Data Communication Networks-IMP Questions with answers

Compare and Contrast Analog and Digital Transmission

Wave Structure

–          Analog

o   an analog waveform

o   similar to sound and light

–          Digital

o   a square waveform

o   only two voltage levels, each representing binary 1s and 0s


–          Analog

o   Transmits by modulating a carrier wave’s frequency, amplitude, or phase

o   Bandwidth will be equal or a multiple of the carrier wave frequency

o   Quadrature Amplitude Modulation

  • Modulates the carrier wave 256 different ways
  • Allows each cycle to represent 8 binary bits
  • Bandwidth is eight times greater than the carrier wave frequency

–          Digital

o   Use high and low voltage levels to represent binary numbers

o   Each cycle can only represent one bit

o   Square waves more resistant to EMI

  • More reliable for long distance transmissions


–          Analog

o   Requires modems to translate digital data in computers to analog waves for transmission over the carrier wave

–          Digital

o   Doesn’t require modems, because data remains digital

o   Pulse Code Modulation

  • Used by digital phones to convert analog voice into digital signals for transmission
  • Voltage of analog wave is measured in regular intervals (sampling)
  • The numerical values of the voltage change over time is recorded and transmitted as digital data
  • Numeric voltage values used to reconstruct analog voice wave at the other end
  • Sampling rate must be at least 2x the highest frequency of the analog wave
  • Digital phones send and receive data at 64000 bps (64Kbps)

Asynchronous V/S Synchronous Transmission


–          NICs not synchronized

–          Only one character (8 bits) at a time


–          NICs synchronized through the preamble

–          Large blocks (multiple bytes) sent at a time

UART (Universal Asynchronous Receiver Transmitter)


–          All transmissions go through the UART

–          Sends a stream of 1s down the line while it is idle

–          Uses start bit (0) and end bit (1) to indicate the start and end of a character

–          Interrupt 14 engaged every time a character needs to be sent

–          Frequent interrupts due to small packet size


–          Don’t go through UART

–          Interrupt 52 engaged every time a block needs to be sent

–          Less frequent interrupts due to larger packet size

Error checking


–          One character per transmission limits error checking to single bit parity

–          Sets parity bit to either 1 or 0 depending on number of 1 bits in the character

–          Number of 1 bits should always be odd, unless a single bit error has occurred

–          Doesn’t detect multi bit errors

–          85% effectiveness


–          Multiple bytes per transmission allows for more effective CRC error checking

–          Entire string divided by 16 or 48 bit polynomial, and remainder is calculated

–          Remainder (CRC checksum) transmitted with data

–          Receiving computer performs same calculation, and compares with CRC that was sent from the sending host

–          99.9999% effective

Describe how DSL and cable modems work

DSL (Digital Subscribers Line)

–          Transmissions are sent and received over standard telephone lines

–          Telephone lines can handle frequencies up to 1,000,000Hz (1MHz)

–          Bandwidth is divided into three frequency ranges

o   0-4000Hz are used for digital telephones.  Transmissions in this range are immediately directed to the telephones

o   4000-100,000Hz are used for sending outbound data

o   100,000-1,000,000Hz used for receiving incoming data

–          The greater range used for incoming data allows for greater downloading bandwidth than uploading bandwidth, which makes it Asymmetrical

o   Most home users download more than they upload

o   Ranges will be more even with Symmetrical DSL

–          Pros:

o   Every home has its own phone line, so bandwidth is not shared between users in the same neighborhood

o   Speeds are always consistent regardless of the activities of the neighboring homes

–          Cons:

o   DSL providers do not have repeaters like Cable providers

o   Higher frequency transmissions become attenuated over longer distances

o   Users located farther from central office will not get the whole downloading frequency range

o   Speed vary depending on distance from central office

Cable Modems

–          Transmission sent over a coaxial cable line

–          Coaxial line can handle signals up to 600MHz

–          Each TV channel is sent over its own frequency

o   A multiple of its base 6MHz signal

o   Some channels (frequencies) are reserved for internet transmissions

–          When the signal from a coaxial cable reaches the cable modem, the modem filters out all the frequencies except for the ones used for internet transmission

–          Pros:

o   Cable internet providers have repeaters in the vicinity of very neighborhood, that boosts the strength of the signals.  Prevents attenuation of internet signal

o   Users get high speeds regardless of their distance from central office

–          Cons:

o   The reserved frequency used for internet transmission is shared between all users in a local neighborhood

o   Speeds can vary depending on the internet usage of your neighbors

o   Speeds are lowered as more people share a common line


What is SDLC and how does it work?

