Lecture #9: Wireless
Transmission. Telephone System.
Contents
 Physical basics of wireless signaling
2





radio waves 5
microwaves
6
8
infrared
9
light
Telephone system:

Structure 10
 Multiplexing 21
 Switching methods 38
1
Physical Basics in
Wireless Signaling



Electromagnetic waves, frequency (f)  Hz and
length (l)  m
Broadcasting and receiving of the electromagnetic
waves - antennas
Vacuum speed of the electromagnetic waves (c).
Speeds of the electrical and light pulses in conductors.
Main relation:
lf = c
(i.e.:
l[m] 104
f[Hz] 30k

1km 100
300k 3M
10
30M
1m
1dm
300M 3G
1cm
30G
1mm
300G)
Differential form of f(l) = c/l is:
f=c/l | d/dl  df = -(c/l2)dl i.e. for finite differences
Df =
(c/l2)Dl
2
Physical Basics in
Wireless Signaling

Since Df = (c/l2)Dl it is possible to calculate from the
physical diagrams the frequency bandwidth of given
material or media: on
Frequency
bandwidth

(2/6) for Dl one has:
l
0.85 mm
1.3 mm
Dl
0.08 mm
0.17 mm
0.185 mm
Df
33.22 THz
30.18 THz
23.1 THz
1.55 mm
Data rate[b]  Df[Hz] and vary from 1b/HzLF to
30-50b/Hz >UHF
3
Electromagnetic Spectrum

Electromagnetic spectrum:
2/11
RadioMicrowave  Infrared  Light  UV  X  Gamma.


ITU standard “radio” bands
Application of bands and correspondence to the
transmission media
International Telecommunication
Union - Radio Communication Sector/
World Agency for Radio Communications



Spectrum allocation: ITU-R/WARC and FCC
Narrow frequency band (Df << f)
Spread spectrum (military but also business
applications; regular hops between frequencies in
wide band for higher security/privacy)
4
Radio Transmission
Radio-frequency waves are:


easy to generate
with high penetration (buildings, forest,
but not hills/mountains)

omnidirectional

with mutual interference

with frequency-dependant properties:
– power losses with the distance
– absorption and reflection (by LF/MF and
HF/VHF) 2/12
5
Microwave Transmission
Microwaves have:

straight spreading (require straight line connection between
the transmitter and receiver: on the Earth surface they need
repeaters on each 100 km)

possibility of ray concentration (focusing)

no interference between non-collinear transmissions



low penetration (buildings and even rain are non
transparent)
possible refraction in the dense atmosphere layers 
multipath fading (phase shifting)
frequency-dependant properties:
–power losses with the distance
–absorption by water (f above 8Ghz/l 40cm and less )
6
Microwave Transmission
Microwave applications:

long-distance phone transmissions (incl. POTS
and cellular phones): mW compete in this area
with fiber optic.
– Advantages: cheap, non-wired communication
– Drawbacks:
crowded
spectrum,
possible
interference with other devices, low level of
security, possible atmospheric disturbance

non licensed free bands
– 902  928 MHz - industrial use (cordless home
devices and distanced controllers); cheapest
transceivers (transmitters or receivers) and lower
level of interference
– 2400  2484 MHz
– 5725  5859 MHz
7
Infrared Waves

1012  1014 Hz band
(invisible light with thermal impact).

Properties:
–
–
–

do not pass solid or non-transparent objects
strong attenuation in relatively short distance of
air
can be focused in a ray but also dispersed in
omnidirectional way
Application
–
–
house remote control devices
implementation of indoor wireless LANs (of
portable computers) and communications.
8
Light Waves





Visible light is called optics.
Optic signals are transmitted usually in form
Light Amplification
of coherent directed light rays by Stimulated Emission of Radiation
(transmitters called LASERs).
Unidirectional transmission needs very precise
alignment between the transmitter and receiver
(photodetector); often application of defocusing
lenses
8
Same Properties like infrared rays
Atmosphere distortions
2/13
9
Telephone System

Public Switched Telephone Network (PSTN) used to for communication media between
computers that cannot share one common LAN
because they are:
– of large number;
– too distanced;
– separated by other property.

