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1. Computer Networks
LEC #2
Communicating Over the
Network
Computer Networks
LEC #8
DataLink Layer

2. Lecture
Outlines
 DataLink Layer.
Framing
Method of Framing
Error Control
Medium Access Control
Next
Lecture

3. The Data Link Layer
Two basic functions:
• Allows the upper layers to access the media using framing.
• Controls how data is placed on the media and received
from the media using media access control (MAC) and
logical link control (LLC) for flow control and error control.

4. Framing
 The data link layer takes the packets it gets from the network layer and
encapsulates them into frames for transmission.
 Framing in the data link layer add a source and destination MAC
addresses.
 Data Link layer translates the physical layer’s raw bit stream into discrete
units (message) called frames.
4

5. Framing: Role of the Header
Header
Data Trailer
Start Address Type/length
Specific bit sequence that indicates to the
receiving device that the frame starts here.
The source and destination
MAC addresses (physical
address). They are only used
for local delivery.
A code identifying the type (fixed size
or variable size of the frame. OR the
total length of the frame.
5

6. Framing: Role of the Trailer
Header Data
Trailer
FCS Stop
Frame Check Sequence (FCS): Used to provide basic error checking, usually
with a Cyclic Redundancy Check (CRC).
Sending device uses an algorithm on the bits of the header and data
portions.
 The resulting value is placed in the FCS field.
Receiving device does the same thing.
 If the values match – no error.
 If the values do not match – frame discarded.
A specific sequence of bits that indicate the end of the frame.

7. Methods of Framing
 To perform framing task there are Different methods :
1. Character Count.
2. Flag byte with Byte Stuffing.
3. Starting and ending flag with byte Stuffing.
4. Physical layer coding violations
7

8. Framing – Character Count
Character Count :
• First field in the frame’s header = the length of the frame.
• Problem:
 Receivers loses synchronization when bits become garbled by
transmission error.
 So, if the bits in the count become corrupted during transmission,
the receiver will think that the frame contains fewer or more bits
than it actually does.
8

9. For the following data create frames using character count with
size 5, 5, 8, and 8 bytes, respectively.
Framing – Character Count - Example
9
1 2 3 4 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
Answer
5 1 2 3 4 5 6 7 8 9 8 0 1 2 3 4 5 6 8 7 8 9 0 1 2 3
Note that the counted number is included in the length of the frame.
Frame-1 Frame-2 Frame-3 Frame-4

10. problem, in the previous example. if the byte count of 5 in the second frame
becomes a 7 due to a single bit flip, the destination will get out of
synchronization. It will then be unable to locate the correct start of the next
frame.
Figure 4. A byte stream. (a) Without errors. (b) With one error.
Framing – Character Count
10

11. Framing – Byte Stuffing
11
Flag byte with Byte Stuffing:
 The data frames can be fixed length or variable length. In variable length
framing, the size of each frame to be transmitted may be different.
So, a pattern of bits is used as a delimiter to mark the end of one frame and
the beginning of the next frame.
 Problem: What happens if the pattern occurs in the message?
 Solution: A byte is stuffed in the message to differentiate from the
delimiter. There should be a strategy so that the receiver does not consider
the pattern as the end of the frame
 a special byte called the escape character (ESC) is stuffed before every
byte in the message (How?)

12. Framing – Byte Stuffing
• To make things simpler we will consider only three types of byte sequences in
the sent data, as :
F : Flag Sequence
E : Escape Sequence
D : Any other Data Sequence
At Sender (Client) Side
1.Data of each frame is first stuffed with 8-bit flag sequence (‘F’) at the beginning and end of
each frame.
2.Next, the data is scanned to see if any similar flag sequence (‘F’) forms a part of it or not. If
yes, then before each such flag sequence, an extra escape sequence (‘E’) is stuffed.
3.Now, if any similar escape sequence (‘E’) is found to form a part of the data to be sent, then
an extra escape sequence (‘E’) is stuffed before the occurrence of each such escape
sequence.
4.Finally, this stuffed data is sent by the sender.

13. Framing – Byte Stuffing
At Receiver (Server) Side
1. The receiver skips over the first and last bytes of data received, as they are
merely for signaling the beginning and end of one frame, respectively and
does not carry any useful data.
2. From the next byte on wards data is scanned and if two escape sequences
(‘E’) are found in succession, the first one is de-stuffed. Similarly, if an
escape sequence is followed by a flag sequence (‘F’), the former is de-
stuffed.
3. This strategy helps the receiver recover the actual sent data, accurately.

14. 14
Framing – Byte Stuffing
The following figure shows the original characters before framing and after
framing using byte stuffing in different situations.

15. 15