1. Introduction to Information Technology
LECTURE 7
THE TELEPHONE SYSTEM: WIRED AND WIRELESS

2. Digitizing Voice We’ve been discussing digital audio
Sampling and Quantizing
Note that voice sampling over digital telephone networks is not as accurate as music sampling.
QUESTION: Assume a voice signal is sampled at a rate of 8000 Hz and quantized to 256 levels at each sample.
What is the bit rate of the signal?
What is the sampling interval in microseconds?
BIT RATE: samples/sec X 8 bits/sample = 64 Kbps
Sampling Interval: 1/8000 = seconds = 125 microseconds.

3. Digitizing Music T=1/f Sampling Rate = 1/44,100 = .0000227 seconds
Assume a song is sampled 44,100 times per second for each of 2 channels and each sample is represented by 2 bytes.
44,100 samples/second * 16 bits/sample * 2 channels
= 1,411,200 bits per second (Versus 64 Kbps for voice)
What’s the sampling interval?
T=1/f
Sampling Rate = 1/44,100 = seconds
(compared to seconds for voice)

4. The Telephone Network
Chapter 13
The telephone system is perhaps the most taken for granted system in the world
Instant, real-time audio communications
Ubiquitous, interconnected, reliable
Known as POTS – Plain Old Telephone Service

5. The Public Switched Telephone Network (PSTN)
Transmission Systems
Switching Systems
Customer Equipment
Customer Equipment
Local Loop
Local Loop
Trunks

6. A Switched Network Home GMU CO CO Tandem Switch 0-5 miles
0-1000s of miles
Tandem
Switch
Tandem
Switch

7. The Telephone System – Analog and Digital
Most telephone calls are analog from the telephone in the home to the first telephone switching office (central office).
Most telephones are within 5 miles of a “central office.”
At a central office, most incoming telephone lines are connected to equipment that converts the incoming voice to digital (A/D conversion) and the outgoing voice to analog (D/A conversion).

8. Circuit Switching
Not long after A. G. Bell invented the telephone, a problem surfaced
How do you interconnect multiple users?
Connecting every user to every other user would require n(n-1) connections (where n is the number of users).
Users sharing the same line would obviously be problematic. The solution? Circuit Switching--the temporary establishment of a path
Circuit-based networks: those in which a path is maintained between the users for the duration of the call.
Packet-based networks: those in which individually addressed packets of information are sent into a communications system, and are individually forwarded until they reach the recipient.

9. Completing the Call
10 digit dialing (in the U.S.) – Traditional origination:
3 digit area code -- describes a particular geographical area--although this is changing (Overlay and 800, 877, 900)
3 digit prefix code -- describes a Central Office (usually) also called an “Exchange” in phone company lingo
4 digit code for the instrument
SS7 (Signaling System 7) is a network “above” the normal telephone network—in the old days used in-band signaling which was vulnerable to fraud--SS7 is out-of-band or common channel signaling
It is a packet network that connects switches
SS7 detects when the call is completed, line is busy, trunks are busy, or call can’t be completed for some reason (U.S. PSTN is sized to support Mother’s Day)

10. PBX Private Branch Exchange (PBX)
Most large businesses have a PBX on premises.
Private Branch Exchange (PBX)
Supports an organization with less than 7-digit dialing
PBX “hangs” off the PSTN
“Want an outside line? Dial 9.”
PBX can be real or virtual
PBX switch might physically reside at company site
The switch might physically reside with the telephone company

11. Future of the Telephone System?
Obviously many duplicative communications systems coexist.
The telephone system
Cellular
Cable
The Internet
Satellite
Why can’t one system handle voice, video, and data?
“Convergence” technologies address the migration of different information technologies onto a single integrated, ubiquitous network.
Wouldn’t this be more economical?

12. Voice over Internet Protocol (VoIP)
VoIP is one implementation of convergence.
Voice conversations carried over the Internet along with other Internet traffic.
Why do this?
How much are you charged to make a long distance call?
How much are you charged to view a website in Germany?
VoIP = Voice over Internet Protocol

13. Voice and Data Traffic are Different
VOICE TRAFFIC
DATA TRAFFIC
Continuous
Synchronous
Constant Bandwidth
Bursty
Asynchronous
Varying Bandwidth ?
Circuit Switching
Packet Switching

14. VoIP Challenges 1) DELAY
While traditional circuit switching of voice has some delay, IP routing involves much more data handling and therefore more delay.
Internet packet loss due to over loaded routers is expected and is addressed through retransmission. The retransmitted packet may arrive too late to be available for the synchronous reconstruction of the voice waveform.
If the transmission rate slows down enough, again it won’t be available for reconstruction of the voice waveform.
2) PACKET LOSS
3) VARIABLE RATE
Packet buffering (streaming) addresses 2 & 3 but contributes to problem 1.
QOS = Quality of Service

15. Cellular Networks Wireless telephony
Discussed in Chapter 13 of Text
Wireless telephony
Just another medium for voice communications
Also used for other types of information transfer
Pictures
Text Messaging
Main advantage of wireless - mobility

16. Cellular History “Radio telephones” have existed for 60 years
Bell Labs invented the first mobile radio system 50 years ago
Improvements made in the mid 1960s
Early radio telephone systems:
One base station covered a city
Some number of channels allocated by the FCC
For example, in 1976, the New York City network could support 12 channels, serving 543 paying customers
Limited capacity related to spectrum constraints…not much sharing and considerable bandwidth dedicated to a single call
Required considerable power
Supported a limited # of users
“Dead” spots

17. Cellular Concept Every cell uses a different frequency
The cellular concept addressed many of the shortcomings of the first mobile telephones
Frequency reuse
Rather than one base station serving an entire city or region, many low-power base stations are distributed throughout the service area
Each base station is called a “CELL”
Every cell uses a different frequency
In each cell, the number of active users is still restricted by the basic principles of the RF spectrum, but this is less of a problem because the area is small.
The power required is low because each cell is small
Frequencies can be reused in multiple cells
A large city might have hundreds of cells
Key Concept: Frequency Reuse

18. What Makes Cellular Work?
Each cell is a low power radio with a limited range (2-10 miles)
Allows for frequency reuse
Reduces interference over a wide area
As a user moves through a cell, the cell phone (radio) makes a connection to the strongest signal
When the user moves to a cell with a stronger signal, the call is “handed over” to the next cell
The handover is (hopefully) transparent to the user

19. Cellular Concept

20. Relationship to Telephone Network
Cellular technology heavily relies on existing wire based telephone networks
If you dial home from your cell phone, it has to enter the public switched telephone network (PSTN) somewhere
If you dial another cellular user, a Mobile Telephone Switching Office (MTSO) handles the switching
It also is the point where you interface with the PSTN
It also is the brains that tells the system when to hand you over from cell to cell as you move
Individual cell sites are connected by traditional phone lines

21. A Cell Site

22. A Cell Site
MTSO
PSTN

23. Cellular FAQs Why do cellular antennas look the way they do?
Why do antenna sizes vary?
Why aren’t all telephones wireless?
What’s the difference between a cell phone and a cordless phone?
Are cell phones safe?