Local Area Networks I

							                                       HW # 3 Solutions

4.    a) Distinguish between binary and digital transmission.
         In binary signaling, there are only two possible states (voltage levels) to represent
         information. In contrast, in digital signaling, there are a few possible states.

      b) Is binary transmission digital?
         Binary transmission is a special case of digital transmission. Here, “few” is two.

      c) What is desirable about having multiple possible states instead of just two?
         It is desirable to have multiple possible states instead of just two because that allows
         the sender to send multiple bits per clock cycle (whereas binary transmission only
         sends one bit per clock cycle).

      d) What is undesirable about having multiple possible states?
         As the number of possible states increases, the number of errors will increase.

      e) How many more bits can you send every time you double the number of possible
      states?
         Every time you double the number of possible states, you can send only one more bit.

11.   a) Distinguish between electromagnetic interference (EMI), crosstalk interference, and
      terminal crosstalk interference.
         Electromagnetic interference (EMI) is unwanted electrical energy coming from
         external devices, such as electrical motors, fluorescent lights, and even nearby UTP
         cord.
         Crosstalk interference is mutual EMI among wire pairs within a single UTP cord.
         Crosstalk interference is a type of EMI.
         Terminal crosstalk interference is crosstalk interference at the ends of a UTP cord,
         where the wires have to be untwisted to fit into RJ-45 connectors. Terminal crosstalk
         interference is a type of crosstalk interference.

      b) How is EMI controlled?
         EMI is controlled by twisted-pair wiring. Each pair‟s wires are twisted around each
         other several times per inch. Over the first half of the twist the interference might add
         to the signal. Over the second half, however, this same interference would subtract
         from the signal. The interference on the two halves would cancel out, and the net
         interference would be zero.

      c) How is terminal crosstalk interference controlled in general? Explain.



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         Terminal crosstalk interference can be controlled by making sure that installers do not
         untwist UTP wires more than 1.25 cm (half an inch) when adding connectors.

      d) Does this completely eliminate crosstalk interference?
         No, but it reduces it to an acceptable level.


15.   a) What is a wave‟s wavelength?
         A wave‟s wavelength is the physical distance between comparable points (e.g., peak
         to peak) in successive cycles.

      b) What is a wave‟s frequency?
         A wave‟s frequency is the number of times the wave goes through a complete cycle
         per second.

      c) Distinguish between traditional optical fiber transmission, WDM, and DWDM.
         Traditional optical fiber only transmits one light source at one frequency. In contrast,
         WDM (wavelength division multiplexing) replaces the signaling equipment at the
         two ends of traditional optical fiber with signaling equipment that can transmit several
         light sources at slightly different frequencies. DWDM (dense wavelength division
         multiplexing) is fiber that carries more than forty light sources at different
         frequencies.

      d) How do WDM and DWDM reduce costs?
         WDM and DWDM reduce costs by gaining added signal capacity but without the high
         cost of laying new fiber.

      e) In WDM, what is a lambda?
         A lambda (λ) a signal at a particular wavelength.

18.   a) What is a signal‟s bandwidth?
         It is the range of frequencies or bandwidths that a signal contains.

      b) Why is being able to support a wide bandwidth good?
         Wide bandwidth means fast speed.

      c) How does graded index multimode fiber reduce modal dispersion?
         Graded index multimode fiber reduces modal dispersion because its graded index of
         refraction decreases from the center of the core to its circumference. This causes
         waves propagating directly down the middle to travel more slowly than waves
         traveling in the outer areas, partially compensating for modal dispersion.

      d) What is modal bandwidth?



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         It is the product of bandwidth times distances—a measure of quality in optical fiber.

      e) Why is it better to have a greater modal bandwidth?
         It is better to have a greater modal bandwidth because a higher modal bandwidth
         means that a multimode fiber can support greater bandwidth over a given distance,
         which in turn means a greater transmission speed.

      f) If you have fiber quality of 200 MHz-km, how much bandwidth would you have if you
      transmit a signal 200 meters?
         You would have 1,000 MHz of bandwidth.

      g) If you have to transmit 4 km?
         50 MHz.

      h) Is modal bandwidth important for single-mode fiber?
         No. It is only important for multimode fiber.

21.   A building has 60 voice jacks on each floor and 40 data jacks on each floor. a) For
      telephony, how many wires will you run through the vertical riser space for each floor?
         You will run 480 wires for each floor. (60 voice jacks times 8 wires per cord)

      b) How many cords will this require?
         There are 25 pairs in each vertical distribution cord, so ten 25-pair cords (500 wires)
         would be needed. <Each 25-pair cord can only support six 4-pair UTP horizontal
         cords because eight does not divide into 25 evenly. This leaves one unused pair.>

      c) For vertical data wiring if you use UTP?
         Only one 4-pair UTP cord would be needed.

      d) For vertical data wiring if you use optical fiber?
         Only one optical fiber cord would be needed.

      e) On each floor, how many wires will you run horizontally from the telecommunications
      closet to wall jacks?
         100 wall jacks times 8 wires per jack means that you will need to run 800 wires.

      f) How many cords will this require?
         It will be 100 UTP cords—one per wall jack.

22.   a) Where is plenum cabling required?
         Plenum cabling is required any time cables run through airways (plenums), such as air
         conditioning ducts.



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       b) Why is plenum cabling needed?
          Plenum cabling is needed in air ducts in case there is ever a fire in the building.
          Ordinary sheaths on UTP and optical fiber cords are made of polyvinyl chloride
          (PVC), which gives off deadly toxins (PCBs) when it burns. If these toxins are
          released in airways, they will spread rapidly to office areas. Plenum, in contrast, gives
          off a lower amount of dangerous fumes when it burns.

       Case 2

       Mitch Wagner, “eBay Takes Steps to Ensure Site Performance,” InternetWeek, March 19,
1999, http://www.internetwk.com/case/study031599-1.htm. (A new window will open in your
browser.)

       What is eBay‟s business? Why is it facing scalability pressures? What servers does it
       have, and how are the different servers used to provide customer service? Finally, what
       general principle(s) did you learn?

        From the eBay website, people can buy and sell items around the world. They are similar
to a large retail store of various items, but they do not sell the items themselves. The reason why
eBay is facing scalability is due to the fact it keeps increasing in popularity all over the world.
eBay is classified as an online auction website that accepts 600 bids per minute. There are
currently 1.7 million active items with 250,000 items added each day. Due to this traffic, a high
demand for their servers is required, especially at key times, like that featured on the „Today‟
show. The demands for reliability and integrity are very high, because people want their
merchandise and information kept safe. In addition, with all of these users online it fails to meet
average response times.

       The Sun Microsystems Starfire E10000 is used for transactions. A second Starfire and
seven four-processor ES450s are used for searches (using a Thunderstone search engine). There
are 60 Compaq PC servers used to run the website for its “front ends”.

        Principles: It requires servers of multiple sizes and uses to run a high-end eCommerce
website. Companies need to maintain high scalability (in some cases, as in the case of eBay,
leave 50% of the main server unused). High scalability is important for high levels of customer
service.




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