TelecomStudy18

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.           ...

There are three types of HDLC frames : ·          Information transfer frame (I-Frame) ·          Supervisory frame (S-Frame) ·          Unnumbered frame (U-Frame) Information Transfer Frame (I-FRAME) I-Frame is used for transporting user data. It also carries acknowledgement of the received frames. The control field of the I-Frame is as shown in Fig.7. The first bit is 0 which identifies the frame as an I-Frame. The next three bits are the sequence number N(S) of the frame. The fifth is Poll/Final (P/F) bit. Its use is explained later. The last three bits are the sequence number N(R) of the acknowledgement (RR) which is piggy backed on the I-Frame.  S upervisory f rame (S-FRAME) S-Frame does not have data field (Fig.3b) and is used to carry only acknowledgements, requests for retransmission, etc. It is identified by the first two bi...

 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...

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...