Unit 4:
WANs and Wireless Networks
Section 1: Wide Area Network Technologies
In this section you will learn about configuring TCP/IP addresses and
subnet masks.
At the end of this section you should be able to:
1. Identify and describe common Wide Area Network
Technologies used to connect to the Internet
2. Identify and describe common Wide Area Network
Technologies used on public networks
3. Identify Remote Access methods and Virtual Private
Network (VPN) types
Objective 1: Identify and Describe common WAN
Technologies used to connect to the Internet
The main technologies used by most individuals and businesses to connect to the Internet have been
the telephone network, cable systems, or wireless technologies such as satellite and WiMax. In this
section we will look at the wired network options including the telephone and cable networks. In section
2 we will look at Wireless solutions including satellite and WiMax.
Public Switched Telephone Network (PSTN)
Network of lines, carrier equipment providing telephone service often referred to as POTS (plain old
telephone service). In a PSTN system, the customers are connected to a Central Office (CO) in what is
called the Local Loop. The COs are interconnected using common network topologies including Ring,
Bus, Star, and Mesh.
Originally the POTS network carried only analog voice traffic. Today the system has been updated to
carry digital data, and uses computer controlled switching. Dial-up connections were one of the first
ways the public accessed the Internet. While dial up networking is very slow, it is still used today in
remote locations.
– Modem connects computer to distant network
– Works from almost anywhere
The PSTN can support a variety of WAN technologies including DSL, ISDN, and T1 connections. Of these
WAN technologies, DSL is the most rapidly growing technology for Internet connections both for
individuals and businesses as it provides high speed, continuous Internet connect where available.
DSL (digital subscriber line)
DSL operates over PSTN at the physical layer and shares voice and data over same telephone line by
using inaudible high range frequencies (voice uses only 300 – 3000 Hz). In addition, DSL uses advanced
data modulation techniques including amplitude and phase modulation providing for higher data rates.
Today, DSL competes with other PSTN services such as ISDN and T1. In many cases, high speed DSL
combined with Virtual Private Networks (VPNs) is replacing T1 lines.
DSL divides communications into downstream and upstream. In Downstream, the data travels from
carrier’s switching facility to customer. More speed is needed to day to stream audio and video contents
from Web sites such as YouTube. In Upstream, data travels from customer to carrier’s switching facility.
Upstream speed is becoming more important as customers wish to upload information and pictures to
social media sites. There are two major categories of DSL:
Asymmetrical - different upstream and downstream speeds
Symmetrical – Same upstream and downstream speeds
DSL services can be classified into several types based on the category and speed, generally referred to
as xDSL. The types of xDSL include; ADSL, G.Lite, HDSL, SDSL, VDSL, SHDSL. Read pages 318 though 320
to learn about these types of DSL services shown below. The Network + objectives expect you to know
the differences between ADSL, HDSL, and VDSL.
Broadband Cable
Cable companies connectivity option that is based on TV signals coaxial cable wiring. As in the case of
DSL, the cable system Operates at Physical and Data Link layer. Because of the cable bandwidth, the
theoretical transmission speeds are very fast compared to DSL running over standard twisted pair. Cable
may off theoretical transmission speeds of 150 Mbps downstream and up to 10 Mbps upstream.
However, because of overhead, thransmission speed throttling, and multiple customers sharing cable
segments, real transmission speeds are often 10 Mbps downstream and 2 Mbps upstream.
Requires cable modem that may be built into a Wireless Access Point or switch as shown in
Figure 7-18 of the book. The Cable modem modulates, demodulates transmission, reception
signals via cable wiring.
Objective 2: Identify and Describe common WAN
Technologies used on public networks
Another major use of WAN technologies is providing high bandwidth connections between data centers.
Prior to the Internet, these technologies were leased by IT departments to allow connections between
there corporate or organizational networks. In this objective we will look at several of these
technologies.
X.25 ITU standard
As described on page 309, X.25 is an analog, packet-switching technology from 1970s. Initially it was
used to connect terminals located in remote locations to mainframe computers and limited to 64 Kbps
throughput. While 64 Kbps was sufficient for simple terminal connections, by 1992 the X.25 system was
updated to support 2.048 Mbps throughput to support more intensive client server server WAN
requirements.
X.25 pperates at Physical, Data Link, and Network layers, and verifies transmission at every node. As a
result X.25 has excellent flow control and ensures data reliability. However, it is it is too slow for timesensitive or large data transmission applications, and because it operates at the Network layer, it is not
compatible with TCP/IP. Today X.25 is a dead technlogy that has been replaced by Frame Relay.
