TRANSMISSION MEDIA
SYSC 4700
Telecommunications Engineering
David Falconer and Halim Yanikomeroglu
Carleton University
01 & 03 February 2016
Transmission Media
Objectives
– To learn some basic vocabulary: e.g. channels, circuits, trunks, dB, dBm,
dBw.
– To have an idea of the relative bandwidths of some common signals.
– Link budget.
– To learn the basic characteristics, advantages and limitations of common
transmission media.
Channels, Circuits, Trunks
• Channel: Refers to the medium (such as copper wire, radio,
coaxial cable, satellite, or optical fiber) used to convey
information from a transmitter to a receiver.
• Circuit: The complete path between two terminals over which
one-way or two-way communications may be provided. A circuit
may involve several channels. Ex: Twisted-pair copper + coax +
fiber + coax + twisted-pair copper.
• Trunk: A single transmission channel between two switching
centers or nodes, shard by many users.
Typical Bandwidths and Bit Rates Required
Analog Voice
• POTS (plain old telephone system): 300 to 3400 Hz
• AM broadcasting: 10 KHz
• FM broadcasting: 200 KHz
Digital Voice
• PCM DS0: 64 Kb/s
• compressed voice: 8, 16, or 32 Kb/s
• CD: 1.4112 Mb/s (2-channel, 16-bit PCM sampled at 44.1 KHz)
MP3 (MPEG-1 Audio Layer 3): 96 kb/s (FM quality)
192 kb/s (DAB quality)
Typical Bandwidths and Bit Rates Required
•
•
•
•
•
•
•
•
•
•
Analog TV video: 6 MHz
Digital compressed TV video: ~1.5 MHz (~1 Mb/s)
HDTV: 8-15 Mb/s (with MPEG-4 compression)
Video teleconferencing: 384 Kb/s.
Dial-up PC modem: v.34: 28.2 kb/s; v.90: 56 kb/s
High speed data: at least several Mb/s
Blu-ray: 40 Mb/s (max)
IEEE 802.11a, .11g: 54 Mb/s (shared peak rate)
3G Cellular: 2-40 Mb/s (shared peak rate)
4G Cellular: 100 Mb/s – mobile (shared peak rate)
1 Gb/s – nomadic (shared peak rate)
Two Wire and Four Wire Circuits
Four-wire trunk
Subscriber loop:
Multiplexing and switching node
Two-wire twisted copper pair.
Two-wire circuit: A full-duplex communications circuit that utilizes only two metallic
conductors, e.g., a single twisted pair.
Four-wire circuit: A two-way circuit using two paths so arranged that the respective signals are
transmitted in one direction only by one path and in the other direction by the other path.
Note: The four-wire circuit gets its name from the fact that, historically, two conductors were
used in each of two directions for full-duplex operation.
[Wikipedia] A two-wire circuit is characterized by supporting transmission in two directions
simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and
receive. In either case they are twisted pairs. Telephone lines are almost all two wire, while
trunks and switching are almost entirely four wire.
A four-wire circuit is a two-way circuit using two paths so arranged that the respective signals
are transmitted in one direction only by one path and in the other direction by the other path.
Late in the 20th century, almost all connections between telephone exchanges were four-wire
circuits, while conventional phone lines into residences and businesses were two-wire circuits.
Key Points
• Nowadays four wire circuits are used for all circuits except for
most subscriber loops.
• Four wire circuit can be 4 physical wires or a pair of frequency
channels or time slots (depending on transmission medium).
• Four wire digital transmission is prevalent today.
• Media: each type of transmission medium is advantageous in a
particular situation.
TRANSMISSION MEDIA
Subscriber loop:
Twisted copper pair cable.
Twisted copper pair.
Optical fibre.
Radio
Coaxial cable
Optical fibre
Microwave radio
Satellite
Multiplexing and switching node
MULTIPLEX/SWITCH NODE
TX
RX
MUX
To/from other trunks
DEMUX
SWITCH
To/from subscribers
What is wrong with the below figure?
