2
Spring IT120
Homework 2012
1. (9pts) An image is 1280 by 800 pixels. Each pixel requires 2 bytes of data. Assume that the image is
uncompressed. How long does it take to transmit this image over a using an 802.11g wireless access point? How
long does it take to transmit this image over a 10Gbps LAN?
(1280 * 800) * 2 bytes * 8 bits/bytes = 16,384,000bits => total number of bits to be transferred
For 802.11g the transmission speed is 54Mbps (this is mega bits per second):
16,384,000 bits/ (54 Mbps * 1,000,000 bits/Mbits) = .3034 s econds
For 10Gbps LAN the transmission speed is 10Gbps (this is giga bits per second):
16,384,000 bits/ (10Gbps * 1,000,000,000 bits/Gbits) = .00164 seconds
2. (6pts) A 802.11 wireless station in the United States wishes to send a frame to a station in another BSS located in
Europe. What should the receiving station see in the ToDS and FromDS bits and the 4 address fields in the frame
from the sending station?
Given these two BSSs are not in the same geographic location, and internet addressing is used , the WAP
for Europe and the WAP for the US will be attached to a wired distrib ution in order to be sent out via the
router. It is This is case 2 since we are talking about a wired distributed system. Therefore,
ToDS = 0
FromDS = 1
Addr1 = Destination MAC
Addr2 = Sending AP MAC
Addr3 = Source MAC
Addr4 = not used
3. (8pts) Assume you provide the tech support for your office. A colleague reports that his computer is working but
he cant access the network. He says he was not able to access the network yesterday from his machine. You ask
around but no one else seems to have this problem.
a. You conclude that the problem is probably either in the computers NIC card or the cabling between his
work area and the telecommunications room. Why?
Because others in the same network can access the network, it appears to be an isolated situation and
the only hardware/software that others are not using are the users machine (including the NIC card) and
the cable from his machine to the telcom room.
b. Based on these two possibilities, how would you try to isolate the problem further?
-
I would first connect his computer to a known good connection. If it works, the problem is not the NIC
but is the cabling. If it does not work, the problem is the NIC , software, or configuration.
- Checking configuration (see if connected to the network) is the most obvious first step if I suspect
it is the machine and not the cable
i. If I suspect the NIC, I will examine the lights and run diagnostics, including a loopback
test.
ii. If I suspect the software, I could reload it but before I do, I would check the NIC and the
configuration
- If I suspect the cable, I will use connectivity testing to isolate the problem to a patch cable, the
horizontal run, or a connector.
4.
(9pts) Eco Friendly Designs has decided to open an office in Sunnyville, TX. They plan to be here for a while
and rented more space to support future expansion. This includes an outdoor patio where their employees can eat
lunch. The new manager, Mr. Wannabe, has hired you to install cabling and related equipment to connect 12
computers, 2 file servers and 2 network printers to a 100BaseT wired Ethernet LAN.
He has provided the floor plan of the office shown below.
Create a plan that includes as a minimum:
a. A list of all equipment required, including that for the equipment (telecommunications) room. Assume
every computer already has the appropriate NIC card.
b. On the plan, identify the location of the telecom room. Also, show the runs and drops (keep cable lengths
under 90 meters and specify whether they will be in the ceiling, the walls, in a raceway outside the walls
or across the floor.) Assume there is a dropped ceiling and estimate distances if necessary.
IT120
Homework 2
Spring 2012
c. Capability for growth. You should be able to add several more computers and upgrade the LAN without
changing or adding equipment.
Telcom room:
Printers
Printer
- Patch panel
connected to
demarc and
then tied to
the rack.
- Servers and
switch are in
the rack
Could run a column
to support desk out
in the open or run it
under the floor
Note, answer can vary in placement of telcom and runs
Selection of telcom location
- Pick an interior space with following characteristics
o no windows
o centrally located with access to the buildings Demarc for Internet access
o not necessarily near forms of EMI (microwaves in the kitchen)
o can be secured with a lock on the door
o has room for expansion if needed
- Include in this location
o Patch panel connected to Horizontal Runs and patch cables to Rack
o Rack to support at least 2 file servers and a switch (maybe something floor to ceiling for
expansion and can be securely fastened so it wont move if bumped. Rack should be near
patch panel and access to Demarc
o Include proper ventilation (humidity and AC)
o Surge protectors for all equipment
o UPS sufficient to bring servers down gracefully
o DO NOT put printers in this location since employees will need to get their printouts.
The 2 printers are located in non-used offices at opposite ends of the office to
support easy access by employees.
Horizontal Runs (red lines)
- Choose to run 3 horizontal runs through the offices. Two may have been enough but the 3
chases will be easier to adapt to future interior wall changes or targeting separate LANs.
- Drop-downs from each run will support ports marked as green Xs in the diagram.
- Cabling used in runs will be CAT 5e or Cat6. Solid core should be used for better conductivity.
- Trays will be used in the ceiling to support the runs
- Note, the length of the office is less than 90m so all runs meet length criteria
- Patch cables from ports (or patch panel in telcom room) should be the same CAT type used horizontals in
the but twisted copper cores for flexibility should be used.
