CompE560 - Exam
Why does TCP's congestion control mechanism not work when we have a wireless LAN connected to TCP? Name a protocol which fixes the above problem
-TCP views a timeout as congestion but with wireless a timeout does not necessarily mean congestion. -Indirect TCP or SNOOP
Examples associated with the application layer of the OSI model?
-Telnet -FTP
Which of the following correctly describe steps in the data encapsulation process in a protocol stack?
-The transport layer divides a data stream into segments and adds reliability and flow control information. -Packets are created when the network layer adds Layer 3 addresses and control information to a segment
Which two TCP header fields are used to confirm receipt of data?
-acknowledgment number -sequence number
What functions do routers perform in a network?
-path selection -packet switching
Which three pieces of information does a link-state routing protocol use initially as link-state information for locally connected links? (Choose three.)
-the cost of that link -the type of network link -the link router interface IP address and subnet mask
What is the complete range of TCP and UDP well-known ports?
0 to 1023
A class B block having mask of 255.255.192.0
2^2=4 subnets 2^14 hosts
122.45.77.32/20
2^4 for subnet 2^12-2 for host
What would be the interval from which a station might pick its back off slots after the 3rd collision in a Binary Exponential Back off Algorithm?
2n-1 = 7 , therefore [0,7]
Given the following information, find the minimum bandwidth required for the path: FDM Multiplexing, Five devices, each requiring 4000 Hz. , 200 Hz guard band for each device
5x4kHz + 4x200Hz = 20khz + 800hz = 20.8kHz
A PC is downloading a large file from a server. The TCP window is 1000 bytes. The server is sending the file using 100-byte segments. How many segments will the server send before it requires an acknowledgment from the PC?
1000 byte window / 100-byte segment = 10
In a classful, Class B addressing protocol, which address in the given list, does not belong to the same network? 130.31.23.31 130.31.24.22 130.32.23.21 130.31.21.23
130.32.23.21
After how many collisions does the algorithm throw away a frame?
16 consecutive collissions
Does Webmail use POP3, IMAP or neither? If neither, which one is it closer to in spirit? If Webmail were to choose between POP3 or IMAP, which one do you think it would choose for better functionality?
Neither, IMAP is closer in spirit IMAP
At which OSI layer is a logical path created between two host systems?
Network
The routing decision is made by which layer?
Network -routes and determines path
Name 2 approaches that are currently used to fight the scarcity of address space in IP.
Network Address Translation and Sub netting
The data link layer takes the packets from _________ layer and encapsulates them into frames for transmission.
Network Layer -Data from Application layer => Transport Layer -Transport Layer => Converted Segments -Converted Segments => Network Layer = ~ PACKETS
The device that is present in every cell and directly talks to your cell phone
Network interface Card
Is the network layer sufficient if you are not worried reliability from the transport layer.Why?
No, the transport layer has required information for transmitting data. With not transport layer reliability you would function in a udp like state.
Consider the Pure ALOHA, Slotted ALOHA, and non-persistent CSMA. Which one will you use at high load? Name on property of the protocol that makes it the suitable choice?
Non-Persistent CSMA because it is non-aggressive and has high throughput under high load (but higher delay) because it waits a random amount of time (dramatically reducing collisions).
A client server communication is taking place over a trans--‐atlantic cable 'y' kms long. Given that the network functions perfectly and the speed of propagation of Electric signals through the cable is 'b'kms/sec, what is the earliest time in which a response could be received from the server to a request put forth by the client?
One way = y/b, to get a response and assuming no delay at the server, total round trip time to receive a response = 2y/b.
Name an application which might decrease the user experience if Nagle's algorithm is being used. Does it still provide better network utilization? What syndrome does Clarke's solution provide a solution to? How is it different from Nagle's solution?
Online Gaming which requires you to constantly send data. Yes, it does provide better network utilization. Silly window at the receiver. It is applied to the receiver and stops it from advertising small windows, whereas nagles sends a small packet and waits for an ack with a window size
The category of network (in terms of coverage area) of which Bluetooth is a member of?
PAN - Personal Area Network
Arrange smallest to largest network size: LAN, PAN, MAN, WAN
PAN < LAN < MAN < WAN
Name the category of a network (in terms of coverage area) of which Bluetooth is a member:
PAN = Personal Area Network
What is the main difference between SMTP and POP3?
POP is to receive emails and SMTP is to send
Name another error detecting technique and an error correcting technique.
Parity; checksum; crc - error detecting Hamming code; binary convolution codes; reed Solomon -error correcting schemes
The layers of the protocol stack implemented in a layer 3 router.
Physical, Data Link, Network
To deliver a message to the correct application program running on a host, the _______ address must be consulted
Port
State two improvisations that MACAW makes over MACA
RRTS, DS, ACK functionality and Uniform back off counter
What is the virtual channel sensing mechanism called in 802.11?
RTS/CTS
The latency encountered by a packet while it is being put out on the link bit by bit?
Transmission Delay - Time it takes to put packet onto link -Time required to put data bits on wire
Name the fourth layer of the hybrid network protocol stack?
Transport
Which layer defines the functions of logical network wide data transfer?
Transport
An example of "same-layer-interaction" can be the process of TCP on one computer marking a segment as segment 1, and the receiving computer then acknowledging the receipt of segment 1 -
True
CSMA/CD cannot be used for wireless networks (True or False)?
