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. 

TEMS CELLPLANNER UNIVERSAL

TEMS CellPlanner Universal is Ericsson´s tool for mobile radio network planning. It is a highly graphical, easy-to-use, PC-based tool for design, realization, and optimization of mobile radio networks. TEMS CellPlanner Universal helps the user to roll out and expand mobile radio networks, and optimize radio network regarding service availability and service quality. It assists the user in a number of complex tasks, including network dimensioning, traffic planning, site configuration, and frequency planning. TEMS CellPlanner Universal is the key to successfully competing in the market place.

TEMS CellPlanner Universal provides support for WCDMA, GSM 850, GSM 900, GSM 1800, GSM 1900, iDEN, CDMA, CDMA2000 1xRTT, TDMA/AMPS, NMT 450, NMT 900, TACS, and E-TACS. TEMS CellPlanner Universal also provides support for GPRS and EGPRS (EDGE), implemented in GSM system

The modular platform makes it possible to customize TEMS CellPlanner Universal to meet the needs of every customer. TEMS CellPlanner Universal can be used in all phases of a radio networks life cycle, from early planning to the most complex optimization. The combination of the modular platform and a modern programming language also make it possible to add new functionality in a fast and easy way. In addition, open interfaces enable connection to external data sources for easy exchange of data.

TEMS CellPlanner Universal incorporates the following major features:

·         User-friendly, intuitive menu structure and guided work flows.

·         TEMS CellPlanner Universal uses an Ericsson-developed GIS Engine specially developed to fit the needs of a demanding RF planning tool. TEMS CellPlanner Universal can read several map data formats natively, and it also handles multiple resolutions.

·         Propagation prediction modeling and calculation of signal coverage and interference.

·         Allows real network deployment scenarios such as layered network architectures, distributed antennas, repeaters, and macro-/micro cells. It supports both Baseband and Synthesized Frequency hopping with MAIO management.

·         Efficient and sophisticated radio propagation in urban and rural environments including automatic model tuning

·         Direct import of TEMS™ logfiles to tune propagation models or analyze network performance

·         True Multi-technology support.

·         TEMS CellPlanner Universal 5.0 was designed to be a multi standard/multi-technology RF planning tool. All technologies can co-exist in the same session. Each technology follows the same flow, making it easier for the user to understand the tool.

·         Generic import interface

TRANSMISSION in telecommunication

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 and receive but not simultaneously.

-         Transmission flow must halt each time and direction is to be reversed.

-         This halt is called the turn-around time and is typically 8 to 10 ms in the case of leased circuits and 50-500 ms in case of 2W telephone line through Public Switched Telephone Network (PSTN). 

Full-duplex Transmission

It is both way communications. If we set up a communication line with two channels, we have the capability of sending information in both directions at the same time. This is called full duplex transmission system. 

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.

TRANSMISSION ERRORS

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:

·                     Introduction of additional check bits in the data bits to detect and correct content errors.

·                     Establishing procedures of data exchange which enable recovery of corrupted/lost messages.

BIT ERROR RATE (BER)

In analog transmission, signal quality is specified in terms of Signal to Noise ratio (S/N) which is usually expressed in decibels. In digital transmission, the quality of received digital signal is expressed in terms of Bit Error Rate (BER) which is number of errors in a fixed number of transmitted bits. A typical error rate on a high quality leased telephone line is as low as 1 error in 10⁶ bits or simply 1 x 10^-6.