The detail is lost for the small values of the vertical axis!
What is wrong with the below figure?
The detail is lost for the small values of the vertical axis!
Want to show large and small values on the same scale?
Logarithmic versus Linear Scale
What is wrong with the below figure?
The detail is lost for the small values of the vertical axis!
Want to show large and small values on the same scale?
Use logarithmic scale (not linear scale) in the vertical axis.
dB Notation
logc(a x b) = logc(a) + logc(b)
Decibel notation:
logc(a / b) = logc(a) – logc(b)
Field quantities:
Power quantities:
20 log10 (./.)
10 log10 (./.)
In this course: 10 log10 (.)
x  + (increased by 1,000,000 times  increased by 60 dB)
÷  - (decreased by 50 times  decreased by 17 dB)
A [unitless] = (10 log10 A [unitless]) [dB]
A [u] = (10 log10 A[u]) [dBu]
Linear dB
5000
37
400
26
10
10
8
9
5
7
2
3
1
0
0.5
-3
0.125
-9
0.01
-20
0.0005
-33
dB Notation
logc(a / b) = logc(a) – logc(b)
logc(a x b) = logc(a) + logc(b)
Decibel notation:
Field quantities:
Power quantities:
20 log10 (./.)
10 log10 (./.)
In this course: 10 log10 (.)
x  + (increased by 1,000,000 times  increased by 60 dB)
÷  - (decreased by 50 times  decreased by 17 dB)
A [unitless] = (10 log10 A [unitless]) [dB]
A [u] = (10 log10 A[u]) [dBu]
P [W] = (10 log10P[W]) [dBW]
P [mW] = (10 log10P[mW]) [dBm]
P [dBW] = (P+30) [dBm]
Ex: 2 [W] = 3 [dBW]
Ex: 2 [mW] = 3 [dBm]
Ex: 5 [dBW] = 35 [dBm]
10 log10SNR = (10 log10(Psignal [mW] / Pnoise [mW])) [dB]
10 log10SNR = (10 log10Psignal) [dBm] – (10 log10Pnoise) [dBm]
X [dBm] – Y [dBm] = Z [dB];
X [dBm] + Y [dB] = Z [dBm]
Linear dB
5000
37
400
26
10
10
8
9
5
7
2
3
1
0
0.5
-3
0.125
-9
0.01
-20
0.0005
-33
Decibel Unit
• A power ratio X/Y, expressed in decibels (dB) is 10 log(X/Y) =
10logX-10logY (note: X and Y must be proportional to power)
• dBm means power relative to 1 milliwatt; i.e. 10log(power in
milliwatts)
• dBw means power relative to 1 watt; i.e., 10log(power in watts).
• 0 dBw (meaning 1 watt) = 30 dBm.
Link Budgets
Transmitter
Channel
Receiver
Noise
Gain
GTX dB
Path loss PL dB (attenuation)
Transmit
power PTX dBW
Gain
GRX dB
Received power in dBW: PRX = PTX + GTX – PL + GRX (all quantities in dB scale)
Received power in watts: PRX = PTX GTX GRX /PL (all quantities in linear scale)
Note: Attenuation PL usually depends on distance and frequency.
Link Budgets
Transmitter
Channel
Receiver
Noise
Gain
GTX dB
Path loss PL dB (attenuation)
Transmit
power PTX dBW
Gain
GRX dB
Received power in dBW: PRX = PTX + GTX – PL + GRX (all quantities in dB scale)
Received power in watts: PRX = PTX GTX GRX /PL (all quantities in linear scale)
Note: Attenuation PL usually depends on distance and frequency.
