Unit IVData Link Control: • Data link layer is the second layer in ISO OSI model.• The two main functions of DLL are• Data link control which deals with the node to node communication.• Medium access control which deals with how to share a communication medium between more than one devices

• In data link control the DLL performs following functions• Framing of the packets received from network layer.• Managing the flow of frames between two devices connected to a

link.• Performing error control of the frames exchanged across the link.• Software implementations of protocols for smooth and reliable

transmission of frames between nodes.

Unit IVFraming: • At the physical layer there is raw transmission of bits.• It is the job of DLL to pack these bits into frames so that they are distinguishable from one another.• It also inserts senders and receivers address into each frame.• The destination address identifies the correct receiver, while the sending address is used by receiver to send an acknowledgement back to the sender.• The size of the frames can be very large but it is not advantageous to do so because if there is even a single bit error in the received frame the total frame needs to be retransmitted.

Unit IVFraming: • Therefore DLL divides a message into several frames to perform flow and error control efficiently.• There are two types of framing mechanisms• Fixed size Framing: In this type each frame is of the same size. Therefore the size of the frame itself can be used to identify the boundary of a frame. A good example of this is frames used by ATM networks.• Variable size Framing: More common type of framing is variable size framing which is used in LAN’s. In this type we need to identify the boundary of a frame by using separate bits or characters. There are two approaches used for variable size framing

Unit IVFraming: • Character oriented approach (Protocol)• Bit oriented approach (Protocol)• Character oriented Protocols: In this type of protocols a frame is formed by inserting additional information along with the data. Usually it contains• Data in the form of 8 bit characters using ASCII coding.• Source and destination address in the header of a frame.• Additional control information for error detection and correction in the trailer of a frame.• To identify the start an end of a frame a 8 bit flag is added in the beginning and end of a frame.

Unit IVFraming: • The flag field consists of protocol dependent charters which identifies the start and end of a frame.• Figure shows format of a frame in character oriented protocols

• Although character oriented protocols are suitable for exchanging text data. Now we are able to exchange other types of data such as audio, graphics and video data using it.

Unit IVFraming: • This is possible as long as the pattern of the bits used in the flag is not in the data.• If the pattern used for flag is a part of data then receiver might misinterpret data as flag and thereby there is possibility of identifying frame boundaries incorrectly.• To solve this problem a method called as byte stuffing is used in character oriented protocols.• In byte stuffing a special byte is added to the data if there is a character of the same type in data as in the flag.• This additional byte is called as stuffed byte and usually called as escape (ESC) character which has a predefined bit pattern.

Unit IVFraming: • Thus whenever a receiver encounters such a stuffed character, it removes it from the data and treats the next character as data not a delimiting flag.• Another problem encountered in character oriented protocols is what if the text contains one or more escape characters followed by a flag.• To solve this problem the escape characters which are part of data must also be marked by another escape character.• Thus if escape characters are part of data then additional escape characters must be added to show that second one is a part of data.

Unit IVFraming: • Figure shows byte stuffing and unstuffing

Unit IVFraming: • Another problem of character oriented protocols is its 8 bit format.• Today most of the systems follows Unicode format which is of 16 or 32 bits.• Thus in general because of the above mentioned problems today we are moving towards bit oriented protocols.

Unit IVFraming: • Bit oriented Protocols: In these type of protocols the data section of the frame contains sequence of bits rather than characters which are interpreted by upper layers as text, graphics, audio, video etc.• In these protocols the frame contains data, header and trailer which contains additional information such as source and destination address, control information and error detection and correction field.• In these protocols also identifying the boundaries of a frame is important for correct reception of the data.

Unit IVFraming: • Bit oriented protocols uses a special 8 bit sequence of once and zeros 01111110 to identify the start and end of a frame.• Figure shows a frame in bit oriented protocols

• As shown every frame starts and end with a flag pattern of 01111110 with remaining fields between them.

Unit IVFraming: • In bit oriented protocols also there is a problem if the pattern used for flag (01111110) is a part of data.• If it is a part of data receiver might interpret the frame boundaries incorrectly. • To solve this problem a method called as bit stuffing is used.• In this method if the data contains same pattern as flag, only one additional bit is added to the data to distinguish it from flag.• This additional bit added to data is called as stuffed bit.• In this method if data contains a 0 and five consecutive 1’s, a extra 0 bit added to the data to distinguish data from flag.• This added extra 0 is removed by the receiver.

Unit IVFraming: • Figure shows concept of bit stuffing

Unit IVFraming: • Thus if a flag like pattern 01111110 is encountered in data it will be converted to 011111010 by adding extra staffed bit 0.• The real flag 01111110 is not stuffed by the sender and receiver identifies frame boundary correctly.