Frame relay
Frame Relay is an updated version of X.25 packet-switching network that is digital and operates at the
Data Link layer making it compatible with other procols such as TCP/IP.
Unlike X.25, Frame relay is a connectionless system that offers no data delivery guarantee. It has the
following characteristics as described on pages 309 – 310.


Customer chooses data speed
Uses Virtual Circuits that are connections between network nodes that while based on
potentially disparate physical links, appear to be a direct dedicated link.
o PVC – Permanent Virtual Circuit –
 Connection remains between sessions
 Used to connect sites that have large and frequent transmissions.
o SVC – Switched Virtual Circuit
 Connection established with session and disconnected at end of session.
 Used with more infrequent and smaller transmissions.
ISDN
Integrated Service Digital Network (ISDN) is an internation standard established by the ITU in
the mid-1980s for transmitting data over the PSTN using either dial-up or dedicated
connections. While very popular in Europe, due to telephone switch company incompatibilities,
ISDN never took off well in North America.
Read pages 311-312 to learn more about ISDN. Some facts are listed below:
ISDN is an ITU standard for digital data transmitted over PSTN
Relies on PSTN for transmission medium
Exchanges data, voice signals
Dial-up or dedicated connections using exclusively on digital transmission
ISDN protocols at Physical, Data Link, Transport layers making it independent of TCP/IP
Gained popularity in the 1990s to connect WAN locations. Most commonly used in
Europe. ISDN has two channel types:
B channel: “Bearer”
Circuit switching for voice, video, audio: 64 Kbps
D channel: “data”
Packet-switching information: 16 or 64 Kbps
BRI (Basic Rate Interface) connection
–
–
2 B channels that may be bonded to create 128 Kbps circuit
1 D channel for network control information
PRI (Primary Rate Interface) connection
–
–
23 B channels that may be bonded to create one or more circuits with up to 1.544 Mbps
1 D channel for network control information
In the above diagrams, the NT (Network Termination) devices connect the twisted pair wiring at the
customer’s building with the ISDN switch using RJ-11 (standard phone connector) or RJ-45 (Ethernet)
data jack. The TE (terminal equipment) may include cards or stand-alone devices used to connect to
ISDN. The TA (Terminal Adapter) convets digital signals into analog signals for use with ISDN phones and
other analog devices. In this example, the workstation is using an analog modem to connect to the ISDN
system.
ATM (Asynchronous Transfer Mode)
ATM is an updated version of Frame Relay that functions in the Data Link layer and is designed to be
used with different physical and network layer protocols. In this way, ATM is similar to Ethernet except
that is uses a small 53-byte fixed length packet called a Cell. The Cell carries 48 data bytes along with a
5-byte header. For a better explanation of ATM and its use, read pages 324 – 325 in the text book.
Following are some points of interest you should know for the Network + objectives:
ATM Functions in Data Link layer and may be used to carry different network layer protocols
over a variety of physical network systems including SONet.
ATM uses an asynchronous communications method.
– Nodes do not conform to predetermined schemes specifying data transmissions timing
– Each character transmitted with start and stop bits
Specifies Data Link layer framing techniques using fixed packets (cell)
– 48 data bytes plus 5-byte header = 53 bytes
Smaller packet size requires more overhead
– Decrease potential throughput
– Cell efficiency compensates for loss
ATM relies on virtual circuits
– ATM considered packet-switching technology
– Virtual circuits provide circuit switching advantage
• Circuit path setup by switches in advance
– Reliable connection
Allows specific QoS (quality of service) guarantee
– Important for time-sensitive applications such as VOIP and video
Often used on SONET rings
T-Carriers
The previous WAN technologies have all included the Data Link layer in their protocol. However,
T-Carriers and SONET work at the physical layer to transmit bits, and can work with different
Data Link protocols.
T-Carriers are designed to transmit digital data at high speeds over the PSTN. A T-Carrier uses
Time Division Multiplexing (TDM) over 2-wire pairs to divide a single channel into multiple
channels of 64-Kbps. T-Carrier includes T1, fractional T1s, and T3s. A T1 is 24 64-Kbps channels
for a total of 1.544 Mbps. T-carrier signaling can be used on ordinary telephone wire, fiber-optic
cable, or wireless links.
Read pages 313 – 317 in the text book for a good description of T-carriers. Following are some
notes from the textbook and presentation.
