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Transmission Media Chapter 4. Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. Types of media: Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber.

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transmission media chapter 4
Transmission Media Chapter 4
  • Physically connect transmitter and receiver carrying signals in the form electromagnetic waves.
  • Types of media:
    • Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber.
    • Unguided: “wireless” transmission (atmosphere, outer space).

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guided media examples 1
Guided Media: Examples 1
  • Twisted Pair:
    • 2 insulated copper wires arranged in regular spiral. Typically, several of these pairs are bundled into a cable. (What happens if the twist is not regular? Reflection?)
    • Cheapest and most widely used; limited in distance, bandwidth, and data rate.
    • Applications: telephone system (from home to local exchange connection).
    • Unshielded and shielded twisted pair.
    • What is a differential amplifier?

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guided media examples 11
Guided Media: Examples 1
  • Twisted pair – continued
    • Category 3: Unshielded twisted pair (UTP) up to 16MHz.
    • Cat 5: UTP to 100 MHz.
    • Table 4.2. Suppose Cat 5 at 200m (the limit of 100Mbps ethernet is 300m).
      • The dB attenuation at 100m is 22.0. So at 200m, the attenuation is ???. Suppose we transmit at –80dBW. Then the received signal has energy of ????.
      • The near-end crosstalk gain is 32dB per 100m. So the crosstalk energy is ????
      • The SNR is ????? (neglecting thermal noise).

44

–124dBW

–144dBW

20dB

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examples 2
Examples 2
  • Coaxial Cable
    • Hollow outer cylinder conductor surrounding inner wire conductor; dielectric (non-conducting) material in the middle.
    • Less capacitance than twisted pair, so less loss at high frequencies. Also, Coaxial has more uniform impedance.
    • Applications: cable TV, long-distance telephone system, LANs.
    • Repeaters are required every few kilometers at 500MHz.
    • +’s: Higher data rates and frequencies, better interference and crosstalk immunity.
    • -’s: Attenuation at high frequency (up to 2 GHz is OK) and thermal noise.

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examples 3
Examples 3
  • Optical Fiber
    • Thin, flexible cable that conducts optical waves.
    • Applications: long-distance telecommunications, LANs (repeaters every 40km at 370THz!).
    • +’s: greater capacity, smaller and lighter, lower attenuation, better isolation,
    • -’s: Not currently installed in subscriber loop. Easier to make use to current cables than install fiber.

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examples 3 types of fiber
Examples 3 – types of fiber

lower index of refraction

  • Step-index multimode

shorter path

longer path

absorbed

total internal reflection

higher index of refraction

Since the signal can take many different paths, the arrival the received signal is smeared.

Input Pulse

Output Pulse

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examples 3 types of fiber1
Examples 3 – types of fiber
  • Single mode

If the fiber core is on the order of a wavelength, then only one mode can pass.

Wavelengths are 850nm, 1300nm and 1550nm (visible spectrum is 400-700nm). 1550nm is the best for highest and long distances.

Attenuation: -0.2dB/km to -0.8dB/km (if the ocean was made of this glass you could see the floor like you can see the ground from an airplane)

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examples 3 types of fiber2

Input Pulse

Output Pulse

Examples 3 – types of fiber

Even for single mode fiber, a pulse gets smeared.

Solitons are a particular wave pulse that does not disperse.

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fiber repeaters two approaches
Fiber Repeaters : Two Approaches
  • Convert the signal to analog. Convert to digital and then send a transmit received signal.
  • Optical repeater. A nonlinear optical amplifier shapes and amplifies the pulse. A single repeater works for all data rates! (more about optical networks later)

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wavelength division multiplexing wdm
Wavelength-division multiplexing (WDM)
  • Wavelength-division multiplexing
    • Multiple colors are transmitted.
    • Each color corresponds to a different channel.
    • In 1997, Bell Labs had 100 colors each at 10Gbps (1Tbps).
    • Commercial products have 80 colors at 10Gbps.

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fiber vs cable
Fiber vs. Cable
  • Fiber is light and flexible.
  • Fiber has very high bandwidth.
  • Fiber is difficult to install (I can’t do it).
  • Fiber interfaces are more expensive than cable (?)

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wireless transmission

Wireless Transmission

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electromagnetic spectrum

1022

1016

UV

Gamma

-ray

X-ray

Electromagnetic Spectrum

Cell phones put out 0.6 – 3 watts. Light bulbs put out 100 watts.

