Chapter 7
in the
Internet Age
by Alan Dennis
Copyright © 2002 John Wiley & Sons, Inc.
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Chapter 7. Learning Objectives
Understand the overall design of the Internet
Understand DSL and cable modem
Be familiar with wireless services
Be familiar with Internet 2
Chapter 7. Outline
• Introduction
• How the Internet Works
– Basic Architecture, Connecting to an ISP, The
Internet Today
• Internet Access Technologies
– Digital Subscriber Line, Cable Modems, Fixed
Wireless, Mobile Wireless, Future Technologies
• Internet Governance
• Internet 2
• The Best Practice Internet Access Design
• The Internet is not one network but a network of
networks made up of thousands of networks of
national and state government agencies, non-profit
organizations and for-profit companies.
• It exists only to the extent that these networks
agree to use Internet protocols and to exchange
data packets among one another.
• All networks on the Internet must conform to the
TCP/IP standards for the transport and network
layers, without which data communications over
the Internet would not be possible.
How The Internet Works
Basic Architecture: NAPs and national ISPs
• The Internet has a hierarchical structure.
• At the highest level are large national Internet
Service Providers that interconnect through
Network Access Points (NAPs).
• There are about a dozen NAPs in the U.S., run by
common carriers such as Sprint and Ameritech
(Figure 7-1), and many more around the world.
• Regional ISPs interconnect with national ISPs and
provide services to their customers and sell access
to local ISPs who, in turn, sell access to
Basic Architecture: MAEs and local ISPs
• As the number of ISPs has grown, a new type of
network access point, called a metropolitan area
exchange (MAE) has arisen.
• There are about 50 such MAE around the U.S.
• Sometimes large regional and local ISPs also have
access directly to NAPs.
• Indiana University, for example, which provides
services to about 40,000 individuals, connects
directly to the Chicago NAP.
Figure 7-1 Basic Internet architecture
Internet Packet Exchange Charges
• ISPs at the same level usually do not charge
each other for exchanging messages.
• This is called peering.
• Higher level ISPs, however, charge lower
level ones (national ISPs charge regional
ISPs which in turn charge local ISPs) for
carrying Internet traffic.
• Local ISPs, of course, charge individuals
and corporate users for access.
Connecting to an ISP
• ISPs provide access to the Internet through a Point of
Presence (POP).
• Individual users access the POP through a dial-up line
using the PPP protocol.
• The call connects the user to the ISP’s modem pool, after
which a remote access server (RAS) checks the userid and
• Once logged in, the user can send TCP/IP/[PPP] packets
over the telephone line which are then sent out over the
Internet through the ISP’s POP.
• Corporate users might access the POP using a T-1, T-3 or
ATM OC-3 connections provided by a common carrier.
• Figure 7-2 shows an example of a POP using a collapsed
backbone with a Layer-2 switch.
Figure 7-2 Inside an ISP point of presence
From the ISP to the NAP/MAE
• Each ISP acts as an autonomous system, with is own
interior and exterior routing protocols.
• Messages destined for locations within the same ISP are
routed through the ISP’s own network.
• Since most messages are destined for other networks, they
are sent to the nearest MAE or NAP where they get routed
to the appropriate “next hop” network.
• Figure 7-3 shows the connection from the local ISP to the
NAP. From there packets are routed to the next higher
level of ISP.
• Actual connections can be complex and packets
sometimes travel long distances. Each local ISP might
connect to a different regional ISP, causing packets to flow
between cities, even though their destination is to another
local ISP within the same city.
Figure 7-3 Inside an Internet network access point
The Internet in 2002
• Figure 7-4 illustrates the backbone networks of
three national ISPs: Compuserve and CAIS in the
US and iSTAR in Canada.
• Compuserve mostly uses T-3 lines for its
backbone, CAIS uses a mix of T-3 and ATM OC12 lines, while iSTAR uses T-1 lines.
• Compuserve and CAIS meet and peer at the
Chicago NAP, while CAIS and iSTAR peer at the
NAP in London, Ontario.
Figure 7-4 Three national ISPs in North America
Internet Backbones in 2002
• Today, most backbone circuits for national ISPs in
the US are 622 Mbps ATM OC-12 lines.
• The largest national ISPs are planning to convert
to OC-192 (10 Gbps) by the end of 2002.
• A few are now experimenting with OC-768 (40
Gbps) and some are planning to use OC-3072 (160
• Aggregate Internet traffic reached 2.5 Terabits per
second (Tbps) in 2001. It is expected to reach 35
Tbps by 2005.
Internet Access Technologies
Internet Access Technologies
• Most people today are still using 56K dialup lines to access the Internet, but a number
of new access technologies are now being
• The main new access technologies are:
Digital Subscriber Line
Cable Modems
Fixed Wireless (including satellite access)
Mobile Wireless (WAP)
Digital Subscriber Line
• Digital Subscriber Line (DSL) is now being
implemented on a widespread basis because it can
significantly increase the data rates over
traditional telephone lines.
