Transcript Point-to-Point Communication

Point-to-Point Communication
2
Terminal-Host Communication

Components




Terminal
Host (locus of processing)
Transmission line (here, phone line and modem)
Telephone line acts as a point-to-point link
Host
Terminal
Phone
Line
Modem
Modem
3
Terminal Emulation

People Already Have PCs

Host operating systems only work with terminals

Do not want to buy a terminal as well

PCs can emulate (act like) terminals
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Only requires software (a communication
program)

Turns an expensive PC into a cheap terminal
4
VT100 Terminals

VT100 Emulation Only Needs Software


Communications program
Terminal emulation software

Most Hosts Support VT100 Terminals

Lowest Common Denominator



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Slow: maximum speed of 19 kbps, usually slower
Uses inefficient asynchronous ASCII transmission,
discussed later
No graphics or even multiple fonts: plain text only
No color
5
Terminal Emulation Software

File Transfer




Terminal emulation program and host file
transfer program must support the same file
transfer protocol standard



Transfer whole files with error correction
Upload: from PC to host
Download: from host to PC
Kermit
XMODEM, YMODEM
IBM 3270 Terminals
6
Analog and Digital Transmission
Analog
Signal
Digital
Signal
(1101)
Modem
Modulation
Analog
Signal
Digital
Signal
(1101)
Modem
Demodulation
7
Digital and Binary

Digital Transmission

Can have multiple voltage levels, say 4

Change to one at start of each bit cycle

If 4, changes can represent 2 bits each:
00, 01, 10, 11
Voltage
Level

11
10
01
00
Time
8
Baud Rate and Bit Rate

Baud Rate



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Number of times line changes per second
Let baud rate be 4 (4 changes per second)
Let bits per line change be 2
Bit rate = 8 bits per second
Bit rate = x2 Baud rate in this example
11
10
01
00
One Second
9
Wave Characteristics



Amplitude (power)
Frequency (cycles per second, Hertz)
Wavelength (meters)
Wavelength
(meters)
Amplitude
(power)
Frequency (Hz)
One Second
Wave Characteristics

Phase



0 and rising
10
Highest
0 and falling
Lowest
Fully cycle is 360 degrees
Phase is degrees different from reference wave
Human ears cannot hear. Equipment can
Reference
Wave
0o
180
degrees
out of
phase
0o
90o
180o
270o
180o
11
Wave Characteristics

Amplitude

Frequency and Wavelength


Not independent

As frequency rises, wavelength falls
(Shorter guitar strings produce higher notes)

Their product is constant--the speed of light,
sound, etc.
Phase
12
Frequency Modulation
Vary the frequency (wavelength) to represent 1 and 0
Low
Frequency
(0)
Wavelength
Wavelength
High
Frequency
(1)
Frequency
Modulation
(1011)
0
1
1
1
13
Amplitude Modulation
Low
Amplitude
(0)
Amplitude (low)
Amplitude (high)
High
Amplitude
(1)
Amplitude
Modulation
(1011)
14
Phase Modulation
In Phase
(0)
180 degrees
out of phase
(1)
Frequency
Modulation
(1011)
15
Complex Modulation
Vary both amplitude and phase
Several values (not just two) on each dimension
In Phase
High
Amplitude
90 Degrees
Out of Phase,
High Amplitude
Low
Amplitude
180 Degrees Out of Phase
16
Modem Standards
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Modems at Two Ends Must Communicate
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Must follow same standards
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Most modem standards set by ITU-T
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Multiple category of standards:
 Modem speed (modulation)
 Error correction and compression
 Facsimile
 Etc.

When buying a modem, must check for
standard(s) followed in each category
17
Modem Speed Standards

Set by the ITU-T


Govern how modulation is done
Standards for speed governs modulation for
data transmission
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V.92
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V.90
56.6 kbps
V.34
28.8 kbps/33.6 kbps
V.32 bis14.4 kbps
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56.6 kbps plus quick connect,
modem on hold, PCM upstream
18
Modem Speed Standards