True
Packets of the same session may be routed through different paths in TCP and UDP : True or False
True
Slotted ALOHA gives higher throughput than pure ALOHA in high traffic conditions (True or False)?
True
The interframe space lets an ongoing communication retain the channel (True or False)?
True
The process of HTTP asking TCP to send some data and make sure that it is received correctly is an example of adjacent layer interaction -
True
Name the protocol that is used to fetch the IP address of a website from a DNS server What layer of the protocol stack does the above protocol reside?
UDP Transport
What is the main difference between TCP and UDP? Given the above difference, why do we still need UDP? Isn't the IP layer good enough?
Udp = simple (unreliable) message delivery Tcp = reliable bidirectional stream of bytes -Small packet header overhead
Which term is used specifically to identify the entity that is created when encapsulating data inside data link layer headers and trailers? a) packets b) segments c) messages d) frames
d - frames
Which one of the following routing algorithm can be used for network layer design? a) shortest path algorithm b) distance vector routing c) link state routing d) all of the mentioned
d) all of the mentioned
What is the difference between a circuit switched network and a packet switched network? a.) circuit set-up time versus no such overhead b.) dedicated communication channel versus shared communication channel c.) all packets take the same path versus each packet may take a different path d.) all of the mentioned
d.) All of the Mentioned
A small-sized company has 20 workstations and 2 servers. The company has been assigned a group of IPv4 addresses 209.165.200.224/29 from its ISP. What technology should the company implement in order to allow the workstations to access the services over the Internet? a.) static NAT b.) dynamic NAT c.) port address translation d.) DHCP
d.) DHCP
The physical layer provides a.) mechanical specifications of electrical connectors and cables b.) electrical specification of transmission line signal level c.) specification for IR over optical fiber d.) all of the mentioned
d.) all of the mentioned
Header of a frame generally contains : a) synchronization bytes b) addresses c) frame identifier d) all of the mentioned
d.) all of the mentioned Header -Part of the data that contains the required information about the transmission of the file EX => synchronization bytes, addresses, frame identifier
In the slow start phase of the TCP congestion control algorithm, the size of the congestion window increases ___________
exponentially
What is a beneficial feature of the UDP transport protocol?
fewer delays in transmission
Which sublayer of the data link layer performs data link functions that depend upon the type of medium? a) logical link control sublayer b) media access control sublayer c) network interface control sublayer d) none of the mentioned
Medium Access Control -deals with transmission of data packets to and from the network-interface card
A host device needs to send a large video file across the network while providing data communication to other users. Which feature will allow different communication streams to occur at the same time, without having a single data stream using all available bandwidth?
Multiplexing
Three or more devices share a link in __________ connection
Multipoint -3 or more network nodes connected to each other
What is a socket?
the combination of a source IP address and port number or a destination IP address and port number -an endpoint of an inter-process communication flow across a computer network
During a TCP session, a destination device sends an acknowledgment number to the source device. What does the acknowledgment number represent?
the next byte that the destination expects to receive
Which information does a switch use to populate the MAC address table?
the source MAC address and the incoming port
A new university (Class B IP addressing) gets a subnet mask of 255.255.240.0. How many unique subnets can this university have? For each subnet, how many unique hosts can it have? If subneting was not introduced would the unique number of hosts that this university could have be more, less or equal to the number of total unique hosts that the university network can support now?
unique subnets 2^4=16 (240 = 1111 0000) Total host possible 2^12 -each part of the IP is 8 bits! -We have 2^4 bits for the subnets and 2^8 for the hosts If subneting was not introduced , the unique number of hosts that this university could have be equal to 2^16
The maximum window size for data transmission using the Back-Back-N protocol with n-bit frame sequence numbers is:
(2^n) - 1
State the interval from which a station might pick its back-off slots after the 3rd collision in a Binary Exponential Backoff Algorithm? What is the probability of successful transmission after the 3rd collision? What is the probability of successful transmission after the 4th collision?
(2^n) - 1 = (2^3) - 1 = 8 - 1 = 7 [0 , 7] multiple collisions 1-(np(1-p)^(n-1)+(1-p)^n)
Mention one drawback of the LSR protocol?
-Flooding of Packet -High convergence/update overhead
Examples of an Application layer service?
-Remote login -File transfer -Email service
(i) Consider an error free 64--‐kbps satellite channel used to send 512--‐byte data frames in one direction with very short ACKs coming back the other way. What is the maximum throughput for window sizes of 127? (ii) The propagation time is 270 msec.
(i) 64kbps (ii) (512x127)/(270x2) Tdata = 512x8x27 / 64kbps = 8.128 sec Utilization = Tdata / (Tdata + 2xTpd) = 8.128 / (8.128 + 2(.270) ) = .781 Throughput = U x link speed = .781 x 64kbps = 49.9 kbps
(i) A 10-Mbps(Mega bits per second) Ethernet with a single store-and-forward switch in the path, and a packet size of 5000 bits. Each link introduces a propagation delay of 10µs and the switch begins retransmitting immediately after it has finished receiving the packet. ii) Same as (i) but assume the switch implements "cut-through" switching: it is able to begin re- transmitting the packet after the first 200 bits have been received. (iii) Is it possible for there to be two or more TCP connections between these two ports at the same time?
