COMPUTER NETWORKS UNIT 3

Transcription

1 COMPUTER NETWORKS UNIT 3

2 1. Illustrate about Reservation, Polling and Token Passing. [10 marks] Controlled access: In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. The threepopular controlled-access methods are as follows. 1. Reservation: In the reservation method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation minislots in the reservation frame. Each minislot belongs to a station. When a station needs to send a data frame, it makes a reservation in its own minislot. The stations that have made reservations can send their data frames after the reservation frame. The following figure shows a situation with five stations and a five-minislot reservation frame. In the first interval, only stations 1, 3, and 4 have made reservations. In the second interval, only station 1 has made a reservation. 2. Polling: Polling works with topologies in which one device is designated as a primary station and the other devices are secondary stations. All data exchanges must be made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions. It is up to the primary device to determine which device is allowed to use the channel at a given time. The primary device, therefore, is always the initiator of a session. Consider the following figure.

3 If the primary wants to receive data, it asks the secondaries if they have anything to send, this is called poll function. If the primary wants to send data, it tells the secondary to get ready to receive; this is called select function. Select: The select function is used whenever the primary device has something to send. If it has something to send, the primary device sends it. It has to know whether the target device is prepared to receive or not. So the primary must alert the secondary to the upcoming transmission and wait for an acknowledgment of the secondary's ready status. Before sending data, the primary creates and transmits a select (SEL) frame, one field of which includes the address of the intended secondary. Poll: The poll function is used by the primary device to solicit transmissions from the secondary devices. When the primary is ready to receive data, it must ask (poll) each device in turn if it has anything to send. When the first secondary is approached, it responds either with a NAK frame if it has nothing to send or with data (in the form of a data frame) if it does. If the response is negative (a NAK frame), then the primary polls the next secondary in the same manner until it finds one with data to send. When the response is positive (a data frame), the primary reads the frame and returns an acknowledgment (ACK frame), verifying its receipt. 3. Token Passing: In the token-passing method, the stations in a network are organized in a logical ring. In other words, for each station, there is a predecessor and a successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. The current station is the one that is accessing the channel now. The right to this access has been passed from the predecessor to the current station. The right will be passed to the successor when the current station has no more data to send. In this method, a special packet called a token circulates through the ring. The possession

4 of the token gives the station the right to access the channel and send its data. When a station has some data to send, it waits until it receives the token from its predecessor. It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. The station cannot send data until it receives the token again in the next round. Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. For example, if a station that is holding the token fails, the token will disappear from the network. Another function of token management is to assign priorities to the stations and to the types of data being transmitted. And finally, token management is needed to make low- priority stations release the token to high priority stations. Logical Ring: In a token-passing network, stations do not have to be physically connected in a ring; the ring can be a logical one. The following figure show four different physical topologies that can create a logical ring. 2. Distinguish between multilevel TDM, multiple slot TDM and pulse stuffed TDM. [10 marks] Time-division multiplexing (TDM) is a digital multiplexing technique for combining several low-rate channels into one high-rate one. In TDM, time is shared. Each connection occupies a portion of time in the link. One problem with TDM is to handle

5 the disparity in the input data rates. If data rates are not the same, 3 strategies, or a combination of them is used. These 3 strategies are multilevel multiplexing, multipleslot allocation and pulse stuffing. Multilevel multiplexing- This technique is used when the data rate of an input line is a multiple of others. In the above figure, we have 2 inputs of 20kbps and 3 inputs of 40kbps. The first 2 input lines can be multiplexed together to provide a data rate equal to the last three. A second level of multiplexing creates an output of 160kbps. Multiple-Slot Allocation- Multi slot TMD is used when many slots in a frame produce 1 single input line. In the above figure, we have an input line that has a data rate which is a multiple of another input line. The input line with a 50kbps data rate can be given 2 slots in the output. A serial-to-parallel converter is needed to make two inputs out of one. Pulse Stuffing- This technique is used when the bit rates of sources are not multiple integers of each other. In this case the highest input data rate is made the dominant data rate and then add dummy bits to the input lines with lower rates. This will increase their rates. This technique is also called as pulse stuffing, bit padding or bit stuffing.

6 In the above figure, the input with a data rate of 46 is pulse-stuffed to increase the rate to 50kbps. After this it is multiplexed. Multilevel multiplexing Multiple-slot multiplexing Pulse Stuffing Multilevel TDM is used when the data rate of an input line is a multiple of others. Multiple slot TDM is used when many slot in a frame produce 1 single line. Pulse stuffed TDM is used when bit rate of an input line is not a multiple of others. SO we add dummy bits. 3. Perform Hamming Code Error Detection and Correction for the given stream of bits: Generate error at 8 th bit, detect and correct it using Hamming Code. [10 marks] Data : M (data bit) : 8 R (redundancy) :? m+r+1<2 r if r= <2 not true if r= <4 not true if r= <8 not true if r= <16 true Therefore, r = 4 m+ r = 4+8 = 12 r1 r2 M1 r3 M2 M3 M4 r4 M5 M6 M7 M r1=m1+m2+m4+m5+m7= ( ) % 2=0 r2=m1+m3+m4+m6+m7= ( ) % 2=0 r3=m2+m3+m4+m8= ( ) % 2=1 r4=m5+m6+m7+m8= ( ) % 2=1 r1 r2 M1 r3 M2 M3 M4 r4 M5 M6 M7 M