Unit IVFlow and Error Control: • The most important responsibilities of DLL are flow and error control which are required for successful data transfer between the two devices connected to a link.• Collectively these two function are called as data link control.• Flow Control: Flow control is used to coordinate the amount data that a sender can transfer before the acknowledgement of the transmitted data is received.• In most of the protocols flow control is a procedure that tells the sender how much data it can transmit before it must wait for an acknowledgement from the receiver.

Unit IVFlow and Error Control: • Flow control is a mechanism which stops the sender from overwhelming a receiver.• A receiver may have slow speed, less memory because of which it can receive a data at a certain rate.• If sender is transmitting the data at a rate more than receiver can handle it, there are chances of data loss.• Usually the speed of processing the data is slower than the speed of data transmission.• Receiver stores the data received from sender in a block of memory called as buffer.• If this buffer is full, receiver can not handle the incoming data.

Unit IVFlow and Error Control: • Therefore receiver requires a method using which it can tell a sender to stop temporarily transmission of data. This mechanism is implemented with the help of flow control. • Error Control: One of the important function of DLL is detecting the errors and correcting them in the received data.• Error control mechanism allows the receiver to tell the sender about damaged or lost frames and coordinates the retransmission of these frames from sender.

Unit IVFlow and Error Control: • In DLL error control refers to methods of error detection and retransmission of frames.• Error control in DLL is implemented very simply. Each time a receiver receives a frame with error, it transmits a specified frame to the sender.• This process is called as Automatic Repeat Request (ARQ).• When a sender receives a ARQ frame it retransmits the frame or frames which contains errors.

Unit IVData Link Control Protocols: • There are various protocols which are used to implement data link control.• Broadly the protocols are classified into two types• Data link control for noiseless channels• Simplest Protocol• Stop and Wait Protocol

• Data link control for noisy channels• Stop and Wait ARQ• Go back N ARQ• Selective Repeat ARQ

Unit IVData Link Control Protocols: • Protocols for Noiseless Channel: In these protocols it is assumed that the communication channel does not makes any errors. • In such channel no frames are lost or corrupted and duplicated.• Simplest Protocol: In this simple protocol it is assumed that there are no errors. Therefore this protocol does not make any error control.• Also it is assumed that receiver is having sufficient memory to handle the frames received from sender. Therefore there is no flow control mechanism implemented.

Unit IVData Link Control Protocols: • Design of Simplest Protocol: As discussed in this protocol there is no need of flow and error control.• The data link layer at the sender receives data from Network layer, makes a fame of it and sends the frame.• The DLL at the receiver receives a frame from physical layer, extracts data from it and hand over the data to network layer.• The DLL at both the ends provides services to network layer while uses the services of the physical layer such as signaling, multiplexing to transmit and receive the data.• Figure shows the design of this protocol

Unit IVData Link Control Protocols: •

Unit IVData Link Control Protocols: • The sending DLL can not send the data until it receives a packet from network layer and receiving DLL can not deliver a frame to network layer until a frame arrives.• Therefore for data transmission to take place certain events needs to happen.• At the sender a event Request from network layer and at the receiver a event Notification from physical layer needs to take place.• The procedures in both sender and receiver are continuously running and waiting for an event to happen.

Unit IVData Link Control Protocols: • At the sender DLL three functions are defined• Get Data which reads a packet from network layer• Make Frame which constructs a frame• Send Frame which hands over the frame to physical layer

• At the receiver DLL three functions are defined• Receive Frame which receives a frame from physical

layer• Extract Data which reads the data• Deliver Data which delivers data to network layer.

Unit IVData Link Control Protocols: • Figure shows sender side algorithm

• As shown the sender is waiting for and event to occur.

• If the event is request to send the algorithm reads the data from network layer, constructs a frame and transmits frame.

Unit IVData Link Control Protocols: • Figure shows the receiver side algorithm

• At the receiver also the algorithm is waiting for an event to occur.• If the event Arrival Notification occurs the algorithm receives a frame, extracts data from it and delivers it to the network layer

Unit IVData Link Control Protocols: • Figure shows the flow diagram for this algorithm

Unit IVData Link Control Protocols: • Stop and Wait Protocol: In most practical situations the speed of sender and receiver are not same.• If the frames transmitted are at a rate more than the rate at which they are processed at the receiver, frames must be stored at the receiver before they can be processed.• If the receiver is not having enough memory to store the frames, they will be discarded.• To stop the sender from overwhelming the receiver we need a mechanism which will tell the sender to slow down.