(i) To put the packet on the first link it will take: 5000/(10x10^6) = 0.5ms Then it will take another .01 ms for the switch to finish receiving the packet for a total of .51 ms. It will take another .51 ms for the switch to send, and then for the destination to receive all of the packet. The total latency is: 2 ∗ 0.51ms = 1.02ms (ii) To begin we can calculate the time it will take to send the first 200 bits: 200/(10x10^6) = 0.02ms Then .01 ms later the switch can start sending the packet. It will take .5 ms (from (a)) to put the entire packet on the wire (As the switch is sending out the packet, it will continue receiving the rest of the packet). Then it will take .01 ms for the destination to finish receiving the entire packet: 0.02ms + 0.01ms + 0.5ms + 0.01ms = .54ms (iii) NO
(i) The receiver sends back ACK=2048, WIN=2048 (ii) The receiver sends back ACK=2048, WIN = 0 (iii) After sending (ii) the sender does not hear from the receiver for a "long" time
(i) sender does a 2k write and is blocked from further transmission until receiver's buffer has room (ii) sender does nothing, waiting for space to be available in receiver buffer (iii) Sender timeout retransmits data
When comparing, and contrasting the similarities and differences between bridges and switches, which of the following are valid statements?
-Bridges and switches learn MAC addresses by examining the source MAC address of each frame received. -A switch is a multiport bridge
A 802.11 wireless device has two carrier sensing mechanisms implemented - name them
-DCF -PCF
Assume that a client types www.extraordinarydesserts.com in his browser window. Also assume that this particular website uses a CDN to speed up the user experience which means that certain image/audio files (in this example the image black_forest_cake.jpg)are hosted by the CDN's server which are strategically distributed all across the globe. The diagram below outlines the sequence of client-server calls that are made in order display the requested webpage. Each client-server call is defined in the items numbered (i)-(x). Match the client-server call numbered on the link in the diagram below along with their appropriate definition as outlined in items (i)-(x) - 2,7,10,6,5,3,8,4,1 i) Cached black_forest_cake.jpg returned ii)Lookup www.extraordinarydesserts.com iii) Request black_forest_cake.jpg iv) Client told to redirect to i-cdn.com v) Client clicks on an image of a black forest cake vi) HTML page returned vii) IP address of www.extraordinarydesserts.com returned viii)IP address of machine where black_forest_cake.jpg is stored, returned ix) Request black_forest_cake.jpg x)Request HTML page
1-Lookup www.extraordinarydesserts.com 2-IP address of www.extraordinarydesserts.com returned 3-Request HTML page 4-HTML page returned 5-Client clicks on an image of a black 6-Request black_forest_cake.jpg 7-IP address of machine where black_forest_cake.jpg is stored, returned 8-Client told to redirect to i-cdn.com 9-Cached black_forest_cake.jpg returned
As data passes downward through the layers of the protocol stack, it is encapsulated into various formats. Which of the following is the correct order of encapsulation?
1. Segment 2. Packet 3. Frame 4. Bit
When is the randomization interval frozen?
10 consecutive collissions
What is the basic difference between Gigabit Ethernet and basic Ethernet?
10 times faster
Calculate the MDR for a noiseless channel which occupies the spectrum between 1MHz and 7MHz and transmits data using 4-level digital signals [Nyquist MDR = 2H log2 V bps, Shannon MDR = H log2 (1+S/N) bps]
7Mhz - 1Mhz = 6Mhz wide MDR = 2(6x10^6)lg(4) = 2(6x10^6)lg(2^2) = (2)(2)(6x10^6) = 24Mbps
(i) For a 7 layer network protocol stack with each layer generating a 'h' byte header and the application layer generating a 'M' byte message - what is the total number of bytes that leave the bottom--‐most layer? (ii) What fraction of that message comprises of overheads?
7h+m 7h/(7h+m)
Consider a network comprising of 5 routers - A, B, C, D, and E. A is connected directly to B, C, and D with links that provide a delay of 5,4 and 3 respectively. A receives 3 distance vector routing packets from B, C and D which carries the information that B, C and D can reach router E in a delay of 4, 6 and 8 respectively. Draw the delay cost packet that A will create based on this information to every other router in the network.
A 0 - B 5 B C 4 C D 3D E 9 B
What are cookies in the context of computer networks?
A way for a server to store information about processes or statuses locally on your computer. Cookies track your online actions. Cookies are very important to ecommerce
In the layered hierarchy, as the data packet moves from the upper to the lower layers, headers are?
Added -Every layer adds its own header to the packet from the previous layer
Name the layer in the TCP/IP protocol stack which lets a user use applications like email?
Application
This OSI layer defines the interface between a program like Outlook or Firefox and the network communication functions.
Application
Name the mechanism through which link state packets are distributed in the network?
Control Flooding
The routing decision is made by which plane within a router?
Control Plane
What prevents a webpage from being stateless?
