Computer Network Test 1 (ch 1 & 2)

Consider sending a packet from a source host to a destination host over a fixed route. List the delay components in the end-to-end delay. Which of these delays are constant and which are variable?

The delay components are processing delays, transmission delays, propagation delays, queuing delays. All of these delays are fixed, except the queuing delay, which are variable. >> Processing Delay (constant): the time required to examine the packet's header and determine where to direct the packet; includes the time needed to check for bit-level errors in the packet. >> Queuing Delay (Variable): waiting to transmitted onto the link; depends on how many packets were sent before yours that are waiting. >> Transmission Delay (constant): The amount of time required to push all of the packet's bits into the link. R=link bandwidth (bps), L=packet length (bits). -> (transmission delay) time to send bit into link = L/R >> Propagation delay: the time required to propagate from the beginning of the link to router B. It depends on the physical medium of the link (fiber optics, twisted-pair copper wire, and so on). The propagation delay is the distance between two routers divided by the propagation speed. -> Propagation delay = d/s. Where d is the distance between router A and router B and s is the propagation speed of the link(~2x10^8 m/sec)

For a communication session between a pair of processes, which process is the client and which is the server?

The process which initiates the communication is the client; the process that waits to be contacted is the server.

Suppose Host A wants to send a large file to Host B. The path from Host Ato Host B has three links, of rates R_1= 500 kbps, R_2= 2 Mbps and R_3= 1 Mbps.a. Assuming no other traffic in the network, what is the throughput for the file transfer? b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B? c. Repeat (a) and (b), but now with R_2 reduced to 100 kbps.

>> Consider givend data:R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps The throughput for the file transfer=min{R1,R2,R3} =min{500 kbps, 2 Mbps, 1 Mbps} =500 kbps So, the throughput for the file transfer=500 kbps >>Consider given data: The file size= 4 million bytes Convert million bytes to bits =32000000 bits. From (a), Throughput for the file transfer=500 Kbps =500000 bps Dividing the file size by the throughput,roughly how long will it take to transfer the file to Host B: =file size/hroughput for the file transfer =32000000 bits/500000 bps =64 seconds >>Consider the given data: Repeat (a) and (b), but now with R2 reduced to 100 kbps. That means R2=100 kbps , R1 = 500 kbps, and R3 = 1 Mbps The throughput for the file transfer=min{R1,R2,R3} =min{500 kbps, 100 kbps, 1 Mbps} =100 kbps So, the throughput for the file transfer=100 kbps Dividing the file size by the throughput,roughly how long will it take to transfer the file to Host B: =file size/hroughput for the file transfer =32000000 bits/100000 bps =320 seconds

List six access technologies. Classify each one as home access, enterprise access, or wide-area wireless access.

>> Dial up modems: Home >> Fiber optics: Home/Enterprise >> Wifi: Home/Enterprise >> Ethernet: Enterprise >> Cellular data (4G/3G): wide-area >> DSL (Digital subscriber line) (phone wall): Home/small office

What are the five layers in the Internet protocol stack? What are the principal responsibilities of each of these layers?

>> From top to bottom - the application layer, the transport layer, the network layer, the link layer, and the physical layer. >> Application layer: Runs the application. User interacts directly with the application layer. >> Transport layer: Runs on the end systems - connects client to server. Connects the network layer to the application layer because it tells the packet which application to go to (based on pot) >> Network layer: Delivers the packet to the end system. Uses different protocols (ex: IP address protocol) >> Link layer: Delivers from the source of a link to the destination of a link. Sends the packet to the next node in the network. Network layer tells link layer next node in the path and the link layer executes it. (deliver data between 2 individual nodes) >> Physical layer: bits "on the wire"

Which layers in the Internet protocol stack does a router process? Which layers does a link-layer switch process? Which layers does a host process?

>> Routers process network, link, physical layers (layer 1 through 3). >> Link layer switches process link and physical layers (layer 1 through 2). >> the Host process all five layers.

What is the difference between a host and an end system? List several different types of end systems. Is a Web server an end system?

>> They are the same (both provide service to user on a network). >> A web server is an end system (application running on an end system) >> End systems include: printer, web server, mail server, work station, PCs,.... (router is NOT an end system b/c they do not implement the transport layer or application layer)

Why is it said that packet-switched networks use statistical multiplexing? What advantage does a circuit-switched network have over a packet-switched network? What advantages does TDM (Time division multiplexing) have over FDM in a circuit-switched network?

>> Use statistical multiplexing for SHARING of resources. (no wasted resources). >> Circuit-switched: (no sharing) guaranteed performance (call) >> TDM and FDM are circuit-switching. TDM > FDM because each user gets full use of the frequency for a certain period of time. When doing FDM, each user gets access to only a portion of the frequency for unlimited time. >> EX: (FDM) 33 mbps each for 3 users, if only 1 wants to send a packet, they still only send it over that 33 mbps when with statistical multiplexing, (TDM) even if there are 3 users and only 1 wants to send a packet, then they get all 100 mbps to send it. No wasted slots.(TDM)

List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.

>> a) Reliable data transfer TCP provides a reliable byte-stream between client and server but UDP does not. >> b) A guarantee that a certain value for throughput will be maintained: Neither >> c) A guarantee that data will be delivered within a specified amount of time: Neither >> d) Security: Neither

What is the transmission rate of Ethernet LANs? For a certain transmission rate, can the users transmit continuously at that same rate?

All at different speeds (different quantities of bit per second) such as 10 mbps, 100 mbps, 1000 mbps (megabits per second). For one user, yes, you can transmit continuously at the same rate. Otherwise, it's rate for each user = rate/number of users.

What is the difference between network architecture and application architecture?

Network architecture refers to the organization of the communication process into layers (e.g., the five-layer Internet architecture). Application architecture, on the other hand, is designed by an application developer and dictates the broad structure of the application (e.g., client server or P2P).

How long does it take a packet of length 1,000 bytes to propagate over a link of distance 2,500 km, propagation speed 2.5·10^8 m/s, and transmission rate 2 Mbps? More generally, how long does it take a packet of length L to propagate over a link of distance d, propagation speed s, and transmission rate R bps? Does this delay depend on packet length? Does this delay depend on transmission rate?

Propagation delay = d/s = (2,500 km) / (250,000,000 Mb/s) >> 10msec; d/s; no; no

What information is used by a process running on one host to identify a process running on another host?

The IP address of the destination host and the port number of the destination socket.

What is an application-layer message? A transport-layer segment? A network- layer datagram? A link-layer frame?

They are the protocol data unit. >> Application-layer message: data which an application wants to send and passed onto the transport layer. >> Transport-layer segment: generated by the transport layer and encapsulates application-layer message with transport layer header. >> Network-layer datagram: encapsulates transport segment with a network-layer header. >> Link-layer frame: encapsulates network-layer datagram with a link-layer header.

Suppose you wanted to do a transaction from a remote client to a server as fast as possible. Would you use UDP or TCP? Why?

You would use UDP. With UDP, the transaction can be completed in one roundtrip time (RTT) - the client sends the transaction request into a UDP socket, and the server sends the reply back to the client's UDP socket. With TCP, a minimum of two RTTs are needed - one to set-up the TCP connection, and another for the client to send the request, and for the server to send back the reply.