Unit IVData Link Control Protocols: • There must be a feedback from receiver to sender.• In Stop and Wait protocol, a sender sends a frame and waits until it receives an acknowledgement from the receiver. After receiving an acknowledgement it transmits the next fame.• In this protocol also the data transmission is unidirectional but acknowledgement (ACK) frames travels from receiver to sender.

Unit IVData Link Control Protocols: • Design of Stop and Wait Protocol: As discussed in this protocol performs flow control by restricting a sender to transmit one frame at a time and wait for an acknowledgement.• The data link layer at the sender receives data from Network layer, makes a fame of it and sends the frame.• The DLL at the receiver receives a frame from physical layer, extracts data from it and hand over the data to network layer.• After receiving the frame the DLL at receiver sends back an acknowledgement frame back to the sender.• When the sender receives the acknowledgement frame it can transmit the next frame.

Unit IVData Link Control Protocols: • Figure shows the design of this protocol

Unit IVData Link Control Protocols: • In this protocol also certain events needs to occur for data transmission to take place.•At the sender two events Request from network layer for transmission of first frame and Notification from physical layer for receiving the acknowledgement frame needs to take place.• At receiver only one event Notification from physical layer needs to take place. •The procedures in both sender and receiver are continuously running and waiting for an event to happen.

Unit IVData Link Control Protocols: • This protocol require a half duplex channel for carrying data from sender to receiver and for acknowledgement frame from receiver to sender. •At the sender DLL four functions are defined• Get Data which reads a packet from network layer• Make Frame which constructs a frame• Send Frame which hands over the frame to physical layer• Receive Frame which receives acknowledgement frame from physical layer.

• At the receiver DLL three functions are defined• Receive Frame which receives a frame from physical layer• Extract Data which reads the data• Deliver Data which delivers data to network layer.• Send Frame which sends acknowledgement frame to the physical

layer.

Unit IVData Link Control Protocols: • Figure below shows algorithm for sender side

Unit IVData Link Control Protocols: • As shown the sender is waiting for and event to occur.• If the event is request to send the algorithm reads the data from network layer, constructs a frame and transmits frame.• Here the algorithm uses a flag CanSend, which restricts a sender from transmitting next frame until an acknowledgement from receiver is received.• When an acknowledgement is received the sender can transmit the next frame using the same functions.

Unit IVData Link Control Protocols: • Figure shows the receiver side algorithm

• At the receiver also the algorithm is waiting for an event to occur.• If the event Arrival Notification occurs the algorithm receives a frame, extracts data from it and delivers it to the network layer and sends back an acknowledgement frame to the sender.

Unit IVData Link Control Protocols: • Figure shows the flow diagram for Stop and Wait Protocol

•

Unit IVData Link Control Protocols: • Protocols for Noisy Channels: As we know noiseless channels are nonexistent, we need different protocols which considers error detection and correction and flow control mechanism. • We are going to discuss three protocols • Stop and Wait Automatic Repeat Request (ARQ)• Go Back N Automatic Repeat Request (ARQ)• Selective Repeat Automatic Repeat Request (ARQ)

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • For wired networks it is easy to detect collision because, when collision occurs the station receives two signals: its own signal and the signal transmitted by second station.• Also when a collision occurs the energy of the received signal almost doubles and therefore a collision is detected easily.• Thus collision detection algorithm works very good in a wired network.• For wireless networks most of the signal energy is lost in transmission.

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • The received signal has very little energy and therefore the collision detection algorithm does not works properly.• For such networks a carrier sense multiple access algorithm with collision avoidance is invented.• This algorithm uses three strategies for collision avoidance• The Inter-frame space• Contention Window• Acknowledgements

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • Figure shows three strategies of CSMA/CA

• Inter- Frame Space: Using this space the stations try to minimize the collisions.• Even if the channel is found free a station does not transmits immediately.

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • The stations instead waits for a time called as Interframe Space (IFS).• This time allows a signal transmitted by other stations to reach to the other stations, before they starts transmission.• The IFS can be used to prioritize the transmissions from the stations.• A station with high priority is assigned a short IFS time and stations with low priority are assigned long IFS time.

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • Contention Window: The contention window is amount of time divided into slots. • A station which is ready for data transmission chooses random number of time slots as its wait time.• The number of slots in the window changes as per binary exponential algorithm. As per this algorithm if it is set to one time slot for first time then it doubles each time the station can not detect an idle channel after IFS time.• One interesting thing about contention window is that a station needs to sense channel after each time slot.

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • After sensing the channel if it is found busy station does not starts the process immediately. Instead it stops the timer and starts over again when the channel is found idle.• This mechanism gives priority to the station with the longest waiting time.• Acknowledgements: With all these precautions still there are chances of collisions resulting in destroyed data.

