Data Communication - CS601
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Course Outline
- The course will consist of :
- 45 lectures
- 10-14 assignments
- 2exams (1 midterm and 1 final)
- GMDB
- Grading Criteria:
- One Midterm: 35 %
- Final Exam: 45%
- Assignments: 15%
- GMDB: 5%
Textbook
- “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
- “Data and Computer Communication” 6th Edition by William Stallings
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KEY DATA COMMUNICATION TERMINOLOGY
- Link: connects adjacent nodes Wires, Cables, Any thing that physically connects two nodes
- Path: end-to-end route within a network
- Circuit: the conduit over which data travels
- Packetizing: dividing messages into fixed-length packets prior to transmission over a network’s communication media
- Routing: determining a message’s path from sending to receiving nodes
The transmission medium may itself be a network, so route needs to be specified
Network
“A NETWORK is a set of devices (Nodes) connected by Communication Links”
Node: Can be a Computer, Printer or any other device capable of sending or receiving
- The links connecting Nodes are called COMMUNICATION CHANNELS Networks- Why we need them?
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Communication Tasks
There are some key tasks that must be performed in a data communication system Elements can be added, deleted, or merged together
| Transmission System utilization | Interfacing |
| Signal Generation | Synchronization |
| Exchange Management | Error Detection and Correction |
| Flow Control | Addressing |
| Routing | Recovery |
| Security | Network Management |
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Standards
“A standard provides a model for development that makes it possible for a product to work regardless of the individual manufacturer|”
- A great deal of coordination and cooperation is required by the devices to communicate
- A device prepared by a specific manufacturer may not be compatible with the devices prepared by other manufacturers
- Unavailability of standards creates problems and puts a halt to product growth
An example of non-standardized products is AUTOMOBILES
Why Standards are Essential?
Standards are therefore essential in:
- Creating and Maintaining an Open and competitive Market for Equipment Manufacturers
- Guaranteeing National and International Interoperability of Data and Telecommunications Technology and Equipment
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TOPOLOGY
“The Topology is the geometric representation of the relationship of the links and the linking devices (Nodes) in a Network”
Or
“Topology defines the physical or the Logical Agreement of Links in a Network”
Topology of a Network is suggestive of how a network is laid out. It refers to the specific configuration and structure of the connections between the Links and the Nodes. Two or more devices connect to a Link and two or more Links form a Topology
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Ring Topology
Ring Topology Diagram
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Metropolitan Area Networks
- Designed to extend over an entire city
- It may be a single network e.g. Cable TV Network
Or
- Interconnection of a No. of LANs into a larger network
Example: A company can use a MAN to connect the LANs in all of its offices throughout a city
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Critique of OSI Model
Reasoning for OSI not getting Widespread
- Bad Timing(slide) (Apocalypse of Two Elephants)
- David Clarke of MIT
- If standards are written too early: subject is badly understood and bad standards
- If standards are written too late so many companies may have already made investments in doing the same thing with different other ways
- Bad Technology
- Flow control, error control, addressing is multiple
- Session and Presentation(EMPTY), Network and DL(Full)
- Bad Implementations
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Transport Layer
- Responsible for Source-to-Destination delivery of Entire Message
- Network Layer oversees source-to-destination delivery of the entire packets but it does not recognize any relationship b/w those packets
- Network layer treats each packet independently
- Transport Layer ensures that whole message arrives at the destination intact
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Presentation Layer
- This layer is concerned with Syntax and Semantics of info exchange between two systems
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Signals
Need For Signals
- One of the major concerns of Physical layer is moving information in the form of electromagnetic signals across a TX medium
- Information can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user (human or m/c)
- Generally, the info usable to a person or application is not in a form that can be transmitted over a network
- For Example, you cannot roll up a photograph, insert it into the wire and transmit it across the city
- You can transmit however an encoded description of the photograph
- The binary digits must be converted into a form that TX. Medium can accept
- TX. Media work by conducting energy along a physical path. So the data stream of 1s and 0s must be turned into energy in the form of EM signals
Analog and Digital
- Both data and signals that represent them can take either analog or digital form
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Problems 4.3
A Sine wave has a frequency of 6 Hz. What is its period?
