Telecommunications

Panko’s
Business Data Networks and Telecommunications, 6th edition
Copyright 2007 Prentice-Hall
May only be used by adopters of the book
Telecommunications

6-2
Telecommunications
• From
– Data communications
– Telecommunications: Voice and Video Communications

Technical Elements of
the Public Switched
Telephone Network

6-4
Figure 6-1: Elements of the Public Switched
Telephone Network (PSTN)
1. Customer Premises
Equipment
1. Customer Premises Equipment

6-5
Figure 6-2: Customer Premises Equipment
PSTN
PBX
Handset
4-Pair UTP
Telephone Wiring
Site
A typical business site.
The private branch exchange is an internal switch for the site.
4-pair UTP was created for business premises telephone wiring
Company is essentially its own telephone company that connects to the
outside PSTN

6-6
2. & 3. End Office
Switch (Class 5)
2.
Access Line
(Local Loop)
2.
Access Line
(Local Loop)
The Access System consists of
the access line to the customer
(called the local loop)
and termination equipment at the end office
(nearest telephone office switch).

6-7
3. Transport Core
3. Trunk
Line
3.
Switch
The Transport Core connects end office
switches and core switches.
Trunk lines connect switches.

6-8
Figure 6-1: Elements of the PSTN
• Telephone Company Switch

6-9
4. Signaling System
Transport is the actual transmission of voice.
Signaling is the control of calling
(setup, teardown, billing, etc.).
SS7 in the United States
C7 in Europe

6-10
Figure 6-3: Points of Presence (POPs)
In the U.S., competing
carriers connect at
points of presence (POPs).
Local Access and Transport Area (LATA)
Local
Carrier 1
Switch
Local
Carrier 2
Switch
POP
Local
Carrier 1
Customer
Local
Carrier 2
Customer
Other Local Area
Other Country
POP
Long-Distance
Carrier A
International
Carrier X

6-11
Figure 6-4: Circuit Switching
A circuit is an end-to-end
connection between two subscribers.
Capacity is reserv ed on all
trunk lines and switches along the way .
Capacity must be paid f or ev en if it is not used.
The PSTN
has traditionally used
circuit switching.

6-12
Figure 6-5: Voice and Data Traffic
Full-Duplex (Tw o-Way) Circuit
Voice Traffic:
Fairly Constant Use;
Circuit Sw itching Is
Fairly Efficient
Data Traffic:
Short Bursts,
Long Silences;
Circuit Sw itching Is
Inefficient
Full-Duplex (Tw o-Way) Circuit
The reserved capacity of circuit switching
is OK for voice, but not for bursty data transmission.

6-13
Figure 6-6: Dial-Up Circuits Versus Leased Line
Circuits
Dial-Up Circuits Leased Line Circuits
Operation Dial-Up. Separate
circuit for each call.
Permanent circuit,
always on.
Speed for Carrying
Data
Up to 56 kbps
Residence can only
Send up to 33.6 kbps
56 kbps to gigabit
speeds
Number of Voice
Calls Multiplexed
One Several due to
multiplexing
There are two types of circuits between customer premises:
ordinary dial-up circuits and leased line circuits.

6-14
Figure 6-7: Local Loop Technologies
Technology Use Status
1-Pair Voice-Grade
UTP
Residences Already installed
2-Pair Data-Grade
UTP
Businesses for
Lowest-speed
access lines
Must be pulled to the
customer premises
(this is expensive)
Optical Fiber Businesses for
higher-speed
access lines
Must be pulled to the
customer premises
(this is expensive)
Residential 1-pair voice-grade UTP is already installed.
This makes it inexpensive to use
Business 2-pair data-grade UTP and fiber for leased lines
must be installed; this is expensive.

6-15
Figure 6-8: Analog Telephone Transmission
Sound
Wave
Analog
(Analogous)
Electrical Signal
Analog signals rise and fall in intensity with the human voice.
No resistance to errors as there is in digital transmission.
Initially, the entire PSTN was analog.

