Wavelength in GSM

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 earth and in the atmosphere. Television and FM radio are examples of applications, which use lower frequencies.

Higher frequencies, with shorter wavelengths, are better suited to transmission over small distances, because they are sensitive to such problems as obstacles in the line of the transmission path. Higher frequencies are suited to small areas of coverage, where the receiver is relatively close to the transmitter.

The frequencies used by mobile systems compromise between the coverage advantages offered by lower frequencies and the closeness-to-the-receiver advantages offered by use of higher frequencies.

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. 

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. 

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.

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).

What is a cell?

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)

•           Possibility of mechanical tilting antennas (to reduce unwanted interference)

•           Simpler antenna mounting (reduced clearance to prevent interaction with other antennas).

Disadvantages of sector cells are:

•           More equipment required at each site

•           Greater environmental impact (more antennas)

•           Longer frequency re-use distance for a given C/I

•           Increased cell handovers.

Interleaving

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 :

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 phones.

GSM and CDMA systems use Frequency Division Duplexing and corDECT uses Time Division Duplexing.