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WAN Questions

February 6th, 2017 Go to comments

Question 1

Explanation

An example of the output of the “show interface serial …” command is shown below. We can see the line “Encapsulation HDLC”.

showinterfaces.jpg

Question 2

Explanation

PPP supports both synchronous (like analog phone lines) and asynchronous circuits (such as ISDN or digital links). With synchronous circuits we need to use clock rate.

Note: Serial links can be synchronous or asynchronous. Asynchronous connections used to be only available on low-speed (<2MB) serial interfaces, but now, there are the new HWICs (High-Speed WAN Interface Cards) which also support asynchronous mode. To learn more about them please visit http://www.cisco.com/en/US/prod/collateral/modules/ps5949/ps6182/prod_qas0900aecd80274424.html.

Question 3

Explanation

The advantages of leased lines include:
+ Simplicity: Point-to-point communication links require minimal expertise to install and maintain.
+ Quality: Point-to-point communication links usually offer high service quality, if they have adequate bandwidth. The dedicated capacity removes latency or jitter between the endpoints.
+ Availability: Constant availability is essential for some applications, such as e-commerce. Point-to-point communication links provide permanent, dedicated capacity, which is required for VoIP or Video over IP.

The disadvantages of leased lines include:
+ Cost: Point-to-point links are generally the most expensive type of WAN access. The cost of leased line solutions can become significant when they are used to connect many sites over increasing distances. In addition, each endpoint requires an interface on the router, which increases equipment costs.
+ Limited flexibility: WAN traffic is often variable, and leased lines have a fixed capacity, so that the bandwidth of the line seldom matches the need exactly (therefore answer D is not correct). Any change to the leased line generally requires a site visit by ISP personnel to adjust capacity.

(Reference: Connecting Networks Companion Guide Book published by Cisco Networking Academy – Page 54)

Question 4

Question 5

Explanation

The idea behind a WAN is to be able to connect two DTE networks together through a DCE network. The network’s DCE device (includes CSU/DSU) provides clocking to the DTE-connected interface (the router’s serial interface).

wan_dlci_understand

A modem modulates outgoing digital signals from a computer or other digital device to analog signals for a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device. A CSU/DSU is used between two digital lines -> A & D are correct but B & C are not correct.

For more explanation of answer D, in telephony the local loop (also referred to as a subscriber line) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider’s network. Therefore a modem terminates an analog local loop is correct.

local_loop_modem.jpg

Question 6

Explanation

Below is an example of the output of this command:

show_controllers.jpg

The “show controllers serial …” command tells us about the type of the cable (in the case V.35 DTE cable) and the status of the physical layer of the interface. In above output we learn that there is an cable attached on S0/0 interface. If no cable is found we will see the line “No DTE cable” instead.

Comments (26) Comments
  1. Ppetro
    February 6th, 2017

    Q3 answers should be B & C

    From Cisco website:
    The advantages of leased lines include
    Simplicity: Point-to-point communication links require minimal expertise to install and maintain.

    Quality: Point-to-point communication links usually offer high service quality, if they have adequate bandwidth. The dedicated capacity removes latency or jitter between the endpoints.

  2. 9tut
    February 7th, 2017

    @Ppetro: Thanks for your detection. We have just updated that question.

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    February 8th, 2017

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    February 8th, 2017

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    February 8th, 2017

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  7. buddy
    February 20th, 2017

    can someone explain question 4?

  8. CCNAer
    February 21st, 2017

    @Buddy the routers are on separate class C subnets, The Atlanta Router is known to have correct config therefore Brevard router should be in 192.168.10.0/24 network

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    February 24th, 2017

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    February 27th, 2017

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    March 14th, 2017

    hi people if any one has the time could you please explain Q5 of the wan questions in more depth finding it a bit difficult to understand

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  24. rajput……..
    May 14th, 2017

    if someone explain q4 ?

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    May 22nd, 2017

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  26. CC
    May 26th, 2017

    @ rajput

    For Q4, they’re telling you that the Atlanta router configs are okay, the only problem is with Brevard. When you look at the show interface input, you’ll see that Brevard serial interface has an ip 192.168.11.2/24 while Atlanta serial interface has an ip of 192.168.10.1/24. The interfaces need to be in the same subnet for the serial link to work, but they aren’t (11.0-11.255 vs. 10.0-10.255), so the problem would be:

    (e) an incorrect IP address