Materia redes de computadores

Materia redes de computadores


DisciplinaRedes de Computadores20.956 materiais264.961 seguidores
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For store-and-forward delay ,50=L sec1\u3bc< . 
 
d) Store-and-forward delay is small for both cases for typical link speeds. However, 
packetization delay for is too large for real-time voice applications. 1500=L
 
Problem 34 
 
The IP addresses for those three computers (from left to right) in EE department are: 
111.111.1.1, 111.111.1.2, 111.111.1.3. The subnet mask is 111.111.1/24. 
 
The IP addresses for those three computers (from left to right) in CS department are: 
111.111.2.1, 111.111.2.2, 111.111.2.3. The subnet mask is 111.111.2/24. 
 
The router\u2019s interface card that connects to port 1 can be configured to contain two sub-
interface IP addresses: 111.111.1.0 and 111.111.2.0. The first one is for the subnet of EE 
department, and the second one is for the subnet of CS department. Each IP address is 
associated with a VLAN ID. Suppose 111.111.1.0 is associated with VLAN 11, and 
111.111.2.0 is associated with VLAN 12. This means that each frame that comes from 
subnet 111.111.1/24 will be added an 802.1q tag with VLAN ID 11, and each frame that 
comes from 111.111.2/24 will be added an 802.1q tag with VLAN ID 12. 
 
Suppose that host A in EE department with IP address 111.111.1.1 would like to send an 
IP datagram to host B (111.111.2.1) in CS department. Host A first encapsulates the IP 
datagram (destined to 111.111.2.1) into a frame with a destination MAC address equal to 
the MAC address of the router\u2019s interface card that connects to port 1 of the switch. Once 
the router receives the frame, then it passes it up to IP layer, which decides that the IP 
datagram should be forwarded to subnet 111.111.2/24 via sub-interface 111.111.2.0. 
Then the router encapsulates the IP datagram into a frame and sends it to port 1. Note that 
this frame has an 802.1q tag VLAN ID 12. Once the switch receives the frame port 1, it 
knows that this frame is destined to VLAN with ID 12, so the switch will send the frame 
to Host B which is in CS department. Once Host B receives this frame, it will remove the 
802.1q tag. 
 
Problem 35 
 
R1 R2
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R5
0 
1 
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A
R6
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label label dest inter. 
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0
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0
Problem 36 
 
 
R1 R2
D 
R3R4
R5 1 
1
00
A
R6
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label label dest inter. 
 1 12 D 1 
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label label dest interf. 
12 - D 0
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3 12 D 0
1
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label label dest interf. 
4 1 D 0
0
2 4 D 1
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label label dest interf. 
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label label dest interf. 
 3 D 0 
0
0
 
Problem 37 
 
(The following description is short, but contains all major key steps and key protocols 
involved.) 
 
Your computer first uses DHCP to obtain an IP address. You computer first creates a 
special IP datagram destined to 255.255.255.255 in the DHCP server discovery step, and 
puts it in a Ethernet frame and broadcast it in the Ethernet. Then following the steps in 
the DHCP protocol, you computer is able to get an IP address with a given lease time. 
 
A DHCP server on the Ethernet also gives your computer a list of IP addresses of first-
hop routers, the subnet mask of the subnet where your computer resides, and the 
addresses of local DNS servers (if they exist). 
 
Since your computer\u2019s ARP cache is initially empty, your computer will use ARP 
protocol to get the MAC addresses of the first-hop router and the local DNS server. 
 
Your computer first will get the IP address of the Web page you would like to download. 
If the local DNS server does not have the IP address, then your computer will use DNS 
protocol to find the IP address of the Web page. 
 
Once your computer has the IP address of the Web page, then it will send out the HTTP 
request via the first-hop router if the Web page does not reside in a local Web server. The 
HTTP request message will be segmented and encapsulated into TCP packets, and then 
further encapsulated into IP packets, and finally encapsulated into Ethernet frames. Your 
computer sends the Ethernet frames destined to the first-hop router. Once the router 
receives the frames, it passes them up into IP layer, checks its routing table, and then 
sends the packets to the right interface out of all of its interfaces. 
 
Then your IP packets will be routed through the Internet until they reach the Web server. 
 
The server hosting the Web page will send back the Web page to your computer via 
HTTP response messages. Those messages will be encapsulated into TCP packets and 
then further into IP packets. Those IP packets follow IP routes and finally reach your 
first-hop router, and then the router will forward those IP packets to your computer by 
encapsulating them into Ethernet frames. 
Chapter 6 Review Questions 
 
1. In infrastructure mode of operation, each wireless host is connected to the larger 
network via a base station (access point). If not operating in infrastructure mode, a 
network operates in ad-hoc mode. In ad-hoc mode, wireless hosts have no 
infrastructure with which to connect. In the absence of such infrastructure, the 
hosts themselves must provide for services such as routing, address assignment, 
DNS-like name translation, and more. 
 
2. a) Single hop, infrastructure-based 
 b) Single hop, infrastructure-less 
 c) Multi-hop, infrastructure-based 
 d) Multi-hop, infrastructure-less 
 
3. Path loss is due to the attenuation of the electromagnetic signal when it travels 
through matter. Multipath propagation results in blurring of the received signal at 
the receiver and occurs when portions of the electromagnetic wave reflect off 
objects and ground, taking paths of different lengths between a sender and 
receiver. Interference from other sources occurs when the other source is also 
transmitting in the same frequency range as the wireless network. 
 
4. a) Increasing the transmission power 
 b) Reducing the transmission rate 
 
5. APs transmit beacon frames. An AP\u2019s beacon frames will be transmitted over one 
of the 11 channels. The beacon frames permit nearby wireless stations to discover 
and identify the AP. 
 
6. False 
 
7. APs transmit beacon frames. An AP\u2019s beacon frames will be transmitted over one 
of the 11 channels. The beacon frames permit nearby wireless stations to discover 
and identify the AP. 
 
8. False 
 
9. Each wireless station can set an RTS threshold such that the RTS/CTS sequence 
is used only when the data frame to be transmitted is longer than the threshold. 
This ensures that RTS/CTS mechanism is used only for large frames. 
 
10. No, there wouldn\u2019t be any advantage. Suppose there are two stations that want to 
transmit at the same time, and they