DLC services, framing, flow and error control, data link layer protocols: Noiseless channels: simplest protocols, stop-and-wait protocol, Noisy channels, HDLC.

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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