2. CN Syllabus Composition + Evaluation
• Data Communications + Computer Networks
• Unit-1: Data Communication Part + OSI &TCP/IP Model
• Units 2-5: Layer 2 – 7 of OSI model with focus based on
General Layer functionality and
TCP/IP specific reference model under each layer
• CIE – 30 Marks - 10 (CIE-1) + 10 (CIE-2) + 5 (Assignment) + 5 (Quiz)
• SEE – 70 Marks - Mandatory to get 40% marks in end exam paper
3. Computer Networks - SYLLABUS OVERIEW
Unit – 1
◦ Data Communication Components:
◦ Representation of Data Communication ,
◦ Flow of Networks,
◦ Layered Architecture,
◦ OSI and TCP/IP model,
◦ Transmission Media.
◦ Techniques for Bandwidth Utilization:
◦ Line configuration,
◦ Multiplexing – Frequency division, Time division and Wave division,
◦ Asynchronous and Synchronous Transmission,
◦ Introduction to Wired and Wireless LAN
4. Computer Networks - SYLLABUS OVERIEW
Unit – 2
◦ Data Link Layer and Medium Access Sub Layer:
◦ Error Correction and Error Detection:
◦ Fundamentals, Block coding,
◦ Hamming Distance, CRC
◦ Flow Control and Error Control Protocols:
◦ Stop and Wait,
◦ Go Back-N,
◦ ARQ, Selective Repeat ARQ,
◦ Sliding Window,
◦ Multiple Access Protocols:
◦ Pure ALOHA, Slotted ALOHA
◦ CSMA/CD, CSMA/CA
6. Computer Networks - SYLLABUS OVERIEW
Unit – 4
◦ Transport Layer:
◦ Process to Process Communications,
◦ Elements of Transport Layer
◦ Internet Protocols:
◦ UDP – User Datagram Protocol
◦ TCP – Transmission Control Protocol
◦ Congestion and Quality of Service:
◦ QoS improving techniques
7. Computer Networks - SYLLABUS OVERIEW
Unit – 5
◦ Application Layer:
◦ Domain Name System (DNS),
◦ EMAIL - Electronic Mail
◦ SNMP – Simple Network Management Protocol
◦ Basic Concepts of Cryptography:
◦ Network Security Attacks,
◦ Symmetric Encryption
◦ Data Encryption Standards,
◦ Public Key Encryption – RSA (Rivest, Shamir, Adleman)
◦ Hash Function,
◦ Message Authentication
◦ Digital Signature
8. Computer Networks – Suggested Reading -
1. Data Communication and Networking,
4th Edition, Behrouz A. Forouzan, McGrawHill
2. Data and Computer Communication,
8th Edition, William Stallings, Pearson Prentice Hall India
3. Unix Network Programming,
W. Richard Stevens, Prentice Hall / Pearson Education, 2009
9. Computer Networks Lab
PC 632 CS [Credits – 1]
Evaluation: CIE – 25 Marks; SEE – 50 Marks
1. Running and using services/commands like:
◦ tcpdump, netstat, ifconfig, nslookup, ftp, telnet. - Execution at command prompt
◦ Capture ping and traceroute PDUs using a network protocol analyzer and examine
2. Configuration of router, switch. (using real devices or simulators)
3. Socket Programming using UDP and TCP ( E.g. Simple DNS, Date and time Client Server, Echo Client/Server, Iterative &
Concurrent Servers) - Application programs through C Language using Socket API
4. Network Packet Analysis using tools like Wireshark, tcpdump etc.
5. Network Simulation using tools like Cisco Packet Tracer, NetSim, OMNet++, NS2, NS3 etc.
6. Study of Network Simulator(NS) and Simulation of Congestion Control Algorithms using NS. Performance Evaluation of
Routing Protocols using Simulation Tools.
7. Programming using raw sockets.
8. Programming using RPC. - Application programs through C Language
Note: Instructor may add/delete/modify/tune experiments, wherever he/she feels in a justified manner.
10. CN-U-1 - INTRODUCTION
Data refers to information presented in whatever form is agreed
upon by the parties creating and using the data.
Data Communications are the exchange of data between two
devices via some form of transmission medium such as a wire
A network is a set of devices (often referred to as nodes)
connected by communication links. A node can be a computer,
printer, or any other device capable of sending and/or receiving
data generated by other nodes on the network.
11. A Communication Model
• The fundamental purpose of a communications system is the
exchange of data between two parties.