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wireless transmission1
Wireless Transmission
  • Omni-directional – the signal is transmitted uniformly in all directions.
  • Directional – the signal is transmitted only in one direction. This is only possible for high frequency signals.

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terrestrial microwave
Terrestrial Microwave
  • Parabolic dish on a tower or top of a building.
  • Directional.
  • Line of sight.
  • With antennas 100m high, they can be 82 km (50 miles).
  • Use 2 – 40 GHz.
  • 2 GHz: bandwidth 7MHz, data rate 12 Mbps
  • 11 GHz: bandwidth 220MHz, data rate 274 Mbps

The M in MCI is for microwave

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satellite microwave
Satellite Microwave
  • Satellites are repeaters.
  • 1 – 10 GHz. Above 10 GHz, the atmosphere (like rain) attenuates the signal, and below 1 GHz there is too much noise.
  • Typically, 5.925 to 6.425 GHz for earth to satellite and 4.2 to 4.7 GHz for satellite to earth. (Why different frequencies?)
  • A stationary satellite must be 35,784 km (22000 miles) above the earth.
  • The round-trip delay is about ½ a second.

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low earth orbit satellites leo
Low-Earth Orbit Satellites (LEO)
  • Iridium The idea of some executive’s wife while vacationing in the tropics and her cell phone didn’t work..
  • Cost 5 billion dollars.
  • Went out of business in 1999. Sold for $25 million and is still operational.
  • Provides phone, fax, paging, data and navigation WORLD WIDE! (jungle, Afghanistan (both sides), etc.)
  • 66 low orbit satellites. Low Orbit, so they move out of range fast
  • Cool thing. The calls go hop from satellite to satellite before returning to the destination. So they have to track every user.
  • Globalstar 48 LEOs. The call goes to the ground as soon as possible and uses a terrestrial network. So they are simpler. Also, the satellites relay the analog signal. On the ground is a large, sensitive antenna to pick up the weak phone signal.
  • Teledesic. 30 satellites. Data network 100Mbps to 720Mbps. Planned for 2005. Bill Gates and Craig McCaw founders.

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other
Other
  • Cell phones – Omni-directional. GSM-900 uses 900MHz, GSM-1800 and GSM-1900 (PCS). Typical data rate seems to be around 40kbps. But the protocol is specified to 171kbps.
  • 802.11 wireless LANs
    • Omni-directional
    • 802.11b 2.4 GHz (where microwave ovens and cordless phones are) up to 11Mbps
    • 802.11a 5 GHz up to 54Mbps
  • Infrared – Line of sight, short distances.

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spectrum allocation
Spectrum Allocation
  • Some bands are allocated for unlicensed usage (ISM)
    • 900 MHz – cell phones, cordless phones. Is not available in all countries. Bandwidth is 26MHz.
    • 2.4 GHz – cordless phones, 801.11b, Bluetooth, microwave ovens. Is available in most countries. Bandwidth is 83.5 MHz.
    • 5.7 GHz – 802.11a. Is new and relatively uncrowded (so far) but a bit expensive. Bandwidth is 125MHz. (Why can 802.11a transmit at a high data rate?)
  • These are actually several bands.

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spectrum allocation1
Spectrum Allocation

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types of connections
Types of Connections
  • Long-haul – about 1500km (1000 miles) undersea, between major cites, etc. High capacity: 20000-60000 voice channels. Twisted pair, coaxial, fiber and microwave are used here. Microwave and fiber are still being installed.
  • Metropolitan trunks – 12km (7.5 miles) 100,000 voice channels. Link long-haul to city and within a city. Large area of growth. Mostly coaxial, twisted pair and fiber are used here.
  • Rural exchange trunks – 40-160km link towns. Twisted pair, coaxial, fiber and microwave are used here.
  • Subscriber loop – run from a central exchange to a subscriber. This connection uses twisted pair, and will likely stay that way for a long time. Cable uses coaxial and is a type of subscriber loop (it goes from central office to homes). But a large number of people share the same cable.
  • Local area networks (LAN) – typically under 300m. Sizes range from a single floor, a whole building, or an entire campus. While some use fiber, most use twisted pair as twisted pair is already installed in most buildings. Wireless (802.11) is also being used for LAN.

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data encoding chap 5
Data Encoding (Chap. 5)
  • Transforming original signal just before transmission.
  • Both analog and digital data can be encoded into either analog or digital signals.