7 Suppose received r1 r2 M1 r3 M2 M3 M4 r4 M5 M6 M7 M r1=m1+m2+m4+m5+m7= ( ) % 2=0 r2=m1+m3+m4+m6+m7= ( ) % 2=0 r3=m2+m3+m4+m8= ( ) % 2=0 r4=m5+m6+m7+m8= ( ) % 2=0 recieved computed match Binary r1 0 0 yes 0 r2 0 0 yes 0 r3 1 0 no 1 r4 1 0 no =2*2+2*2*2=12 So the error is at 12 th bit After Correction bit sequence is

8 4. Provide the classification of multiple access protocols. Explain ALOHA in detail with clear flowchart. [10 marks] ALOHA ALOHA, the earliest random access method. There are potential collisions in this arrangement. The medium is shared between the stations. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled. Pure ALOHA: The original ALOHA protocol is called pure ALOHA. This is a simple but elegant protocol. The idea is that each station sends a frame whenever it has a frame to send (multiple access). However, since there is only one channel to share, there is the possibility of collision between frames from different stations. There are four stations (unrealistic assumption) that contend with one another for access to the shared channel. The figure shows that each station sends two frames, there are a total of eight frames on the shared medium. Some of these frames collide because

9 multiple frames are in contention for the shared channel. The above figure shows that only two frames survive: one frame from station 1 and one frame from station 3. We need to mention that even if one bit of a frame coexists on the channel with one bit from another frame, there is a collision and both will be destroyed. It is obvious that we need to resend the frames that have been destroyed during transmission. The pure ALOHA protocol relies on acknowledgments from the receiver. When a station sends a frame, it expects the receiver to send an acknowledgment. If the acknowledgment does not arrive after a time-out period, the station assumes that the frame (or the acknowledgment) has been destroyed and resends the frame. A collision involves two or more stations. If all these stations, try to resend their frames after the time-out, the frames will collide again. Pure ALOHA dictates that when the time-out period passes, each station waits a random amount of time before resending its frame. The randomness will help avoid more collisions. We call this time the back off time TB. Pure ALOHA has a second method to prevent congesting the channel with retransmitted frames. After a maximum number of retransmission attempts Kmax, a station must give up and try later. The above figure shows the procedure for pure ALOHA based on the above strategy. The time-out period is equal to the maximum possible round-trip propagation delay, which is twice the amount of time required to send a frame between the two most widely separated stations (2 Tp). The backoff time TB is a random value that normally depends on K (the number of attempted unsuccessful transmissions). The formula for TB depends on the implementation. One common formula is the binary exponential back off. In this method, for each retransmission, a multiplier R = 0 to 2K 1 is randomly chosen and multiplied by Tp (maximum propagation time) or Tfr (the average time required to send out a frame) to find TB. Note that in this procedure, the range of the random numbers increases after each collision. The value of Kmax is usually chosen as 15.

10 Pure ALOHA vulnerable time = 2 x Tfr The throughput for pure ALOHA is S = G x e 2g The maximum throughput Smax = 1/(2e) = when G = (1/2). Slotted ALOHA Slotted ALOHA was invented to improve the efficiency of pure ALOHA. In slotted ALOHA we divide the time into slots of Tfr seconds and force the station to send only at the beginning of the time slot. Figure below shows an example of frame collisions in slotted ALOHA. Slotted ALOHA vulnerable time = Tfr The throughput for slotted ALOHA is S = G x e G. The maximum throughput Smax = when G = 1.

11 5. Describe and distinguish between FDMA and CDMA. [10 marks] FDMA (frequency division multiple access) In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time. Each station also uses a bandpass filter to confine the transmitter frequencies. To prevent station interferences, the allocated bands are separated from one another by small guard bands. FDMA specifies a predetermined frequency band for the entire period of communication. This means that stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA. FDM is a physical layer technique that combines the loads from low bandwidth channels and transmits them by using a high-bandwidth channel. The channels that are combined are low-pass. The multiplexer modulates the signals, combines them, and creates a bandpass signal. The bandwidth of each channel is shifted by the multiplexer. FDMA, on the other hand, is an access method in the data-link layer. The datalink layer in each station tells its physical layer to make a bandpass signal from the data passed to it. The signal must be created in the allocated band. There is no physical multiplexer at the physical layer. The signals created at each station are automatically bandpass-filtered. They are mixed when they are sent to the common channel.

12 CDMA (code division multiple access) CDMA differs from FDMA in that only one channel occupies the entire bandwidth of the link. CDMA simply means communication with different codes. Same frequency is used by every user and simultaneous transmission occurs Every narrowband signal is multiplied by wideband spreading signal, usually known as codeword Every user has a separate pseudo-codeword, i.e., orthogonal to others Only the desired codeword is detected by the receivers and others appear as noise It is mandatory for the receivers to know about the transmitter s codeword. 6. Explain channelization. Give valid examples of FDMA, TDMA and CDMA. [10 Marks] Channelization (or channel partition, as it is sometimes called) is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, among different stations. There are three channelization protocols: FDMA, TDMA, and CDMA. FDMA

13 In frequency-division multiple access (FDMA), the available bandwidth is divided into frequency bands. Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time. Each station also uses a bandpass filter to confine the transmitter frequencies. To prevent station interferences, the allocated bands are separated from one another by small guard bands. Figure shows the idea of FDMA. FDMA, on the other hand, is an access method in the data-link layer. The datalink layer in each station tells its physical layer to make a bandpass signal from the data passed to it. The signal must be created in the allocated band. There is no physical multiplexer at the physical layer. The signals created at each station are automatically bandpassfiltered. They are mixed when they are sent to the common channel. Early cellular telephony mostly used FDMA analogue transmission. Walkie talkies and mobile networks for closed user groups often use FDMA. Another example of FDMA is AM or FM radio broadcasting, where each station has its own channel. TDMA

