Page 1 Discussion ▪ You have been tasked to design an organizational LAN. The organizations Manager has asked you to provide a LAN that will make use of twelve (12) PCs, a photocopier, two laser printer and four ink jet printers. In addition, some employees will need to access the LAN from home to transfer files and to do remote printing. There is currently no network in the organization. The printers are attached to particular desktops, and the photocopier is not attached to any computers. ▪ Using a physical star topology, draw a network diagram of your proposed LAN. Label all hardware components in your diagram. Support your network diagram, by providing an explanation of the following: i. Equipment, software, and cabling that will be needed to connect each device to the LAN ii.Details of configuration of the hardware, software, and communications capabilities required at the LAN and home user ends of the connection. Page 2 A company network structure Page 3 Remote-access VPNs Page 4 Page 5 Review Data Communication Telecommunication Networking Transfer of Data/Information from A to B 1. Source/Destination 2. Transmiter 3. Receiver 4. Message 5. medium The exchange of A network is a set of data/information in any form devices/nodes connected by (voice, data, text, images, communication links. audio, video) over networks. Page 6 Elements of a Communication System Guided/Unguided Communications Channel Source Transmitter Signal Signal Transmission system Source system Source: generates data to be transmitted Transmitter: Converts data into transmittable signals Receiver Transmission System: Carries data Destination Signal Destination system Receiver: Converts received signal into data Destination: Takes incoming data Protocols and Standards Page 7 Electromagnetic Spectrum for Transmission Media Page 8 Media Summary: Backbone Network (BN), LAN, WAN Page 9 A Transmission System’s Communication channel ▪ Transmitter ⚫ ⚫ Converts information into signal suitable for transmission Injects energy into communications medium or channel ⚫ Telephone converts voice into electric current ⚫ Modem converts bits into tones ▪ Receiver ⚫ ⚫ Receives energy from medium Converts received signal into form suitable for delivery to user ⚫ Telephone converts current into voice ⚫ Modem converts tones into bits Page 10 Encoding and Modulation ▪ Encoding is about representing a signal, while modulation is about changing a signal. ▪ Encoding is used for efficient transmission and storage, while modulation is used for long-distance communication. Page 11 Goals of Line Coding ▪ Self-synchronization. ▪ The ability to recover timing from the signal itself. ▪ Long series of ones and zeros could cause a problem. ▪ Low probability of bit error. ▪ The receiver needs to be able to distinguish the waveform associated with a mark from the waveform associated with a space, even if there is a considerable amount of noise and distortion in the channel. ▪ Spectrum that is suitable for the channel. ▪ In some cases DC components should be avoided if the channel has a DC blocking capacitance. ▪ The transmission bandwidth should be minimized. Page 12 Line coding schemes Unipolar Polar Line Coding All signal levels are on one side of the time axis - either above or below. The voltages are on both sides of the time axis. Bipolar defines three voltage methods: positive, negative, and zero. Multilevel increase the number of data bits per symbol thereby increasing the bit rate Multitransitional Codes can be created that are differential at the bit level forcing transitions at bit boundaries. Page 13 Unipolar ▪ All signal levels are on one side of the time axis - either above or below ▪ NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission. ▪ Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption. Page 14 Polar - NRZ ▪ The voltages are on both sides of the time axis. ▪ Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0. ▪ There are two versions: ▪ NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the other ▪ NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not. Page 15 Polar - RZ ▪ The Return to Zero (RZ) scheme uses three voltage values. +, 0, -. ▪ Each symbol has a transition