Cookies
The __________ of a signal is the difference between its highest and lowest frequencies
Bandwidth -Range of frequencies within a given band
Application developer has permission to decide the following on transport layer side: A. Transport layer protocol B. Maximum buffer size C. Both of the mentioned D. None of the mentioned
C. BOTH MENTIONED Transport Layer Protocol Maximize Buffer SIze
A client process P needs to make a TCP connection to a server process S. Consider the following situation: the server process S executes a socket(), a bind() and a listen() system call in that order, following which it is preempted. Subsequently, the client process P executes a socket() system call followed by connect() system call to connect to the server process S. The server process has not executed any accept() system call. Which one of the following events could take place? A. connect() system calls returns successfully B. connect() system call blocks C. connect() system calls returns an error D. connect() system calls results in a core dump
C. connect() system calls returns an error
The channel contention mechanism of Ethernet
CSMA/CD
Assume that a client types www.cnn.com in his browser window. Also assume that this particular website uses the Akamai CDN services to speed up the user experience which means that certain image/audio files (in this example the image taliban_rampage.mov)are hosted by the CDN's server which are strategically distributed all across the globe. The following is a jumbled up sequence of client-server calls that are made in order to display the requested webpage and the video file. Arrange them in the right sequence. i) Cached taliban_rampage.mov returned ii) Ask for the IP address of www.cnn.com from the DNS server iii) Request file taliban_rampage.mov from Akamai-CDN server iv) Client told to redirect to Akamai-cdn.com v) Client clicks on a video clip vi) IP address of www.cnn.com returned vii) P address of machine where taliban_rampage.mov is actually stored, returned viii) Request taliban_rampage.mov ix) Request main HTML page from server x) Client enters www.cnn.com in his browser window xi) IP address of Akamai-cdn is fetched xii) HTML page returned
Client enters www.cnn.com in his browser window Ask for the IP address of www.cnn.com from the DNS server IP address of www.cnn.com returned Request main HTML page from server HTML page returned Client clicks on a video clip Request taliban_rampage.mov IP address of machine where taliban_rampage.mov is actually stored, returned IP address of Akamai-cdn is fetched Client told to redirect to Akamai-cdn.com Request file taliban_rampage.mov from Akamai-CDN server
Name the specific problem of the DVR algorithm that LSR was designed to overcome?
Count to Infinity
A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop--‐and--‐wait give an efficiency of at least 50%.
Frame size/data rate > 2 times delay => 160 bits or more. x/4k>.04 => .00025x> .04
What is the karn-Partridge algorithm?
It's a way to determine round trip time
Name the routing protocol that encounters the "count to infinity" problem
DVR
In which layer are frame errors corrected?
Data Link Layer
In which layer of the protocol stack are MAC addresses used for addressing?
Data Link Layer
The __________ of a channel is the maximum information rate that a channel is able to transmit/receive
Data Rate Rate = Data / Time
Frames of 1000 bits are sent over a 10^6 bps duplex link between two hosts. The propagation time is 25ms. Frames are to be transmitted into this link to maximally pack them in transit (within the link). What is the minimum number of bits (i) that will be required to represent the sequence numbers distinctly? Assume that no time gap needs to be given between transmission of two frames.
Delay = 1 Frame = 1000 bits / 10^6 bps = 1 / 10^3 = 1ms Propogation = 25ms -Sender can transfer 25 frames before 1st frame reaches destination => # of bits needed to represent 25 = 5 bits 16 8 4 2 1 1 1 0 0 1 = 25
A small-sized company has 20 workstations and 2 servers. The company has been assigned a group of IPv4 addresses 209.165.200.224/29 from its ISP. What technology should the company implement in order to allow the workstations to access the services over the Internet? static NAT, dynamic NAT, port address translation , DHCP
Dynamic Nat 20 workstations + 2 servers = 22. 29-24=5 bits for hosts 2^5 = 32 hosts, so static NAT could theoretically work
E-mail is
Elastic application
The scheme commonly implemented to ensure fairness and success of data transmission in case of collissions in the CSMA/CA protocol is called _________
Exponential Backoff
A multiplexing technique used that chops up frequency to accommodate several users
FDMA
What is the language used to create a webpage?
HTML
Name the protocol that supports webpage transfer and email transfer.
HTTP-Webpage transfer SMTP-Email Transfer
The IEEE standard for Wifi?
IEEE 802.11 - 11Mbps transmission = 2.4Ghz band
DNS uses UDP instead of TCP i.e. if a DNS packet is lost, there is no automatic recovery. Does this cause a problem, and if so, how is it resolved?
If the packet is lost, the application sends out another request if it doesn't get a response. No ACKs are used since it is UDP -Resents request after timeout
How can a device have more than one IP address?
If we have more than one Ethernet network (another network device)
What issue does the SNOOP protocol address?
Improves TCP performance by suppressing duplicate TCP acks
Suppose that a 24-Mbps 802.11g LAN is transmitting 64-byte frames back-to-back over a radio channel with a bit error rate of 10^-7. How many frames per second will be damaged on average?
In one second, 24,000,000 bits (10^-7 bits destroyed/bit) = 2.4 bits destroyed 2.4 (3) frames / sec worst case, best case is 1 frame/sec if those 2.4 (3) bits fall in one frame.
This is a time-sensitive service
Internet telephony
Can a computer have two DNS names that fall in different top-level domains? If yes, give an example.
It could host a web page for cnn.com via IIS and also yahoo.com via apache hosting
What does a client do when it has UDP datagrams to send?
It just sends the datagrams
Which socket API function converts an unconnected active TCP socket into passive socket?
Listen
08:00:27:00:E8:BA is an example of what type of addresses (choose two)? MAC, IP, Physical, Logical
MAC and Physical
Name the email protocol which allows the sender to send fancy text and audio-video content in emails
MIME (Multi-Purpose Internet Mail Extensions)
Name the receiver side and sender side solution to the silly window syndrome.
