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Physical-MAC
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- 1TransmitterConverts info into a signal suitable for transmission by injecting energy into a comm medium.
E.g.
-Pair of copper wires
-Coaxial cable
-Radio
-Infrared
-Light in optical fiber - 2ReceiverReceives energy from medium and converts signals into form suitable of delivery to user
- 3Receiver IMPARIMENTS-Spurious noise
-Interference from other signals
-Distortion
-Signal attenuation - 4Analogue-Long Distance Comm (Repeaters)=>Uses repeaters but restoration not perfect as distortion not completely eliminated, noise + interference only partially removed
=>More signal passes through repeaters, quality decreases - 5AmplifiersCompensate for energy loss on channel.
Amplifies everything including signal AND noise - 6Digital Long-Distance Comm (Regenerators)=>Regenerator recovers original data sequence, then retransmits on next segment, so as if new signal being transmitted every time
=> Probability of error very small - 7Digital Sys vs Analogue SysDigital Systems have advantage of:
-Less power usage
-Longer distance
-Lower system cost
-Monitoring, encryption protocols, etc. - 8DistortionRefers to degradation of signal in some way
The longer the wire is, the longer it takes for signal to propagate, and different wires allow different speeds, so one can run into the next due to delays and distort. - 9Signal Encoding TechniquesHow do we encode in efficient and reliable manner?
>Receiver must extract timing from incoming signal
>Inclusion of error detecting and correcting:
Want to encode so you can tell there is an error (via introduce additional bits into raw data stream). Receiving end needs to be able to check if that is what its suppose to receive. - 10Noise>Presence of noise limits accuracy of measurement of received signal.
>Errors occur if signal separation is comparable to noise level.
[BER] Bit Error Rate increases with decreasing bit-to-noise ratio - 11Shannon Channel Capacity(refer to OneNote notes)
- 12Fundamental Issues with TransmissionInfo bearing capacity - amplitude response and bandwidth depend on distance; susceptibility to noise produces error rates
- 13Characteristics of Wireless Media>Signal propagates in limited directionality
>Interference possible so spectrum regulated
>Limited bandwidth
>User can move
>No physical connection between network and users
>Simple infrastructure: antennas and transmitters - 14Characteristics of Wired Media>Signal energy contained and guided within medium
>Spectrum can be re-used in seperate media
>Extremely high bandwidth
>Complex infrastructure: ducts, conduits, poles, etc. - 15AttenuationDistance is key factor
Wired - has exponential dependance on distance
Wireless - has logarithmic dependance on distance (maintained for longer distance) - 16Twisted Pair>Two insulated copper wires arranged in spiral pattern to reduce interference
>More twists = stronger resistance to interference
>Low cost
>Much higher bit rates possible at shorter distances
>Used in ADSL (high speed internet access) - 17Coaxial Cable-Inner conductor surrounded by insulator layer
-High interference immunity
-Higher bandwidth than twisted pair
-Long distance telephone communication - 18Optical Fiber ILight source generates pulse of light through optical fiber
-Very long distances +1000km and very high speeds +40Gbps, nearly error free
-Distance is less of a cost factor - 19Optical Fiber IILight can reflect back into core if projected at critical angle.
MULTIMODE: LED, thicker core and shorter reach, dis: lasers on diff paths so can interfere and cause dispersion which limits bandwidth
SINGLE MODE: Lasers, very thin core one path, expensive but higher speeds - 20Optical Fiber IIIAD:
-Low attenuation and noise immunity
-Extremely high bandwidth
-No corrosion, compact and lighter than copper wire
DIS:
-Limited bend radius, will break if arc'd a certain way or light wont reflect, etc.