in the middle. Either from high to zero or from low to zero. ▪ This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth. ▪ No DC components or baseline wandering. ▪ Self synchronization - transition indicates symbol value. ▪ More complex as it uses three voltage level. It has no error detection capability. Page 16 Polar - Biphase: Manchester and Differential Manchester ▪ Manchester coding consists of combining the NRZ-L and RZ schemes. ▪ Every symbol has a level transition in the middle: from high to low or low to high. Uses only two voltage levels. ▪ Differential Manchester coding consists of combining the NRZ-I and RZ schemes. ▪ Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not. Page 17 Bipolar - AMI and Pseudoternary ▪ Code uses 3 voltage levels: - +, 0, -, to represent the symbols (note not transitions to zero as in RZ). ▪ Voltage level for one symbol is at “0” and the other alternates between + & -. ▪ Bipolar Alternate Mark Inversion (AMI) - the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V. ▪ Pseudoternary is the reverse of AMI. Page 18 Bipolar C/Cs ▪ It is a better alternative to NRZ. ▪ Has no DC component or baseline wandering. ▪ Has no self synchronization because long runs of “0”s results in no signal transitions. ▪ No error detection. Page 19 Multilevel Schemes ▪ In these schemes we increase the number of data bits per symbol thereby increasing the bit rate. ▪ Since we are dealing with binary data we only have 2 types of data element a 1 or a 0. ▪ We can combine the 2 data elements into a pattern of “m” elements to create “2m” symbols. ▪ If we have L signal levels, we can use “n” signal elements to create Ln signal elements. Page 20 Code C/Cs ▪ Now we have 2m symbols and Ln signals. ▪ If 2m > Ln then we cannot represent the data elements, we don’t have enough signals. ▪ If 2m = Ln then we have an exact mapping of one symbol on one signal. ▪ If 2m < Ln then we have more signals than symbols and we can choose the signals that are more distinct to represent the symbols and therefore have better noise immunity and error detection as some signals are not valid. Page 21 Representing Multilevel Codes ▪ We use the notation mBnL, where m is the length of the binary pattern, B represents binary data, n represents the length of the signal pattern and L the number of levels. ▪ L = B binary, L = T for 3 ternary, L = Q for 4 quaternary. Page 22 Multitransition Coding ▪ Because of synchronization requirements we force transitions. This can result in very high bandwidth requirements -> more transitions than are bits (e.g. mid bit transition with inversion). ▪ Codes can be created that are differential at the bit level forcing transitions at bit boundaries. This results in a bandwidth requirement that is equivalent to the bit rate. ▪ In some instances, the bandwidth requirement may even be lower, due to repetitive patterns resulting in a periodic signal. Page 23 (a) Punched Tape Volts (b) Unipolar NRZ 1 1 0 1 0 0 Mark (hole) Mark (hole) space Mark space space (hole) 1 BINARY DATA Mark (hole) A 0 Tb Time A (c) Polar NRZ 0 -A A (d) Unipolar RZ 0 A (e) Bipolar RZ 0 -A A (f) Manchester NRZ 0 -A Binary Signaling Formats Page 24 Todays Agenda The Data Communications Interface Data Link Control Protocols ▪ Understanding of: ▪ Understanding of: Asynchronous and Synchronous Transmission; Flow Control; Model of Frame Transmission; Interfacing; Characteristics of Interface; V.24/EIA-232-F; Mechanical Specification; Electrical Specification; Stop and Wait; ▪ Functional Specification; Procedural Specification; ISDN Physical Interface & Electrical Specification; Error Detection; Sliding Window Flow Control; ▪ High-Level Data Link Control (HDLC): Characteristics and operation Page 25 Transmission ▪ The process of transferring data from one place to another. ▪ The two main types of data transmission used in computer networking: ▪ Parallel: ▪ More than one bit sent at a time ▪ Serial: ▪ One bit sent at a time 1. synchronous transmission, 2. asynchronous transmission, 3. Isochronous Page 26 Bitwise Data Transmission ▪ Data transmission requires: ▪ Encoding bits as energy ▪ Transmitting energy through medium ▪ Decoding energy back into bits ▪ Energy