Unit IVCarrier Sense Multiple Access with Collision Avoidance (CSMA/CA): • In addition data may get corrupted during transmission.• A positive acknowledgement and time out timer can guarantee that the frame is received correctly.• Figure shows the procedure for CSMA/CA.• One thing should be noted from figure that a channel must be sensed before and after IFS. • Also it should be sensed during the contention.

Unit IV

Unit IV• Controlled Access: In controlled access mechanism of multiple access, stations consults each other before they transmit to decide which station has right to transmit data. A station can not transmit data unless it is authorized by other stations for transmission.• There are three methods in this category• Reservation• Polling• Token Passing

Unit IV• Reservation: In this method a station needs to make reservation before sending data. • Time is divided into intervals and in each time interval a reservation frame precedes the data frame send in that interval.• In this method if there are N stations in a network then there are exactly N mini slots of time in a reservation frame.• When a station want to send a frame it needs to reserve a mini slot in reservation frame.• The stations that have made reservations in reservation frame can send data after reservation frame.

Unit IV• Figure below shows reservation access mechanism

• As shown there are five stations and therefore there are five mini slots in reservation frame.• For first reservation frame three stations 1, 3, and 4 have made reservations therefore they transmit data following reservation frame.• For second reservation frame only one station 1 have made reservation and therefore only station 1 transmits data.

Unit IV• Polling: This method of controlled access works with those topologies in which one station is designated as primary and other are secondary.• All data transfers are to be made through primary station even though the destination of data is secondary station.• The primary station is always initiator of data transfers and it controls the link.• Primary station determines which station can transmit data and at what time.• Figure shows the working mechanism of polling

Unit IV•

• If the primary station wants to send data it selects a secondary for data transmission and if primary station wants to receive data it polls a secondary asking them whether they want to transmit the data.

Unit IV• This method uses two functions for controlling the access

• Select: The select function is used by primary when it wants to transmit data. Since primary station controls the link it knows when link is available for data transmission.

• Before sending data the primary station creates and transmits a SEL frame. One frame of SEL frame contains address of the intended secondary station.

• After receiving frame secondary station sends back acknowledgement frame.

Unit IV• Poll: This function is used by primary when it want to receive data from secondary.

• Primary station asks (Poll) each secondary station whether it has data to transmit.

• If the secondary does not want to transmit data it replies to polling frame with NAK frame, otherwise it responds to Poll frame with data frame to primary.

• When primary receives a data fame it responds it with an acknowledgment frame.

• Thus primary keeps on polling each station in the network for data transfers.

Unit IV• Token Passing: In this method of controlled access all stations in a ring are connected in a logical ring.

• Thus each station in a ring has a predecessor and successor station. A predecessor station is logically before a station and successor station is logically after a station.

• The right of transmission is passed from predecessor to current station and from current station to successor station.

• The right to access a channel is controlled using a special frame called as token.

Unit IV• When a station want to transmit data it must have token with itself.

• The token is passed from predecessor to current station and from current station to successor.

• When a token is with a station it transmits the data and after completion of transmission it passes the token to its successor.

• If the station does not want to transmit data it simply passes the token to its successor.

• In this way token keeps on revolving around the ring and data transmission takes place turn by turn.

Unit IV• In order to work properly this method require token management.

• Token management is required for controlling the time for which a station on the ring can transmit data, what if a station fails having token with it, what to do if a token is lost or destroyed.

• Another function of token management is to assign priorities to certain stations.

• A high priority station must get chance for data transmission before low priority station.

Unit IV• In token passing network, the ring need not be physical one. The stations are connected in a ring logically.

• Figure shows four possible ways to connect stations in a logical ring.

• In a physical ring topology a station sends the token to its successor. The token can not be seen by other stations.

• Therefore the addressing is not an issue in this topology.

Unit IV•

Unit IV• The problem with this configuration is that if one of the link between two stations breaks the total network collapses.

• In dual ring topology there are two rings which operates in opposite direction.

• The second ring is a stand by ring which is to be used only in case of emergencies.

• If the primary ring is broken then the system automatically switches to the second ring to provide uninterrupted operation to the system.

Unit IV• The problem with this configuration is that if one of the link between two stations breaks the total network collapses.

• In dual ring topology there are two rings which operates in opposite direction.

• The second ring is a stand by ring which is to be used only in case of emergencies.

• If the primary ring is broken then the system automatically switches to the second ring to provide uninterrupted operation to the system.

Unit IV• The bus ring topology is also called as bus topology.

• In this topology all the stations are connected to the central bus.

• The stations