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

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: 8000 samples/sec X 8 bits/sample = 64 Kbps Sampling Interval: 1/8000 = .000125 seconds = 125 microseconds.

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 = .0000227 seconds (compared to .000125 seconds for voice)

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

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

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

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).

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.

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)

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

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?

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

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

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

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

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

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

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

Cellular Concept

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

A Cell Site

A Cell Site MTSO PSTN

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?