SIGNAL ENCODING

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

(b)        Biphase-M Coding

(c)         Biphase-S Coding

(d)        Differential Manchester Coding.

Manchester Coding

There '1' is represented as the clock pulse itself and '0' as inverted clock pulse. It is widely used in local area networks. Fig.21 shows representation of '1' and '0'.

Bi-phase M Coding

  There is always a transition in the beginning of a bit interval and binary '1' is having additional transition in the middle of the bit interval.

Bi-phase S Coding

There is a transition at the beginning of a bit interval and binary '0' is having additional transition in the middle of the bit interval.

Differential Manchester Coding

There is always a transition in the middle of the bit interval and Binary '0' has additional transition in the beginning of the bit interval. Let us see Fig.22 in which bit sequence 10100110 has been shown in different RZ codes. 

TRANSMISSION CODES in Telecom

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 2ⁿ characters. 

Some commonly used codes are :

1.             Baudot code

2.             ASCII code

3.             BCDIC code

4.             EBCDIC Code

ASCII Code (American Standard Code for Information Interchange)

It is an eight-bit code which consists of seven information bits and one bit for parity checking. This is most widely used data code. Seven information bits gives us 128 combinations, which allows us to encode a full keyboard of the computer.

-          52 alphabets (capital and small).

-          0-9 (10 numbers).

-          Punctuation marks

-          Additional graphic and control characters.

BCDIC (Binary Code Decimal Interchange Code)

It is a six-bit code that is used as an internal code by some computers. With 6 information bits, we can have 2⁶ = 64 possible code combinations. For data transmission, code is implemented as 7-bit code containing 6 information bits and one parity bit.

EBCDIC (Extended Binary Coded Decimal Interchange Code)

It is a 8-bit code in which all the 8-bits are used for information (unlike ASCII), giving 256 possible code combinations. EBCDIC is used as an internal machine code in some of the computers. 

TYPES OF HDLC FRAMES

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. 

Supervisory frame (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 bits of the control field (Fig. 8). These two bits are 10 in an S-Frame. The next two bits (SS) are used to indicate four supervisory states. Receive Ready (RR), Receive Not Ready (RNR), Reject (REJ) and Selective Reject (SREJ).

The fifth bit is Poll/Final bit. The last three bits are the sequence number associated with the supervisory states RR, RNR, REJ and SREJ indicated in the SS-Bits of the control field. Note that an acknowledgement (RR) can be sent either on a supervisory frame or piggybacked on an I-Frame as mentioned earlier. On the other hand RNR, REJ and SREJ are sent only through a supervisory frame.

Unnumbered Frame (U-Frame)

The U-Frames do not have data field as S-Frames. They are used for link establishment, termination, mode setting and other control functions. Controls field of an unnumbered frame is shown in Fig.9.

The first two bits of the control field are 11 which identify an unnumbered frame. The fifth bit is Poll/Final bit. The rest five bits are called modifier bits. They specify the control function. 

High Level Data Link Control (HDLC)

 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 circuits.

·         Two way alternate operation over half duplex or full duplex circuits.

·         Synchronous and asynchronous communication.

·         Communication between primary stations and between primary and secondary stations.

·         Full data transparency.

Types of Stations

To make HDLC protocol applicable in various possible network configurations, three types of stations have been defined.

·         Primary station

·         Secondary station

·         Combined station

Communication can be between a primary station and one or more secondary stations . The primary station has the responsibility of link management, i.e. activating and disconnecting the communication link. The secondary stations operate under the control of the primary station. The frames sent by a primary station are called commands and the frames sent by secondary station are called responses. as showing in fig.

A combined station can act as a primary as well as secondary station, i.e. it is capable of link management function, sending and receiving both commands and responses. Such a communication situation occurs when it is between two logical equal stations. 

ERROR DETECTION & COORECTION

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 depends on the distance between any two valid code words. For example, if the valid code words are separated by a distance 4, upto three errors in a code word can be detected. By adding certain number of redundant bits and properly choosing the algorithm for generating them, we ensure some minimum distance between any two valid code words and, therefore, the error detection capability.

ERROR CORRECTION

After an error is detected, there are two approaches to correction of errors :

·          Reverse Error Correction (REC).

·          Forward Error Correction (FFC).

In the first approach, the receiver requests for retransmission of the code word. In the second approach, the code set is so designed that it is possible for the receiver to not only detect but correct the errors also without requesting for retransmission. The receiver either locates the errors by analysing the received code word and reverses the erroneous bits. An alternative way is to search the most likely correct code word. When an error is detected, the distances of all the valid code words from the received invalid code word are measured. The nearest valid code word is the most likely correct version of the received word (Fig.3). If the minimum distance between valid code words is D, upto D/2-1 errors can be corrected. More than D/2-1 errors will cause the received code word to be nearer to the wrong valid code word.

different between TRANSMISSION AND COMMUNICATION

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)               Asynchronous Communication.

Synchronous Communication

In Synchronous communication the exchange of information is in a well disciplined manner, e.g. if A want to send some information to B, it can do so only when B permits it to send. Similarly, vice-versa is true. There is complete synchronisation of dialogues, i.e. each message of the dialogue is either a command or a response. Physical transmission of data may be in synchronous or asynchronous mode already decided between A and B.

Asynchronous Communication

In Asynchronous communication the exchange of information is in less disciplined manner, e.g. A and B can send messages whenever they wish to do so. Physical transmission of data may be in synchronous / asynchronous mode.

Thus, we see that Simplex Transmission is one way communication (OW), Half Duplex Transmission is two way Alternate Communication (TWA), and Full Duplex Transmission is two way Simultaneously Communication (TWS).