CHAPTER 5
WAN Technologies
This chapter covers the following official Network+ objective:
Compare and contrast WAN technologies.
This chapter covers CompTIA Network+ objective 2.5. For more information on the official Network+ exam topics, see the “About the Network+ Exam” section in the Introduction.
If you think of networking as something that can be represented on a plane, there would be two ends of the spectrum. At one end of the spectrum, there would be small networks of only a few nodes and devices connected together. At the other end of the spectrum is the wide-area network (WAN): an amalgamation of multiple local-area networks (LANs) creating an entity that is only as strong as its weakest link.
This chapter focuses mostly on the WAN end of the spectrum, but also covers such small network practices such as dial-up connectivity. It also looks at best practices and safety practices associated with networking today.
WAN Technologies
Compare and contrast WAN technologies.
CramSaver
If you can correctly answer these questions before going through this section, save time by skimming the Exam Alerts in this section and then completing the Cram Quiz at the end of the section.
1. What are T-lines used for, and what is the maximum speed of T1 and T3?
2. What are the X.25 transmission speed restrictions?
3. What is the difference between circuit switching and packet switching technologies?
Answers
1. T-carrier lines create point-to-point network connections for private networks. T1 lines offer transmission speeds of up to 1.544 Mbps, whereas T3 lines offer transmission speeds of 44.736 Mbps.
2. X.25 is restricted to transmission rates of 56 Kbps or 64 Kbps with digital implementations.
3. Circuit switching networking offers a dedicated transmission channel that is reserved until it is disconnected. Packet switching enables packets to be routed around network congestion.
Many of today’s network environments are not restricted to a single location or LAN. Instead, many networks span great distances, becoming wide-area networks (WANs). When they do, hardware and software are needed to connect these networks. This section reviews the characteristics of various WAN technologies.
Integrated Services Digital Network
ISDN has long been an alternative to the slower modem WAN connections but at a higher cost. ISDN enables the transmission of voice and data over the same physical connection.
ISDN connections are considerably faster than regular modem connections. To access ISDN, a special phone line is required. This line usually is paid for through a monthly subscription. You can expect these monthly costs to be significantly higher than those for traditional dial-up modem connections.
To establish an ISDN connection, you dial the number associated with the receiving computer, much as you do with a conventional phone call or modem dial-up connection. A conversation between the sending and receiving devices is then established. The connection is dropped when one end disconnects or hangs up. The line pickup of ISDN is fast, enabling a connection to be established, or brought up, much more quickly than a conventional phone line.
ISDN has two defined interface standards: Basic Rate Interface (BRI) and Primary Rate Interface (PRI).
BRI
BRI ISDN uses three separate channels—two bearer (B) channels of 64 Kbps each and a delta channel of 16 Kbps. B channels can be divided into four D channels, which enable businesses to have eight simultaneous Internet connections. The B channels carry the voice or data, and the D channels are used for signaling.
Note
BRI ISDN channels can be used separately using 64 Kbps transfer or combined to provide 128 Kbps transfer rates.
PRI
PRI is a form of ISDN that generally is carried over a T1 line and can provide transmission rates of up to 1.544 Mbps. PRI is composed of 23 B channels, each providing 64 Kbps for data/voice capacity, and one 64 Kbps D channel, which is used for signaling.
Comparing BRI and PRI ISDN
Table 5.1 compares BRI to PRI ISDN.
ExamAlert
ISDN is considered a leased line because access to ISDN is leased from a service provider.
TABLE 5.1 BRI to PRI ISDN Comparison
|
Characteristic |
BRI |
PRI |
|
Speed |
128 Kbps |
1.544 Mbps |
|
Channels |
2B+D |
23B+D |
|
Transmission carrier |
ISDN |
T1 |
Note
It is recommended that you know how PRI compares to BRI and their basic characteristics shown in Table 5.1.
Leased Lines
T-carrier lines are high-speed dedicated digital lines that can be leased from telephone companies. This creates an always-open, always-available line between you and whomever you choose to connect to when you establish the service.
T-carrier lines can support both voice and data transmissions and are often used to create point-to-point private networks. Because they are a dedicated link, they can be a costly WAN option. Four types of T-carrier lines are available:
T1: Offers transmission speeds of 1.544 Mbps and can create point-to-point dedicated
digital communication paths. T1 lines have commonly been used for connecting LANs.
In North America, DS (digital signal) notation is used with T-lines to describe the
circuit. For all practical purposes, DS1 is synonymous with T1.
T2: Offers transmission speeds of 6.312 Mbps. It accomplishes this by using 96 64 Kbps
B channels.
T3: Offers transmission speeds of up to 44.736 Mbps, using 672 64 Kbps B channels. Digital
signal 3 (DS3) is a more accurate name in North America, but T3 is what most refer
to the link as.
ExamAlert
When you take the exam, think of DS3 and T3 as synonymous.
T4: Offers impressive transmission speeds of up to 274.176 Mbps by using 4,032 64 Kbps
B channels.
ExamAlert
Of these T-carrier lines, the ones commonly associated with networks and the ones most likely to appear on the exam are the T1 and T3 lines.
Note
Because of the cost of a T-carrier solution, you can lease portions of a T-carrier service. This is known as fractional T. You can subscribe and pay for service based on 64 Kbps channels.
T-carrier is the designation for the technology used in the United States and Canada. In Europe, they are called E-carriers, and in Japan, J-carriers. Table 5.2 describes the T/E/J carriers.
TABLE 5.2 Comparing T/E/J Carriers
|
Name |
Transmission Speed |
|
T1 |
1.544 Mbps |
|
T1C |
3.152 Mbps |
|
T2 |
6.312 Mbps |
|
T3 |
44.736 Mbps |
|
T4 |
274.176 Mbps |
|
J0 |
64 Kbps |
|
J1 |
1.544 Mbps |
|
J1C |
3.152 Mbps |
|
J2 |
6.312 Mbps |
|
J3 |
32.064 Mbps |
|
J3C |
97.728 Mbps |
|
J4 |
397.200 Mbps |
|
E0 |
64 Kbps |
|
E1 |
2.048 Mbps |
|
E2 |
8.448 Mbps |
|
E3 |
34.368 Mbps |
|
E4 |
139.264 Mbps |
|
E5 |
565.148 Mbps |
ExamAlert
Ensure that you review the speeds of the T1, T3, E1, and E3 carriers.
