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EE 482: Communication Networks
Lecture 6
Framing in Data Link Layer
Instructor
Dr. Naif Almakhdhub
Original slides prepared by:
Dr. Basil AsSadhan
Reading: Section 9.1 (Forouzan)
5.4 (Leon-Garcia, Widjaja)
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Data Link Layer
Reference: J. Kursoe and K. Ross
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Outline
• Data Link Layer Functions
• Framing
• Types of Frames
– Fixed Length Frames
– Variable Length Frames
• Stuffing
– Bit Stuffing
– Byte Stuffing
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From Signals to Packets
Analog Signal
Digital Signal
Bit Stream
Packets
0 0 1 0 1 1 1 0 0 0 1
0100010101011100101010101011101110000001111010101110101010101101011010111001
Header/Payload
Packet
Transmission
Sender
Header/Payload
Header/Payload
Receiver
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Data Link Layer Functions
• Framing
– Map stream of bits into frames
– Add header, mark and detect frame boundaries
• Error control and recovery
– Error detection and correction through coding
– Retransmission
• Flow control
– Avoid sender outrunning receiver
– Receiver controls when sender can send a frame
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Data Link Layer Functions (Cont.)
• Medium Access Control (MAC)
– Control which frame to be sent next over link
• Multiplexing multiple flows
• Easy for point-to-point links
• Harder for multi-access links: who gets to send?
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Framing
• The physical layer carries information in the form of
stream of bits
• The data link layer maps stream of bits into frames
• How does a receiver detect the boundaries of a frame?
0111010001110111110111011001110
A: Mark the beginning and end of frames
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Types of Frames
• Fixed length frames
– E.g., ATM , SONET
– Need to only mark beginning of frame (not covered)
• Variable length frames
– E.g., Ethernet
– Need to mark beginning and end of frame (covered)
or
– Beginning of frame and specify length (not covered)
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Two Examples of Protocols
• HDLC: High-Level Data Link Control
– Bit-oriented data link control
• PPP: Point-to-Point Protocol
– Character-oriented version of HDLC
• HDLC & PPP are used in point-to-point communication
– E.g., two routers connected by a long-distance link
Details of these two protocols are in
Sections 11.3 and 11.4 (Forouzan)
5.5 and 5.6 of (Leon-Garcia, Widjaja)
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Flag-Based Frames
• Can have arbitrary number of bits in a frame
HDLC frame
Flag
Address
Control
Information
FCS
Flag
any number of bits
• Flag: the byte 0x7E (01111110)
• What to do if the flag occurs inside a frame?
A: Bit stuffing
FCS: Frame Check Sequence – used to detect/correct bit errors
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Bit Stuffing in HDLC
• Flag: the byte 0x7E (01111110)
• Transmitter stuffs a 0 after five consecutive 1s
– E.g., 11111111 -> 111110111
– E.g., 11111011 -> 111110011
This is done regardless of what comes after the five 1s!

Receiver unstuffs the 0 after five 1s
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Example: Bit Stuffing & De-Stuffing
(a)
Data to be sent
0110111111111100
After stuffing and framing
01111110 01101111101111100001111110
(b)
Another set of data received
01111110000111011111011111011001111110
After destuffing and deframing
*000111011111-11111-110*
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Byte-Stuffing in PPP
•
•
•
•
•
•
PPP is character-oriented version of HDLC
PPP uses similar frame structure as HDLC
But payload contains an integer number of bytes
Flag is the same: 0x7E (01111110)
Define a control escape octet 0x7D (01111101)
If the flag or control escape values appear inside a frame
– It is replaced with 0x7D followed by original octet XORed with 0x20 (00100000)
Data to
be sent
7E
41
41
7D
7D
5D
42
7E
42
7D
50
70
5E
46
50
After stuffing
and framing
70
46
7E
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