CompTIA Network+ N10-006 Cert Guide (2015)

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Satellite

Many rural locations lack the option of connecting to an IP WAN or to the Internet via physical media (for example, a DSL modem or a broadband cable modem connection). For such locations, a satellite WAN connection, as shown in Figure 7-12, might be an option.

Figure 7-12 Satellite WAN Sample Topology

Most satellites used for WAN connectivity are in orbit above the earth’s equator, about 22,300 miles high. Therefore, if a customer in North America, for example, had a clear view of the southern sky, she would be able to install a satellite dish and establish a line-of-sight communication path with the orbiting satellite.

The satellite would then relay transmissions back and forth between the customer’s site and the service provider’s ground station. The ground station could then provide connectivity, via physical media, to an IP WAN or to the Internet.

Two significant design considerations include the following:

Delay: Radio waves travel at the speed of light, which is 186,000 miles per second, or 3 * 10⁸ meters per second. This speed is specifically the speed of light (and radio waves) in a vacuum; however, for the purposes of this discussion, assume these commonly known values, even though, technically, the speed of light (and radio waves) is a bit slower when traveling through air, as opposed to traveling through a vacuum. Although these are fast speeds, consider the distance between a customer and the satellite. If a customer were located 2,000 miles north of the equator, the approximate distance between the customer site and the satellite could be calculated using the Pythagorean Theorem: d²=2000² + 22,300². Solving the equation for d, which is the distance between the customer and the satellite, yields a result of approximately 22,390 miles.

A transmission from a customer to a destination on the Internet (or IP WAN) would have to travel from the customer to the satellite, from the satellite to the ground station, and then out to the Internet (or IP WAN). The propagation delay alone introduced by bouncing a signal off of the satellite is approximately 241 ms (that is, (22,390 * 2) / 186,000 = .241 seconds = 241 ms). And to that, you have to add other delay components, such as processing delay (by the satellite and other networking devices), making the one-way delay greater than one-fourth of a second, and therefore the round-trip delay greater than one-half of a second. Such delays are not conducive to latency-sensitive applications, such as Voice over IP (VoIP).

Sensitivity to weather conditions: Because communication between a customer’s satellite dish and an orbiting satellite must travel through the earth’s atmosphere, weather conditions can impede communications. For example, if a thunderstorm is in the vicinity of the customer location, that customer might temporarily lose connectivity with her satellite.

Based on these design considerations, even though satellite WAN technology offers tremendous flexibility in terms of geographical location, more terrestrial-based solutions are usually preferred.

Plain Old Telephone Service

The Public Switched Telephone Network (PSTN) is composed of multiple telephone carriers from around the world. An end-user location (for example, a home or business) gets to the PSTN by connecting to its local telephone company, known as a local exchange carrier (LEC). Analog connections (both voice and data connections) using the PSTN are referred to as plain old telephone service (POTS) connections.

With the PSTN as we know it today, you can place a telephone call to just about anywhere in the world from just about anywhere in the world. Although the bandwidth available on the PSTN is limited, the PSTN is such an expansive network, it is more likely to be available in a given location that other wired WAN solutions. So, the benefit of availability has the trade-off of performance.

A POTS connection can be used to access the Internet (or an IP WAN) by connecting a computer to a modem with a serial cable (using a DB-9 [9 pin] or DB-25 [25 pin] RS232/EIA232 serial port, USB with adapter or using a computer with an internal modem), connecting the modem to a POTS phone line, and dialing in to a service provider. The service provider can then connect to the Internet (or an IP WAN), as shown in Figure 7-13.

Figure 7-13 Dial-Up Modem Sample Topology

As previously stated, the performance of a POTS connection (using a dial-up modem) is limited. Although modems are rated as 56-Kbps modems, in the United States and Canada, a modem’s upstream data rate is limited to 48.0 Kbps, and its downstream data rate is limited to 53.3 Kbps. These limits are imposed not based on a technical limitation, but based on regulations from these countries’ communications commissions.

Table 7-2 offers a collection of common terms used when working with POTS connections, for both voice and data.

Table 7-2 Common POTS Terms

Integrated Services Digital Network

Integrated Services Digital Network (ISDN) is a digital telephony technology that supports multiple 64-Kbps channels (known as bearer channels [B channels]) on a single connection. ISDN was popular back in the 1980s and was used to connect private branch exchanges (PBXs), which are telephone switches owned by and operated by a company, to a CO. ISDN has the capability to carry voice, video, or data over its B channels. ISDN also offers a robust set of signaling protocols: Q.921 for Layer 2 signaling and Q.931 for Layer 3 signaling. These signaling protocols run on a separate channel in an ISDN circuit (known as the delta channel, data channel, or D channel).

