Page 1
Discussion
▪ You have been tasked to design an organizational LAN. The organizations
Manager has asked you to provide a LAN that will make use of twelve (12)
PCs, a photocopier, two laser printer and four ink jet printers. In addition,
some employees will need to access the LAN from home to transfer files
and to do remote printing. There is currently no network in the
organization. The printers are attached to particular desktops, and the
photocopier is not attached to any computers.
▪ Using a physical star topology, draw a network diagram of your proposed
LAN. Label all hardware components in your diagram. Support your
network diagram, by providing an explanation of the following:
i. Equipment, software, and cabling that will be needed to connect
each device to the LAN
ii.Details of configuration of the hardware, software, and
communications capabilities required at the LAN and home user
ends of the connection.
Page 2
A company network structure
Page 3
Remote-access VPNs
Page 4
Page 5
Review
Data Communication
Telecommunication
Networking
Transfer of Data/Information
from A to B
1. Source/Destination
2. Transmiter
3. Receiver
4. Message
5. medium
The exchange of
A network is a set of
data/information in any form devices/nodes connected by
(voice, data, text, images,
communication links.
audio, video) over networks.
Page 6
Elements of a Communication System
Guided/Unguided
Communications Channel
Source
Transmitter
Signal
Signal
Transmission system
Source system
Source: generates
data to be transmitted
Transmitter:
Converts data into
transmittable signals
Receiver
Transmission
System:
Carries data
Destination
Signal
Destination system
Receiver: Converts
received signal into data
Destination: Takes
incoming data
Protocols and Standards
Page 7
Electromagnetic Spectrum for Transmission
Media
Page 8
Media Summary: Backbone Network (BN), LAN,
WAN
Page 9
A Transmission System’s Communication
channel
▪ Transmitter
⚫
⚫
Converts information into signal suitable for transmission
Injects energy into communications medium or channel
⚫ Telephone converts voice into electric current
⚫ Modem converts bits into tones
▪ Receiver
⚫
⚫
Receives energy from medium
Converts received signal into form suitable for delivery to user
⚫ Telephone converts current into voice
⚫ Modem converts tones into bits
Page 10
Encoding and Modulation
▪ Encoding is about representing a signal, while modulation is about
changing a signal.
▪ Encoding is used for efficient transmission and storage, while
modulation is used for long-distance communication.
Page 11
Goals of Line Coding
▪ Self-synchronization.
▪ The ability to recover timing from the signal itself.
▪ Long series of ones and zeros could cause a problem.
▪ Low probability of bit error.
▪ The receiver needs to be able to distinguish the waveform associated with a
mark from the waveform associated with a space, even if there is a
considerable amount of noise and distortion in the channel.
▪ Spectrum that is suitable for the channel.
▪ In some cases DC components should be avoided if the channel has a DC
blocking capacitance.
▪ The transmission bandwidth should be minimized.
Page 12
Line coding schemes
Unipolar
Polar
Line Coding
All signal levels are on one side of the time axis - either
above or below.
The voltages are on both sides of the time axis.
Bipolar
defines three voltage methods: positive, negative, and zero.
Multilevel
increase the number of data bits per symbol thereby increasing
the bit rate
Multitransitional
Codes can be created that are differential at the bit level
forcing transitions at bit boundaries.
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Unipolar
▪ All signal levels are on one side of the time axis - either
above or below
▪ NRZ - Non Return to Zero scheme is an example of this
code. The signal level does not return to zero during a
symbol transmission.
▪ Scheme is prone to baseline wandering and DC
components. It has no synchronization or any error
detection. It is simple but costly in power consumption.
Page 14
Polar - NRZ
▪ The voltages are on both sides
of the time axis.
▪ Polar NRZ scheme can be
implemented with two
voltages. E.g. +V for 1 and -V
for 0.
▪ There are two versions:
▪ NZR - Level (NRZ-L) - positive
voltage for one symbol and
negative for the other
▪ NRZ - Inversion (NRZ-I) - the
change or lack of change in
polarity determines the value of a
symbol. E.g. a “1” symbol inverts
the polarity a “0” does not.
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Polar - RZ
▪ The Return to Zero (RZ) scheme uses
three voltage values. +, 0, -.
▪ Each symbol has a transition in the
middle. Either from high to zero or
from low to zero.
▪ This scheme has more signal
transitions (two per symbol) and
therefore requires a wider bandwidth.
▪ No DC components or baseline
wandering.