Communication properties of switched phone
lines:
– low data rate - 104 b/S (108 b/S for direct wire)
– high error rate - 10-5 Err/b (10-12 Err/b for direct
wires)
10
Telephone System Structure

2/14

2/15
Development of the Telephone system from
non-switched fully interconnected graph to
centralized switching system and then to
hierarchical system of [routable] switches
Structure of the connectivity of the hierarchical
switched telephone system:
– local loops  twisted pair  analog
– toll connecting trunks  coax, mW, fiber  analog,
digital
– intertoll trunks  coax, mW, fiber  digital, analog
11
Telephone System Transmission Technologies
• The transmission and switching are shifting towards fully optical
systems:
all electric
optical transmission
electric switching
all optical
12
Analog Transmission
In analog transmission the physical
quantity of the signal carrying the information
(amplitude, frequency, phase) varies in a direct
relationship to the information to be
transmitted.
 The nature of information transfer is
continuous.

13
Digital Transmission






In digital transmission the continuous signal is first
sampled by slicing the signal at regular intervals into
discrete values.
Signal samples are then quantified, their amplitudes
are approximated to the nearest pre-defined value.
Quantified samples are coded into binary words.
After this, the binary words are transmitted.
The length of a coded binary sample depends on that
how accurate the quantization must be.
For example, if the signals are presented with 256
different (voltage) levels, the coded binary word is 8
bits long (28 = 256).
14
Digital Transmission
Samples
Quantization
011
100
010
Binary words
15
Telephone System - Digital
vs. Analog Signaling

Digital vs. Analog signaling in Telephone system:
–
–
–
–
–
–
–
stronger attenuation but
possibility for multiple consecutive signal regeneration
possibility congestion of data rates on the lines
possibility for 1:1 data transmission (incl. image, voice,
stored data) between two points (i.e. high QoS)
easier equipment implementation
possibility for automation of maintenance
the ability to add redundant information (e.g. for error
detection and correcting purposes) to the true
information stream …
17
16
Telephone System - Digital
vs. Analog Signaling

Digital vs. Analog signaling in Telephone
system - continue:
–
–
not so sensitive for interference as the analogue
transmission.
the binary representation of the signal can be
transmitted, for example, by frequency
modulation when a frequency F1 represents the
binary digit ‘0’ and a frequency F2 is associated
with the binary ‘1’.
17
Local Loops by Intercomputer Communication


Doubled digital-to/from-analog conversion in local
loops of switched lines
Main distortions:
2/17

attenuation A(log10d, f)  dB/km - signal power losses
and signal shape wrapping

delay - different for the signal frequency components 
overlapping of the components of different consecutive
bits

noise - caused by line/relay commutations and
junctions, signal inductive interference or random
energy sources
18
Local Loops Modulation/Demodulation
Modems: Digital-to-analog-and-analog-to-digital
converters. Analog coding is called modulation.
Digital [de]coding is called demodulation.
 Analog signal is transmitted by sine carrier of
f = 12kHz. Modulation (i.e. analog coding) can
2/18 be performed by carrier tension amplitude, by
carrier frequency shifting or by carrier phase
shifting. Nyquist’s Theorem boundary: 3kHz
local loop bandwidth transmits up to 6kHz carrier
 Advanced modulation techniques transmit multiple
(3-4) bits per baud by combination of amplitude
2/19 and phase shifting (QAM - Quadrature Amplitude
Modulation).
19

Local Loops - Circuitry

Electrical Modem/Computer interfaces
(DCE/DTE interface):
– RS232C (CCITT: V.24 standard): serial interface,
25 pin connectors including lines for in/out bit
stream, control, synchronization, clock. Null
modem - crossed transmit/receive data lines for
computer/computer (no modem) interface.
– RS232 variations ( RS422, RS423, RS 449 difference in ground lines for the signals)
2/18

Optical interfaces - for arising new telecom
services
 FTTH (Fiber To The Home) - straight but expensive
 FTTC (Fiber To The Curb) - fiber to junction box (for
20
group of users); end connection[s] is twisted pair or coax.
Multiplexing


Multiplexing by Physical layer means splitting off (and
thus sharing) the media transmission capacity
between multiple users/processes/channels
2 methods:
– multiplexing by frequency - FDM. Frequency spectrum is
divided into smaller bands which are assigned to each
active user. Example: Radio broadcasting stations over fixed
frequency channel (static assignment/multiplexing)
– multiplexing by time - TDM. The channel time is split into
[small] even periods, that are repeatedly assigned to a fixed
(for given instant) list of active users/channels - in roundrobin manner
21
Frequency Division
Multiplexing