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digital transmission terminology
Digital Transmission Terminology
  • Data element: bit.
  • Signaling element: encoding of data element for transmission.
  • Unipolar signaling: signaling elements have same polarization (all + or all -).
  • Polar signaling: different polarization for different elements.

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more terminology
More Terminology
  • Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec.
  • Modulation rate = baud rate (rate at which signal levels change).

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approach 1 nrz

Switch when a 1 occurs

Approach 1: NRZ
  • But how do you know when to sample?
    • Phase-locked-loop (PLL) – measures the difference when transitions occur on the wire and when they occur on a local adjustable oscillator, and then make adjustments accordingly.
    • YOU MUST HAVE TRANSISTIONS TO LOCK ON TO.

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multilevel binary
Multilevel Binary

opposite direction

Pros:

No DC component.

Can be used to force transitions (to help PLL).

Cons:

We are using 3 levels and could send ?? bits instead of 1

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scrambling to help the pll
Scrambling – to help the PLL
  • If there are not enough transitions, the PLL may have problems.
  • So we force extra transitions when there are not enough.
  • Approach 1 – Use special coding so that long strings of zeros (or ones) don’t occur.

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scrambling to help the pll1
Scrambling – to help the PLL
  • Approach 2 – Use multilevel binary and set illegal transitions to long strings of zeros.
  • Here, if an octet of zeros occurs, send a special illegal sequence.
  • The receiver must be able to interpret this special sequence.

used in long-distance transmission

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biphase differential manchester self clocking
Biphase – Differential Manchester(Self-Clocking)

A transition always occurs in the middle of the period.

A zero is represented by a transition occurring at the beginning of the period.

A one is represented by no transition at the beginning of the period.

0

0

1

1

always a transition in the middle

Used in CD players and Ethernet

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methods to encode digital signals
Methods to Encode Digital Signals
  • NRZ
  • Multilevel binary
  • Manchester
  • Issues:
    • DC?
    • Self Clocking?
    • How big is the spectrum?

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sending digital signals over analog e g modem
Sending Digital Signals over Analog (e.g. Modem)
  • Amplitude shift keying (ASK) (Amplitude Modulation)
  • Frequency shift keying (FSK) (Frequency modulation)
  • Phase shift keying (PK) (Phase Modulation)
  • Modems use phase and amplitude of them.

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modulation techniques
Modulation Techniques

ASK

FSK

PSK

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phase shift keying
Phase-shift Keying
  • Quadrature phase-shift keying (QPSK) - send 2 bits.

90

0

180

270

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qam quadrature amplitude modulation
QAM - Quadrature Amplitude Modulation

constellation diagrams

90

90

0

180

0

180

270

270

QAM-16

(16 levels, how many bits)

QAM - 64

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slide35
V32

128 bits: 6 data and 1 parity (error correction)

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how fast is v32

Use 2400 sample each way - duplex

Definition: a duplex connection means that we can send data in both directions at the same time.

A simplex or half-duplex connection only sends data in one direction at a time.

How fast is V32?

The phone system transmits 300 to 3400 Hz

So what bandwidth can we use. How fast can we send symbols?

So 2400 * 6 = 14400 bps

What is the baud rate?

V.34 2400 baud - with 12 data bits/symbol

V.34 2400 baud – with 14 data bits/symbol

That’s the fastest there is!

To get 56K you send at 4000 baud (if the phone system can handle it)

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digital subscriber lines dsl
Digital Subscriber Lines (DSL)
  • ADSL – A for asymmetric, faster down load speed than up.
  • The 56kbps or 33kbps is because of a filter installed at the end office.
  • If this filter is removed, then the full spectrum of the twisted pair is available.
  • But, if you are far from the office, then you can’t get a very high data rate because…?
  • The DSL standard goes up to 8 Mbps down and 1 Mbps up.

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slide38
DSL

A total of 256 4kHz channels

Upstream

downstream

empty

25kHz (channel 6)

voice

(channel 0)

250 parallel channels: Each data channel uses QAM 16 (with 1 parity bit).

The quality of each channel is monitored and adjusted.

So channels may transmit at different speeds

What is the maximum data rate?