▪ Self synchronization - transition
indicates symbol value.
▪ More complex as it uses three voltage
level. It has no error detection
capability.
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Polar - Biphase: Manchester and
Differential Manchester
▪ Manchester coding consists of
combining the NRZ-L and RZ
schemes.
▪ Every symbol has a level transition in
the middle: from high to low or low to
high. Uses only two voltage levels.
▪ Differential Manchester coding
consists of combining the NRZ-I
and RZ schemes.
▪ Every symbol has a level transition in
the middle. But the level at the
beginning of the symbol is
determined by the symbol value. One
symbol causes a level change the
other does not.
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Bipolar - AMI and Pseudoternary
▪ Code uses 3 voltage levels: - +,
0, -, to represent the symbols
(note not transitions to zero as
in RZ).
▪ Voltage level for one symbol is
at “0” and the other alternates
between + & -.
▪ Bipolar Alternate Mark Inversion
(AMI) - the “0” symbol is
represented by zero voltage and
the “1” symbol alternates
between +V and -V.
▪ Pseudoternary is the reverse of
AMI.
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Bipolar C/Cs
▪ It is a better alternative to NRZ.
▪ Has no DC component or baseline wandering.
▪ Has no self synchronization because long runs of
“0”s results in no signal transitions.
▪ No error detection.
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Multilevel Schemes
▪ In these schemes we increase the number of data
bits per symbol thereby increasing the bit rate.
▪ Since we are dealing with binary data we only have 2
types of data element a 1 or a 0.
▪ We can combine the 2 data elements into a pattern of
“m” elements to create “2m” symbols.
▪ If we have L signal levels, we can use “n” signal
elements to create Ln signal elements.
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Code C/Cs
▪ Now we have 2m symbols and Ln signals.
▪ If 2m > Ln then we cannot represent the data
elements, we don’t have enough signals.
▪ If 2m = Ln then we have an exact mapping of one
symbol on one signal.
▪ If 2m < Ln then we have more signals than symbols
and we can choose the signals that are more distinct
to represent the symbols and therefore have better
noise immunity and error detection as some signals
are not valid.
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Representing Multilevel Codes
▪ We use the notation
mBnL, where m is the
length of the binary
pattern, B represents
binary data, n
represents the length of
the signal pattern and L
the number of levels.
▪ L = B binary, L = T for 3
ternary, L = Q for 4
quaternary.
Page 22
Multitransition Coding
▪ Because of synchronization
requirements we force transitions. This
can result in very high bandwidth
requirements -> more transitions than
are bits (e.g. mid bit transition with
inversion).
▪ Codes can be created that are differential
at the bit level forcing transitions at bit
boundaries. This results in a bandwidth
requirement that is equivalent to the bit
rate.
▪ In some instances, the bandwidth
requirement may even be lower, due to
repetitive patterns resulting in a periodic
signal.
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(a) Punched Tape
Volts
(b) Unipolar NRZ
1
1
0
1
0
0
Mark
(hole)
Mark
(hole)
space
Mark space space
(hole)
1
BINARY DATA
Mark
(hole)
A
0
Tb
Time
A
(c) Polar NRZ
0
-A
A
(d) Unipolar RZ
0
A
(e) Bipolar RZ
0
-A
A
(f) Manchester NRZ 0
-A
Binary Signaling Formats
Page 24
Todays Agenda
The Data Communications Interface
Data Link Control Protocols
▪ Understanding of:
▪ Understanding of:
 Asynchronous and Synchronous
Transmission;
 Flow Control; Model of Frame
Transmission;
 Interfacing; Characteristics of Interface;
V.24/EIA-232-F; Mechanical Specification;
Electrical Specification;
 Stop and Wait;
▪ Functional Specification; Procedural
Specification; ISDN Physical Interface &
Electrical Specification;
 Error Detection;
 Sliding Window Flow Control;
▪ High-Level Data Link Control (HDLC):
Characteristics and operation
Page 25
Transmission
▪ The process of transferring data from one place to another.
▪ The two main types of data transmission used in computer
networking:
▪ Parallel:
▪ More than one bit sent at a time
▪ Serial:
▪ One bit sent at a time
1. synchronous transmission,
2. asynchronous transmission,
3. Isochronous
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Bitwise Data Transmission
▪ Data transmission requires:
▪ Encoding bits as energy
▪ Transmitting energy through medium
▪ Decoding energy back into bits
▪ Energy can be electric current, radio, infrared, light, smell, etc.
