1. Telecommunications Chapter 6 Updated January 2009
Raymond Panko’s Business Data Networks and Telecommunications, 7th edition May only be used by adopters of the book

2. The Public Switched Telephone Network (PSTN)
Carriers
Telephony
Television

3. 6-1: Elements of the Public Switched Telephone Network (PSTN)
Customer premises
Equipment (CPE) consists
Of telephones, wires,
And other infrastructure
on the customer premises.
It is owned by the customer.
1. Customer Premises
Equipment
1. Customer Premises Equipment

4. 6-2: Customer Premises Equipment at a Business Site
Most businesses have a PBX (private branch exchange).
It acts like an internal switchboard
Businesses use 4-pair UTP for in-building telephone wiring.
Have long used 4-pair UTP for telephony.
Only recently was this 4-pair UTP used for data.

5. 6-1: Elements of the PSTN 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). 2.
Access Line
(Local Loop) 2.
Access Line
(Local Loop)
2. & 3. End Office
Switch (Class 5)

6. 6-1: Elements of the PSTN 3. Transport Core 3. Switch 3. Trunk Line
The Transport Core connects end office
switches and core switches.
Trunk lines connect switches.

7. 6-1: Elements of the PSTN
Telephone Company Switch

8. 6-1: Elements of the PSTN 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

9. Transport Versus Signaling
The carriage of voice during a conversation
Signaling
Supervisory communication to set up a connection, monitor connection quality, collect billing information, closing a connection, etc.
A frequent point of confusion

10. 6-3: Points of Presence (POPs)
Local, long-distance, and international
carriers connect at POPs
(points of presence)
This permits their subscribers
to call one another.

11. Circuits

12. 6-4: Circuit Switching

13. 6-5: Voice and Data Traffic
Voice uses about 30% of capacity, on average.
Data only uses about 5% of capacity, on average.
Circuit switching is not too wasteful for voice,
but it is very wasteful for data transmission.

14. 6-6: Dial-Up Circuits Versus Leased Line Circuits
Is it a circuit with reserved capacity?
Yes, by definition
Operation
Dial-up. Separate circuit for each call
Permanent circuit, always on
Speed for Carrying Data
Up to 33.6 kbps
56 kbps to gigabit speeds
Number of Simultaneous Voice Calls per Circuit
One
Several due to multiplexing

15. 6- 7: Time Division Multiplexing (TDM) in T1 Lines

16. 6- 7: Time Division Multiplexing (TDM) in T1 Lines

17. 6- 7: Time Division Multiplexing (TDM) in T1 Lines

18. 6- 7: Time Division Multiplexing (TDM) in T1 Lines
Calculation
Each conversation gets an 8-bit time slot in each frame
There are 8,000 frames per second
So each conversation gets 64 kbps

19. 6-8: Local Loop Technologies
Technology
Use
Status
1-Pair Voice-Grade UTP
Residences
Already installed, so no installation cost
2-Pair Data-Grade UTP
Businesses for high- speed access lines
Must be pulled to the customer premises. (This is expensive)
Optical Fiber
Note: Within buildings, corporate telephony uses 4-pair UTP

20. Analog versus Digital Transmission

21. 6-9: Analog Telephone Transmission
Handset
Speaking creates pressure waves, which hit the microphone in the handset.
The microphone generates an analogous electrical signal.
This is called an analog signal.

22. 6-10: The PSTN: Mostly Digital with Analog Local Loops
The PSTN today is almost entirely digital.
This includes switches (3) and trunk lines (4).
It also includes digital leased access lines to businesses (5).

23. 6-10: The PSTN: Mostly Digital with Analog Local Loops
Only the residential telephone (1) and the
1-pair voice-grade UTP line going to residences (2)
are analog today.
Digital subscriber lines (which we will see later)
Send digital signals over these 1-pair VG UTP lines.

24. 6-11: Codec at the End Office Switch
A codec at the end office switch translates between the
analog customer signals and digital signals in the PSTN core
ADC is analog to digital conversion.
DAC is digital to analog conversion.

