TelecomStudy18

There are many different types of electromagnetic waves. These electromagnetic waves can be described by a sinusoidal function, which is characterized by wavelength. Wavelength (l) is the length of one complete oscillation and is measured in meters (m). Frequency and wavelength are related via the speed of propagation, which for radio waves is the speed of light (3 x10^8 m/s or meters per second). The wavelength of a frequency can be determined by using the following formula: Wavelength = Speed / Frequency Thus, for GSM 900 the wavelength is: Wavelength = 3×10^8 m/s / 900 MHz            Wavelength = 300,000,000 m/s  / 900,000,000 Wavelength = 0.33 m (or 33 cm) From this formula it can be determined that the higher the frequency, the shorter the wavelength. Lower frequencies, with longer wavelengths, are better suited to transmission over large distances, because they bounce on the surface of the e...

We can represent bits as digital electrical signals in many ways. Data bits can be coded into following two types of codes : (a)                  Non Return to Zero (NRZ Codes). (b)                  Return to Zero (RZ Codes) NRZ Codes In this type of codes, the signal level remains constant during a bit duration. There are 3 types of NRZ codes. NRZ-L Coding Bit is represented as a voltage level which remains constant during the bit duration. NRZ-M Coding A transition in the beginning of a bit interval whenever there is a 'Mark. NRZ-S Coding A transition in the beginning of a bit interval whenever there is a 'Space'. Let us see the following bit stream 10100110 into three different types of NRZ codes  RZ Codes: Following are the RZ Codes (a)         Manchester Coding ...

All data communication codes are based on the binary system (1s and 0s). A message can be encoded into a meaningful string of 1s and 0s that can be transmitted along a data line and decoded by a receiver. The string of 1s and 0s is meaningful because it is defined by a code that is known to both the source and the receiver. Code is limited by the number of bits (binary digits) it contains, e.g. one-bit code means that we can have 2 characters so that we can encode the letter A by '0' and B by '1'. Similarly, a 2 bit code will enable us to handle 4 characters. Thus, a n-bit code enables us to handle 2 n characters.  Some commonly used codes are : 1.              Baudot code 2.              ASCII code 3.              BCDIC code 4.           ...

For understanding the data communication following terminology is discussed: - ·         Communication lines The medium that carries the message in a data communication system, example of  A 2W telephone line.    Communication Channel A channel is defined as a means of one way transmission. It can carry information in either direction but in only one direction at a time, e.g. A hose pipe. It can carry water in either direction, but the direction of flow depends on which end of pipe is connected to the water tap.  Simplex Transmission 1.     Message always flows in one direction only. 2.     An input Terminal can only receive and never transmit. 3.     An O/P Terminal can only transmit and never receive. Half Duplex Transmission -         A half duplex channel can transmit an...

 HDLC  was developed by ISO and has become the most widely accepted data link protocol. It offers a high level of flexibility, adaptability, reliability and efficiency of operation for today as well as tomorrow's synchronous data communication needs. ADCCP developed by ANSI is almost similar to HDLC, IBM'S SDLC is a proper subset of HDLC and level 2 of X-25 is a permissible option of HDLC. In this chapter, we shall study the basic features and operation of HDLC protocol. Certain liberties have been taken in the level of completeness of description so as not to cloud the overall picture with the details. GENERAL FEATURES of hdlc    HDLC is a bit oriented data link control protocol which satisfies wide variety of data link control requirements including: ·          Point-to-point and point-to-multipoint links. ·          Two way simultaneous communication over full duplex circui...

ERROR DETECTION When a code word is transmitted, one or more of its bits may be reversed due to signal impairment. The receiver can detect these errors if the received code word is not one of the valid code word of the code set. If the corrupted received word becomes another valid code word, the error cannot be detected. When error occurs, the distance between the transmitted and received code words is equal to the number of erroneous bits . as showing in given below figure. TRANSMITTED CODE WORD RECEIVED CODE WORD NUMBER OF ERRORS DISTANCE 11001100 11001110 1 1 10010010 00011010 2 2 10101010 10100100 3 3 In other words the valid code words must be separated by a distance more than 1 else even a single bit error will generate another valid code word and the error will not be detected. The number of errors which can be detected...

Errors are introduced in the data bits during their transmission across a sub network. These errors can be categorised into : ·           Content errors ·           Flow integrity errors Content errors are the errors in the content or a message, e.g. a "1" may be received as "0". This type of errors gets introduced due to impairment of the electrical signal in the transmission media. Flow integrity errors refer to missing blocks of data. For example, a data block may be lost in the sub-network due to its having been delivered to a wrong destination. In voice communication, the listener can tolerate a good deal of signal corruption during transmission. But data is very sensitive to errors. Measures are, therefore, built into a data communication system to counteract the effect of errors. These measures include: ·             ...

Let us now understand the difference between transmission and communication. Transmission means physical movement of information from one point to another. Communication means meaningful exchange of information between the communicating devices. Example Two persons, one knowing English language only and the other knowing French language only cannot communicate with each other. Here transmission is taking place, but communication is not there. Therefore, for communication, we need much more than the transmission. For communication, we must have the same language, i.e. Data codes should be understood both by transmitter and the receiver. Moreover, receiver should be in a position to receive, i.e. Timing is also very important. We have two types of communication : (1)                Synchronous Communication. (2)                Asynchrono...

A cell is a base transceiver service area as seen by the mobile station (MS). A cell uses a specific set of frequencies.    There are two  types of cells: Omni cells: An omni cell is a cell where the antenna transmits omni-directional. The coverage area of an omni cell is in principle a hexagon/circle, but in reality a rough pattern. Sector cells: A sector cell is a cell where the antenna transmits directional.  Examples of sector cell types are: -           2-sector cells (e.g. for highways) -           3-sector cells. The following figure shows examples of different cell types    Sector vs. omni cells Advantages of sector cells are (compared to omni cells): •           Increased coverage area per site (by the use of higher gain antennas) •   ...

Interleaving is a simple, but powerful, method of reducing the effects of burst errors and recovering bits when burst errors occur. The symbols (output of Forward Error Correction Coder) from each group are interleaved in a pattern that the receiver knows. The interleaver is located at the BTS and in the phone.

Duplexing : Duplexing is the technique by which the send and receive paths are separated over the medium, since transmission entities (modulator, amplifiers, demodulators) are involved. There are two types of duplexing.\ 1. Frequency Division Duplexing FDD 2. Time Division Duplexing TDD        Frequency Division Duplexing FDD Different Frequencies are used for send and receive paths and hence there will be a forward band and reverse band. Duplexer is needed if simultaneous transmission (send) and reception (receive) methodology is adopted .Frequency separation between forward band and reverse band is constant Time Division Duplexing (TDD) TDD uses different time slots for transmission and reception paths. Single radio frequency can be used in both the directions instead of two as in FDD. No duplexer is required. Only a fast switching synthesizer, RF filter path and fast antenna switch are needed. It increases the battery life of mobile phon...