Receiver = Clark's solution closes the window until another segment of maximum segment size (MSS) can be received or the buffer is half empty. Sender = Nagle's solution requires that the sender send the first segment even if it is a small one, then that it wait until an ACK is received or a maximum sized segment (MSS) is accumulated
What is the method of DNS querying called, when a server does not have the requested information but goes and finds it somewhere, then reports back?
Recursive query - work on server Iterated - work on user
What is the shortest frame interval in a WLAN?
SIFS
For n-bit frame sequence number, the window sizes for stop-n-wait, Go-Back-N and Selective reject protocols are? :
STOP-N-WAIT -sender sends and waits for acknowledgment -once acknowledgment reaches sender, it transmits the next packet in row GO-BACK-N -Sends N packets equal to window size, once entire window is sent, sender waits for cumulative ACK to send more packets SELECTIVE -Sends packets of window size N and receiver acknowledges whether packets were received in order or not
Which two flags in the TCP header are used in a TCP three-way handshake to establish connectivity between two network devices? (Choose two.) RST, FIN, SYN, ACK, URG, PSH
SYN and ACK
What does a sender do when a receiver blocks the sender from sending further data at the transport layer?
Sender waits to be allowed to send more data
Every communication is identified by a unique quadruple at the Transport layer. Name the four entities in this quadruple.
Source ip, source port, destination ip, destination port
The use of switches and hubs for network connectivity?
Switches increase the number of collision domains in the network
A multiplexing technique used that chops up time to accommodate several users
TDMA
A system has a "y" layer protocol stack. Applications generate messages of length "l" bytes. Each layer adds a header of length "h" bytes. What fraction of the network bandwidth is filled with headers?
The message is just l and for layer "y" we have a header "h" -total message size = I+yh -fraction of bandwidth wasted on headers is yh/(I+yh)
Name the latency encountered by a packet while it is being put out on the link bit by bit
Transmission Delay
What best describes the operation of distance vector routing protocols?
They send their routing tables to directly connected neighbors.
What is the main purpose of using a layered architecture for our network protocol?
To standardize interfaces and isolate functionalities. Allows us to replace each layer independently if we need to without affecting the other layers (ie..put in a new version of TCP if you need to).
Every byte in the TCP layer is numbered
True, it assigns a number to each byte transmitted
What is hand--off in a cellular phone system?
When you change from one AP/tower to a following tower with no noticeable interruption to the user.
Does the area over which a network sprawls govern the choice of the transmission medium? Yes or No
Yes
Does the number of users and amount of traffic on a network shape the design and feasibility of a MAC algorithm? Yes or No
Yes
Is frame number required even in a stop-n-wait protocol?
Yes
Is it possible for a WLAN comprising of 5 wireless nodes arranged all in a straight line, to have two simultaneous transmissions between two separate sets of transmitters and receivers? Why or why not?
Yes if they're in a straight line A B C D E B can transmit to A & D can transmit to E simultaneously
Does star topology offer higher reliability and bandwidth than the bus and ring topologies? Yes or No
Yes. Star topology benefit is that a failure of one node doesn't affect the other nodes
Ten signals, each requiring "A" Khz and separated by "B" Hz guard bands are multiplexed on a single channel using FDM. How much minimum bandwidth is required for the channel?
[(10^4)A + 9B] Hz -10 signals requires 9 guard bands
What does a client application select for a TCP or UDP source port number?
a random value in the range of the registered ports
Frames of 1000 bits are sent over a 1--Mbps channel. Propagation delay on the link is 270 msec. Ignoring the header bits and assuming that ACK bits are always included within the 1000 bits of data frames, answer the following questions: a) The minimum amount of time required to receive an ACK back once a data frame has been sent b) The number of frames that can be sent within the above time frame (as computed in (a)) in the 3 different protocols namely Stop--and--Wait, Go--Back--N, and Selective Reject (assume a 3 bit sequence number for the last 2 protocols)
a) 1000/1Mbps +270msec*2=.541 secs b) assume windows size sequence =max stop and wait = 1 each packet takes 1 msec to send window 2n-1=7 go back n= 7 window size 2n-1 = 4 selective reject= 4
The following is a snippet from a DNS table at a DNS server. Study it carefully to answer the following questions: Flits.cs.vu.nl 86400 IN HINFO Sun Unix Flits.cs.vu.nl 86400 IN A 130.37.16.112 Flits.cs.vu.nl 86400 IN A 192.31.231.165 Flits.cs.vu.nl 86400 IN MX 1 flits.cs.vu.nl Flits.cs.vu.nl 86400 IN MX 2top.cs.vu.nl www.cs.vu.nl 86400 IN CNAME star.cs.vu.nl a) What kind of a machine is "Flits"? b) What are the IP addresses of Flits? c) What are the choices given for handling email sent to flits.cs.vu.nl? d) Explain the aliasing used when users want to access www.cs.vu.nl.
a) Sun Unix (HINFO specifies type of machine) b) 130.37.16.112 and 192.31.231.165 (The ones that say 'A') c) Flits or Top (The ones that say MX) d) star.cs.vu.nl (Anything that says CNAME is an alias)
Assume we have allocated 10 bits for denoting an IP address. a) How many unique IP addresses can be generated? b) If the first 5 bits denoted the network address and the last 5 bits the host address with the first two bits of the network address part preset to '01' what would be the range of addresses possible (in hex decimal dotted notation - pair two bits and represent them in their decimal representation, each such pair separated by dots)? c) Assume further that the first few bits of the host part were reserved for a subnet ID. An address along with its subnet mask reads as follows: 1.0.0.3.0/7. Given the above information, deduce how many bits are allocated for the subnet ID. d) For the given IP address, what is the subnet ID?