Most data modems are also fax modems


V.14
V.29
14.4 kbps
9,600 bps
Error Correction and Compression

ITU-T Standards
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V.42
Error detection and correction
V.42 bisData compression (up to 4:1)
V.44
Data compression (20 to 120%
more than V.42 bis)
Independent of speed standards (but V.44 only
with V.92)
Microcom Standards
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Microcom Network Protocol (MNP)
Both error correction and compression
Several levels
Independent of speed standards
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20
Modem Intelligence
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Computer Can Send Commands to Modem
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Hayes Developed the first Command Set
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Dial a number, including how long to wait, etc.
Called intelligent modems
Most modems follow the same command set
We call them “Hayes compatible”
Commands start with “AT”
Other Standards for Fax Modems
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Class 1 and Class 2: extensions to Hayes
21
Telephone Bandwidth is Limited
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Telephone Transmission
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Cuts off sounds below 300 Hertz
Cuts off sounds above about 3,400 Hz
Bandwidth is the difference between the highest
and lowest frequencies (3400-300): about 3,100
Hz
Sound
Loudness
Bandwidth
3,100 Hz
0
300
3400
Frequency (Hz)
20,000
22
Telephone Bandwidth is Limited
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Speed is Limited

Maximum speed is related to bandwidth
(Shannon’s Law)
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Maximum speed for phone lines for transmission
is a little over 30 kbps

So modems can’t get much faster
23
Another Look at Compression

With 4:1 Compression, a V.34 Modem Can
Receive Data at 115.2 kbps from the PC

However the ~30 kbps limit of the phone
system is not exceeded. Still transmit at 33.6
kbps.
~35 kbps
Maximum
Compression
in Modem
115.2 kbps
33.6 kbps
24
56 kbps Analog Modems

From home, you transmit

Analog-to-Digital Converter (ADC)
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
Filters your signal to a bandwidth of ~3.1 kHz
This limits you to 33.6 kbps
PC
V.34
modem
33.6 kbps
ADC
Telephone
Network
25
56 kbps Analog Modems
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But ISP Can Connect Digitally
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PC
Signal travels through phone system at 56 kbps
At user end, digital-to-analog converter (DAC)
Sends signal to analog modem at wide bandwidth
Modem can receive at 56 kbps
56 kbps
modem
DAC
Telephone
Network
56 kbps
Digital
Link
ISP
56 kbps Modems

What they can do

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
Problems
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Send at 33.6 kbps (V.92 with PCM upstream can
go up to 48 kbps)
Receive at 56 kbps (V92 with V.44 compression
can go up to 120 kbps)
past: competing standards from Rockwell, U.S.
Robotics (V.90 ended them)
present: ISPs must support V.92 (all support V.90)
Users and ISPs


Users V.90 analog modem or V.92
ISPs V.90 digital modem or V.92
26
27
56 Kbps Modems

Telephone company

No changes needed, although ...

Many not have an internal ADC conversion
between ISP and customer (some do)

May not have long transmission line from last
switch to the customer premises (local loop)

Not all phone lines to customer premises will
support 56 kbps modems

Even when they do, speeds may only be 40-50
kbps
28
Half-Duplex Transmission
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Sender and receiver must take turns sending
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Like an old one-lane road
No interruption for error handling or flow control
A
B
Time 1
Only one side
May communicate
A does
A
B
Time 2
Only one side
May communicate
B does
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Full-Duplex Transmission
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Both Sides May Transmit Simultaneously
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Needed for error correction, flow control
Now almost universal in modem communication
A
B
Time 1
Both sides may communicate
Both do
A
B
Time 2
Both sides may communicate
A does
30
Asynchronous Transmission
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ASCII Character Set
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7-bit is the standard
8-bit extended ASCII is popular
Bits transmitted backward
Parity for Error Detection
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Only for 7-bit ASCII
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Start/Stop Bits for Framing
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Each frame is exactly 10 bits long
31
Asynchronous Transmission
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ASCII Character Set
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Created for sending printed American text
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Each character is a 7-bit code (e.g., 1010101)
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This allows 2^7 or 128 possible characters
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Printing characters: A, a, !, <, %, etc.
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Control codes: XOFF tells other side to pause
32
Asynchronous Transmission
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8-bit ASCII
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Used in PCs: 8 bits per character (10101010)
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Used for word processing format codes
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Used in graphics that stores data in bytes
33
PC Serial Port
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Bit Transmission of ASCII Characters
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Transmits last bit first
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If you wish to send 1111000,
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The serial port transmits 0001111
34
Parity

For 7-bit ASCII Only (No Parity = 8-bit ASCII)
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Transmit an 8th bit per character
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Even parity: sum of data and parity bits is
even
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To send 1110000 (odd), send 00001111
To send 1111000 (even), send 00011110
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Odd parity: sum is odd
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If error is detected, the character is simply
discarded. No way to ask for retransmission
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Start and Stop Bits
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When the Data Line is at Rest
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It is kept in the “1” state
So “11110000” would look like 111111100001111
“00001111” would also look like 11111100001111
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How can you tell where a character begins?
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Solution
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Add a start bit (always 0) to change the line state
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End with a stop bit (always 1) to guarantee at least
a one-bit rest (1) against which to detect the next
start bit (0)
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The Final Asynchronous Frame
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Always 10 bits
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Start, 7 data bits, parity, stop, or
Start, 8 data bits, stop
Start
Parity
0 1 1 1 0 0 0 1 1 1
7-bit ASCII Character
Start
0 1 1 1 0 0 0 1 1 1
8-bit ASCII Character
Stop
Stop
37
Flow Control

