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1
Telephony: Internal and External
Chapter 6
Copyright 2003 Prentice-HallPanko’s Business Data Networks and Telecommunications, 4th edition
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Figure 6.1: Telephony
Telephone Service is Expensive for Corporations
LANs Use Traditional Telephone Building Wiring
Telephone Technology is Basis for Much Wide Area Networking
Telephone Regulation and Carriers Affect Wide Area Networking
Desire for Converged Services: Integrated Management of Voice and Data Networks
3
Figure 6.2: Internal PBX-Based Telephone Network
Multi-floorOffice
Building
4
Figure 6.2: Internal PBX-Based Telephone Network
1. Equipment Room in Ground Floor or Basement
2. ToPhone
Co.
3. EntranceFacility
5. PBXInternal Telephone
Switch
6. WireBundle
(Many Pairs)
7. VerticalRiserSpace
4. TerminationEquipment
5
Figure 6.2: Internal PBX-Based Telephone Network
3. TelecommunicationsCloset on Floor 2. Wire Bundle
4. Cross-Connect Device
5. Horizontal Distribution
1. VerticalDistribution
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Figure 6.2: Internal PBX-Based Telephone Network
1. Horizontal DistributionOne 4-Pair UTP Cord
2. Final DistributionAlong or Through Wall
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Figure 6.3: LAN Building Wiring
1. Equipment Room in Ground Floor or Basement
2. ToWAN
3. EntranceFacility
5. CoreSwitch
(Chassis)
6. VerticalRiserSpace4. Router
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Figure 6.3: LAN Building Wiring
3. TelecommunicationsCloset on Floor
2. Optical FiberOne Pair per Floor
4. Workgroup Switch
5. Horizontal Distribution
1. VerticalDistribution
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Figure 6.3: LAN Building Wiring
1. Horizontal DistributionOne 4-Pair UTP Cord
Horizontal and Final Distributionare the Same as in Telephony
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Building Cabling Management
Structured Cabling Plans
Testing Inexpensive just to test whether wires are
connected properly
More expensive to test for signal quality
Documentation and Neatness Critical to avoid chaos in wiring
11
Figure 6.4: Public Switched Telephone Network (PSTN)
Customer Premises(Residential)
Customer Premises(Business)
Switching OfficeCentral Office
End OfficeDigital
Access LineLocal Loop
Analog SignalingSingle Twisted Pair
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Figure 6.4: Public Switched Telephone Network (PSTN)
SwitchingHierarchy
Trunk LineDigital
Class 4
Class 3
Class 5End
Office
Class 5End
Office
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Figure 6.4: Public Switched Telephone Network (PSTN)
CircuitEnd-to-End Connection
Between Two Subscribers
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Figure 6.5: Trunk Line Technologies
Trunk Line Speed
North American Digital Hierarchy
56 kbps (DS0 Signaling) 56 kbps (sometimes 64 kbps)
T1 (DS1 Signaling) 1.544 Mbps
T3 (DS3 Signaling) 44.7 Mbps
CEPT Multiplexing Hierarchy
64 kbps 64 kbps
E1 2.048 Mbps
E3 34.4
Connect Pairs of Switches
15
Figure 6.5: Trunk Line Technologies
Trunk Line Speed
SONET/SDH*
OC3/STM1 156 Mbps
OC12/STM4 622 Mbps
OC48/STM16 2.5 Gbps
OC192/STM64 10 Gbps
OC768/STM256 40 Gbps
Notes: SONET and SDH speeds are multiples of 51.84 Mbps.OCx is the SONET designation.STMx is the SDH designation.
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Figure 6.6: SONET/SDH Dual Rings
SONET/SDH Ring
SONET/SDH Ring
TelephoneSwitch
TelephoneSwitch
TelephoneSwitch
TelephoneSwitch
Rings can beWrapped if a
Trunk lineIs Broken
(Ch. 5)
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Figure 6.7: Circuit Switching
3. Circuit Switching:Circuit Reserved for
Duration of Call
2. AcrossMultiple
Access Lines,Switches, andTrunk Lines
1. CircuitEnd-to-End ConnectionSubscriber-Subscriber
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Figure 6.7: Circuit Switching
Dedicated Capacity Full capacity always available to pair of
subscribers during their call
Wasted if not used, and callers pay for whether use it or not
Good for voice, in which there is almost always someone talking
Expensive for data, which is bursty, having short transmissions mixed with long silences
Time
DataBurst
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Figure 6.8: The PSTN: Mostly Digital with Analog Local Loops
Original Telephone Network: All Analog
LocalLoop
(Analog)
ResidentialTelephone(Analog)
Switch(Analog)
Switch(Analog)
Switch(Analog)
LocalLoop
(Analog)
BusinessTelephone(Analog)
Trunk Line(Analog)
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Figure 6.8: The PSTN: Mostly Digital with Analog Local Loops
LocalLoop
(Analog)
ResidentialTelephone(Analog)
Switch(Digital)
Switch(Digital)
Switch(Digital)
LocalLoop
(Digital)
PBX(Digital)
Trunk Line(Digital)
Today’s Telephone Network: Predominantly Digital
