The Physical Layer
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Transcript The Physical Layer
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
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Magnetic Media
Twisted Pair
Coaxial Cable
Fiber Optics
WIRELESSTRANSMISSION
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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