ISDN circuits are classified as either a basic rate interface (BRI) circuit or a primary rate interface (PRI) circuit:

BRI: A BRI circuit contains two 64-Kbps B channels and one 16-Kbps D channel. Although such a circuit can carry two simultaneous voice conversations, the B channels can be logically bonded into a single VC (using the multilink interface feature of PPP as discussed earlier in this chapter) to offer a 128-Kbps data path.

PRI: A PRI circuit is an ISDN circuit built on a T1 or E1 circuit. Recall that a T1 circuit has 24 channels. Therefore, if a PRI circuit is built on a T1 circuit, the ISDN PRI circuit has 23 B channels and 1 64-Kbps D channel. The 24th channel in the T1 circuit is used as the ISDN D channel (the channel used to carry the Q.921 and Q.931 signaling protocols, which are used to set up, maintain, and tear down connections).

Also, recall that an E1 circuit has 32 channels, with the first channel being reserved for framing and synchronization and the seventeenth channel being served for signaling. Therefore, an ISDN PRI circuit built on an E1 circuit has 30 B channels and one D channel, which is the seventeenth channel.

Figure 7-14 depicts the constituent elements of an ISDN network.

Figure 7-14 ISDN Sample Topology

Some ISDN circuits are four-wire circuits, and some are two-wire. Also, some devices in an ISDN network might not natively be ISDN devices, or they might need to connect to a four-wire ISDN circuit or a two-wire ISDN circuit. As a result of all these variables, an ISDN network, as pictured in Figure 7-14, categorizes various reference points in the network and various elements in the network. Table 7-3 presents some definitions of these reference points and elements.

Table 7-3 ISDN Network Reference Points and Elements

Frame Relay

Although it is starting to wane in popularity because of the proliferation of technologies such as cable modems and DSL connections, for many years Frame Relay was the WAN technology of choice for many companies. Frame Relay offers widespread availability and relatively low cost compared to leased lines.

Figure 7-15 shows a sample Frame Relay topology. Frame Relay sites are interconnected using virtual circuits (VCs). So, a single router interface can have multiple VCs. For example, in Figure 7-15, notice that the New York router has two VCs (as indicated by the dashed lines) emanating from a single interface. One VC is destined for the Austin router, and the other VC is destined for the Orlando router. These VCs could be point-to-point circuits, where the VC between New York and Austin belongs to the same IP subnet, and the VC between New York and Orlando belongs to a separate subnet. Alternatively, the connection from New York to Austin and Orlando could be a point-to-multipoint connection, where all routers belong to the same subnet.

Figure 7-15 Frame Relay Sample Topology

Frame Relay is a Layer 2 technology, and a router uses locally significant identifiers for each VC. These identifiers are called data-link connection identifiers (DLCIs). Because DLCIs are locally significant, DLCIs at the different ends of a VC do not need to match (although they could). For example, note the VC that interconnects New York with Orlando. From the perspective of the New York router, the VC is denoted with a DLCI of 103. However, from the perspective of the Orlando router, the same VC is referenced with a DLCI of 301.

If a VC is always connected, it is considered to be a permanent virtual circuit (PVC). However, some VCs can be brought up on an as-needed basis, and they are referred to as switched virtual circuits (SVCs).

Unlike a dedicated leased line, Frame Relay shares a service provider’s bandwidth with other customers of its service provider. Therefore, subscribers might purchase an SLA (previously described) to guarantee a minimum level of service. In SLA terms, a minimum bandwidth guarantee is called a committed information rate (CIR).

During times of congestion, a service provider might need a sender to reduce his transmission rate below its CIR. A service provider can ask a sender to reduce his rate by setting the backward explicit congestion notification(BECN) bit in the Frame Relay header of a frame destined for the sender that needs to slow down. If the sender is configured to respond to BECN bits, it can reduce its transmission rate by as much as 25 percent per timing interval (which is 125 ms by default). Both CIR and BECN configurations are considered elements of Frame Relay Traffic Shaping (FRTS). A device that does packet shaping is referred to as a packet shaper.

Another bit to be aware of in a Frame Relay header is the discard eligible (DE) bit. Recall that a CIR is a minimum bandwidth guarantee for a service provider’s customer. However, if the service is not congested, a customer might be able to temporarily transmit at a higher rate. However, frames sent in excess of the CIR have the DE bit in their header set. Then, if the Frame Relay service provider experiences congestion, it might first drop those frames marked with a DE bit.

Asynchronous Transfer Mode

Like Frame Relay, Asynchronous Transfer Mode (ATM) is a Layer 2 WAN technology that operates using the concept of PVCs and SVCs. However, ATM uses fixed-length cells as its protocol data unit (PDU), as opposed to the variable frames used by Frame Relay.

