Telecommunications Chapter 6 Updated January 2009

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 So far, we have looked at wired and wireless communication WITHIN the company’s premises, that is, in LANs. In this chapter, we will begin moving outside the firm’s walls. First, we will look at the telephone network, which wide area networks use for some, most, or all of their transmission. Second, we will look at Internet access alternatives from residences and businesses.

The Public Switched Telephone Network (PSTN)
Carriers Telephony Television The telephone network is officially the “Public Switched Telephone Network”. It is available to the public—that, is, anyone. It uses switches to link telephone subscribers together.

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. Customer premises equipment, as the name suggests, is equipment on the customer’s site—residential homes and apartments and business buildings. This equipment is owned by the customer. [Actually, until the 1970s and 1980s, the telephone company owned the telephones and wires in homes and business buildings.] 1. Customer Premises Equipment 1. Customer Premises Equipment

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. <Read the boxed text and point to the red-enclosed areas.>

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) <Read the boxed text and point to the red-enclosed area.>

6-1: Elements of the PSTN 3. Transport Core 3. Switch 3. Trunk Line
<Read the boxed text and point to the red-enclosed area.> The Transport Core connects end office switches and core switches. Trunk lines connect switches.

6-1: Elements of the PSTN Telephone Company Switch
Here is a picture of a telephone company switch.

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 Here is a distinction that students tend to forget easily. <Read the text in the box.>

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. <Read the slide.> A frequent point of confusion

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. <Read the boxed text and point to the red-enclosed area.>

Circuits Data networks use packet switching to deliver messages.
Telephone networks traditionally have used something simpler, called circuit switching.

6-4: Circuit Switching <Read the text.>

6-5: Voice and Data Traffic
<Read the text in the box.> 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.

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 <Read the text in the box.>

6- 7: Time Division Multiplexing (TDM) in T1 Lines
<Read the red-enclosed text and then point to the red-enclosed areas.>

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 <Read the slide.>

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 <Read the table.> <Emphasize that 1p VG UTP is already installed, so there is no installation cost when using it. Other types of wiring have to be pulled to the customer premises.> <Emphasize the difference between wiring within firms and wiring in the local loop.> Note: Within buildings, corporate telephony uses 4-pair UTP

Analog versus Digital Transmission
In this book, we have been looking primarily at digital transmission, in which different states represent 1s and 0s. The PSTN was created to use a much older form of transmission called analog transmission.

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. <Read the text in the box.>

6-10: The PSTN: Mostly Digital with Analog Local Loops
<Read the text in the box.> 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).

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. <Read the text in the box.>

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. <Read the text in the box.>

6-12: Frequency Division Multiplexing (FDM) in Microwave Transmission
Box Microwave provides Point-to-point Terrestrial Transmission <Read the red-enclosed text.>

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 <Read the text in the 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

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 <Read the text in the box.>

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 <Read the text in the box.>

Box In each sampling period, only the amplitude of the signal Is sampled <Read the text in the box.>

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. <Read the text in the box.>

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 <Read the text in the box.>

6-14: Digital-to-Analog Conversion (DAC)
Box So far, we have been looking at how analog-to-digital conversion works. In the other direction, the codec has to do digital-to-analog conversion. <Read the text in the 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.

Cellular Telephony Nearly everyone today has a cellular telephone.
In this section, we will look at how your mobile phone works.

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 <Read the text in the box.>

6-15: Cellular Telephony Cellsites
Here are some pictures of cellular telephone towers. The antennas are the rectangular metal boxes. They are not as strongly directional as dish antennas, but they are still strongly direction.

A mobile telephone switching office (MTSO) coordinates activity among the cellsites. The MTSO also connects mobile customers with wired PSTN customers via a POP. <Read the text in the box.>

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. Here is something that was not in the book. Obviously, cellsites need to connect to the MTSO. <Read the text>

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. <Read the text in the box.>

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. <Read the text in the box.>

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. <Read the text in the box.>

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. <Read the text in the box.>

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 We have seen handoffs and roaming in both wireless LANs and cellular telephony. Sadly, these two terms are not used consistently in the two contexts. <Read the text in the table.>

Voice over IP (VoIP) You may use Skype or some other program to talk over the Internet at low cost or even no cost. This is called voice over IP, because you are using an IP network, namely the Internet. Companies also use VoIP, both within their buildings and outside their buildings. They do not always use the Internet for transmission.

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. <Read the text in the boxes.>

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 <Read the text in the box.>

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. <Read the text in the box.>

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. <Read the text.>

6-18: VoIP Signaling and Transport
Again, signaling is the transmission of supervisory messages. Transport is the actual transmission of voice. <Read the text in the box.>

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. <Read the text in the box.>

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. <Read the text in the box.>

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 <Read the text in the box.>

Wired “Last Mile” Services
Telephone Modems ADSL Modem Service Cable Modem Service Fiber to the Home You need to get to the Internet from your home or apartment. Businesses need to get to the Internet from their sites. We will begin looking at three traditional ways to connect to the Internet. All of them used wired transmission.

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 <Read the text.>

Telephone Service and Cable TV 1950s brought cable television service Used coaxial cable with a central wire and a coaxial conductive ring or mesh <Read the text.>

Telephone Service and Cable TV A static situation emerged Telephone companies controlled broadcast telephone service Cable companies controlled television delivery service <Read the text.>

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 <Read the text.> Telephone Line

Telephone modems Limited to 33.6 kbps sending / 56 kbps receiving Cannot use your telephone for calls while using the telephone modem <Read the text.>

6-21: Asymmetric Digital Subscriber Line (ADSL)
<Read the text in the box.> 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

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) <Read the text.>

<Read the text in the box.> Unlike telephone modem services, ADSL provides simultaneous voice and data. The phone line is not tied up

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. <Read the text in the box.>

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 <Read the text in the box.>

End office switch needs a DSLAM (DSL access multiplexer) Connects voice calls to the PSTN Connects data calls to a data network <Read the text in the box.>

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 <Read the text in the box.>

6-22: Cable Modem Service Optical fiber brings signals
to and from the neighborhood. Thick coaxial cables carry signals in the neighborhood. <Read the text in the box.>

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. <Read the text in the box.>

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 <Read the text.>

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

Wireless Access Service
Figure 6-23: Wireless Technologies for the Last Mile We have looked at wired access to the Internet. We will no look at wireless access technologies to get you to the Internet.

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 <Read the text.>

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

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 <Read the text.>

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 <Read the text.>

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 <Read the text.>

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 <Read the text.>

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 <Read the text.>

The Market Situation Having looked at technologies, we need to turn the situation in the marketplace for services.

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 <Read the text.>

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 <Read the text.>

Topics Covered

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)

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

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

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

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)

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

Copyright © 2009 Pearson Education, Inc. Publishing as Prentice Hall
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Printed in the United States of America. Copyright © 2009 Pearson Education, Inc. Publishing as Prentice Hall

Telecommunications Chapter 6 Updated January 2009

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