S305 Assignment 1 PDF

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Assignment 1 1. Explain how a message is transmitted from one computer to another using layers. The user creates a message using the application layer by clicking a link which translates the message into HTTP. The transport layer then takes the HTTP Packet and puts it in a TCP Segment to be sent to the network layer. The network layer then puts the TCP packet into an IP packet which contains the message, HTTP packet, and TCP segment which then go off into the data link layer. The data link layer then uses the Ethernet frame to instruct the hardware in what to do with the packets it has received. Finally, the physical layer is simply the cable that connects you to the network, which then signals pulses to the server. The server receives the message then reverses the process above, and sends the webpage in the packet through the data link layer and so forth until it is stripped to the bare message which contains the webpage you requested. 2. Describe how a Web browser and Web server work together to send a Web page to a user. A web browser needs a web server in order to process requests to retrieve URLs or access a website. The web browser is simply an application that issues messages and requests indicated by the user. It uses HTTP requests that contain the URL and sends it to the web server where the server generates an HTTP response and sends back the webpage or error message to the browser. 3. Describe how data can be transmitted using amplitude modulation The data that gets transmitted becomes amplitude, which changes the height of the wave that data travels. This allows for more bits to be sent. It is more susceptible to more errors. 4. Describe how data can be transmitted using frequency modulation The data gets transmitted by using more waves per second. Sending more bits at a time. 5. Describe how data can be transmitted using phase modulation The phase modulation allows for faster transfer of data as it shifts when the waves should be sent. 6. Briefly describe how continuous ARQ works When a computer sends a message it does not wait for an ACK after sending the message, instead it keeps sending the next message and then only then resends a message if an NAK came back for that certain message. This helps control the flow/rate at which messages are being sent as to not overload data storage.

7. Clarence should do error checking. There might be something wrong with the network that is caused by external factors such as white noise, impulse, or attenuation. These can be checked by using parity, check sum, or CRC error checking methods. 8. How does TCP/IP perform address resolution from URLs into network layer addresses? Using DNS, URLs are converted in IP Addresses on the Network Layer. When the computer does not know the IP Address of a webpage, the name servers provide DNS services that store IP addresses that then give the client the IP address. The more complex route involves the resolving server contacting root servers, which then contact TLD server which finally asks the authoritative server to send the correct IP Address to the client. Each response though goes back to the resolving server to tell it next steps of who to contact essentially. 9. How does TCP/IP perform address resolution from IP addresses into data link layer addresses? ARP is used to translate IP from DNS to data link layer, then TCP sends out broadcast and asks the subnet to retrieve a computer with the IP Address, and then sends an ARP response with the data link address which allows the sender to transmit the message to the data link layer. 10. What is the transmission efficiency of a 5,000-byte file sent in response to a Web request HTTP, TCP/IP, and Ethernet? Assume that the HTTP packet has 100 bytes in addition to the 5,000-byte file. Assume that the max- imum packet size is 1,200 bytes. Hint: Remember from Chapter 4 that efficiency = user data/total transmission size. 5000/(5100+(9*64))= 88.09% 11. Suppose that a client computer (128.192.98.130) in Building B in Figure 5-17 requests a large Web page from the Web server 2 in the Data Center (www2 .anyorg.com). Assume that the client computer has just been turned on and does not know any addresses other than those in its configuration tables. Assume that all gateways and Web servers know all network layer and data link layer addresses. a. Explain what messages would be sent and how they would flow through the network to deliver the Web page request to the server. The computer sends a DHCP request which gives the IP address of the computer, the subnet mask, Default Gateway IP, and DNS address. The computer also sends out a DNS request to the name server, and send a HTTP request to the Web server. Since they are using HTTP, TCP will be used in the transport layer. The source for TCP varies, and the destination will go to port 80. Then the IP address of the client (128.192.98.130) acts as the IP source and destination is 128.192.50.1 of the Data Center Router. Then in the Ethernet layer, the mac address is the source from the client 00-0C-00-33-3A-1B which has a destination of 00-0C-00-33-3A-B4 for the first hop. The destination from the first hop becomes the source of the 2nd hop and its destination is 00-0C-00-33-3A-DC, in turn it becomes source of 3 rd hop and its destination is 000C-00-33-3A-1D.

b. Explain what messages would be sent and how they would flow through the network as the Web server sent the requested page to the client. Now the process from web server to the client is similar to client to web server. The only difference is that it is in reverse, where destinations and source are swapped all the way to the application layer. There is also no DHCP or need for DNS the opposite way since it knows all the IP addresses, etc. c. Describe, but do not explain in detail, what would happen if the Web page contained several graphic images (e.g., GIF [Graphics Interchange Format] or JPEG [Joint Photographic Experts Group] files). The client would send the original HTTP request for the web server, alongside the DHCP and DNS items. With the original web page HTTP request, it will send additional HTTP requests for each image once the web page is loaded....