As shown in Figure 7-16, an ATM cell contains a 48-byte payload and a 5-byte header. Table 7-4 describes the fields of an ATM header.

Figure 7-16 ATM Cell Structure

Table 7-4 ATM Header Fields

An ATM cell’s 48-byte payload size resulted from a compromise between the wishes of different countries as an international standard for ATM was being developed. Some countries, such as France and Japan, wanted a 32-byte payload size because smaller payload sizes worked well for voice transmission. However, other countries, including the United States, wanted a 64-byte payload size because they felt such a size would better support the transmission of both voice and data. In the end, the compromise was to use the average of 32 bytes and 64 bytes (that is, 48 bytes).

Although ATM uses VCs to send voice, data, and video, those VCs are not identified with DLCIs. Rather, ATM uses a pair of numbers to identify a VC. One of the numbers represents the identifier of an ATM virtual path. A single virtual path can contain multiple virtual circuits, as shown in Figure 7-17.

Figure 7-17 ATM Virtual Circuits

Also note in Figure 7-17 that a virtual path is labeled with a virtual path identifier (VPI), and a virtual circuit is labeled with a virtual circuit identifier (VCI). Therefore, an ATM VC can be identified with a VPI/VCI pair of numbers. For example, 100/110 can be used to represent a VC with a VPI of 100 and a VCI of 110.

Figure 7-18 provides an example of an ATM network topology. Notice that interconnections between ATM switches and ATM endpoints are called user-network interfaces (UNI), while interconnections between ATM switches are called network-node interfaces (NNIs).

Figure 7-18 ATM Sample Topology

Multiprotocol Label Switching

Multiprotocol Label Switching (MPLS) is growing in popularity as a WAN technology used by service providers. This growth in popularity is due in part to MPLS’s capability to support multiple protocols on the same network—for example, an MPLS network can accommodate users connecting via Frame Relay or ATM on the same MPLS backbone—and MPLS’s capability to perform traffic engineering (which allows traffic to be dynamically routed within an MPLS cloud based on current load conditions of specific links and availability of alternate paths).

MPLS inserts a 32-bit header between Layer 2 and Layer 3 headers. Because this header is shimmed between the Layer 2 and Layer 3 headers, it is sometimes referred to as a shim header. Also, because the MPLS header resides between the Layer 2 and Layer 3 headers, MPLS is considered to be a Layer 2 1/2 technology.

The 32-bit header contains a 20-bit label. This label is used to make forwarding decisions within an MPLS cloud. Therefore, the process of routing MPLS frames through an MPLS cloud is commonly referred to as label switching.

Figure 7-19 shows a sample MPLS network. Table 7-5 defines the various MPLS network elements shown in the figure.

Figure 7-19 MPLS Sample Topology

Table 7-5 MPLS Network Elements

An MPLS frame does not maintain the same label throughout the MPLS cloud. Rather, an LSR receives a frame, examines the label on the frame, makes a forwarding decision based on the label, places a new label on the frame, and forwards the frame to the next LSR. This process of label switching is more efficient than routing based on Layer 3 IP addresses. The customer using a provider’s network and the MPLS transport across that network is not normally aware of the details of the exact MPLS forwarding that is done by the service provider.

Overlay Networks

In today’s environments, when virtually every device has connectivity to the Internet, using the basic connectivity of the Internet can also provide wide-area network (WAN) solutions. An example of this is establishing connectivity to the Internet and then building a virtual private network (VPN) between a computer or device on one part of the Internet and a computer or device on another part of the Internet. This is an example of an “overlay” network because the VPN is overlaid on top of another network (in this case the Internet). The benefit of a virtual private network is that authentication and encryption can be done so that anyone on the Internet who may happen to see packets associated with the VPN will not be able to decrypt or understand them without the correct keys, which keeps the VPN content confidential.

For a small company that didn’t want to purchase explicit WAN connectivity between two or more sites, they could simply purchase Internet connectivity and build site-to-site VPNs or remote-access VPNs for their WAN connectivity. We discuss VPNs again in Chapter 12, “Network Security.”

Real-World Case Study

Acme Inc. has its headquarters in a building where multiple service providers are offering high-speed serial and Ethernet-based connectivity options. The company has decided that for the connectivity between the headquarters and the two branch offices, it will use a service provider that is using MPLS-based services. The MPLS connectivity will be delivered to the customer’s CE routers as Ethernet connections. This same service provider will be providing the Internet access, as well, for all three locations. For fault tolerance, the company decided to also purchase Internet access using a serial HDLC connection with a second service provider. In the event the primary service provider fails, the headquarters and two branch office sites will connect to each other using site-to-site VPNs and the Internet as the backbone network for the VPN.

