1. Core 3: Communication Systems

3. There are three terms that we will consider in relation to the speed of communication. Bits per second (bps) Baud Rate & Bandwidth These are not interchangeable terms and are often used to describe bps. We will look at the actual meaning of each and their relationship to each other.

4. Bits per Second The number of bits (1 or 0) transferred each second. The speed of binary data transmission, bps, is NOT bytes per second. (B=bytes, b=bits). For example a speed of 2400bps means that 2400 binary digits can be transferred each section.

5. Baud Rate  The number of signal events occurring each second along a communication channel. Equivalent to the number of symbols per second. A symbol is 1 complete wavelength of the transmission. This is also known as a baud.

6. Bandwidth  Bandwidth is the difference between the highest and lowest frequencies in a transmission channel. Hence it is expressed in hertz (Hz), kilohertz (KHz) or megahertz (MHz). Frequency, measured in Hz, means cycles per second. Each cycle is a complete wavelength.  For example, 20 Hz = 20 complete waves per second. Bandwidth is a measurement of capacity, not speed. Having a large bandwidth refers to having a lot of channels, not having faster channels.

8. Error Checking Methods  Previously we mentioned that the TCP protocol includes a checksum within the header of each segment, IP includes a checksum of the header fields and that an Ethernet frame includes a 32-bit CRC. In this section we will explore what these terms mean.

9. Parity bit checks  Early modems transmitted and received each character separately as its 7 bit ASCII code. For a simple packet like this then a simple parity bit check would be adequate. Larger data packets utilise more sophisticated error checks like checksum or CRC.

10. Parity bit checks contd.  Parity bits are a single bit added before or after data so that the number of 1s is either odd or even. For example: Byte 1011011(1) is sent using even parity. There for the 8 th parity bit is (1).  When the byte is sent the software will label the byte saying that the parity bit is the last digit, and that the amount of 1s should be even. The receiving software counts the amount of ones, and if they are even, counts the message as successfully received.

11. Checksum  Checksum is a method of error checking that uses a calculation or a sum. Simple checksum involves adding all bytes as if they were integers within a message (this means translating all the binary digits into decimal digits and adding them).  The sum is sent. The receiver calculates the sum of bytes and compares checksum value sent.

12. Checksum contd.  Checksum detects errors at approximately 99.9985% with a 16 bit checksum and 99.999999977% with a 32 bit checksum.  For detailed examples and explanations please see pge. 251-253 of the text.

13. CRC or Cyclic Redundancy Checking  CRC is similar to checksum in that a value is sent in the header and checked by the receiver. However, CRC uses a much more sophisticated calculation to determine the value sent. CRC uses division, checksum uses addition. CRC converts the entire message into a number which is divided by a predetermined value. The remainder is the CRC value.

14. CRC or Cyclic Redundancy Checking  With 16 bit CRC we can expect better than 99.9985% of errors to be detected and when using a 32 bit CRC we expect more than 99.999999977% of errors to be detected.  For a more detailed explanation and examples of CRC please see pge 253-257 of the text.