Telecom Systems Chae Y. Lee

Communication Link

NASA to Test First Mars - Earth Laser Communication Link

Telecom Systems Chae Y. Lee

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Where we are now…

Notions of network designLayers of Internet:

Application/Transport/Network/LinkBut, how to transfer actual data?

Application

Transport

Network

Link

Telecom Systems Chae Y. Lee

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Contents: Links

Issues in Link DesignLink ExamplesLink CapacityFraming

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Issues in Link Design

Operations:Single or multiple channels; Media Access ControlHalf-Duplex of Full-Duplex

Characteristics:Bit rate – sometimes called ‘bandwidth’DistanceBit Error Rate

Implementation:Encoding and Modulation (bits into signals)Framing (package bits into messages and addressing)Error detectionReliability with ARQ

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Common Media

Cat 3 UTP (10 Mb/s, 100m)Cat 5 UTP (100 Mb/s, 100m)Multimode fiber (100 Mb/s, 2km)Single mode fiber (2 Gb/s, 40km)

Telecom Systems Chae Y. Lee

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Common Telco Circuits

ISDN (2x64 kb/s)T1 (1.544 Mb/s)T3 (44.736 Mb/s)OC3 (155.521 Mb/s)OC12 (622.080 Mb/s)OC48 (2.48832 Gb/s)

Telecom Systems Chae Y. Lee

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Link Examples

Distance (km) vs. Bit Rate (Mbps)

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Link Examples

Components:

Link0100..10 0110..10

ErrorCoding

ChannelCoding Modulation

ErrorDecoding

ChannelDecoding

De-modulation

Medium

0100011

0100011

01000111 1001010101

01000111 1001010101

e.g., parity e.g., 8b/10b e.g., OOK

1001010101

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Link Examples

Types of Modulation:Baseband: Bits -> signals at about same rate

Advantage: SimplicityTwo subtypes:

With self-clocking: enables large packets – efficientWithout self-clocking: one character at a time

Broadband: bits drive a high-frequency carrierAdvantages:

Makes antenna effective (e.g., Radio)Enables frequency division multiplexing (e.g., CATV)

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Link Examples

Baseband: Self-clockingNRZI: non return to zero

“stay [0]/transition [1]” codingdifferential code (adjacent transitions)good noise immunity

Manchester0 = down-transition; 1 = up-transitionNo DC component

MLT-30 = zero volt; 1 = alternate +1/-1No DC componentNarrower spectrum

0 1 00 1 1

0 1 00 1 1

0 1 00 1 1

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Link Examples

Optical: Self-Clocking4B/5B or 8B/10BBasic Idea:Introduce enough transitions so that the signal

contains the clock

Telecom Systems Chae Y. Lee

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Link Examples

Broadband: FSK and BPSK

FSK – Frequency Shift KeyingBit 0 and Bit 1 are encoded as different frequencies

BPSK – Binary Phase Shift KeyingBit 0 and Bit 1 are encoded as different phases

Telecom Systems Chae Y. Lee

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Channel Capacity

C = Maximum possible reliable data rate(with best possible transmission scheme)

C is limited by noiseShannon: In copper and wireless

C = BW log2 (1+S/N) bits per second*BW = bandwidth of channelS is signal power, N is noise powerLog2 is the logarithm in base 2

*Assumes additive white Gaussian noise

Telecom Systems Chae Y. Lee

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Framing

Package bits into messagesRecognize the beginning and end of each messageByte-oriented protocols

BISYNC (BSC), DDCMP, IMP-IMP, PPP** Common mode

Bit-oriented protocolsHDLC

“Other”-oriented protocolsSONET

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Summary

Link layer: Encoding and Modulation (bits into signals)FramingError detectionReliability with ARQ: Stop-and-wait, Sliding window

Channel capacity