T3 Lines
For a time, the speeds offered by T1 lines were sufficient for all but a few organizations. As networks and the data they support expanded, T1 lines did not provide enough speed for many organizations. T3 service answered the call by providing transmission speeds of 44.736 Mbps.
T3 lines are dedicated circuits that provide high capacity; generally, they are used by large companies, ISPs, and long-distance companies. T3 service offers all the strengths of a T1 service (just a whole lot more), but the cost associated with T3 limits its use to the few organizations that have the money to pay for it.
Fiber, SONET, and OCx Levels
In 1984, the U.S. Department of Justice and AT&T reached an agreement stating that AT&T was a monopoly that needed to be divided into smaller, directly competitive companies. This created a challenge for local telephone companies, which were faced with the task of connecting to an ever-growing number of independent long-distance carriers, each of which had a different interfacing mechanism. Bell Communications Research answered the challenge by developing Synchronous Optical Network (SONET), a fiber-optic WAN technology that delivers voice, data, and video at speeds starting at 51.84 Mbps. Bell’s main goals in creating SONET were to create a standardized access method for all carriers within the newly competitive U.S. market and to unify different standards around the world. SONET is capable of transmission speeds from 51.84 Mbps to 2.488 Gbps and beyond.
One of Bell’s biggest accomplishments with SONET was that it created a new system that defined data rates in terms of Optical Carrier (OCx) levels. Table 5.3 lists the OCx levels you should be familiar with.
ExamAlert
Before taking the exam, review the information provided in Table 5.3. Be sure that you are familiar with OC-3 and OC-192 specific transmission rates.
TABLE 5.3 OCx Levels and Transmission Rates
|
OCx Level |
Transmission Rate |
|
OC-1 |
51.84 Mbps |
|
OC-3 |
155.52 Mbps |
|
OC-12 |
622.08 Mbps |
OC-24 |
1.244 Gbps |
|
OC-48 |
2.488 Gbps |
|
OC-96 |
4.976 Gbps |
|
OC-192 |
9.953 Gbps |
|
OC-768 |
39.813 Gbps |
Note
Optical carrier (OCx) levels represent the range of digital signals that can be carried on SONET fiber-optic networks. Each OCx level defines the speed at which it operates.
Synchronous Digital Hierarchy (SDH) is the European counterpart to SONET.
ExamAlert
When you take the exam, equate SDH with SONET.
A passive optical network (PON) is one in which unpowered optical splitters are used to split the fiber so it can service a number of locations, and it brings the fiber either to the curb, the building, or the home. It is known as a passive system because there is no power to the components and consists of an optical line termination (OLT) at the split and a number of optical network units (ONUs) at the end of each run (typically near the end user). It can be combined with wavelength division multiplexing and is then known as WDM-PON.
A form of multiplexing optical signals is dense wavelength-division multiplexing (DWDM). This method replaces SONET/SDH regenerators with erbium doped fiber amplifiers (EDFAs) and can also amplify the signal and enable it to travel a greater distance. The main components of a DWDM system include the following:
Terminal multiplexer
Line repeaters
Terminal demultiplexer
Note
Chapter 6, “Cabling Solutions,” discusses several other methods of multiplexing.
An alternative to DWDM is coarse wavelength-division multiplexing (CWDM). This method is commonly used with television cable networks. The main thing to know about it is that it has relaxed stabilization requirements; thus, you can have vastly different speeds for download than upload.
ExamAlert
Make sure that you associate CWDM with television cabling.
Frame Relay
To understand Frame Relay, it is important to understand some ancient history and X.25. X.25 was one of the original packet-switching technologies, but today it has been replaced in most applications by Frame Relay. Various telephone companies, along with network providers, developed X.25 in the mid-1970s to transmit digital data over analog signals on copper lines. Because so many entities had their hands in the development and implementation of X.25, it works well on many kinds of networks with different types of traffic. X.25 is one of the oldest standards, and therein lies both its greatest advantage and its greatest disadvantage. On the upside, X.25 is a global standard that can be found all over the world. On the downside, its maximum transfer speed is 56 Kbps—which is reasonable when compared to other technologies in the mid-1970s but slow and cumbersome today. However, in the 1980s a digital version of X.25 was released, increasing throughput to a maximum of 64 Kbps. This, too, is slow by today’s standards.
Because X.25 is a packet-switching technology, it uses different routes to get the best possible connection between the sending and receiving device at a given time. As conditions on the network change, such as increased network traffic, so do the routes that the packets take. Consequently, each packet is likely to take a different route to reach its destination during a single communication session. The device that makes it possible to use the X.25 service is called a packet assembler/disassembler (PAD), which is required at each end of the X.25 connection.
At its core, Frame Relay is a WAN protocol that operates at the physical and data link layers of the OSI model. Frame Relay enables data transmission for intermittent traffic between LANs and between endpoints in a WAN.
Frame Relay was designed to provide standards for transmitting data packets in high-speed bursts over digital networks, using a public data network service. Frame Relay is a packet-switching technology that uses variable-length packets. Essentially, Frame Relay is a streamlined version of X.25. It uses smaller packet sizes and fewer error-checking mechanisms than X.25; consequently, it has less overhead than X.25.
A Frame Relay connection is built by using permanent virtual circuits (PVCs) that establish end-to-end communication. This means that Frame Relay is not dependent on the best-route method of X.25. Frame Relay can be implemented on several WAN technologies, including 56 Kbps, T1, T3, and ISDN lines.
To better understand how it works, look at some of the components of Frame Relay technology. All devices in the Frame Relay WAN fall into two primary categories:
Data terminal equipment (DTE): In the Frame Relay world, the term DTE refers to terminating equipment located within a company’s network. Termination equipment
includes such hardware as end-user systems, servers, routers, bridges, and switches.
Data circuit-terminating equipment (DCE): DCE refers to the equipment owned by the carrier. This equipment provides the switching
services for the network and therefore is responsible for actually transmitting the
data through the WAN.
As previously mentioned, Frame Relay uses virtual circuits to create a communication channel. These virtual circuits establish a bidirectional communication link between DTE devices. Two types of virtual circuits are used with Frame Relay:
Permanent virtual circuit (PVC): A permanent dedicated virtual link shared in a Frame Relay network, replacing a hard-wired
dedicated end-to-end line.
Switched virtual circuit (SVC): Represents a temporary virtual circuit established and maintained only for the duration
of a data transfer session.
Figure 5.1 shows the components of a Frame Relay network.