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Figure 6.9: Codec at the End Office Switch
Subscriber Access Line is Analog
Switch is Digital
Codec Converts Between Them
Codec
DigitalInternalSignal
DigitalSwitch
Local LoopAccess Line
End Office
AnalogSubscriber
Signal ADC
DAC
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Figure 6.10: Sampling for Analog-to-Digital Conversion (ADC)
Codec First Bandpass Filters the Voice Signal Cuts of all energy below about 300 Hz Cuts off all energy above about 3,400 Hz Bandwidth of about 3.1 kHz
Signal
Energy Distribution forHuman Speech
O Hz 300 Hz ~3,400 Hz 20 kHz
Bandwidth (~3.1 kHz)
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Figure 6.10: Sampling for Analog-to-Digital Conversion (ADC)
Codec Constantly samples the intensity of the analog
voice signal from the customer
Sample
1/8,000 sec
0110010
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Figure 6.10: Sampling for Analog-to-Digital Conversion (ADC)
Codec Divides each second into 8,000 sampling periods Only measures intensity of voice signal in each
Sample
1/8,000 sec Sampling Period
IntensityValue
0110010
25
Figure 6.10: Sampling for Analog-to-Digital Conversion (ADC)
Codec Measures voice intensity as an 8-bit intensity
(loudness) value (0-255) Overall, sends 8 bits 8,000 times per second (64
kbps)Sample
1/8,000 sec Sampling Period
IntensityValue
0110010
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Figure 6.10: Sampling for Analog-to-Digital Conversion (ADC)
Codec This is why telephone channels are 64 kbps
Designed for digitized voiceCarrier often “steals” 8 kbps for
supervisory signaling, so 56 kbps
8,000 samples/second * 8 bits/sample = 64 kbps
64 kbps – 8 kbps = 56 kbps
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Figure 6.11: Digital-to-Analog Conversion (DAC)
00000100 00000011 00000111
Arriving Digital SignalFrom Telephone Switch(8000 Samples/Second)
Generated Analog SignalFor Subscriber Line
DAC
1/8000Second(8 bits)
Sounds smooth if there are enough samples
per second
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Question
If you have an audio CD player, it contains a(n) _____. a. Analog-to-digital converter
b. Digital-to-analog converter
c. Both
d. Neither
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Figure 6.12: Cellular Telephony
B
E
H
D
I
G
L
K
F
C
M
A
J
N
P
HandoffO
PSTNMobile TelephoneSwitching Office
Cellsite
1. Divide AreaInto Cells
2. CellphoneCommunicatesVia Cellsites,
MTSO
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Figure 6.12: Cellular Telephony
B
E
H
D
I
G
L
K
F
C
M
A
J
N
P
O
3. UseChannel
47
PSTN
Cellsite
1. Reuse Channels in Non-Adjacent Cells
2. UseChannel 47
In Cell A
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Figure 6.12: Cellular Telephony
B
E
H
D
I
G
L
K
F
C
M
A
J
N
P
O
PSTN
Cellsite
1. Use A, D, and F
2. ReuseChannel 47
NextIn What Cells?
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Importance of Channel Reuse
Channels are Scarce Only ~800 in First Generation Cellular Systems
Only ~2,500 More for Second Generation Systems
You Can Only Have an Average of 20 Customers per Channel Assumes each will use the system 5% of the time
(generous)
Only 16,000 to 50,000 customers without channel reuse
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Importance of Channel Reuse
Can Reuse Channel in Nonadjacent Cells In adjacent cells, signals will interfere
To tell how roughly how many times you can reuse a channel on average
Divide the number of cells by 7Rough estimate but very useful20 cells / 7 = 3 (Round off: It’s not exact)100 cells / 7 = 14
You can multiply the possible number of subscribers by this factor
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Importance of Channel Reuse
Example 800 channels
20 cells
3 (20/7) channel reuse factors
2,400 effective channels (800 x 3)
20 subscribers/channel maximum for good service
48,000 maximum subscribers (2,400 x 20)
35
Importance of Channel Reuse
Example 2,400 channels 100 cells
36
Compression
Multiplies Number of Possible Subscribers by About a Factor of 3 2,400 channels 100 cells
37
Figure 6.12: Cellular Telephony
B
E
H
D
I
G
L
K
F
C
M
A
J
N
P
HandoffO
PSTNMobile TelephoneSwitching Office
1.Automatic
Handoff BetweenCellsites as
Phone TravelsBetween
Cells
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Handoffs versus Roaming
Handoff Moving between cells within a single cellular
system
Cellular telephony
802.11 wireless LANs
Roaming Moving between systems
Cellular telephony: use cellphone in another city
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Figure 6.13: Generations of Cellular Service
Generation First 2nd 2.5G 3G
Technology Analog Digital Digital Digital
Data TransferRate
Data TransferIs Difficult
10 kbps*20 kbps to384 kbps
384 kbpsto 2 Mbps
Channels ~800~800 +2,500
~800 +2,500
?
Cells/ ChannelReuse
Large/Medium
Large/Medium
and Small/High
Basedon 2G
?
