CS412 Introduction to Computer Networking & Telecommunication Data Link Layer Part I – Designing Issues and Elementary Protocols Chi-Cheng Lin, Winona State University

Topics Introduction Framing Error Control Flow Control Elementary Data Link Protocols

Introduction Algorithms to achieve reliable, efficient communication between two adjacent machines at the layer Adjacent: Physically connected by a communication channel

Introduction Issues Services to network layer Framing Error control Determining how bits are grouped into frames Error control Flow control

Functions of the Data Link Layer Relationship between packets and frames.

Services to Network Layer Transferring data between network layers of machines

Placement of Data Link Protocol

Services Unacknowledged connectionless service Appropriate for Low error rate, higher layers recover errors Real-time traffic e.g., speech, video Most LANs Acknowledged connectionless service Useful over unreliable channels e.g., wireless systems

Services Acknowledged connection-oriented service Guarantees Each frame sent is received without error All frames sent are received in right order  Network layer always receives correct packets in the order in which the packets were sent Three phases: Connection establishment Variables and counters initialization Frame transmission Connection release Variables, buffers, resources freed up

Framing Fact Data link layer detects/corrects errors Raw bit stream delivered by physical layer is not error free Data link layer detects/corrects errors Framing Computing checksum Handling error if any

Framing How about inserting time gaps? Approaches Trust no time!! Character count Starting/ending character w/ character stuffing Starting/ending flags w/ bit stuffing Physical layer coding violations

Character Count A field in header specifies number of characters in a frame. Problem?

Starting/Ending Characters w/ Character Stuffing A frame delimited by flag bytes. Four examples of byte sequences before and after stuffing.

Starting/Ending Flags w/ bit Stuffing Flag byte (special bit pattern): 01111110 Problem: 6 consecutive 1s in data Solution: Bit Stuffing: Inserting a 0 after 5 consecutive 1s Original Data After Stuffing After received and destuffing

Physical Layer Coding Violations Encoding on physical medium Framing by invalid physical code Use some code that does not represent 0 nor 1 Example Manchester encoding in 802 LAN standard (Ch. 4) Combination of approaches is possible E.g., character count + one other method

Error Control Using acknowledgement Positive Negative Problem: In some cases, sender waits for acknowledgement forever Solution: Timer Problem: Duplicate transmission Solution: Sequence number

Positive Acknowledgement Sender sends a message, waits for acknowledgement from receiver, and then sends next message There’s no free lunch, though  overhead, delay How does it work?

Reliability and Acknowledgement Case 1: no error Sender Receiver Case 2: data lost Sender Receiver Time Data Time Data X Timeout Ack. Data Ack. Timeout and retransmission

Reliability and Acknowledgement Case 3: data error Sender Receiver Case 4: ack. lost Sender Receiver Time Data Time Data Error Timeout Timeout X Data Data Ack. Ack. Timeout and retransmission New problem? Duplicate Solution: Sequence number

Flow Control Needed Problem Solution When frames are transmitted faster than receiver can accept, frames will be lost Solution Flow control by feedback mechanism

Elementary Data Link Protocols Key Assumptions Network, data link, and physical layers are independent processes communicating by sending messages Machine A wants to send a long stream of data to machine B over a reliable, connection-oriented service

Data Structures and Primitives Just think of it as a class containing data only…

Unrestricted Simplex Protocol Utopia protocol Assumptions Unidirectional data transmission Transmitting/receiving network layers are always ready Processing time is ignored Infinite buffer space No errors

Unrestricted Simplex Protocol - Sender

Unrestricted Simplex Protocol - Receiver

Simplex Stop-and-Wait Protocol Assumptions Unidirectional data transmission Transmitting/receiving network layers are always ready Finite processing speed Finite buffer capacity No errors Problem: Sender sends too fast Stop-and-wait Senders sends one frame and then waits for an acknowledgement before processing

Simplex Stop-and-Wait Protocol - Sender

Simplex Stop-and-Wait Protocol - Receiver

Simplex PAR Protocol For noisy channel Positive acknowledgement w/ retransmission Sender waits for a positive acknowledgement before advancing to the next data item A. k. a. ARQ (Automatic Repeat reQuest)

PAR Protocol Assumptions Timer + sequence number Unidirectional data transmission Transmitting/receiving network layers are always ready Finite processing speed Finite buffer capacity Errors, can be detected Timer + sequence number Size (i.e., # bits) of sequence number?

PAR - Sender

PAR - Receiver