Lecture 4
Continuation of transmission basics
Chapter 3, pages 75-96
Dave Novak School of Business University of Vermont
Sources: 1) Network+ Guide to Networks, Dean 2013 2) Comer, Computer Networks and Internets, 2004 3) Other sources cited within the lecture slides
Objectives
Line coding ModulationAM, FM, Phase Shift
Multiplexing
FDM, TDM, WDM
Broadband –vs- Baseband
How do computers communicate?
At a very basic level computers usebinary digits
(bits)
to represent data / informationBits are transmitted over some medium
Electrical current over copper cable
Pulses of light over fiber optic cable
How do computers communicate?
How can data be represented byelectrical signals?
Can be generally explained via local
asynchronous communication (RS-232) Example: electrical voltage over copper wires
How do computers communicate?
Simple electronic communication systemsElectric current is used to encode data
For example: Negative voltage represents a 1 and positive voltage represents a 0
Transmit a “1” by transmitting negative voltage over a copper wire
Local asynchronous communication
RS-232 (EIA) emphasizes need for standards and
illustrates how they are used in networking
Most widely accepted standard for transferring data across copper wires
Defines serial, asynchronouscommunication
Serial –
-Local asynchronous communication
There are limitations to hardware…Electronic devices cannot produce an exact
voltage or change from one voltage to another instantly
Wires are not perfect conduits Signal loses energy as it travels Takes time to change voltage
Local asynchronous communication
Transmission hardware is typically ratedin
baud
Baud = the signaling rate at which data are
sent through a channel measured in transactions per second
In simple RS-232 baud rate = the bit rate,
as one bit is transferred per signal transition
9600 baud = 9600 bps
This is not true for more complex coding schemes
Local asynchronous communication
Bit rate –vs- baud rate – they aredirectly related to one another
Bit rate – number bits transmitted per sec
Baud rate – number of signaling elements
per sec
Depending on the signaling level or
modulation technique, more than one bit
can be transferred per sec Bit rate = baud/sec x # of bits/baud
Modems
Hardware circuit that accepts sequence
of data bits and applies modulation to a carrier wave is called a
modulator
Hardware circuit that accepts amodulated carrier wave and recreates the sequence of bits used to modulate the carrier is called a
demodulator
To support full duplex transmission, eachsystem needs both – these are combined into a single device called a
modem
Modems
Different types of modems including RF
(wireless) and optical fiber modems
Most familiar with 2-wire dialup modems
Half duplex – take turns sending info
Use a carrier that is an audible tone to mimic a telephone
Note that the term
modem
is not limitedto the dialup device
Modern modems use a combination of
modulation techniques to transmit multiple bits per baud
Some Basic Issues
Why don’t we just use simple RS232 to
transmit 1’s and 0’s?
Desire to transmit large amounts of data
over long distances at really high speeds
Multiple conversion processes as different
types of data travel over different physical networks (for example sending analog data over a digital network)
Transmission errors
Advanced transmission concepts
Line Encoding Modulation Multiplexing
Line encoding
Different line encoding schemes are
used to transmit
digital data
using adigital signal
Improve bit-rate
Decrease bit-error rates
Digital data / digital signals Encoding schemes can vary by
Layer 1 and 2 standards (which also impact
media and distance)
Line Encoding
Modulation
Encode digital data onto a continuous
analog carrier wave by modulating (altering one or more properties of the carrier wave) the signal
Digital datausing analog signals
1) Frequency Modulation 2) Amplitude Modulation 3) Phase-shift Modulation
Analog data using digital signals
4) Pulse Code Modulation
Digital data / analog signal
a) Digital (binary) signalbeing represented b) Amplitude modulation (AM) c) Frequency modulation (FM) d) Phase-Shift modulation (PSM)
Analog data / digital signal
Most common technique for encoding
analog data using digital signals is Pulse
Multiplexing
Technique that allows multiple signals to
be transmitted simultaneously over a single medium
Media is separated into multiple channels or
subchannels
This can be done virtually and/or physically Individual signals from different sources can be
combined into a single complex signal and then the separate signals are recovered at the receiving end
Multiplexing depends on signal type
(analog / digital) and the media used
Multiplexing
Why is multiplexing so important in data
communications?