FIGURE 5.1 A Frame Relay network
Asynchronous Transfer Mode
Introduced in the early 1990s, Asynchronous Transfer Mode (ATM) was heralded as a breakthrough technology for networking because it was an end-to-end solution, ranging in use from a desktop to a remote system. Although it was promoted as both a LAN and WAN solution, ATM did not live up to its hype due to associated implementation costs and a lack of standards. The introduction of Gigabit Ethernet, which offered great transmission speeds and compatibility with existing network infrastructure, further dampened the momentum of the ATM bandwagon. ATM has, however, found a niche with some ISPs and is also commonly used as a network backbone. It provides functionality that combines the benefits of both packet switching and circuit switching.
ATM is a packet-switching technology that provides transfer speeds ranging from 1.544 Mbps to 622 Mbps. It is well suited for a variety of data types, such as voice, data, and video. Using fixed-length packets, or cells, that are 53 bytes long, ATM can operate more efficiently than variable-length-packet packet-switching technologies such as Frame Relay. Having a fixed-length packet allows ATM to be concerned only with the header information of each packet. It does not need to read every bit of a packet to determine its beginning and end. ATM’s fixed cell length also makes it easily adaptable to other technologies as they develop. Each cell has 48 bytes available for data, with 5 bytes reserved for the ATM header.
ATM is a circuit-based network technology because it uses a virtual circuit to connect two networked devices. Like Frame Relay, ATM is a circuit-based network technology that also uses PVCs and SVCs. PVCs and SVCs were discussed in the preceding section.
ATM is compatible with the most widely used and implemented networking media types available today, including single-mode and multimode fiber, coaxial cable, unshielded twisted-pair, and shielded twisted-pair. Although ATM can be used over various media, the limitations of some of the media types make them impractical choices for deployment in an ATM network. ATM can also operate over other media, including FDDI, T1, T3, SONET, OC-3, and Fibre Channel.
Copper Versus Fiber
The WAN technology used is often based on the infrastructure on which it is built, and the infrastructure—in turn—is based on the WAN technology that you want to deploy. The symbiotic relationship between the technology and the media is one of great interdependence.
The two most common media in use with WANs are copper and fiber. Both are explored in much more detail in the next chapter, “Cabling Solutions,” but Table 5.4 provides an overview of the technologies discussed so far and the media that support them.
TABLE 5.4 Comparing WAN Technologies
ExamAlert
For the Network+ exam, be sure that you can identify the characteristics of the various WAN technologies from Table 5.4.
Other WAN Technologies
Table 5.4 lists the most popular WAN technologies used today, but you should be aware of several others as well:
PPP: Point-to-Point Protocol is a data link protocol that is used to establish a connection
between two nodes. PPP works with plain old telephone service (POTS), ISDN, fiber links such as SONET, and other faster connections, such as T1. PPP does
not provide data security, but it does provide authentication using the Challenge Handshake Authentication Protocol (CHAP). A PPP connection allows remote users to log on to the network and have access as
though they were local users on the network. PPP by itself does not provide for any
encryption services for the channel. As you might have guessed, the unsecure nature
of PPP makes it largely unsuitable for WAN connections. To counter this issue, other
protocols have been created that take advantage of PPP’s flexibility and build on
it. For example, PPP can be used with the Encryption Control Protocol (ECP). You should make sure that all your PPP connections use secure channels, dedicated
connections, or high-speed connections.
PPPoE: Point-to-Point Protocol over Ethernet offers the capability to encapsulate PPP frames
inside Ethernet frames. It was first utilized to tunnel packets over a DSL connection
to an ISP’s IP network. Although it has been around for many years, it is still widely
used by DSL Internet providers.
Note
PPPoE is not used by cable (or fiber) Internet providers.
ExamAlert
Be sure you understand the WAN technology characteristics of service for MPLS, PPPoE, PPP, DMVPN and SIP trunk.
Multilink PPP: Building off of PPP, Multilink PPP allows you to configure multiple links to act
as one, thus increasing the speed of the connection. This technology has gained popularity
with the cloud, but still suffers from problems inherent with PPP.
MPLS (Multiprotocol Label Switching): Used in high-performance-based telecom networks, MPLS is a technology that uses short
path labels instead of longer network addresses to direct data from one node to another.
These “labels” are used to identify shorter virtual links between nodes instead of
endpoints. MPLS supports technologies such as ATM, Frame Relay, DSL, T1, and E1.
GSM/CDMA: The Global System for Mobile Communications (GSM) can work with code division-multiple access (CDMA) to provide various means of cell phone coverage. The methods that can be used include
LTE/4G, HSPA+, 3G, or Edge.
DMVPN: The Dynamic Multipoint Virtual Private Network offers the capability to create a
dynamic-mesh VPN network without having to preconfigure all the possible tunnel endpoints.
The hubs are statically configured, and then it dynamically builds out a hub-and-spoke
network by finding, and accepting, new spokes, thus requiring no additional configuration
on the hubs or the spokes.
Metro-Ethernet (Metropolitan Ethernet): This is nothing more than an Ethernet-based MAN (metropolitan-area network). Various
levels of deployment can be implemented, but all have limitations based on the underlying
technology.
SIP Trunk: With SIP trunking, the Session Initiation Protocol is used as a streaming media service
with Voice over Internet Protocol (VoIP) to provide telephone services and unified
communications. It requires the use of SIP-based private branch exchanges (IP-PBX)
and Unified Communications software applications, such as voice, video, and other
streaming media applications (desktop sharing, shared whiteboard, web conferencing,
and the like).
Cram Quiz
1. Your company currently uses a standard communication link to transfer files between LANs. Until now, the transfer speeds have been sufficient for the amount of data that needs to be transferred. Recently, a new application was purchased that requires a minimum transmission speed of 1.5 Mbps. You have been given the task to find the most cost-effective solution to accommodate the new application. Which of the following technologies would you use?
A. T3
B. X.25
C. T1
D. BRI ISDN
2. Which of the following best describes the process to create a dedicated circuit between two communication endpoints and direct traffic between those two points?
A. Multiplexing
B. Directional addressing
C. Addressing
D. Circuit switching
3. Which of the following statements are true of ISDN? (Choose the two best answers.)
A. BRI ISDN uses 2 B+1 D channels.
B. BRI ISDN uses 23 B+1 D channels.
C. PRI ISDN uses 2 B+1 D channels.
D. PRI ISDN uses 23 B+1 D channels.
4. You have been hired to establish a WAN connection between two offices: one in Vancouver and one in Seattle. The transmission speed can be no less than 2 Mbps. Which of the following technologies could you choose?