14 In time-division multiple access (TDMA), the stations share the bandwidth of the channel in time. Each station is allocated a time slot during which it can send data. Each station transmits its data in its assigned time slot. The main problem with TDMA lies in achieving synchronization between the different stations. Each station needs to know the beginning of its slot and the location of its slot. This may be difficult because of propagation delays introduced in the system if the stations are spread over a large area. To compensate for the delays, we can insert guard times. In TDMA, the bandwidth is just one channel that is timeshared between different stations. TDM IS USED IN TELEPHONE COMMUNICATION A channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. CDMA Code-division multiple access (CDMA) was conceived several decades ago. Recent advances in electronic technology have finally made its implementation possible. CDMA differs from FDMA in that only one channel occupies the entire bandwidth of the link. It differs from TDMA in that all stations can send data simultaneously; there is no timesharing.

15 In CDMA, one channel carries all transmissions simultaneously. CDMA is based on coding theory. Each station is assigned a code, which is a sequence of numbers called chips, Tata Indicom, reliance virgin mobile are the examples for CDMA phones.

16 7. Perform CRC for the given set of bits: with the key [10 Marks] 8. What is a controlled access? Explain 3 protocols used in controlled access with valid examples. [10 Marks] In controlled access, the stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. The three controlled-access methods are reservation, polling and token-passing. Reservation: In the reservation method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation minislots in the reservation frame. Each minislot belongs to a station. When a station needs to send a data frame, it makes reservation in its own minislot. The stations that have made reservations can send their data frames after the reservation frame. The figure shows a situation with five stations and a five minislot reservation frame. In the first interval, only stations 1, 3, and 4 have made reservations. In the second interval, only station 1 has made a reservation.

17 Polling: Polling works with topologies in which one devices is designated as a primary station and the other devices are secondary stations. All data exchanges must be made through the primary device even when the ultimate destination is a secondary device. The primary device controls the link; the secondary devices follow its instructions. It is up to the primary device to determine which device is allowed to use the channel at a given time. The primary device, therefore, is always the initiator of a session. If the primary wants to receive data, it asks the secondaries if they have anything to send; this is called poll function. If the primary wants to send data, it tells the secondary to get reading to receive; this is called select function. Token-passing:

18 In the token-passing method, the stations in a network are organized in a logical ring. In other words, for each station, there is a predecessor and successor. The predecessor is the station which is logically before the station in the ring; the successor is the station which is after the station in the ring. In this method, a special packed called a token circulates through the ring. The possession of the token gives the station the right to access the channel and send its data. In a token-passing network, stations do not have to be physically connected in a ring; the ring can be a logical one. In the physical ring topology, when a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is or emergencies only. If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. In the bus ring topology, also called a token bus, the stations are connected to a single cable called a bus. They make a logical ring because each station knows the address of its successor. When a station has finished sending its data, it releases the token and inserts the address of its successor in the token. In the star ring topology, the physical topology is a star. There is a hub that acts as the connector. The wiring inside the hub makes the ring; the stations are connected to this ring through the two wire connections. 9. Write a shot note on CSMA/CD and CSMA/CA with valid figures and example. [10 Marks] CSMA/CD: Carrier Sense Multiple Access with Collision Detection The CSMA method does not specify the procedure following a collision. Augments the algorithm to handle the collision. In this method, a stations monitors the medium after

19 frame it sends a frame to see if the transmission was successful. If so, the station is finished. If, however, there is collision, the frame is sent again. At time t 1, station A has executed its persistence procedure and starts sending the bits of its frame. At time t 2, station C has not yet sensed the first bit sent by A. Station C executes its persistence procedure and starts sending the bits in its frame, which propagate both to the left and to the right. The collision occurs sometime after time t 2 ' Station C detects a collision at time t 3 when it receives the first bit of A's frame. Station C immediately (or after a short time, but we assume immediately) aborts transmission. Station A detects collision at time t 4 when it receives the first bit of C's frame; it also immediately aborts transmission. Looking at the figure, we see that A transmits for the duration t 4 - t 1 ; C transmits for the duration t 3 - t 2 ' Later we show that, for the protocol to work, the length of any frame divided by the bit rate in this protocol must be more than either of these durations. At time t 4, the transmission of A s frame, though incomplete, is aborted at time t 3, the transmission of B's frame, though incomplete, is aborted. The minimum frame size is related to the distance which the network spans, the type of media being used and the number of repeaters which the signal may have to pass through to reach the furthest part of the LAN. Together these define a value known as the Ethernet Slot Time, corresponding to 512-bit times at 10 Mbps. Procedure: at the flow diagram for CSMA/CD in the following figure. It is similar to the one for the ALOHA protocol, but there are differences.

20 The first difference is the addition of the persistence process. We need to sense the channel before we start sending the frame by using one of the persistence processes we discussed previously (non-persistent, I-persistent, or p-persistent). The second difference is the frame transmission. In ALOHA, we first transmit the entire frame and then wait for an acknowledgment. In CSMA/CD, transmission and collision detection is a continuous process. We constantly monitor in order to detect one of two conditions: either transmission is finished or a collision is detected. Either event stops transmission. Energy Level: level of energy in a channel can have three values: zero, normal, and abnormal. At the zero level, the channel is idle. At the normal level, a station has successfully captured the channel and is sending its frame. At the abnormal level, there is a collision and the level of the energy is twice the normal level. A station that has a frame to send or is sending a frame needs to monitor the energy level to determine if the channel is idle, busy, or in collision mode. Throughput of CSMA/CD is greater than that of pure or slotted ALOHA. The maximum throughput occurs at a different value of G and is based on the persistence method and the value of p in the p-persistent approach. For 1-persistent method the maximum throughput is around 50 percent when G =1. For nonpersistent method, the maximum throughput can go up to 90 percent when G is between 3 and 8. Example, A network using CSMA/CD has a bandwidth of 10 Mbps. If the maximum propagation time (including the delays in the devices and ignoring the time needed to send a jamming signal, as we see later) is S, what is the minimum size of the frame?

