The physical layer is responsible for transporting a raw bit stream between machines using various physical transmission media. There are two categories of media: guided and unguided. Guided media uses cabling to guide signals along a specific path, including twisted pair, coaxial cable, and optical fiber. Unguided media like radio and lasers transmit without physical cabling. Twisted pair cable uses two twisted wires to cancel noise and is commonly used for Ethernet. Coaxial cable contains a shielded inner conductor to protect signals. Optical fiber uses thin glass cores and cladding to guide light pulses using total internal reflection.

1. The Physical Layer
Chapter 2
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

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.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

3. Transmission Media
There are two categories of transmission media:
Guided media
Unguided media
a) 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.
b) 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.
c) There re three basic types of Guided Media:
– Twisted pair
– Coaxial cable
– Optical fiber
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

4. Guided Transmission Data
• Magnetic Media
• Twisted Pair
• Coaxial Cable
• Fiber Optics
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

5. Twisted pair
a) The wires in Twisted Pair cabling are twisted together in pairs.
a) Each pair would consist of a wire used for the positive data signal and a wire used for the
negative data signal.
b) Any noise that appears on one wire of the pair would occur on the other wire
c) The noise appears on both wires; it cancels itself.
d) 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.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

6. Unshielded Twisted Pair (UTP) Cable
Computer Networks, Fifth Edition by
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7. a) 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...)
b) STP or Shielded Twisted Pair is used with the traditional Token Ring cabling .
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

8. Twisted Pair
(a) Category 3 UTP.
(b) Category 5 UTP.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

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

10. Coaxial Cable
Computer Networks, Fifth Edition by
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11. Coaxial Cable
A coaxial cable.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

12. 3. Optical fiber
a) Optical Fiber consists of thin glass fibers that can carry information at frequencies in the
visible light spectrum and beyond.
b) 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.
c) A typical Core diameter is 62.5 microns (1 micron = 10-6 meters).
d) Typically Cladding has a diameter of 125 microns.
e) Coating the cladding is a protective coating consisting of plastic, it is called the Jacket
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

13. Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
Computer Networks, Fifth Edition by
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14. Fiber Optic Cable
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

15. a) 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.
b) 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.
c) The core refracts the light and the cladding reflects the light.
d) The core refracts the light and guides the light along its path.
e) The cladding reflects any light back into the core and stops light from escaping through it
f) Optical Transmission Modes
Computer Networks, Fifth Edition by
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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.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

17. a) 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.
a) 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.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

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!
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

19.  Single Mode has separate distinct Refractive Indexes for the cladding and core.
a) The light ray passes through the core with relatively few reflections off the cladding.
b) Single Mode is used for a single source of light (one color) operation.
c) It requires a laser and the core is very small: 9 microns.
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David

20. Advantages of Optical Fiber:
a) Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -
Electromagnetic Interference)
b) Security: cannot tap into cable.
c) Large Capacity due to BW (bandwidth)
d) Longer distances than copper wire
e) Smaller and lighter than copper wire
f) Faster transmission rate
Disadvantages of Optical Fiber:
a) Physical vibration will show up as signal noise
b) Limited physical arc of cable. Bend it too much & it will break!
c) Difficult to splice
d) At higher transmission capacity, it is cheaper than copper. At lower transmission capacity,
it is more expensive.
e) Media versus Bandwidth
f) The following table shows the bandwidths for the different guided transmission media
Cable type Bandwidth
Twisted Pair 100 MHz
Coaxial Cable 600 MHz
Optical Fiber 1 GHz
Computer Networks, Fifth Edition by
Andrew Tanenbaum and David