Data link control

Data Link Control

1. Framing 2. Flow and Error Control3. Protocols4. Noiseless Channels5. Noisy Channels6. HDLC7. Point-to-Point Protocol

FramingFramingData link layer needs to pack bits into frames, so Data link layer needs to pack bits into frames, so that each frame is distinguishable from anotherthat each frame is distinguishable from another

Separate a message from one source to a destination, or from other messages to other destinations, by adding a sender address and a destination address

Fixed-size framing: ATM (Chapter 18)

Variable-size framingNeed a way to define the end of the frame and the beginning of the next

Character-oriented approach and bit-oriented approachDr. Md. Abdur Razzaque

Green Networking Research GroupDept. of Computer Science and Engineering, University of Dhaka

Character-Oriented ProtocolsCharacter-Oriented ProtocolsFrame structureFrame structure

• Byte stuffing: process of adding 1 extra byte whenever Byte stuffing: process of adding 1 extra byte whenever there is a flag or escape character in the textthere is a flag or escape character in the text

Dr. Md. Abdur RazzaqueGreen Networking Research Group

Dept. of Computer Science and Engineering, University of Dhaka

Universal Coding!!!

Bit-Oriented ProtocolsBit-Oriented ProtocolsFrame structureFrame structure

• Bit stuffing: process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data



Flow and Error ControlFlow and Error ControlData link control = flow control + error controlFlow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgementError control in the data link layer is based on automatic repeat request (ARQ), which is the retransmission of dataACK, NAK(Negative ACK), Piggybacking (ACKs and NAKs in data frames)



Noiseless Channels: Simplest Noiseless Channels: Simplest ProtocolProtocol

Simplest protocol with no flow or error control



Simplest ProtocolSimplest ProtocolSender-site algorithm

• Receiver-site algorithm



Stop-and-Wait ProtocolStop-and-Wait ProtocolSimple tokens of ACK and flow control added



Stop-and-Wait ProtocolStop-and-Wait ProtocolSender-site algorithm

• Receiver-site algorithm



Stop-and-Wait Protocol: Stop-and-Wait Protocol: ExampleExample



Noisy Channels: Stop-and-Wait Noisy Channels: Stop-and-Wait ARQARQ

Stop-and-wait Automatic Repeat Request (ARQ)Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expiresIn Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmeticAcknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected.



Stop-and-Wait ARQStop-and-Wait ARQ



Stop-and-Wait ARQStop-and-Wait ARQSender-site algorithm



Stop-and-Wait ARQStop-and-Wait ARQReceiver-site algorithm



Stop-and-Wait ARQ: ExampleStop-and-Wait ARQ: Example



Go-Back-N ARQGo-Back-N ARQPipelining improves the efficiency of the transmissionIn the Go-Back-N Protocol, the sequence numbers are modulo 2m, where m is the size of the sequence number field in bitsThe send window is an abstract concept defining an imaginary box of size 2m − 1 with three variables: Sf, Sn, and Ssize

The send window can slide one or more slots when a valid acknowledgment arrives.



Go-Back-N ARQGo-Back-N ARQReceive window for Go-Back-N ARQThe receive window is an abstract concept defining an imaginary box of size 1 with one single variable Rn. The window slides when a correct frame has arrived; sliding occurs one slot at a time.



Go-Back-N ARQGo-Back-N ARQSliding windows, Timers, ACK, Resending a frame



Go-Back-N ARQ: Send Window SizeGo-Back-N ARQ: Send Window SizeIn Go-Back-N ARQ, the size of the send window must be less than 2m; the size of the receiver window is always 1Stop-and-Wait ARQ is a special case of Go-Back-N ARQ in which the size of the send window is 1



Go-Back-N ARQ: Sender AlgorithmGo-Back-N ARQ: Sender Algorithm



Go-Back-N ARQ: Receiver AlgorithmGo-Back-N ARQ: Receiver Algorithm



Go-Back-N ARQ: Example 1Go-Back-N ARQ: Example 1



Go-Back-N ARQ: Example 2Go-Back-N ARQ: Example 2



Selective Repeat ARQSelective Repeat ARQSender window size

• Receive window size



Selective Repeat ARQSelective Repeat ARQ



Selective Repeat ARQ: Window Selective Repeat ARQ: Window SizeSize

The size of the sender and receiver window must be at most one-half of 2m



Selective Repeat ARQ: Sender-Site Selective Repeat ARQ: Sender-Site AlgorithmAlgorithm