21 Solution The frame transmission time is T fr = 2 x T p =51.2 µs. This means, in the worst case, a station needs to transmit for a period of 51.2 µs to detect the collision. The minimum size of the frame is 10 Mbps 51.2 µs = 512 bits or 64 bytes. This is actually the minimum size of the frame for Standard Ethernet. o CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance CSMA/CA is that a station needs to be able to receive while transmitting to detect a collision. When there is a collision, the station receives two signals: its own signal and the signal transmitted by second station. In a wired network, the received signal has almost the same energy as the sent signal this means that in a collision, the detected energy almost doubles. In a wireless network, much of the sent energy is lost in transmission. The received signal has very little energy. Therefore, a collision may add only 5 to 10 percent additional energy. This is not useful for effective collision detection. CSMA/CA was invented to avoid collisions on wireless networks. Collisions are avoided through the use of CSMAICA's three strategies: the interframe space, the contention window, and acknowledgments. Interframe Space (IFS): First, collisions are avoided by deferring transmission even if the channel is found idle. When an idle channel is found, the station does not send immediately. It waits for a period of time called the interframe space or IFS. Even though the channel may appear idle when it is sensed, a distant station may have already started transmitting. Contention Window: The contention window is an amount of time divided into slots. A station that is ready to send chooses a random number of slots as its wait time. The number of slots in the window changes according to the binary exponential back-off strategy. Acknowledgment: With all these precautions, there still may be a collision resulting in destroyed data. In addition, the data may be corrupted during the transmission. The positive acknowledgment and the time-out timer can help guarantee that the receiver has received the frame.

22 Procedure: the channel needs to be sensed before and after the IFS. The channel also needs to be sensed during the contention time. For each time slot of the contention window, the channel is sensed. If it is found idle, the timer continues; if the channel is found busy, the timer is stopped and continues after the timer becomes idle again. CSMA/CA and Wireless Networks: CSMA/CA was mostly intended for use in wireless networks. However, it is not sophisticated enough to handle some particular issues related to wireless networks, such as hidden terminals or exposed terminals.

23 10. Explain Flow Control with the help of the following protocols. a) Stop and Wait. b) Automatic Repeat Request (ARQ). [10 Marks] Whenever an entity produces items and another entity consumes them, there should be a balance between production and consumption rates. If the items are produced faster than they can be consumed, the consumer can be overwhelmed and may need to discard some items. If the items are produced more slowly than they can be consumed, the consumer must wait, and the system becomes less efficient. Flow control is related to the first issue. We need to prevent losing the data items at the consumer site. Stop- and wait Protocol This protocol is a connection-oriented protocol called the Stop-and-Wait protocol, which uses both flow and error control. Both the sender and the receiver use a sliding window of size 1. The sender sends one packet at a time and waits for an acknowledgment before sending the next one. To detect corrupted packets, we need to add a checksum to each data packet. When a packet arrives at the receiver site, it is checked. If its checksum is incorrect, the packet is corrupted and silently discarded. The silence of the receiver is a signal for the sender that a packet was either corrupted or lost. Every time the sender sends a packet, it starts a timer. If an acknowledgment arrives before the timer expires, the timer is stopped and the sender sends the next packet (if it has one to send). If the timer expires, the sender resends the previous packet, assuming that the packet was either lost or corrupted. This means that the sender needs to keep a copy of the packet until its acknowledgment arrives. Note that only one packet and one acknowledgment can be in the channels at any time. Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Any time an error is detected in an exchange, specified frames are retransmitted. This process is called automatic repeat request (ARQ). The protocols for noisy and noiseless channels are given below:

24 Stop and wait ARQ: The sending device keeps a copy of the last frame transmitted until it receives an acknowledgement for that frame. A data frame uses a seqno(sequence number); an ACK frame uses an ackno(acknowledgment number). The sender has a control variable, which we call Sn (sender, next frame to send), that holds the sequence number for the next frame to be sent(0 or 1). The receiver has a control variable, which we call Rn(Receiver, next frame expected), that holds the number of the next frame expected. When a frame is sent, the value of Sn is incremented(modulo-2) which means if it is 0, it be comes 1 and vice versa. When a frame is received, the value of Rn is incremented(modulo-2), which means if it is 0, it becomes 1 and vice versa. Three events can happen at the sender site; one event can happen at the receiver site. Variable Sn points to the slot that matches the sequence number of the frame that has been sent, but not acknowledged; Rn points to the slot that matches the sequence number of the expected frame.

25 Go-Back-N ARQ: Multiple frames can be in transit in the forward direction, and multiple acknowledgements in the reverse direction. The idea is similar to Stop-and-Wait ARQ; the difference is that the send window allows us to have as many frames in transition as there are slots in the send window.