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digital transmission receiver side issues
Digital Transmission: Receiver-Side Issues
  • Clocking: determining the beginning and end of each bit.
    • Transmitting long sequences of 0’s or 1’s can cause synchronization problems.
  • Signal level: determining whether the signal represents the high (logic 1) or low (logic 0) levels.
    • S/N ratio is a factor.

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comparing digital encoding techniques
Comparing Digital Encoding Techniques
  • Signal spectrum: high frequency means high bandwidth required for transmission.
  • Clocking: transmitted signal should be self-clocking.
  • Error detection: built in the encoding scheme.
  • Noise immunity: low bit error rate.

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digital to analog encoding
Digital-to-Analog Encoding
  • Transmission of digital data using analog signaling.
  • Example: data transmission of a PTN.
  • PTN: voice signals ranging from 300Hz to 3400 Hz.
  • Modems: convert digital data to analog signals and back.
  • Techniques: ASK, FSK, and PSK.

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amplitude shift keying
Amplitude-Shift Keying
  • 2 binary values represented by 2 amplitudes.
  • Typically, “0” represented by absence of carrier and “1” by presence of carrier.
  • Prone to errors caused by amplitude changes.

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frequency shift keying
Frequency-Shift Keying
  • 2 binary values represented by 2 frequencies.
  • Frequencies f1 and f2 are offset fromcarrier frequency by same amount in opposite directions.
  • Less error prone than ASK.

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phase shift keying1
Phase-Shift Keying
  • Phase of carrier is shifted to represent data.
  • Example: 2-phase system.
  • Phase shift of 90o can represent more bits: aka, quadrature PSK.

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analog to digital encoding
Analog-to-Digital Encoding
  • Analog data transmitted as digital signal, or digitization.
  • Codec: device used to encode and decode analog data into digital signal, and back.
  • 2 main techniques:
    • Pulse code modulation (PCM).
    • Delta modulation (DM).

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pulse code modulation 1
Pulse Code Modulation 1
  • Based on Nyquist (or sampling) theorem: if f(t) sampled at rate > 2*signal’s highest frequency, then samples contain all the original signal’s information.
  • Example: if voice data is limited to 4000Hz, 8000 samples/sec are sufficient to reconstruct original signal.

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pcm 2
PCM 2
  • Analog signal -> PAM -> PCM.
    • PAM: pulse amplitude modulation; samples of original analog signal.
    • PCM: quantization of PAM pulses; amplitude of PAM pulses approximated by n-bit integer; each pulse carries n bits.

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delta modulation dm
Delta Modulation (DM)
  • Analog signal approximated by staircase function moving up or down by 1 quantization level every sampling interval.
  • Bit stream produced based on derivative of analog signal (and not its amplitude): “1” if staircase goes up, “0” otherwise.
  • Parameters: sampling rate and step size.

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analog to analog encoding
Analog-to-Analog Encoding
  • Combines input signal m(t) and carrier at fc producing s(t) centered at fc.
  • Why modulate analog data?
    • Shift signal’s frequency for effective transmission.
    • Allows channel multiplexing: frequency-division multiplexing.
  • Modulation techniques: AM, FM, and PM.

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amplitude modulation am
Amplitude Modulation (AM)
  • Carrier serves as envelope to signal being modulated.
  • Signal m(t) is being modulated by carrier cos(2p fct).
  • Modulation index: ratio between amplitude of input signal to carrier.

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angle modulation
Angle Modulation
  • FM and PM are special cases of angle modulation.
  • FM: carrier’s amplitude kept constant while its frequency is varied according to message signal.
  • PM: carrier’s phase varies linearly with modulating signal m(t).

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spread spectrum 1
Spread Spectrum 1
  • Used to transmit analog or digital data using analog signaling.
  • Spread information signal over wider spectrum to make jamming and eavesdropping more difficult.
  • Popular in wireless communications

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spread spectrum 2
Spread Spectrum 2
  • 2 schemes:
    • Frequency hopping: signal broadcast over random sequence of frequencies, hoping from one frequency to the next rapidly; receiver must do the same.
    • Direct Sequence: each bit in original signal represented by series of bits in the transmitted signal.

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transmission modes
Transmission Modes
  • Assuming serial transmission, ie, one signaling element sent at a time.
  • Also assuming that 1 signaling element represents 1 bit.
  • Source and receiver must be in sync.
  • 2 schemes:
    • asynchronous and
    • synchronous transmission.

