l8 Physical and link layers

l8 Physical and link layers - Lecture 8: Physical and Link...

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Lecture 8: Physical and Link Layers
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Topics Today Physical layer: chips versus bits Link layer and media access control (MAC) Ethernet Hubs and Switches MPLS
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Physical layer: chips versus bits
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Protocol Layering Session Transport Network Link Physical Presentation Application 7 6 5 4 3 2 1
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Protocol Layering Session Transport Network Link Physical Presentation Application 7 6 5 4 3 2 1
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Physical Layer (Layer 1) Responsible for specifying the physical medium - Category 5 cable (Cat5): 8 wires, twisted pair, RJ45 jack - WiFi wireless: 2.4GHz Responsible for specifying the signal - 100BASE-T: 5-level pulse amplitude modulation (PAM-5) - 802.11b: Binary and quadrature phase shift keying (BPSK/QPSK) Responsible for specifying the bits - 100BASE-T: 4-to-6 bit-to-chip encoding, 3 chip symbols - 802.11b: Barker code (1-2Mbps), complementary code keying (5.5-11Mbps)
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Specifying the signal Chips versus bits - Chips: data (in bits) at the physical layer - Bits: data above the physical layer Physical layer specifies Analog signal chip mapping - On-off keying (OOK): voltage of 0 is 0, +V is 1 - PAM-5: 000 is 0, 001 is +1, 010 is -1, 011 is -2, 100 is +2 - Frequency shift keying (FSK) - Phase shift keying (PSK) - Don’t worry about this too much now: we’ll cover it in greater depth when we look at wireless
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How fast can you transmit information? Depends on bandwidth and Signal/Noise ration Shannon: Channel capacity C = B log 2 (1 + S/N ) - B is bandwidth of line - S and N are average signal & noise power For any transmission rate R < C , can have arbitrarily low error rate Example: Telephone line - 3 KHz b/w, 30 db S/N = 10 30 / 10 = 1000 - C 30 Kbps (so 56 Kbps modems need better S/N ratio) Crude intuition for Shannon - Sample rate B - V voltage levels encode log 2 V bits, so bits/sample log 2 (1 + )
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Straw man: On-off keying Bits NRZ 0 0 1 0 1 1 1 1 0 1 0 0 0 0 1 0 To transmit 0 bit, sent 0 V , to transmit 1, sent +5 V - A bit is a chip in this scheme OOK a form of Amplitude Shift Keying (ASK) - Bits are encoded in amplitude of the signal - Can also have frequency shift keying (FSK) - And phase shift keying (PSK) Also an example of non-return to zero (NRZ) - E.g., four 1 bits transmitted by asserting +5 V for 4 clock ticks
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NRZ drawbacks Consecutive 1s or 0s are problematic Non signal could be interpreted as 0s (or vice versa) “Baseline wander” problem - Where is threshold between low and high? - Could compare signal to average value, but avg. will drift Sender and receiver need synchronized clocks - Otherwise, can experience “bit slip”
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Non-return to Zero Inverted (NRZI) Encode 1 with transition from current signal Encode 0 by staying at same level At least solves problem of consecutive 1s
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l8 Physical and link layers - Lecture 8: Physical and Link...

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