25. 6-12: Frequency Division Multiplexing (FDM) in Microwave Transmission
Box
Microwave provides
Point-to-point
Terrestrial Transmission

26. For telephone transmission, a filter at the end office switch
6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
For telephone transmission, a filter at the end office switch
Bandpass filters the voice to fit into 4 kHz channels.
Even when microwave is not used, this saves capacity

27. More precisely, it cuts off all signal below about 300 Hz
6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
0 to 4 kHz
More precisely, it cuts off all signal below about 300 Hz
and above about 3,400 Hz
This gives “guard bands” below 300 Hz
And from 3,400 Hz to 4 Hz
Voice still sounds good because most energy is 30 Hz to 3,400 Hz

28. 6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
The signal is
Sampled 8,000
Times per second.
So each sample
Is 1/8000 second

29. 6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
In each
sampling period,
only the amplitude
of the signal
Is sampled

30. 6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
The filter can
distinguish 256
loudness levels.
Each loudness
level is represented
as a binary number
between 0 and 255.
0 =
1 =
255=
This requires one
octet of storage
per sample.

31. 6-13: Analog-to-Digital Conversion (ADC): Bandpass Filtering and Pulse Code Modulation (PCM)
Box
This sampling method,
which is called pulse
code modulation (PCM),
produces 8 bits
per sample.
Times 8,000
samples per second –
this means
64 kbps per conversation

32. 6-14: Digital-to-Analog Conversion (DAC)
Box
Signals arriving from the PSTN are digital.
The DAC converts the 8 bits of each sample into a loudness level.
Not smooth, but sounds smooth at 8,000 samples/second.

33. Cellular Telephony

34. 6-15: Cellular Technology
Customer has a mobile phone.
A city is divided into small
geographic regions called cells.
Each cell has a cellsite
with an antenna and equipment
to serve mobile phones in the cell

35. 6-15: Cellular Telephony
Cellsites

36. 6-15: Cellular Technology
A mobile telephone switching office (MTSO)
coordinates activity among the cellsites.
The MTSO also connects mobile customers
with wired PSTN customers via a POP.

37. 6-15: Cellular Technology
New
Cellsites connect to the MTSO using a landline or a point-to-point radio system called microwave.
Here is a microwave dish.
It is covered with cloth, which does not interfere with radio transmission.

38. 6-15: Cellular Technology
Channels can be reused in different cells.
This permits more customers to be served.
Serving more customers through channel reuse
Is the whole reason for cellular service.

39. 6-15: Cellular Technology
GSM cellular technology cannot use the
same channel in adjacent cells.
CDMA can use the same channel in adjacent
cells, providing more channel reuse
and so more customers.

40. 6-15: Cellular Technology
When a mobile phone travels between cells,
it is handed off to the cellsite in the new cell.
In this figure, there is handoff between
the cellsite in Cell O and the cellsite in Cell P.

41. 6-15: Cellular Technology
In handoff, a mobile phone moves from one cell
to another cell in the same city.
In roaming, a mobile phone is taken to a different city.

42. 6-16: Handoff and Roaming in 802
6-16: Handoff and Roaming in Wireless Networking and Cellular Telephony
WLANs
Cellular Telephony
Relationship
Handoff and roaming mean the same thing
Handoff and roaming mean different things
Handoffs (means the same in both)
Wireless host travels between access points in an organization
Mobile phone travels between cellsites in the same city
Roaming (means different things)
Mobile phone travels to a different city

43. Voice over IP (VoIP)

44. 6-17: Voice over IP (VoIP) In voice over IP (VoIP),
calls are digitized, packetized, and
transported over an IP network:
either an internal IP network or the Internet.

45. 6-17: Voice over IP (VoIP) The user either has a
PC with multimedia hardware
and VoIP software or
an IP telephone that can be
plugged into an IP network
via a wall jack.
Either must have a codec

46. 6-17: Voice over IP (VoIP) A media gateway connects
a VoIP network to the PSTN.
This gives VoIP users access
To PSTN users.
The media gateway must translate
between both signaling technology
and transport technology.

47. VoIP
VoIP means that a firm does not have to maintain two networks—an IP network for data and a circuit- switched voice network.
This should reduce costs considerably by only requiring the maintenance of a single network.
In addition, VoIP’s packet switching should be more efficient than the PSTN’s circuit switching.
But companies have concerns about sound quality and the high availability expected of telephone service.

48. 6-18: VoIP Signaling and Transport
Again, signaling is the transmission
of supervisory messages.
Transport is the actual
transmission of voice.

49. 6-18: VoIP Signaling and Transport
The most popular SIGNALING protocol in VoIP is SIP. This figure shows how a sender initiates a connection using SIP.
The initiator sends a SIP INVITE message to its SIP proxy server.
The initiator’s SIP proxy server passes the INVITE to the receivers’ server.
The receiver’s SIP proxy server passes the INVITE to the receiver.
If the receiver accepts the INVITE, the conversation begins.