Solution
Problems 4.5
A Sine wave completes one cycle in 4 seconds. What is its frequency?
Solution:
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Frequency Spectrum / Bandwidth
- Frequency Spectrum: of a signal is the collection of all the component frequencies it contains
- It is shown using a Frequency domain graph
- Bandwidth: of a signal is the width of the frequency spectrum
- In other words ,Bandwidth refers to the range of the component frequencies and Frequency Spectrum refers to the elements within that range
How to calculate Bandwidth?
- To calculate Bandwidth, subtract the lowest frequency from the highest frequency
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Conversions
Introduction
- Information must be transformed into signals before it can be transported across the communication media
- How info is transformed depends on its original format and on the format used by the communication device
- If you want to send a letter by a smoke signal, you need to know which smoke patterns make which words in your message before building the fire
- Words are the Information and the puffs of smoke are representation of that information
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Conversions
Return to Zero (RZ)
- Any time, data contains long strings of 1’s or 0’s, Rx can loose its timing
- In unipolar, we have seen a good solution is to send a separate timing signal but this solution is both expensive and full of error
- A better solution is to somehow include synch in encoded signal somewhat similar to what we did in NRZ-I but it should work for both strings of 0 & 1
- One solution is RZ encoding which uses 3 values : Positive, Negative and Zero
- Signal changes not b/w bits but during each bit
- Like NRZ-L , +ve voltage means 1 and a –ve voltage means 0, but unlike NRZ- L, half way through each bit interval, the signal returns to zero
- A 1 bit is represented by positive to zero and a 0 is represented by negative to zero transition
- The only problem with RZ encoding is that it requires two signal changes to encode one bit and therefore occupies more BANDWIDTH
- But of the 3 alternatives we have discussed, it is most effective value
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Conversions
Types of Digital-to-Digital Encoding
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Conversion
Digital To Analog Conversion
- Process of changing one of the characteristics of an analog signal based on the info in a digital signal
- When you Tx data from one computer to the other using a public telephone line
- Original data is digital but because telephone wires carry analog signal, original data must be converted
- Digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 to binary 0
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Frequency Shift Keying (FSK)
- Frequency of signal is varied to represent binary 1 or 0
- The frequency of the signal during each bit duration is constant and depends on the bit (0 or 1)
- Both peak amplitude and phase remains constan
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Conversions
Analog To Analog Conversion
- Representation of Analog information by an Analog signal
- For Example: Radio
Analog To Analog Conversion Methods
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Introduction
Q: How do we relay encoded data from the generating device to the next device?
A: Interface
- Defined by several popular standards
- Physical layer of the OSI model
- Mechanical/electrical/functional specifications
Digital Data Transmission
Do we send one bit at a time or do we group bits into larger groups and if so, How?
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DTE-DCE Interface
- There are 4 basic functional units involved in communication of data:
- A DTE and DCE on one end
- A DTE and DCE on the other end
- DTE: Any device that is a source of or destination of digital data
- DCE: Any device that transmits/receives signal through network
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Other Interface Standards
- Both Data Rate and Cable LENGTH are restricted by EIA 232
- Data rate to 20 Kbps and Cable length to 50 feet
- To meet need of users requiring more speed or distance , the EIA and ITU-T has introduces additional standards: EIA 49, EIA 530 and X.21
EIA 449
- Mechanical specifications of EIA 448 define a combination of two connectors , on with 37 pins ( DB 37) and one with 9 pins (DB 9) for a combined 46 pins
- The functional specifications of EIA 449 give the DB 37 pins properties similar to those of the DB 25.