6-16
Figure 6-9: The PSTN: Mostly Digital with Analog
Local Loops
Trunk Line
(Digital)
Local
Loop
(Analog)
Local
Loop
(Digital)
PBX
(Digital)
Residential
Telephone
(Analog)
Today's Telephone Netw ork: Predominantly Digital
Switch
(Digital)
Switch
(Digital)
Switch
(Digital)
Today, everything is digital except for the
local loop access line and residential telephones.
The actual local loop line can carry either analog or digital signals,
but the equipment at both ends is analog.

6-17
Figure 6-10: Codec at the End Office Switch
Analog Signal
Local Loop
Telephone
Home
Codec
DAC Digital Switch
ADC
End Office
Digital
Signal
A codec at the end office translates between
residential analog and PSTN digital signaling.
ADC = analog to digital conversion
DAC = digital to analog conversion

6-18
Figure 6-11: Frequency Division Multiplexing
(FDM) in Microwave Transmission
Channel 1 / Circuit A
Channel 2 / Circuit D
Channel 3 / Circuit C
Channel 4 / Unused
Channel 5 / Circuit E
Each circuit is sent in a separate channel.
If channel bandw idth is large,
there w ill be few er channels.
Voice uses 4 kHz-w ide channels
to allow more channels.
Microwave uses
radio transmission
for PSTN trunk lines
Box:
Codec Operation

6-19
Figure 6-12: Analog-to-Digital Conversion (ADC): Bandpass
Filtering and Pulse Code Modulation (PCM)
Subscriber
Analog Electrical
Signal
Analog Voice
Signal
Filter at
End Office Switch
Step 1: Bandpass Filtering
At the end office, the voice signal is bandpass-filtered
to limit its bandwidth to 4 MHz.
This permits more calls to be multiplexed on trunk lines
Filter at
End Office Switch
Box:
Codec
Operation

6-20
0 Hz
Signal
300 Hz 3,400 Hz (3.4 kHz) 20 kHz
Frequency
Bandw idth (3.1 kHz)
Energy Distribution of
Human Speech Along the
Frequency Spectrum
Step 1: Bandpass Filtering
Actually, to provide a safety margin, the signal
is filtered to between about 300 Hz and 3.4 kHz
instead of from 0 Hz to 4 kHz.
Box:
Codec Operation

6-21
Signal
Amplitude
Time
Analog
Signal
Sample
Intensity of Sample
(125/255 or 01111101)
Duration
of Sample
(1/8000 sec.)
0
255 (maximum)
Step 2: Pulse Code Modulation (PCM) Sampling
Nyquist found that signals must be
sampled at twice their highest frequency.
For a top frequency of 4 kHz,
there must be 8,000 samples per second.
Each sample is 1/8000 second.
Box:
Codec Operation

6-22
Signal
Amplitude
Time
Analog
Signal
Sample
Intensity of Sample
(125/255 or 01111101)
Duration
of Sample
(1/8000 sec.)
0
255 (maximum)
In each sampling
period, the intensity
of the signal is
measured.
In pulse code
modulation, the
signal is measured
as one of 256
intensity levels.
One byte stores
one sample.
Box:
Codec Operation

6-23
Signal
Amplitude
Time
Analog
Signal
Sample
Intensity of Sample
(125/255 or 01111101)
Duration
of Sample
(1/8000 sec.)
0
255 (maximum)
Pulse Code
Modulation (PCM)
produces
8,000 one-byte
samples per second.
This is 64 kbps
of data.
Box:
Codec Operation

6-24
ADC Recap
• First, Bandpass-Filter the Incoming Signal to 4 kHz
– Really about 300 Hz to 3.4 kHz
– To reduce transmission requirements
• The Codec then Uses PCM for the Conversion
– Samples at twice the highest frequency (4 kHz so 8,000
samples/second)
– Loudness is recorded with 8 bits per sample (to give 256
loudness levels)
– Generates 64 kbps of traffic (8 bits/sample times 8,000
samples per second)
Box:
Codec Operation

