The document discusses the Data Link Layer (DLL) of computer networks, detailing its services, protocols, and functionalities such as framing, flow control, and error control. It explains different protocols like HDLC and PPP, covering aspects such as connection-oriented versus connectionless protocols, framing methods, and control field structures. Key concepts include the processes of framing, piggybacking for efficiency, and error detection mechanisms through checksums.

1. CH-11
DATA LINK LAYER:
DATA LINK CONTROL
- ASST. PROF. MEENAKSHI PAUL
G. N. KHALSA COLLEGE

2. OUTLINE
11.1 DLC SERVICES
11.1.1 Framing
11.1.2 Flow and Error Control
11.1.3 Connectionless and Connection-Oriented
11.2 DATA-LINK LAYER PROTOCOLS
11.2.1 Simple Protocol
11.2.2 Stop-and-Wait Protocol
11.2.3 Piggybacking
11.3 HDLC
11.3.1 Configurations and Transfer Modes
11.3.2 Framing
11.4 POINT-TO-POINT PROTOCOL (PPP)
11.4.1 Services
11.4.2 Framing
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3. 11.1 DLC SERVICES
 The data-link layer is divided into two sub-layers
 Logical Link Control (LLC)/DLC
 Media Access Control (MAC)
 The LLC sub-layer handles error control, flow
control, framing, and MAC sub-layer addressing.
 The MAC sub-layer is the lower of the two sub-
layers of the Data Link Layer.
 MAC handles access to shared media, such as
Token passing or Ethernet. 3

4. 11.1.1 FRAMING
 It is process of wrapping data with certain info before
sending out.
 Frame size can be variable or fixed.
 In fixed-size framing, there is no need of delimiter.
 In variable-size framing, there is need of delimiter.
 This can be done using two approaches
 Character-Oriented Approach
 Bit-Oriented Approach
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5. 11.1.1 FRAMING
CONTD..
 Character-Oriented Framing(or Byte-Oriented):
 In COF framing, data carried 8-bit characters from a coding system
such as ASCII.
 A frame typically consists of
 Data from the upper layer
 Flag: indication for start and end of a frame, composed of special
characters (1-byte)
 Header: source/destination addresses, as well as other control
information
 Trailer: error detection/correction code
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6. BYTE STUFFING
 Process of adding extra byte whenever there is an
escape or a flag character in the data
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7. BIT-ORIENTED FRAMING
 In bit-oriented framing, data is transmitted as a
sequence of bits that can be interpreted in the upper
layers.
 Need a delimiter to separate one frame from the other.
 Each frame begins and ends with special bit pattern
called flag byte (01111110).
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8. BIT STUFFING
 Process of adding extra bit to ensure flag sequence does not appear in the
data.
 When sender’s DLL finds 5 consecutive 1’s in data stream, stuffs 0.
 When receiver sees 5 1’s followed by 0, de-stuffs.
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01111110

9. BYTE VS. BIT ORIENTED
 Framing in byte-oriented protocols
 Framing in bit-oriented protocols
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10. FLOW CONTROL AND ERROR
CONTROL
 Flow control
 It is the process of managing the rate of data transmission
 It prevent a fast sender from overwhelming a slow
receiver.
 Error control
 It is the process of detecting and correcting data frames that
have been corrupted or lost during transmission.
 Retransmission of frames whenever error is detected or
frame is lost.
 Allows receiver to inform sender of lost or duplicate frames
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11. 11.1.3 CONNECTIONLESS AND CONNECTION-
ORIENTED
 A DLC protocol can be either connectionless
or connection-oriented.
 Connectionless Protocol
 No connection is established between two nodes.
 Each frame is independent.
 There is no connection between frames.
 Frames are not numbered
 Connection-Oriented Protocol
 A logical connection must be established between the two nodes( setup,
data transfer and teardown)
 The frames are numbered and sent in order
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12. 11.2 DATA-LINK LAYER PROTOCOLS
 Data link layer have four protocols
 Simple,
 Stop-and-Wait,
 Go-Back-N, and
 Selective-Repeat that deal with flow and error control
 First two protocols still are used at the data-link
layer
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13. 11.2.1 SIMPLE PROTOCOL
 A simple protocol with neither flow nor error control.
 We assume that the receiver can immediately handle any frame it receives.
 The receiver can never be overwhelmed with incoming frames.
 The DLL sender gets a packet from its network layer, makes a frame out of it, and
sends the frame.
 The DLL at the receiver receives a frame from the link, extracts the packet from the
frame, and delivers the packet to its network layer.
 The data-link layers of the sender and receiver provide transmission services for their
network layers.
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14. FLOW DIAGRAM SIMPLE PROTOCOL
 It is very simple.
 The sender sends frames one after another without even
thinking about the receiver.
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15. 11.2.2 STOP-AND-WAIT
PROTOCOL
 It uses both flow and error control.
 The sender will send one frame at a time.
 The sender will stop and wait for the acknowledgment (ACK) from the
receiver.
 After receiving ACK sender send the next packet to receiver.
 Receiver responds with an ACK when it successfully receives a frame
 Both Data and ACK frames must be numbered
 When sender does not receive an ack within certain time, it assumes
frame is lost, then retransmits the same frame.
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16. 11.2.2 STOP-AND-WAIT
PROTOCOL
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17. 11.2.3 PIGGYBACKING
 Piggybacking is the process of converting two
way process to one way process.
 In this sender send both (Data+ACK) to the
receiver.
 Data and acknowledgement are send in a single
frame is called Piggybacking.
 Piggybacking saves bandwidth.
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18. 11.2.3 PIGGYBACKING
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19. 11.3 HDLC
 High-level Data Link Control
 Bit-oriented protocol
 Support both
 Point-to-point links
 Multipoint links
 It implements the Stop-and-Wait protocol
 HDLC provides two common transfer modes
 Normal Response Mode (NRM)
 Asynchronous Balanced Mode (ABM)
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20. NORMAL RESPONSE MODE
(NRM)
 In NRM, the configuration is unbalanced.
 Have one primary station and multiple secondary stations.
 A primary station can send commands; a secondary station can only
respond.
 Used in both point-to-point and multi-point link
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21. ASYNCHRONOUS BALANCE MODE (ABM)
 In ABM, the configuration is balanced
 Supports only point-to-point links
 Each station is both primary and secondary
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22. 11.3.2 HDLC FRAMES
 HDLC defines three types of frames:
 Information frames (I-frames)
 Supervisory frames (S-frames)
 Unnumbered frames (U-frames)
 I-frames 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. 22

