The Physical Layer

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Chapter 2 The Physical Layer
The lowest layer of reference model. It
defines the mechanical, electrical, and timing
interfaces to the network.
1
BANDWIDTH AND INFORMATION CAPACITY
Bandwidth is the span of frequencies within the spectrum
occupied by a signal and used by the signal for conveying
information.
Carrying information requires bandwidth.
2
NOISELESS CHANNEL: NYQUIST BIT
RATE
bit rate  2  bandwidth log2 L
L is the number of signal levels used to represent
data.
Increasing the levels of a signal may reduce
the reliability of the system.
3
NOISY CHANNEL: SHANNON CAPACITY
The theoretical highest data rate for a noisy channel
signal power
Capacity bandwidthlog2 (1 
)
Noise power
S
C  B log2 (1  )
N
where capacity is in bits/second, bandwidth is in hertz,
and signal and noise powers are measured in the same
physical units, such as watts. Bits are fundamental units of
information.
4
USING BOTH LIMITS
The Shannon capacity gives us the
upper limit;
the Nyquist formula tells us how many
signal levels we need.
5
GUIDED TRANSMISSION DATA
•
•
•
•
Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
WIRELESSTRANSMISSION
•
•
•
•
•
The Electromagnetic Spectrum
Radio Transmission
Microwave Transmission
Infrared and Millimeter Waves
Lightwave Transmission
6
TWISTED PAIRS
Category 5e UTP cable with four twisted pairs
7
COAXIAL CABLE
A coaxial cable
8
POWER LINES
A network that uses household electrical wiring.
9
FIBER CABLES
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
10
DIGITAL SUBSCRIBER LINES (3)
A typical ADSL equipment configuration.
11
FIBER TO THE HOME
Passive optical network for Fiber To The Home.
12
MODEMS
(a) A binary signal
(b) Amplitude modulation
13
(c) Frequency modulation
(d) Phase modulation
SIGNAL ENCODING TECHNIQUES
Digital
data, digital signal(Ethernet)
Analog data, digital signal(PCM)
Digital data, analog signal(ADSL)
Analog data, analog signal(phone)
14
LINE CODING
SCHEMES
15
LINE CODES
16
POLAR (+ AND – VOLTAGES) NRZ
17
BIPHASE


Manchester
 Transition in middle of each bit period
 Transition serves as clock and data
 Low to high represents one
 High to low represents zero
 Used by IEEE 802.3
Differential Manchester
 Mid-bit transition is clocking only
 Transition at start of a bit period represents zero
 No transition at start of a bit period represents one
 Note: this is a differential encoding scheme
 Used by IEEE 802.5
18
MANCHESTER ENCODING
19
Effect of lack of synchronization
4.20
TQ 6. The waveform of following figure belongs to a
Manchester encoded binary data stream.
Determine the beginning and end of bit periods
(i.e., extract clock information) and give the data
sequence.
21
BIPOLAR (+, 0, - VOLTAGES) SCHEMES
Alternate Mark Inversion (AMI): 1s are
represented by alternating positive and
negative voltages.
22
Note
In mBnL schemes, a pattern of m data
elements is encoded as a pattern of n
signal elements in which 2m ≤ Ln.
23
Multilevel: 2B1Quaternary scheme, used in DSL
4.24
Multilevel: 8Binary6Ternary scheme, used in 100Base-T4
25
Figure 4.12
Multilevel: 4D-PAM5 scheme
4.26
Multitransition: MLT-3 scheme, G-bit Ethernet
4.27
Table 4.1
Summary of line coding schemes
4.28
SIMPLE CIRCUIT SWITCHED NETWORK
29
PACKET SWITCHED NETWORK
30
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