Ask the Other Device to Pause (or Slow
Down)
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ASCII


In asynch, usually done by sending ASCII control
codes

XOFF tells other side to pause

XON tells the other device to resume
Serial Port

Signals on the pins control when PC, modem can
transmit
Signal and Propagation
38
A signal is a disturbance in the media that propagates
(travels) down the transmission medium to the receiver
If propagation effects are too large, the receiver will not be
able to read the received signal
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39
Binary-Encoded Data
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Computers store and process data in binary
representations
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Binary means “two”
There are only ones and zeros
Called bits
1101010110001110101100111
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Binary-Encoded Data
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Non-Binary Data Must Be Encoded into
Binary
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Text
Integers (whole numbers)
Decimal numbers
Alternatives (North, South, East, or West, etc.)
Graphics
Human voice
etc.
Hello
11011001…
3-40
41
Layering Perspective
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Where is binary data encoding done?
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
It is done at the application layer, not at the
physical layer.
Where is signaling done

It is done at the physical layer
3-41
42
Signaling
On/Off Signaling
43
On/off signaling is used in optical fiber
The light is turned on during a clock cycle for a 1
The light is turned off during a clock cycle for a 0
There are two signaling states—on and off
This is called binary signaling
This is a simple type of signaling
3-43
Binary Voltage Signaling in 232 Serial Ports44
1
The high state (0) is anything from +3 to +15 volts
The low state (1) is anything from -3 to -15 volts
3-44
45
Relative Immunity to Errors in Binary Signaling
Binary signaling gives some immunity to errors.
This is one of its major attractions.
3-45
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UTP Propagation
Unshielded Twisted Pair wiring
Unshielded Twisted Pair (UTP) Wiring 47

UTP Characteristics

Inexpensive and to purchase and install

Dominates media for access links between computers
and the nearest switch
3-47
Unshielded Twisted Pair (UTP) Wiring 48

Standards

The TIA/EIA-568 standard governs UTP wiring in the
United States

In Europe, the comparable standard is ISO/IEC 11801
3-48
4-Pair UTP Cord with RJ45 Connector49
3.
8-pin
RJ-45
Connector
1.
UTP cord
Industry standard pen
2.
8 Wires
organized
as 4
twisted
pairs
UTP cord
3-49
Unshielded Twisted Pair (UTP)
Wiring

50
Cord Organization


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
A length of UTP wiring is a cord
Each cord has eight copper wires
 Each wire is covered with dielectric
(nonconducting) insulation
The wires are organized as four pairs
 Each pair’s two wires are twisted around each
other several times per inch
There is an outer plastic jacket that encloses the
four pairs
3-50
Unshielded Twisted Pair (UTP)
Wiring

51
RJ-45
Jack
Connector


RJ-45 connector is the standard
connector
Plugs into an RJ-45 jack in a NIC, switch, or wall
jack
RJ-45
Jack
8-pin RJ-45 connectors
3-51
52
Attenuation and Noise
Power
1.
4.
Signal Noise Spike
5.
Error
2.
Signalto-Noise
Ratio (SNR)
1.
2.
3.
4.
5.
6.
3.
Noise Floor
(Average Noise level)
2.
Noise
Distance
The signal attenuates (falls in power) as it propagates
There is noise (random energy) in the wire that adds to the signal
The average noise level is called the noise floor
Noise is random. Occasionally, there will be large noise spikes
Noise spikes as large as the signal cause errors
You want to keep the signal-to-noise ratio high
3-52
53
Limiting UTP Cord Length

Limit UTP cord length to 100 meters

This keeps the signal-to-noise ration (SNR) high

This makes attenuation and noise problems
negligible

Note that limiting cord lengths limits BOTH noise
and attenuation problems
100 Meters Maximum
Cord Length
3-53
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UTP Wiring

Electromagnetic Interference (EMI)

Electromagnetic interference is electromagnetic
energy from outside sources that adds to the signal


From fluorescent lights, electrical motors,
microwave ovens, etc.
The problem is that UTP cords are like long radio
antennas

