1. Communication Networks NETW 501
Lecture 4
Introduction to Data Link Layer
Course Instructor: Dr.-Ing. Maggie Mashaly
C3.220

2. Data Link Layer Data Link Layer provides a service for Network Layer
(transfer of data from the network layer of a sender to the network layer of a receiver)
Data Link Layer uses the Physical Layer
(to transmit bits of Data Link Frames over the physical medium)
LLC
MAC
Network
Physical

3. Data Link Layer Functions
Data Link layer is responsible for the following:
Framing (Grouping Bits into Frames)
Error Control
Flow Control
Medium Access Control

4. Framing Physical Layer deals with bit streams
Framing reflects the ability of the Data Link Layer to identify frame boundaries
Framing Methods
Character Count
Starting and Ending Flags with Bit Stuffing

5. Framing – Character Count
Uses a Field in the header that specifies the number of characters in the frame
Frame 1
6 Characters
Frame 2
5 Characters
Frame 3
8 Characters 6 5 8
If there is mistake in reading the header
Synchronization is Lost
The receiver has no way to know where does the frame start afterwards

6. Framing - Starting and Ending Flags with Bit Stuffing
Each Frame begins and ends with a special bit pattern (Flag Byte)
Bit Stuffing
If Data has 5 Consecutive 1S  ADD 0
Data
Flag Byte
Flag Byte
Flag Byte
Data

7. References NETW 501 Lectures slides by Assoc. Prof. Tallal El-Shabrawy
“Communication Networks 2nd Edition”, A. Leon-Garcia and I. Widjaja, McGraw Hill, 2013
“Computer Networks 4th Edition”, A. S. Tanenbaum, Pearson International