Basic concept of multiplexing
Image Source: http://www.cadvision.com/blanchas/Intro2dcRev2/page96.html
Multiplexing
Frequency Division Multiplexing (FDM)
Wave Division Multiplexing (WDM)
Same concept as FDM but applied to fiber
where optical signals are used
Time Division Multiplexing (TDM)
Frequency Division Multiplexing (FDM)
Inherently an analog technology Uses different frequency ranges over
single medium
Total bandwidth is divided into subchannels
consisting of smaller segments of available bandwidth
Carrier wave used by each sender/receiver
pair operates within a unique frequency band to avoid interfering with other transmissions
Frequency Division Multiplexing (FDM)
Wave Division Multiplexing
Frequency division applied to lightwaves as opposed to radio frequencies
Combining separate wavelengths of
data into a light stream on a single fiber carrier
Time Division Multiplexing (TDM)
Primarily a digital technology thatseparates different data streams by time (as opposed to frequency)
1) Synchronous time division (STD)
(slotted)
2) Statistical
Time division multiplexing
Synchronous time division (slotted) Sources takes turns in a round robin
fashion
Great for telephone transmission
Source A
Source B
Source C
Source A Source B Source C Source A Source B Source C Source A Multiplexor Multiplexor
Destination
Time division multiplexing
Statistical multiplexing Take turns like STD, but does not waste
slots if source has no data to send
Source A
Source B
Source C
Source A Source A Source A Source A Source B Source B Source A Multiplexor Multiplexor
Destination
Some Basic Issues
To transmit an analog signal, the
bandwidth of the signal must match the bandwidth of the transmission channel
To transmit a digital signal, the bit rate
of the signal must be within the bit rate range of the transmission channel
Form of transmission
Baseband and broadband are different
forms of transmission
Baseband transmission uses digital
signaling
Broadband transmission uses analog
Baseband
Baseband systems can transmit one
signal / one channel at a time
Each transmission requires full use of the
medium so when one node is transmitting the other nodes must wait their turn Ethernet uses baseband signaling
Why would the most popular LAN technology use this signaling technique?
Baseband
CharacteristicsConverts digital signal to voltage without
using different frequency channels
Single frequency
Bi-directional signal flow (but not
simultaneously)
Entire bandwidthof cable used to transmit
single data stream
Multiple signals can be sent using Time
Division Multiplexing (TDM)
Baseband
If baseband requires the use of the
entire cable for single transmission, how can multiple computers on a LAN communicate simultaneously?
Broadband
Broadband systems can transmit multiple
signals over many different channels simultaneously
Each channel uses a different frequency
band to transmit
Cable TV uses broadband signaling
Broadband
CharacteristicsCommunications medium split into multiple
channels
Multiple signals are transmitted over a single
medium simultaneously
Can send and receive simultaneously
Analog transmission
Signal flow is unidirectional on each channel
Uses Frequency Division Multiplexing (FDM)
Broadband
If broadband only supports
uni-directional transmission, how do 2 (or more) devices communicate?
Broadband versus baseband
Broadband networks carry more datathan baseband networks
Using analog signaling over multiple
frequency bands
Broadband networks are more expensive
and complex than baseband networks
Use multiplexing
Broadband networks are better suited for
long-distance communication than baseband networks
Broadband, Baseband, Analog, Digital
What is the signaling technique
used oncommunication channel?
Phone line (DS0) – broadband channel
T-1 carrier – baseband channel
Ethernet – baseband
WiFi - broadband
What are the form of the data
transmitted
over the channel?Can be either analog or digital
Broadband, Baseband, Analog, Digital
Transmit analog data over broadband
channel
Transmit analog data over baseband
channel
Transmit digital data over broadband
channel
Transmit digital data over baseband
channel
Summary
Transmission basics Analog –vs- Digital Simple transmission RS232 Line coding ModulationAM, FM, Phase Shift
Multiplexing
FDM, TDM, WDM
