TEAM TMK

Kathleen Jorgensen

Miao Miao

Tarek El Shaarani

56KB Modem Data Representation

Original Question:

How would the string CSE370 be represented at the physical level of a 56KB modem?

Exact Question Answering:

How does the string CSE370 transmit from a 56KB modem to another 56KB digital modem over an analog line? 

Glossary

            56kbps: 56, 000 bits per second          

ASCII: a standard for assigning numerical values to the set of letters and numbers in the Roman alphabet.

Parity: The even or odd number of 1’s or 0’s in a binary code, often used to determine the integrity of data especially after transmission.

Background/Assumptions behind Answer

Due to the complexity and un-availability of protocol specific algorithms the following information is a generalized view of how modems transmit data.
 

Answer

Character Recognition

A computer recognizes characters in ASCII. ASCII assigns a decimal value to every character. The computer translates the decimal value to a binary 8-bit sequence. ASCII characters are 7 bits, where the 8^th-bit is used to specify parity. Figure 1 illustrates the ASCII Decimal values of each of the characters in the string “CSE370”.

A modem takes binary (digital) encoded information stored on a computer and transmits it across the phone line to another computer’s modem. A modems representation of strings is not in binary sequences but rather in analog waveforms. A modem must therefore convert a binary sequence of digits into an analog waveform.  

Figure 1 ASCII/Decimal/Binary Character Conversion

            Digital to Analog Conversion

Once in digital format, the modem uses certain protocols to translate the signal from digital to analog.  There are two common methods to perform this conversion; AM and FM. AM stands for Amplitude modulation, and FM stands for Frequency modulation.

·        Amplitude Modulation: A binary digit of one corresponds to a high wave amplitude, whereas a binary digit of zero corresponds to zero amplitude. Each digit represents a wave period.

Example:

·        Frequency Modulation: Each period represents a bit. A high frequency correlates to a ‘1’ and a low frequency represents a ‘0’.

Example:

Now that the modem has translated the binary digit sequence into an analog signal it is ready to transfer the information from the sender computer to the receiver computer.

Transmission over a digital line

Modems and the Public Switched Telephone Line (PSTN)        

           A modem transmits data from one computer to another by sending analog signals to the Public Switched Telephone Line (PSTN). The PSTN
           was originally designed for voice communications but it was later determined to be more efficient to send digital information.  Hence, once the
           modem generates and sends its analog signal over the telephone line it is intercepted by an analog to digital converter (ADC) of the sender’s
           internet service provider.  The ADC uses a method called quantization to translate analog signals to binary. The ADC samples the analog
          signal at twice the frequency, or 8000 times due to the Nyquest Rate Law. Analog waveforms are continuous whereas digital waveforms are
          discrete therefore the conversion from analog to digital is an approximation. Having to do the approximation as accurately as possible limits
          modem speeds.  Once converted to digital form, the PSTN passes the digital waveform to the receiver’s Internet service provider. The digital
         signal is passed through a digital to analog converter (DAC), which converts the data back into analog, the correct representation for modems.
         The receiving modem then receivers the  analog waveform and translates it into the corresponding binary sequence of digits.

Analog to Digital Conversion

·        Given an analog signal in Amplitude Modulation form a conversion to digital is made by representing high amplitudes as a binary 1, and flat amplitudes as a binary 0.

·        Given an analog signal in Frequency Modulation form a conversion to digital is made by representing high frequencies with a 1 and low frequencies with a zero.

Example of binary signal:

Result

             In AM format, the string “CSE370” is physically interpreted by a modem as follows:

Notice that each character is 8 bits long.  The last bit is appended in order to make the total number of 1’s. This happens mostly for debugging and alignment purposes (8-bit word lengths are universal to all modems). Suppose a noise destroys a 1 bit or blows up a 0, the number of 1’s will be odd.

          Data Correction and Compression Overview  

As a result of transmission, bit errors will occur throughout the process of transmission due to noise in the line or other external factors         that may affect the transmission medium. To correct this problem, standards have been set to correct the bits which might have been         incorrectly transmitted. One of the most common is the V.42 error correction standard. Techniques used to correct errors in data             include:

·        Flow Control: Binary codes are sent between machines to signal the readiness to receive or send data.

·        Link Access Procedure for Modems (LAPM) and Parity: The LAPM procedure is used in parallel with a Cyclic Redundancy Check (CRC) detect bit errors based on a checksum that is saved to the packets that are sent out, while Parity adds bits to the data packets, making them either odd or even to detect any errors that have resulted during transmission.

On the other hand, data must also be compressed using certain algorithms to make transmission faster and make it easier to collect data on errors as they are received by either party during transmission. Examples of standards used with 56k modems are v.42bis, which is used with the V.42 error correction standard, and MNP proprietary protocols for compression and correction.

Standards

Over the years, modems have become faster and have rapidly switched standards used for transmission over telephone wires. Just before 56k modems emerged, the V.34 standard which eventually ran at a rate of 33.6kbits/second (kbps), became the fallback standard for new 56k modems. Two different standards evolved for 56k modems, one by 3com and U.S. Robotics called the X2 technology, and another by Lucent Technologies called K56Flex. These two new standards could outrun older V.34 modems based on the fact that digital phone lines where being used, as this eliminated the previous problems that occurred with line noise on analog lines. These two technologies eventually were combined together to form the V.90 standard using the connection sequence from K56flex and modulation from x2.

At a last effort to increase performance on 56k modems, a V.92 standard is currently being introduced with the new V.44 compression standard that will enable faster dial-up speeds and added features such as Modem-On-Hold/Call-Waiting (placing the internet connection on hold while using the phone line).

           Conclusions and Comments

Based on the information found, we concluded that modems have evolved dramatically since they were first introduced. More specifically, 56k modems have undergone many standards, all of which are not all universally supported. Therefore, it may be difficult to illustrate how a signal is sent through those patented compression and correction standards and algorithms.

However, the basic concept of how a modem transmits data stays the same. Starting with a digital signal at the source computer and ending with another digital signal at the remote computer, the data undergoes conversions between digital and analog. Newer technology (all digital networks) is allowing for direct digital signal communication.

References

                1. http://gallery.uunet.be/Cedric.Walravens/technology/56kexplain.html

            2. http://www.whpress.com/jm/extern/modem.html

3. http://www.cs.williams.edu/~cs105/s01/text/ch3/DigitalTrans_15.html
            4. http://ourworld.compuserve.com/homepages/g_knott/elect95.htm

            5.  http://www.comm.toronto.edu/~karen/projects/22.ITUV90/startup_procedure.html

            6. http://www.gaoresearch.com/resources/articles/v90.html (More on V.90 modems)

            7. http://www.macntosh.com/V.92.html (More on V.92 modems)