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Multiplexing And Error Checking
Multiplexing • Multiplexing • Allow multiple signals to travel through one medium • Types • Frequency division multiplexing • Synchronous time division multiplexing • Statistical Multiplexing
Frequency Division Multiplexing • Many channels of communication can be transmitted through one medium • Each Channel will transmit on their own frequency. • Television and radio transmit much the same way • Each channel is then de-multiplexed according to the frequency of each communication channel • Used for Analog Signals
Synchronous Time Division Multiplexing (STDM) • Used for Digital signals • Allows multiple channels of communication over one communication line. • Each channel or computer will take a turn to send a piece of a message over the line within a frame. • In a round robin technique each channel effectively transmits and places a byte into a frame to be transmitted. • Three types of STDM’s that are used today are • T-1 multiplexing • ISDN multiplexing • SONET/SDH multiplexing
Example of STDM Notice the Blank transmissions in Frames 2,3 and 4
Example of the De-Multiplexing technique Because Data in each frame is sent in a specific order, de-multiplexing the original message becomes easy.
Synchronous Time Division Multiplexing • On the receiving end, each byte within the frame is re-assembled according to the order that they were sent (Each byte from each channel is Always Sent in the same order with in a frame). • Disadvantage – even when a channel (or computer) does not transmit data, zeros are transmitted in its place (creates overhead). • The communication line is then under utilized and active channels have to wait for the transmission of blank spaces before transmitting more data.
Statistical Multiplexing • Solves the problems addressed by the Synchronous time division multiplexing • Each computer will now send data in data blocks. • Each Data Block will contain an address corresponding to the channel it belongs to • Each Data Block will contain the data following the address of each channel • Each block is than placed into a frame to be sent on the communication line as before • Data is from the data blocks within the frame are then re-assembled according to the addresses • Blank transmissions for idle channels are no longer needed since addressing is used for each channel.
What is a packet • Provides a means for a sender and a receiver to coordinate the transmission of data • Gives a way to break the data in pieces or small blocks called packets and transmit them over a network • Gives a way to check for errors and only require the “pieces” that caused the error to be re-transmitted. • Dividing messages in packets enables computer equipment and network devices to share expensive networking equipment. • A network device can only hold a network resource long enough to transmit the packet
Packets (Frames) in Time division Multiplexing • Bytes from each channel are placed into a frame to be transmitted over one communication line. • Each channel will take a “turn” in transmitting over a communication line • Each Frame will have information that marks the beginning and the end of the Frame called the SOH and EOT characters • For Statistical Multiplication the channel number are added to the Data blocks within the frame to eliminate the need to transmit 0’s for idle channels.
Hardware Frames • Each frame will have a SOH and an EOT character marking the beginning and the end of the frame. • Disadvantages • This adds extra characters and data that is transmitted over the communication line. • These special characters may appear in the data of the frame and may cause an error in transmission
Structure of a frame • The following is an example of a STDM Frame and a statistical TDM Frame • Notice the Address portion of the Data Blocks in with in the frame for Statistical TDM Note: of course this is a general example. Frames have a lot more information than just what is illustrated bellow Example of STDM Frame Example of Statistical TDM
Byte Stuffing • Helps prevent EOT and SOH being detected in the data portion of the frame or packet • Change the data as it is sent to prevent the appearance of the network control character in the data section of the frame. • Using substitute characters in the data stream to represent the special EOT and SOH characters. • Receiver will then replace the special characters with the corresponding SOH and EOT characters within the data stream
Transmission Errors • Errors may occur in the packets or frames during data transmissions • These errors are often caused by noise on the transmission medium. • A form of error checking is needed to provide reliable communications • Two basic types include • Simple parity • Checksum
Even Parity • Even Parity will check the number of 1’s in a character. • If there are an odd number of ones the parity bit will be turned on to make it even • If there are an even number of ones, the parity bit will remain off to keep the number of ones in the byte even.
Odd Parity • Works in the same manner as even parity • If the character or byte contains an odd number of ones, the parity bit is set to 0. • If the character or byte contains an even number of ones, the parity bit is turned on to maintain odd parity.
Simple parity in general • The number of ones in a byte are maintained according to the parity standard chosen. • Both the sender and receiver must use the same standard for this to work. • Often can detect a spike that may occur in a transmission line that will change one of the bits in the byte. • Will not be able to detect two spikes or when two bits change within a byte.
Longitudal Or Vertical Parity Check • A byte is added that checks the parity of a group of bytes. • Parity bits at the end of the byte are also checked • Solves the problem when two spikes occur. • Does not solve all transmission errors that may occur.
Simple Checksum • Treats the data as an integer. • Sums up all the integers and places this sum at the end of the packet. • Solves a lot of the problems encountered by simple parity • Does not solve all the problems for checking errors in transmission of data • The receiver will make the same calculations and then compare the result with the checksum in the packet.
CRC error checking • Uses a more complex method of errors in the data packet. • Calculates a polynomial from the byte of data. • The calculated polynomial is then divided by the generating polynomial. • The remainder is then attached to the end of the packet. • The receiver divides the incoming packet (remainder and all) with the same generating polynomial. If a zero is the result, no error is detected. Otherwise an error occured
CRC Generating Polynomials • Generating Polynomial • An industry approved bit string used to create CRC remainders. • Types include • CRC-12 • CRC-16 • CRC-CCITT • CRC-32
CRC usually conducted in Hardware • Division of polynomials are easier to implement in the hardware • Hardware needed include • Shift registers • Exclusive OR gates • When shifting bits left a division occurs. • Each XOR Gate will be place between registers according to the generating polynomial that will be used
Shift registers and the XOR Gates for CRC • The generated polynomial here is highlighted in red • The XOR gates are placed only in the spaces where the base and exponents of the polynomial may appear (notice that x^3 does not have a gate). • As bits shift in from the right to the left, they are xored with bits that are shifting out of the last register.
Shifting continued • The shift registers are usually the size of the checksum • The same hardware can be used for calculating the checksum and checking it. • When calculating the checksum the incoming data is padded with 0’s the size of the checksum • The resulting number in the registers become the checksum remainder.
Shifting Continued • When checking the checksum, the same process is run on the incoming data including the checksum • The result in the registers when the process should be zero when there are no errors • If the result is greater than zero, an error occurred
CRC Conclusion • It is implemented quickly through the use of hardware • It is the closest to 100 percent reliability in finding errors in data transmissions • Is used widely in the Internet, LAN, and WAN networks • Ethernet uses CRC-32 • Internet uses CRC-16
Types of Errors • Vertical errors • Errors that occur in multiple bytes of information • Vertical parity check helps solve most of these errors but not all. • Burst errors • Occur in or around the same location in a data stream. • These errors occur sporadically and are hard to detect. • CRC error checking can detect both these types of errors the best.
Checking for errors in the frame format • Errors can also be checked by examining the format of the frame as it arrives to the receiver • If the frame is not in the correct format, reject the frame • Frames that have errors are discarded and new frames are requested and then sent