Telecom Systems Chae Y. Lee
Communication Link
NASA to Test First Mars - Earth Laser Communication Link
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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
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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)
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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)
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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
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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
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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
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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
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