can be electric current, radio, infrared, light, smell, etc. ▪ Transmitter and receiver must agree on encoding scheme and transmission timing Page 27 Parallel Transmission ▪ Multiple data bits transferred at the same time over separate media ▪ Generally used with wired medium with multiple independent wires ▪ Signals on all wires are synchronized ▪ Each wire carries the signal for one bit ▪ All wires operate simultaneously ▪ Other wires allow sender and receiver to coordinate ▪ Wires placed in a single large cable Page 28 Parallel Transmission ▪ Advantages ▪ High speed ▪ Faster than serial as more wires! ▪ Match to underlying hardware ▪ Computers and communication hardware use parallel circuitry internally Page 29 Serial Transmission ▪ Sends one bit at a time ▪ Most communications system use serial mode ▪ Cheaper to extend over long distances ▪ Fewer wires ▪ Intermediate electronic components are cheaper ▪ Never a timing problem caused by one wire being slightly longer than another ▪ Because only one wire ▪ Sender and receiver need small amount of hardware to convert data from parallel form used in the device to serial form used on the wire Page 30 Serial Transmission ▪ Transmission Order: Bits and Bytes ▪ Which bit should be sent across a medium first? ▪ Most Significant Bit (MSB) ▪ Big Endian ▪ Least Significant Bit (LSB) ▪ Little Endian ▪ Which byte should be sent first?? ▪ Byte order and bit order can be chosen independently Page 31 Transmission Order Ethernet technology specifies that data is sent byte bigendian and bit little-endian Page 32 Timing of Serial Transmission Three broad categories of serial transmission ▪ Asynchronous ▪ Transmission can occur at any time ▪ Arbitrary delay between the transmission of two data items ▪ Synchronous ▪ Continuous transmission ▪ No gap between transmission of data items ▪ Isochronous ▪ Transmission occurs at regular intervals ▪ Fixed gap between transmission of two data items Page 33 Asynchronous Transmission ▪ One definition of asynchronous: transmitter and receiver do not explicitly coordinate each data transmission ▪ Transmitter can wait arbitrarily long between transmissions ▪ Used, for example, when transmitter such as a keyboard may not always have data ready to send ▪ Asynchronous may also mean no explicit information about where data bits begin and end ▪ E.g. when we send individual ASCII characters Page 34 Transmission Timing Problems ▪ Encoding scheme leaves several questions unanswered: ▪ How long will voltage last for each bit? ▪ How soon will next bit start? ▪ How will the transmitter and receiver agree on timing? ▪ Later : Self-clocking codes (e.g. Manchester Encoding) ▪ Standards specify operation of communication systems ▪ Devices from different vendors that adhere to the standard can interoperate ▪ Example organizations: ▪ International Telecommunications Union (ITU) ▪ Electronic Industries Association (EIA) ▪ Institute for Electrical and Electronics Engineers (IEEE) Page 35 Asynchronous Transmission • Allows physical medium to be idle for an arbitrary amount of time between transmissions • Good for applications that generate data at random • E.g. keyboard connected to a computer • BUT – lack of coordination between sender and receiver • Receiver does not know when the sender will transmit • transmit when data are ready • variable delays between transmissions • no sender-receiver coordination beforehand • Technically, the electrical signal does not contain information about where individual bits begin and end • Therefore common for technologies to specify a preamble • Few extra bits before each data item to tell receiver data are coming Page 36 Using Electric Current to Send Bits ▪ Simple idea - use varying voltages to represent 1s and 0s ▪ One common encoding use negative voltage for 1 and positive voltage for 0 ▪ In following figure, transmitter puts positive voltage on line for 0 and negative voltage on line for 1 Page 37 Transmission Timing Problems ▪ Encoding scheme leaves several questions unanswered: ▪ How long will voltage last for each bit? ▪ How soon will next bit start? ▪ How will the transmitter and receiver agree on timing? ▪ Later : Self-clocking codes (e.g. Manchester