Computer Networks Module 3: Data Link Layer (Framing) Dr. Vikram Shete St. Francis Institute of Technology

Framing Data link layer needs to pack bits into frames, so that 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

Framing Fixed-size framing: ATM Variable-size framing –Need a way to define the end of the frame and the beginning of the next –Character-oriented approach and bit-oriented approach

Character-Oriented Protocols Frame structure Byte stuffing: process of adding 1 extra byte whenever there is a flag or escape character in the text

Bit-Oriented Protocols Frame structure Bit stuffing: process of adding one extra 0 whenever five consecutive 1s follow a 0 in the data

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

Noiseless Channels: Simplest Protocol Simplest protocol with no flow or error control

Simplest Protocol Sender-site algorithm

Receiver-site algorithm Simplest Protocol

Stop-and-Wait Protocol Simple tokens of ACK and flow control added

Stop-and-Wait Protocol Sender-site algorithm

Receiver-site algorithm Stop-and-Wait Protocol

Stop-and-Wait Protocol: Example

Noisy Channels: Stop-and-Wait ARQ 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 expires In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic Acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected.

Stop-and-Wait ARQ

Sender-site algorithm Stop-and-Wait ARQ

Receiver-site algorithm

Stop-and-Wait ARQ: Example

Stop-and-Wait ARQ: Sequence Number Sequence no. starts from 0 and goes to 2 m -1 and then repeats Eg: Given bandwidth is 1Mbps and 1 bit round trip time is 20 ms. Frame size is 1000 bits. What is the BW – Delay product? What is % utilization of the link?

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Go-Back-N ARQ Pipelining improves the efficiency of the transmission The send window is an abstract concept defining an imaginary box of size 2 m − 1 with three variables: S f, S n, and S size The send window can slide one or more slots when a valid acknowledgment arrives.

Go-Back-N ARQ

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

Go-Back-N ARQ Sliding windows, Timers, ACK, Resending a frame

Go-Back-N ARQ: Send Window Size In Go-Back-N ARQ, the size of the send window must be less than 2 m ; the size of the receiver window is always 1 Stop-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: Send Window Size

Go-Back-N ARQ: Sender Algorithm

Go-Back-N ARQ: Receiver Algorithm

Go-Back-N ARQ: Example 1

Go-Back-N ARQ: Example 2

Selective Repeat ARQ Sender window size Receive window size

Selective Repeat ARQ

Selective Repeat ARQ: Window Size The size of the sender and receiver window must be at most one-half of 2 m

Selective Repeat ARQ: Sender-Site Algorithm

Selective Repeat ARQ: Receiver-Site Algorithm

Selective Repeat ARQ: Example

Piggybacking To improve the efficiency of the bidirectional protocols Piggybacking in Go-Back-N ARQ

HDLC High-level Data Link Control Two common transfer mode: normal response mode (NRM) and asynchronous balanced mode (ABM)

HDLC: Frames I(information)-frames, S(supervisory)-frames, U(unnumbered frame)- frames Flag field: 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: 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: Frames Control field for S-frame Receive ready (RR), Receive not ready (RNR), Reject (REJ) Selective reject (SREJ)

HDLC: Frames Control field for U-frame

HDLC: Example 1 Connection and disconnection

HDLC: Example 2 Piggybacking without error

HDLC: Example 3 Piggybacking with error

HDLC: Bit Stuffing and Unstuffing

Point-to-Point Protocol: PPP One of the most common protocols for point-to-point access Many Internet users who need to connect their home computer to the server of an Internet service provider use PPP A point-to-point link protocol is required to control and manage the transfer of data PPP defines/provides –the format of the frame to be exchanged between devices –how two devices negotiate the establishment of the link and the exchange of data –how network layer data are encapsulated in the data link frame –how two devices can authenticate each other –multiple network layer services –connection over multiple links –Network address configuration But, several services are missing for simplicity –no flow control, simple error control (detection and discard), no sophisticate addressing for multipoint configuration

PPP Frame Flag: the same as HDLC, but it treated as a byte because of PPP is a byte-oriented protocol Address: (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

PPP: Transition States

PPP: 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 Frame

LCP: Common Options Options are inserted in the information field of the configuration packets

Authentication 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 authentication –Password Authentication Protocol (PAP) –Challenge Handshake Authentication Protocol (CHAP)

Password Authentication Protocol (PAP)

Challenge Handshake Authentication Protocol (CHAP) Three-way hand-shaking authentication protocol with greater security than PAP

Network Control Protocol: NCP PPP 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 Packet IP Datagram in a PPP frame

Multiple PPP

Example (1)

Example (2)