Objectives. Lecture 4. How do computers communicate? How do computers communicate? Local asynchronous communication. How do computers communicate?

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1 Lecture 4 Continuation of transmission basics Chapter 3, pages Dave Novak School of Business University of Vermont Objectives Line coding Modulation AM, FM, Phase Shift Multiplexing FDM, TDM, WDM Broadband vs- Baseband Sources: 1) Network+ Guide to Networks, Dean ) Comer, Computer Networks and Internets, ) Other sources cited within the lecture slides How do computers communicate? At a very basic level computers use binary digits (bits) to represent data / information Bits are transmitted over some medium Electrical current over copper cable Pulses of light over fiber optic cable How do computers communicate? How can data be represented by electrical signals? Can be generally explained via local asynchronous communication (RS-232) Example: electrical voltage over copper wires How do computers communicate? Simple electronic communication systems Electric current is used to encode data For example: Negative voltage represents a 1 and positive voltage represents a 0 Transmit a 1 by transmitting negative voltage over a copper wire Local asynchronous communication RS-232 (EIA) emphasizes need for standards and illustrates how they are used in networking Most widely accepted standard for transferring data across copper wires Defines serial, asynchronous communication Serial Asynchronous - 1

2 Local asynchronous communication There are limitations to hardware Electronic devices cannot produce an exact voltage or change from one voltage to another instantly Wires are not perfect conduits Signal loses energy as it travels Takes time to change voltage Local asynchronous communication Transmission hardware is typically rated in baud Baud = the signaling rate at which data are sent through a measured in transactions per second In simple RS-232 baud rate = the bit rate, as one bit is transferred per signal transition 9600 baud = 9600 bps This is not true for more complex coding schemes Local asynchronous communication Bit rate vs- baud rate they are directly related to one another Bit rate number bits transmitted per sec Baud rate number of signaling elements per sec Depending on the signaling level or modulation technique, more than one bit can be transferred per sec Bit rate = baud/sec x # of bits/baud Modems Hardware circuit that accepts sequence of data bits and applies modulation to a carrier wave is called a modulator Hardware circuit that accepts a modulated carrier wave and recreates the sequence of bits used to modulate the carrier is called a demodulator To support full duplex transmission, each system needs both these are combined into a single device called a modem Modems Different types of modems including RF (wireless) and optical fiber modems Most familiar with 2-wire dialup modems Half duplex take turns sending info Use a carrier that is an audible tone to mimic a telephone Note that the term modem is not limited to the dialup device Modern modems use a combination of modulation techniques to transmit multiple bits per baud Some Basic Issues Why don t we just use simple RS232 to transmit 1 s and 0 s? Desire to transmit large amounts of data over long distances at really high speeds Multiple conversion processes as different types of data travel over different physical networks (for example sending analog data over a digital network) Transmission errors Increase bit transfer rate 2

3 Advanced transmission concepts Line Encoding Modulation Multiplexing Line encoding Different line encoding schemes are used to transmit digital data using a digital signal Improve bit-rate Decrease bit-error rates Digital data / digital signals Encoding schemes can vary by Layer 1 and 2 standards (which also impact media and distance) Line Encoding Modulation Encode digital data onto a continuous analog carrier wave by modulating (altering one or more properties of the carrier wave) the signal Digital data using analog signals 1) Frequency Modulation 2) Amplitude Modulation 3) Phase-shift Modulation Analog data using digital signals 4) Pulse Code Modulation Digital data / analog signal Analog data / digital signal a) Digital (binary) signal being represented b) Amplitude modulation (AM) Most common technique for encoding analog data using digital signals is Pulse Code Modulation (PCM) c) Frequency modulation (FM) d) Phase-Shift modulation (PSM) Source: 3

4 Multiplexing Technique that allows multiple signals to be transmitted simultaneously over a single medium Media is separated into multiple s or subs This can be done virtually and/or physically Multiplexing Why is multiplexing so important in data communications? Individual signals from different sources can be combined into a single complex signal and then the separate signals are recovered at the receiving end Multiplexing depends on signal type (analog / digital) and the media used Basic concept of multiplexing Multiplexing Frequency Division Multiplexing (FDM) Wave Division Multiplexing (WDM) Same concept as FDM but applied to fiber where optical signals are used Time Division Multiplexing (TDM) Image Source: Frequency Division Multiplexing (FDM) Frequency Division Multiplexing (FDM) Inherently an analog technology Uses different frequency ranges over single medium Total bandwidth is divided into subs consisting of smaller segments of available bandwidth Carrier wave used by each sender/receiver pair operates within a unique frequency band to avoid interfering with other transmissions Source: 4

5 Wave Division Multiplexing Frequency division applied to light waves as opposed to radio frequencies Combining separate wavelengths of data into a light stream on a single fiber carrier Time Division Multiplexing (TDM) Primarily a digital technology that separates different data streams by time (as opposed to frequency) 1) Synchronous time division (STD) (slotted) 2) Statistical Time division multiplexing Synchronous time division (slotted) Sources takes turns in a round robin fashion Great for telephone transmission Source A Time division multiplexing Statistical multiplexing Take turns like STD, but does not waste slots if source has no data to send Source A Destination Destination Source B Source A Source A Source A Source A Source B Source B Source A Source B Source A Source B Source C Source A Source B Source C Source A Source C Source C Some Basic Issues To transmit an analog signal, the bandwidth of the signal must match the bandwidth of the transmission To transmit a digital signal, the bit rate of the signal must be within the bit rate range of the transmission Form of transmission Baseband and broadband are different forms of transmission Baseband transmission uses digital signaling Broadband transmission uses analog signaling 5

6 Baseband Baseband systems can transmit one signal / one at a time Each transmission requires full use of the medium so when one node is transmitting the other nodes must wait their turn Ethernet uses baseband signaling Why would the most popular LAN technology use this signaling technique? Baseband Characteristics Converts digital signal to voltage without using different frequency s Single frequency Bi-directional signal flow (but not simultaneously) Entire bandwidth of cable used to transmit single data stream Multiple signals can be sent using Time Division Multiplexing (TDM) Baseband If baseband requires the use of the entire cable for single transmission, how can multiple computers on a LAN communicate simultaneously? Broadband Broadband systems can transmit multiple signals over many different s simultaneously Each uses a different frequency band to transmit Cable TV uses broadband signaling Broadband Characteristics Communications medium split into multiple s Multiple signals are transmitted over a single medium simultaneously Can send and receive simultaneously Analog transmission Signal flow is unidirectional on each Uses Frequency Division Multiplexing (FDM) Broadband If broadband only supports unidirectional transmission, how do 2 (or more) devices communicate? 6

7 Broadband versus baseband Broadband networks carry more data than baseband networks Using analog signaling over multiple frequency bands Broadband networks are more expensive and complex than baseband networks Use multiplexing Broadband networks are better suited for long-distance communication than baseband networks Broadband, Baseband, Analog, Digital What is the signaling technique used on communication? Phone line (DS0) broadband T-1 carrier baseband Ethernet baseband WiFi - broadband What are the form of the data transmitted over the? Can be either analog or digital Broadband, Baseband, Analog, Digital Transmit analog data over broadband Transmit analog data over baseband Transmit digital data over broadband Transmit digital data over baseband Summary Transmission basics Analog vs- Digital Simple transmission RS232 Line coding Modulation AM, FM, Phase Shift Multiplexing FDM, TDM, WDM Broadband vs- Baseband 7

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