The key elements of this model are:
• Source - generates data to be transmitted
• Transmitter - converts data into transmittable signals
• Transmission System - carries data from source to destination
• Receiver - converts received signal into data
• Destination - takes incoming data
12. Data Communication Model
"Data Communications”, deals with the most fundamental aspects of the
communications function, focusing on the transmission of signals in a reliable
and efficient manner.
Example: Electronic Mail: User A sending an email message m to user B.
Steps for this process:
1. User A keys in message m comprising bits g buffered in source PC memory
2. Input data is transferred to I/O device (transmitter) as sequence of bits g(t)
using voltage shifts
3. transmitter converts these into a signal s(t) suitable for transmission
4. whilst transiting media signal may be impaired so received signal r(t) may
differ from s(t)
5. receiver decodes signal recovering g’(t) as estimate of original g(t)
which is buffered in destination PC memory as bits g’ being the received
15. Communications Tasks
Transmission system utilization Addressing
Signal generation Recovery
Synchronization Message formatting
Exchange management Security
Error detection and correction Network management
16. Communications Tasks
• Key tasks that must be performed in a data communications system:
• transmission system utilization - need to make efficient use of transmission facilities typically
shared among a number of communicating devices
• a device must interface with the transmission system
• once an interface is established, signal generation is required for communication
• there must be synchronization between transmitter and receiver, to determine when a signal
begins to arrive and when it ends
• there is a variety of requirements for communication between two parties that might be collected
under the term exchange management
• Error detection and correction are required in circumstances where errors cannot be tolerated
17. Communications Tasks
• Flow control is required to assure that the source does not overwhelm the destination by sending data faster
than they can be processed and absorbed
• addressing and routing, so a source system can indicate the identity of the intended destination, and can
choose a specific route through this network
• Recovery allows an interrupted transaction to resume activity at the point of interruption or to condition prior
to the beginning of the exchange
• Message formatting has to do with an agreement between two parties as to the form of the data to be
exchanged or transmitted
• Frequently need to provide some measure of security in a data communications system
• Network management capabilities are needed to configure the system, monitor its status, react to failures
and overloads, and plan intelligently for future growth
31. LAYERED ARCHITECTURE: NEED AND ADVANTAGES
• Allows Complex problems are decomposed in to small manageable units.
• Implementation details of the layer are abstracted.
• Separation of implementation and specification.
• Layers work as one by sharing the services provided by each other.
•Layering allows reuse functionality i.e., lower layers implement common once.
•Provide framework to implement multiple specific protocols (rules) per layer
•Provides Modularity with Clear Interfaces.
• Has Implementation Simplicity, Maintainability, Flexibility and Scalability.
• Support for Portability.
• Provides for Robustness
32. ISO - OSI MODEL
• International Standards Organization (ISO) - is a multinational body
to worldwide agreement on international standards.
• An ISO standard that covers all aspects of network communications is the
Open Systems Interconnection (OSI) model.
• It was first introduced in the late 1970s.
• OSI model has seven layers. ----->
46. TCP/IP REFERENCE MODEL /PROTOCOL
The layers in the TCP/IP reference model is FOUR in comparison to the OSI
The original TCP/IP protocol suite was defined as having four layers:
host-to-network, internet, transport, and application.
But when TCP/IP is compared to OSI, we can say that the TCP/IP protocol suite
is made of five layers:
physical, data link, network, transport, and application.
48. Comparison of ISO-OSI model and TCP/IP
1. Layers: 7 in OSI ; 5 in TCP/IP
2. Model vs Implementation: In OSI first model was designed followed by
Implementation. In TCP/IP first implemented then design followed
3. In OSI : Clear definition of Services, Interface and Protocols. Not so in TCP/IP
4. In OSI Network layer is both Connection Oriented and Connectionless and
Transport Layer is only connection oriented. In TCP/IP network layer is
connectionless and Transport Layer is both connection oriented and
5. In TCP/IP Session and Presentation layers are missing, this functionality is done
by Application layer.
6. TCP/IP is the defacto protocol used in internet. OSI is mostly a theoretical
Four levels of addresses are used in an internet employing the TCP/IP protocols:
• Physical Addresses
• Logical Addresses
• Port Addresses
• Specific Addresses
51. Physical Addresses
• Physical Address – It is of 6-bytes (12 hexadecimal digits).
• Also called MAC ADDRESS.
