Encoding

1
Encoding
2
Objectives

• Understand basic encoding schemes
• Understand basic framing schemes
• Know bit stuffing
• Ethernet lab introduction

3
Non-Return to Zero (NRZ)
0 0 1 0 1 1 1 1 0 1 0 0
0 0 1 0
Bits
NRZ

• Problem Consecutive 1s or 0s
• Low signal (0) may be interpreted as no signal
• High signal (1) leads to baseline wander
• Unable to recover clock

4
NRZI and Manchester

• Non-return to Zero Inverted (NRZI) Make a
  transition from the current signal to encode a
  one, and stay at the current signal to encode a
  zero
• solves the problem of consecutive ones.
• Manchester Transmits the XOR of the NRZ encoded
  data and the clock only 50 efficient.

5
Example
Manchester encoding allows the clocks to be
synchronized NRZI eliminates consecutive 1s and
baseline wander Consecutive zeros can still make
it difficult to recover clock
Bits
0 0 1 0 1 1 1 1 0 1 0 0 0
0 1 0
NRZ
Clock
Mancester
NRZI
6
4B/5B

• Problem consecutive zeros
• Idea Every 4 bits of data is encoded in a 5-bit
  code, with the 5-bit codes selected to have no
  more than one leading 0 and no more than two
  trailing 0 (i.e., never get more than three
  consecutive 0s).
• Resulting 5-bit codes are then transmitted using
  the NRZI encoding. Achieves 80 efficiency.
• We already dealt with consecutive 1s with NRZI

7
4B/5B

• At most one zero on each end
• data
• 1111 1000 0011 1101
• 4B/5B
• 11101 10010 10101 11011
• NRZI
• 0 10110 11100 11001 01101

8
Framing
9
Overview

• Problem Breaking sequence of bits into a frame
• Must determine first and last bit of the frame
• Typically implemented by network adapter
• Adapter fetches (deposits) frames out of (into)
  host memory

10
Four Approaches

• Clock Based
• fixed length frames, high reliability required
• Sentinels
• Special character to delineate frames, replace
  character in data stream
• Character Count
• Frame length at certain position in frame
• Physical layer invalid codes
• requires physical layer redundancy

11
Byte-Oriented Protocols

• Sentinel Approach
• PPP protocol uses 0x7e01111110 as the flag byte
  to delimit a frame
• When a 0x7e is seen in the payload, it must be
  escaped to keep it from being seen as an end of
  frame

16
8
8
8
8
14
Addr
Flag
Payload
Checksum
Flag
Prot
Cont
12
PPP Escape Character

• To escape a character first send the escape
  character ''. Then send the character to be
  escaped XORed with 001000002. To escape '', send
  the escape character '' followed by the ASCII
  value of '' (011111102) XORed with 001000002.

13
Bit-Oriented Protocols

• HDLC High-Level Data Link Control (also SDLC and
  PPP)
• Delineate frame with a special bit-sequence
  01111110

14

• Bit Stuffing
• Sender any time five consecutive 1s have been
  transmitted from the body of the message, insert
  a 0.
• Receiver should five consecutive 1s arrive, look
  at next bit(s)
• if next bit is a 0 remove it
• if next bits are 10 end-of-frame marker
• if next bits are 11 error

15
State Machine
0
1
0
1
0
1
0
1
0
1
Insert 0
16
Bit stuffing Example

• Original Data
• 001111111000011111100
• Bit Stuffed
• 00111110110000111110100
• Receiver
• 0011111011000011111010001111110

End of Frame
17
Ethernet
18
Overview

• History
• Developed by Xerox PARC in mid-1970s
• Roots in Aloha packet-radio network
• Standardized by Xerox, DEC, and Intel in 1978
• Similar to IEEE 802.3 standard
• Manchester encoding, synchronous transmission

19
Frame Format

• Addresses
• Unique, 48-bit unicast address assigned to each
  adaptor
• Example 802be4b12
• Broadcast all 1s
• Multicast first bit is 1

20
Reading

• "GIVE ATTENDANCE TO READING"
• by Brad Wilcox, Associate Professor, Brigham
  Young University
• Have you ever read a book and then seen a movie
  made from the book? Were you disappointed? Most
  people are. That's because reading is a mentally
  active experience while watching TV or movies is
  a mentally passive one.
• When we watch TV or a video, we do not have to
  imagine the setting. It is shown to us. We do
  not have to imagine what the main characters look
  like. They are shown to us. We do not even have
  to imagine the kissing scenes because--like it or
  not--those are shown to us too. On the other
  hand, reading allows us to use our brains and
  become involved intellectually.

21
Reading
If you are a reader you are following Paul's
counsel to "give attendance to reading" in your
youth (1 Timothy 413). Keep up the great
work. Keep reading magazines, newspapers, cereal
boxes, bumper stickers--anything you can get your
eyes on. Keep reading good books--famous ones,
fun ones, sad ones, spiritual ones. Keep
reading and you will come to understand why the
comedian Groucho Marx once said, "Outside of a
dog, man's best friend is a book inside of a
dog, it's too dark to read. This applies in
particular to the CS460 Book. Make it your
friend.