Single phone voice-grade channel seizes 3kHz band
(sound transmission in band 50/3000 Hz)
By FDM 4kHz band is allocated for each channel (3kHz
signal + 2*500Hz separation bands)
The standard FDM: group of 12 channel bands *
4kHz placed in band 60108kHz (60 + 12*4 = 108) . Option: second group in the band 1260kHz. 2/24
Overlapping between the adjacent channels (although
the separation bands) because of nonperfect filtration.
Mastergroup = 5 supergroups = 25 groups = 300
vice-grade channels.
22




Frequency Division
Multiplexing
FDM variation used in fiber optics: WDM - Wavelength
Division Mux.
Same physical principle: junction of two light transmitting
fiber by a diffraction grid (e.g. optic prism) in one fiber
transmitting the combined (two wavelengths/frequencies)
signal to a common end point. The signal separation at the
end point by diffraction grid.
2/25
WDM properties:
– passive system (no frequency/wavelength shifting)
– reliability
WDM based switches - light switching is harder then
electricity switching. 1:n light switches are either of type
“Passive star” (for n~102) or some kind of optical tunable
filters like interferometrs (for n~106)
[24] 23
[Interferometers]

Interferometers isolate a specific portion of the
electromagnetic spectrum. Unlike prism or grid
monochromators, interferometers are not dispersive
instruments, but use interference to selectively transmit a
certain wavelength. Two interferometer designs:
Schematic of a MachZender interferometer
Schematic of a
Fabry-Perot etalon
24
WDM



WDM is the technique for fiber optics that uses a similar
principle than FDM, except the channel discriminator is a
wavelength instead of time.
Each input data stream is converted into separate
wavelength. Each application creates a channel that operates
at a separate wavelength. After that, the WDM system
combines and at the same time transmits the channels
through the same optical fiber. Since each wavelength is
completely isolated from the other, protocols can be mixed
within the same link.
This is a great benefit to WDM, since FDM creates highspeed time slots in the form of frames or cells, which allow
multiple applications to share the channel only if all
applications are of the same platform.
25
WDM



Stable WDM systems today multiplex just a few
wavelengths over fiber to extend capacity. However, some
systems can now increase a fiber's capacity by
multiplexing 32 wavelengths and more. Still newer
multiplexers entering the market this year promise 40 to
80 discrete optical channels.
WDM is a costly solution, although it works with the
existing fiber infrastructure, since it requires additional
equipment like transmitters and optical amplifiers for each
wavelength. That equipment is quite expensive nowadays.
The WDM market is predicted to increase dramatically over
the next ten years as telephone companies, cable
television and other carriers maximize their optical-fiber
26
network capacity.
WDM

Besides the telephone system trunks
WDM is suitable for future LAN
network environment:
ATM
...
ETHERNET
ATM
...
ETHERNET
WDM
WDM
TOKEN
RING
FDDI
FIBER
CHANNEL
FDDI
TOKEN
RING
FIBER
CHANNEL
27
Time Division Multiplexing
Applicable only in digital electronics (while
FDM is applicable only for analog data) - i.e.
TDM condenses just digital data for the
telephone company trunks.
 Digitizing the analog signal by codec
(Coder-DECoder).
 PCM - Pulse Code Modulation
 In the telephone systems PCM is performed
with rate 8000S-1 samples in periods (i.e.
8kHz) of 125mS. According Niquist’s theorem
8kHz = 2*4kHz (telephone channel
bandwidth - for avoiding information losses
28
and information overload).

Pulse Code Modulation
T1-carrier Method (USA, Japan): multiplexes 24
voice
channels
in
Round-robin
for
each
125mS
2/26
period [By data communication the 24th channel is
reserved for a synchronization pattern].

– The instant state of each channel is coded by 7b + 1b
control.
– Transmission rate per each channel is 64kb/S
(=8000*8b; 56kb/S data + 8kb/S control)
– Frame size 193b (=24*8b + 1b framing code)
– Transmission rate per multiplexed channel 1.544Mb/S
(=193b*8000)

E1-carrier Method: multiplexes 32 channels (30
data + 2 control/synchronization) in Round-robin
for each 125mS period .
– Transmission rate per multiplexed channel 2.048Mb/S
29
(=32ch*8b*8000)
Pulse Code Modulation


2/27
Differential PCM - codes not the amplitude but
the difference between the two latest
amplitudes. Differences of 7b-coded magnitude
(0127) can be coded with less bits (i.e. 5b
code difference 0  32 - 25% of the maximum
of the magnitude for single sampling step).
Delta PCM - Differential PCM modification - 1b
code for the difference: “0” means
magnitude[t] = magnitude[t-1]-1; “1” means
magnitude[t] = magnitude[t-1]+1.
– Possibility for delay of the modulated signal to the
analog original.