26 Selective Repeat ARQ: Go-Back-N ARQ simplifies the process at the receiver site. The receiver keeps track of only one variable, and there is no need to buffer out-of-order frames; they are simply discarded. However, this protocol is very inefficient for a noisy link. In a noisy link a frame has a higher probability of damage, which means the resending of multiple frames. This resending uses up the bandwidth and slows down the transmission. For noisy links, there is another mechanism that does not resend N frames when just one frame is damaged; only the damaged frame is resent. This mechanism is called Selective Repeat ARQ.

27 11. Define TDMA. [2 Marks] Time-division multiple access (TDMA) is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. [1] The users transmit in rapid succession, one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital Cellular (PDC) and iden, and in the Digital Enhanced Cordless Telecommunications (DECT) standard for portable phones. It is also used extensively in satellite systems, combat-net radio systems, and PON networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below. TDMA frame structure showing a data stream divided into frames and those frames divided into time slots TDMA is a type of time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes particularly difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers. 12. What is Go-Back-N Automatic Repeat Request? [2 Marks] To improve the efficiency of transmission (filling the pipe), multiple frames must be in transition while waiting for acknowledgement i.e. we need to let more than one frame be outstanding to keep the channel busy while the sender is waiting for acknowledgement. The Go-Back-N Automatic Repeat Request is a protocol which helps to achieve this goal. With the help of this protocol we can send several frames before receiving acknowledgements. We keep a copy of these frames until the acknowledgements arrive.

28 13. List different types of Error Detection Schemes. [2 Marks] Simple Parity check Two-dimensional Parity check Checksum Cyclic redundancy check 14. Define Piggy backing. [2 Marks] In two-way communication, whenever a data frame is received, the receiver waits and does not send the control frame (acknowledgement or ACK) back to the sender immediately. The receiver waits until its network layer passes in the next data packet. The delayed acknowledgement is then attached to this outgoing data frame. This technique of temporarily delaying the acknowledgement so that it can be hooked with next outgoing data frame is known as piggybacking. 15. What is stop and wait ARQ? [2 Marks] It is also referred to as alternating bit protocol, is a method in telecommunications to send information between two connected devices. It ensures that information is not lost due to dropped packets and that packets are received in the correct order. It is the simplest automatic repeat-request (ARQ) mechanism. A stop-and-wait ARQ sender sends one frame at a time; it is a special case of the general sliding window protocol with transmit and receive window sizes equal to one and greater than one respectively. After sending each frame, the sender doesn't send any further frames until it receives an acknowledgement (ACK) signal. After receiving a valid frame, the receiver sends an ACK. If the ACK does not reach the sender before a certain time, known as the timeout, the sender sends the same frame again. The timeout countdown is reset after each frame transmission Rules: Sender: o Send one data packet at a time. o Send next packet only after receiving acknowledgement for previous Receiver: o send acknowledgement after receiving and consuming of data packet. o After consuming packet acknowledgement need to be sent (Flow control)

29 16. Give an example for burst error. [2 Marks] The term burst error means that 2 or more bits in the unit have changed from 1 to 0 or from 0 to 1. Example, was sent, but was received. Note that burst error does not necessarily mean that the error occur in consecutive bits. The length of the burst is measured from the first corrupted bit to the last corrupted bit. Some bits in between may not have been corrupted. Data sent Bits changed by burst error Data received Length of burst error = 8

30 The length of burst error is measured from first changed bit to last changed bit. Although some bits are unchanged in between. Burst error is most likely to occur in a serial transmission. The noise occurring for a longer duration affects multiple bits. The number of bits affected depends on the data rate & duration of noise. For e.g. if data rate is 1 kbps, a noise of 1/100 second can affect 10 bits. 17. What is burst error? [2 Marks] When more than a single bit of data unit gets corrupted it is known as Burst error. In comparison of single-bit errors, burst errors are more likely to occur. Because as we know that the duration of noise is generally longer than the duration of transferring 1bit, that means with longer duration noise can corrupt more than 1 bit easily. Number of bit affected depends on the data rate and duration of noise. 18. What is CRC? [2 Marks] Cyclic codes are special linear block codes with one extra property. In a cyclic code, if a codeword is cyclically shifted (rotated), the result is another codeword. A category of cyclic codes called the cyclic redundancy check (CRC) is used in networks such as LANs and WANs. CRC is based on binary division. In CRC, a sequence of redundant bits, called cyclic redundancy check bits, are appended to the end of data unit so that the resulting data unit becomes exactly divisible by a second, predetermined binary number. 19. What is reservation? [2 Marks] In the reservation method, a station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. A station needs to make a reservation before sending data. Time is divided into intervals. In each interval, a reservation frame precedes the data frames sent in that interval. If there are N stations in the system, there are exactly N reservation mini-slots in the reservation frame

31 20. What is a single bit error? [2 Marks] As name suggest single-bit errors occur when a single bit gets changed during transmission of data due to interference in network communication. The Term Single-Bit error means that only 1 bit of a given data unit (such as a byte, character, or packet) is changed from 1 to 0 or from 0 to 1 Forouzan Single bit error Single-bit errors are least likely type of error because their duration or noise is normally longer than duration of 1 bit. 21. Compare and contrast Slotted ALOHA and Pure ALOHA. [6 Marks]

32 Difference Between Pure ALOHA and Slotted ALOHA Pure ALOHA and Slotted ALOHA both are the Random-Access Protocols, that are implemented on the Medium Access Control (MAC) layer, a sublayer of Data Link Layer. The purpose of the ALOHA protocol is to determine that which competing station must get the next chance of accessing the multi-access channel at MAC layer. The main difference between Pure ALOHA and Slotted ALOHA is that the time in Pure Aloha is continuous whereas, the time in Slotted ALOHA is discrete. Comparison Chart BASIS COMPARISON FOR PURE ALOHA SLOTTED ALOHA Introduced Introduced by Norman Abramson and his associates at Introduced by Roberts in the University of Hawaii in Frame Transmission The user can transmit the data frame whenever the station has the data to be transmitted. The user has to wait till the next time slot start, to transmit the data frame.

