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Topic 3a: The Physical Layer
Our goals:
• Understand physical channel
fundamentals
– Physical channels can carry data
proportional to bandwidth and signal level
and inverse proportion to noise level.
– Modulation encodes digital data into an
analogue channel.
– Baseband vs. Broadband channel use.
– Synchronous vs. Asynchronous.
DJG V
Physical Channels / The Physical Layer these example physical channels are also known as Physical Media Twisted Pair (TP)
- (^) two insulated copper wires - (^) Category 3: traditional
phone wires, 10 Mbps
Ethernet
25Gbps Ethernet
- (^) Shielded (STP)
- (^) Unshielded (UTP) Coaxial cable:
- (^) two concentric copper conductors
- often bi-directional
- (^) baseband:
- (^) single channel on cable
- (^) eg. legacy Ethernet
- (^) broadband:
cable
- HFC (Hybrid Fiber Coax) Fiber optic cable:
- (^) point-to-point transmission
- (^) (10’s-100’s Gbps)
- (^) low error rate
- (^) immune to electromagnetic noise
Physical Channel Characteristics
- Fundamental Limits - ● Delay : speed-of-light in medium and distance travelled ● Fidelity : signal-to- noise ratio (SNR)
- Bandwidth : a measure of the range of frequencies of sinusoidal signal that channel supports
Bandwidth:
- (^) E.g., a channel that supports sinusoids from 1
MHz to 1.1 MHz has a bandwidth of 100 KHz
- (^) “supports” in this context means “comes out
the other end of the channel”
- (^) some frequencies supported better than others
- (^) analysing what happens to an arbitrary
waveform is done by examining what happens
to its component sinusoids → Fourier analysis
Bandwidth and SNR are our key resources.
Basic Channel (L1) Structure
L1 Physical layer - conveys bits
L2 Data link layer – understands framing, addresses and MAC
The distinction is often not entirely clear
System can work better if there is no clear boundary
Making the decision ONCE about what a packet contains is always better
In information-theory terms.
Noise : Enemy of Communications
Attenuation, External Noise,
Systematic, non-systematic,
digitization, interference, reflection, ….
Link Power Budget
●
Launch power - the peak energy emitted per second (Watts).
●
Medium loss - how much the physical medium attenuates the signal, generally by converting it to heat
and typically expressed per unit length.
● Scattering loss - how much of the launched power 'hits' the receiver assuming no medium loss.
●
Receiver self-noise - the noise generated inside the receiver. For most modulation schemes, the
received far-end signal must be several dB higher than the receiver-self noise, effectively limiting the
length of a practical channel.
- (^) can be minimised by cooling the receiver, as used in radio telescopes.
● Near-end crosstalk (NEXT) - for a duplex system, how much of the locally transmitted power finds its
way into the local receiver, acting like another form of noise,
- (^) potential to overcome the far-end signal.
- (^) can sometimes be cancelled out by subtraction of a filtered version of the transmitted signal.
● Pickup noise - signals from other systems and energy sources that find their way into the receiver.
- (^) generally be minimised by Faraday screening (put a metal box around the sensitive electronics
- (^) boxes also restrict unintentional emissions, as required by law.
Digital Channel Modulation
• Baud rate, R, is
the rate at which
symbols can be
transmitted.
• Data rate, D (or
bit rate), is the
equivalent
number of binary
digits conveyed.
• Eg. 4-level signal
gives 2 bits per
baud interval
D = 2R
These are modulation schemes from the 1950’s. Modern are far more complex!
Optical Fibre System
Further Common Channels
0. Fibre
1. Asynchronous RS
2. Block-coded (synchronous, baseband)
3. QPSK radio (synchronous, broadband)
4. OFDM (synchronous, FDM internally, baseband or broadband)
5. CDMA (synchronous, code-division multiplexing (CDM)
Also: Scrambling for security, transitions and
whiteness – (likely not lectured 2526)
Asynchronous Example: RS
UART data configurations and data format (from MSD).
0 1 0 0 1 0 0 1 1 1 Line Coding – Block Coding Example Name 4b 5b Description 0 0000 11110 hex data 0 1 0001 01001 hex data 1 2 0010 10100 hex data 2 3 0011 10101 hex data 3 4 0100 01010 hex data 4 5 0101 01011 hex data 5 6 0110 01110 hex data 6 7 0111 01111 hex data 7 8 1000 10010 hex data 8 9 1001 10011 hex data 9 A 1010 10110 hex data A B 1011 10111 hex data B C 1100 11010 hex data C D 1101 11011 hex data D E 1110 11100 hex data E F 1111 11101 hex data F Name 4b 5b Description Q -NONE- 00000 Quiet I -NONE- 11111 Idle J -NONE- 11000 SSD # K -NONE- 10001 SSD # T -NONE- 01101 ESD # R -NONE- 00111 ESD # H -NONE- 00100 Halt 0 1 0 1 0 1 0 0 1 1
Block coding transfers data with a fixed
overhead: 20% less information per Baud in
the case of 4B/5B
So to send data at 100Mbps; the line rate
(the Baud rate) must be 125Mbps.
1Gbps uses an 8b/10b codec; encoding
entire bytes at a time but with 25% overhead
Data to send
Line-(Wire) representation
DtoA (DAC) and AtoC (ADC)
Recall from Digital Electronics
Conversion errors can occur in both directions
e.g.
Noise leads to incorrect digitization
Insufficient digitization resolution leads to information loss
Our ‘decision flip-flop’ was a 1-bit ADC
Copper (Coax/Twisted-pair) - Power Budget Transmit power might be about 0 dBm (eg 1 volt peak-to-peak, which in rms terms, is 1.3 mW given a 100 ohm characteristic impedance). All of the transmitted power enters the cable, so there is no scattering loss, apart from a small amount of radiation from unbalance in the cable twisting. The receiver sensitivity (before self-noise dominates) might be 10 mV peak-to-peak, which is 40dB less, or roughly -40 dBm. Cable path loss might be about 0.2 dB/m, giving a maximum length restriction of 200 metres. The near-end cross talk (NEXT) will be about -30 dB in many cases, so the full length cannot be used in duplex cables: ● (^) Perhaps cancellation of the known-to-be-transmitted signal can give another 10 dB of margin ● (^) But problematic if multiple pairs in the same cable are being driven at once, as in gigabit Ethernet. ● (^) High quality cabling have a tight NEXT specification (eg -44 dB for CAT 6), which may make the channel not limited by NEXT. Pick-up should be negligible in most cases, especially if shielded twisted pair or coax is being used. ● (^) But if operating next to an electric blast furnace or ● (^) EMP bomb testing range, there could be problems. [Optical fibre does not suffer from that!]
The Radio (or Wireless) Spectrum (or Ether)