6-25
Figure 6-13: Digital-to-Analog Conversion (DAC)
To Customer:
Generated “analog” signal
(Sounds smooth because
the sampling rate
is very high)
From digital PSTN network:
Arriving digital signal
from the PSTN Core
(8,000 Samples/Second)
00000100 00000011 00000111
DAC
at End
Office
Switch
One 8-Bit
Sample
One 8-Bit
Sample
Box:
Codec Operation

6-26
Figure 6-14: Cellular Telephony
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel
47
Mobile Telephone Sw itching Office
Cellsite
PSTN
HandoffIn cellular technology, the region
is divided into smaller cells.
In each cell, a cellsite serves
cellphones in the cell.

6-27
• Cellsites

6-28
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel
47
Cellsite
PSTN
Handoff
Channels can be reused in different cells.
Channel reuse supports more customers.
This is the reason for using cells.
(Channel 47 is reused in cells A, D, and F)

6-29
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel
47
Cellsite
PSTN
Handoff
When a subscriber moves from one
cell to another in a cellular system,
this is called a handoff.
When a subscriber moves from
one city to another, this is roaming.
(In WLANs, handoffs and roaming
mean the same thing.)

6-30
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Channel
47
Cellsite
PSTN
Handoff
The Mobile Telephone
Switching Office (MTSO)
coordinates the cellsites and
implements signaling and handoffs.
The MTSO also connects
cellphones to the PSTN
(called the wireline network).

6-31
Cellular Technologies
• GSM is the worldwide standard for cellular voice
– Uses time division multiplexing (TDM)
– Uses 200 kHz channels
– Divides each second into many frame periods
– Divides each frame into 8 slots
– Gives same slot in each frame to a conversation
Slot 1
Conversation
A
Slot 2
Conversation
B
Slot 8
Conversation
H
……
Slot 1
Conversation
A
Time Frame 1 Frame 2

6-32
• Cannot use the same channel in adjacent cells
– So can only reuse a channel about every 7 cells
– For example, suppose there are 50 cells
• Channel can be reused 50 / 7 times
• This is 7 (not precise, so round things off)
• So each channel can support 7 simultaneous
customers in these 7 cells

6-33
• Code Division Multiple Access (CDMA)
– Also used in the United States
– A form of spread spectrum transmission
– Unlike traditional spread spectrum technology, multiple
users can transmit simultaneously
– 1.25 MHz channels
– Can support many users per channel
• Can use the same channel in adjacent cells
– So can only reuse a channel in every cell

6-34
Figure 6-15: Voice over IP (VoIP)
PC with
Multimedia Hardw are
and VoIPSoftware
IPTelephone
w ith
Codec and
TCP/IPFunctionality
Media
Gatew ay
PSTN
Internet
VoIP carries telephone calls over
LANs and the Internet
With IP, there is no wasted capacity
as there is with circuit switching.
This reduces cost.

6-35
PC with
and VoIPSoftware
IPTelephone
w ith
Codec and
TCP/IPFunctionality
Media
Gatew ay
PSTN
Internet
Stations can be special IP telephones
with IP functionality
Or a PC with multimedia hardware
and VoIP software
IP phones need a codec to convert
voice analog signals from the microphone
into digital IP signals

6-36
PC with
and VoIPSoftware
IPTelephone
w ith
Codec and
TCP/IPFunctionality
Media
Gatew ay
PSTN
Internet
A media gateway connects a
VoIP network to the PSTN
Handles transport and signaling differences

6-37
Figure 6-16: Speech Codes
Codec Transmission Rate
G.711 64 kbps (pulse code modulation)
G.721 32 kbps (adaptive PCM)
G.722 46, 56, or 64 kbps
G.722.1 24, 32 kbps
G.723.1A 5.3, 6.3 kbps
There are several codec standards.
They differ in transmission rate, sound quality, and latency.
Both sides must use the same codec standard.