33 BASIS COMPARISON FOR PURE ALOHA SLOTTED ALOHA Time In Pure ALOHA the time is continuous. In Slotted ALOHA the time is discrete. Successful Transmission The probability of successful transmission of the data frame is: S= G* e^-2g The probability of successful transmission of the data frame is: S= G*e^-G Synchronization The time is not globally synchronized. The time here is globally synchronized. Throughput The maximum throughput occurs at G = 1/2 which is 18%. The maximum throughput occurs at G = 1 which is 37%. Definition Of Pure ALOHA Pure ALOHA is introduced by Norman Abramson and his associates at the University of Hawaii in early The Pure ALOHA just allows every station to transmit the data whenever they have the data to be sent. When every station transmits the data without checking whether the channel is free or not there is always the possibility of the collision of data frames. If the acknowledgment arrived for the received frame, then it is ok or else if the two frames collide (Overlap), they are damaged.

34 If a frame is damaged, then the stations wait for a random amount of type and retransmits the frame till it transmits successfully. The waiting time of the each station must be random and it must not be same just to avoid the collision of the frames again and again. The throughput of the Pure ALOHA is maximized when the frames are of uniform length. The formula to calculate the throughput of the Pure ALOHA is S-=G*e^-2G, the throughput is maximum when G=1/2 which is 18% of the total transmitted data frames. Definition Of Slotted ALOHA After the pure ALOHA in 1970, Roberts introduced an another method to improve the capacity of the Pure ALOHA which is called Slotted ALOHA. He proposed to divide the time into discrete intervals called time slots. Each time slot corresponds to the length of the frame. In contrast to the Pure ALOHA, Slotted ALOHA does not allow to transmit the data whenever the station has the data to be send. The Slotted ALOHA makes the station to wait till the next time slot begins and allow each data frame to be transmitted in the new time slot.

35 Synchronization can be achieved in Slotted ALOHA with the help of a special station that emits a pip at the beginning of every time slot as a clock does. The formula to calculate the throughput of the Slotted ALOHA is S=G*e^-G, the throughput is maximum when G=1 which is 37% of the total transmitted data frames. In Slotted ALOHA, 37% of the time slot is empty, 37% successes and 26% collision. Key Differences Between Pure ALOHA and Slotted ALOHA 1. Pure ALOHA was introduced by Norman and his associates at the university of Hawaii in On the other hand, Slotted ALOHA was introduced by Roberts in In pure ALOHA, whenever a station has data to send it transmits it without waiting whereas, in slotted ALOHA a user wait till the next time slot beings to transmit the data. 3. In pure ALOHA the time is continuous whereas, in Slotted ALOHA the time is discrete and divided into slots. 4. In pure ALOHA the probability of successful transmission is S=G*e^-2G. On the other hand, in slotted ALOHA the probability of successful transmission is S=G*e^-G. 5. The time of sender and receiver in pure ALOHA is not globally synchronized whereas, the time of sender and receiver in slotted ALOHA is globally synchronized. 6. The maximum throughput occurs at G=1/2 which is 18 % whereas, the maximum throughput occurs at G=1 which is 37%.

36 Conclusion: The Slotted ALOHA is somewhat better than the Pure ALOHA. As the probability of collision is less in Slotted ALOHA as compared to Pure ALOHA because the station waits for the next time slot to begin which let the frame in a previous time slot to pass and avoids the collision between the frames. 22. Write a short note on Selective Repeat ARQ. Explain all the cases of packet loss. How does Selective Repeat ARQ react to packet loss? [6 Marks] In a noisy link a frame has a higher probability of damage, which means the resending of multiple frames. This resending uses up the bandwidth and slows down the transmission. For noisy links, there is another mechanism that does not resend N frames when just one frame is damaged; only the damaged frame is re-sent. This mechanism is called Selective Repeat ARQ. Window size of sender and receiver is the same but shorter than that of Go-Back-N. The smaller window size means less efficiency but the fact that there are fewer duplicate frames, compensates for that.

37 In selective repeat ARQ, the size of the sender and receiver window must be at most one-half of 2 m. CASES OF PACKET LOSS When a frame is lost in transmission o When sender is transmitting data, one or more frames can be dropped due to hindrances in the network. This may lead to loss of information in the receiver end. The sender window has an individual timer for each frame which resends the frame if acknowledgement is not received until the time out. The frame is re-sent after the rest of the frames in current slot is transmitted. When an acknowledgement is lost in transmission o If due to any hindrances in the network, the acknowledgement packet is lost and not received by sender before the time-out, the sender assumes its fault and resends the frame. The frame is re-sent after the remaining frames in current slot is sent. The receiver on receiving the duplicated frame, drops it and re-sends the acknowledgement. Thus, completing entire cycle of data transmission. Selective Repeat ARQ reaction to Packet Loss

38 Here, each frame sent or re-sent has a timer thus it is numbered. The timer for frame 0 starts at the first request, but stops when the ACK for this frame arrives. The timer for frame 1 starts at the second request, restarts when a NAK arrives, and finally stops when the last ACK arrives. At the receiver site we need to distinguish between the acceptance of a frame and its delivery to the network layer. At the second arrival, frame 2 arrives and is stored and marked, but it cannot be delivered because frame 1 is missing. At the next arrival, frame 3 arrives and is marked and stored, but still none of the frames can be delivered. Only at the last arrival, when finally, a copy of frame 1 arrives, can frames 1,2 and 3 be delivered to the network layer. There are two conditions for the delivery of frames to the network layer: First, a set of consecutive frames must have arrived. Second, the set starts from the beginning of the window. After the first arrival, there was only one frame and it started from the beginning of the window. After the last arrival, there are three frames and the first one stats from the beginning of the window.