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asynchronous xmission 1
Asynchronous Xmission 1
  • Avoid synchronization problem by including sync information explicitly.
  • Character consists of a fixed number of bits, depending on the code used.
  • Synchronization happens for every character: start (“0”) and stop (“1”) bits.
  • Line is idle: transmits “1”.

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asynchronous xmission 2
Asynchronous Xmission 2
  • Example: sending “ABC” in ASCII

0 10000010 1 0 01000010 1 0 110000 1 1111…

  • Timing requirements are not strict.
  • But problems may occur.
    • Significant clock drifts + high data rate = reception errors.
  • Also, 2 or more bits for synchronization: overhead!

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synchronous xmission 1
Synchronous Xmission 1
  • No start or stop bits.
  • Synchronization via:
    • Separate clock signal provided by transmitter or receiver; doesn’t work well over long distances.
    • Embed clocking information in data signal using appropriate encoding technique such as Manchester or Differential Manchester.

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synchronous xmission 2
Synchronous Xmission 2
  • Need to identify start/end of data block.
  • Block starts with preamble (8-bit flag) and may end with postamble.
  • Other control information may be added for data link layer.

8 -bit

flag

8 -bit

flag

Control

Control

Data

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data link layer
Data Link Layer
  • So far, sending signals over transmission medium.
  • Data link layer: responsible for error-free (reliable) communication between adjacent nodes.
  • Functions: framing, error control, flow control, addressing (in multipoint medium).

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flow control
Flow Control
  • What is it?
    • Ensures that transmitter does not overrun receiver: limited receiver buffer space.
    • Receiver buffers data to process before passing it up.
    • If no flow control, receiver buffers may fill up and data may get dropped.

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stop and wait
Stop-and-Wait
  • Simplest form of flow control.
    • Transmitter sends frame and waits.
    • Receiver receives frame and sends ACK.
    • Transmitter gets ACK, sends other frame, and waits, until no more frames to send.
  • Good when few frames.
  • Problem: inefficient link utilization.
    • In the case of high data rates or long propagation delays.

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sliding window 1
Sliding Window 1
  • Allows multiple frames to be in transit at the same time.
  • Receiver allocates buffer space for n frames.
  • Transmitter is allowed to send n (window size) frames without receiving ACK.
  • Frame sequence number: labels frames.

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sliding window 2
Sliding Window 2
  • Receiver ack’s frame by including sequence number of next expected frame.
  • Cumulative ACK: ack’s multiple frames.
  • Example: if receiver receives frames 2,3, and 4, it sends an ACK with sequence number 5, which ack’s receipt of 2, 3, and 4.

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sliding window 3
Sliding Window 3
  • Sender maintains sequence numbers it’s allowed to send; receiver maintains sequence number it can receive. These lists are sender and receiver windows.
  • Sequence numbers are bounded; if frame reserves k-bit field for sequence numbers, then they can range from 0 … 2k -1 and are modulo 2k.

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sliding window 4
Sliding Window 4
  • Transmission window shrinks each time frame is sent, and grows each time an ACK is received.

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example 3 bit sequence number and window size 7
Example: 3-bit sequence number and window size 7

A B

0 1 2 3 4 5 6 7 0 1 2 3 4... 0 1 2 3 4 5 6 7 0 1 2 3 4

0

1

2

0 1 2 3 4 5 6 7 0 1 2 3 4

0 1 2 3 4 5 6 7 0 1 2 3 4

RR3

0 1 2 3 4 5 6 7 0 1 2 3 4

0 1 2 3 4 5 6 7 0 1 2 3 4

3

0 1 2 3 4 5 6 7 0 1 2 3 4

4

5

0 1 2 3 4 5 6 7 0 1 2 3 4

RR4

6

0 1 2 3 4 5 6 7 0 1 2 3 4

0 1 2 3 4 5 6 7 0 1 2 3 4

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digital analog encoding
Digital/Analog Encoding

Encoding:

g(t)

g(t)

(D/A)

Encoder

Digital Medium

Decoder

Source

Destination

Source System

Destination System

Modulation:

g(t)

g(t)

(D/A)

Modulator

Analog Medium

Demodulator

Source

Destination

Source System

Destination System

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encoding considerations
Encoding Considerations
  • Digital signaling can use modern digital transmission infrastructure.
  • Some media like fiber and unguided media only carry analog signals.
  • Analog-to-analog conversion used to shift signal to use another portion of spectrum for better channel utilization (frequency division mux’ing).

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