50. 6-18: VoIP Signaling and Transport
VoIP transport consists of a stream of VoIP packets.
Each VoIP packet contains a short amount codec-encoded voice.
There is no time to wait for error correction, so UDP is used.
The Real Time Protocol (RTP) header “fixes” weaknesses of UDP.
First, the RTP has a sequence number to place packets in order.
Second, RTP has a time stamp so that the voice steam
can be played back at the correct time.

51. 6-19: VoIP Codecs The two phones must use the same codec
to encode and decode voice.
They must agree on
one of several standard
codec protocols
through negotiation.
Generally,
more compression
gives lower
sound quality but
lowers transmission cost
Codec
Transmission Rate
G.711
64 kbps
G.721
32 kbps
G.722
48, 56, 64 kbps
G.722.1
24, 32 kbps
G.723
5.33, 6.4 kbps
G.723.1A
5.3, 6.3 kbps
G.726
16, 24, 32, 40 kbps
G.728
16 kbps
G.729AB
8 kbps

52. Wired “Last Mile” Services
Telephone Modems
ADSL Modem Service
Cable Modem Service
Fiber to the Home

53. 6-20: “Traditional” Technologies for the Last Mile
The access line to your home
Traditionally, a 1-pair VG UTP line from the telephone company
In the 1960s, a few businesses started getting 2-pair data-grade UTP and optical fiber
Given the cost of upgrading the 1-pair VG UTP plant, 1- pair VG UTP seemed eternal

54. 6-20: “Traditional” Technologies for the Last Mile
Telephone Service and Cable TV
1950s brought cable television service
Used coaxial cable with a central wire and a coaxial conductive ring or mesh

55. 6-20: “Traditional” Technologies for the Last Mile
Telephone Service and Cable TV
A static situation emerged
Telephone companies controlled broadcast telephone service
Cable companies controlled television delivery service

56. 6-20: “Traditional” Technologies for Data Transmission in the Last Mile
Telephone modems
Convert digital computer signals to analog and send these over the telephone access line
They also convert incoming analog signals into digital signals for the computer
Digital
Computer
Signal:
Analog
Telephone
Signal:
Telephone
Modem
Telephone Line

57. 6-20: “Traditional” Technologies for the Last Mile
Telephone modems
Limited to 33.6 kbps sending / 56 kbps receiving
Cannot use your telephone for calls while using the telephone modem

58. 6-21: Asymmetric Digital Subscriber Line (ADSL)
Like telephone modems,
ADSL also uses the existing 1-pair voice-grade
UTP line going to the home;
but it offers higher speeds than telephone modems

59. Telephone Modems and ADSL
Both use the 1-pair VG UTP line running to the subscriber’s home
Already installed, so no extra cost of running a new line
Telephone modems send analog signals
This is what the traditional telephone system expects
ADSL
Send digital signals for data (digital subscriber line)
Requires special equipment at the end office switch (DSLAM)

60. 6-21: Asymmetric Digital Subscriber Line (ADSL)
Unlike telephone modem services,
ADSL provides simultaneous voice and data.
The phone line is not tied up

61. 6-21: Asymmetric Digital Subscriber Line (ADSL)
Speed is asymmetric.
Faster downstream (to home) speed
than upstream (from the home) speed.
This is ideal for World Wide Web downloads.
Speeds are increasing rapidly in both directions.

62. 6-21: Asymmetric Digital Subscriber Line (ADSL)
Home user needs
a splitter for each
telephone outlet
Connects a phone
to the splitter voice port
Connects an
ADSL modem
To the splitter data port

63. 6-21: Asymmetric Digital Subscriber Line (ADSL)
End office switch
needs a DSLAM
(DSL access multiplexer)
Connects voice calls
to the PSTN
Connects data calls
to a data network

64. Cable Modem 6-22: Cable Modem Service Cable modem service is provided
by the cable television company,
not by a telephone company
Generally is faster than ADSL
but also more expensive

65. 6-22: Cable Modem Service Optical fiber brings signals
to and from the neighborhood.
Thick coaxial cables carry signals in the neighborhood.

66. 6-22: Cable Modem Service Thin coaxial drop cables carry
signals from the trunk cable
to individual residences.
Subscriber needs a cable modem
to receive data service.