• Historically, voice telephone circuits have had
only a limited capacity for data communications
because they were constrained by the 4 kHz
bandwidth voice channel.
• Most local loop telephone lines actually have a
much higher intrinsic bandwidth capability and
can therefore carry data at much higher rates.
DSL Topology (see Figure 7-5)
• DSL provides both a voice circuit and a point-topoint full –duplex data circuit.
• DSL installations commonly use line splitters to
separate the voice and data channels.
• Data from the splitter goes to the DSL modem, which
sends Ethernet frames for the customer’s LAN.
• In the local end office, the data stream from the local
loop goes to the main distribution facility (MDF)
which splits off the voice to the PSTN
• The data stream first goes to a DSL Multiplexer
(DSLAM) which combines it with other DSL signals
before sending it on the ISP.
Figure 7-5 DSL architecture
DSL Multiplexing
• One thing all DSL services have in common is that
they use Frequency Division Multiplexing to
divide available bandwidth into three channels.
• The channels are separated by guardbands which
are dead spaces that separate the channels so they
don’t interfere with each other (see Figure 7-6).
• The three channels are:
– A relatively small voice channel (0-4 kHz)
– An upstream channel with a 300 to 700 kHz
– A downstream channel with a 1000 to 10000
KHz bandwidth
Figure 7-6 DSL multiplexing
Types of DSL
• DSL services are quite new and not all
common carriers offer them.
• Two general categories of DSL services
have so far emerged ADSL and VDSL.
• Asymmetric DSL (ADSL) provides
different data rates to (up to 640 Kbps)
and from (up to 8.4 Mbps)
• Maximum data rates also depend on the
distance from the customer premises to
the carrier’s end office. (see Figure 7-7).
of Local Loop
Upstream Rate
18,000 feet
1.5 Mbps
384 Kbps
16,000 feet
2.0 Mbps
384 Kbps
12,000 feet
6.1 Mbps
384 Kbps
9,000 feet
8.4 Mbps
640 Kbps
Note: E1 and E2 are the European standard services
similar to T1 and T2 services in North America
Figure 7-7 ADSL data rates
Very-High-Data-Rate DSL (VDSL)
• VDSL is a high-speed member of the DSL
family designed for local loops of 4500 feet or
less, but the protocol is not yet standardized.
• The upstream and downstream data rates for
VDSL’s channels depend on the distance of
the end user from the nearest telephone
exchange and are listed in Figure 7-8.
of Local Loop
4,500 feet
13 Mbps
1.6 Mbps
3,000 feet
26 Mbps
2.3 Mbps
1,000 feet
52 Mbps
2.3 Mbps
Figure 7-8 Anticipated VDSL data rates
Cable Modems
• The most important high speed alternative to DSL
today is “cable modem”, a digital service offered
by cable television companies.
• Although not a formal standard, the Data Over
Cable System Interface Specification (DOCSIS)
is now a widely accepted industry standard for
cable modem communications and has led to the
production of standardized equipment.
• Most cable companies provide their services using
hybrid fiber coax (HFC) networks that combine
optical fiber backbones with coax cable access
Cable Modem Topology (Figure 7-9)
• Cable modems use shared multipoint circuits. Data
is split off by the cable splitter, then the cable
modem translates the data into 10BaseT frames.
• The coax cable leaving the customer premises
connects to a fiber node, which converts the
converts the coaxial cable’s electrical signal into a
light signal using an opto-electrical converter.
• Two circuits connect to the fiber node:
– The upstream circuit connects to the cable modem
termination system (CMTS), which is then connects to
the ISP
– The downstream circuit connects to a combiner where it
is combined with the incoming cable signal.
Figure 7-9 Cable modem architecture
Fixed Wireless
• Fixed Wireless is another “dish-based” microwave
transmission technology.
• It requires “line of sight” access between
• Both point-to-point and point-multipoint forms are
• Multipoint forms connect a multiplexed group of
users from a single location to the wireless service
provider’s network (e.g., an apartment building).
• Data access speeds range from 1.5 to 11 Mbps
depending on the vendor.
Fixed Wireless (Figure 7-10)
• Fig. 7-10 is an example of fixed wireless technology.
• Transmissions travel between transceivers at the customer
premises and ISP’s wireless access office.
• Incoming signals at the customer site are first demultiplexed and then sent to the MDF where the signal is
combined with voice transmissions.
• This combined signal is then distributed to individual
customer premises where a line splitter separates out the
voice communications.
• The data transmission is then sent to a DSL modem which is
connected to a hub on the customer’s LAN.
• The transceiver at the wireless access office is connected to
a router which then sends outgoing packets over the
Figure 7-10 Fixed wireless architecture
Satellite Internet Access
• For Internet access via satellite, a small satellite
dish is installed outside the home or office.
• Satellite Internet services usually provide
downstream data rates of about 500 kbps and 128
kbps upstream.
• One problem with the service is propagation
delay, due to distance the signal must travel,
resulting in relatively slow response times.
