Data Comm Final

Front 1

Differentiate between an analog and a digital electromagnetic signal.

Back 1

An analog signal is one in which the signal intensity varies in a smooth fashion over time (There are no breaks or discontinuities in the signal). A digital signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level.

Front 2

How many radians are there in a complete circle of 360 degrees

Back 2

2(Pie)Radians

Front 3

What is the relationship between the wavelength and frequency of a sine wave?

Back 3

A wavelength is the distance between two points of corresponding phase of two consecutive cycles (The distance occupied by a single cycle- distance between 2 successive peaks of a wave) Frequency is the number of cycles that pass an observer in a given time

Front 4

Define fundamental frequency

Back 4

When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is referred to as the fundamental frequency

Front 5

What is attenuation

Back 5

When an electromagnetic signal is transmitted along any medium, it gradually becomes weaker at greater distances

Front 6

What is noise?

Back 6

The addition of unwanted electromagnetic energy that is inserted somewhere between the transmitter and the receiver. The latter, undesired signals are noise

Front 7

What is white noise?

Back 7

Thermal noise in uniformly distributed across the frequency spectrum and hence is often referred to as white noise

Front 8

Describe the phenomenon known as multipath

Back 8

It is when wireless signals can be obstructed by obstacles, in which it then bounces off of and it reaches the receiving antenna by several different paths

Front 9

Define channel capacity

Back 9

The rate at which data can be transmitted over a given communication path, or channel

Front 10

Distinguise among analog data, analog signaling, and analog transmission.

Back 10

Analog data take on a continuous values on some interval (Ex- temperature and pressure measurements). Analog signaling is a continuously varying electromagnetic wave that may be transmitted over a variety of media, depending on frequency. Analog transmission is a means of transmitting analog signals without regard to their contentl the signals may represent analog data (such as voice) or digital data (such as data that pass through a modem). In either case the analog signal will suffer attenuation.

Front 11

Distinguish among digital data, digital signaling, and digital transmission.

Back 11

Digital Data take on discrete values; examples are text, integers, and binary data. Digital signaling is a sequence of voltage pulses that may be transmitted over a wire medium; for example, a constant positive voltage may represent binary 0, and a constant negative value may represent binary 1. Advantages of digital signaling is its cheaper and less susceptible to noise interference. Digital Transmission is concerned with the content of the signal. digital transmission can only go a limited distance before attenuation endangers the integrity of the line, therefore repeaters are used.

Front 12

What is differential encoding?

Back 12

The signal is decoded by comparing the polarity of adjacent signal elements rather than determining the absolute value of a signal element

Front 13

What function does a modem perform

Back 13

It encodes digital information- it modulates one of the three characteristics of the carrier:amplitude, frequency, or phase, or some combination of these

Front 14

Indicate 3 major advantages of digital transmission over analog transmission.

Back 14

Cost, Security and Privacy, and Integration

Front 15

How many binary values represented in amplitude shift keying, and what is the limitation of this approach?

Back 15

2- It is susceptible to sudden gain changes and is a rather inefficient modulation technique

Front 16

How is the transmission of a single character differentiated from the transmission of the next character in asynchronous transmission?

Back 16

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. When no character is being transmitted the line between transmitter and receiver is idle.

Front 17

What is a major disadvantage of asynchronous transmission?

Back 17

The larger the block of bits, the greater the cumulative timing error. It also requires an overhead of 2 to 3 bits per character.

Front 18

Define error control

Back 18

A technique in which the transmission and reception ends provide data link control to provide mechanisms by which the two sides cooperate in the retranmission of frames that suffer from errors on the first try. It is based on Error detection, positive acknowledgement, retransmission after timeout, and negative acknowledgment and retransmision

Front 19

What is multiplexing?

Back 19

This is when two communicating stations will not utilize the full capacity of a data link. They then in turn share the capacity

Front 20

The set of time slots or the frequency allocated to a single source is called what?

Back 20

synchronous TDM

Front 21

How is interference avoided by using frequency division multiplexing?

Back 21

To prevent interference, the channels are separated by guard bands, which are unused portions of the spectrum

Front 22

Define upstream and downstream with respect to subscriber lines

Back 22

upstream is when the information travels from the customer to the carrier and downstream is when the information travels from the carrier's central office to the customer's site

Front 23

Explain how synchronous time division multiplexing(TDM) works.

Back 23

Six signal sources are fed into a multiplexer, which interleaves the bits from each signal by taking turns transmitting bits from each of the signals in a round-robin fashion.

Front 24

What is the difference between passive and active security threats?

Back 24

A passive attack attempts to learn or make use of information from the system but does not affect system resources. An active attack attempts to alter system resources or affect their opertation`

Front 25

List and briefly define categories of passive and active security threats.

Back 25

Passive- Eavesdropping, or monitoring transmissions.Release of message contents-is when a telephone conversation, an email, or a transferred file may contain sensitive or confidential info. We would like to prevent an opponent from learning the contents of these transmissions. Traffic Analysis is when the information is encrypted, so even if they get the info they cannot read it. Active Attacks-- Masquerade- takes place when 1 entity pretends to be a different entity. Replay- a passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect. Modification of messages- some portion of a legitimate message is altered, or that messages are delayed or reordered, to produce an unauthorized effect. Denial of service- prevents or inhibits the normal use of management of communications facilities.

Front 26

What are the DES and triple DES?

Back 26

Data Encryption Standard- was the original dominant encryption algorithm since its introduction. Seen as it only uses a 56-bit key, it became to slow for computers and became obsolete. Triple DES extended the live of DES which involves repeating the basic DES algorithm three times, using either 2 or 3 unique keys, for a key size of 112 or 168 bits. The main drawback of 3DES is that the algorithm is relatively sluggish in software. Another drawback is that both DES and 3DES use a 64-bit block size . For reasons of both efficiency and security, a larger block size is desirable

Front 27

How is the AES expected to be an improvement over triple DES

Back 27

It should have at least a block length of 128 bits and support for key lengths of 128, 192, and 256 bits. It would have increased security strength and improved efficiency

Front 28

Explain traffic padding.

Back 28

a function that produces ciphertext output continuously, even in the absence of plaintext.

Front 29

List and briefly define various approaches to message authentication

Back 29

authentication using symmetric encryption- here we assume that the sender and receiver share a key, then only the genuine sender would be able to successfully encrypt a message for the other participant. Message Authentication without Message Encryption- an authentication tag is generated and appended to each message for transmission. The message itself is not encrypted and can be read at the destination independent of the authentication function at the destination. Message Authentication Code- using a secret key to generate a small block of data, that is appended to the message. Both the sender and receiver know the code and must enter it to send and receive the data.

Front 30

What is a secure hash function?

Back 30

also known as one-way hash function- its function is to produce a "fingerprint" of a larger file,message, or other block of data. Used to ensure integrity of the information.

Front 31

Explain the difference between symmetric encryption and public-key encryption

Back 31

Public key uses two separate keys, instead of only one key. The use of 2 keys has profound consequences in the areas of confidentiality, key distribution, and authentication.

Front 32

What are the distinctions among the terms public key, private key, and secret key?

Back 32

public key- is made public for others to use. It relies on one key for encryption and a different but related key for decryption-- Private Key- is known only to its owners. -- Secret Key- ?

Front 33

What is a digital signature

Back 33

an authentication mechanism that enables the creator of a message to attach a code that acts as a signature. The signature guarantees the source and integrity of the message