- The major functional difference b/w 25 and 37 pin connectors is that all functions relating to the secondary channel have been removed from DB 37
- Because the secondary channel is seldom used, EIA 449 separates those functions out and puts them in the second, 9 pin connector (DB9)
- In this way, a second channel is available to systems that need it
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Modem Standards
- Bell modems
- First commercial modems by Bell Telephone Co.
- Developed in early 1970s
- ITU-T modem standards
- V-series: Today’s most popular modem standards
- Bell modem compatible: V.21/22/23/26/27/29
- Intelligent modems
- Hayes (or Hayes-compatible) modems
- Modem is controlled by instructions (AT commands)
- Automatic answering, dialing, etc.
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Cable Modems
- Data rate limitation of traditional modems is mostly due to the narrow BW of the local loop telephone line (4Khz)
- If higher BWs are available, one can design a modem that can handle much higher data rates
- Fortunately, cable TV provides residential premises with a coaxial cable that has a BW of up to 750 MHz and sometimes even more
- This BW is normally divided into 6MHz bands using FDM
- Each band provides a TV Channel
- Two bands can be left aside to allow a user to download and upload the information from the internet
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Shielded Twisted Pair (STP)
- Has a metal foil or braided-mesh covering that encases each pair of insulated conductors
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Single Mode Fiber
- Uses step index fiber and a highly focused source of light that limits beams to a small range of angles all close to the horizontal
- Single Mode fiber is manufactured with a much smaller Diameter than Multimode
- All of the beams arrive at the destination together and can be recombined without distortion to the signal
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Propagation of Specific Signals
- The type of propagation used in radio transmission depends upon the frequency of the signal
- Each frequency is suited for a specific layer of atmosphere and is most efficiently transmitted and received by technologies adapted to that layer
Very Low Frequency (VLF) (Figure)
- VLF waves are propagated as surface waves through air
- Do not suffer much attenuation in TX but are susceptible to high levels of atmospheric noise I.e. electricity and heat
- Used for Long-range radio navigation and Submarine Communication
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Transmission Impairments
TX Media are not perfect
Imperfections cause impairments in the signal through the medium
This means that the signal at the beginning and the end of the medium are not the same
What is sent is not what is received
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Introduction
- Whenever the TX capacity of a medium linking 2 devices is greater than the TX needs of the devices, the link can be shared
- Example: Large Water pipe can carry water to several separate houses at once
- Multiplexing is the set of techniques that allows simultaneous TX of multiple signals across a single data link
- As data communication usage increases, traffic also increases
- We can add a new line each time a new channel is needed
- Or we can install higher capacity links and use each to carry multiple signals
- All current TX media i.e. Coax, Optical fiber have high available BWs
- Each of these has carrying capacity far in excess of that needed for one signal
- If TX capacity of a link is greater than the TX needs of devices attached to it, the excess capacity is wasted
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Bit Stuffing
- It is possible to connect devices of different data rates to synchronous TDM
- For Example, device A uses one time slot, while the faster device B uses two slots
- The Time slot length is FIXED
- Therefore data rates should be integer multiples of each other
- For example, we can accommodate a device that is 5 times faster than the other device by giving it five slots to one for each of the other devices
- We cannot accommodate a device which is five and a half times faster using this method because we cannot introduce half a time slot into a frame
- When the speeds are not integer multiples of each other, they can be made to behave as if they were
- This is done by a technique known as BIT STUFFING
- In bit stuffing, MUX adds extra bits to a device
- For Example, if we have one device with a bit rate of 2.75 times that of other devices, we can add enough bits to raise this rate to 3 times that of others
- The extra bits are then discarded by the Demultiplexer
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Telephone Services
Common carrier Services & Hierarchies
- Telephone companies began by providing their subscribers with ANALOG services using ANALOG networks
- Later digital services were introduced