▪ Transmitter and receiver must agree on encoding scheme and
transmission timing
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Parallel Transmission
▪ Multiple data bits transferred at the same time over separate
media
▪ Generally used with wired medium with multiple independent
wires
▪ Signals on all wires are synchronized
▪ Each wire carries the signal for one bit
▪ All wires operate simultaneously
▪ Other wires allow sender and receiver to coordinate
▪ Wires placed in a single large cable
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Parallel Transmission
▪ Advantages
▪ High speed
▪ Faster than serial as more wires!
▪ Match to underlying hardware
▪ Computers and communication hardware use parallel circuitry internally
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Serial Transmission
▪ Sends one bit at a time
▪ Most communications system use serial mode
▪ Cheaper to extend over long distances
▪ Fewer wires
▪ Intermediate electronic components are cheaper
▪ Never a timing problem caused by one wire being slightly longer than
another
▪ Because only one wire
▪ Sender and receiver need small amount of hardware to convert data
from parallel form used in the device to serial form used on the wire
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Serial Transmission
▪ Transmission Order: Bits and Bytes
▪ Which bit should be sent across a medium first?
▪ Most Significant Bit (MSB)
▪ Big Endian
▪ Least Significant Bit (LSB)
▪ Little Endian
▪ Which byte should be sent first??
▪ Byte order and bit order can be chosen independently
Page 31
Transmission Order
Ethernet technology specifies that data is sent byte bigendian and bit little-endian
Page 32
Timing of Serial Transmission
Three broad categories of serial transmission
▪
Asynchronous
▪ Transmission can occur at any time
▪ Arbitrary delay between the transmission of two data items
▪
Synchronous
▪ Continuous transmission
▪ No gap between transmission of data items
▪
Isochronous
▪ Transmission occurs at regular intervals
▪ Fixed gap between transmission of two data items
Page 33
Asynchronous Transmission
▪ One definition of asynchronous: transmitter and
receiver do not explicitly coordinate each data
transmission
▪ Transmitter can wait arbitrarily long between transmissions
▪ Used, for example, when transmitter such as a keyboard
may not always have data ready to send
▪ Asynchronous may also mean no explicit information
about where data bits begin and end
▪ E.g. when we send individual ASCII characters
Page 34
Transmission Timing Problems
▪ Encoding scheme leaves several questions unanswered:
▪ How long will voltage last for each bit?
▪ How soon will next bit start?
▪ How will the transmitter and receiver agree on timing?
▪ Later : Self-clocking codes (e.g. Manchester Encoding)
▪ Standards specify operation of communication systems
▪ Devices from different vendors that adhere to the standard can
interoperate
▪ Example organizations:
▪ International Telecommunications Union (ITU)
▪ Electronic Industries Association (EIA)
▪ Institute for Electrical and Electronics Engineers (IEEE)
Page 35
Asynchronous Transmission
• Allows physical medium to be idle for an arbitrary amount of time between
transmissions
• Good for applications that generate data at random
• E.g. keyboard connected to a computer
• BUT – lack of coordination between sender and receiver
• Receiver does not know when the sender will transmit
• transmit when data are ready
• variable delays between transmissions
• no sender-receiver coordination beforehand
• Technically, the electrical signal does not contain information about where
individual bits begin and end
• Therefore common for technologies to specify a preamble
• Few extra bits before each data item to tell receiver data are coming
Page 36
Using Electric Current to Send Bits
▪ Simple idea - use varying voltages to represent 1s and 0s
▪ One common encoding use negative voltage for 1 and positive
voltage for 0
▪ In following figure, transmitter puts positive voltage on line for 0 and
negative voltage on line for 1
Page 37
Transmission Timing Problems
▪ Encoding scheme leaves several questions unanswered:
▪ How long will voltage last for each bit?
▪ How soon will next bit start?
▪ How will the transmitter and receiver agree on timing?