23. 11.3.2 HDLC FRAMES
 HDLC frame may contain up to six fields
 A beginning flag field, an address field, a control field, an information field, a
frame check sequence (FCS) field, and an ending flag field.
Information frame (I-frame)
Supervisory frame (S-frame)
Unnumbered frame (U-frame)
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Frame Check Sequence
(error detection code)

24. 11.3.2 HDLC FRAMES
Flag field.
 This field contains synchronization pattern 01111110, 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 the network layer or management information.
 Its length can vary from one network to another.
FCS field.
 The frame check sequence (FCS) is the HDLC error detection field.
 It can contain either a 2- or 4-byte CRC.
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25. CONTROL FIELD FOR I-FRAMES
 I-frames carry user data from the network layer
 Can carry flow- and error-control information (piggybacking)
 0, 1 bit, means the frame is an I-frame
 N(S), 3 bits, define the sequence number (0 to 7) of the frame
 N(R), 3 bits, acknowledgment number when piggybacking is used
 P/F field is a single bit with a dual purpose
 when it is set (bit = 1) and can mean poll or final
 poll when the frame is sent by a primary station to a secondary
 final when the frame is sent by a secondary to a primary
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26. CONTROL FIELD FOR S-FRAMES
 Receive ready (RR).
 00 is code
 This kind of frame acknowledges the receipt of a safe and sound frame or group of
frames.
 N(R) field defines the acknowledgment number.
 Receive not ready (RNR).
 10 is code, It gives acknowledgment of frames
 It also announces that the receiver is busy and cannot receive more frames ask
sender to slow down.
 Reject (REJ).
 01 is code, this is a NAK frame which can be used in Go-Back-N-ARQ
 Selective reject (SREJ).
 11 is code, This Nak is used in Selective Repeat ARQ
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27. CONTROL FIELD FOR U-FRAMES
 Unnumbered frames are used to exchange session management
and control information
 Do not contain N(S) or N(R) in control field.
 U-frame contains two code fields, one two bit and other three bit.
 These five bits can create upto 32 different U-frames.
 Purpose of P/F bit is same
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28. U-FRAME CODES
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29. 11.4 POINT-TO-POINT PROTOCOL (PPP)
 Point-to-Point Protocol
 Byte-oriented protocol
 Most common protocol for point-to-point access
 Dial-up access
 ADSL (Asymmetric Digital Subscriber Line)
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30. 11.4.1 SERVICES PROVIDED BY PPP
 Defining the frame format of the data to be transmitted.
 Defining the procedure of establishing link between two points and
exchange of data.
 Stating the method of encapsulation of network layer data in the frame.
 Stating authentication rules of the communicating devices.
 Providing address for network communication.
 Providing connections over multiple links.
 Supporting a variety of network layer protocols by providing a range os
services.
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31. 11.4.1 SERVICES NOT PROVIDED BY PPP
 PPP does not provide flow control
 A sender send frames one after another without concern of receiver.
 PPP has a very simple mechanism for error control.
 A CRC field is used to detect errors.
 If the frame is corrupted, it is silently discarded; the upper-layer protocol needs to
take care of the problem.
 Lack of error control and sequence numbering may cause a packet to be received
out of order.
 PPP does not provide a sophisticated addressing mechanism to handle frames in
a multipoint configuration.
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32. 11.4.2 PPP FRAME FORMAT
 The fields of a PPP frame are −
 Flag − 1 byte that marks the beginning and the end of the frame. The bit pattern of the flag is 01111110.
 Address − 1 byte which is set to 11111111 in case of broadcast.
 Control − 1 byte set to a constant value of 11000000.
 Protocol − 1 or 2 bytes that define the type of data contained in the payload field.
 Payload − This carries the data from the network layer. The maximum length of the payload field is 1500
bytes. However, this may be negotiated between the endpoints of communication.
 FCS − It is a 2 byte or 4 bytes frame check sequence for error detection. The standard code used is CRC
(cyclic redundancy code)
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