• Every byte (2 hexadecimal digits) is separated by a colon
• Example: 07:01:02:01:2C:4B
• Physical Addresses Change Hop by Hop
52. Logical Addresses
•Network with two
•Each device (computer
or router) has a pair of
addresses (logical and
physical) for each
•Each device connected
to one link – 1 pair of
address. For router 3
• Also called IP
53. Port addresses
• Port Addresses are for
• Port and Logical
addresses remain same
for source to
55. Transmission Media Introduction
• Transmission medium – It is the physical path between transmitter and receiver
• It is of two types / categories / classes –
• Guided media – Electromagnetic waves are guided along a solid medium
Eg: Copper Twisted Pair, Copper Coaxial Cable, and Optical fiber
• Unguided media – wireless transmission occurs through the atmosphere, space, water
• Characteristics and Quality of data transmission is determined by both characteristics of
Medium and Signal
• For guided media - Medium is more important for data transmission
• For Unguided media - Bandwidth of the signal produced by the transmitting antenna is more
- One key property is directionality of the signal.
Signals at lower frequencies are omni-directional and at higher
frequencies can be focused into a directional beam
56. Data Transmission System Design : Data rate & Distance are
the key factors
Design Factors Determining Data Rate and Distance
• higher bandwidth gives higher data rate
• impairments, such as attenuation, limits the distance - Twisted Pair -> Coaxial Cable -> Optical Fiber
• overlapping frequency bands can distort or wipe out a signal – More in Unguided than Guided medium.
• more receivers introduces more attenuation - in case of shared link with multiple attachments. Not in
number of receivers
58. Transmission Characteristics of Guided
Frequency Range Typical
Typical Delay Repeater
Twisted pair (with
0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km
0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km
Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km
Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km
In Guided Media ,transmission capacity, in terms of either data rate or bandwidth, depends
critically on the distance and on whether the medium is point-to-point or multipoint.
62. Twisted Pair
Twisted pair is the least expensive and most widely used guided transmission medium.
• consists of two insulated copper wires arranged in a regular spiral pattern
• a wire pair acts as a single communication link
• pairs are bundled together into a cable
• most commonly used in the telephone network and for communications
• within buildings
63. Twisted Pair - Transmission Characteristics
5km to 6km
can use either
analog or digital
2km to 3km
interference and noise
64. Unshielded vs. Shielded Twisted Pair
Unshielded Twisted Pair (UTP)
• ordinary telephone wire
• easiest to install
• suffers from external electromagnetic interference
Shielded Twisted Pair (STP)
• has metal braid or sheathing that reduces interference
• provides better performance at higher data rates
• more expensive
• harder to handle (thick, heavy)
66. Near End Crosstalk - occurs in Twisted Pair
• Coupling of signal from one pair of conductors to another
• Occurs when transmit signal entering the link couples back to the
receiving pair - (near transmitted signal is picked up by near
67. Coaxial Cable
Coaxial cable can be used over longer distances and support more stations on a shared line
than twisted pair.
• consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor
• is a versatile transmission medium used in a wide variety of applications
• used for TV distribution, long distance telephone transmission and LANs
68. Coaxial Cable – Transmission
- closer if
extends up to
• repeater every
1km - closer for
69. Optical Fiber
Optical fiber is a thin flexible medium capable of guiding an optical ray.