▪ Later : Self-clocking codes (e.g. Manchester Encoding)
▪ Standards specify operation of communication systems
▪ Devices from different vendors that adhere to the standard can interoperate
▪ Example organizations:
▪ International Telecommunications Union (ITU)
▪ Electronic Industries Association (EIA)
▪ Institute for Electrical and Electronics Engineers (IEEE)
Page 38
RS-232
▪ Standard for transfer of characters across copper
wire
▪ Produced by EIA
▪ Full name is RS-232-C
▪ RS-232 defines serial, asynchronous communication
▪ Serial - bits are encoded and transmitted one at a time (as
opposed to parallel transmission)
▪ Asynchronous - characters can be sent at any time and bits
are not individually synchronized
Page 39
Details of RS-232
▪ Components of standard:
▪ Connection must be less than 50 feet
▪ Data represented by voltages between +15v and -15v
▪ 25-pin connector, with specific signals such as data,
ground and control assigned to designated pins
▪ Specifies transmission of characters between, e.g., a
terminal and a modem
▪ Transmitter never leaves wire at 0v; when idle, transmitter
puts negative voltage (a 1) on the wire
Page 40
Identifying asynchronous
characters
▪ Transmitter indicates start of next character by
transmitting a one
▪ Receiver can detect transition as start of character
▪ Extra one called the start bit
▪ Transmitter must leave wire idle so receiver can
detect transition marking beginning of next
character
▪ Transmitter sends a zero after each character
▪ Extra zero call the stop bit
▪ Thus, character represented by 7 data bits
requires transmission of 9 bits across the wire
Page 41
Start, Stop Bits
Typically one of the data bits might be a parity bit
(7N1, 8E1)…
Page 42
Timing
▪ Transmitter and receiver must agree on
timing of each bit
▪ Agreement accomplished by choosing
transmission rate
▪ Measured in bits per second
▪ Detection of start bit indicates to receiver
when subsequent bits will arrive
▪ Hardware can usually be configured to
select matching bit rates
▪ Switch settings
▪ Software
▪ Autodetection
Page 43
Transmission Rates
▪ Baud rate measures number of signal changes per
second
▪ Bits per second measures number of bits transmitted
per second
▪ In RS-232, each signal change represents one bit, so
baud rate and bits per second are equal
▪ If each signal change represents more than one bit,
bits per second may be greater than baud rate
▪ This is the case with modems nowadays!
▪ More on this when we look at modulation
Page 44
Framing
▪ Start and stop bits represent framing of each character
▪ If transmitter and reciver are using different speeds, stop bit
will not be received at the expected time
▪ Problem is called a framing error
▪ RS-232 devices may send an intentional framing error called a
BREAK
Page 45
Duplex
▪ Two endpoints may send data
simultaneously - full-duplex
communication
▪ Requires an electrical path in each direction
▪ If only one endpoint may send data – halfduplex communications or simplex
▪ Pin 2 - Receive (RxD)
▪ Pin 3 - Transmit (TxD)
▪ Pin 4 - Ready to send (RTS)
▪ Pin 5 - Clear to send (CTS)
▪ Pin 7 - Ground
Page 46
Limitations on Transmission
▪ Limitations on wires makes waveforms look like:
▪ Longer wire, external interference may make signal
look even worse
▪ RS-232 standard specifies how precise a waveform
the transmitter must generate, and how tolerant the
receiver must be of imprecise waveform
Page 47
Page 48
Interfaces
▪ Transmission of data from the source to a device or from a
device to the destination
▪ Parallel transmission:
Multiple lines carrying bits simultaneously
▪ High data rate, but expensive
▪ Serial transmission
Bits transmitted serially
▪ Synchronous vs. Asynchronous
Page 49
Serial I/O Protocols
▪ Synchronous:
A master clock controls the transmission as
a continuous stream
▪ Asynchronous:
Random delays between data pieces
Synchronous
Requires processing
to extract clock
Asynchronous
No clock recovery needed
Overhead applies to entire block c.a. 20% overhead/character
Error detection and correction
built into protocol
Error detection possible,
correction done separately
Page 50
Asynchronous Protocols
▪ RS-232-C
▪ 20MA Current Loop
▪ RS-422, RS-423, RS-485
RS: Recommended Standard by EIA
(Electronic Industries Association)
1, 1½, 2
Stop Bits
Mark
Space
Start
Bit
5 to 8 Data Bits
LSB First
110 - 19.2k bps
Page 51
Start and Stop Bits
▪ Start bit permits local synchronization
▪ Stop bit provides validity check and the opposite
level for the start bit
▪ Implementation with 16X clock
…
See beginning
of start bit
…
Starting from 8th tick,
sample every 16th tick
Page 52
RS-232-C Interface
▪ EIA in cooperation with Bell Systems,
independent modem and computer
manufacturers
▪ Standard for interface between Data Terminal
Equipment (DTE) and Data Communication
Equipment (DCE) employing serial bit
interchange
Telephone
Network
DTE
DTE
DCE
RS-232-C
DCE
RS-232-C
Page 53
RS-232-C
▪ Standards contain
▪ Electrical signal characteristics
▪ Interface mechanical characteristics
▪ Functional description of interchange circuits
▪ Standard subsets for specific groups of communication
systems applications
▪ Mechanical
▪ DB-25 or DB-9 connectors
▪ Cable
▪ Female connected to DTE, male to DCE
▪ Maximum 15 meters
Page 54
▪ Lines/Pins:
RS-232-C
1 Shield Shield
7 GND
Signal ground
2 XMIT
Transmit from DTE to DCE (Modem)
3 RCV
Receive from DCE (Modem)
4 RTS
Request to send, from terminal to modem
5 CTS
Clear to send, from modem to terminal
6 DSR
Data set ready, from modem to terminal
Data set (modem) online
20 DTR
Data terminal ready, from term. to modem
Tie to power
22 RI
Ring indicator, from modem to terminal
“Say hello!”