- Nowadays carriers are even thinking about changing their service lines digital too
- Soon the entire n/w will be digital
- For now both types of services are available and both FDM and TDM are in use
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T Lines
T Lines for Analog Lines
- T Lines are digital lines designed for digital data
- However they can also be used for analog transmission (Telephone connections)
- Analog signals are first sampled and the Time Multiplexed
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Types of Errors
Single Bit & Burst
- In a single-bit error, a 0 is changed to a 1 or a 1 to a 0
- In a burst error multiple bits are changed
- For Example, a 0.01 second burst of impulse noise on a TX with a data rate of 1200 bps might change all or some of 12 bits of information
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Error Detection And Correction Methods
Longitudinal Red Check(LRC)
- In LRC, a block of bits is organized in a table (rows and columns)
- For example instead of sending 3 its, we organize them in a table made of 4 rows and 8 columns
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Error Correction And Detection Method
CHECKSUM
- Error detection method used by the Higher Layers
- Like VRC, LRC, CRC, Checksum is also based on the concept of redundancy
One’s Complement
Finding one’s complement
- Invert every 1 to 0 and 0 to 1
- A and –A are one’s complement of each other
- +A = 1010 → -A = 0101
- +0 = 0000 → -0 = 1111
- Error detection method used by the Higher Layers
- Like VRC, LRC, CRC, Checksum is also based on the concept of redundancy
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Hamming Code
Redundancy Bits (r)
- r must be able to indicate at least m+r+1 states
- m+r+1 states must be discoverable by r bits
- Therefore, 2r ≥ m+r+1
- If m=7, r=4 as 24 ≥ 7+4+1
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Line Discipline
Enquiry / Acknowledgement (ENQ/ACK)
- Used primarily in systems where there is no question of wrong receiver getting the transmission
- In other words when there is a Dedicated Link b/w the two devices so that the only device that can receive data is the intended one
- Enquiry / Acknowledgement (ENQ/ACK)
- ENQ/ACK coordinates who may start a transmission and whether or not the intended recipi nt is ready and enabled
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SLIDING WINDOW ARQ
Among several popular mechanisms for error control two protocols are important:
- Go-back-n ARQ
- Selective Reject ARQ
Three features are added to sliding window flow control to allow for the retransmission of the lost or the damaged frames:
- The sending device keeps copies of the transmitted frames until all of them have been acknowledged
- In addition to ACK frames, receiver also has the option of NAK frames, if data has been received damaged
- Because sliding window is a continuous TX mechanism, both ACK and NAK frames
must be numbered for identification
- ACK frames carry the number of the next frame expected
- ACK 5 tells sender that all frames up to frame 5 are received
- NAK frames carry the number of the damaged frame itself
- If data frames 4 and 5 are damaged, NAK 4 and NAK 5 must be sent
- ACK frames carry the number of the next frame expected
- Like sender in stop-and wait ARQ, the sliding window ARQ is also equipped with a timer in the sender to deal with lost ACKs
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YMODEM
YMODEM is similar to X-MODEM with only the following major differences:
- 1024-Byte data unit
- Two CANsto abort Transmission
- ITU-T CRC-16 for Error Checking
- Multiple files can be sent simultaneously
ZMODEM
- Newer Protocol
- Combines features of
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Bit-Oriented Protocols
- In character-oriented protocols, bits are grouped into predefined patterns forming characters
- By comparison, bit-oriented protocols can pack more information into shorter frames
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Local Area Network
A local area network is a data communication system that allows a number of independent devices to communicate directly with each other in a limited geographical area
Architectures for LANS
Dominated by 4 architectures:
- Ethernet, Token Bus, Token Ring
- Standards of IEEE and a part of its Project 802
- Fiber Distributed Data Interface (FDDI)
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Ethernet
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Token Ring
- Token Ring allows each station to send one frame per turn
- Access method: Token passing
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Internet
- An internet is a generic term used to mean an interconnection of individual networks
- To create an internet, we need networking devices called routers and gateways
- An internet is different from the Internet
- Internet is the name of a specific worldwide network
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