Noise power in watts: k T B F (all quantities in linear scale)
where k = 1.38 x 10-23 (Boltzmann’s constant); T = 273+°C
Noise power in dBW: N = - 228.6 + 10log10(273 + C°) + 10log10(B) +F
where °C = temp. in degrees centigrade; B = signal bandwidth in Hz; F=noise figure in dB
Link Budgets
Transmitter
Channel
Receiver
Noise
Gain
GTX dB
Path loss PL dB (attenuation)
Transmit
power PTX dBW
Gain
GRX dB
Received power in dBW: PRX = PTX + GTX – PL + GRX (all quantities in dB scale)
Received power in watts: PRX = PTX GTX GRX /PL (all quantities in linear scale)
Note: Attenuation PL usually depends on distance and frequency.
Noise power in watts: k T B F (all quantities in linear scale)
where k = 1.38 x 10-23 (Boltzmann’s constant); T = 273+°C
Noise power in dBW: N = - 228.6 + 10log10(273 + C°) + 10log10(B) +F
where °C = temp. in degrees centigrade; B = signal bandwidth in Hz; F=noise figure in dB
Signal to noise ratio (SNR) in dB = PRX - N
SNR in linear: PRX/N
Link Budgets
Transmitter
Channel
Receiver
Noise
Gain
GTX dB
Path loss PL dB (attenuation)
Transmit
power PTX dBW
Gain
GRX dB
Received power in dBW: PRX = PTX + GTX – PL + GRX (all quantities in dB scale)
Received power in watts: PRX = PTX GTX GRX /PL (all quantities in linear scale)
Note: Attenuation PL usually depends on distance and frequency.
Noise power in watts: k T B F (all quantities in linear scale)
where k = 1.38 x 10-23 (Boltzmann’s constant); T = 273+°C
Noise power in dBW: N = - 228.6 + 10log10(273 + C°) + 10log10(B) +F
where °C = temp. in degrees centigrade; B = signal bandwidth in Hz; F=noise figure in dB
Signal to noise ratio (SNR) in dB = PRX - N
SNR in linear: PRX/N
Link budget: Determine minimum transmit power etc. which satisfy these
equations and give a required minimum SNR.
VF (Voice Frequency) Cable Paired Metallic Conductors
• Aerial (on poles): exposed to damage and environment
• Buried: more expensive to install; trouble locating
• Pro
– Compact, fairly high capacity (up to 1000 pairs per cable) - but low
compared to coax and optical fiber
– Quality controlled in factory
• Con
– High loss
– Subject to crosstalk interference from neighbouring pairs in same cable
Coaxial Cable
•
•
•
•
Aerial or buried
Large bandwidth (~1000 MHz), high capacity
Self shielding
Relatively expensive
Attenuation (dB\km.)
[Fig. 7.20 from
“Telecommunication
System Engineering”
2nd ed., by R.L.
Freeman, Wiley, Ny.
1989]
Frequency (MHz)
Attenuation of 0.375 in. coax cable
Summary
• Twisted copper pairs used in telephone subscriber loop plant:
– Attenuation and usable bandwidth depends on length and thickness.
– Many pairs in a cable in close proximity - subject to crosstalk interference.
• Coax used in CATV plant:
– Much lower attenuation and much higher usable bandwidth for same length.
– Insulated - little or no interference.
– More expensive and bulky, but can carry more payload.
Radio
•
•
•
•
Microwave Line of Sight (LOS)
Cellular, WLAN, WiMax, sensor, ad hoc, PAN
Dispatch, paging
Pro
– Low real estate and installation cost (don’t need to install wires or fibers to
every possible customer), good coverage
– Mobility
• Con
– Subject to interference and propagation disturbances
– Security, privacy a concern
Line of Sight (LOS) Radio Propagation
Free space path loss in linear scale: (4πd/λ)^2
Free space path loss (FSPL) in dB
=10 log10 (input power/output power)
= -147.6 + 20 log10 f + 20 log10 d
where frequency f is in Hertz and distance d is in metres.
PL is terrestrial radio links: PL = A + 20 log10 f + 10n log10 d
where n (>2) is the propagation exponent.
