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C H A P T E R 4
Fundamentals of WANs
In the previous chapter, you learned more details about OSI Layers 1 and 2, and how Ethernet LANs perform the functions defined by the two lowest OSI layers. In this chapter, you will learn about how wide-area network (WAN) standards and protocols also implement OSI Layers 1 and 2. The OSI physical layer details are covered, along with two popular WAN data link layer protocols, High-Level Data Link Control (HDLC) and Frame Relay.
“Do I Know This Already?” Quiz
The purpose of the “Do I Know This Already?” quiz is to help you decide whether you really need to read the entire chapter. If you already intend to read the entire chapter, you do not necessarily need to answer these questions now.
The ten-question quiz, derived from the major sections in “Foundation Topics” portion of the chapter, helps you determine how to spend your limited study time.
Table 4-1 outlines the major topics discussed in this chapter and the “Do I Know This Already?” quiz questions that correspond to those topics.
Table 4-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundations Topics Section
OSI Layer 1 for Point-to-Point WANs
OSI Layer 2 for Point-to-Point WANs
Packet-Switching Services
Questions Covered in This Section
1–3, 6
4, 5, 7
8–10
CAUTION The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark this question wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security.
78 Chapter 4: Fundamentals of WANs
1. Which of the following best describes the main function of OSI Layer 1 protocols?
a. Framing
b. Delivery of bits from one device to another c. Addressing
d. Local Management Interface (LMI) e. DLCI
2. Which of the following typically connects to a four-wire line provided by a telco?
a. Router serial interface b. CSU/DSU
c. Transceiver
d. Switch serial interface
3. Which of the following typically connects to a V.35 or RS-232 end of a cable when cabling a leased line?
a. Router serial interface b. CSU/DSU
c. Transceiver
d. Switch serial interface
4. Which of the following functions of OSI Layer 2 is specified by the protocol standard for PPP, but is implemented with a Cisco proprietary header field for HDLC?
a. Framing
b. Arbitration c. Addressing
d. Error detection
e. Identifying the type of protocol that is inside the frame
5. Which of the following WAN data link protocols on Cisco routers support multiple Layer 3 protocols by virtue of having some form of Protocol Type field?
a. PPP
b. HDLC c. LAPB
“Do I Know This Already?” Quiz 79
d. LAPD e. SDLC
f. None of the above
6. On a point-to-point WAN link between two routers, what device(s) are considered to be the DTE devices?
a. The routers
b. The CSU/DSUs
c. The central office equipment
d. A chip on the processor of each router e. None of the above
7. Imagine that Router1 has three point-to-point serial links, one link each to three remote routers. Which of the following is true about the required HDLC addressing at Router1?
a. Router1 must use HDLC addresses 1, 2, and 3.
b. Router1 must use any three unique addresses between 1 and 1023. c. Router1 must use any three unique addresses between 16 and 1000.
d. Router1 must use three sequential unique addresses between 1 and 1023. e. None of the above.
8. What is the name of the Frame Relay field used to identify Frame Relay Virtual Circuits?
a. Data-link connection identifier b. Data-link circuit identifier
c. Data-link connection indicator d. Data-link circuit indicator
e. None of the above
9. Which of the following is true about Frame Relay virtual circuits?
a. Each VC requires a separate access link.
b. Multiple VCs can share the same access link.
c. All VCs sharing the same access link must connect to the same router on the other side of the VC.
d. All VCs on the same access link must use the same DLCI.
80 Chapter 4: Fundamentals of WANs
10. Which of the following defines a SONET link speed around 155 Mbps?
a. T1 b. T3 c. DS3
d. DS155 e. OC-3
f. OC-12 g. OC-48 h. OC-155
The answers to the “Do I Know This Already?” quiz are found in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes and Q&A Sections.” The suggested choices for your next step are as follows:
8 or less overall score—Read the entire chapter. This includes the “Foundation Topics” and “Foundation Summary” sections and the Q&A section.
9 or 10 overall score—If you want more review on these topics, skip to the “Foundation Summary” section and then go to the Q&A section. Otherwise, move to the next chapter.
OSI Layer 1 for Point-to-Point WANs 81
Foundation Topics
As you read in the previous chapter, the OSI physical and data link layers work together to deliver data across a wide variety of types of physical networks. LAN standards and protocols define how to network between devices that are relatively close together—hence the term local in the acronym LAN. WAN standards and protocols define how to network between devices that are relatively far apart—in some cases, even thousands of miles apart— hence the term wide-area in the acronym WAN.
LANs and WANs both implement the details of OSI Layers 1 and 2. Some details are different, but many of the concepts are the same. In this chapter, because you just finished reading about LANs, I will compare WANs to LANs whenever possible, to point out the similarities and differences.
In the CCNA ICND Exam Certification Guide, you will read more about the details of WANs, including the configuration details on Cisco routers.
OSI Layer 1 for Point-to-Point WANs
The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as “sending bits.” Higher layers encapsulate the data, as described in Chapter 2, “The TCP/IP and OSI Networking Models.” No matter what the other OSI layers do, eventually the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards and protocols used to create the physical network and to send the bits across that network.
A point-to-point WAN link acts like a trunk between two Ethernet switches in many ways. For perspective, look at Figure 4-1, which shows a LAN with two buildings and two switches in each building.
As a brief review, remember that Ethernet uses a twisted pair of wires to transmit and another twisted pair to receive, to reduce electromagnetic interference. You typically use straight-through Ethernet cables between end user devices and the switches. For the trunk links between the switches, you use crossover cables because each switch transmits on the same pair, so the crossover cable connects one device’s transmit pair to the other device’s receive pair. The lower part of the figure reminds you of the basic idea behind a crossover cable.
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