• various glasses and plastics can be used to make optical fibers
• has a cylindrical shape with three sections – core, cladding, jacket
• widely used in long distance telecommunications
• performance, price and advantages have made it popular to use
70. Optical Fiber - Benefits
◦ data rates of hundreds of Gbps
smaller size and lighter weight
◦ considerably thinner than coaxial or twisted pair cable
◦ reduces structural support requirements
◦ not vulnerable to interference, impulse noise, or crosstalk
◦ high degree of security from eavesdropping
greater repeater spacing
◦ lower cost and fewer sources of error
71. Optical Fiber - Transmission
• uses total internal reflection to transmit light
• effectively acts as wave guide for 1014 to 1015 Hz (this covers portions of infrared &
• Light sources used:
• Light Emitting Diode (LED)
• cheaper, operates over a greater temperature range, lasts longer
• Injection Laser Diode (ILD)
• more efficient, has greater data rates
• has a relationship among wavelength, type of transmission and achievable data rate
73. Optical Fiber Transmission Modes
Light from a source enters the cylindrical glass or plastic core. Rays at shallow angles are
reflected and propagated along the fiber; other rays are absorbed by the surrounding
material. This form of propagation is called step-index multimode
Varying the index of refraction of the core, a third type of transmission, known as
Reducing the radius of the core to the order of a wavelength, only a single angle or mode
can pass: the axial ray. We have the single-mode propagation
76. Wireless Transmission Frequencies
• referred to as microwave frequencies
• highly directional beams are possible
• suitable for point to point transmissions
• also used for satellite
• suitable for omnidirectional applications
• referred to as the radio range
3 x 1011 to 2
• infrared portion of the spectrum
• useful to local point-to-point and multipoint applications within confined areas
electrical conductors used to
radiate or collect electromagnetic
same antenna is often used for
energy impinging on
converted to radio
fed to receiver
energy by antenna
78. Radiation Pattern
•power radiated in all directions
•does not perform equally well in all directions
• as seen in a radiation pattern diagram
•an isotropic antenna is a point in space that radiates power
• in all directions equally
• with a spherical radiation pattern
80. Antenna Gain
•measure of the directionality of an antenna
•power output in particular direction verses that produced by an
•measured in decibels (dB)
•results in loss in power in another direction
•effective area relates to physical size and shape
81. Terrestrial Microwave
most common type is a parabolic
dish with an antenna focusing a
narrow beam onto a receiving
located at substantial heights above
ground to extend range and
transmit over obstacles
uses a series of microwave relay
towers with point-to-point
microwave links to achieve long
82. Terrestrial Microwave Applications
• used for long haul telecommunications, short point-to-point links
between buildings and cellular systems
• used for both voice and TV transmission
• fewer repeaters but requires line of sight transmission
• 1-40GHz frequencies, with higher frequencies having higher data rates
• main source of loss is attenuation caused mostly by distance, rainfall
84. Satellite Microwave
• a communication satellite is in effect a microwave relay station
• used to link two or more ground stations
• receives on one frequency, amplifies or repeats signal and transmits on
• frequency bands are called transponder channels
• requires geo-stationary orbit
• rotation match occurs at a height of 35,863km at the equator
• need to be spaced at least 3° - 4° apart to avoid interfering with each other
• spacing limits the number of possible satellites
87. Satellite Microwave Applications
private business networks
◦ satellite providers can divide capacity into channels to lease to individual business users
◦ programs are transmitted to the satellite then broadcast down to a number of stations
which then distributes the programs to individual viewers
◦ Direct Broadcast Satellite (DBS) transmits video signals directly to the home user
◦ Navstar Global Positioning System (GPS)
88. Transmission Characteristics
• the optimum frequency range for satellite transmission is 1 to 10 GHz
• lower has significant noise from natural sources
• higher is attenuated by atmospheric absorption and precipitation
• satellites use a frequency bandwidth range of 5.925 to 6.425 GHz from earth
to satellite (uplink) and a range of 3.7 to 4.2 GHz from satellite to earth
• this is referred to as the 4/6-GHz band
• because of saturation the 12/14-GHz band has been developed (uplink: 14 - 14.5 GHz; downlink: 11.7 -
89. Broadcast Radio
radio is the term used to encompass frequencies in the range of 3kHz to 300GHz
broadcast radio (30MHz - 1GHz) covers
• FM radio
• UHF and VHF television
• data networking applications
limited to line of sight
suffers from multipath interference
◦ reflections from land, water, man-made objects
• achieved using transceivers that modulate noncoherent infrared light
• transceivers must be within line of sight of each other directly or via
• does not penetrate walls
• no licenses required
• no frequency allocation issues
• typical uses:
• TV remote control
92. Wireless Propagation Ground Wave
• ground wave propagation follows the contour of the earth
and can propagate distances well over the visible horizon
• this effect is found in frequencies up to 2MHz
• the best known example of ground wave communication is AM radio
93. Wireless Propagation Sky Wave
• sky wave propagation is used for amateur radio, CB radio, and international broadcasts
such as BBC and Voice of America
• a signal from an earth based antenna is reflected from the ionized layer of the upper
atmosphere back down to earth
• sky wave signals can travel through a number of hops, bouncing back and for the between the
ionosphere and the earth’s surface
94. Wireless Propagation Line of Sight
• ground and sky wave propagation modes do not operate above 30
MHz - - communication must be by line of sight
velocity of electromagnetic wave is a function of the density of the medium
through which it travels
• ~3 x 108 m/s in vacuum, less in anything else
speed changes with movement between media
index of refraction (refractive index) is
◦ varies with wavelength
◦ density of atmosphere decreases with height, resulting in bending of radio waves
96. Line of Sight Transmission
Free space loss
• loss of signal
• from water vapor
• bending signal
97. Free Space Loss : which can be expressed in terms of the ratio of the
radiated power Pt to the power Pr received by the antenna or, in decibels, by taking 10
times the log of that ratio.