T-Carriers operate at the Physical layer and consist of T1s, fractional T1s, T3s
Single channel divided into multiple channels
– Using TDM (time division multiplexing) over two wire pairs. One pair for
transmiting and the other for receiving.
– Telephone wire, fiber-optic cable, wireless links
Smart Jacks along with CSU/DSU are used to terminate T-carrier wire pairs at the customer’s
demarc (demarcation point) which may be inside or outside building. The demarc also acts
as a connection monitoring point.
SONET (Synchronous Optical Network)
Read pages SONETs operate at the Physical layer and offer the following four key strenght when
compared to other WAN technologies:
– WAN technology integration
– Fast data transfer rates
– Simple link additions, removals
– High degree of fault tolerance due to the dual-ring system. If a cable or site is down, the
dual ring allows the data to be transferred to all other devices. This process is referred
to as “self healing”.
SONET is a synchronous network system that where nodes conform to a central timing scheme. SONET
uses a double-ring topology over fiber cable to provide fault tolerance or self-healing. In the SONET ring,
a connection begins and ends at the telecommunication carrier’s facility as shown below:
As shown in the diagram below, SONET rings terminate at a multiplexer and provide easy ring additions,
and removals.
SONET can carry data from a variety of WAN technologies including ISDN, T1, and ATM. The data rat ofa
particular SONET ring is indicated by its OC (Optical Carrier) level. You will be asked about some of these
OC levels in the Network + certification.
Because of its high cost and bandwidth, SONETs are usually used only by large corporations and
telecommunication companies. Read pages 325 – 327 for a complete description of the SONET
technology.
Objective 3: Identify Remote Access methods and Virtual
Private Network (VPN) types
Remote access provides the ability for a user to connect to another computer or network from a remote
location. In the past this was done primarily by using the dial-up PSTN to connect to a remote access
server. The remote access server would then verify the user’s name and password and allow the remote
computer to become a part of the local network as shown below.
Remote Access Protocols
There are two major types of remote access protocols used to support dial-up connections – SLIP and
PPP. When using these protocols, workstations connect using serial connection and encapsulate higherlayer networking protocols, in lower-layer data frames as shown below.
Read pages 328-333 for a description of dial-up networking and the SLIP and PPP remote access
protocols. Some differences are noted below:
SLIP is an earlier protocol designed to cary IP packets only
Harder to set up
Supports only asynchronous data
PPP carries many different Network layer packets
Automatic set up
Performs error correction, data compression, supports encryption
Supports asynchronous and synchronous transmission
Today, PPP is used when connecting to the Internet through a dial-up connection. A variation of
PPP called PPPoE (PPP over Ethernet), is used when connecting to the Internet using DSL or
cable modems.
Remote Desktop Infrastructure
As described in Unit 3, Remote Desktop is a remote access protocol that allows a user to open a
window to the desktop of another computer and then operation that system the same as if they
were at the remote computer’s console. Following is some notes from pages 333-334 of the
textbook.
Windows client and server operating systems
Relies on RDP (Remote Desktop Protocol)
Application layer protocol
Uses TCP/IP to transmit graphics, text quickly
Carries session, licensing, encryption information
Exists for other operating systems
Not included in Windows home editions
Uses Port 3389 by default
Today, the remote desktop infrastructure allows a server computer to run multiple systems
called virtual computers. Each virtual computer is a Windows desktop environment that can be
accessed from systems called “thin clients”. A thin-client is a device that acts like a terminal to
the virtual computer running on the server. From the thin-client a user can run programs and
access data just as if they were sitting at the actual computer. BYOD systems can also run the
thin-client software, making it possible for a user to access their desktop computer from a table
or notebook system.
VPNs (Virtual Private Networks)
A VPN replaces the older dial-up network system by using the Internet to connect to a remote access
server rather than a dial-up connection. A VPN creates a secure “tunnel” which acts like a phone
connection to transfer all data packets from the remote computer to the remote access server as shown
below.
Read pages 336 – 338 for an explanation of VPNs. Following are notes based on the presentation and
texbook materials:
Types of VPN tunnels
PPTP (Point-to-Point Tunneling Protocol)
Microsoft
Authentication and minimal encryption
L2TP (Layer 2 Tunneling Protocol)
Developed by Cisco
Uses IPSec encryption at Network Layer to provide strong security
SSL VPN
Uses SSL encryption at Presentation level
• Establishes connection at Browser level
An SSL VPN (Secure Sockets Layer virtual private network) is a form of VPN that can be used with
a standard Web browser.