A. T1
B. PPPoE
C. T3
D. ISDN
5. On an ISDN connection, what is the purpose of the D channel?
A. It carries the data signals.
B. It carries signaling information.
C. It enables multiple channels to be combined to provide greater bandwidth.
D. It provides a temporary overflow capacity for the other channels.
6. Which of the following circuit-switching strategies does ATM use? (Choose the two best answers.)
A. SVC
B. VCD
C. PVC
D. PCV
7. Due to recent cutbacks, your boss approaches you and demands an alternative to the company’s costly dedicated T1 line. Only small amounts of data require transfer over the line. Which of the following are you likely to recommend?
A. ISDN
B. FDDI
C. A public switched telephone network
D. X.25
8. Which of the following technologies requires a logical connection between the sending and receiving devices?
A. Circuit switching
B. Virtual-circuit packet switching
C. Message switching
D. High-density circuit switching
9. Which technology uses short path labels instead of longer network addresses to direct data from one node to another?
A. MPLS
B. Metropolitan Ethernet
C. DMVPN
D. PPP
1. C. A T1 line has a transmission capability of 1.544 Mbps and is considerably cheaper than a T3 line. X.25 and BRI ISDN cannot provide the required transmission speed.
2. D. Circuit switching is the process of creating a dedicated circuit between two communications endpoints and directing traffic between those two points. None of the other answers are valid types of switching.
3. A, D. BRI ISDN uses 2 B+1 D channels, which are two 64 Kbps data channels, and PRI ISDN uses 23 B+1 D channels. The D channel is 16 Kbps for BRI and 64 Kbps for PRI.
4. C. The only possible answer capable of transfer speeds above 2 Mbps is a T3 line. None of the other technologies listed can provide the transmission speed required.
5. B. The D channel on an ISDN link carries signaling information, whereas the B, or bearer, channels carry the data.
6. A, C. ATM uses two types of circuit switching: PVC and SVC. VCD and PCV are not the names of switching methods.
7. C. When little traffic will be sent over a line, a public switched telephone network (PSTN) is the most cost-effective solution, although it is limited to 56 Kbps. All the other WAN connectivity methods accommodate large amounts of data and are expensive compared to the PSTN.
8. B. When virtual-circuit switching is used, a logical connection is established between the source and the destination device.
9. A. Used in high-performance-based telecom networks, MPLS is a technology that uses short path labels instead of longer network addresses to direct data from one node to another. Metropolitan Ethernet is nothing more than an Ethernet-based MAN (metropolitan-area network). DMVPN offers the capability to create a dynamic-mesh VPN network without having to preconfigure all the possible tunnel endpoints. PPP is a data link protocol that is used to establish a connection between two nodes. PPP works with plain old telephone service (POTS), ISDN, fiber links such as SONET, and other faster connections, such as T1.
CramSaver
If you can correctly answer these questions before going through this section, save time by skimming the Exam Alerts in this section and then completing the Cram Quiz at the end of the section.
1. What is VHDSL commonly used for?
2. True or false: DSL using regular phone lines transfers data over the same copper wire.
3. What is the difference between a one-way and a two-way satellite system?
4. What hardware is located at the demarcation point?
Answers
1. VHDSL supports high-bandwidth applications such as VoIP and HDTV.
2. True. DSL using regular phone lines transfers data over the same copper wire.
3. A one-way satellite system requires a satellite card and a satellite dish installed at the end user’s site. This system works by sending outgoing requests on one link using a phone line, with inbound traffic returning on the satellite link. A two-way satellite system, in contrast, provides data paths for both upstream and downstream data.
4. The hardware at the demarcation point is the smart jack, also known as the network interface device (NID).
Internet access has become an integral part of modern business. You can obtain Internet access in several ways. Which type you choose often depends on the cost and what technologies are available in your area. This section explores some of the more common methods of obtaining Internet access.
Note
The term broadband often refers to high-speed Internet access. Both DSL and cable modems are common broadband Internet technologies. Broadband routers and broadband modems are network devices that support both DSL and cable.
DSL Internet Access
Digital subscriber line (DSL) is an Internet access method that uses a standard phone line to provide high-speed Internet access. DSL is most commonly associated with high-speed Internet access. Because it is a relatively inexpensive Internet access, it is often found in homes and small businesses. With DSL, a different frequency can be used for digital and analog signals, which means that you can talk on the phone while you upload data.
For DSL services, two types of systems exist: asymmetric digital subscriber line (ADSL) and high-rate digital subscriber line (HDSL). ADSL provides a high data rate in only one direction. It enables fast download speeds but significantly slower upload speeds. ADSL is designed to work with existing analog telephone service (POTS) service. With fast download speeds, ADSL is well suited for home-use Internet access where uploading large amounts of data isn’t a frequent task.
In contrast to ADSL, HDSL provides a bidirectional high-data-rate service that can accommodate services such as videoconferencing that require high data rates in both directions. A variant of HDSL is very high-rate digital subscriber line (VHDSL), which provides an HDSL service at very high data transfer rates.
DSL arrived on the scene in the late 1990s and brought with it a staggering number of flavors. Together, all these variations are known as xDSL:
Asymmetric DSL (ADSL): Probably the most common of the DSL varieties is ADSL, which uses different channels
on the line. One channel is used for POTS and is responsible for analog traffic. The
second channel provides upload access, and the third channel is used for downloads.
With ADSL, downloads are faster than uploads, which is why it is called asymmetric DSL.
Note
ADSL2 made some improvements in the data rate and increased the distance from the telephone exchange that the line can run. ADSL2+ doubled the downstream bandwidth and kept all the features of ADSL2. Both ADSL2 and ADSL2+ are compatible with legacy ADSL equipment.
Symmetric DSL (SDSL): Offers the same speeds for uploads and downloads, making it most suitable for business
applications such as web hosting, intranets, and e-commerce. It is not widely implemented
in the home/small business environment and cannot share a phone line.
ISDN DSL (IDSL): A symmetric type of DSL commonly used in environments in which SDSL and ADSL are
unavailable. IDSL does not support analog phones.
Rate-adaptive DSL (RADSL): A variation on ADSL that can modify its transmission speeds based on signal quality.
RADSL supports line sharing.