67. ADSL Versus Cable Modem Service
Generally, cable modem service is somewhat faster and more expensive than ADSL service
However, price and performance ranges overlap
And performance is increasing rapidly
In cable modem service, all subscribers in a neighborhood must share the speed
However, cable modem speed to the neighborhood is very high, so cable modem subscribers usually still get higher-than-ADSL speeds
And other subscribers cannot read a subscriber’s transmissions, which are encrypted

68. Fiber to the Home
Some carriers are beginning to replace their 1-pair voice grade UTP residential wiring with optical fiber
This is called fiber to the home or fiber to the premises
Download speeds of 100 Mbps or more
Substantially more expensive than DSL service

69. Wireless Access Service
Figure 6-23: Wireless Technologies for the Last Mile

70. 6-23: Wireless for the Last Mile
3G Cellular Data Transmission
2G cellular service is for voice, texting, and photographs
Can send data via a cellular modem, but only at 10 kbps
3G cellular was created to send data faster
Most current services offer low DSL speeds at higher prices
2 Mbps to 3 Mbps speeds are arriving but will be even more expensive
Consumer usage is dominating with downloading music, videos, and games

71. 6-23: Wireless for the Last Mile
Cellular companies are using many 3G technologies
Cellular companies will eventually introduce faster 4G service
100 Mbps or more
Beginning to converge on Long-Term Evolution (LTE)
Which is IP-based
New

72. 6-23: Wireless for the Last Mile
WiMAX Metropolitan Area Networks
Designed to compete with DSL, cable modem service, and 3G and 4G cellular service
Designed to serve a metropolitan area
Users can get service anywhere, not just at hotspots

73. 6-23: Wireless for the Last Mile
WiMAX Metropolitan Area Networks
Promises to be faster than 3G service at lower cost
Beginning with 1 to 4 Mbps and will be faster
Mobile subscribers with omnidirectional antennas will receive speeds at the lower end
Fixed subscribers in homes will have directional antennas and speeds will be at the higher end

74. 6-23: Wireless for the Last Mile
WiMAX Metropolitan Area Networks
Standards created by the WiMAX Forum
WiMAX depends on the IEEE standard but goes beyond it
A single MAC-layer standard for all service bands between 2 GHz to 11 GHz
WiMAX forum is initially developing profiles for the 2.3, 2.5, 3.5, and 5.8 GHz licensed bands
WiMAX carriers want licensed bands for higher quality service

75. 6-23: Wireless for the Last Mile
WiMAX Metropolitan Area Networks
Uses advanced technologies
Scalable OFDM, MIMO, adaptive antennas systems (AAS) that do beam forming, and cellular organization for its base stations
WiMAX technology provides high-quality service
TDM gives each subscriber its fair share of the capacity

76. 6-23: Wireless for the Last Mile
Satellite Access Service
Very expensive because of long transmission distance to satellites
Hundreds to thousands of miles from the user site
One-way transmission, which is used in television delivery, is not too expensive
Two-way data transmission is complex and therefore expensive

77. The Market Situation

78. 6-24: The Market Situation
The Triple Play
The goal of access carriers
Telephony companies
Cable television companies
Wireless access companies
Provide telephony, data, and video in a package
Video is the hardest
People want multiple incoming TV signals
They also want HDTV

79. 6-24: The Market Situation
The International Situation
United States ranks 16th internationally in broadband speed and availability
Korea and Japan provide 50 Mbps speeds or faster at prices comparable to U.S. prices (for lower speeds)
Leadership in speed brings leadership in applications

80. Topics Covered

81. 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
Coming: fiber to the home (FTTH)

82. PSTN Transmission Circuit Switching Analog and Digital Transmission
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

83. 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

84. VoIP To allow voice to be carried over data networks
Converge voice and data networks
Phone or user’s computer contains a codec
Transport: UDP header followed by RTP header
Signaling: H.323 and SIP

85. Last Mile Services Wired Access Wireless Access Telephone Modems
Asymmetric Digital Subscriber Line (ADSL)
Cable Modem Service
Fiber to the home
Wireless Access
3G and 4G Cellular Data Service
WiMAX (based on and e)
Satellite service (expensive and so rare)

86. The Market Situation Triple Play International Situation
Traditional telephone companies, cable companies, and even wireless companies want to provide voice, data, and television
International Situation
U.S. is not a leader in broadband data service or cellular telephone service

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