99. • Line configuration,
• Multiplexing – Frequency division, Time division and
Techniques for Bandwidth Utilization:
100. Line Configuration - Topology
•Physical arrangement of stations on medium
• Point to Point - two stations
• such as between two routers / computers
• Multi point - multiple stations
• traditionally mainframe computer and terminals
• now typically a local area network (LAN)
Note: Two characteristics that distinguish various data link
configurations : Topology and Whether the link is half duplex or full
duplex [Data Flow].
101. Line Configuration - Topology
• In point-to-point each
terminal has a separate I/O
Port and transmission link
102. Line Configuration - Duplex
• classify data exchange as half or full duplex
• half duplex (two-way alternate)
• only one station may transmit at a time
• requires one data path
• full duplex (two-way simultaneous)
• simultaneous transmission and reception between two stations
• requires two data paths
• separate media or frequencies used for each direction
• or echo canceling ( can be used for transmitting using a single line)
•Under the simplest conditions, a medium can carry only one signal at any moment in
•For multiple signals to share one medium, the medium must somehow be divided,
giving each signal a portion of the total bandwidth.
•Whenever the bandwidth of a medium linking two devices is greater than the
bandwidth needs of the devices, the link can be shared.
•Efficiency can be achieved by multiplexing;
i.e., sharing of the bandwidth between multiple users.
•Transparent to the User
-- It is the set of techniques that allows the (simultaneous) transmission of
multiple signals across a single data link.
-- Two or more simultaneous transmissions on a single circuit.
Figure: Dividing a link into channels
106. Multiplexing Techniques/Categories
The current techniques include :
1. FDM: Frequency Division Multiplexing
2. WDM: Wavelength Division Multiplexing
3. TDM: Time Division Multiplexing - Digital
a. Synchronous b. Statistical
107. Frequency Division Multiplexing
• It is an analog multiplexing technique that combines analog signals. Uses
the concept of modulation
• Assignment of non-overlapping frequency ranges to each “user” or signal
on a medium. Thus, all signals are transmitted at the same time, each
using different frequencies.
109. Frequency Division Multiplexing
• Analog signaling is used to transmit the signals due to which it is more
susceptible to noise.
• It is the oldest multiplexing technique.
• Examples of FDM:
Broadcast radio and television,
AMPS cellular phone systems
110. FDM Process
--A multiplexor accepts inputs and
assigns frequencies to each
--It is attached to a high-speed
--A corresponding multiplexor, or
demultiplexor, is on the end of the
high-speed line and separates
the multiplexed signals.
111. FDM Process
--Each signal is modulated to a different carrier frequency
--Carrier frequencies separated so signals do not overlap (guard bands)
e.g. broadcast radio.
--Channel allocated even if no data
116. Dense Wavelength Division Multiplexing
• DWDM which is often called WDM multiplexes multiple data streams onto
a single fiber optic line.
Data Transmission through a single fiber optic line
117. Dense Wavelength Division Multiplexing (DWDM)
• Different wavelength lasers (called lambdas) transmit the multiple signals.
• Each signal carried at a different rate, combines(30, 40, more?) signals
onto one fiber.
118. Wavelength Division Multiplexing
1997 Bell Labs
◦ 100 beams
◦ Each at 10 Gbps
◦ Giving 1 terabit per second (Tbps)
Commercial systems of 160 channels of 10 Gbps now available
Lab systems (Alcatel) 256 channels at 39.8 Gbps each
◦ 10.1 Tbps
◦ Over 100km
119. Time Division Multiplexing (TDM)
•TDM is a digital multiplexing technique for combining several low-rate
digital channels into one high-rate one.
• Data rate of medium exceeds data rate of digital signal to be
• Multiple digital signals interleaved in time
• May be at bit level of blocks
120. Time Division Multiplexing (TDM)
Sharing of the signal is accomplished by dividing available transmission
time on a medium among users.
123. TDM Types/Forms
•Time division multiplexing comes in two basic forms:
•1. Synchronous time division multiplexing
•2. Statistical, or Asynchronous time division multiplexing.
124. Synchronous TDM
The original time division multiplexing.
The multiplexor accepts input from attached devices in a round-robin fashion
and transmit the data in a never ending pattern.