6-38
Figure 6-17: VoIP Protocols
Transport is the transmission of voice
(carries codec data).
Signaling is call supervision.
Signaling: SIP or H.323
(Call setup, breakdown, accounting, and other superv isory tasks)
IP
Hdr
UDP
Hdr
RTP
Hdr
Codec Data
Stream
Transport
(Voice transmission)PC with Multimedia and
VoIP Sof tware
IP Telephone
VoIP Transport Packet

6-39
IP
Hdr
UDP
Hdr
RTP
Hdr
Codec Data
Stream
Transport
VoIP Sof tware
IP Telephone
2. The UDP header is followed by a
Real Time Protocol (RTP) header, which contains
a sequence number and timing information.
Receiver uses timing information to smooth out sound playback.
1. VoIP transport packets use UDP at the transport layer.
(There is no time for retransmissions to repair errors.)
The receiver puts in fill sounds for lost packets.
3.
The application
message is a
codec data
stream

6-40
IP
Hdr
UDP
Hdr
RTP
Hdr
Codec Data
Stream
Transport
VoIP Sof tware
IP Telephone
Signaling is call supervision.
The H.323 signaling standard came first for VoIP signaling.
SIP is simpler and now dominates VoIP signaling

6-41
Video over IP
• The Other VoIP
– It’s not just voice over IP
– Video Telephones
– Video Conferencing
• PC to PC
• Multiparty
• Sometimes room-to-room
– Video Downloads on Demand

6-42
Figure 6-18: Residential Internet
Access Services
• Telephone Modems
• Broadband Internet Access
• Asymmetric Digital Subscriber Line (ADSL)
• Cable Modem Service
• 3G Cellular Data Service
• WiMAX (802.16d and 802.16e)
• Broadband over Power Lines
• Fiber to the Home (FTTH)
Note:
Speeds and Prices
Change Rapidly

6-43
Figure 6-19:
Telephone Modem Connection to an ISP
Client A
Telephone
Modem
Telephone
PSTN (Digital)
33.6 kbps
Digital
Analog
Access
Line
Analog
56 kbps
Telephone modems
convert digital computer
signals to analog
telephone signals.

6-44
Figure 6-19:
Digital Leased Line
(No Modem)
ISP
PSTN (Digital)
56 kbps
Digital
33.6 kbps
ISP does not have a modem.
It has a digital leased line so
can send at 56 kbps.
(There is no bandpass
filtering on digital leased lines.)

6-45
Figure 6-19:
Client A
Telephone
Modem
Digital Access Line
(No Modem)
Telephone
ISP
PSTN (Digital)
33.6 kbps
56 kbps
Digital
Analog
Access
Line
Digital
Analog
56 kbps
Circuit
Dial-up circuits connect the client with the ISP.
56 kbps downstream, 33.6 kbps upstream
33.6 kbps

6-46
Telephone Modem Limitations
• Very low transmission speeds
– Long delays in downloading webpages
• Subscriber cannot simultaneously use the
telephone line for voice calls
• Still used by 30% to 40% of Internet users.

6-47
Figure 6-20: Amplitude Modulation
Client A
Binary Data
Modem
Amplitude Modulation
Telephone
Modulated Analog
Signal
1 0 1 1
PSTN
Serial
Cable
Telephone
Cable
Modulation is the conversion of binary computer signals
into analog signals that can travel over an ordinary access line.
Demodulation, at the other ends, converts the modulated
signals back to digital computer signals.

