| 1: | How many twisted pairs of wires are typically used in a leased line between two routers? |
| A1: | Answer: Two pairs are used, one for transmission in each direction. |
| 2: | Imagine that you wanted to create a WAN link between two routers (Router A and Router B) in cities that are 1000 miles apart. How long are the cables that the telco needs to run? |
| A2: | Answer: The telco would probably just need to run a cable from its local CO in each city to the office building in that city where each router resides. Those cables might be several miles long. The telco would not need to run a cable between cities because it already has those cables installed. |
| 3: | What is a WAN link? |
| A3: | Answer: A WAN link is a physical transmission medium between two end points, as created by a telco. WAN links typically connect devices that are not in the same building or office campus. |
| 4: | What are some of the other names for a WAN link? |
| A4: | Answer: Leased line, leased circuit, point-to-point link, and serial link 4-wire circuit |
| 5: | Explain whether point-to-point WAN links need something like the Ethernet CSMA/CD algorithm. |
| A5: | Answer: There is no need for an algorithm like CSMA/CD for WAN links. A router that is connected to a WAN link can send anytime it wants to send. |
| 6: | What kind of physical interface does a router use to connect to a WAN link? |
| A6: | Answer: A serial interface |
| 7: | Imagine that a WAN link has been installed between routers A and B. The physical cable from the telco was plugged directly into the serial interfaces on the routers. What feature must be included in the serial interface cards on these routers to allow the cable to be connected directly to the routers? |
| A7: | Answer: An internal CSU/DSU must be installed on the card. The engineer who orders the router would make that choice when ordering. |
| 8: | What is the main function of the CSU/DSU, as covered in this chapter? |
| A8: | Answer: To control the transmission speed on the link |
| 9: | What is the base transmission speed typically used for WAN links that run at relatively lower speeds (up to 1.5 Mbps)? |
| A9: | Answer: WAN links typically use a transmission speed of 64 Kbps, or a multiple of that number, up through 24 times 64 Kbps, or about 1.5 Mbps. |
| 10: | What are the two most popular data link protocols used on WAN links? |
| A10: | Answer: HDLC and PPP |
| 11: | What does HDLC do to alert a router that is receiving a frame that an error has occurred? |
| A11: | Answer: HDLC, as well as PPP, includes an FCS field in the trailer. |
| 12: | Of the two more popular WAN link data link protocols, which was developed later and has more features? |
| A12: | Answer: PPP |
| 13: | Explain whether IP ARP is needed on WAN links. Why or why not? |
| A13: | Answer: There is no need for IP ARP on a WAN link. The PPP and HDLC address fields do not carry a meaningful number because anything sent out a point-to-point WAN link always goes to the same place. There is no need to identify the recipient of the frame anyway. |