Examples of STDM: T-1, ISDN telephone lines,
SONET (Synchronous Optical NETwork)
When one device generates data at a faster rate than other devices –
then the multiplexor must either sample the incoming data stream from
that device more often than it samples the other devices, or buffer the
faster incoming stream.
•When a device has nothing to transmit, the multiplexor must still insert a piece of data
from that device into the multiplexed stream So that the receiver may stay
synchronized with the incoming data stream
•The transmitting multiplexor can insert alternating 1s and 0s into the data stream.
The process of taking a group of bits from each input line for multiplexing
is called interleaving.
We interleave bits (1 - n) from each input onto one output.
129. TDM Link Control
• No headers and trailers
• Data link control protocols not needed
• Flow control
–Data rate of multiplexed line is fixed
–If one channel receiver can not receive data, the others must carry on
–The corresponding source must be quenched
–This leaves empty slots
• Error control
–Errors are detected and handled by individual channel systems
•To ensure that the receiver correctly reads the incoming bits,
i.e., knows the incoming bit boundaries to interpret a “1” and a
“0”, a known bit pattern is used between the frames.
•The receiver looks for the anticipated bit and starts counting bits
till the end of the frame.
•Then it starts over again with the reception of another known
•These bits (or bit patterns) are called synchronization bit(s).
•They are part of the overhead of transmission.
133. Data Rate Management
• Synchronizing data sources
• Not all input links maybe have the same data rate.
• Some links maybe slower. There maybe several different input link speeds
• Data rates from different sources not related by simple rational number
• Clocks in different sources drifting
• Three strategies that can be used to overcome the data rate mismatch:
• Multilevel, Multislot and Pulse Stuffing
134. Data Rate Management
• Multilevel: used when the data rate of the input links are multiples of
135. Data Rate Management
Multislot: used when there is a GCD between the data rates. The higher bit rate channels are allocated
more slots per frame, and the output frame rate is a multiple of each input link.
136. Data Rate Management
• Pulse Stuffing: used when there is no GCD between the links. The
slowest speed link will be brought up to the speed of the other links by bit
insertion, this is called pulse stuffing.
–Outgoing data rate (excluding framing bits) higher than sum of
–Stuff extra dummy bits or pulses into each incoming signal until it
matches local clock
–Stuffed pulses inserted at fixed locations in frame and removed
137. Inefficient use of Bandwidth
• Sometimes an input link may have no data to transmit then, one or more
slots on the output link will go unused.
• Thus wasting bandwidth
139. Statistical TDM or Asynchronous TDM
•In Synchronous TDM many slots are wasted
•Statistical TDM allocates time slots dynamically based on
•Multiplexer scans input lines and collects data until frame
•Data rate on line lower than aggregate rates of input lines
141. Statistical TDM
• A statistical multiplexor transmits only the data from active
workstations (or why work when you don’t have to).
• If a workstation is not active, no space is wasted on the multiplexed
143. Statistical TDM
To identify each piece of data,
an address is included.
If the data is of variable size,
a length is also included.
144. Statistical TDM
•A statistical multiplexor does not require a line over as high a
speed line as synchronous time division multiplexing since STDM
does not assume all sources will transmit all of the time!
•Good for low bandwidth lines (used for LANs)
•Much more efficient use of bandwidth!
145. • Asynchronous and Synchronous Transmission,
• XDSL – X Digital Subscriber Line
A, S, H, V
Asymmetric, Symmetric, High Data Rate, Very High Data Rate
Techniques for Bandwidth Utilization:
146. Transmission of Data between 2 devices
Types: Asynchronous and Synchronous
•Transmission of a stream of bits from one device to another across a
transmission link involves cooperation and agreement between the two
•Timing problems require a mechanism to synchronize the transmitter
• receiver samples stream at bit intervals
• if clocks not aligned and drifting will sample at wrong time after
sufficient bits are sent
•Two solutions to synchronizing clocks
• Asynchronous transmission
• Synchronous transmission
147. Asynchronous Transmission
• Here each character of data is treated independently.
• Timing problem is avoided by not sending long, uninterrupted
streams of bits. So data is sent character by character.
• Each character begins with a start bit that alerts the receiver that
a character is arriving. The receiver samples each bit in the
character and then looks for the beginning of the next character. [
does not work with long blocks of data as receiver clock may go out
of sync with the transmitter’s clock.
148. Asynchronous Transmission
• When no character is being transmitted, the line between transmitter and receiver is in an idle state (binary 1
• The beginning of a character is signaled by a start bit with a value of binary 0.
• This is followed by the 5 to 8 bits that actually make up the character.