*Sufficient for Short Message Service (SMS) and wireless Web accessusing the Wireless Access Protocol (WAP) or i-mode
40
Figure 6.13: Generations of Cellular Service
Generation First 2nd 2.5G 3G
WorldStandardization(and therefore
roaming)
PoorGood(GSM)
Basedon 2G
? (W-CDMA,CDMA-2000,
and othersystems
may compete)
U.S.Standardization
Good(AMPS)
Poor (GSM,CDM, TDMA,
& CDPD)
Basedon 2G
?
41
Figure 6.14: Regulation and Deregulation
Regulation Carriers: carry signals between customer premises
Rights of Way: government permission to lay wire
Monopoly: Service was originally provided by a single telephone carrier
Regulation: This monopoly carrier was regulated to prevent abuse of the monopoly
Tariffs specify a service’s specific service parameters and pricing to prevent discrimination and guarantee service parameters
42
Figure 6.14: Regulation and Deregulation
Deregulation Initially, regulated protected monopoly service
Deregulation: remove protections & restrictions
To increase competition, lowering prices
Varies by country
Varies by service within countriesData, long-distance, and customer premises
deregulation is high.Local voice service deregulation is low.
43
Figure 6.15: Regulation and Carriers
Carriers Public Telephone and Telegraph (PTT) authority is
the traditional domestic monopoly carrier in most countries.
Domestic transmission: within a countryUK: British TelecomsJapan: NTT
44
Figure 6.15: Regulation and Carriers
Carriers In the United States
U.S. is divided into regions called local access and transport areas (LATAs)
About 200 LATAs nationwideSmall states have just one LATALarge states have 10 to 20 LATAs
LATA
45
Figure 6.15: Regulation and Carriers
Carriers
In the United StatesLocal exchange carriers (LECs) provide
service within a LATAIncumbent LEC (ILEC) is the traditional
monopoly carrier in the LATACompetitive LEC (CLEC) is a new
competitor
LATA
LEC
ILEC CLEC
46
Figure 6.15: Regulation and Carriers
LATA LATAIXC
Carriers In the United States
Inter-exchange carriers (IXCs) provide service between LATAs
LEC versus IXC distinction is used by data carriers as well as voice carriers.
47
Figure 6.15: Regulation and Carriers
Carriers
In the United StatesPoint of Presence (POP) is a place in a
LATA where all carriers interconnect to provide integrated service to all customers LATA
POPILEC
CLEC IXCIXC
48
Figure 6.15: Regulation and Carriers
International Service (Between Pairs of Countries)
Provided by international common carriers (ICCs)
Allowed carriers, prices, and conditions of service are settled through bilateral negotiation between each pair of countries
Country 1 Country 2ICC
49
Figure 6.15: Regulation and Carriers: Recap
U.S. Intra-LATA
LECsILECCLECs
Inter-LATAIXCs
Most of the WorldPTTs for domestic service
ICCs
50
Figure 6.16: Converged Services
Integrate Voice and Data Networks Often referred to as voice over IP (VoIP) and IP
telephony
51
Figure 6.16: Converged Services
Provide Voice and Data on a Single Network
Save money compared to traditional telephony Reducing staff and economies of scale in
purchasing with one network
Encoding voice to less than 64 kbps so fewer bits need to be sent
Packet switching to reduce transmission costs for bit sent
52
Figure 6.16: Converged Services
Other advantages Computer-telephony integration (CTI)
Provide integrated voice and data applications
E.g., when a customer calls, their information can be brought up on-screen
53
Figure 6.17: PBX-PBX IP Telephony
OrdinaryTelephone
OrdinaryTelephone
Frame Relay, ATM, orthe Internet
IPPacket
PBX withIP Telephony Module
PBX withIP Telephony Module
54
Figure 6.16: Converged Services
Implementation PBX-to-PBX connectivity is easy and saves money
on long-distance calls
LAN implementations are more difficult, less well-developed, and may not save money
55
Figure 6.16: Converged Services
Will Cost Savings be Realized? Convergence justified by gap between high long-
distance and international telephone charges and possible savings through IP telephony
Falling traditional telephone prices are reducing the gap
Packet transmission inefficiency is reducing the theoretical savings and therefore the gap
Does the remaining gap justify convergence?
Telephone Price
VoIP Price
Gap
56
Figure 6.16: Converged Services
Service Quality Availability (less than the PSTN’s 99.999%)
Sound qualitylatency produces pausesMillisecond-to-millisecond inconstancy of
speed produces jitter Sound quality is addressed by using a single ISP to
connect all sitesService Level Agreements (SLAs)
57
Figure 6.18: Using a Single ISP for VoIP
ISP 1 Backbone ISP 2 Site A Site B
Site C
No Congestion in Site A-C CommunicationBecause All Traffic Passes through a Single ISP.
It Avoids the Congested Internet Backbone.
Congestion in Site A-B CommunicationBecause of Passage Through Internet Backbone.
58
Voice/Data Cultural Concerns
Data Networking Concerns Will all this voice traffic mean that my data cannot
get through well?
Telephony Concerns Will people get the same level of voice quality?
Will they get secondary services (3-party calling, call waiting, etc.)
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