Very high bit rate DSL (VHDSL or VDSL): An asymmetric version of DSL and, as such, can share a telephone line. VHDSL supports
high-bandwidth applications such as VoIP and HDTV. VHDSL can achieve data rates up
to approximately 10 Mbps, making it the fastest available form of DSL. To achieve
high speeds, VHDSL uses fiber-optic cabling.
High bit rate DSL (HDSL): A symmetric technology that offers identical transmission rates in both directions.
HDSL does not allow line sharing with analog phones.
Why are there are so many DSL variations? The answer is quite simply that each flavor of DSL is aimed at a different user, business, or application. Businesses with high bandwidth needs are more likely to choose a symmetric form of DSL, whereas budget-conscious environments such as home offices are likely to choose an option that enables phone line sharing at the expense of bandwidth. In addition, some of the DSL variants are older technologies. Although the name persists, they have been replaced with newer DSL implementations. When you work in a home/small office environment, you should expect to work with an ADSL system.
Table 5.5 summarizes the maximum speeds of the various DSL options. Maximum speeds are rarely obtained.
TABLE 5.5 DSL Speeds
|
DSL Variation |
Upload Speed* |
Download Speed* |
|
ADSL |
1 Mbps |
3 Mbps |
|
ADSL2 |
1.3 Mbps |
12 Mbps |
|
ADSL2+ |
1.4 Mbps |
24 Mbps |
|
SDSL |
1.5 Mbps |
1.5 Mbps |
|
IDSL |
144 Kbps |
144 Kbps |
|
RADSL |
1 Mbps |
7 Mbps |
|
VHDSL |
1.6 Mbps |
13 Mbps |
|
HDSL |
768 Kbps |
768 Kbps |
*Speeds may vary greatly, depending on the technologies used and the quality of the connection.
ExamAlert
For the exam, focus on ADSL as you study, but be able to put it in perspective with other varieties.
Note
DSL using regular phone lines transfers data over the same copper wire. The data and voice signals are sent over different frequencies, but sometimes the signals interfere with each other. This is why you use DSL filters. A DSL filter works by minimizing this interference, making for a faster and cleaner DSL connection.
DSL Troubleshooting Procedures
Troubleshooting DSL is similar to troubleshooting any other Internet connection. The following are a few things to check when users are experiencing problems with a DSL connection:
Physical connections: The first place to look when troubleshooting a DSL problem is the network cable connections.
From time to time, these cables can come loose or inadvertently be detached, and they
are often overlooked as the cause of a problem. DSL modems typically have a minimum
of three connections: one for the DSL line, one for the local network, and one for
the power. Make sure that they are all plugged in appropriately.
The network interface card (NIC): While you are checking the cable at the back of the system, take a quick look to
see whether the network card LED is lit. If it is not, something could be wrong with
the card. It might be necessary to swap out the network card and replace it with one
that is known to be working.
Drivers: Ensure that the network card is installed and has the correct drivers. Many times,
simply using the most up-to-date driver can resolve connectivity issues.
Protocol configuration: The device you are troubleshooting might not have a valid IP address. Confirm the
IP address by using the appropriate tool for the operating system (and version of
IP whether it be IPv4 or IPv6) being used—for example, ipconfig or ifconfig. If the system requires the automatic assignment of an IP address, confirm that the
system is automatically set to obtain an IP address. It might be necessary to use
the ipconfig /release and ipconfig /renew commands to get a new IP address.
DSL LEDs: Each DSL box has an LED on it. The light sequences are often used to identify connectivity
problems or problems with the box itself. Refer to the manufacturer’s website for
specific information about error codes and LEDs, but remember the basics. A link light
should be on to indicate that the physical connection is complete, and a flashing
LED indicates that the connection is active.
ExamAlert
When troubleshooting remote connectivity on a cable or DSL modem, use the LEDs that are always present on these devices to aid in your troubleshooting process.
Ultimately, if none of these steps cures or indicates the cause of the problem, you might have to call the DSL provider for assistance.
Cable Broadband
Cable broadband Internet access is an always-on Internet access method available in areas that have digital cable television. Cable Internet access is attractive to many small businesses and home office users because it is both inexpensive and reliable. Most cable providers do not restrict how much use is made of the access, but they do control the speed. Connectivity is achieved by using a device called a cable modem. It has a coaxial connection for connecting to the provider’s outlet and an unshielded twisted-pair (UTP) connection for connecting directly to a system or to a hub, switch, or router.
Cable providers often supply the cable modem, with a monthly rental agreement. Many cable providers offer free or low-cost installation of cable Internet service, which includes installing a network card in a PC. Some providers also do not charge for the network card. Cable Internet costs are comparable to DSL subscription.
Most cable modems offer the capability to support a higher-speed Ethernet connection for the home LAN than is achieved. The actual speed of the connection can vary somewhat, depending on the utilization of the shared cable line in your area.
ExamAlert
A cable modem generally is equipped with a medium-dependent interface crossed (MDI-X) port, so you can use a straight-through UTP cable to connect the modem to a system.
One of the biggest disadvantages of cable access (by DSL providers, at least) is that you share the available bandwidth with everyone else in your cable area. As a result, during peak times, performance of a cable link might be poorer than in low-use periods. In residential areas, busy times are evenings and weekends, and particularly right after school. In general, though, performance with cable systems is good, and in low-usage periods, it can be fast.
Note
The debate between cable and DSL has gone on for years. Although cable modem technology delivers shared bandwidth within the local neighborhood, its speeds are theoretically higher but influenced by this shared bandwidth. DSL delivers dedicated local bandwidth but is sensitive to distance that impacts overall performance. With the monthly costs about the same, the decision of which to use is often based on the package and bundle specials offered by local providers.
Cable Troubleshooting Procedures
In general, cable Internet access is a low-maintenance system with few problems. When problems do occur, you can try various troubleshooting measures:
Check the user’s end: Before looking at the cable modem, make sure that the system is configured correctly
and that all cables are plugged in. If a hub, switch, or router is used to share the
cable Internet access among a group of computers, make sure that the device is on
and correctly functioning.
Check the physical connections: Like DSL modems, cable modems have three connections: one for the cable signal, one
for the local network, and one for the power. Make sure that they are all appropriately
plugged in.
Ensure that the protocol configuration on the system is valid: If an IP address is assigned via Dynamic Host Configuration Protocol (DHCP), the absence of an address is a sure indicator that connectivity is at fault. Try
obtaining a new IP address by using the appropriate command for the operating system
platform you use. If the IP addresses are statically configured, make sure that they
are correctly set. Trying to use any address other than that specified by the ISP
might prevent a user from connecting to the network.