39 23. Explain the protocol of Go-Back-N ARQ. Explain all the cases of packet loss. How does Go-Back-N react in each case? [6 Marks] Go-Back-N protocol is a sliding window protocol. It is a mechanism to detect and control the error in datalink layer. During transmission of frames between sender and receiver, if a frame is damaged, lost, or an acknowledgement is lost then the action performed by sender and receiver is explained in the following content. Damaged Frame If a receiver receives a damaged frame or if an error occurs while receiving a frame then, the receiver sends the NAK ( negative acknowledgement) for that frame along with that frame number, that it expects to be retransmitted. After sending NAK, the receiver discards all the frames that it receives, after a damaged frame. The receiver does not send any ACK (acknowledgement) for the discarded frames. After the sender receives the NAK for the damaged frame, it retransmits all the frames onwards the frame number referred by NAK. Lost frame The receiver checks the number on each frame, it receives. If a frame number is skipped in a sequence, then the receiver easily detects the loss of a frame as the newly received frame is received out of sequence. The receiver sends the NAK for the lost frame and then the receiver discards all the frames received after a lost frame. The receiver does not send any ACK (acknowledgement) for that discarded frames. After the sender receives the NAK for the lost frame, it retransmits the lost frame referred by NAK and also retransmits all the frames which it has sent after the lost frame.

40 Lost Acknowledgement If the sender does not receive any ACK or if the ACK is lost or damaged in between the transmission. The sender waits for the time to run out and as the time run outs, the sender retransmits all the frames for which it has not received the ACK. The sender identifies the loss of ACK with the help of a timer. The ACK number, like NAK (negative acknowledgement) number, shows the number of the frame, that receiver expects to be the next in sequence. The window size of the receiver is 1 as the data link layer only require the frame which it has to send next to the network layer. The sender window size is equal to w. If the error rate is high, a lot of bandwidth is lost wasted. 24. Explain HDLC Frame Format. [6 Marks] High-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. HDLC defines three types of frames: information frames (I-frames), supervisory frames (S-frames), and unnumbered frames (U-frames). Each type of frame serves as an envelope for the transmission of a different type of message. Iframes are used to data-link user data and control information relating to user data (piggybacking). S-frames are used only to transport control information. U-frames are reserved for system management. Information carried by U-frames is intended for managing the link itself. A beginning flag field, an address field, a control field, an information field, a frame check sequence (FCS) field, and an ending flag field. In multiple-frame transmissions, the ending flag of one frame can serve as the beginning flag of the next frame. Flag field. This field contains synchronization pattern , which identifies both the beginning and the end of a frame. Address field. This field contains the address of the secondary station. If a primary station created the frame, it contains a to address. If a secondary station creates the frame, it contains a from address. The address field can be one byte or several bytes long, depending on the needs of the network. Control field. The control field is one or two bytes used for flow and error control. Information field. The information field contains the user s data from

41 the network layer or management information. Its length can vary from one network to another. FCS field. The frame check sequence Information Transfer Format: The frame is used to transmit end-user data between two devices. Supervisory Format: The control field performs control functions such as acknowledgment of frames, requests for re-transmission, and requests for temporary suspension of frames being transmitted. Its use depends on the operational mode being used. Unnumbered Format: This control field format is also used for control purposes. It is used to perform link initialization, link disconnection and other link control functions. THE POLL/FINAL BIT (P/F): The 5th bit position in the control field is called the poll/final bit, or p/f bit. It can only be recognized when it is set to 1. If it is set to 0, it is ignored. The poll/final bit is used to provide dialogue between the primary station and secondary station. The primary station uses P=1 to acquire a status response from the secondary station. The P bit signifies a poll. The secondary station responds to the P bit by transmitting a data or status frame to the primary station with the P/F bit set to F=1. The F bit can also be used to signal the end of a transmission from the secondary station under Normal Response Mode. THE INFORMATION FIELD: This field is not always in a HDLC frame. It is only present when the Information Transfer Format is being used in the control field. The information field contains the actually data the sender is transmitting to the receiver. 25. What is hamming distance? Explain with example. How does that relate to error detection? [6 Marks] A Hamming distance in information technology represents the number of points at which two corresponding pieces of data can be different. It is often used in various kinds of error correction or evaluation of contrasting strings or pieces of data.

