CS-4333 COMPUTER NETWORK EXAM
QUESTIONS WITH COMPLETE
ANSWERS
4B/5B - Answer-- 4B/5B, attempts to address the inefficiency of the Manchester
encoding without suffering from the problem of having extended durations of high or low
signals.
- The idea of 4B/5B is to insert extra bits into the bit stream so as to break up long
sequences of 0s or 1s. Specifically, every 4 bits of actual data are encoded in a 5-bit
code that is then transmitted to the receiver; hence, the name 4B/5B.
- The 5-bit codes are selected in such a way that each one has no more than one
leading 0 and no more than two trailing 0s.
- Thus, when sent back-to-back, no pair of 5-bit codes results in more than three
consecutive 0s being transmitted.
- The resulting 5-bit codes are then transmitted using the NRZI encoding, which
explains why the code is only concerned about consecutive 0s—NRZI already solves
the problem of consecutive 1s.
- Note that the 4B/5B encoding results in 80% efficiency.
Chapter 2.3 Framing - Answer-Byte-Oriented Protocols (PPP)
Bit-Oriented Protocols (HDLC)
Clock-Based Framing (SONET)
Byte-Oriented Protocols (PPP) - Answer-8 bits - Flag
8 bits - Address
8 bits - Control
16 bits - Protocol
1500 bytes by default - Payload
16 bits - Checksum
8 bits - Flag
Bit-Oriented Protocols (HDLC) - Answer-8 bits - Starting Sequence
16 bits - Header
Body
16 bits - CRC
8 bits - Ending Sequence
Clock-Based Framing (SONET) - Answer-- Clock-based framing In a clock-based
system, a series of repetitive pulses are used to maintain a constant bit rate and keep
the digital bits aligned in the data stream.
- SONET (Synchronous Optical Network) is a synchronous system in which all the
clocks in the network are synchronized back to a master clock reference.
, Chapter 2.4.1 Checksum (TEST with calculate) - Answer-- one's complement arithmetic.
- The idea behind the Internet checksum is very simple—you add up all the words that
are transmitted and then transmit the result of that sum. The result is the checksum.
The receiver performs the same calculation on the received data and compares the
result with the received checksum.
If any transmitted data, including the checksum itself, is corrupted, then the results will
not match, so the receiver knows that an error occurred.
Chapter 2.4.2 CRC (TEST with calculate) - Answer-
Chapter 2.5 Reliable Transmission - Answer-Stop-and-Wait
Sliding Window
Concurrent Logical Channels
Stop-and-Wait - Answer-The simplest ARQ scheme is the stop-and-wait algorithm. The
idea of stop-and-wait is straightforward: After transmitting one frame, the sender waits
for an acknowledgment before transmitting the next frame. If the acknowledgment does
not arrive after a certain period of time, the sender times out and retransmits the original
frame.
Disadvantage:
fairly slow: the sender can send at most one new packet per RTT.
not robust: if the ack can get lost, when the receiver gets a packet, the receiver cannot
tell if it is a retransmission or a new packet.
Sliding Window - Answer-The sliding window is a technique for sending multiple frames
at a time. It controls the data packets between the two devices where reliable and
gradual delivery of data frames is needed. It is also used in TCP (Transmission Control
Protocol).
Sliding window protocol has two types:
1. Go-Back-N ARQ
2. Selective Repeat ARQ
Concurrent Logical Channels - Answer-The idea underlying the ARPANET protocol,
which we refer to as concurrent logical channels, is to multiplex several logical channels
onto a single point-to-point link and to run the stop-and-wait algorithm on each of these
logical channels. There is no relationship maintained among the frames sent on any of
the logical channels, yet because a different frame can be outstanding on each of the
several logical channels the sender can keep the link full.
Ethernet (Chapter 2.6) - Answer-- Collision
- Aloha Network
- Repeater
- Transceiver
- Standard 802.3
- Adaptor
QUESTIONS WITH COMPLETE
ANSWERS
4B/5B - Answer-- 4B/5B, attempts to address the inefficiency of the Manchester
encoding without suffering from the problem of having extended durations of high or low
signals.
- The idea of 4B/5B is to insert extra bits into the bit stream so as to break up long
sequences of 0s or 1s. Specifically, every 4 bits of actual data are encoded in a 5-bit
code that is then transmitted to the receiver; hence, the name 4B/5B.
- The 5-bit codes are selected in such a way that each one has no more than one
leading 0 and no more than two trailing 0s.
- Thus, when sent back-to-back, no pair of 5-bit codes results in more than three
consecutive 0s being transmitted.
- The resulting 5-bit codes are then transmitted using the NRZI encoding, which
explains why the code is only concerned about consecutive 0s—NRZI already solves
the problem of consecutive 1s.
- Note that the 4B/5B encoding results in 80% efficiency.
Chapter 2.3 Framing - Answer-Byte-Oriented Protocols (PPP)
Bit-Oriented Protocols (HDLC)
Clock-Based Framing (SONET)
Byte-Oriented Protocols (PPP) - Answer-8 bits - Flag
8 bits - Address
8 bits - Control
16 bits - Protocol
1500 bytes by default - Payload
16 bits - Checksum
8 bits - Flag
Bit-Oriented Protocols (HDLC) - Answer-8 bits - Starting Sequence
16 bits - Header
Body
16 bits - CRC
8 bits - Ending Sequence
Clock-Based Framing (SONET) - Answer-- Clock-based framing In a clock-based
system, a series of repetitive pulses are used to maintain a constant bit rate and keep
the digital bits aligned in the data stream.
- SONET (Synchronous Optical Network) is a synchronous system in which all the
clocks in the network are synchronized back to a master clock reference.
, Chapter 2.4.1 Checksum (TEST with calculate) - Answer-- one's complement arithmetic.
- The idea behind the Internet checksum is very simple—you add up all the words that
are transmitted and then transmit the result of that sum. The result is the checksum.
The receiver performs the same calculation on the received data and compares the
result with the received checksum.
If any transmitted data, including the checksum itself, is corrupted, then the results will
not match, so the receiver knows that an error occurred.
Chapter 2.4.2 CRC (TEST with calculate) - Answer-
Chapter 2.5 Reliable Transmission - Answer-Stop-and-Wait
Sliding Window
Concurrent Logical Channels
Stop-and-Wait - Answer-The simplest ARQ scheme is the stop-and-wait algorithm. The
idea of stop-and-wait is straightforward: After transmitting one frame, the sender waits
for an acknowledgment before transmitting the next frame. If the acknowledgment does
not arrive after a certain period of time, the sender times out and retransmits the original
frame.
Disadvantage:
fairly slow: the sender can send at most one new packet per RTT.
not robust: if the ack can get lost, when the receiver gets a packet, the receiver cannot
tell if it is a retransmission or a new packet.
Sliding Window - Answer-The sliding window is a technique for sending multiple frames
at a time. It controls the data packets between the two devices where reliable and
gradual delivery of data frames is needed. It is also used in TCP (Transmission Control
Protocol).
Sliding window protocol has two types:
1. Go-Back-N ARQ
2. Selective Repeat ARQ
Concurrent Logical Channels - Answer-The idea underlying the ARPANET protocol,
which we refer to as concurrent logical channels, is to multiplex several logical channels
onto a single point-to-point link and to run the stop-and-wait algorithm on each of these
logical channels. There is no relationship maintained among the frames sent on any of
the logical channels, yet because a different frame can be outstanding on each of the
several logical channels the sender can keep the link full.
Ethernet (Chapter 2.6) - Answer-- Collision
- Aloha Network
- Repeater
- Transceiver
- Standard 802.3
- Adaptor