• The bits of the character are transmitted beginning with the least significant bit.
• Then the data bits are usually followed by a parity bit, set by the transmitter such that the total number of
ones in the character, including the parity bit, is even (even parity) or odd (odd parity).
• The receiver uses this bit for error detection.
• The final element is a stop element, which is a binary 1.
• A minimum length for the stop element is specified, and this is usually 1, 1.5, or 2 times the duration of an
• No maximum value is specified since the stop element is the same as the idle state, so the transmitter will
continue to transmit the stop element until it is ready to send the next character.
149. Asynchronous Transmission
• Example: Say the receiver is fast by
• Thus, the receiver samples the
incoming character every 94 µs
(based on the transmitter's clock).
• Thus the last sample is erroneous.
150. Asynchronous Transmission - Merits
•Simple & cheap
•Overhead of 2 or 3 bits per char (~20%)
•Example: For an 8-bit character with no parity bit, using a
1-bit-long stop element, two out of every ten bits convey
no information but are there merely for synchronization;
thus the overhead is 20%.
•Good for data with large gaps (keyboard)
151. Synchronous Transmission
•Block of data transmitted sent as a frame
• [includes a starting and an ending flag, and is transmitted in a steady stream without start and stop codes. The
block may be many bits in length. ]
•Clocks must be synchronized [to avoid drift]
• can use separate clock line
• or embed clock signal in data
•Need to indicate start and end of block of data for the receiver to sync
• use preamble and postamble bits
• Data plus preamble, postamble, and control information are called a frame (exact frame format
depends of DLL procedure).
• More efficient (lower overhead) than Asynchronous (20% more overhead).
• Preamble, Postamble and control field would mostly less than 100 bits.
Internet Access Technology:
Upstream and Downstream
• Internet access technology refers to a data communications system that
connects an Internet subscriber to an ISP
• such as a telephone company(DSL) or cable company
• Most Internet users follow an asymmetric pattern
• a subscriber receives more data from the Internet than sending
• a browser sends a URL that comprises a few bytes
• in response, a web server sends content
• Upstream to refer to data traveling from a subscriber to an ISP
• Downstream to refer to data traveling from an ISP in the Internet to a
Narrowband and Broadband Access Technologies
• A variety of technologies are used for Internet access
• They can be divided into two broad categories based on the data rate they
• In networking terms, network bandwidth refers to data rate
• Thus, the terms narrowband and broadband reflect industry practice
Narrowband Access Technologies
• Narrowband Technologies
• refers to technologies that deliver data at up to 128 Kbps
• For example, the maximum data rate for dialup noisy phone lines is 56 Kbps
and classified as a narrowband technology
• the main narrowband access technologies are given below
Broadband Access Technologies
• Broadband Technologies
• generally refers to technologies that offer high data rates, but the exact boundary
between broadband and narrowband is blurry
• many suggest that broadband technologies deliver more than 1 Mbps
• but this is not always the case, and may mean any speed higher than dialup
• the main broadband access technologies are given below
Digital Subscriber Line (DSL) Technologies
• DSL is one of the main technologies used to provide high-speed data communication services over a
• DSL variants are given below
• Because the names differ only in the first word, the set is collectively referred to by the acronym
• Currently, ADSL is most popular
The Local Loop
• Local loop describes the physical connection between a telephone company
Central Office (CO) and a subscriber
• consists of twisted pair and dialup call with 4 KHz of bandwidth
• It often has much higher bandwidth; a subscriber close to a CO may be able
to handle frequencies above 1 MHz
LOCAL LOOP Technologies
• Electric local loop(POTS lines): Voice, ISDN, DSL
• Optical local loop: Fiber Optics services such as FiOS
• Satellite local loop: communications satellite and cosmos Internet connections
of satellite televisions (DVB-S)
• Cable local loop: Cablemodem
• Wireless local loop (WLL): LMDS, WiMAX, GPRS, HSDPA, DECT
167. Asymmetrical DSL (ADSL)
• ADSL is an asymmetric communication technology designed for
residential users; it is not suitable for businesses
• ADSL is an adaptive technology.
•Link between subscriber and network
•Uses currently installed twisted pair cable
–Can carry broader spectrum
–1 MHz or more
168. Asymmetrical DSL (ADSL)
• ADSL divides up the available frequencies in a line on the assumption that
most Internet users look at, or download, much more information than
they send, or upload.
• The system uses a data rate based on the condition of the local loop line.
• Speed: Most existing local loops can handle bandwidths up to 1.1 MHz.
