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2004.3.25.
 Make-up class for 2004.3.23.?
3/27
or 4/15
 HW#2
Be
careful when you submit; don’t broadcast
your solution!
 Chapter 2.1.~2.4.
1
Problems to Consider
When Connecting Node Directly
 Encoding
 Framing
 Error Detection
 Reliable Delivery
 Media Access Control
2
Point to Point Data Link Control
 one sender, one receiver, one link: easier
than broadcast link:
 no Media Access Control
 no need for explicit MAC addressing
 e.g., dialup link, ISDN line
 popular point-to-point DLC protocols:
 PPP (Point-to-Point Protocol)
 HDLC (High level data link control)
 SONET (Synchronous Optical Network)
3
Encoding
 Many ways to “modulate”
process
of encoding source data onto a
carrier signal with frequency f
Digital data -> digital signal
Digital data -> analog signal
Analog data -> digital signal
Analog data -> analog signal
4
Digital Data -> Digital Signal Encoding
5
Digital Data -> Digital Signal




NRZ
Manchester
NRZI
4Bit/5Bit
6
Framing
 Definition?
Task
of recognizing where a frame begins and
ends
Byte-oriented: PPP
Bit-oriented: HDLC
Clock-based framing: SONET
7
PPP Data Frame
 Flag: delimiter (framing)
 Address: does nothing (only one option)
 Control: does nothing; in the future possible
multiple control fields
 Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
8
PPP Data Frame
 info: upper layer data being carried
 check: cyclic redundancy check for error
detection
9
Byte Stuffing
 “data transparency” requirement: data
field must be allowed to include flag
pattern <01111110>
 Q: is received <01111110> data or flag?
 Sender: adds (“stuffs”) extra < 01111110>
byte after each < 01111110> data byte
 Receiver:
 two 01111110 bytes in a row: discard
first byte, continue data reception
 single 01111110: flag byte
10
Byte Stuffing
flag byte
pattern
in data
to send
flag byte pattern plus
stuffed byte in
transmitted data
11
Bit-Oriented Protocols
 HDLC
Similar
to PPP
Uses bit stuffing
12
Clock-Based Framing
 SONET
Used
mostly for digital transmission over
optical fiber

13
Acknowledgements
 Slides ###
copyright
Ross
1996-2000, Jim Kurose and Keith
14
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