Check the indicator lights on the modem: Most cable modems have indicator lights that show the modem’s status. Under normal
conditions, a single light labeled Ready or Online should be lit. Most cable providers
give the user a modem manual that details the functions of the lights and what they
indicate in certain states. Generally, any red light is bad. Flashing LEDs normally
indicate traffic on the connection.
Cycle the power on the modem: Cycling the power on the modem is a surefire way to reset it.
Call the technical support line: If you are sure that the connectors are all in place and the configuration of the
system is correct, the next step is to call the technical support line of the cable
provider. If the provider experiences problems that affect many users, you might get
a message while you’re on hold, informing you of that. If not, you eventually get
to speak to someone who can help you troubleshoot the problem. One of the good things
about cable access is that the cable company can remotely monitor and reset the modem.
The cable company should tell you whether the modem is correctly functioning.
Unless the modem is faulty, which is not that common, by this point the user should be back on the Internet, or at least you should fully understand why the user cannot connect. If the problem is with the cable provider’s networking equipment, you and the user simply have to wait for the system to come back on.
Broadband Security Considerations
Whether you use DSL or cable Internet access, keep a few things in mind. Each of these technologies offers always-on service. This means that even when you are away from your computer, it still connects to the Internet. As you can imagine, this creates a security risk. The longer you are online, the better the chances that someone can remotely access your system.
The operating systems in use today all have some security holes that attackers wait to exploit. These attacks often focus on technologies such as email or open TCP/UDP ports. Combining OS security holes with an always-on Internet technology is certainly a dangerous mix.
Today, DSL and cable Internet connections must be protected by mechanisms such as firewalls. The firewall offers features such as packet filtering and Network Address Translation (NAT). The firewall can be a third-party software application installed on the system, or it can be a hardware device.
In addition to a firewall, it is equally important to ensure that the operating system you use is completely up to date in terms of updates and security patches. Today’s client systems typically offer automatic update features that alert you when a new security update or patch is available.
If you diligently follow a few security measures, both DSL and cable Internet can provide safe Internet access.
Dial-up
Although it’s somewhat slow, one means to connect to the Internet or a remote network from a remote location where broadband access is not available may still be the good old telephone line and a modem: either an internal PCIe or external USB one. Because the same line used for a household phone is used for dial-up access, it is called the plain old telephone system (POTS) method of access. Although many parts of the world are served by broadband providers offering services such as those discussed so far in this chapter, some people still must (or choose to) connect with a dial-up modem.
Internet access through a phone system requires two things: a modem and a dial-up access account through an ISP. Dial-up modems are devices that convert the digital signals generated by a computer system into analog signals that can travel across a phone line. A computer can have either an internal or external modem. External USB modems tend to be less problematic to install and troubleshoot because they don’t require reconfiguration of the host system. Internal modems use one of the serial port assignments (that is, a COM port) and therefore must be configured not to conflict with other devices.
The second piece of the puzzle, the dial-up ISP account, can easily be obtained by contacting one of the many local, regional, or national ISPs. Most ISPs offer a range of plans normally priced based on the amount of time the user is allowed to spend online. Almost without exception, ISPs offer 56 Kbps access, the maximum possible under current standards. Most ISPs also provide email accounts, access to newsgroup servers, and often small amounts of web space.
It is a good idea to carefully research an ISP choice. Free services exist, but they generally restrict users to a certain number of online hours per month or use extensive banner advertising to pay for the services.
Another big consideration for dial-up Internet access is how many lines the ISP has. ISPs never have the same number of lines as subscribers; instead, they work on a first-come, first-served basis for dial-up clients. This means that sometimes users get busy signals when they try to connect. Before signing up for a dial-up Internet access account, ask the company what its ratio of lines to subscribers is, and use that figure as part of your comparison criteria.
With a modem and an ISP account, you are ready to connect. But what happens if things don’t go as you plan? Welcome to the interesting and sometimes challenging world of troubleshooting dial-up connections.
Dial-up Troubleshooting Procedures
Troubleshooting a dial-up connection problem can be tricky and time-consuming because you must consider many variables. Of the remote connectivity mechanisms discussed in this chapter, you are far more likely to have problems with a POTS connection than with any of the others. The following are some places to start your troubleshooting under various conditions.
Note
In some cases, users may not even use an ISP; instead, they may directly dial another system on the corporate network. In that case, all the troubleshooting steps in this section apply. The exception is that you must rely on the technical support capabilities of the person responsible for the remote system rather than the ISP if you have a problem.
If the user cannot dial out, try the following:
Check physical connections: The most common problem with modem connections is that something has become unplugged;
modems rarely fail after they initially work. For an external modem, you also need
to verify that the modem has power and that it is connected to the correct COM port.
Check that the line has a dial tone: You can do this by plugging a normal phone into the socket to see whether you can
dial out. Also, a modem generally has a speaker, and you can set up the modem to use
the speaker so that you can hear what is going on.
If the user can dial out but cannot connect to the network, try the following:
Make sure that the user is dialing the correct number: This suggestion sounds obvious, but sometimes numbers change or are incorrectly entered.
Call the ISP: You can call the ISP to determine whether it is having problems.
Check the modem speaker: Find out whether you get busy signals from the ISP by turning on the modem speaker.
If the user can dial out and can get a connection but is then disconnected, try the following:
Make sure that the modem connection is correctly configured: The most common modem configuration is 8 data bits, 1 stop bit, and no parity (commonly
called eight-one-none).
Check the username and password: Make sure that the correct username and password combination is configured for the
dial-up connection.
Verify that the connection settings are correct: Pay particular attention to things such as the IP address. Nearly all ISPs assign
IP addresses through DHCP, and trying to connect with a statically configured IP address
is not permitted.
Make sure that the user has not exceeded a preset connection time limit: Some ISPs restrict the number of monthly access hours. If the user has such a plan,
check to ensure that some time credit is left.
Try specifying a lower speed for the connection: Modems are designed to negotiate a connection speed with which both devices are comfortable.
Sometimes, during the negotiation process, the line can be dropped. Initially setting
a lower speed might get you a connection. You can then increase the modem speed to
accommodate a better connection.
The Public Switched Telephone Network
The Public Switched Telephone Network (PSTN), often considered a POTS, is the entire collection of interconnected telephone wires throughout the world. Discussions of the PSTN include all the equipment that goes into connecting two points, such as the cable, the networking equipment, and the telephone exchanges.