Encoding) ▪ Standards specify operation of communication systems ▪ Devices from different vendors that adhere to the standard can interoperate ▪ Example organizations: ▪ International Telecommunications Union (ITU) ▪ Electronic Industries Association (EIA) ▪ Institute for Electrical and Electronics Engineers (IEEE) Page 38 RS-232 ▪ Standard for transfer of characters across copper wire ▪ Produced by EIA ▪ Full name is RS-232-C ▪ RS-232 defines serial, asynchronous communication ▪ Serial - bits are encoded and transmitted one at a time (as opposed to parallel transmission) ▪ Asynchronous - characters can be sent at any time and bits are not individually synchronized Page 39 Details of RS-232 ▪ Components of standard: ▪ Connection must be less than 50 feet ▪ Data represented by voltages between +15v and -15v ▪ 25-pin connector, with specific signals such as data, ground and control assigned to designated pins ▪ Specifies transmission of characters between, e.g., a terminal and a modem ▪ Transmitter never leaves wire at 0v; when idle, transmitter puts negative voltage (a 1) on the wire Page 40 Identifying asynchronous characters ▪ Transmitter indicates start of next character by transmitting a one ▪ Receiver can detect transition as start of character ▪ Extra one called the start bit ▪ Transmitter must leave wire idle so receiver can detect transition marking beginning of next character ▪ Transmitter sends a zero after each character ▪ Extra zero call the stop bit ▪ Thus, character represented by 7 data bits requires transmission of 9 bits across the wire Page 41 Start, Stop Bits Typically one of the data bits might be a parity bit (7N1, 8E1)… Page 42 Timing ▪ Transmitter and receiver must agree on timing of each bit ▪ Agreement accomplished by choosing transmission rate ▪ Measured in bits per second ▪ Detection of start bit indicates to receiver when subsequent bits will arrive ▪ Hardware can usually be configured to select matching bit rates ▪ Switch settings ▪ Software ▪ Autodetection Page 43 Transmission Rates ▪ Baud rate measures number of signal changes per second ▪ Bits per second measures number of bits transmitted per second ▪ In RS-232, each signal change represents one bit, so baud rate and bits per second are equal ▪ If each signal change represents more than one bit, bits per second may be greater than baud rate ▪ This is the case with modems nowadays! ▪ More on this when we look at modulation Page 44 Framing ▪ Start and stop bits represent framing of each character ▪ If transmitter and reciver are using different speeds, stop bit will not be received at the expected time ▪ Problem is called a framing error ▪ RS-232 devices may send an intentional framing error called a BREAK Page 45 Duplex ▪ Two endpoints may send data simultaneously - full-duplex communication ▪ Requires an electrical path in each direction ▪ If only one endpoint may send data – halfduplex communications or simplex ▪ Pin 2 - Receive (RxD) ▪ Pin 3 - Transmit (TxD) ▪ Pin 4 - Ready to send (RTS) ▪ Pin 5 - Clear to send (CTS) ▪ Pin 7 - Ground Page 46 Limitations on Transmission ▪ Limitations on wires makes waveforms look like: ▪ Longer wire, external interference may make signal look even worse ▪ RS-232 standard specifies how precise a waveform the transmitter must generate, and how tolerant the receiver must be of imprecise waveform Page 47 Page 48 Interfaces ▪ Transmission of data from the source to a device or from a device to the destination ▪ Parallel transmission: Multiple lines carrying bits simultaneously ▪ High data rate, but expensive ▪ Serial transmission Bits transmitted serially ▪ Synchronous vs. Asynchronous Page 49 Serial I/O Protocols ▪ Synchronous: A master clock controls the transmission as a continuous stream ▪ Asynchronous: Random delays between data pieces Synchronous Requires processing to extract clock Asynchronous No clock recovery needed Overhead applies to entire block c.a. 20% overhead/character Error detection and correction built into protocol Error detection possible, correction done separately Page 50 Asynchronous Protocols ▪ RS-232-C ▪ 20MA Current Loop ▪ RS-422, RS-423, RS-485 RS: Recommended Standard by EIA (Electronic Industries