a.) 2^10 b.) 1.0.0.0.0-> 1.3.3.3.3 c.) 2 bits d.) 01
Suppose that two stations 'A' and 'B' are contending for the channel and 'B' has just finished a conversation with another node in the network. Based on this information, answer the following questions: a) How long (give the IFS name) will A sense the channel for idleness before it transmits data? b) How long will B sense the channel for idleness before it transmits data? c) Assume that another station C who is within hearing range of A is also sensing the channel around the same time as A and B. C waits for the requisite IFS period of time and then chooses a backoff time of 10 slots before transmission. Assume that A starts its transmission at a time that corresponds to the middle of slot period number 5. What does C do then?
a.) D.I.F.S. b.) DIFS + Back Off c.) waits DIFS & Counts from 4
What are two features of a link-state routing protocol? (Choose two.) a. The database information for each router is obtained from the same source. b. Routers send triggered updates in response to a change. c. Routers create a topology of the network by using information from other routers. d. Paths are chosen based on the lowest number of hops to the designated router. e. Routers send periodic updates only to neighboring routers.
b. Routers send triggered updates in response to a change. c. Routers create a topology of the network by using information from other routers. d. Paths are chosen based on the lowest number of hops to the designated router.
AM and FM are examples of this modulation scheme? a.) digital data to digital signal b.) analog data to analog signal c.) digital data to analog signal d.) analog data to digital signal
b.) analog data to analog signal
The physical layer concerns with?
bit by bit delivery
The network layer concerns with a) bits b) frames c) packets d) segments
c) packets
Which one of the following allows a user at one site to establish a connection to another site and then pass keystrokes from local host to remote host? A. HTTP B. FTP C. telnet D. none of the mentioned
c. telnet
Which one of the following protocol delivers/stores mail to receiver server?
simple mail transfer protocol
Example of an internet standard protocol for managing devices on IP network?
simple network management protocol
Which control bit is involved in setting up a TCP session?
syn and ack bits
Which factor determines TCP window size?
the amount of data the destination can process at one time
What problem of WLANs does the MACA protocol address? What problem does it give rise to?
uses rts(request to send), and cts(confirmed to send), to assure that they avoid the hidden station problem and the exposed station problem. -Tries to reduce frequencies of collisions while providing a plan if a collision does occur -gives rise to the hidden node problem -"hidden station" = stations in a wireless network have limited range -transmissions can overlap causing data to become lost
Ten signals, each requiring 4000Hz and separated by 400Hz guard bands are multiplexed on a single channel using FDM(frequency Division multiplexing). How much minimum bandwidth is required for the channel?
4000(10)+400(9)= 43600Hz where 10 is number of signals and 9 is the number of guard bands req between 10 signals.
"At low load, pure ALOHA performs better than slotted ALOHA in terms of packet delay "- Refute or validate the statement.
-With pure ALOHA, transmission can start instantly -At low load, no collisions are expected so the transmission is likely to be successful -With slotted ALOHA, it has to wait for the next slot. This introduces half a slot time of delay -At low load, pure ALOHA performs better than slotted because it doesn't need any global time synchronization whereas slotted ALOHA does -Yes, and because slotted help multiples users on the link
What is the difference between network layer data delivery versus transport layer data delivery?
-machine to machine vs port to port -IP address based vs (IP, Port Number) based -distance based versus reliability based Transport = data to another computer Network = distribution
There are n stations in a LAN using ALOHA protocol. Each station attempts to transmit with a probability p in each time slot. What is the probability that ONLY one station transmits in a given time slot?
1 Transmission no collision np(1-p)^(n-1)
Assume a x-Mbps Ethernet network with a single store-and-forward switch in the path, and a packet size of y bits. Each link introduces a propagation delay of z us and the switch begins retransmitting immediately after it has finished receiving the packet. The delay encountered by the packet is?
1 packet = y / (x 10^6) total time to store = [(y / x) + z] time to send = [(y / x) + z] total time = (2/10^6)[(y / x) + z]
How much time does it take to transmit each bit on a 10-Mmps network?
1 second = 10x10^6 bits 1 bit = (1 second / 10x10^6) = (1/10)x10^-6 seconds 0.1 us
Find the delay encountered by data packets in the two scenarios below: (i) Circuit switched network sending an x-bit message over a k-hop path with propagation delay of d sec/hop and a data rate of b bps (ii) Packet switched network with the same network characteristics with packet size of p bits.
1.) With circuit switching, at t = s the circuit is set up; at t = s + x /b the last bit is sent; at t = s + x /b + kd the message arrives. 2.) With packet switching, the last bit is sent at t = x /b. -To get to the final destination, the last packet must be retransmitted k − 1 times by intermediate routers -each retransmission taking p /b sec -total delay is x /b + (k − 1)p /b + kd -Packet switching is faster if s > (k − 1)p /b.
A sender wants to transmit a 10-bit binary data (1101011011). The sender uses CRC for error detection and hence uses a bit pattern of 10011 as the divisor. How many bits are actually transmitted by the sender? If all bits are received correctly, what is the bit pattern that the receiver receives?
