Physical Layer Two - Local and Wide Area Networks - Lecture Slides, Slides of Social Work

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Introduction to LAN/WAN

Physical Layer (contd)

Modulation Modes

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amplitude-shift

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frequency-shift

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phase-shift modulation

• shift by 45, 135,

225, 315 degree(2bits/interval).

Modems: Constellation diagrams

(a) QPSK.(b) QAM-16.(c) QAM-64.

Digital Transmission

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Analog circuits require amplifiers, and eachamplifier adds distortion and noise to thesignal.

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Digital amplifiers regenerate an exact signal

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Integrate all traffic

Analog Data/Digital Signals

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Although most local loops are analog, endoffices increasingly use digital circuits forinter-trunk lines. A codec (coder/decoder) isa device that converts an analog signal intoa digital signal.

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To convert analog signals to digital signals,many systems use Pulse Code Modulation(PCM)

Pulse Code Modulation

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PCM

Convert analog to digital (done by codec)

Uses sampling (snapshots of waveform)

PCM samples the 4kHz signal 8,000 times persecond. (Nyquist theorem)

Each sample measures the amplitude of the signal,converting it into an n-digit integer value.

The digital channel carries these n-digit encodings.

Wireless Local Loops

Architecture of an LMDS system.

• FCC: 1.3 GHz of spectrum for LMDS

IEEE 802.

Multiplexing

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Problem: Given a channel of large capacity, howdoes one subdivide the channel into smaller logicalchannels for individual users? Multiplex manyconversations over same channel.

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Three flavors of solution: 1.Frequency division multiplexing (FDM)2.Time division multiplexing (TDM)3.Statistical multiplexing

Frequency Division Multiplexing

(a) The original bandwidths.(b) The bandwidths raised in frequency.(b) The multiplexed channel.

Time division multiplexing

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Use time slicing to give each user the fullbandwidth, but for only a fraction of asecond at a time (analogous to time sharingin operating systems).

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Problem?if the user doesn't have data to sent duringhis time slice, the bandwidth is not used(e.g., wasted).

Statistical multiplexing

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Allocate bandwidth to arriving packets ondemand.

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Advantage:leads to the most efficient use of channelbandwidth because it only carries usefuldata. Channel bandwidth is allocated topackets that are waiting for transmission,and a user generating no packets doesn't useany of the channel resources.

Switching

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Circuit SwitchingUsed in current telephone system

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Message Switching

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Packet SwitchingUsed in the next generation telephonesystem--broadband ISDN system

Circuit Switching

(Fig2-35)

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1.Once a call setup has been completed, the user sees a setof virtual wires between communicating endpoints.

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2.The user sends a continuous stream of data, which the channel guarantees to deliver at a known rate.

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3.Data transmission handled elegantly using TDM or FDM.

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4.Call setup required before any data can be sent.

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5.Call termination required when parties complete call.

Message Switching

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No physical copper path is established in advancebetween communicating endpoint.

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Entire message stored at each node. Each message isreceived in its entirety,inspected for errors and thenforwarded.

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A network using this technique is called a store-and-forward network.

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Memory requirements at intermediate nodes