ExamAlert
Although PSTN is not specifically listed as an exam objective, you need to know how it compares with other technologies. Know that if money is a major concern, the PSTN is the method of choice for creating a WAN.
The modern PSTN is largely digital, with analog connections existing primarily between homes and local phone exchanges. Modems are used to convert the computer system’s digital signals into analog so that they can be sent over the analog connection.
Using the PSTN to establish WAN connections is a popular choice, although the significant drawback is the limited transfer speeds. Transfer on the PSTN is limited to 56 Kbps with a modem and 128 Kbps with an ISDN connection, and it is difficult to share large files or videoconferencing at such speeds. However, companies that need to send only small amounts of data remotely can use the PSTN as an inexpensive alternative for remote access, particularly when other resources such as the Internet are unavailable.
Satellite Internet Access
Many people take DSL and cable Internet access for granted, but these technologies are not offered everywhere. Many rural areas do not have cable Internet access. For areas where cheaper broadband options are unavailable, a limited number of Internet options are available. One of the primary options is Internet via satellite.
Satellite access provides a viable Internet access solution for those who cannot get other methods of broadband. Satellite Internet offers an always-on connection with download speeds considerably faster than an old dial-up connection. Satellite Internet access does have a few drawbacks, though, such as cost and high latency. Latency is the time it takes for the signal to travel back and forth from the satellite.
Although satellite Internet is slower and costlier than DSL or cable, it offers some attractive features, the first of which is its portability. Quite literally, wherever you go, you have Internet access with no phone lines or other cables. For businesses with remote users and clients, the benefit is clear. But the technology has a far-reaching impact; it is not uncommon to see recreational vehicles (RVs) with a satellite dish on the roof. They have 24/7 unlimited access to the Internet as they travel.
Many companies offer satellite Internet services; a quick Internet search reveals quite a few. These Internet providers offer different Internet packages that vary greatly in terms of price, access speeds, and service. Some target businesses, whereas others aim for the private market.
Two different types of broadband Internet satellite services are deployed: one-way and two-way systems. A one-way satellite system requires a satellite card and a satellite dish installed at the end user’s site. This system works by sending outgoing requests on one link using a phone line, with inbound traffic returning on the satellite link. A two-way satellite system, in contrast, provides data paths for both upstream and downstream data. Like a one-way system, a two-way system uses a satellite card and a satellite dish installed at the end user’s site; bidirectional communication occurs directly between the end user’s node and the satellite.
Home satellite systems are asymmetric; that is, download speeds are faster than upload speeds. A home satellite system is likely to use a modem for the uplink traffic, with downloads coming over the satellite link. The exact speeds you can expect with satellite Internet depend on many factors. As with other wireless technologies, atmospheric conditions can significantly affect the performance of satellite Internet access. One additional consideration for satellite Internet is increased propagation time—how long it takes the signal to travel back and forth from the satellite. In networking terms, this time is long and therefore is an important consideration for business applications.
Home Satellite Troubleshooting Procedures
Your ability to troubleshoot satellite Internet connections might be limited. Home satellite Internet is a line-of-sight wireless technology, and the installation configuration must be precise. Because of this requirement, many satellite companies insist that the satellite be set up and configured by trained staff members. If you install a satellite system in a way that does not match the manufacturer’s recommendations, you might void any warranties.
Given this limitation, troubleshooting satellite connections often requires you to concentrate less on connectivity issues and more on physical troubleshooting techniques. Perhaps more than for any other Internet technology, calls to technical support occur early in the troubleshooting process. Satellite Internet has a few aspects that you should be aware of:
Rain fade: Refers to signal loss due to moisture interference. The general rule is that the
smaller the dish, the more susceptible it is to rain fade. Homes and small businesses
use small dishes.
Latency: Refers to the time lapse between sending or requesting information and the time it
takes to return. As you might expect, satellite communication experiences high latency
due to the distance it has to travel.
Line-of-sight: Despite the distance, satellite is basically a line-of-sight technology. This means
that the path between the satellite dish and the satellite should be as unobstructed
as possible.
Wireless Internet Access
Not too long ago, it would have been inconceivable to walk into your local coffee shop with your laptop under your arm and surf the Web while drinking a latte. Putting aside that beverages and laptops don’t mix, wireless Internet access has become common.
Wireless Internet access is provided by an ISP providing public wireless Internet access known as hotspots. Hotspots offer Internet access for mobile network devices such as laptops, handheld computers, and cell phones in airports, coffee shops, conference rooms, and so on. A hotspot is created using one or many wireless access points near the hotspot location.
Client systems might need to install special application software for billing and security purposes; others require no configuration other than obtaining the network name (service set identifier [SSID]). Hotspots do not always require a fee for service, because companies use them as a marketing tool to lure Internet users to their businesses.
Hotspots are not everywhere, but finding them is not difficult. Typically, airports, hotels, and coffee shops advertise that they offer Internet access for customers or clients. In addition, ISPs list their hotspot sites online so that they are easily found.
Establishing a connection to a wireless hotspot is a straightforward process. If not equipped with built-in wireless capability, laptops require an external wireless adapter card. With the physical requirements of the wireless card taken care of, connect as follows:
1. When you arrive at the hotspot site, power up your laptop or other mobile device. In some instances, you might need to reboot your system to clear out old configuration settings if it was on standby.
2. The card might automatically detect the network. If this is the case, configuration settings, such as the SSID, are automatically detected, and the wireless Internet is available. If Internet access is free, there is little else to do; if it is a paid-for service, you need to enter a method of payment. One thing to remember is to verify that you use encryption for secure data transfer.
3. If for some reason the wireless settings are not automatically detected, you need to open your wireless NIC’s configuration utility and manually set the configurations. These can include setting the mode to infrastructure, inputting the correct SSID, and setting the level of encryption used.
In addition to using an ISP, some companies, such as hotels and cafes, provide wireless Internet access by connecting a wireless router to a DSL or cable Internet connection. The router becomes the wireless access point to which the users connect, and it enables clients to connect to the Internet through the broadband connection. The technology is based on the 802.11 standards, typically 802.11n/ac today, and client systems require only an internal or external wireless adapter.
Note
Want more information on wireless? Chapter 7, “Wireless Solutions,” covers wireless technologies in detail.
Termination Points
To work properly, a network must have termination points. These endpoints stop the signal and prevent it from living beyond its needed existence. For the exam, CompTIA wants you to be familiar with a number of termination-related topics, all of which are discussed in the sections that follow.