6-48
Figure 6-20: Amplitude Modulation
Client A
Binary Data
Modem
Amplitude Modulation
Telephone
Modulated Analog
Signal
1 0 1 1
PSTN
Serial
Cable
Telephone
Cable
In amplitude modulation, there are
two amplitude (loudness levels)—
one for 1 and one for 0
1011 is loud-soft-loud-loud

6-49
Figure 6-21: Asymmetric Digital Subscriber Line
(ADSL)
PC
ADSL
Modem Single Pair of
Voice-Grade
UTPWires
Splitter DSLAM
Telephone
Telephone Company
End Office Sw itch
Subscriber
Premises
Data
WAN
PSTN
ADSL ALSO uses the existing residential local loop technology.
Inexpensive because no need to pull new wires, but
1-pair voice-grade UTP is not designed for high-speed transmission.

6-50
(ADSL)
PC
ADSL
Modem Single Pair of
Voice-Grade
UTPWires
Splitter DSLAM
Telephone
Telephone Company
End Office Sw itch
Subscriber
Premises
Data
WAN
PSTN
1.
Subscriber needs an ADSL modem.
Also needs a splitter for each
telephone wall outlet.
2.
Telephone carrier needs a digital subscriber line
access multiplexer (DSLAM) to separate the two signals.

6-51
(ADSL)
Unlike telephone modems, ADSL service
provides simultaneous voice and data transmission.
PC
ADSL
Modem
Ordinary Telephone
Service
Single Pair of
Voice-Grade
UTPWires
Dow nstreamData
Up to 1.5 Mbps
UpstreamData
Up to 512 kbps
Splitter DSLAM
Telephone
Telephone Company
End Office Sw itch
Subscriber
Premises
Data
WAN
PSTN
Downstream Data
Up to 3 Mbps

6-52
(ADSL)
PC
ADSL
Modem
Ordinary Telephone
Service
Single Pair of
Voice-Grade
UTPWires
Dow nstreamData
Up to 1.5 Mbps
UpstreamData
Up to 512 kbps
Splitter DSLAM
Telephone
Telephone Company
End Office Sw itch
Subscriber
Premises
Data
WAN
PSTN
Downstream Data
Up to 3 Mbps
Speed is asymmetric
Faster downstream than upstream
(Up to 3 Mbps versus up to 512 kbps)
Ideal for Web access
Acceptable for e-mail
Good for residential use

6-53
PC
Cable
Modem
Subscriber Premises
Neighborhood
Splitter Cable Telev ision
Head End
Optical
Fiber to
Neighborhoods
ISP
Coaxial Cable
in Neighborhood
(Shared Throughput)
Coaxial Cable
Drop Cable
UTP
or
USB
Maximum download
throughput is about 5 Mbps
Figure 6-22: Cable Modem Service
Cable modem service brings high-speed
optical fiber lines to the neighborhood.

6-54
PC
Cable
Modem
Subscriber Premises
Neighborhood
Splitter Cable Television
Head End
Optical
Fiber to
Neighborhoods
ISP
Thick Coaxial Cable
in Neighborhood
(Shared Throughput)
Thin
Coaxial Cable
Drop Cable
UTP
or
USB
In the neighborhood,
thick coaxial cable
brings service to
households.
This bandwidth is
shared by
everyone in the
neighborhood.
A thin coax line
goes to each
home’s
cable modem.

6-55
PC
Cable
Modem
Subscriber Premises
Neighborhood
Splitter Cable Telev ision
Head End
Optical
Fiber to
Neighborhoods
ISP
Thick Coaxial Cable
in Neighborhood
(Shared Throughput)
Thin
Coaxial Cable
Drop Cable
UTP
or
USB
Maximum download
throughput is about 5 Mbps
Downstream speeds up to 5 Mbps.
Upstream speeds up to about 1 Mbps.