• For example, a to get a response from a Web
server, a signal must travel from the user’s site to
the satellite, then down to the ISP and back, or
about 90,000 miles, ½ second at the speed of light.
Mobile Wireless
• 1G cell phones of the 1980s were analog (1G).
• Digital 2G cell phones followed in the mid-1990s but
they are only capable of low speed data
communications (ca. 14.4 kbps).
– The Global System for Mobile communications (GSM) is
the most popular 2G mobile phone standard.
• 3G wireless, officially known as UMTS, is intended
to be higher speed, but is not yet standardized.
– Enhanced Data GSM Environment (EDGE) is a possible
3G standard, with a proposed data rate of 384 kbps.
– Other proposals are for 2Mbps 3G services.
• Discussions on 4G at even higher data rates have
already begun.
Mobile Wireless Protocols
• Mobile wireless uses the wireless
application protocol (WAP) used by the
wireless application environment (WAE).
• WAP uses WAE and Wireless Markup
Language (WML) instead of HTTP and
• These protocols streamline access to the
web since the low speed and small screen
mobile networking environment is order to
make Web access practical.
Basic WAP Architecture (Figure 7-11)
• WAP clients (e.g., cell phone or palm computer) run a WAP
program called a WAE user agent that generates WAE
requests and sends them to the WAP gateway.
• The WAP gateway transceiver next passes the requests to a
wireless telephony application (WTA) server.
• The server sends WAE responses back to the WAP client.
• If the client has requested a Web page, the WAE request is
sent to a WAP proxy which translates both outgoing requests
from WAE to HTTP and incoming HTTP responses back into
• The WAE responses are then sent back to the WTA server
which, in turn, sends them back to the WAP client.
Fig 7-11 Mobile wireless architecture for WAP applications
Future Access Technologies
• Two key future Internet access technologies are:
• Passive Optical Networking (PON)
– PON, part of fiber-to-the-home will unleash the
potential of fiber optics to home users.
– Passive optical splitters don’t require electricity,
lowering cost, but limiting its maximum distance to
about 10 miles.
– WDM is also used so hundreds or thousands of
channels are possible at very high speeds.
• Ethernet to the Home
– Gives home users 10BaseT or 100BaseT connections.
– is now doing this in several large US cities.
– The common carrier installs TCP/IP routers connected
to an Ethernet MAN.
Internet Governance
ISOC and Internet Governance
• The Internet Society (ISOC) is the closest thing to an
“owning” organization that exists for the Internet.
• ISOC is an open society whose members include 175
organizational and 8,000 professional members worldwide.
• ISOC works in three areas:
– In public policy by participating in national and
international debates on issues such as censorship,
copyright laws, privacy and universal access.
– In education, ISOC provides training and education
programs aimed at improving Internet infrastructure in
developing nations.
– In standards, ISOC works through four inter-related
standards bodies: IETF, IESG, IAB and IRTF.
4 ISOC-related Standards Bodies
• Internet Engineering Taskforce (IETF) includes network
designers, vendors, and researchers who develop new
Internet architecture. IETF sends out requests for comment
(RFCs) which form the basis of new Internet standards.
• Internet Engineering Steering Group (IESG) is responsible
for technical management of IETF activities and standards
and is governed by rules ratified by ISOC trustees. Each
IETF group is chaired by an IESG member.
• Internet Architecture Board (IAB) provides strategic
direction by promoting which actions the IETF and IESG
should take. The IAB also elects the IETF chair and all IESG
members out of the IETF nominating committee’s list.
• Internet Research Taskforce (IRTF) works through small
research groups focused on specific research topics. IETF
generally works on short-term issues, IRTF works on longterm ones related to Internet protocols, applications,
architecture, and technology.
Internet 2
Internet 2 (Figure 7-12)
• New networks are being developed to develop future Internet
technologies including:
– The very high performance Backbone Network Service
(vBNS) run by Worldcom. 34 universities participate.
– The Abilene network (also called Internet 2) is being
developed by the University Corporation for Advanced
Internet Development (UCAID).
– CA*Net3 is the Canadian government initiative.
• Access is through Gigapops, similar to NAPs, but which
operate at very high speeds (622 Mbps to 2.4 Gbps) using
SONET, ATM and IPv6 protocols (see Figure 7-13).
• Protocol development focuses on issues like Quality of
Service and multicasting.
• New applications include tele-immersion and
Figure 7-12 Gigapops and high speed backbones
of Internet 2/Abilene, vBNS, and CA*Net 3
Fig. 7-13 Inside the Pacific Northwest gigapop
The Best Practice Internet Access
• If mobility is important, then WAP is really
the only viable alternative at present.
• Choosing between cable modem, DSL and
satellite is difficult because each technology
has shared elements that can impact
performance and vary from location to
• The main factors to investigate are:
– number of users on shared segments
– Capacity from the ISPs POP to the Internet
End of Chapter 7

Chapter 1. Introduction to Data Communications