42 For example: Take two code words. One code word is and the other one is By hamming distance, what we mean is that we have to see in how many bit positions the two code words are different. So, simply XOR the two code words code word code word = XOR hamming distance = 5 If we XOR these, we get Now we count the number of 1s in this XOR, which is 5. So we say that their hamming distance is 5. If the two corresponding bits are different in the two code words 0 and 1,1 and 0, then we get a 1 in this XOR.if they are the same like 1 1or 0 0, we get 0. This hamming distance is the number positions where the two codes differ. A key issue in designing any error correcting code is making sure that any two valid code words are sufficiently dissimilar so that corruption of a single bit (or possibly a small number of bits) does not turn one valid code word into another. To measure the distance between two code words, we just count the number of bits that differ between them. If we are doing this in hardware or software, we can just XOR the two code words and count the number of 1 bit in the result. This count is called the Hamming distance. The key significance of the hamming distance is that if two code words have a Hamming distance of d between them, then it would take d single bit errors to turn one of them into the other. For a set of multiple code words, the Hamming distance of the set is the minimum distance between any pair of its members. Minimum Hamming distance for error detection To design a code that can detect d single bit errors, the minimum Hamming distance for the set of code words must be d + 1 (or more). That way, no set of d errors in a single bit could turn one valid code word into some other valid code word. Minimum Hamming distance for error correction

43 To design a code that can correct d single bit errors, a minimum distance of 2d + 1 is required. That puts the valid code words so far apart that even after bit errors in d of the bits, it is still less than half the distance to another valid code word, so the receiver will be able to determine what the correct starting code word was. 26. Describe Data Link Control. [6 Marks] The data link control (DLC) deals with procedures for communication between two adjacent nodes node-to-node communication no matter whether the link is dedicated or broadcast. Data link control functions include framing and flow and error control. Framing: Data transmission in the physical layer means moving bits in the form of a signal from the source to the destination. The physical layer provides bit synchronization to ensure that the sender and receiver use the same bit durations and timing. Framing in the data-link layer separates a message from one source to a destination by adding a sender address and a destination address. The destination address defines where the packet is to go; the sender address helps the recipient acknowledge the receipt. Framing Approaches: Character-Oriented Framing Bit-Oriented Approach Error Control Error control at the data-link layer is normally very simple and implemented using one of the following two methods. In both methods, a CRC is added to the frame header by the sender and checked by the receiver. In the first method, if the frame is corrupted, it is silently discarded; if it is not corrupted, the packet is delivered to the network layer. This method is used mostly in wired LANs such as Ethernet. In the second method, if the frame is corrupted, it is silently discarded; if it is not corrupted, an acknowledgment is sent (for the purpose of both flow and error control) to the sender.

44 Flow Control Whenever an entity produces items and another entity consumes them, there should be a balance between production and consumption rates. If the items are produced faster than they can be consumed, the consumer can be overwhelmed and may need to discard some items. If the items are produced more slowly than they can be consumed, the consumer must wait, and the system becomes less efficient. Flow control is related to the first issue. We need to prevent losing the data items at the consumer site. 27. Write a note on ALOHA. [6 Marks] The Aloha protocol was designed as part of a project at the University of Hawaii. It provided data transmission between computers on several of the Hawaiian Islands using radio transmissions. Communications was typically between remote stations and a central sited named Menehune or vice versa. All message to the Menehune were sent using the same frequency. When it received a message intact, the Menehune would broadcast an ack on a distinct outgoing frequency. The outgoing frequency was also used for messages from the central site to remote computers. All stations listened for message on this second frequency. Pure Aloha Pure Aloha is an unslotted, fully-decentralized protocol. It is extremely simple and trivial to implement. The ground rule is - "when you want to talk, just talk!". So, a node which wants to transmits, will go ahead and send the packet on its broadcast channel, with no consideration whatsoever as to anybody else is transmitting or not.

45 One serious drawback here is that, you dont know whether what you are sending has been received properly or not (so as to say, "whether you've been heard and understood?"). To resolve this, in Pure Aloha, when one node finishes speaking, it expects an acknowledgement in a finite amount of time - otherwise it simply retransmits the data. This scheme works well in small networks where the load is not high. But in large, load intensive networks where many nodes may want to transmit at the same time, this scheme fails miserably. This led to the development of Slotted Aloha. Slotted Aloha This is quite similar to Pure Aloha, differing only in the way transmissions take place. Instead of transmitting right at demand time, the sender waits for some time. This delay is specified as follows - the timeline is divided into equal slots and then it is required that transmission should take place only at slot boundaries. To be more precise, the slotted-aloha makes the following assumptions: All frames consist of exactly L bits. Time is divided into slots of size L/R seconds (i.e., a slot equals the time to transmit one frame). Nodes start to transmit frames only at the beginnings of slots. The nodes are synchronized so that each node knows when the slots begin. If two or more frames collide in a slot, then all the nodes detect the collision event before the slot ends.

46 In this way, the number of collisions that can possibly take place is reduced by a huge margin. And hence, the performance become much better compared to Pure Aloha. collisions may only take place with nodes that are ready to speak at the same time. But nevertheless, this is a substantial reduction. Carrier Sense Mutiple Access Protocols In both slotted and pure ALOHA, a node's decision to transmit is made independently of the activity of the other nodes attached to the broadcast channel. In particular, a node neither pays attention to whether another node happens to be transmitting when it begins to transmit, nor stops transmitting if another node begins to interfere with its transmission. As humans, we have human protocols that allow allows us to not only behave with more civility, but also to decrease the amount of time spent "colliding" with each other in conversation and consequently increasing the amount of data we exchange in our conversations. Specifically, there are two important rules for polite human conversation: Listen before speaking: If someone else is speaking, wait until they are done. In the networking world, this is termed carrier sensing - a node listens to the channel before transmitting. If a frame from another node is currently being transmitted into the channel, a node then waits ("backs off") a random amount of time and then again senses the channel. If the channel is sensed to be idle, the node then begins frame transmission. Otherwise, the node waits another random amount of time and repeats this process. If someone else begins talking at the same time, stop talking. In the networking world, this is termed collision detection - a transmitting node listens to the channel while it is transmitting. If it detects that another node is transmitting an interfering frame, it stops transmitting and uses some protocol to determine when it should next attempt to transmit.