6-56
ADSL versus Cable Modem Service
• Do Not Over-Stress the Importance of Sharing
– Cable modem service usually is still faster than ADSL
service
– DSLAM sharing can slow ADSL service too
• The Bottom Line Today:
– Cable modem service usually is faster
– ADSL service usually is cheaper
• ADSL offers more speed-price options
• Both are improving rapidly in terms of speed and
(sometimes) price

6-57
Figure 6-23: Third-Generation (3G)
Cellular Data Services
• Cellphone connects to computer via a cellphone
modem or USB
• Traditional GSM and CDMA
– Limited to only about 10 kbps
– Far too slow for usability

6-58
Figure 6-23: Third-Generation (3G) Cellular
Data Services
• Both GSM and CDMA are evolving
• Second Generation (now dominant)
– Only 10 kbps data transmission
• Third Generation
– Low end: comparable to telephone modem service
– High end: comparable to low-speed DSL service
• Future
– Speeds comparable to high-end DSL or cable modem
service
– 100 Mbps or more (fast enough for good video)

6-59
Access Services
• WiMax (802.16)
– Wireless Internet access for metropolitan areas
– Basic 802.16d standard: ADSL speeds to fixed locations
• Will use dish antennas
• Just reaching the market
– 802.16e will extend the service to mobile users
• Will use omnidirectional antennas

6-60
Access Services
• Satellite Internet
Access
– Very expensive
– Often needed to
serve rural areas
New

6-61
Access Services
• Broadband over Power Lines
– Broadband data from your electrical company
– It already has transmission wires and access to
residences and businesses
– It can modulates data signals over electrical power
lines
– It works, but has very limited availability and is slow
– Especially promising for rural areas

6-62
Access Services
– Carrier runs fiber to the home
– Provides speeds of tens of megabits per second for high-
speed video, etc.
• Less if fiber only goes to the curb (FTTC)
• Or to the neighborhood (FTTN)
– Much faster than other residential internet access
services
– Could dominate residential (and business) Internet
access in the future

6-63
Internet Access and VoIP
• Most ISPs are Planning to or Already Provide VoIP
Telephone Service
– An alternative to the local telephone company service
– Media gateways will interconnect with the PSTN
– Should be less expensive that traditional phone service
– Questions remain
• Voice quality and reliability
• 911 and 911 location discovery
• Regulation and taxation
• Laws that require wiretapping with warrants

6-65
Telecommunications
• Data Communications versus Telecommunications
• The PSTN’s Technical Elements
– Customer premises equipment (PBX and 4-pair UTP)
– Access system (local loop)
– Transport core
– Signaling (call setup and management)
• POP to interconnect carriers

6-66
Telecommunications
• Access Lines
– For residences, 1-pair voice-grade UTP
• DSL uses existing residential access lines to carry data
by changing the electronics at each end (DSL modem in
the home and DSLAM at the end office switch)
• DSL is cheap because 1-p VG UTP is already in place
– For businesses,
• 2-pair data-grade UTP for speeds up to a few Mbps
• Optical fiber for faster speeds
• Usually must be pulled into place, so expensive
– Eventually, fiber to the home (FTTH), FTTC, FTTN

6-67
PSTN Transmission
• Circuit Switching
– Reserved capacity end-to-end
– Acceptable for voice, but not for bursty data transmission
– Dial-up and leased line circuits
• Analog and Digital Transmission
– Analog signals on the local loop
– ADC and DAC at the end office switch
– ADC: bandpass filtering and sampling for 64 kbps
– DAC: sample values are converted to sound levels

6-68
Cellular Telephony
• Cells Allow Channel Reuse
– Channel reuse allows more customers to be served with
a limited number of channels
• GSM: most widely used technology for cellular
telephony
• CDMA for greater channel reuse
• Handoffs and Roaming

6-69
VoIP
• To allow voice to be carried over data networks
• Converge voice and data networks
• Phone needs a codec
• Transport: UDP header followed by RTP header
• Signaling: H.323 and SIP
• Video over IP

6-70
Residential Internet Access
Services
• Telephone Modems
• Asymmetric Digital Subscriber Line (ADSL)
• Cable Modem Service
• 3G Cellular Data Service
• WiMAX (802.16 and 802.16e)
• Broadband Over Power Lines

Telecommunications

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