Predictive PCM - Delta PCM modification magnitude[t] = magnitude[t-1, t-2,...]
30
Hierarchy of the Time
Division Multiplexing

Higher order multiplexing of the sequential
bits of T1 channels - Round-robin for
2/28
several T1 channels

USA standard hierarchy:
24ch  T1 4*T1T2
6*T2T3
Known as PDH (Plesiochronous
Digital Hierarchy). Coax, fiber
and radio links are applied in
PDH connections over 2 Mbps
7*T3T4
1.544Mb/S 6.312Mb/S 37.400Mb/S 260.500Mb/S

CCITT (ITU-T) standard hierarchy:
32ch  T1 4*T1T2
4*T2T3
4*T3T4
4*T4T5
2.048Mb/S 8.848Mb/S 34.304Mb/S 139.264Mb/S 565.148Mb/S
31
SONET Standard





SONET (Synchronous Optical NETwork) - standard
for optical long-distance telephone traffic (CCITT
[ITU-T] equivalent: SDH - Synchronous Digital
[33]
Hierarchy)
SONET is the North American standard for
synchronous multiplexing that corresponds to
SDH
ITU-T has based the standardization work of
SDH on SONET standards
In principle, SDH is an international extension
of SONET
The terminology in SDH and SONET is similar
32
and in some cases identical
SDH

Synchronous Digital Hierarchy (SDH)
standardized by ITU-T has several improvements
compared to the PDH:
– Direct pointer based access to data without need of layers
of hierarchical branching equipment
– Centralized remote control of network elements
– Increased use of the physical network
– Shorter delivery time for leased lines


SDH is a standardized multiplexing hierarchy for both
plesiochronous and synchronous purposes.
The multiplexing levels in the SDH can be divided
into two groups:
– multiplexing levels (virtual containers, VC) and line signal
levels
– (synchronous transport modules, STM).
33
SONET Standard

SONET defines
– internetworking parameters: wavelength /
frequen-cies, timing and framing
– Operations, Administrations and Maintenance
(OAM) support - tools and procedures.


SONET is synchronous system with one
master clock and TDM data congestion
method (i.e. one carrier frequency in entire
bandwidth) standard bit lasting varies 10-7%.
SONET is channel switching model. [ATM is
cell switching model with asynchronous
arrivals of cells].
34
SONET System Model
SONET system structure: fiber-connected
multiplexers, switches and repeaters.
2/29
 Sections, lines and paths.
 Physical topology is mesh; logical topology is
ring (any path).
 SONET frame: 810B/6480b frame, frame
?
interval 125mS i.e. 8000frames/S = 6.48MB/S
Why Path overload
is aligned with SPE = 51.84Mb/S channel speed - STS-1
generated and
and Section/Line
generated and
read
at the start/
Signal-1).
overloads are aligned(Synchronous Transport
read at the start/
end of the section
with the frames
end of the line
 Each SONET frame contains:

2/30
– Header of Section overhead followed by Line overhead
and Path overhead.
– Payload [SPE (Synchronous Payload Envelope)] - user data
(might be empty!) pointed by the 3 leading bytes of the Line
35
overhead. (SPE contains 9B of Path overload spread by each
SONET Multiplexing

2/31
1st stage: The standard T1, 2, 3 input streams are
converted to standard STS-1 channel (rounded to the
STS-1 rate of 51.84 Mb/S). The input channels to a
multiplexer are called tributaries. Multiplexing is Byte-level
Round-robin of all the tributaries. Further steps of
multiplexing:
– 3 of STS-1 tributaries are multiplexed to STS-3 155.52 Mb/S channel rate
– 4 of STS-3 tributaries are multiplexed to STS-12 622.08 Mb/S channel
SONET
rate
and SDH
multiplex – 4 of STS-12 tributaries are multiplexed to STS-48 2.4883 Gb/S channel
rate
data rates