8 CD
Carrier Detect, from modem to terminal
“I hear the other end”
*
Page 55
RS-232-C
▪ *
▪ Originally designed for half-duplex control
▪ For full-duplex, tie both RTS and CTS true
▪ If RTS and CTS tied together, it means that RTS is OK if other end is plugged in
▪ If CTS is connected to CD, it is OK to talk if both modems are connected
Page 56
Null Modem
▪ Direct connection between two DTEs, e.g.,
terminal and computer, or two computers
directly
DTE
DTE
Shield
Shield
GND
GND
XMIT
RCV
XMIT
RCV
RTS
RTS
CTS
CTS
DTR
DSR
DTR
DSR
Page 57
RS-232-C
▪ Electrical specification:
15 V
Control “ON”
Space
Logical 0
3V
Undefined
Area
Receiver
side
-3 V
12 V
5V
0V
Transmitter
side
-5 V
Control “OFF”
Mark
Logical 1
-12 V
-15 V
15V
3V
-3V
-15V
1
0
1
0
Page 58
RS-232-C
▪ Open circuit ≤ 25V
▪ Driver must be able to sustain short circuit current without
damage; short circuit current ≤ 0.5A
▪ Voltage change not faster than 30V/μs, +3V/-3V transition not
to exceed 1ms or 4% of bit time
▪ Terminator capacitance ≤ 2500pF including cable
Page 59
RS-232-C
▪ Electrical Problems:
▪ ±12V supply needed, inconvenient
▪ Cable capacitance: Maximum 50 ft if cable is 40-50pF/ft!
▪ Ground reference
▪ System has poor common-mode noise rejection
▪ Cross-talk and increase of bias distortion
▪ Especially bad if clock lines used (SYNC)
▪ Not suitable for long distances
→ Motivation for new standards RS-422, 423
Page 60
RS-423
▪ Use RS-449 for functional and mechanical aspects
▪ Created for transition from RS-232 to RS-422
▪ Uses unpopular 37-pin connectors per RS-449
▪ Unbalanced like RS-232-C
▪ All signals use a common return to complete the
circuit
▪ Valid margins: +2V/+6V and -2V/-6V
▪ For less than 20kbps
Page 61
RS-422
▪ Use RS-449 for functional and mechanical aspects
▪ Fully balanced, differential inputs
▪ Supports data rates  20kbps
Length
4k
(ft)
90k
1k
100
10
10k
100k
1M
10M
Baud rate
• Using 24G Twisted-pair, 100Ω load
– Amplitude drop less than 6dB
– Rise time less than ½ bit time
Page 62
RS-485
▪ Like 422, 485 is also balanced
▪ 485 handles multiple drivers and receivers
▪ Better common-mode noise rejection (-7 to +12 Volts)
▪ Sensitivity of ±200mV in receivers
▪ Drivers give up to 5 volts balanced output
▪ Can stand contention, driver shuts down by itself
▪ High input resistance (12K ohms)
▪ Hysteresis of 50 mv to overcome diff. noise
Page 63
20mA Current Loop
▪ Historically, current controlled encoding
▪ Now implemented with optoisolators
▪ High immunity to noise
▪ Distance limited by voltage available
▪ If source has 20V and 750Ω internal resistance, we can add
300Ω wire resistance and still get 18mA → 3650ft.
▪ Pros:
▪ High common mode rejection and high isolator
▪ Cons:
▪ Not standardized
▪ Creates crosstalk in adjacent wires
Page 64