Association) 1, 1½, 2 Stop Bits Mark Space Start Bit 5 to 8 Data Bits LSB First 110 - 19.2k bps Page 51 Start and Stop Bits ▪ Start bit permits local synchronization ▪ Stop bit provides validity check and the opposite level for the start bit ▪ Implementation with 16X clock … See beginning of start bit … Starting from 8th tick, sample every 16th tick Page 52 RS-232-C Interface ▪ EIA in cooperation with Bell Systems, independent modem and computer manufacturers ▪ Standard for interface between Data Terminal Equipment (DTE) and Data Communication Equipment (DCE) employing serial bit interchange Telephone Network DTE DTE DCE RS-232-C DCE RS-232-C Page 53 RS-232-C ▪ Standards contain ▪ Electrical signal characteristics ▪ Interface mechanical characteristics ▪ Functional description of interchange circuits ▪ Standard subsets for specific groups of communication systems applications ▪ Mechanical ▪ DB-25 or DB-9 connectors ▪ Cable ▪ Female connected to DTE, male to DCE ▪ Maximum 15 meters Page 54 ▪ Lines/Pins: RS-232-C 1 Shield Shield 7 GND Signal ground 2 XMIT Transmit from DTE to DCE (Modem) 3 RCV Receive from DCE (Modem) 4 RTS Request to send, from terminal to modem 5 CTS Clear to send, from modem to terminal 6 DSR Data set ready, from modem to terminal Data set (modem) online 20 DTR Data terminal ready, from term. to modem Tie to power 22 RI Ring indicator, from modem to terminal “Say hello!” 8 CD Carrier Detect, from modem to terminal “I hear the other end” * Page 55 RS-232-C ▪ * ▪ Originally designed for half-duplex control ▪ For full-duplex, tie both RTS and CTS true ▪ If RTS and CTS tied together, it means that RTS is OK if other end is plugged in ▪ If CTS is connected to CD, it is OK to talk if both modems are connected Page 56 Null Modem ▪ Direct connection between two DTEs, e.g., terminal and computer, or two computers directly DTE DTE Shield Shield GND GND XMIT RCV XMIT RCV RTS RTS CTS CTS DTR DSR DTR DSR Page 57 RS-232-C ▪ Electrical specification: 15 V Control “ON” Space Logical 0 3V Undefined Area Receiver side -3 V 12 V 5V 0V Transmitter side -5 V Control “OFF” Mark Logical 1 -12 V -15 V 15V 3V -3V -15V 1 0 1 0 Page 58 RS-232-C ▪ Open circuit ≤ 25V ▪ Driver must be able to sustain short circuit current without damage; short circuit current ≤ 0.5A ▪ Voltage change not faster than 30V/μs, +3V/-3V transition not to exceed 1ms or 4% of bit time ▪ Terminator capacitance ≤ 2500pF including cable Page 59 RS-232-C ▪ Electrical Problems: ▪ ±12V supply needed, inconvenient ▪ Cable capacitance: Maximum 50 ft if cable is 40-50pF/ft! ▪ Ground reference ▪ System has poor common-mode noise rejection ▪ Cross-talk and increase of bias distortion ▪ Especially bad if clock lines used (SYNC) ▪ Not suitable for long distances → Motivation for new standards RS-422, 423 Page 60 RS-423 ▪ Use RS-449 for functional and mechanical aspects ▪ Created for transition from RS-232 to RS-422 ▪ Uses unpopular 37-pin connectors per RS-449 ▪ Unbalanced like RS-232-C ▪ All signals use a common return to complete the circuit ▪ Valid margins: +2V/+6V and -2V/-6V ▪ For less than 20kbps Page 61 RS-422 ▪ Use RS-449 for functional and mechanical aspects ▪ Fully balanced, differential inputs ▪ Supports data rates 20kbps Length 4k (ft) 90k 1k 100 10 10k 100k 1M 10M Baud rate • Using 24G Twisted-pair, 100Ω load – Amplitude drop less than 6dB – Rise time less than ½ bit time Page 62 RS-485 ▪ Like 422, 485 is also balanced ▪ 485 handles multiple drivers and receivers ▪ Better common-mode noise rejection (-7 to +12 Volts) ▪ Sensitivity of ±200mV in receivers ▪ Drivers give up to 5 volts balanced output ▪ Can stand contention, driver shuts down by itself ▪ High input resistance (12K ohms) ▪ Hysteresis of 50 mv to overcome diff. noise Page 63 20mA Current Loop ▪ Historically, current controlled encoding ▪ Now implemented with optoisolators ▪ High immunity to noise ▪ Distance limited by voltage available ▪ If source has 20V and 750Ω internal resistance, we can add 300Ω wire resistance and still get 18mA → 3650ft. ▪ Pros: ▪ High common mode rejection and high isolator ▪ Cons: ▪ Not standardized ▪ Creates crosstalk in adjacent wires Page 64
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