CS 313 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

2
Introduction

Algorithms to achieve reliable, efficient
communication between two adjacent
machines at the layer
Adjacent:
Physically connected by a communication
channel
3
Introduction

Issues
Providing well-defined services to network
layer
Framing
Determining how bits are grouped into frames
Error control
Flow control
4
Functions of the Data Link Layer

Relationship between packets and
frames.
5
Services to Network Layer

Transferring data between network
layers of machines
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Services

Unacknowledged connectionless service
Appropriate for
Low error rate
 Let higher layer(s) recover errors
Real-time traffic, e.g., speech, video
Most LANs, such as Ethernet

Acknowledged connectionless service
Useful over unreliable channels
Each frame sent individually acknowledged
e.g., wireless systems, e.g. 802.11 (WiFi)
7
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
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Framing

Fact
Raw bit stream delivered by physical layer
is not error free

Data link layer detects/corrects errors
Framing
Computing checksum
Handling error if any
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Framing

Approaches
Byte count
Flag bytes with byte stuffing
Flag bits with bit stuffing
Physical layer coding violations
10
Character Count
A field in header specifies number of
characters in a frame.
 Problem?

11
Flag Bytes with Byte Stuffing
A frame delimited by flag bytes
 Four examples of byte sequences before
and after stuffing

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Flag Bits with Bit Stuffing




Each frame begins and ends with special
bit pattern (flag byte): 01111110
Problem: 6 consecutive 1s in data
Solution: Bit Stuffing: inserting a 0 after 5
consecutive 1s
Used in USB
Original Data
After
Stuffing
After received
and destuffed
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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 classic Ethernet

Combination of approaches is possible
E.g., Ethernet and 802.11 use preamble
(well-defined pattern) + byte count
14
Error Control

Using acknowledgement
Positive
Negative
Problem: In some cases, sender waits
for acknowledgement forever
 Solution: Timer
 Problem: Duplicate transmission
 Solution: Sequence number

15
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?

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Reliability and Acknowledgement

Case 1: no error

Case 2: data lost
Sender Receiver
Time
Data
Sender
Time
Receiver
Data
X
Timeout
Ack.
Data
Ack.
Timeout and retransmission
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Reliability and Acknowledgement

Case 3: data error
Sender
Time

Case 4: ack. lost
Receiver
Data
Time
Error
Timeout
Sender
Data
Ack.
Timeout and retransmission
Timeout
Receiver
Data
X
Data
Ack.
New problem? Duplicate
Solution: Sequence number
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Flow Control
Needed
 Problem

When frames are transmitted faster than
receiver can accept, frames will be lost

Solution
Flow control by feedback mechanism
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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
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Implementation of Physical, Data
Link, and Network Layers
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Data Structures and Primitives
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Data Structures and Primitives
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Data Structures and Primitives
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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
25
Unrestricted Simplex Protocol - Sender
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Unrestricted Simplex Protocol - Receiver
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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
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Simplex Stop-and-Wait Protocol - Sender
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Simplex Stop-and-Wait Protocol - Receiver
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Simplex PAR Protocol
For noisy channel
 Positive acknowledgement w/
retransmission (PAR)
 Sender waits for a positive
acknowledgement before advancing to
the next data item
 A. k. a. ARQ (Automatic Repeat
reQuest)

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PAR Protocol

Assumptions
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?
32
PAR Protocol – Sender
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PAR Protocol – Sender (Cont’d)
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PAR Protocol – Receiver
35