1101011011 => Divisor = 10011 1101011011(0000) / 10011 last 4 = 1110 # of redundant bits = divisir - 1 = 5-1 = 4 bits 1.) Actual # of bits transmitted = 10 + 4 = 14 bis 2.) Transmitted Sequence 1101011011(1110) CRC at the END
In a noiseless channel, if we were to send TV signals 'm' MHz wide, digitally encoded in four levels what data rate can we achieve in bps?
2(m)log(2^2)=4m Mbps
The frequency channel on which an IEEE 802.11 AP operates in the U.S. is somewhere between __________ and __________ Ghz
2.4 and 5 Ghz
If a binary signal is sent over a 3-kHz channel whose signal-to-noise ratio is 20dB, what is the approximate maximum achievable data rate? [Nyquist MDR = 2H log2 V bps, Shannon MDR = H log2 (1+S/N) bps]
20db = 10log(S/N) S/N = 10^2 = 100 Shannon MDR = (3x10^3)lg(1+100) = (3x10^3)lg(101) = ~(3x10^3)lg(2^7) = ~21Kbps
The maximum window size for data transmission using the selective reject protocol with n-bit frame sequence numbers is: Why is it shorter than the Go-Back-N protocol?
2^(n-1) Need to maintain additional packets past rejected packet in buffer. If the window in Selective Reject was bigger, it could erroneously accept packets.
Two neighboring nodes (A and B) use a sliding window protocol with a 2-bit sequence number. As the ARQ mechanisms, go-back-N is used with a window size of 3 frames. Assuming A is transmitting and B is receiving, show the window positions for 'A' for the following succession of events: (A) Before A sends any frames (B) After A sends frames 0,1,2 and receives ACK from B for 0,1 and 2 C) After A sends frames 3, 4, 5 and B sends RR4 and it is received by A
2^2 = 4 size = 3 A.) A = [0 1 2 ] 3 0 B = [0 1 2 ] 3 0 B.) A = 0 1 2 | [ 3 4 5 ] B = 0 1 2 | [ 3 4 5 ] C.) A = 0 1 2 3 4 | 5 [ 6 7 0 ] B = 0 1 2 3 4 | 5 [ 6 7 0 ]
What is the purpose of a timer at the sender in an ARQ system?
After the time times out without receiving ACK, the sender will resend the packet
State the major differences between CSMA and CSMA/CD?
CSMA after collision occur, it will continue transmitting the message CSMA/CD will stop after it senses there has been a collision
Which one of the following is the multiple access protocol for channel access control? a) CSMA/CD b) CSMA/CA c) ALOHA d) All of the mentioned
CSMA/CD -Deals with detection after a collision has occurred CSMA/CA -Deals with preventing collission
Give 1 reason as to why WLANs do not use CSMA/CD
CSMA/CD -works well by detecting data/collision though a medium such as wired networks -In wireless transmission data can be sent/received however, the "detection" of data being sent in that medium cannot -Radios cannot receive and transmit on the same frequency at the same time, so CSMA/CD cannot be used.
Which of the following statements is TRUE about CSMA/CD: a. IEEE 802.11 wireless LAN runs CSMA/CD protocol b. Ethernet is not based on CSMA/CD protocol c. CSMA/CD is not suitable for a high propagation delay network like satellite network d. There is no contention in a CSMA/CD network
CSMA/CD is not suitable for high propogation delay networks like satellite network -Requires sender be transmitting at least till the first bit reaches the receiver, so it can detect collisions if any
Which one of the following task is not done by data link layer? a) framing b) error control c) flow control d) channel coding
Channel Coding -function of Physical Layer -Data Link mainly deals with framing, error control & flow
The mechanism of data transfer which first sets up a dedicated channel between the sender and the receiver?
Circuit Switching
Distinguish between a circuit switched network and a packet switched network?
Circuit Switching Network -all packets of the message sent through the same router Packet Switching Network -each packet might be sent through different route
CRC stands for : a) cyclic redundancy check b) code repeat check c) code redundancy check d) cyclic repeat check
Cyclic Redundancy Check -Code that is added to a data which helps identify any error that occurred during the transmission of the data
Name the layer where CRC is performed
Data Link
Which sub-layer is concerned with channel contention mechanisms?
Data Link -Medium Access Control = Data Layer
The physical layer translates logical communication requests from the __________ into hardware specific operations a.) data link layer b.) network layer c.) transport layer d.) application layer
Data Link Layer -Physical Layer accepts data or information from the data link layer and converts it into hardware specific operations
Under what conditions does a packet switched network have a lower delay than a circuit switched network with the following network characteristics: x bit message, k hop path, d sec/hop of propagation delay, b bps of data rate, s secs of circuit set up time, p bits/pkt
Delay for Packet Switching x / b + kd + (k-1)p/b Delay for Circuit Switching x/b + kd + Circuit Setup (k-1)p/b * Circuit Setup Time
Which one of the following is a data link protocol? a) Ethernet b) Dijkstra c) Post Office Protocol d) Distance Vector Routing
Ethernet SDLC = Synchronous Data Link Control HDLC = High Level Data Link Control SLIP = Serial Line Interface Protocol PPP = Point to Point Protocol
Which term is used specifically to identify the entity that is created when encapsulating data inside data link layer headers and trailers?