Selective Repeat ARQ: Receiver-Site AlgorithmSelective Repeat ARQ: Receiver-Site Algorithm



Selective Repeat ARQ: ExampleSelective Repeat ARQ: Example



PiggybackingPiggybackingTo improve the efficiency of the bidirectional protocolsPiggybacking in Go-Back-N ARQ



HDLCHDLC• High-level Data Link Control• Two common transfer mode: normal response mode (NRM) and

asynchronous balanced mode (ABM)



HDLC: FramesHDLC: Frames• I(information)-frames, S(supervisory)-frames, U(unnumbered frame)-

frames• Flag field: 01111110 to identify both the beginning and the end of a

frame and serve as synchronization pattern for receiver• FCS field: 2- or 4-byte ITU-T CRC for error detection



HDLC: FramesHDLC: Frames• Control Field: 1- or 2-byte segment of the frame used for flow and

error control• Determine the type of frame and define its functionality• Control field for I-frame: P/F (poll/final bit for primary/secondary)



HDLC: FramesHDLC: Frames• Control field for S-frame• Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective

reject (SREJ)



HDLC: FramesHDLC: Frames• Control field for U-frame



HDLC: Example 1HDLC: Example 1• Connection and disconnection



HDLC: Example 2HDLC: Example 2• Piggybacking without error



HDLC: Example 3HDLC: Example 3• Piggybacking with error



HDLC: Bit Stuffing and HDLC: Bit Stuffing and UnstuffingUnstuffing



Point-to-Point Protocol: Point-to-Point Protocol: PPPPPPOne of the most common protocols for point-to-point accessMany Internet users who need to connect their home computer to the server of an Internet service provider use PPPA point-to-point link protocol is required to control and manage the transfer of dataPPP defines/provides

the format of the frame to be exchanged between deviceshow two devices negotiate the establishment of the link and the exchange of datahow network layer data are encapsulated in the data link framehow two devices can authenticate each othermultiple network layer servicesconnection over multiple linksNetwork address configuration

But, several services are missing for simplicityno flow control, simple error control (detection and discard), no sophisticate addressing for multipoint configuration



PPP FramePPP Frame• Flag: 01111110 the same as HDLC, but it treated as a byte because of

PPP is a byte-oriented protocol• Address: 11111111 (broadcast address)• Control: No need because PPP has no flow control and limited error

control• PPP is a byte-oriented protocol using byte stuffing with the escape byte

01111101



PPP: Transition StatesPPP: Transition States



PPP: MultiplexingPPP: Multiplexing• PPP uses another set of other protocols to establish the link, authenticate

the parties, and carry the network layer data• Three sets of protocols defined for powerful PPP: LCP, two APs, several

NCPs



LCP: Encapsulated in a FrameLCP: Encapsulated in a Frame



LCP: Common OptionsLCP: Common Options

Options are inserted in the information field of the configuration packets



AuthenticationAuthentication

Authentication means validating the identity of a user who needs to access

PPP is designed for use over dial-up links

⇒ User authentication is necessary

PPP has two protocols for authenticationPassword Authentication Protocol (PAP)

Challenge Handshake Authentication Protocol (CHAP)



Password Authentication Password Authentication Protocol (PAP)Protocol (PAP)



Challenge Handshake Challenge Handshake Authentication Protocol (CHAP)Authentication Protocol (CHAP)

• Three-way hand-shaking authentication protocol with greater security than PAP



Network Control Protocol: Network Control Protocol: NCPNCPPPP is a multiple-network layer protocol.

It can carry a network data packet from protocols defined by the Internet, OSI, Xerox, DECnet, AppleTalk, Novel

IPCP (IP Control Protocol)

Configures the link used to carry IP packets in the Internet



IPCP PacketIPCP Packet

IP Datagram in a PPP frame



Multiple PPPMultiple PPP



Example (1)Example (1)



Example (2)Example (2)



Data link control

Engineering

Transcript of Data link control