Expansion should not be a problem. Additional runs add ed to support the outline of existing offices with
sufficient ports to support additional equipment. Make sure the switch/hub can support the number of
ports shown and the rack can support additional equipment.
IT120
5.
Homework 2
Spring 2012
(9pts) Mr. Wannabe of Eco Friendly Design has just found out that their main competitor, Friendly Ecosystem
Design, has moved in next door on the wall adjacent to the patio. He was thrilled with the network installation
you proposed in the previous example and has contacted you to add a security system to monitor Eco Friendly
Designs patio. The new security system incorporates many remote video cameras (you do not have to place
them). Live video feeds can be provided to any networked PC running security application software.
Evaluate the network you designed. Your customer is concerned that the new security system will slow down his
normal business operations, so he wants you to investigate upgrading the network to Gigabit Ethernet while
minimizing the changes to his existing network. His concern is cost and down time with this installation.
While answering the questions below, make any necessary assumptions about your clients network and be sure to
state those assumptions.
Answers can vary but the overall recommendation must be consistent.
a. Do you recommend using UTP or fiber? Why?
I recommend using UTP because the existing cables can be reused. The existing network, 100BaseT, uses
CAT5e or better cables of less than 100m in length. This same cabling can be used in 1000BaseT.
b. Exactly which Ethernet do you recommend? (1000BaseT, 1000BaseCX, 1000BaseSX, or 1000BaseLX)
I recommend 1000BaseT for the reasons listed above.
c. What equipment must be upgraded? (Consider the equipment room, the cabling and the work areas.)
I assume that the existing equipment room contains at least one 100 Mbps switch. These switches will
have to be upgraded since we havent specified what machines will support live feed software.
As discussed above, the cabling is probably OK.
The exact connection of the new security cameras to the network was not discussed, but I assume the
cameras will feed into some security computers. These computers will probably also require Gigabit
NICs. I assume that at least some of the hosts on the network have only 100Mbps NICs and will need new
Gigabit NICs. At this point, I would need to discuss with Mr. Wannbe and have him identify the machines
that need to support live feed. I would suggest these machines be in locked rooms or at least have the
data stored on a server in the telcom room. This may require a separate server.
6. (9pts) Mr. Wannabe of Eco Friendly Design has need of your services again. He has been told by a number of
employees they would like to do their work on the patio on sunny days. He has asked you to research what
wireless network options will fit their needs but is again concerned about cost.
Review the floor plan from the previous question and consider the following needs in your recommendation.
The network should be able to support up to 20 computers
The network must be secure against unauthorized wireless access, but allow authorized visitors to join
with minimal configuration
The network should use widely available industry-standard technology
a. What 802.11 standard would you recommend? Why?
I would recommend 802.11n, since it is the most modern, is compatible with earlier hardware, and has good
performance.
b. What equipment will be necessary?
I would create a network in infrastructure mode. It will require a wireless access point and wireless NICs for each
computer. The access point would be wired to the distribution system (existing Ethernet system we did in
problem 3). One access point would be sufficient since there is an effective distance of 300feet for 802.11n. If you
do use multiple WAPs dont forget to use different channels.
c. How would you provide security? Would you make any special suggestions considering the upstairs
competition? Why?
I would enable WPA2 encryption. Visitors computers can easily be configured with the encryption key.
I do not recommend MAC filtering, since it would require reconfiguring the WAP for each visitor.
A more secure means would be to setup a Radius Security solution with guest accounts that have restricted
access.
IT120
Homework 2
Spring 2012
Extra Credit
1. (2pts) An Ethernet frame contains 20 bytes of data. Calculate the percentage of the frame that is overhead (include
the preamble in the frame).
#overhead bytes = 8(preamble) +6(dest addr) +6(source addr) +2(length/type) + 4(CRC)
=26bytes
20 bytes of data is insufficient for the frame size so padding is needed
64 bytes is the minimum data size therefore
64 bytes (18(without preamble+20(data))bytes = 26bytes of padding
The total frame(64bytes) plus preamble (8bytes) = 72bytes of which only 20bytes is actual data
%overhead = # overhead bytes / total bytes *100%
=(72-20)/72 * 100%
=72.2% is overhead
2. (2pts) The standard wired Ethernet does not use any acknowledgement frames but Wireless Ethernet (IEEE
802.11) requires an ACK frame in the protocol described in class. Explain why there is a need for the difference
(in 5 sentences or less).
Wired Ethernet (802.3) uses CSMA/CD and can detect collisions by monitoring the medium. Wireless
Ethernet (802.11) uses CSMA/CA and depends upon the lack of acknowledgement to determ ine that a
collision has occurred. Therefore, 802.11 requires RTS/CTS/ACK.
3. (1pt) A lower has a bandwidth of 15MHz and a higher frequency of 20MHz. What is the medium frequency?
BW = upper freq lower freq
lower freq = upper freq BW
lower freq = 20MHz 15MHz
= 5MHz