Demarc, Demarc Extension, and Smart Jacks
A network’s demarcation point is the connection point between the operator’s part of the network and the customer’s portion of the network. This point is important for network administrators because it distinguishes the portion of the network that the customer is responsible for from the section the owner is responsible for. For example, for those who have high-speed Internet, the boundary between the customer’s premises and the ISP typically is mounted on the wall on the side of the home. However, high-speed service providers support everything from the cable modem back to their main distribution center. This is why, if a modem fails, it is replaced by the ISP and not by the customer. This is true for the wiring to that point as well.
Knowing the location of the demarcation point is essential because it marks the point between where the customer (or administrator) is responsible and where the owner is. It also identifies the point at which the customer is responsible should a problem occur, and who should pay for that problem. The ISP is responsible for ensuring that the network is functional up to the demarcation point. The customer/administrator is responsible for ensuring that everything from that point is operational.
The demarcation point is the point at which the ISP places its services in your network. There is not always a choice of where this demarcation is placed. This means that a company might have six floors of offices, and the demarcation point is in the basement—impractical for the network. This is when you need a demarcation extension, which extends the demarcation point to a more functional location. This might sound simple, but it involves knowledge of cabling distances and other infrastructure needs. The demarcation extension might be the responsibility of the administrator, or for a fee, owners might provide extension services.
As you might imagine, you need some form of hardware at the demarcation point. This is the smart jack, also known as the network interface device (NID). The smart jack performs several primary functions:
Loopback feature: The loopback feature is built in to the smart jack. Like the Ethernet loopback cable,
it is used for testing purposes. In this case, the loopback feature enables remote
testing so that technicians do not always need to be called to visit the local network
to isolate problems.
Signal amplification: The smart jack can amplify signals. This feature is similar to that of the function
of repeaters in an Ethernet network.
Surge protection: Lightning and other environmental conditions can cause electrical surges that can
quickly damage equipment. Many smart jacks include protection from environmental situations.
Remote alarms: Smart jacks typically include an alarm that allows the owner to identify if something
goes wrong with the smart jack and therefore the connections at the demarcation point.
ExamAlert
Demarcation point is the telephone company or ISP term for where their facilities or wires end and where yours begin.
CSUs/DSUs
A channel service unit/data service unit (CSU/DSU) acts as a translator between the LAN data format and the WAN data format. Such a conversion is necessary because the technologies used on WAN links are different from those used on LANs. Some consider a CSU/DSU a type of digital modem. But unlike a normal modem, which changes the signal from digital to analog, a CSU/DSU changes the signal from one digital format to another.
A CSU/DSU has physical connections for the LAN equipment, normally via a serial interface, and another connection for a WAN.
ExamAlert
Traditionally, the CSU/DSU has been in a box separate from other networking equipment. However, the increasing use of WAN links means that some router manufacturers are now including CSU/DSU functionality in routers or are providing the expansion capability to do so.
Verify Wiring Installation and Termination
After a segment of network cable has been placed where it needs to go, whether run through the plenum or connecting a patch cable, the final task is wiring termination. Termination is the process to connect the network cable to the wall jack, plug, or patch panel. Termination generally is a straightforward process. You can quickly see if the wiring and termination worked if the LED on the connected network card is lit. Also, if you connect a client system, you can ping other devices on the network if all works.
Note
Cabling topics, such as patch cables and plenums, are discussed in Chapter 6, “Cabling Solutions.”
If you run the wiring and complete termination, but a system cannot access the network and the link light is not lit, you should look for a few things when troubleshooting the wiring installation and termination.
Verify that the termination and wiring installation link light on the device (switch/NIC) is not lit:
If you connect a patch cable to a PC or a switch, and no link light is lit, verify
that the patch cable is good by switching it with a known working one.
If it is a homemade patch cable, ensure that the RJ-45 connector is properly attached.
Ensure that the RJ-45 connector is properly seated in the wall jack and NIC or switch
port.
If no link light is lit when you connect to a switch, change to another port on the
switch. Sometimes a single port can be faulty.
Verify that the correct patch cable is used. It is possible that a rollover cable
or crossover cable has been accidentally used.
Verify that the cables used are the correct standard. For example, the patch cable
should be a 568A or 568B.
If the link light on a device is lit and intermittent problems occur, check the following:
Try replacing the cable with a known working one.
Verify where the network cable is run. Ensure that a plenum-rated cable is used if
it runs through ceilings or ductwork.
Look for heavy bends or partial breaks in the network cable.
Verify that shielded cabling is used in areas of potentially high interference.
Check the length of the cable run. Remember, the total run of cable should be about
100 meters. If the patch cable or the cable between the wall jack and the wiring closet
is too long, intermittent signals can occur.
Cram Quiz
1. Which of the following technologies require dial-up access? (Choose the best answer.)
A. Fiber
B. ISDN
C. Packet switching
D. DMVPN
2. Which of the following is an advantage of ISDN over a public switched telephone network?
A. ISDN is more reliable.
B. ISDN is cheaper.
C. ISDN is faster.
D. ISDN uses 53 Kbps fixed-length packets.
3. Which of the following is the time lapse between sending or requesting information and the time it takes to return?
A. Echo
B. Attenuation
C. Bandwidth
D. Latency
4. What is the speed usually offered with dial-up service?
A. 1 Gbps
B. 256 Kbps
C. 144 Kbps
D. 56 Kbps
5. What device acts as a translator between the LAN data format and the WAN data format?
A. SIP Trunk
B. PRI
C. MPLS
D. CSU/DSU
1. B. ISDN requires a dial-up connection to establish communication sessions.
2. C. One clear advantage that ISDN has over the PSTN is its speed. ISDN can combine 64 Kbps channels for faster transmission speeds than the PSTN can provide. ISDN is no more or less reliable than the PSTN. ISDN is more expensive than the PSTN.
3. D. Latency refers to the time lapse between sending or requesting information and the time it takes to return.
4. D. Almost without exception, ISPs offer 56 Kbps access, the maximum possible under current dial-up standards.
5. D. A channel service unit/data service unit (CSU/DSU) acts as a translator between the LAN data format and the WAN data format. Such a conversion is necessary because the technologies used on WAN links are different from those used on LANs.
What’s Next?
For the Network+ exam, and for routinely working with an existing network or implementing a new one, you need to identify the characteristics of network media and their associated cabling. Chapter 6, “Cabling Solutions,” focuses on the media and connectors used in today’s networks and what you are likely to find in wiring closets.