Frames
Let the size of congestion window of a TCP connection be 32 KB when a timeout occurs. The round trip time of the connection is 100 msec and the maximum segment size used is 2 KB. The time taken (in msec) by the TCP connection to get back to 32 KB congestion window is _________.
MSS= 2KB, RTT= 100 ms and Cwnd= 32KB. No error given so assume channel is error free. Slow Start Threshold ( SSth)= Cwnd/2= 16KB. Now upto 16KB , window size increase exponentially and after that Additive increase will take place. at time T, 1 MSS is sent and at T+100, 1 ack is received. We know that window size increase as per number of Acks received. so window size become 2MSS( 4KB). at time T+100, 2 MSS sent and at T+200, 2 acks received.Now window size become 2MSS+ 2MSS= 4MSS ( 8KB) at Time T+200, 4 MSS sent and at T+ 300 ,4 acks received. Now window size become 4MSS+ 4MSS= 8MSS ( 16KB). upto this point of time , window size is 16KB and total time taken is 300ms. This window size is equal to SSth .So now from this point of time, additive increase in window size will take place.Aditive increase refers to the increaseing window size with 1 MSS each time. Now it is easy to calculate that from 16KB to 32KB , we need to send 8MSS( 16KB) and each of the MSS will take 100 ms of RTT. so after 8 more RTTs( 800 ms), the window size will become 32 KB. total time for this process is 300+ 800= 1100 ms.
Transport layer protocols used for real time multimedia, file transfer, DNS and e-mail, respectively are?
Multimedia = UDP File Transfer = TCP DNS and E-mail = UDP and TCP
A single channel is shared by multiple signals by a.) analog modulation b.) digital modulation c.) multiplexing d.) none of the mentioned
Multiplexing -MUX = multiple analog or digital are combined over a shared medium
Which transmission media has the highest transmission speed in a network?
Optical Fiber -1000Mbps
Name in order, the layers of the OSI (open systems interconnection)model protocol stack starting from layer 1
People.Don't.Need.Those.Anyway 1) Physical 2) Data Link 3) Network 4) Transport 5) Application
Name the layer to which the BPSK protocol belongs to:
Physical
Which layer defines the standards for cabling and connectors?
Physical -Cables & Connections -transmit of mediums, connector physical, etc
The portion of data link layer that interfaces with the PHYSICAL layer is called? a.) physical signaling sublayer b.) physical data sublayer c.) physical address sublayer d.) none of the mentioned
Physical Signaling Sublayer MAC = physical signaling sublayer -encoding, reception, decoding
The technique of temporarily delaying outgoing acknowledgements so that they can be hooked onto the next outgoing data frame is called: a) piggybacking b) cyclic redundancy check c) fletcher's checksum d) none of the mentioned
Piggybacking -acknowledgement is temporarily delayed so as to be hooked with the next outgoing data frame
Name the latency encountered by a packet while it traverses the medium
Propagation Delay
A set of rules that governs data communication?
Protocols -Set of rules and regulations that allow a network of nodes to transmit and receive information
A __________ is a device that forwards packets between networks by processing the routing information included in the packet
Router -performs traffic directing functions on the internet
The Data Link Layer transforms the physical layer, a raw transmission facility, to a reliable link and is responsible for node-node delivery - True or False
True
Some of the functionalities of the Data Link Layer includes the following: True or False a. Framing b. Physical Addressing c. Flow Control d. Error Control e. Access Control
True Framing - divides stream of bits from network layer into measurable data Physical Addressing - DATA LINK LAYER adds heads to define send/receive Flow Control - Data rate must be the same on both sides Error Control - Trailer added to end of frame to detect damaged frames Access Control - When 2 or more devices are connected to the same link
Name the layers of the hybrid protocol stack that executes the following tasks: a) Helps you compose an email: b) Ensures timely delivery of that email from the source server to your machine: c) Routes the above email using the best possible route through the Internet: d) Ensures that the electromagnetic signals traverse the wire in a secure and decodable fashion: e) Deals with reliable data delivery issues within the local area network:
a) Helps you compose an email: application layer b) Ensures timely delivery of that email from the source server to your machine:Transport c) Routes the above email using the best possible route through the Internet:Network d) Ensures that the electromagnetic signals traverse the wire in a secure and decodable fashion: Physical Layer e) Deals with reliable data delivery issues within the local area network: Data Link Layer
The ____ translates internet domain and host names to IP address. a) domain name system b) routing information protocol c) network time protocol d) internet relay chat
a) domain name system
On which layer of the protocol stack do the following devices operate? a. Application gateway: b. Routers: c. Bridges: d. Hubs:
a. Application gateway: Application Layer b. Routers: Network Layer c. Bridges: Data Link Layer d. Hubs: Physical Layer
In a computer network, nodes are
all of the following: -computer that originates the data -computer that routes the data -the computer that terminates the data Nodes -Physical Devices that able to send, receive, and/or forward information
The physical layer is responsible for a.) line coding b.) channel coding c.) modulation d.) all of the mentioned
d.) all of the mentioned Physical -line coding, channel coding, and modulation needs for transmission
Wireless transmission can be done via a.) radio waves b.) microwaves c.) infrared d.) all of the mentioned
d.) all of the mentioned Wave Range = 3 kHz to +300 GHz -wireless allows for wider band for modulation signals
