Category Archives: Data Communications Systems

What is a ransomware attack?

Ransom + Software = Ransomware

Ransomware will either:

  • lock the computer to prevent the user from normal usage or
  • encrypt the data that it stored on the victim’s hard disk to prevent access.

Ransomware is a type of malware that forces its victim to pay the ransom through certain online payment methods so as to grand access to their system or to decrypt their data.

Payment is often required in the form of cryptocurrency such as Bitcoin.

In order to minimize the impact of ransomware maintain an offline backup of end-user data.

Example of Ransomware: Locky, CryptoLocker, Cerber, Simplelocker.

Prefix Code

Prefix code is defined as a code in which no codeword is a prefix of a any other codeword.

Example 1:

Assume the Huffman code:

A=11111, B=11110, C=1110, D=1101, E=1100, F=10, G=01, H=00

We have codeword 00 and no other codeword starts with 00. The five-bit codewords 11111 and 11110 start with 1111, which is not a codeword.

Example 2:

A=0, B=10, C=110, D=111

We have codeword 0 and no other codeword starts with 0. No codeword starts with 10. Therefore is a prefix code.

Time-Division Duplexing (TDD)

Time-Division Duplexing (TDD) is a transmission method that uses one channel for transmitting and receiving, separating them by different time slots. No guard band is used. Since the transmitter and the receiver do not operate at the same time, there is no possibility of interference between the two.

TDD is like a narrow bridge in a two-way road. Vehicles moving in one lane must wait the vehicles coming from the opposite direction to cross the bridge.

Time-Division Duplexing (TDD) is also known as Half Duplex. In half duplex transmission data can travel in either direction but not simultaneously.

Example of a Half Duplex system is the walkie-talkie.

LTE supports both FDD and TDD, [1].

LTE stands for Long Term Evolution.


[1] Erik Dahlman, Stefan Parkvall, Johan Skold, 4G: LTE/LTE-Advanced for Mobile Broadband, Academic Press

Time-Division Multiple Access (TDMA)

TDMA users share a common frequency channel but use the channel for only a very short time. They are given a time slot and only allowed to transmit during that time slot. When all available time slots in a given frequency are used, the next user must be assigned a time slot on another frequency. These time slices are so small that the human ear does not perceive the time slicing. Examples of TDMA-based systems are the following:

  • Global System for Mobile communications (GSM) in Europe
  • TDMA IS-136 in US


In TDMA one user can transmit and receive data only during the predetermined time interval.


[1] Independent Component Analysis, Erkki Oja et al, John Wiley & Sons

[2] UMTS, Javier Sanchez, Mamadou Thioune, John Wiley & Sons


RSA SecurID uses two-factor authentication to determine we are who we say we are. RSA SecurID combines

  • something we know (our PIN) with
  • something we have (our RSA SecurID token that generates the tokencode)

0005 RSA SecurID token

Figure: RSA SecurID token



Multifactor authentication

Multifactor authentication means submitting at least two out of three forms of identification.

  • First factor: something you know (e.g. password, PIN)
  • Second factor: something you have (e.g. Credit/Debit card, Mobile phone, Software/Hardware token )
  • Third factor: something you are (e.g. signature, fingerprint)

Visitor Location Register (VLR)

Some networks provide Visitor Location Registers (VLR) at their MSCs (Mobile Switching Center), such that, one an MS (Mobile Station) roams from a different traffic area, the subscriber information from its HLR (Home Location Register) can be transmitted to and temporarily stored in the VLR. VLRs prevent unnecessary, potentially long-distance, repeated communication with the HLR.

Home Location Register (HLR)

Each MSC (Mobile Switching Center) has access to a database called Home Location Register (HLR). This contains information on all subscribers who are initially registered within its traffic area. This information includes the subscriber identity numbers, MS (Mobile Station) identity numbers, service subscription details, usual and present traffic area locations plus authentication and encryption keys.