In contrast to the traditional Internet Protocol Security (IPsec) VPN, an SSL VPN does not require
the installation of specialized client software on the end user's computer.
Used to give remote users with access to Web applications, client/server applications and
internal network connections.
OpenVPN Tunnel
OpenVPN is an SSL VPN and as such is not compatible with IPSec, L2TP, or PPTP.
–
The IPSec protocol is designed to be implemented as a modification to the IP stack in
kernel space, and therefore each operating system requires its own independent
implementation of IPSec.
–
By contrast, OpenVPN's user-space implementation allows portability across operating
systems and processor architectures, firewall and NAT-friendly operation, dynamic
address support, and multiple protocol support including protocol bridging.
–
There are advantages and disadvantages to both approaches. The principal advantages
of OpenVPN's approach are portability, ease of configuration, and compatibility with
NAT and dynamic addresses. The learning curve for installing and using OpenVPN is on
par with that of other security-related daemon software such as SSH.
Chapter 7 Review Questions
1. Which of the following WAN topologies comes with the highest availability and the
greatest cost?
Page 304
2. Which of the following elements of the PSTN is most likely capable of transmitting only
analog signals?
Page 307
3. A customer calls your ISP’s technical support line, complaining that his connection to the
Internet usually goes as fast as 128 Kbps, but today it is only reaching 64 Kbps. He adds
that he has tried dialing up three different times with the same result. What type of
connection does this customer have?
Page 311
4. What is the purpose of ISDN’s D channel?
Page 311
5. Suppose you work for a bank and are leasing a frame relay connection to link an
automatic teller machine located in a rural grocery store with your bank’s headquarters.
Which of the following circuits would be the best option, given the type of use this
automatic teller machine will experience?
Page 309
6. On an ISDN connection, what device separates the voice signal from the data signals at
the customer premises?
Page 312
7. Which of the following WAN technologies operates at Layer 3 of the OSI model?
Page 322
8. What technique enables DSL to achieve high throughput over PSTN lines?
Page 318
9. Suppose you establish a home network and you want all three of your computers to share
one broadband cable connection to the Internet. You decide to buy a router to make this
sharing possible. Where on your network should you install the router?
Page 322
10. How does ATM differ from every other WAN technology described in this chapter?
Page 324
11. You work for an Internet service provider that wants to lease a T3 over a SONET ring.
What is the minimum Optical Carrier level that the SONET ring must have to support the
bandwidth of a T3?
Page 327
12. Which two of the following are asymmetrical versions of DSL?
Page 318
13. What technique does T1 technology use to transmit multiple signals over a single
telephone line?
Page 313
14. Where on the PSTN would you most likely find a DSLAM?
Page 321
15. The science museum where you work determines that it needs an Internet connection
capable of transmitting and receiving data at 12 Mbps at any time. Which of the
following T-carrier solutions would you advise?
Page 313
16. A local bookstore that belongs to a nationwide chain needs a continuously available
Internet connection so that staff can search for the availability of customer requests in the
database stored at the bookstore’s headquarters. The maximum throughput the store needs
is 768 Kbps. Which of the following options would best suit the store?
Page 319
17. What part of a SONET network allows it to be self-healing?
Page 325
18. Which of the following may limit a DSL connection’s capacity?
Page 319
19. You work for a consulting company that wants to allow telecommuting employees to
connect with the company’s billing system, which has been in place for 10 years. What
do you suggest as the most secure and practical means of providing remote LAN access
for this application?
Page 32920. Why is broadband cable less commonly used by businesses than DSL or T-carrier
services?
Page 32121. You’re troubleshooting a problem with poor performance over a WAN connection at your
office. Looking at the smart jack, you see the Tx light is blinking green and the Rx light
is not illuminated. What can you conclude about the problem?
Page 315
22. Your company has decided to order ADSL from its local telecommunications carrier. You
call the carrier and find out that your office is located 17,000 feet from the nearest CO.
Given ADSL’s potential throughput and your distance from the CO, what is the maximum
downstream throughput you can realistically expect to achieve through this connection?
Page 319 (Note: This applies to “Full Rate” ADSL, whose maximum potential
downstream throughput is 8 Mbps.)
23. In which of the following situations would you use RDP?
Page 33324. You have decided to set up a VPN between your home and your friend’s home so that
you can run a private digital telephone line over your DSL connections. Each of you has
purchased a small Cisco router for terminating the VPN endpoints. Which of the
following protocols could you use to create a tunnel between these two routers?
Page 337
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