They pick up EMI energy nicely

When they carry signals, they also send EMI
energy out from themselves
3-54
Electromagnetic Interference (EMI)55
and Twisting
Electromagnetic
Interference (EMI)
UTP is twisted
specifically to reduce EMI
Twisted
Wire
Interference on the Two Halves of a Twist Cancels Out
3-55
56
Crosstalk Interference and Terminal Crosstalk
Interference
Untwisted
at Ends
Signal
Crosstalk Interference
Terminal crosstalk interference
Terminal Crosstalk normally is the biggest EMI problem for UTP
Interference
3-56
Interference Hierarchy

57
EMI is any interference

Signals in adjacent pairs interfere with one another
(crosstalk interference). This is a specific type of EMI

Crosstalk interference is worst at the ends, where the
wires are untwisted. This is terminal crosstalk
interference—a specific type of crosstalk EMI
EMI
Crosstalk Interference
Terminal Crosstalk
Interference
3-57
58
Terminal Crosstalk Interference

Terminal crosstalk interference dominates
interference in UTP

Terminal crosstalk interference is limited to an
acceptable level by not untwisting wires more
than a half inch (1.25 cm) at each end of the
cord to fit into the RJ-45 connector

This reduces terminal crosstalk interference
to a negligible level.
1.25 cm or 0.5 inches
3-58
59
Shielded Twisted Pair Wiring (STP)

We have been talking about unshielded twisted
pair wiring.

Is there a shielded twisted pair wiring?




Yes. It has a metal mesh shield around each pair to
reduce cross-talk interference
It also has a metal mesh shield around the
four pairs to reduce external EMI
It is no longer used extensively because UTP,
which is much less expensive, was found to be
good enough for normal environments
However, we will see that Cat 7 wiring uses STP
3-59
2
60
UTP Limitations

Limit cords to 100 meters


Do not untwist wires more than 1.25 cm (a half
inch) when placing them in RJ-45 connectors


Limits BOTH noise AND attenuation problems to an
acceptable level
Limits terminal crosstalk interference to an acceptable
level
Neither completely eliminates the problems but
they usually reduce the problems to negligible
levels
3-60
3-18: Serial Versus Parallel
Transmission
61
3-61
62
Optical Fiber
Transmission
Light through Glass
Spans Longer Distances than UTP
Optical Fiber Transceiver and Strand
63
An optical fiber strand has a thin glass core
This core is 8.3, 50, or 62.5 microns in diameter
This glass core is surrounded by a tubular glass cladding
The outer diameter of the cladding is 125 microns,
regardless of the core’s diameter
The transceiver injects laser light into the core
3-63
Optical Fiber Transceiver and Strand
64
When a light wave ray hits the core/cladding boundary,
there is perfect internal reflection. There is no signal loss
3-64
Roles of UTP and Optical Fiber in LANs
65
3-65
Two-Strand Full-Duplex Optical Fiber Cord 66
with SC and ST Connectors
Cord
Two
Strands
A fiber cord has
two-fiber strands
for full-duplex
(two-way)
transmission
SC Connectors
ST Connectors
3-66
Full-Duplex Optical Fiber Cord with SC and ST
67
Connectors
SC Connector
(push and click)
ST Connector
(bayonet connectors:
push and click)
In contrast to UTP, which always uses RJ-45 connectors,
there are several optical fiber connector types
SC and ST are the most popular
3-67
68
Optical Fiber Strand
In optical fiber transmission, light is expressed in nanometers.
The transceiver transmits at 850 nm, 1,310 nm, or 1,550 nm
Shorter-wavelength (850 nm) transceivers are less expensive
Longer-wavelength (1,310 or 1,550 nm) light travels farther for a given speed
For LAN fiber, 850 nm provides sufficient distance and dominates
3-68
69
Multimode Fiber and Single-Mode Fiber
Multimode fiber has a thick core (50 or 62.5 microns in diameter)
Light can only enter the core at certain angles, called modes
Modes traveling straight through arrive faster than
modes that bounce against the cladding several times
3-69
70
Radio Propagation
Radio Propagation
71
Radio signals also propagate as waves.
As noted earlier, radio waves are measured in hertz (Hz),
which is a measure of frequency.
Radio usually operates in the MHz and GHz range.
3-71
Omnidirectional and Dish Antennas
72
3-72
Wireless Propagation Problems
73
UTP and optical fiber propagation are fairly predictable.
However, radio suffers from many propagation effects.
This makes radio transmission difficult to manage.
3-73
74
Topology
Network topology is the physical
arrangement of a network’s computers,
switches, routers, and transmission lines
It is a physical layer concept:




Point-to-point
Star
Bus
Ring