If either of the branch offices loses all connectivity to the Internet (and to the service providers and PLS network), the router at each branch office will be connected to the PSTN so that in a worst-case scenario dial-up connectivity will be established to those routers for management purposes. The dial-up connectivity over the public telephone network would use PPP encapsulation and CHAP authentication.

Remote workers who need to access either the branch or the headquarter locations can do so by using their computer and a VPN connection going to the router or firewall at the site they want to connect to. To enable this, the remote workers would need connectivity to the Internet, which could be through DSL, cable modem, dial-up, or a wireless service provider network that is available to the remote worker, such as a cell phone company that provides data services.

Summary

The main topics covered in this chapter are the following:

This chapter identified the three categories of WAN connections: dedicated leased lines, circuit-switched connections, and packet-switched connections.

Data rates of various WAN technologies were contrasted.

Various types of WAN media were identified. These types could be categorized as either physical media (including unshielded twisted pair (UTP), coaxial cable, fiber-optic cable, and electric power lines) or wireless technologies (including cellular phone, satellite, WiMAX, HSPA+, and radio technologies).

The basic theory and operation of various WAN technologies were discussed, including dedicated leased line, digital subscriber line (DSL), cable modem, Synchronous Optical Network (SONET), satellite, plain old telephone service (POTS), Integrated Services Digital Network (ISDN), Frame Relay, Asynchronous Transfer Mode (ATM), and Multiprotocol Label Switching (MPLS).

Exam Preparation Tasks

Review All the Key Topics

Review the most important topics from inside the chapter, noted with the Key Topic icon in the outer margin of the page. Table 7-6 lists these key topics and the page numbers where each is found.

Table 7-6 Key Topics for Chapter 7

Complete Tables and Lists from Memory

Print a copy of Appendix D, “Memory Tables” (found on the DVD), or at least the section for this chapter, and complete the tables and lists from memory. Appendix E, “Memory Table Answer Key,” also on the DVD, includes the completed tables and lists so you can check your work.

Define Key Terms

Define the following key terms from this chapter, and check your answers in the Glossary:

dedicated leased line

circuit-switched connection

packet-switched connection

optical carrier

T1

E1

T3

E3

channel service unit/data service unit (CSU/DSU)

Point-to-Point Protocol (PPP)

Password Authentication Protocol (PAP)

Challenge-Handshake Authentication Protocol (CHAP)

Microsoft Challenge-Handshake Authentication Protocol (MS-CHAP)

Point-to-Point Protocol over Ethernet (PPPoE)

Microsoft Routing and Remote Access Server (RRAS)

digital subscriber line (DSL)

cable modem

Synchronous Optical Network (SONET)

satellite (WAN technology)

Public Switched Telephone Network (PSTN)

plain old telephone service (POTS)

telco

local loop

central office (CO)

tip and ring

demarc

Integrated Services Digital Network (ISDN)

basic rate interface (BRI)

primary rate interface (PRI)

Frame Relay

Asynchronous Transfer Mode (ATM)

Multiprotocol Label Switching (MPLS)

customer premise equipment (CPE)

edge label switch router (ELSR)

label switch router (LSR)

Review Questions

The answers to these review questions are in Appendix A, “Answers to Review Questions.”

1. ISDN is considered to be what type of WAN connection?

a. Dedicated leased line

b. Circuit-switched connection

c. Packet-switched connection

d. Cell-switched connection

2. What is the data rate of an OC-3 connection?

a. 51.84 Mbps

b. 622 Mbps

c. 155.52 Mbps

d. 159.25 Gbps

3. Which of the following WAN technologies commonly use unshielded twisted pair (UTP)? (Choose three.)

a. Cable modem

b. ISDN

c. DSL modem

d. POTS dial-up modem

4. How many channels on an E1 circuit are available for voice, video, or data?

a. 23

b. 24

c. 30

d. 32

5. Which PPP authentication method provides one-way authentication and sends credentials in clear text?

a. WEP

b. MS-CHAP

c. PAP

d. CHAP

6. What DSL variant has a distance limitation of 18,000 ft. between a DSL modem and its DSLAM?

a. HDSL

b. ADSL

c. SDSL

d. VDSL

7. What kind of network is used by many cable companies to service their cable modems and contains both fiber-optic and coaxial cabling?

a. Head-end

b. DOCSIS

c. Composite

d. HFC

8. What locally significant identifier is used by a Frame Relay network to reference a virtual circuit?

a. VPI/VCI

b. DLCI

c. TEI

d. MAC

9. How big is the payload portion of an ATM cell?

a. 5 bytes

b. 48 bytes

c. 53 bytes

d. 64 bytes

10. What is the size of an MPLS header?

a. 4 bits

b. 8 bits

c. 16 bits

d. 32 bits