169. ADSL Design
– Greater capacity downstream than upstream
• Frequency division multiplexing
– Lowest 25kHz for voice
• Plain old telephone service (POTS)
– Use echo cancellation or FDM to give two bands
– Use FDM within bands
– The region above 25kHz is used for data transmission
– Upstream: 64kbps to 640kbps
– Downstream: 1.536Mbps to 6.144Mbp
• Range 5.5km
172. Two standards for ADSL
1. Discrete multitone (DMT)
2. Carrierless amplitude/phase (CAP)
173. CAP - three distinct bands:
1. Voice channel - 0 to 4 KHz
2. Upstream channel - 25 and 160 KHz
3. Downstream channel - 1.5 MHz
Minimizes the possibility of interference between the channels on
one line, or between the signals on different lines
175. Discrete Multitone
• Multiple carrier signals at different frequencies
• Some bits on each channel
• 4kHz subchannels
• Send test signal and use subchannels with better signal to noise ratio
• 256 downstream subchannels at 4kHz (60kbps)
– Impairments bring this down to 1.5Mbps to 9Mbps
178. ADSL Distance Limitations
•ADSL is a distance-sensitive technology
•The limit for ADSL service is 18,000 feet (5,460 meters)
•At the extremes of the distance limits, ADSL customers may
see speeds far below the promised maximums
•customers nearer the central office have faster connections
and may see extremely high speeds
179. OTHER TYPES OF DSL:
• SDSL -- Symmetric DSL
Used mainly by small businesses & residential areas
Bit rate of downstream is higher than upstream
• HDSL -- High-bit-rate DSL
Used as alternative of T-1 line
Uses 2B1Q encoding
Less susceptible to attenuation at higher frequencies
Unlike T-1 line (AMI/1.544Mbps/1km), it can reach 2Mbps
180. OTHER TYPES OF DSL:
• VDSL -- Very high bit-rate DSL
Uses DMT modulation technique
Effective only for short distances(300-1800m)
Speed: downstream: 50 - 55 Mbps upstream: 1.5-2.5 Mbps
•In 1985, the Computer Society of the IEEE started a
project, called Project 802.
•Purpose was to set standards to enable
intercommunication among equipment from a variety of
•Project 802 is a way of specifying functions of the
physical layer and the data link layer of major LAN
190. Unicast and multicast addresses
The least significant bit of the first byte defines the type of address.
If the bit is 0, the address is unicast; otherwise, it is multicast.
The broadcast destination address is a special case of the multicast address in which all
bits are 1s.
191. Define the type of the following destination addresses:
a. 4A:30:10:21:10:1A b. 47:20:1B:2E:08:EE
To find the type of the address, we need to look at the second hexadecimal
digit from the left. If it is even, the address is unicast. If it is odd, the address is
multicast. If all digits are F’s, the address is broadcast. Therefore, we have the
a. This is a unicast address because A in binary is 1010.
b. This is a multicast address because 7 in binary is 0111.
c. This is a broadcast address because all digits are F’s.
192. Example shows how the address 47:20:1B:2E:08:EE is sent out on
The address is sent left-to-right, byte by byte; for each byte, it is sent right-to-
left, bit by bit, as shown below:
CHANGES IN THE STANDARD
The 10-Mbps Standard Ethernet has gone through several changes before
moving to the higher data rates.
These changes actually opened the road to the evolution of the Ethernet
to become compatible with other high-data-rate LANs.
Fast Ethernet was designed to compete with LAN protocols such as FDDI
or Fiber Channel.
IEEE created Fast Ethernet under the name 802.3u.
Fast Ethernet is backward-compatible with Standard Ethernet, but it can
transmit data 10 times faster at a rate of 100 Mbps.
211. GIGABIT ETHERNET
• The need for an even higher data rate resulted in the design of the Gigabit
Ethernet protocol (1000 Mbps). The IEEE committee calls the standard
• In the full-duplex mode of Gigabit Ethernet, there is no collision;
• the maximum length of the cable is determined by the signal
in the cable.
217. IEEE 802.11 - Wireless LAN Standard
IEEE has defined the specifications for a wireless LAN, called IEEE
802.11, which covers the physical and data link layers.
A BSS without an AP is called an ad hoc network;
a BSS with an AP is called an infrastructure network.
Bluetooth is a wireless LAN technology designed to connect devices of
different functions such as telephones, notebooks, computers, cameras,
printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network,
which means that the network is formed spontaneously.