Non-Line of Sight Radio
Propagation
Diffraction
Reflection
Impulse response: Frequency response:
Obstruction
(shadowing)
Scattering,
diffraction
“Multipath”
Sample Link Budget for a Radio Link
Noise
Transmitter
Receiver
Distance d
Given: Signal centre frequency f = 4 GHz
Distance d = 20 km.
Total antenna gains and cable losses = Gt + Gr = 24 dB
Bandwidth = B = 10 MHz
Noise figure = 10 dB; Temp = 17°C
Required SNR = 20 dB
Sample Link Budget for a Radio Link
Noise
Transmitter
Receiver
Distance d
Given: Signal centre frequency f = 4 GHz
Distance d = 20 km.
Total antenna gains and cable losses = Gt + Gr = 24 dB
Bandwidth = B = 10 MHz
Noise figure = 10 dB; Temp = 17°C
Required SNR = 20 dB
Free space path loss (FSPL) in dB
= -147.6 + 20 log10 f + 20 log10 d = 130.4 dB
[f is in Hz and d is in m.]
Noise power in dBW = - 228.6 + 10log10(273 + C°) + 10log10(B) +F = -124 dBW
Required received power in dBW = SNR + Noise Power = 20 – 124 = -104 dBW
Required transmit power = Required received power + Path Loss - Gains
= -104 + 130.4 - 24
= 2.4 dBW = 1.74 watts
Shadowing
• Path loss variation due to obstructions, combined with distance
• Path loss (dB) =avg. path loss+random path loss
Avg. path loss
Path
Loss
(dB)
Measured path loss
Free space path loss
Distance (log scale)
Fiber Optic Cable
• Most long-haul trunks in Canada and U.S. are now carried on
optical fiber.
• Huge capacity.
• High installation cost (but low cost per circuit in large capacity
systems).
• Immune to electromagnetic interference.
• Low attenuation.
• Long life.
• Small size - makes better use of cable ducts.
• Excellent for digital signals.
Satellites
• Microwave frequency repeaters in geostationary orbits (~40,000
km.)
• Large coverage; e.g., all of Canada.
• Large capacity, high quality.
• Extensively used by TV networks.
• Capacity shared by multiple access.
• Excellent for providing fixed or mobile coverage in remote areas,
across oceans (although fiber is taking over).
Orbits (LEO, GEO, MEO)
• Low Earth Orbit (LEO)
500km-1500km from earth  low delay
Non-stationary, satellites rise and fall (orbit time ~ 90 minutes)
Smaller satellites with less transponders (~100 kg to 500 kg)
Closer  lower launch costs and requirements
Closer  smaller antennas, lower power requirements (ground terminals, satellite)
Need many, many satellites to provide global, uninterrupted coverage (example: original Teledesic
needed 800! Skybridge requires 80)
- Hand-off required, complex signal routing needed
-
• Geo-stationary Earth Orbit (GEO)
-
30,000 km - 40,000 km from earth  larger delay
Appear stationary to earth observer (Clarke’s orbit!)
Larger satellites with many transponders (~ 1 to 20 tons!)
Further  higher launch costs and requirements
Further  larger antennas, Kwatts of power requirements
Theoretically, can cover earth with 3 satellites
No-hand-off needed, simpler signal routing
• Medium Earth Orbit (MEO)
- 4,000-12,000 km from earth
Satellite Types
• Bent Pipe Satellites
– Simple Repeaters in the sky
– Receive a signal on frequency F1, amplify it, transmit it on frequency F2
– Almost all of today’s commercial satellites are bent pipe
• Regenerative Satellites
– Deploy on-board processing (OBP)
– Receive signals, demodulate them, make intelligent decision on where to
send them, modulate them and transmit them
– Restricted to a few military satellites, they are becoming the preferred type
of the next generation communication satellites
– Iridium is a Regenerative satellite
MEDIA COSTS AND BANDWIDTHS
Twisted pair
1000
Relative
transmission
cost per
circuit mile
Coax
Microwave
100
Optical fibre
10
1
10
100
1000
10000
100000
Number of equivalent voice circuits