2/32
2nd stage: scrambling (disarranging
the components of the
transmission in order to make unintelligible to interception and avoid long
sequences of “0” or “1” states to run out of the synchronizing clock; back
arranging based on header information)
3rd stage: Electric-to-optic conversion to optical carrier
signal denoted OC-n. (when carries STS-n channel) E.g.
OC-3 (starting), OC-9, -12, -36, -48.
36
SONET Physical Layer

2/33
4 sublayers architecture:
– Photonic sublayer specifies the physical
properties of the light rays and fiber guides.
– Section sublayer transmits standard frames
between each two consecutive repeaters by
amplifying and regeneration of the single bits.
– Line sublayer multiplexes several tributaries in
a channel and demultiplexes it in the next
consecutive multiplexer. Repeaters are
transparent to this sublayer.
– Path sublayer organize end-to-end connection
between source and destination multiplexers.37
Switching Methods - Circuit
Switching

Circuit switching - establishment of temporary
dedicated physical connection from-end-to-end before
and during the whole transfer. The connection exists:
– from the moment of realization of the transmission initiative
– till the moment of request for end of the transfer by either of
the parties (or for some technical reason/problem).

2/34

Structure of circuit switching: user device
(phone/computer…); end switching office; hierarchy of
intermediate switching offices.
Time diagram of the processes by circuit switching:
– propagation time for each signal (in copper 5mSm-6)
– request/acknowledge signals before setting up the connection
(by phones ack={signal, vabusy}); request delayed by:
2/35
• time for propagation
• time for finding outgoing trunks in the switching offices

Dedicated channel: no possibility of interruptions and
clipped signal until the end of the connection
38
Switching Methods –
Connectionless Switching

2/34b

– propagation time for each message
– queuing delay in the consecutive routers (interactive
communications (e.g. phone calls) have priority over noninteractive ones  undefined delays for intercomputer
communications; possibility for perceivable delays by interactive
communications.
2/35b


2/35c
Message switching: transmission of arbitrary long
buffered blocks - “messages” - to the next switching
office which is actually a router.
Time diagram of the processes by message switching:
Packet switching: transmission of small fixed size
buffered blocks - “packets”. Demonopolization of the
channels capacity
Time diagram of the processes by packet switching:
– propagation time for each packet (shorter than by the message sw.)
– channel congestion, reduced delay; possible perceivable brakes in the
39
interactive communications
Connection-oriented vs. Connectionless [Packet] Switching
40
Switching Systems Hierarchy

Basically tree structure of hierarchy
levels (AT&T  5 levels) but:
– the nearer the root - the denser
connections in that level (up to full graph
of connections)
– interlevel shortcuts (“direct trunks) for
overloaded routes
2/37

Service Access Points are at the lowest
hierarchy level (at tree leaves)
41
Switching Systems - Crossbars

nxn (in parallel computers: nxm) crossbar
(“crosspoint”) connects n inputs full duplex to n
2/38 outputs - one stage switch.



The matrix of elementary switches has n2
components.
The elementary switches are automatically
controlled in manner : in:om requires im:on; no
impossible combinations (full graph of
connectivity)
Delays
– by switching - hardly depends on hardware; down to
mS
– by signal propagation - no delay (commutation
42
independence)
Switching Systems InterLAN Crossbars
43
Switching Systems Multistage Switches







Multistage switches consist of s stages
Each stage i consists of of ni independent
n:k (e.g. 2:2, 4:4) crossbar elements
(CE)
Static interstage connection scheme
(usually so called Banyan switching)
NO intrastage and interstage loops
Less hardware than crossbars: 8:8
crossbar needs 64 elementary switches;
8:8 Banyan multistage needs 12 CE by 4
elementary switches = 48, BUT
Possible rejected connections
Delays:
– by switching = delay for one
switching element
– by signal propagation = delay for one
switching element x s
44
Switching Systems - Time
Division Switching

2/40

Components: n-input multiplexer; n-slots interchange
buffer with associated n-pointers mapping table and noutput demultiplexer
Switching method:
 Round robin scanning (multiplexing) of n input lines and storing
the content in the indexed buffer
 Reordering the contents of the buffer according to the scheme
in the mapping table in n steps [for double space buffers: in one
step  input and output process overlap - “conveyer”]
 Output the contents of the reordered buffer to n serial output
lines (demultiplexing)

Delays:
– by reordering: for non-conveyered switching the
delay depends on n; for conveyered switching the
delay depends on the R/W RAM cycle of the buffer
45
Electromagnetic spectrum
46
Radio Transmission
47
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