do not pass into the water. If the angle of incident is great, light passes through the media but is bent or refracted. • The core refracts the light and the cladding reflects the light. • The core refracts the light and guides the light along its path. • The cladding reflects any light back into the core and stops light from escaping through it • Optical Transmission Modes

16. Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection.

17. There are three types of fiber optic transmission modes a) Step Index b) Grade Index c) Single Mode • Step Index has a large core the light rays tend to bounce around, reflecting off the cladding, inside the core. • This causes some rays to take a longer or shorted path through the core. Some take the direct path with hardly any reflections while others bounce back and forth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. Typical Core: 62.5 microns.

18. Grade Index has a gradual change in the Core's Refractive Index. • This causes the light rays to be gradually bent back into the core path. • This is represented by a curved reflective path in the attached drawing. • The result is a better receive signal than Step Index. LED light sources are used. • Typical Core: 62.5 microns. • Both Step Index and Graded Index allow more than one light source to be used . Multiple channels of data can be run simultaneously!

19. Single Mode has separate distinct Refractive Indexes for the cladding and core. • The light ray passes through the core with relatively few reflections off the cladding. • Single Mode is used for a single source of light (one color) operation. • It requires a laser and the core is very small: 9 microns.

20. Advantages of Optical Fiber: • Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -Electromagnetic Interference) • Security: cannot tap into cable. • Large Capacity due to BW (bandwidth) • Longer distances than copper wire • Smaller and lighter than copper wire • Faster transmission rate Disadvantages of Optical Fiber: • Physical vibration will show up as signal noise • Limited physical arc of cable. Bend it too much & it will break! • Difficult to splice • At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive. • Media versus Bandwidth • The following table shows the bandwidths for the different guided transmission media

21. Wireless Transmission Wireless transmissions use radio waves or infrared light to transmit data. We can subdivide wireless networking technology into three basic types corresponding to three basic networking scenarios: Local area networks (LANs): one or more wireless machines will function as members of a cable-based LAN. A LAN with both wireless and cable-based components is called a hybrid. Extended local networks. A wireless connection serves as a backbone between two LANs. For instance, a company with office networks in two nearby but separate buildings could connect those networks using a wireless bridge. Mobile computing. A mobile machine connects to the home network using cellular or satellite technology. Spaces where cabling would be impossible or inconvenient. These include open lobbies, inaccessible parts of buildings, older buildings, historical buildings where renovation is prohibited, and outdoor installations. People who move around a lot within their work environment. Temporary installations. These situations include any temporary department set up for a specific purpose that soon will be torn down or relocated. People who travel outside of the work environment and need instantaneous access to network resources.

22. Spread-spectrum radio transmission is a technique originally developed by the military to solve several communication problems. Spread-spectrum improves reliability, reduces sensitivity to interference and jamming, and is less vulnerable to eavesdropping than single-frequency radio. Spread-spectrum transmission uses multiple frequencies to transmit messages. Two techniques employed are frequency hopping and direct sequence modulation.1- Frequency hopping switches (hops) among several available frequencies staying on each frequency for a specified interval of time. The transmitter and receiver must remain synchronized during a process called a hopping sequence in order for this technique to work. Range for this type of transmission is up to two miles outdoors and 400 feet indoors. Frequency hopping typically transmits at up to 250 Kbps, although some versions can reach as high as 2 Mbps. 2- Direct sequence modulation breaks original messages into parts called chips which are transmitted on separate frequencies. The intended receiver knows which frequencies are valid and can isolate the chips and reassemble the message. Direct sequence modulation systems operating at 900 MHz support bandwidths of 2-6 Mbps.