–          Synchronous Data Link Control (developed by IBM)

–          A uniquely efficient protocol due to a low number of overhead control bits

–          Commonly used in banking machines

–          Two types of nodes used in SDLC

o   Primary nodes control the operation of their child nodes (secondary nodes)

o   Primary will poll secondaries in sequence, to see if they have data to send

o   Secondaires will only send data when they are polled by the primary

o   Only one secondary will send data to its primary at a time

o   Completely free of collisions

SDLC frame format

–          Starts and ends with a flag byte (01111110)

–          A unique bit pattern used to indicate the start and end of a frame

–          No set frame length in SDLC so all data is read between flags, regardless of length

–          Protocol Transparency (Bit Stuffing)

o   Used to prevent the accidental occurrence of another flag sequence

o   Protocol checks the bit stream for a string of 5 consecutive 1s

o   If a string longer than 5 is found, a 0 bit is ‘stuffed’ after every 1 consecutive 1s to break the chain.

o   The receiving computer will know that the 0 bit is a stuffing bit an will remove it reaches the destination

–          Address frame (8-16 bits)

o   Every secondary node connected to a common primary will have a unique address

o   The address will be of the secondary node the data is coming from, or being sent too

–          Control frame

o   3 types control frames

o   Information frames

  • [frame sequence #][P/F][# frames before ack][0]
  • Always end in a 0
  • Requires an information field
  • Bits 1-3 = Frame sequence number is the number of frames sent
  • Bits 5-7 = Number of frames sent before acknowledgement
  • Bit 4 = poll/final
  • If information is being sent from a primary to secondary, the bit is a poll bit, turned on when the primary is polling a secondary
  • If information is being sent from a secondary to the primary, it is final frame bit. It is turned on when it is the last frame in the sequence.

o   Supervisor Frames

  • [frame sequence #][P/F][2 bit code][01]
  • Always ends in a 01
  • Have no information field
  • Bits 1-3 are frame sequence number
  • Bit 4 is the poll/final bit
  • Bits 5-6 are used for a 2 bit code
  • Positive acknowledgement
  • Negative acknowledgement
  • Flow control

o   Unnumbered Frames

  • [3 bit code][P/F][2 bit code][11]
  • Always ends in a 11
  • Sometimes contain information fields
  • 5 bit code allows for up to 32 different codes to be sent
  • Can be used in initialize secondary terminals

–          Information frames (optional)

o   Contains Path Information Unit (PIU) or Exchange Identification (XID), along with any other information that needs to be sent

–          Frame Checksum

o   A CRC checksum used by the receiving terminal to verify data integrity


What is X.25 and how does it work?

–          A packet switching protocol

–          Developed by the ITU (International Telecommunication Union)

–          Very reliable and often used in the retail industry

–          Relatively inexpensive to set up

–          Slow due to large amount of flow control and error checking

–          X.25 only used to transfer data into and out of the packet switching network

–          Different protocols used within the network itself

X.25 data transfer

–          Data passed to X.25 protocol is broken down into packets

o   Default packet size is 128 bits

–          Each packet contains a 3-4 byte header

o   A virtual circuit ID

–          Establishing a connection

o   The terminal will send the ‘call request’ packet to the central server automatically when it powers on and uses server’s physical address 14 bytes

o   The packet will take the path least used in the packet switching network

o   The central server will respond with ‘call accept’ packet and send it along the same path

o   The terminal will then assign a number to that virtual path and use that address to send and receive data from the server

  • Each terminal can have up to 32 virtual paths by default

o   Terminal will terminate its connections when powered down by sending ‘call cancel’ frame to the server

o   Every time the terminal powers up, it will use a different virtual path

–          Extensive fault tolerance and flow control

o   Every time a packet reaches a switch, it is copied, and is checked for errors

o   If an error is found, it requests the copy from the previous switch

o   Extensive error checking and flow control slows down the network

What is Frame Relay and how does it work?

–          A high speed packet switching protocol

–          Developed by ITU (International Telecommunication Union)

–          Based on X.25

–          Designed to handle large bursts of data

o   Not constant bit-rate transmissions

–          Designed to be very fast

o   No control information is used

o   Error handling and flow control discarded for greater speed

o   Frame only contains [flag][address][data][CRC][flag]

Frame Relay Addressing

–          2-4 bytes long

–          Byte 1

o   First 6 bits – Data Link Connection ID (DLCI)

o   Bit 7 – Command/Response (not used)

o   Bit 8 – extended address (EA) bit (turned on to indicate next byte is also used for addressing)

–          Byte 2

o   First 4 bits – DLCI extension

o   Bit 5 – Backward Explicit Congestion Notification (BECN)

  • Not used

o   Bit 6 – Forward Explicit Congestion Notification (FECN)

  • Not used

o   Bit 7 – Discard Eligibility (DE)

o   Bit 8 – EA bit (turned on when next byte is used for addressing)


–          Frame Relay Assembler Disassembler

–          Used to connect LANs to FR networks

–          FR protocol has no flow control.  Flow control is done by FRAD

–          Prevents users from flooding the network with very long frames

–          No flow control is present beyond the FRAD

–          Network provider assigns a Committed Information Rate (CIR) to each LAN connected to the network, which specifies max frame size

–          Frames that exceed the CIR have their DE bit set to 1

–          When a switch becomes congested, frames with the DE bit set to 1 are the first to be dropped

Error checking

–          Frame Relay switch will verify the DLCI and perform CRC error checking

–          If an error is found, the frame is dropped, but the sending host is not notified


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