-Mechanical vibrations become signal noise
-New signal types can be dispersed - 21Regenerators + Optical AmplifiersOptical amplifiers amplify optical signal
Impairments in optical amp limit max num of optical amplifiers on path
>Severe problems with WDM systems - 22Radio Transmission IRadio Signals: antenna transmits signal that radiates in air space
-Info embedded in carrier signal using modulation e.g. QAM
-Com without tethering e.g. cellular phones, satellite transmissions - 23Radio Transmission IIProblems:
Multipath prop causes fading
Interference from other users - 24Error Control Basic approaches [ARQ, FEC]Digital transmission introduces errors, and services require reliability (data apps require error free transfer)
Error control ensures data stream is transmitted with a certain level of accuracy despite errors - 25Error Control Concepts-'Codewords' all transmitted data blocks that satisfy a pattern
-If received block doesn't satisfy pattern, it is error
-Redundancy - only subset of possible blocks can be codeword - 26Single Parity Check I>Parity uses ASCII code
>All odd-numbered patterns are detectable
>All even-numbered patterns are undetectable - 27Single Parity Check IIInformation (7 bits): 0,1,0,1,1,0,0
Parity Bit: = 0 + 1 + 0 + 1+ 1 + 0 + 0 = 1
So now there are even number of 1's
Code Word: 0,1,0,1,1,0,0,1
If error like so: 0,1,1,1,1,0,0,1 with 3rd bit wrong, # of 1's = 5 = odd, means detectable - 28Single Parity Check IIIRedundancy: Adds 1 redundant bit per k info bits. Overhead = 1/(k+1)
Coverage: all error patterns with odd# of errors can be detected - 29What classifies good code?>If codewords are close to each other, then detection failures will occur
>Good codes should maximise seperation between codewords - 30Two Dimensional Parity Check>More parity bits improve coverage
>Arrange info as columns, single parity bit to each column
>Add final parity column
>Last column = check bits for each row
>Last row = check bits for each column - 31Data Link ControlTwo important functions:
-Flow Control
-Error Control
FLOW CONTROL: enables users to regulate data flow (invokes ACK)
Two methods:
1) STOP AND WAIT
2) SLIDING WINDOW - 32STOP AND WAITSender sends out a frame, and waits for an ACK from the receiver before sending out another frame
AD: Simple, no out of order frames
DIS: Inefficient as only one frame on channel at a time, must wait at least 2tprop + tframe - 33SLIDING WINDOW I-Transmits several frames (depending on window size) without receiving ACK
-If receive ACK within window time, basically gives go ahead to send out next window of frames - 34SLIDING WINDOW IIAD: Full capacity of bandwidth is used, and better performance/greater throughput compared to stop and wait
DIS: ambiguous - 35ERROR CONTROLMethod used to detect and correct errors during transmission of frames
-Utilises ACK and NACK
Automated Repeat Request (ARQ)
Three types:
-Stop and Wait
-Go back N
-Selective repeat - 36Stop and Wait (ARQ)Source sends a single frame - if doesn't get an ACK, sends all frames again.
AD: However requires less calculation, as just simply need to resend a quantity of frames
Dis: Very inefficient in most scenarios - 37Go-Back-NIf sent out 10 packets, and packet 5 is lost, will resend packets 5,6,7,8,9,10
AD: More efficient and doesn't require re-order at receivers end - 38Selective Repeat ARQOnly the lost or damaged frame is retransmitted
AD: More efficient from a utilisation point of view
DIS: Complex to implement are calculate; if were to compare tax on server, would be easier just to use stop and wait - 39Pipe shit• Synchronous bit pipe
• Sending side of DLC supplies the sending side modem bits at a fixed rate (one bit per T seconds)
• Idle fill (dummy bits) when no data
• Intermittent Synchronous bit pipe
• Supplies synchronously when there is data to be sent • Sends nothing when no data
• Receiver complications:
• need to distinguish between 0, 1 and idle
• Re-synchronise with sender at end of idle period
• Asynchronous character pipe
• Bits within a character sent at a fixed rate • Characters separated by variable delays - 40HDLC High Level Data Link ControlBit oriented code transparent synchronous data link layer protocol developed by ISO.
Frames transmitted over synchronous/asynchronous serial comm links - 41HDLC IILinks have no mechanism to mark beginning or end of a frame, so beginning and end of each frame has to be identified
Done using unique sequence of bits as a frame delimiter or flag, encode data to ensure flag seq never seen inside frame
Each frame begins and ends with a frame delimiter. A frame delimiter at end of a frame may also mark start of the next frame
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- Fichas sobre Physical-MAC, creado por Eric Wojcik el 08/06/2018.
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- Eric Wojcik
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